Guidelines for the Interpretation of Dissolved Gas Analysis (DGA) for PaperInsulated Underground Transmission Cable SystemsTechnical Report Guidelines for the Interpretation of Dissolved Gas Analysis (DGA) for Paper-Insulated Underground Transmission Cable Systems 1000275 Final Report, September 2000 EPRI Project Manager W. Zenger EPRI • 3412 Hillview Avenue, Palo Alto, California 94304 • PO Box 10412, Palo Alto, California 94303 • USA 800.313.3774 • 650.855.2121 •
[email protected] • www.epri.com DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: (A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR (B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT. ORGANIZATION(S) THAT PREPARED THIS DOCUMENT Detroit Edison Company ORDERING INFORMATION Requests for copies of this report should be directed to the EPRI Distribution Center, 207 Coggins Drive, P.O. Box 23205, Pleasant Hill, CA 94523, (800) 313-3774. Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc. EPRI. ELECTRIFY THE WORLD is a service mark of the Electric Power Research Institute, Inc. Copyright © 2000 Electric Power Research Institute, Inc. All rights reserved. CITATIONS This report was prepared by Detroit Edison Company 2000 Second Ave. Detroit, MI 48226 Principal Investigators N. Singh O. Morel This report describes research sponsored by EPRI. The report is a corporate document that should be cited in the literature in the following manner: Guidelines for the Interpretation of Dissolved Gas Analysis (DGA) for Paper-Insulated Underground Transmission Cable Systems, EPRI, Palo Alto, CA: 2000. 1000275. iii REPORT SUMMARY Laminar dielectric underground transmission cables represent a utility investment of approximately $20 billion. Protection of this investment depends on proper condition monitoring to avoid unscheduled outages, which can amount to hundreds of thousands of dollars per incident. The dissolved gas analysis (DGA) technique—which has been successfully applied to transformers—has now proven itself a cost-effective alternative for condition assessment of these paper-insulated underground transmission cable systems. This guide explains the basic elements of successful DGA application to such systems, with emphasis on proper data interpretation. Background Laminar dielectric cables include high-pressure fluid-filled (HPFF), self-contained liquid-filled (SCLF), and high-pressure gas-filled (HPGF) cable systems. Insulation for these cables consists of cellulose paper and a dielectric fluid, both of which govern the condition and life of these cables (assuming that the cable’s steel pipe or lead sheath maintains integrity and protects the insulating system from moisture ingress). Under the thermal and electrical stresses experienced by the cable, the insulating system generates gases such as carbon dioxide, carbon monoxide, hydrogen, and lower and higher hydrocarbon gases. The type, distribution, and concentration of these gases in cable fluid samples form the basis of DGA condition assessment. Recognizing the potential for DGA condition assessment of installed laminar dielectric underground transmission cable systems, EPRI initiated the first DGA project at Detroit Edison in 1983. Detroit Edison has continued work under EPRI sponsorship, resulting in several comprehensive reports based on extensive laboratory, field, and Waltz Mill cable testing studies. Objective To develop a guide for applying the DGA technique to condition assessment of paper-insulated underground transmission cable systems. Approach Developers of this guide drew from the extensive laboratory and field work performed by Detroit Edison on more than 6000 samples. This field work focused on condition assessment of a variety of cable designs, vintages, and accessories using DGA techniques. Detroit Edison performed the work for EPRI, Empire State Electric Energy Research Corporation (ESEERCO), and numerous North American, South American, European, and Southeast Asian utilities; however, the vast majority of data relates to U.S. HPFF cable systems. v Results The DGA technique—which can be applied to energized HPFF and HPGF cables—offers a number of unique advantages for distinguishing between a satisfactory and a problem cable system and assessing the severity of any problems detected. First, the technique is easy to apply in the field, with a sampling time of under 15 minutes. It is reliable, has proven extremely costeffective, and requires only minimal lightweight field equipment. Moreover, DGA is highly sensitive to electrical and thermal stresses and can detect gases at very low levels, with relatively modest equipment investment. The DGA process is, in fact, accumulative, meaning it is able to detect activity that has already occurred or is intermittent in nature. Finally, the DGA technique may prove viable for use in performing remaining life assessments. DGA success depends on several factors, including proper fluid sampling and handling, accurate chemical analysis, sound interpretation of the generated DGA data, and a knowledge of cable history. This guide addresses such factors, aided by tables, illustrations, and photographs so that utilities can cost-effectively apply the DGA approach for condition monitoring of installed paper-insulated transmission cable systems. A flowchart defines use of the DGA technique for the entire cable system—including terminations, trifurcators, splices, and cable runs. In addition to these features, the guide also provides training material for engineers and technicians to use in maintaining cable systems via the DGA process. EPRI Perspective Utilities are facing erosion of cable expertise through the loss of experienced personnel, many of whom were involved in the development, installation, and maintenance of laminar dielectric high-voltage cable systems. The need for diagnostic capabilities becomes all the more imperative due to the advancing age of such cable systems. It is particularly noteworthy that nearly 15% of these cables are reaching or have exceeded their design life of 40 years and 30% are more than 30 years old. Rapidly emerging competition in the utility industry dictates minimum expenditures to prolong the use of existing cable assets in order to defer cable replacements. As a part of its continuing efforts to ameliorate this situation, EPRI plans to issue a set of guides that will readily train new engineers and technicians in the operation and maintenance of underground transmission systems. This first training guide describes the application of DGA techniques to condition monitoring of installed paper-insulated transmission cable systems and the encouraging results obtained to date. Related EPRI research includes Dissolved-Gas Analysis Method for HPFF Paper Cable (EL-7488-L); Dissolved Gas Analysis (DGA) by EPRI Disposable Oil Sampling System (EDOSS) (TR-111322); and Transmission Cable Life Evaluation and Management (TR-111712). Keywords Transmission cables Condition assessment Dissolved gas analysis vi ABSTRACT Condition monitoring of installed laminar dielectric underground transmission cables is crucial if utilities are to remain competitive, protect their $20 billion investment in these systems, and defer additional investments in cable replacement. Proper condition monitoring will also help prevent unscheduled outages that run into hundreds of thousands of dollars per incident. Utilities are increasingly using the dissolved gas analysis (DGA) technique—which has been successfully applied to transformers—for condition monitoring of these paper-insulated underground transmission cable systems, with encouraging results. Under the thermal and electrical stresses experienced by the cable, the insulating system generates gases such as carbon dioxide, carbon monoxide, hydrogen, and lower and higher hydrocarbon gases. The type, distribution, and concentration of these gases in cable fluid samples form the basis of DGA condition assessment. The DGA technique—which can be applied to energized HPFF and HPGF cables—offers a number of unique advantages for distinguishing between a satisfactory and a problem cable system and assessing the severity of any problems detected. First, the technique is easy to apply in the field, with a sampling time of under 15 minutes. It is reliable, has proven extremely costeffective, and requires only minimal lightweight field equipment. Moreover, DGA is highly sensitive to electrical and thermal stresses and can detect gases at very low levels, with relatively modest equipment investment. The DGA process is, in fact, accumulative, meaning it is able to detect activity that has already occurred or is intermittent in nature. Finally, the DGA technique may prove viable for use in performing remaining life assessments. Recognizing the potential for DGA condition assessment of paper-insulated underground transmission cable systems, EPRI initiated the first DGA project at Detroit Edison in 1983. This guide explains the basic elements of successful DGA application to these cable systems, with emphasis on proper data interpretation. Detroit Edison has continued work under EPRI sponsorship, resulting in several comprehensive reports based on extensive laboratory, field, and Waltz Mill cable testing studies. vii PREFACE Utilities are facing erosion of cable expertise through the loss of experienced personnel, many of whom were involved in the development, installation and maintenance of laminar dielectric high voltage cable systems, namely, High Pressure Fluid-Filled (HPFF), High-Pressure Gas-Filled (HPGF) and Self Contained Liquid-Filled (SCLF) cable systems. As a part of its continuing efforts to ameliorate this situation, EPRI has planned a set of guides that will readily train new engineers and technicians alike in the operation and maintenance of such underground transmission systems. The first training guide issue consists of the interpretation of Dissolved-Gas Analysis (DGA). The aim of this Guide is to explain through simple language, illustrations, and pictures, the basic elements involved in the successful application of DGA to assess the condition of laminar dielectric transmission cable systems. Included in the basic elements are: • • • • • • Fluid sampling Fluid sample handling Fluid sample storage Fluid sample transportation Chromatographic gas analysis Interpretation of results and recommendations It will be assumed that the reader knows little about DGA but has some basic understanding of the various laminar dielectric cables, cable accessories and general cable installation. ix ACKNOWLEDGMENTS The valuable contribution of Dr. Reza Ghafurian of Consolidated Edison Company of New York, Mr. Mohammad Khajavi of Los Angeles Department of Water and Power (LADWP) and Mr. Takashi Kojima of BC Hydro toward the preparation of this guide is gratefully acknowledged. Thanks are in order for many utility engineers, too numerous to mention, in North America and overseas who provided opportunities to develop a DGA database. The continued support and guidance of Mr. Walter Zenger of EPRI is deeply appreciated. xi CONTENTS 1 INTRODUCTION.................................................................................................................. 1-1 2 FLUID OR GAS SAMPLING FOR CABLE SYSTEMS, INCLUDING HANDLING, STORAGE AND TRANSPORTATION.................................................................................... 2-1 EDOSS Method .................................................................................................................. 2-1 EDOSS Sampling Procedure ......................................................................................... 2-4 Vial Number Code.......................................................................................................... 2-6 EPOSS Method .................................................................................................................. 2-6 EPOSS Sampling Procedure ......................................................................................... 2-7 Cell Filling ...................................................................................................................... 2-8 Disassembling................................................................................................................ 2-8 Application of DGA to Laminar Dielectric Cables, Splices and Terminations....................... 2-9 Sampling of HPFF & SCLF Cable Accessories................................................................... 2-9 Sampling of HPFF Terminations ....................................................................................... 2-10 Sampling HPGF Cables.................................................................................................... 2-11 3 GAS ANALYSIS OF COMBUSTIBLE AND NON-COMBUSTIBLE GASES: AVAILABLE METHODS, INCLUDING EPRI METHODS........................................................ 3-1 Dynamic Headspace........................................................................................................... 3-1 Static Headspace ............................................................................................................... 3-2 EDOSS System .................................................................................................................. 3-3 4 INFLUENCE OF ORIGINAL FLUID QUALITY WITH EMPHASIS ON ELECTRIC AND THERMAL ACTIVITY; EFFECT OF THE TYPE OF FLUID, NATURAL VERSUS SYNTHETIC HYDROCARBONS ............................................................................................ 4-1 5 INTERPRETATION OF DGA DATA .................................................................................... 5-1 6 FLOW-CHART ON SAMPLING PROCEDURE, PROBLEM LOCATION AND DGA INTERPRETATION................................................................................................................. 6-1 xiii 7 CONCLUSIONS................................................................................................................... 7-1 A GLOSSARY ........................................................................................................................A-1 xiv LIST OF FIGURES Figure 2-1 Glass Sampling Vial .............................................................................................. 2-2 Figure 2-2 Two Views for Quick-Connect Vial Holder ............................................................. 2-3 Figure 2-3 Sampling Tool Showing the Metallic Housing for Needle Support and the 3-Way Valve .................................................................................................................... 2-3 Figure 2-4 Quick-Connect Coupler and Vial, Before and After Sampling ................................ 2-4 Figure 2-5 Schematic Diagram for EDOSS Sampling Procedure............................................ 2-5 Figure 2-6 Schematic of Duplicate Fluid Sampling by EPOSS Method Along With Fittings and Valves .......................................................................................................... 2-7 Figure 2-7 DGA Sampling by EPOSS..................................................................................... 2-8 Figure 2-8 Sampling of a 345 kV Cable Splice by EDOSS ................................................... 2-10 Figure 2-9 Sampling Kit for HPGF Cables ............................................................................ 2-12 Figure 2-10 Field Sampling of a HPGF Cable....................................................................... 2-13 Figure 3-1 Headspace Analyzer Showing the Sampling Vials and the 50-Sample Carrousel......................................................................................................................... 3-3 Figure 3-2 Gas Chromatographs Utilized in DGA ................................................................... 3-4 Figure 3-3 Schematic Diagram of a Gas Chromatograph for Dissolved Gas Analysis ............ 3-4 Figure 5-1 Bar Chart Showing Normal Gas Concentrations for HPFF Splices ........................ 5-2 Figure 6-1 Flow-Chart for Fluid Sampling and DGA Interpretation.......................................... 6-2 xv LIST OF TABLES Table 1-1 Estimated Circuit Length and Investment of Laminated Underground Transmission Cables in the U. S...................................................................................... 1-2 Table 4-1 Effect of Peroxide Content on Gas Content of New Fluids...................................... 4-2 Table 4-2 Gas Concentration for Fluid Samples From Various Sources ................................. 4-3 Table 5-1 Gas Concentration Limits for Splices and Cable Runs of Static HPFF Cables........ 5-6 Table 5-2 Gas Concentration Limits for Terminations of HPFF Cables................................... 5-7 Table 5-3 Gas Concentration Limits for Forced-Cooled HPFF Cables.................................... 5-8 Table 5-4 Gas Concentration Limits for SCLF Cable Splices.................................................. 5-9 Table 5-5 Gas Concentration Limits for Terminations of SCLF Cables ................................. 5-10 Table 5-6 Gas Concentration Limits for HPGF Cables (200 psi Nitrogen) ............................ 5-11 Table 5-7 DGA Schedule for Splices and Terminations ........................................................ 5-12 Table 5-8 Gas Concentrations and Ratios Included in the Six-Digit Code for the Interpretation of Dissolved Gas Analysis of HPFF Cables and Accessories................... 5-13 Table 5-9 Fault Diagnosis of Cables Through Dissolved Gas Analysis................................. 5-16 Table 5-10 Six Digit Code Showing Arcing Pathway to a 345 kV Cable Failure.................... 5-16 Table 5-11 Example of Application of Condition Assessment Code to HPFF Cable Termination – DGA Data ............................................................................................... 5-17 Table 5-12 Example of Application of Condition Assessment Code to HPFF Cable Splice – DGA Data......................................................................................................... 5-18 xvii 1 INTRODUCTION Laminar dielectric underground transmission cables, which are an integral part of the electric power system in the United States, represent a utility investment of approximately 20 billion dollars. It is essential to protect this investment. This can be accomplished by proper condition monitoring to avoid unscheduled outages, which can amount to several hundred thousands of dollars for laminar dielectric underground transmission cable systems. The need of this diagnosis becomes imperative all the more due to the advancing age of such cable systems. It is noteworthy that nearly 15% of these cables are reaching or have exceeded their design life of 40 years and 30% are over 30 years old. The rapidly emerging competitive utility climate also dictates that prolonged and trouble-free use of cable assets be made with minimum expenditures, deferring cable replacements. The laminar dielectric cables include High-Pressure Fluid-Filled (HPFF), Self-Contained LiquidFilled (SCLF) and High-Pressure Gas-Filled (HPGF) cable systems. The three key components of such cables are: conductor, insulation, and a metallic covering, namely, steel pipe for HPFF and HPGF cables and lead or aluminum sheath for SCLF cables. The insulation consists of cellulosic paper and a dielectric fluid. This combination governs the condition and life of these cables, assuming that the steel pipe or lead sheath maintains integrity, protecting the insulating system from moisture ingress. Whereas fluids of high (3500 SUS @ 100 F) and low (125-600 SUS @ 100 F) viscosities serve respectively as the impregnating and pipe filling pressuring (200 psi) medium in a HPFF cable, a single dielectric liquid of low (60 SUS @ 100 F) viscosity is used in SCLF cable at a pressure in the 15-50 psi range. The pipe filling pressuring medium for a HPGF cable is nitrogen at 200 psi, the viscosity of the highly viscous impregnating fluid being 1000 to 3000 SUS at 210 F. The insulating system generates gases such as carbon dioxide, carbon monoxide, hydrogen, and lower and higher hydrocarbon gases under thermal and electrical stresses experienced by the cable. The type, distribution and concentration of such gases form the basis of condition assessment of laminar dielectric cables through DGA on fluid samples removed from such cables. The year of introduction, voltage class along with the estimated average age, circuit lengths and present-day Investment dollars are given in Table 1-1 for the three types of cables. While the present market for SCLF is limited, HPFF cables are still being purchased by U. S. utilities for new, replacement and re-routed circuits. Recently, HPGF cables have been receiving a renewed interest at 115 through 138 kV as they offer the rugged and reliable pipe-type construction without raising environmental concern because of free dielectric fluid. 1-1 Introduction Table 1-1 Estimated Circuit Length and Investment of Laminated Underground Transmission Cables in the U. S. Present Day Investment (in $millions) $15,000 $750 $1,500 Cable Type HPFF, 69-345 kV HGFF, 69-138 kV SCLF, 69-525 kV Year of Introduction 1935 1941 1927 Circuit Length (in miles) 2,900 300 450 Average Age (Years) 25 35 45 The Dissolved-Gas Analysis (DGA) technique, which has been successfully applied to transformers, is being increasingly considered for laminar dielectric transmission cables, with encouraging results. This technique offers the following unique advantages: • • • • • • • • Easy-to-apply in the field, sampling time is under 15 minutes Fast and reliable approach Highly cost effective, requiring minimal light weight field equipment Applicable to energized HPFF and HPGF cables, a particularly desirable feature High sensitivity to electrical and thermal stresses, with discerning capability Gases detectable at very low levels with relatively modest equipment investment Accumulative process able to detect activity that has already occurred or is intermittent in nature Potential for remaining life assessment The success of DGA depends on several factors such as proper fluid sampling, handling, accurate chemical analysis and sound interpretation of the generated DGA data. In addition, a knowledge of cable history: failures, repairs, cable additions, re-routing, fluid leaks, make-up fluid or nitrogen as for HPGF cables, initial quality of pipe filling fluid for HPFF or impregnating fluid for SCFF cables, disposition of dielectric fluid reservoir(s) as well as any common reservoirs, presence of hot-spots and loading is helpful. The initial quality of nitrogen for HPGF cables is important for condition assessment through gas analysis. Data interpretation is of utmost importance as improper interpretation can lead to wrong decisions that can be very expensive. While there is some commonality in transformer and cable DGA, marked differences exist as a result of distinct designs, materials and operations of the two electrical products. Accordingly, the transformer DGA experience cannot be directly applied to cables. Application of transformer guidelines can shut down otherwise satisfactorily operating cables. The guide draws from the extensive laboratory as well as field work (over 6,000 samples) performed by Detroit Edison for EPRI, ESEERCO and numerous North American and overseas utilities toward the condition assessment of a variety of cable designs, vintages, and their 1-2 Introduction accessories through DGA since the early 1980s. It should be stressed that vast majority of these data relates to HPFF cable systems. The purpose of this guide is to address these various factors with the aid of tables, illustrations and pictures so that it can be conveniently and profitably applied to monitor the condition of inservice laminar dielectric transmission cable systems by means of DGA. It is also meant to serve as a training guide for engineers and technicians for the maintenance of such cable systems through DGA. The various steps and procedures involved in the sampling, storage, handling and transportation of in-service cable fluids have been discussed, facilitating the role of field technicians. Particular attention has been paid to the different sampling approaches relating to splices, terminations and cable runs so that DGA can be properly performed on these three components. The interpretation aspects and methodology of DGA are addressed along with the frequency of sampling. Depending on the condition of the cable system, the gas concentration limits are classified into four categories, namely, normal, acceptable, concern and action level. These levels are arranged in increasing order of severity. The normal and acceptable levels are self-explanatory. Concern level means that the concentration of key gases is high enough to require close monitoring through DGA. The action level category requires prompt attention, which includes inspection and possibly repairs. It is advisable to consult with experts before the expensive decision to open the cable system is made. A flow-chart delineating the DGA technique for the entire cable system, including terminations, trifurcators, splices and cable runs has been presented to aid in the application of DGA, including both static and forced-cooled lines. 1-3 2 FLUID OR GAS SAMPLING FOR CABLE SYSTEMS, INCLUDING HANDLING, STORAGE AND TRANSPORTATION The importance of correct fluid sampling under service conditions cannot be overemphasized; the results are no better than a representation of the fluid through proper chemical analysis. The removed sample should faithfully correspond to the fluid within the cable system being diagnosed and not the fittings involved. The sampling should cover both the cable and its accessories together with the trifurcators and the pressurizing system. Splices and terminations usually have fluid sampling ports. Unlike static lines, it is enough to sample a circulating line at a single point. Any point along a cable run can be reached by fluid drainage. A termination always has at least one sampling port (or valve). If a valve is not available at the splice, it can be readily provided. The sampling of terminations from the top port implies de-energization. However, the splices of HPFF cables can be sampled without de-energization but this does not hold for the splices of SCLF cables due to safety considerations. A recent Japanese development allows the sampling of splices in energized SCLF cables1. While the same general sampling approach is used for cables, splices and terminations, there are certain differences with respect to these individual products and the type of laminar cable. The choice of sampling system is governed by the DGA method, for example, the conventional ASTM DGA method (D 3612) utilizes both glass syringes and stainless steel cylinders, and Shimadzu method employs only the former. EDOSS Method The EPRI method named EDOSS (EPRI Disposable Oil Sampling System), which is now exclusively utilized by Detroit Edison for DGA, uses small evacuated glass vials. The EDOSS method along with its sampling vials and chemical instrumentation will be described in detail. It should be noted that the EDOSS method is a much simpler and inexpensive version of the previous EPRI method called EPOSS (EPRI Pressurized Oil Sampling System) which had been extensively used for laboratory, field and Waltz Mill investigations. Unlike the EPRI DGA methods, the ASTM D 3612 and Shimadzu DGA methods were primarily developed for power transformers and they continue to serve the transformer industry well. However, the higher gas content in cables coupled with the presence of nitrogen in HPFF and 1 “Development of oil sampling devices for energized oil filled cable system” Okada M. Tenaka, Tamura K. Tsuji, Jicable, pp. 272-277 (1999) 2-1 Fluid or Gas Sampling for Cable Systems, Including Handling, Storage and Transportation SCLF cables can lead to the formation of a fairly large size bubble after sampling. This bubble presents some difficulties in the subsequent gas analysis process, impacting the results to some extent. Gases with low solubility such as hydrogen, nitrogen and carbon monoxide tend to largely collect in the bubble, giving rise to higher or lower concentrations, depending whether or not the bubble is included. Contrary to the recommended ASTM practice, this bubble is often pushed out from the glass syringe in the field while keeping the dielectric fluid. There is no correct way of treating the bubble in the Shimadzu method, as there is no provision to handle the bubble in the sampling loop. The EDOSS vial overcomes the bubble difficulties as the analysis system is designed to operate on the headspace principle, which inherently accounts for both the gaseous and liquid phases. The EDOSS vial consists of a disposable evacuated crimp-top 22 mm x 75 mm glass cell with a nominal capacity of 20 cm3. However, only about 6 cm3 of fluid sample is needed. The vial is sealed with a suitable elastomeric plug, which is secured with a metallic crimp cap. A photograph of the vial is shown in Figure 2-1. This vial requires two more important components to accomplish the sampling process, namely, the quick-connect coupler modified to hold the disposable crimp-top vial and a suitable adapter incorporating a hollow needle through which the fluid is admitted into the vial and a 3-way valve. A metallic housing protects the needle. The quick-connect coupler (or vial holder) and the metallic housing containing the needle and the 3-way valve are shown respectively in Figures 2-2 and 2-3. Two vials, supported by the quick-connect coupler, are shown in Figure 2-4, one of which shows the collected fluid sample. A schematic diagram of the key components of the EDOSS system, as they are put together in sequence for sampling, is illustrated in Figure 2-5. Figure 2-1 Glass Sampling Vial 2-2 Fluid or Gas Sampling for Cable Systems, Including Handling, Storage and Transportation Figure 2-2 Two Views for Quick-Connect Vial Holder Figure 2-3 Sampling Tool Showing the Metallic Housing for Needle Support and the 3-Way Valve 2-3 Fluid or Gas Sampling for Cable Systems, Including Handling, Storage and Transportation Figure 2-4 Quick-Connect Coupler and Vial, Before and After Sampling EDOSS Sampling Procedure The sampling procedure is schematically shown in Figure 2-5, and it illustrates in sequence the various steps involved in the sampling process. It is recommended to take duplicate samples at each location. The steps for duplicate sampling are described as follows: • Attach the sampling tool to the equipment being sampled utilizing proper reduction fittings. With the 3-way valve handle pointing downward, drain about ½ quart of fluid to flush the fittings by opening he main valve of the equipment. After flushing, point the 3-way valve handle upward to direct fluid through the needle. Let some fluid flush through the needle before taking the first sample. Maintain the 3-way valve open during the entire sampling operation. Make sure the rubber seal (gray disc in Figure 2-2) is in place inside the quick-connect vial holder. Secure the empty vial to the quick-connect vial holder. To do so, place the vial in the holder while holding the outer-ring down and then releasing it. With fluid running through the needle, gently push the quick-disconnect holder with a vial into the needle housing. Keep the quick-connect vial holder aligned with the needle to prevent any damage to the needle. Once in place, the upper edge of the vial holder should align with the upper edge of the needle housing, refer to Figure 2-5. Admit fluid up to about the center blue line of the vial as shown in Figure 2-5. To retrieve the vial, simply pull the vial straight up from the housing and then remove it from the quick-disconnect. Repeat the procedure with a second vial while keeping the initial fluid flow through the needle. After the second sample is taken, close the main valve and then turn the 3-way valve handle downward to release fluid pressure before disassembling. Restore the corrosion protection tape to protect the main valve • • • • • 2-4 Fluid or Gas Sampling for Cable Systems, Including Handling, Storage and Transportation • Zep Brake Wash spray can be used to clean the sampling device. Be careful, Zep Brake Wash is very flammable. Place the vials with the red crimp down in the foam filled boxes supplied by Detroit Edison and ship without delay. Fluid flush Fluid flush Fluid level Figure 2-5 Schematic Diagram for EDOSS Sampling Procedure 2-5 Fluid or Gas Sampling for Cable Systems, Including Handling, Storage and Transportation Vial Number Code The disposable sampling vials are under vacuum, with a shelf life of about 2 months. The prepared vials are coded with a 9-digit identification number before shipment. The first four digits indicate the month and day (February 2 as 0202) of shipment. Digits 5 and 6 indicate the fractional part of the empty vial weight and the last three digits a sequential number 001 through 999 to identify the vial. There is no need to use the vials in sequential order. EPOSS Method In the previous EPOSS method, a small volume of cable fluid (~20 cm3) is taken in a glass cylinder, which had been previously evacuated to a low pressure (<0.1 mtorr). With the carefully chosen components of this sampling system, the internal pressure can be maintained below detectability limits of the gas analysis system for about one month. The EPOSS cell has a sturdy 3-arm valve fitted with zero clearance o-ring face fittings. Figure 2-6 shows duplicate EPOSS cells; two cells are used in case one cell breaks in transit, leaks or there is a sampling error. Through this valve and with an adapter, the cell can be connected to the sampling equipment, permitting trouble-free sampling. As the fluid is admitted into the EPOSS cell, dissolved gases rapidly evaporate from the fluid until equilibrium is obtained, as is true for its modified version, the EDOSS cell. The evolved gases remain confined in the cell until the analysis is completed. This prevents accidental contamination and gas losses through bubble formation. The gas analysis is carried out in the same cell, which becomes the extraction vessel. As a result, this novel method provides for a simple sampling process with no liquid handling needed during sampling and analysis. 2-6 Fluid or Gas Sampling for Cable Systems, Including Handling, Storage and Transportation Three-Way Valves VCO to Pipe Thread Adapter 1/4" VCO Connectors Sample Inlet Ball Valve Flush Outlet EPOSS Cells Magnetic Stirrers Figure 2-6 Schematic of Duplicate Fluid Sampling by EPOSS Method Along With Fittings and Valves EPOSS Sampling Procedure The procedure to perform DGA sampling by the EPOSS method is described below. A photograph, showing the EPOSS system during fluid sampling from HPFF terminations, is given in Figure 2-7. The EPOSS sampling cells are under vacuum. Do not open the cell top valve unless the sample is being collected. The T-branch under the valve is always open for drainage and is not affected by the status of the valve. • • • Connect together two EPOSS cells plus the ball valve at one end, as shown in Figures 2-6 and 2-7. Attach the valves to the sampling port using the supplied VCO to ¼″ male adapter. You must supply your own fittings to attach the VCO to ¼″ male NPT to the equipment sampling valve. 2-7 Fluid or Gas Sampling for Cable Systems, Including Handling, Storage and Transportation • • Once the cells are attached to the sampling port, open the ball valve fully and then slowly open the valve attached to the equipment being sampled. Drain sufficient fluid to flush all connecting fittings and manifolds (about a quart should be sufficient). At this point, you can take the sample in the amber bottles and syringe. Cell Filling • When ready to take the fluid sample in the EPOSS cell, slowly open the cell valve, allowing sufficient fluid in the cell to reach the red line (1″ from the bottom). Disassembling • • • Once the fluid sample has been taken, close the valve connecting to the equipment being sampled and open the ball valve to release the pressure in the sampling manifold. Remove the EPOSS cells, drain some of the fluid in the T-branch and reinstall the plastic seals on either side of the EPOSS cell. Identify the location of the sample by writing on the supplied labels. Figure 2-7 DGA Sampling by EPOSS 2-8 Fluid or Gas Sampling for Cable Systems, Including Handling, Storage and Transportation Application of DGA to Laminar Dielectric Cables, Splices and Terminations The sampling procedure for HPFF and SCLF cables and splices is essentially the same. However, sampling of terminations requires a different procedure. The sampling of HPGF cables requires steel cylinders because the sample of gas has to be taken at operating pressure. Each procedure is addressed below. Sampling of HPFF & SCLF Cable Accessories Although there are no general rules for the installation of valves in the splice casing of HPFF cables, most splices have two valves, a 2″ gate valve at the top and a 1″ gate valve at the bottom, Figure 2-8. Samples for DGA can be taken from either the top or bottom valve. However, a top sample is preferred for convenience. Before taking the cable fluid sample from these large valves, it is essential to drain sufficient fluid to ensure that the sample reflects the fluid within the splice. It should be noted that the sample taken from the splice does not represent the condition of the cable away from the splice for a static line. To assess the condition of the cable through DGA, fluid has to be drained. Knowing the fluid volume per foot of cable pipe, any location in the cable within adjoining splices can be reached by removing the proper amount of fluid. The direction of fluid movement is governed by the configuration of the pumping plant(s). Whereas two pumping plants will enable fluid movement in either direction, cable systems with only one pumping plant can allow movement only in the direction away from the pumping plant. However, this limitation can be readily overcome by selecting the next splice away from the pumping plant. It should be noted that DGA is the only available technique that allows condition assessment at any point away from the splice in HPFF and SCLF cables. Fluid movement has been successfully applied to assess cable condition2. The sampling of SCLF cables is essentially the same as that of HPFF cables, except that the access valves have smaller sizes. Unlike HPFF cables, SCLF cables have to be de-energized for sampling for safety. “Accurate Gas Analysis Reveals Damaged Cable Section” Innovators with EPRI Technology, IN-101579, October (1992) 2 2-9 Fluid or Gas Sampling for Cable Systems, Including Handling, Storage and Transportation Figure 2-8 Sampling of a 345 kV Cable Splice by EDOSS Sampling of HPFF Terminations There is limited exchange between the cable and termination fluid. The check valves provided at the bottom of the termination limit this exchange. The long narrow annular space between the inner porcelain wall and the cable and stress assembly does not facilitate convective fluid movement. Once dissolved gases are formed, they tend to stay in the same location. The sampling of terminations should reflect the entire length of the termination. The condition assessment of HPFF terminations should be taken seriously due to the physical hazards involved in the shattering of HPFF terminations. Terminations always have a valve at the top. In most cases, this is a small ¼″ NPT valve that is normally utilized to bleed the termination after installation. Some terminations are fitted with a sampling valve in the base spool, below the stress cone region. A valve at the end of the riser section, a few feet from the bottom of the termination, is in provided in some cases. Due to the restricted fluid flow between the terminations and the pipe, samples taken from the riser pipe do not provide a representative sample. The fluid emerging from the raiser valve originates from the cable pipe and might have only a very small amount of gases emanating from the termination. 2-10 Fluid or Gas Sampling for Cable Systems, Including Handling, Storage and Transportation The valve at the termination base spool offers a good sample due to its proximity to the stress cone region; this is invariably true for capacitor graded designs employed at 230 and 345 kV. A second sample should always be taken from the top termination valve, followed by a third sample, as fluid is drained from the top.. These samples allow the assessment of the upper & middle regions of the termination. For terminations without a bottom valve, sampling from the top with drainage is the only option. This evidently requires de-energization of the line. With the exception of the sampling of a 345 kV termination with at the bottom value, the sampling of any 138 kV or 230 kV termination requires de-energization regardless of accessing the bottom or top value or safety considerations. The clearances are too small to allow live-line sampling of terminations. Drainage is needed to cover the entire length of the termination. Considering all these designs, different procedures must be planned ahead in order to obtain good samples. Details of the different sampling scenarios are given as follows: 1. Only top valve, 138 kV HPFF termination • • • • • Take sample from the top valve without drainage Drain 1 to 2 gallons Take a second sample from the top valve Drain another 1 to 2 gallons Take a third sample, after draining 1 to 2 gallons 2. One top valve and one bottom valve, 138 kV HPFF termination • • • • Take sample from the bottom valve without drainage Take first sample from the top valve without drainage Drain 2 to 3 gallons Take second sample from the top valve 230 kV and 345 kV terminations will invariably have valves both at the bottom and top. However, the drained volume between samples should be increased to account for the larger volume and length of termination. Sampling HPGF Cables The absence of a liquid dielectric renders the sampling of HPGF type cables quite simple to perform. This is also true for the chromatographic analysis. Although the gas sample is taken and maintained at system pressure, the analysis must be conducted at atmospheric pressure, resulting in a dilution equivalent to the ratio of system pressure to atmospheric pressure. Accordingly, the analysis results must be multiplied by this ratio before the data can be compared to those from HPFF type cables. Despite the pressure correction, results from HPGF cables are significantly lower than those of HPFF cables due to the diffusion of the generated gases along the pipe, which does not allow the accumulation of gases at a location in HPGF cables. Moreover, the 2-11 Fluid or Gas Sampling for Cable Systems, Including Handling, Storage and Transportation amount of the total insulating fluid for HPGF cables is significantly lesser than that HPFF cables, due to the replacement of he pipe fluid by nitrogen gas. Stainless steel cylinders are utilized to sample HPGF lines. These cylinders have two valves, one at each end as shown in Figure 2-9. The set-up for field sampling of HPGF cables is shown in Figure 2-10. The gas sample is taken by connecting one end of the cylinder to the sampling port at the splice or pipe. Often, valves are provided at the risers. With both cylinder end-valves open, open the cable valve and allow gas to flush through the cylinder. Let gas purge for 2 to 3 seconds and then close the outermost valve followed by the other valve. Then close the cable valve and disconnect the cylinder. It is recommended to sample a HPGF cable at all the splices along the line. An energized HPGF cable can be sampled at its splices, as is true for the case of a HPFF cable. Figure 2-9 Sampling Kit for HPGF Cables 2-12 Fluid or Gas Sampling for Cable Systems, Including Handling, Storage and Transportation Figure 2-10 Field Sampling of a HPGF Cable 2-13 3 GAS ANALYSIS OF COMBUSTIBLE AND NON-COMBUSTIBLE GASES: AVAILABLE METHODS, INCLUDING EPRI METHODS The analysis of gases dissolved in dielectric fluid presents two main challenges: • • Gases must be extracted from the liquid matrix Technique must be sufficiently sensitive to detect trace level gas concentrations Gases separated from the liquid matrix can be separated and quantified utilizing standard gas chromatographic techniques with readily available columns and detectors. However, samples cannot be directly injected into a separation column due to high fluid viscosity and elevated boiling point (low vapor pressure) of the fluid. Hence, sample preparation techniques must be employed to allow the separation of the gaseous components from the liquid. Gases can be separated from the liquid matrix in two ways. Each is addressed below. Dynamic Headspace Gases are stripped from the liquid sample by bubbling a carrier gas through a column containing the sample. The carrier gas with the extracted gases is injected into a gas chromatograph for gas separation and measurement. Commercial application of dynamic headspace is represented by Shimadzu TOGAS (Transformer Oil Gas Analysis System). This equipment, supplied by Shimadzu, consists of a dedicated gas chromatograph connected in tandem with a gas extraction column for transformer DGA. In this unit, the sample is injected directly into a stripping column. The outlet of the stripping column is connected to a chromatographic column(s) for the separation and measurement of gaseous components. Since it is impossible to reach 100% gas extraction for various gases, the extraction efficiency must be determined for a characteristic fluid sample. The extraction efficiency is expected to vary with the viscosity and nature of the fluid. This extraction efficiency is determined by standard gas in oil solutions. Since mineral oil is invariably used in power transformers, only one determination of extraction efficiency is needed. However, several fluids with varying viscosities are employed in cables. The determination of the extraction efficiency is needed in each case. The fluid samples must be collected in 50 cm3 glass syringes. This system has good accuracy, repeatability and is fast. 3-1 Gas Analysis of Combustible and Non-Combustible Gases: Available Methods, Including EPRI Methods Static Headspace In static headspace, the liquid sample is injected into an evacuated constant temperature container of known volume. After phase equilibrium, the headspace is analyzed by gas chromatography. The equilibrium concentration of each component in the liquid phase is calculated from the known concentration of the gas phase and their solubility values (Henry’s Law). The final concentration of gases in the original sample is then obtained. This is expressed in terms of total gas content (gas in gas phase + gas in liquid phase) in cubic centimeters at STP (standard temperature, 273°C & pressure, 1 atm) or NTP (normal temperature, 298°K & 6 3 3 pressure, 1 atm) conditions. A factor of 1x10 is applied to transform the cm gas per cm oil ratio to volume parts per million. A gas concentration in ppm at STP is about 10% smaller than ppm at NTP. Two DGA methods based on static headspace are available: the traditional ASTM procedure (ASTM D-3612) and automatic headspace analyzers. Headspace is the volume of gas in contact with a fluid in a confined vessel. Headspace analyzers are devices that allow the sampling of the headspace for final analysis by gas chromatography. The ASTM D-3612 requires a fluid sample of about 50 cm3 , which is injected into an evacuated glass container of about 500 cm3. With such a fluid volume to headspace ratio, only a small amount of gas remains in the fluid. The extracted gas is later compressed with mercury into a graduated burette to determine its volume. Due to its hazardous nature, mercury is not favored in utility laboratories. A portion of the extracted gas is subsequently injected into a gas chromatograph to determine its gas composition. When fluid samples are taken from high pressure cables with glass syringes, bubbles are formed. According to the ASTM standard, the entire sample including the bubble must be injected in the extraction equipment. Commercially available headspace analyzers allow unmanned analysis of a large number of samples, Figure 3-1. The control of temperature and pressure, in addition to the unmanned handling of gases, leads to a significant increase in accuracy and reproducibility; both are important in the effective application of gas analysis. In headspace chromatography, the fluid sample has to be introduced in a glass vial. This is accomplished by taking the field sample in glass syringes and then transferring to the glass vial in the laboratory. This two step process reduces the accuracy and adds to the cost of the analysis. A novel-sampling device developed under EPRI sponsorship has reduced the two step process to a one step. The developed method, called EDOSS (EPRI Disposable Oil Sampling System), enables the direct filling of the glass vial in the field. The glass vials are discarded after analysis and hence the 'disposable' designation. The glass sampling vials are pre-evacuated to under 1 mtorr (one micron of Hg), weighed, labeled and then delivered to the users in foam packed cardboard boxes with 18 vials each. Each vial weighs about 17 g. Details of the sampling procedure have been provided in Chapter 2. 3-2 Gas Analysis of Combustible and Non-Combustible Gases: Available Methods, Including EPRI Methods Procedures for the calculation of the original gas concentration in the fluid sample consist of basic mass balances. This requires the value of the equilibrium pressure after extraction and gas solubility data at equilibrium temperature. Day to day gas chromatograph calibration is achieved with certified gas blends. Solubility must be determined for each different fluid. Solubility corrections are needed for hydrocarbon gases C2 and higher. Figure 3-1 Headspace Analyzer Showing the Sampling Vials and the 50-Sample Carrousel EDOSS System The EDOSS system corresponds to a static headspace approach. It uses commercially available equipment with some modifications. However, the sampling system, an important part of DGA, is custom-made. In the EDOSS method, the utilization of two gas chromatographs in parallel significantly reduces the time required to perform the analysis, Figure 3-2. Basically, a gas chromatograph is a device in which a carrier gas is forced through a separation column(s) connected to a gas detector(s). The gas sample, placed in a sampling loop, is swept by the carrier gas stream before it enters the separation column(s), Figure 3-3. Analysis of H2, O2, N2, CH4, CO and CO2 is performed by a gas chromatograph as shown in Figure 3-3, while another gas chromatograph is used for the separation of hydrocarbons C2 through C4 hydrocarbons. 3-3 Gas Analysis of Combustible and Non-Combustible Gases: Available Methods, Including EPRI Methods Figure 3-2 Gas Chromatographs Utilized in DGA Sample Loop Column 2 6-W ay Valve Sample In Thermal Flame Methanize Ionizatio Detector 6-W ay Valve Conductivit Detector Column 1 Hydroge Hydroge Oxygen Flow Controller Pressure Regulator Flow Restriction Carrier Gas ( ) Figure 3-3 Schematic Diagram of a Gas Chromatograph for Dissolved Gas Analysis 3-4 4 INFLUENCE OF ORIGINAL FLUID QUALITY WITH EMPHASIS ON ELECTRIC AND THERMAL ACTIVITY; EFFECT OF THE TYPE OF FLUID, NATURAL VERSUS SYNTHETIC HYDROCARBONS The condition of the original pipe fluid has a bearing on the interpretation of the DGA results. Accordingly, it is essential to utilize a gas-free fluid so that DGA can be properly performed. Fluid for bulk use is always delivered in large railways or truck tanks under a 5 psi nitrogen blanket. It should be noted that the fluid for some of the older installations was delivered under a nitrogen blanket containing appreciable amounts of saturated hydrocarbon gases such as methane, ethane, propane and butanes. As a result, there are many old operating HPFF cables with large concentrations of such gases. Although the presence of these gases has no effect on the performance of the cable, it can interfere with DGA interpretation. Another source of gas contamination found in new fluids is due to the presence of oxidation products, namely, peroxides. Several important diagnostic gases such as hydrogen, carbon oxides and hydrocarbons are generated by the decomposition of peroxides at temperatures as low as 3 80°C . Although the yield of these gases is not large, occasionally concentrations of 100 ppm and 1000 ppm of hydrogen and carbon oxides, respectively, have been observed in new fluids taken directly from tank-cars. Formation of peroxide compounds results from fluid handling operations involving high temperatures and small amounts of air, for example, during fluid distillation processes. Some peroxide compounds are very unstable and their decomposition can result in gas formation at relatively low temperatures. Peroxide compounds can be readily eliminated through treatment with activated clays (Fuller’s earth) or silica. Portable degassing units can be fitted with absorption columns to recondition dielectric fluids and reduce dissipation factor and peroxide level of used fluids. Alkylbenzene fluids are characterized by a low peroxide content, below 1 ppm. However, polybutene fluids tend to have a comparatively large peroxide content, 2 to 5 ppm. It is recommended that new polybutene fluids should have a low peroxide content, not exceeding 2 to 3 ppm. A high peroxide content will lead to high content of carbon oxides and hydrogen. This 3 effect is shown in Table 4-1 . 3 Singh, N., Morel, O.E., Singh, S.K. and Rodenbaugh, T.J., “Predictive Maintenance of Fluid-Filled Taped Cables through a Novel Dissolved Gas Analysis Method: US Field Experience”, CIGRE 15/21/33-16 (1996) 4-1 Influence of Original Fluid Quality With Emphasis on Electric and Thermal Activity; Effect of the Type of Fluid, Natural Versus Synthetic Hydrocarbons The dissolved gas analysis data for fluids with high peroxide content and non-standard nitrogen blanket are compared to a well degassed fluid in Table 4-2. The first column shows the gas composition of a fluid sample taken directly from a degassing unit. This is considered to be a high quality, gas-free fluid sample. Column two represents a new fluid with high peroxide oxidation products. This fluid, a polybutene, was sampled directly from a tank-car at a U.S. utility. This situation is not desirable from a DGA standpoint. Table 4-1 Effect of Peroxide Content on Gas Content of New Fluids CO (v/v ppm) 0 541 3 Fluid DO 100 06CS 2 1 CO2 (v/v ppm) 129 1,336 186 i-C4H8 (v/v ppm) 101 1,325 761 Actual Peroxide (w/w ppm) 0.2 2.6 0.3 Estimated Peroxide (w/w ppm) 0.2 3.8 0.9 4 DO 500 1 2 3 4 40 Low viscosity alkylbenzene Medium viscosity polybutene Medium viscosity blend, 75% alkybenzene: 25% polybutene Peroxide content estimated from the yield of carbon oxides 4-2 Influence of Original Fluid Quality With Emphasis on Electric and Thermal Activity; Effect of the Type of Fluid, Natural Versus Synthetic Hydrocarbons Table 4-2 Gas Concentration for Fluid Samples From Various Sources Unused Degassed Fluid (ppm) 0 0.3 0 0 0.4 0 0.6 0.8 5.4 0 0 0 530 Nil Fluid From Tank Car Showing High Peroxide Content (ppm) 23 7 2 0 14 7 23 28 85 243 822 100 179,056 Nil In-Service Fluid With NonStandard Nitrogen Blanket (ppm) 53,850 11,840 2 0 4,383 11 238 365 10 14 424 23 77,462 Nil Gas Methane Ethane Ethylene Acetylene Propane Propylene Isobutane n-Butane Isobutylene C. Monoxide C. Dioxide Hydrogen Nitrogen Oxygen Column three shows the DGA for a 230 kV cable operating satisfactorily but showing very high methane and ethane levels. The large concentrations of methane, ethane and propane compared to ethylene and propylene should be noted. The former gases essentially came with the original fluid (a mineral oil), which did not have a standard nitrogen blanket during transportation and/or storage in that this nitrogen blanket contained appreciable concentrations of saturated hydrocarbons as opposed to the recommended pure nitrogen. The non-saturated hydrocarbons, namely, ethylene and propylene were probably formed in-situ. 4-3 5 INTERPRETATION OF DGA DATA The insulation system of laminar dielectric cables is composed of paper insulation and a dielectric fluid. These two components are subjected to both electrical and thermal stresses during cable operation. As a result, a large number of combustible and non-combustible gases are generated. These gases, which include carbon dioxide, carbon monoxide, hydrogen and lower and higher hydrocarbon gases, remain dissolved in the dielectric fluid in a liquid state, and hence the name “dissolved gases”. Once formed, they remain in place, even if, the cause of generation has disappeared. This confers a unique advantage upon DGA over partial discharges that disappear due to many factors such as modifications of the internal surface of the void, or its pressure or shape. While carbon oxides along with minute amounts of hydrogen and methane are evolved from paper, the dielectric fluid yields hydrogen, methane, ethane, ethylene, acetylene, propane, propylene, n-butane, isobutane, isobutylene, t-2-butene and 1-butene. The pipe fluid can also yield carbon oxides, depending on its peroxide content as discussed in Chapter 4. Cables operating in a normal manner have a low dissolved gas content. The presence of high gas concentration indicates that the cable system is experiencing unusual electrical and/or thermal activity. This has been demonstrated both by laboratory and field studies under EPRI sponsorship. While the laboratory studies may not establish a quantitative relationship between laboratory and field data, they clearly interrelate the specific types of gases and their relative levels generated under electrical and /or thermal conditions encountered in field conditions. The type, concentration, and distribution of gases are governed by the specific nature of the problem faced by a cable. Certain gases can be associated uniquely with either electrical or thermal activity, thereby, giving clues as to the condition of the cable. Dissolved gases do not only lend themselves to accurate measurements but their generation is also highly sensitive to thermal and electrical stresses. Moreover, such gases can be detected at extremely low levels with relative ease and modest chemical instrumentation, unlike partial discharges that require elaborate and sophisticated field equipment. Carbon dioxide and carbon monoxide are related to the thermal condition of paper, the rest of gases evolve from fluid. In normally operating cables, the concentration of carbon dioxide is much larger than that of carbon monoxide. However, the carbon dioxide to carbon monoxide ratio varies as the paper ages. The rate of carbon dioxide evolution tends to decrease with time, but the rate of carbon monoxide tends to increase. Thus the ratio of carbon dioxide to carbon monoxide decreases as the paper ages, giving a clue as to the degradation of paper. Whereas hydrogen is associated with low level electrical activity in the fluid, acetylene is related to strong electrical activity involving visible arching. Electron bombardment of hydrocarbon chains leads 5-1 Interpretation of DGA Data to the removal of hydrogen from the hydrocarbon chain, leading to an increase in hydrocarbon unsaturation. A similar effect is seen from exposure to elevated temperature, wherein the stability of saturated compounds such as ethane and propane decreases, favoring the formation of ethylene and propylene. Hydrogen and methane tend to increase with temperature. When hydrocarbons are subjected to pronounced ionization activity, acetylene and some carbonized fluid along with other unsaturated (double bonds) species such as ethylene, propylene, and butenes are formed. Typical normal gas concentrations for HPFF cable splices are shown in bar-chart form, Figure 5-1. Acetylene, the single most important diagnostic gas, should be zero for cables operating in a normal fashion. Even a few ppm of can be a cause of concern, requiring attention. It is worth pointing out that in the IEEE transformer DGA guide the level of acetylene is being significantly reduced to less than 5 ppm. The presence of acetylene is always accompanied by an increase in the concentration of ethylene, frequently making the ratio ethane to ethylene closer to one or smaller. The generation of isobutylene is connected with the thermal decomposition of the dielectric fluid, particularly polybutenes for which isobutylene is the starting material. It should be emphasized that both the levels and ratios of gases are important. It is essential to consider the entire pattern of gases rather than rely on a few individual gases for proper data interpretation. Methane Ethane Ethylene Acetylene Propane Propylene Hydrogen C. Monoxide C. Dioxide 0 20 0 70 100 200 50 1000 100 500 200 400 600 800 1000 Gas Concentration (ppm) Figure 5-1 Bar Chart Showing Normal Gas Concentrations for HPFF Splices While sound DGA experience can readily distinguish between a normal and problem cable, periodic monitoring to establish gas generation trends and rates is essential for cases posing concern and problem. The gas generation rates should be periodically determined to establish 5-2 Interpretation of DGA Data whether or not a particular cable is maintaining dielectric integrity or deteriorating. Such periodic monitoring also confirms, if any previous unusual electric or thermal activity has ceased. The cable per se, splices and terminations are the sources of gases. It has been observed that in rare circumstances the components of the pumping plant can generate gases that can confuse the DGA picture. However, this invariably involves hydrogen generation as a result of the presence of galvanized fittings or rusting of associated pipes and/or fittings with the reservoir and/or piping. Minute amounts of oxygen in water can accelerate the production of hydrogen. Compared to cables, splices and terminations are the most likely sources of gases. It is also important that the initial dielectric fluid is basically free of gases. This should be ensured in the beginning, as discussed in Chapter 4. For a new fluid, acetylene and hydrogen must be both zero, with the remaining gases generally below 10 ppm. Before nitrogen blanket became the normal practice for cable fluid shipment, ethane, propane or methane were sometimes involved as a part of the nitrogen blanket for such gases are readily available in the refinery. It has been observed that some older cables could have larger amounts of such gases. Terminations can yield significantly larger concentrations of some key gases due to the nature of the electric field, relatively smaller fluid volume, and possible exposure to external/internal over-voltages. Thus, the limits of key gases for terminations are significantly different from those of the splices and cables, with terminations yielding higher limits. It should be noted that for SCLF terminations with reservoirs, the fluid is free to move out of the termination into the reservoir, resulting in a dilution effect contrary to the case of HPFF terminations and SCLF terminations without direct connection to the reservoir. Static Versus Forced-Cooled Cables The influence of forced-cooling on DGA is discussed for the case of HPFF cables as this mode of operation is confined only to HPFF cables in the U.S. The gas concentration limits for forcedcool lines are significantly lower than those for the static lines. Gases generated at a given location in a forced-cooled cable are rapidly mixed and distributed throughout the large volume of the free fluid as a result of fluid circulation. The dilution factor depends on the total volume of the pipe fluid, which is proportional to the cable and return line length, and the number of gas sources, mostly the splices. The individual gas limits for a splice are the same for both types of lines. Therefore, the gas limits for a forced-cooled line must be calculated by the application of a dilution factor that incorporates the effect of mixing. Dissolved gases, which are mostly localized once formed, generally affect the fluid to a distance of about 150 ft , on either side of the source of generation. This corresponds to a fluid volume of about 750 gallons (2.5 gallons/ft.). Assuming a 5-mile long line, the dilution factor becomes (5 [mile] x 5,280 [ft./mile/300 [ft.] = 88). The dilution factor becomes 44 for a 2.5-mile long line. Assuming 2 splices per mile, a 5-mile line will have 10 splices. For a splice giving 10 ppm of acetylene, the forced-cooled line would have approximately 0.11 (10/88) ppm of acetylene. If two splices are equally involved, each yielding 10 ppm, this line will have .22 ppm of acetylene. 5-3 Interpretation of DGA Data This illustrates the marked differences between static and forced-cooled lines. It follows that to identify a problem splice, one has to shutdown the cable for several months. Forced-cooled lines lend themselves more readily to the determination of the rate of gas generation. It is sufficient to sample a forced-cooled line at a single point along the line to determine the average gas concentration; the inlet or outlet of the pump is a convenient aboveground location. However, for a static case, one has to sample each splice to identify the problem and establish trends. A problem on a cable length can be established by moving fluid as DGA samples are taken for a static line. It is not possible to distinguish whether a splice or a cable is involved in a circulating line; only the stoppage of the fluid circulation can allow gas accumulation at the problem spot. The principle of the gas ratios holds for both static and forced-cooled cables. However, the presence of splices complicates the picture. If one splice is yielding an equal amount of carbon dioxide and carbon monoxide, this may not be true for the rest of splices, and this makes the application of ratios difficult. The levels of gases can provide significant clues to the condition of forced-cooled cables. Tables 5-1 through 5-6 provide the normal, acceptable, concern and action levels together with two key gas ratios for various types of paper-insulated transmission cables and their accessories. The term normal means that the cable system is operating in a satisfactory manner. Such cables are characterized by low gas concentrations, requiring long sampling intervals. The gas concentrations in the acceptable condition are higher than the corresponding values for the normal condition, as shown in column 3 of Table 5-1. Under these conditions, small amounts of acetylene are present as well larger concentrations of other gases, however, such levels are low enough not to pose any concern. The frequency of sampling should be increased to ensure that the cable system continues to be acceptable. The concern levels signify that the gas concentrations have increased enough to pose concern, calling for a further increase in sampling frequency. The schedule of sampling is discussed later. The action level category requires prompt attention, ranging from close monitoring of the system to inspection and repairs. It is advisable to consult with experts before the expensive decision to open the cable system is made. Because of the safety hazards associated with the potential shattering of porcelain, a termination problem requires prompt visual examination. The gas levels and the ratios presented in these tables, according to the status of various types of cable systems, are based on laboratory and field data. Based on the values of gas concentrations given in tables 5-1 through 5-6, the DGA schedule for splices and terminations is presented with the recommended action in Table 5-7. While the laboratory studies sponsored by EPRI at Detroit Edison have been helpful, this guide is primarily based on DGA data predominantly generated on HPFF cable systems in the field. Because of the limited sampling of SCLF circuits, it has not been possible to refine the gas concentrations for SCLF cable systems to the extent of HPFF cable systems. However, the concept of carbon dioxide-to-carbon monoxide and ethane-to-ethylene ratio is equally applicable to both types of cable systems. Accordingly, the user is urged a degree of caution in the application of individual gas limits for concern and action levels for SCLF cable splices (Table 5-4), particularly the latter. While the gas levels for terminations of both HPFF and SCLF 5-4 Interpretation of DGA Data are in a better agreement, it is not the case for acetylene concentration in the splices of both types of systems. The significantly higher level of acetylene observed in SCLF cable splices is attributed to the very small volume of fluid between the lead sheath and the overall body of the insulated splice. Unlike its HPFF splice counterpart, the fluid being sampled at the splice casing is not diluted, hence the higher concentration of acetylene for SCLF cable splices. As mentioned earlier in Chapter 5, both the level and the overall pattern of individual gases is important for data interpretation. Accordingly, the data presented in each table should be considered in its entirety. However, the two key gases in order of importance are acetylene and hydrogen. While the gases are a direct consequence of electrical and thermal activity, DGA cannot identify the location of the gas generation source, particularly the electrical type, across the cable cross-section. Thus, DGA will not readily identify if the source is confined to skidwires for HPFF cables and the outer regions of the insulation of both HFF and SCLF cable systems as opposed to the inner parts of the cable. It appears that the higher concentrations of acetylene and hydrogen are confined not to the inner cable insulation but rather the outer layers and shielding. Every effort should be made to understand such a situation, which is not common, through regular DGA. 5-5 Interpretation of DGA Data Table 5-1 Gas Concentration Limits for Splices and Cable Runs of Static HPFF Cables Normal Range (ppm) 0 – 1,000 0 0 – 300 0 – 1,000 0 – 400 0 – 300 0 – 100 0 – 500 Acceptable (ppm) < 10,000 <1 < 500 < 5,000 < 1,000 < 500 < 200 Concern Level (ppm) 10,000 – 40,000 1–5 500 – 1,000 5,000 – 10,000 1,000 –4,000 500 – 1,000 200 – 500 Action Level (ppm) 40,000 + 5+ 1,000 + 10,000 + 4,000 + 1,000 + 500 + Gas Hydrogen (H2) Acetylene (C2H2) C. Monoxide (CO) C. Dioxide(CO2) Methane (CH4) Ethane (C2H6) Ethylene (C2H4) Propane (C3H8) Isobutylene (C2H8) Polybutene Fluids Alkylbenzene Fluids Mineral Oils Oxygen (O2) Nitrogen (N2) Gas Ratios 1 CO2/CO C2H6/C2H4 (1) 0 – 1,500 0 – 100 0 – 200 0 0 – 80,000 < 5,000 < 500 < 1,000 5,000 – 10,000 500 – 1,000 1,000 – 2,000 10,000 + 1,000 + 2,000 + 5 – 10 8 – 10 >1 >1 0.75 – 1 0.75 – 1 < 0.5 < 0.5 Ratios only apply to gas concentration levels larger than 50 ppm for carbon oxides and larger than 20 ppm for hydrocarbons 5-6 Interpretation of DGA Data Table 5-2 Gas Concentration Limits for Terminations of HPFF Cables Normal Range (ppm) 0 – 1,000 0 0 – 300 0 – 1,000 0 – 400 0 – 300 0 – 100 0 – 500 Acceptable (ppm) < 1,500 < 30 < 300 < 5,000 < 1,000 < 500 < 200 Concern Level (ppm) 1,500 – 10,000 30 – 150 300 – 2,000 5,000 – 10,000 1,000 – 4,000 500 – 1,000 200 – 500 Action Level (ppm) 10,000 + 150 + 2,000 + 10,000 + 4,000 + 1,000 + 500 + Gas Hydrogen (H2) Acetylene (C2H2) C. Monoxide (CO) C. Dioxide(CO2) Methane (CH4) Ethane (C2H6) Ethylene (C2H4) Propane (C3H8) Isobutylene (C2H8) Polybutene Fluids Alkylbenzene Fluids Mineral Oils Oxygen (O2) Nitrogen (N2) Gas Ratios CO2/CO C2H6/C2H4 (1) 1 0 – 1,500 0 – 100 0 – 200 0 0 – 80,000 < 5,000 < 500 < 1,000 5,000 – 10,000 500 – 1,000 1,000 – 2,000 10,000 + 1,000 + 2,000 + 5 – 10 8 – 10 >1 >1 0.75 – 1 0.75 – 1 < 0.5 < 0.5 Ratios only apply to gas concentration levels larger than 50 ppm for carbon oxides and larger than 20 ppm for hydrocarbons 5-7 Interpretation of DGA Data Table 5-3 Gas Concentration Limits for Forced-Cooled HPFF Cables Normal Range (ppm) 0 – 500 0 0 – 100 0 – 500 0 – 200 0 – 200 0 – 50 0 – 200 Acceptable (ppm) < 1,500 <2 < 300 < 1,000 < 500 < 500 < 100 Concern Level (ppm) 1,500 – 3,000 2 – 10 300 – 500 1,000- 5,000 500 – 1,000 500 –1,000 100 - 500 Action Level (ppm) 3,000 + 10 + 500 + 5,000 + 1,000 + 1,000 + 500 + Gas Hydrogen (H2) Acetylene (C2H2) C. Monoxide (CO) C. Dioxide(CO2) Methane (CH4) Ethane (C2H6) Ethylene (C2H4) Propane (C3H8) Isobutylene (C2H8) Polybutene Fluids Alkylbenzene Fluids Mineral Oil Oxygen (O2) Nitrogen (N2) Gas Ratios (1) CO2/CO C2H6/C2H4 (1) 0 – 2,000 0 – 200 0 – 200 0 0 – 80,000 < 5,000 < 500 < 500 5 – 10 8 – 10 >1 >1 0.75 – 1 0.75 – 1 < 0.5 < 0.5 Ratios only apply to gas concentration levels larger than 50 ppm for carbon oxides and larger than 20 ppm for hydrocarbons 5-8 Interpretation of DGA Data Table 5-4 Gas Concentration Limits for SCLF Cable Splices Normal Range (ppm) 0 – 600 0 0 – 300 0 – 1,000 0 – 400 0 – 300 0 – 100 0 – 500 Acceptable (ppm) < 1,000 1- 5 < 500 < 5,000 < 1,000 < 500 < 200 Concern Level (ppm) 1,000 – 3,000 5 – 25 500 – 1,000 5,000 – 10,000 1,000 – 4,000 500 – 1,000 200 – 500 Action Level (ppm) 3,000 + 25 + 1,000 + 10,000 + 4,000 + 1,000 + 500 + Gas Hydrogen (H2) Acetylene (C2H2) C. Monoxide (CO) C. Dioxide(CO2) Methane (CH4) Ethane (C2H6) Ethylene (C2H4) Propane (C3H8) Isobutylene (C2H8) Polybutene Fluids Alkylbenzene Fluids Mineral Oils Oxygen (O2) Nitrogen (N2) Gas Ratios CO2/CO C2H6/C2H4 (1) 1 0 – 1,500 0 – 100 0 – 200 0 0 - 80,000 < 5,000 < 500 < 1,000 5,000 – 10,000 500 – 1,000 1,000 – 2,000 10,000 + 1,000 + 2,000 + 5-10 8-10 >1 >1 0.75 – 1 0.75-1 < 0.5 <0.5 Ratios only apply to gas concentration levels larger than 50 ppm for carbon oxides and larger than 20 ppm for hydrocarbons 5-9 Interpretation of DGA Data Table 5-5 Gas Concentration Limits for Terminations of SCLF Cables Normal Range (ppm) 0 – 1,000 0–2 0 – 300 0 – 1,000 0 – 400 0 – 300 0 – 100 0 – 500 Acceptable (ppm) < 2,000 <5 < 500 < 5,000 < 1,000 < 500 < 200 Concern Level (ppm) 2,000 – 5,000 5 – 50 500 – 1,000 5,000 – 10,000 1,000 –4,000 500 – 1,000 200 - 500 Action Level (ppm) 5,000 + 50 + 1,000 + 10,000 + 4,000 + 1,000 + 500 + Gas Hydrogen (H2) Acetylene (C2H2) C. Monoxide (CO) C. Dioxide(CO2) Methane (CH4) Ethane (C2H6) Ethylene (C2H4) Propane (C3H8) Isobutylene (C2H8) Polybutene Fluids Alkylbenzene Fluids Mineral Oils Oxygen (O2) Nitrogen (N2) Gas Ratios (1) CO2/CO C2H6/C2H4 (1) 0 – 1,500 0 – 100 0 – 200 0 0 – 80,000 < 5,000 < 500 < 1,000 5,000 – 10,000 500 – 1,000 1,000 – 2,000 10,000 + 1,000 + 2,000 + 5 – 10 8 – 10 >1 >1 0.75 – 1 0.75 – 1 < 0.5 < 0.5 Ratios only apply to gas concentration levels larger than 50 ppm for carbon oxides and larger than 20 ppm for hydrocarbons 5-10 Interpretation of DGA Data Table 5-6 Gas Concentration Limits for HPGF Cables (200 psi Nitrogen) Normal Range (1) (ppm) 0 – 75 0 0 – 15 0 – 75 0 – 30 0 – 15 0 – 80 0 – 36 Acceptable (ppm) < 700 < 0.07 < 75 < 150 < 75 < 40 < 15 Concern Level 1 (ppm) 700 – 2,200 0.07 - 0.4 75 – 150 150 – 350 75 – 300 40 – 75 15 – 40 Action Level 1 (ppm) 2,200 + 0.4 + 150 + 350 + 300 + 75 + 40 + Gas Hydrogen (H2) Acetylene (C2H2) C. Monoxide (CO) C. Dioxide(CO2) Methane (CH4) Ethane (C2H6) Ethylene (C2H4) Propane (C3H8) Isobutylene (C2H8) Polybutene Fluids Alkylbenzene Fluids Mineral Oils Oxygen Gas Ratios CO2/CO C2H6/C2H4 (1) 2 0 – 100 0 – 10 0 – 20 0 < 350 < 40 < 80 350 - 750 40 - 100 80 – 200 750 + 100 + 200 + 5 – 10 8 – 10 >1 >1 0.75 – 1 0.75 – 1 < 0.5 < 0.5 Gas concentrations must be multiplied by the ratio of cable operation pressure in psi to atmospheric pressure in psi. The numbers given in this table have already been corrected Ratios only apply to gas concentration levels larger than 40 ppm for carbon oxides and larger than 10 ppm for hydrocarbons (2) 5-11 Interpretation of DGA Data Table 5-7 DGA Schedule for Splices and Terminations Accessory Splice Normal 2 to 4 years, depending on cable voltage class Concern 6 months to 1 year Action Establish whether or not the splice is the source of gas. Drain fluid to obtain sample from the cable, both sides if possible. Two pumping plants or one pumping plant and a return line enable inspection of both sides through fluid movement. Consult with experts before opening the splice. Consider changing cable length in question Cable Run 1 to 2 years, if a problem has been identified 2 to 4 years 6 months to 1 year Termination 6 months Open termination for visual inspection/ rebuild. To further enhance the interpretation of DGA data, a code combining gas levels and ratios has been developed based on over 6,000 field samples relating to cables splices, cable runs, trifurcators, reservoirs and terminations. This Code basically holds for static HPFF cable systems, which form the bulk of U.S. paper cable systems. While the code is helpful for SCLF, HPGF and forced-cool cable systems, it is recommended to refer to the applicable Tables, 5-1 through 5-6 for such cable systems. Similar to the code developed originally by Rogers for transformers and later modified in the C57.104 IEEE Standard for transformer DGA data interpretation, this Guide contains six digits. Compared to transformers, cables operate in an oxygen-free, sealed system from which gases cannot escape. Cables also operate at significantly higher electrical stresses than transformers and the designs, materials, operating conditions and fluid volumes are markedly different in the two products. Accordingly, the type, distribution and concentration vary significantly, with cables showing much larger gas concentrations. For instance, hydrogen levels close to 100,000 ppm have been occasionally observed for HPFF cables, however, this gas rarely exceeds 2,000 ppm in transformers. Likewise, acetylene levels over 150 ppm have been observed for in-service HPFF cable systems, a situation uncommon for transformers. The code presented in this work has six digits. The definition of each digit, according to the severity of the problem, is given in Table 5-8 below: 5-12 Interpretation of DGA Data Table 5-8 Gas Concentrations and Ratios Included in the Six-Digit Code for the Interpretation of Dissolved Gas Analysis of HPFF Cables and Accessories Digit 1 Description Acetylene Concentration (ppm) Acceptable 1 2 3 2 Ethylene/Ethane Ratio Acceptable 1 2 3 Acceptable 1 Acceptable 1 2 3 Acceptable 1 2 Acceptable 1 Level Splices: <1 1<5 5 <10 =>10 < 0.5 0.5 < 1.0 1.0 < 2.0 => 2.0 <4 => 4 Magnitude =0 =1 =2 =3 =0 =1 =2 =3 =0 =1 =0 =1 =2 =3 Terminations: 0 < 30 =0 30 < 100 =1 100 < 150 =2 >= 150 =3 3 4 Hydrogen/Methane Ratio Hydrogen Concentration (ppm) < 5,000 5,000 < 10,000 10,000 < 25,000 => 25,000 < 2,500 =0 2,500 < 5,000 = 1 => 5,000 =2 > 1.4 1 <= 1.4 <1 =0 =1 =2 5 Total concentration of saturated hydrocarbons and isobutylene. Among the saturated HC are: methane, ethane, propane and butanes (ppm) Carbon Dioxide/Carbon Monoxide Ratio 6 The utilization of gas concentrations and gas ratios given in Table 5-8 offers the assessment of the condition of cable system along with the severity of the problem involved, as follows: Digit One Acetylene concentration forms the first digit of this code. Acetylene concentrations under 1 ppm in splices and cable runs or under 30 ppm in terminations are deemed acceptable. However, acetylene should not normally be present in splices and cable runs, and it is often the case. An increase in acetylene concentration with time signifies a potential problem. Compared to splices, the higher acetylene concentration limits for terminations results from the nature of the electrical field, over-voltage conditions experienced by terminations, limited fluid volume associated with terminations, and the restricted fluid movement. Acetylene is produced in discharges involving arcing. Acetylene can also be generated by thermal exposure alone, provided the temperature reaches over 600°C, which is not practical for cables. Discharges can take place external to the cable insulation such as skid-wires, loose ground connections, arcing over insulated flanges, and circulating currents. In these cases, the 5-13 Interpretation of DGA Data presence of acetylene is not associated with the insulation condition, but can eventually lead to failure. Digit Two Digit two represents the magnitude of the ratio between ethylene and ethane. A ratio ranging from 0.1 to 0.125 means a normal condition. The increase of this ratio up to 0.5 represents an acceptable condition, the assigned code being 0. As this ratio exceeds 0.5, codes of 1, 2, and 3 with the corresponding ratio ranges are provided, Table 5-8. Ethylene is produced by the thermal decomposition of the dielectric fluid. This gas can be generated in fairly large concentrations at temperatures above 150°C. However, such temperatures are not likely in cable systems. Ethylene observed in cable systems results primarily from electrical discharges that expose minute fluid volumes to elevated temperatures. As the intensity of the discharge increases i.e., as more current is involved, both the temperature and volume of the affected fluid increases and so does the yield of ethylene. Ethylene-to-ethane ratios close to, and over 1, are a cause of concern, except for cases where the concentrations of both ethane and ethylene are so small (under 10 ppm) that a high ratio can result from an error in analysis. To overcome this situation, digit 5 involving the total amount of saturated hydrocarbon gases is utilized. Digit Three Digit three represents the hydrogen to methane ratio. This ratio has been introduced to distinguish between electrical and thermal origin of gases. A high ratio with more hydrogen than methane generally indicates the presence of partial discharges, where a highly limited generation of heat is involved. The opposite is expected from purely thermal effects. Digit Four Digit four represents the total concentration of hydrogen. Compared to transformers where hydrogen seldom exceeds 2000 ppm, HPFF cable systems yield large concentrations of hydrogen. Hydrogen concentration of 10,000 ppm is acceptable for splices and cables. Starting with zero code corresponding to 5000 ppm of hydrogen, codes 1 though 3 have been presented to cover hydrogen levels up to 25,000 ppm and above. It should be noted that extraordinarily large hydrogen concentrations of the order of 100,000 ppm have been observed for cable runs and splices of HPFF cable systems in some cases. Although the generation of hydrogen is attributed to low level partial discharges in fluid resulting in large hydrogen concentrations, such an activity apparently does not impact the cable life. Nevertheless, the presence of large amounts of hydrogen should be carefully addressed, with periodic monitoring. The appearance of acetylene, together with large concentration of hydrogen, is an indication of increased discharge activity. It is noteworthy that hydrogen is also evolved as a corrosion product of iron. Both moisture and oxygen are required to produce hydrogen; the former should be measured when large concentrations of hydrogen are involved utilizing a glass bottle as opposed a glass syringe so that the settling of water can be observed. This situation has 5-14 Interpretation of DGA Data been encountered for a pumping plant reservoir with rusted fittings, and a HPFF cable with moisture content above 110 ppm. Digit Five Code five indicates the level of total saturated hydrocarbon gases. The hydrocarbons included in this code are methane, ethane, propane, iso and normal butane and isobutylene. Isobutylene is not a saturated hydrocarbon but this gas can be found in large amounts in polybutene fluids for which it forms the starting material. It is common to find large amounts of saturated hydrocarbons in fluid samples taken from early vintage cables. These hydrocarbons were used in varying distributions by the supplier as part of the blanket gas during storage and/or handling of these fluids. It is also likely that older cables did not utilize properly degassed fluids, resulting in high concentration of such saturated hydrocarbons. Thus digit five has been introduced to distinguish such cases. Digit Six The last digit, namely Code 6, represents the ratio between the concentration of carbon dioxide and carbon monoxide. Thermal decomposition of paper evolves large amounts of carbon oxides, however, the concentration of carbon dioxide is always larger than that of carbon monoxide. Although age and temperature can decrease this ratio, it is always greater than one based solely on thermal considerations. Compared to carbon dioxide, the presence of electrical activity can increase the yield of carbon monoxide, resulting in a ratio smaller than one. This has been observed in cases where thermal mechanical bending (TMB) is suspected in cable splices. In addition, laboratory investigations under EPRI sponsorship have shown that the yield of carbon monoxide is always higher than that of carbon dioxide when paper is exposed to electrical discharges. A CO2/CO ratio below one combined with the presence of acetylene indicates that both paper and fluid are being affected by electrical discharges. The six digits along with the corresponding codes with the increasing order of severity of the problem faced by the cable are given in Table 5-9. The propagation of a failure has been followed through this coding system for a 345 kV HPFF cable that failed at Waltz Mill, Table 5-10. The pathway shown in Table 5-10 indicates a continuous increase in digits 1 and 2. Digit 3 remains at 1, indicating that the problem is of electrical nature and no overheating is involved. The level of hydrogen in this cable did not show a significant increase. The level of total saturated hydrocarbons was low as this fluid was thoroughly degassed before filling the pipe. A significant decrease in the CO2/CO ratio was observed as the cable approached failure. However, due to the unusually high background of carbon dioxide, this ratio could not become less than the first cutoff point of 1.4 as shown in Table 5-8. The large concentration of carbon dioxide originated from the high temperature utilized to accelerate the aging of the test cable. Tables 5-11 and 5-12 demonstrate the application of the interpretation code system to splices and terminations for several HPFF systems. 5-15 Interpretation of DGA Data Table 5-9 Fault Diagnosis of Cables Through Dissolved Gas Analysis Digit-1 Digit-2 Digit-3 Digit-4 Digit-5 Digit -6 C2H2 0 to 1 0 1 0 1 1 1 1 2 3 3 3 4 4 C2H4 C2H6 0 0 1 2 to 3 0 1 1 2 to 3 1 to 3 2 to 3 2 3 2 3 H2 CH4 0 to 1 0 1 1 0 0 1 0 to 1 0 to 1 0 0 1 1 0 to 1 H2 0 0 0 0 0 0 0 0 0 0 2 to 3 0 0 0 TSHC 0 1 to 3 0 0 0 2 0 0 0 0 2 0 0 2 CO2 CO 0 0 1 0 to 1 0 1 0 0 0 0 1 0 0 0 Normal Normal, hydrocarbon contamination from original oil Acceptable low intensity discharge activity Acceptable, reversed ethane/ethylene ratio, repeat analysis Acceptable, low level acetylene Low level discharge activity with saturated hydrocarbon contamination Low intensity discharge activity Discharges in oil High acetylene, strong discharge activity Arcing Sustained strong arcing activity Discharges in oil Arcing in oil (skid wires) Strong or sustained arcing activity Comments Table 5-10 Six Digit Code Showing Arcing Pathway to a 345 kV Cable Failure Digit-1 Sample 13 month 15 month 17 month 18 month 18 month (failure) C2H2 0 1 1 1 4 Digit-2 C2H4 C2H6 0 0 1 2 3 Digit-3 H2 CH4 1 1 1 1 1 Digit-4 H2 0 0 0 0 0 Digit-5 TSHC 0 0 0 0 0 Digit-6 CO2 CO 0 0 0 0 0* * Excessive CO2 background to reach cutoff point 5-16 Interpretation of DGA Data Table 5-11 Example of Application of Condition Assessment Code to HPFF Cable Termination – DGA Data Voltage Class Cooling Type Sample date Methane Ethane Ethylene Acetylene Propane Propylene Isobutane Nbutane Isobutylene Hydrogen C. Monoxide C. Dioxide C2H4/C2H6 H2/CH4 Sat. + Isobutylene CO2/CO Digit-1 (C2H2) Digit-2 (C2H4/C2H6) Digit-3 (H2/CH4) Digit-4 (H2) Digit-5 (Tsat) Digit-6 (CO2/CO) Condition 345 kV Static Dec-89 102 134 54 45 155 46 68 99 98 783 159 828 0 8 557 5.21 0 0 1 0 0 0 Normal termination 138 kV Static Apr-93 156,906 57,752 384 10 22,687 33 6,411 3,023 28,030 858 21 115 0 0 271,787 5.58 0 0 0 0 2 0 Normal termination with original oil contamination 345 kV FC Sep-96 Gases (ppm) 18,561 673 3,608 283 2,597 305 20 65 3,887 427 7,022 326 1,953 469 551 225 22,519 1,425 2,769 919 335 291 903 413 Ratios: 1 1 0 1 50,529 3,277 2.69 1.42 Codes: 0 1 1 2 0 1 0 0 2 1 0 0 Normal termination Discharges in with original oil oil contamination and high ethylene/ethane ratio 138 kV Static Sep-95 345 kV Static Nov-89 777 172 201 118 201 110 93 46 149 2,018 321 857 1 3 1,392 2.67 2 2 1 0 0 0 Strong discharge activity 138 kV Static Jan-95 81,848 13,691 14,494 113.8 14,027 31,351 7,895 1,455 81,704 22,321 820 1,009 1.06 0.27 199,166 1 2 2 0 2 2 1 Sustained arcing activity 138 kV Static Apr-93 159,898 60,626 407 146 22,236 35 6,755 3,174 29,588 975 26 116 0 0 279,103 4.44 2 0 0 0 2 0 Discharges in oil 345 kV Static Sep-92 3,530 596 1,620 182 463 1,442 606 296 2,106 3,499 488 403 3 1 7,301 0.83 3 2 0 1 2 2 Sustained arcing activity 5-17 Interpretation of DGA Data Table 5-12 Example of Application of Condition Assessment Code to HPFF Cable Splice – DGA Data Voltage Class Cooling Type Sample Date Methane Ethane Ethylene Acetylene Propane Propylene Isobutane Nbutane Isobutylene Hydrogen C. Monoxide C. Dioxide C2H4/C2H6 H2/CH4 Sat. + Isobutylene CO2/CO Digit-1 (C2H2) Digit-2 (C2H4/C2H6) Digit-3 (H2/CH4) Digit-4 (H2) Digit-5 (Tsat) Digit-6 (CO2/CO) Condition 138 kV Static Sep-91 46 6 3 1 9 4 1 2 7 75 27 156 0 2 69 5.78 0 0 1 0 0 0 Normal 138 kV Static Dec-94 Gases (ppm) 76 253 4,204 65 65 1,329 84 225 12 0 0 0 132 45 588 67 126 101 4 4 425 159 91 140 18 66 6,130 1,786 108 2,929 474 18 156 8,402 119 706 Ratios: 1 3 0 23 0 1 296 432 12,676 17.73 6.72 4.51 Code: 0 0 0 2 2 0 1 0 0 0 0 0 0 0 2 0 0 0 Acceptable, Acceptable, Acceptable, reversed ratio reversed ratio possible ethane/ethylene ethane/ethylene contamination of original oil 138 kV Static Mar-95 230 kV Static Nov-98 138 kV Static Mar-92 44,731 16,524 0 0 4,809 34 919 1,140 41,554 1,198 167 242 0 0 108,537 1.45 0 0 0 0 2 0 Acceptable, possible contamination of original oil 220 kV Static May-99 0 4 3 0 11 3 1 3 15 24,355 32 180 1 32 5.70 0 1 0 2 0 0 H2 evolution from low level PD activity or metal corrosion by free water 138 kV Static Dec-96 12 23 47 14 43 83 7 17 44 0 7 38 2 0 130 5.80 3 2 0 0 0 0 Discharges in oil 345 kV Static Jun-88 20,776 1,074 1,387 147 309 820 196 104 2,335 14,278 1,068 1,042 1 1 24,690 0.98 3 2 0 2 2 2 Arcing 5-18 6 FLOW-CHART ON SAMPLING PROCEDURE, PROBLEM LOCATION AND DGA INTERPRETATION The application of DGA depends on three inter-dependent steps, namely, fluid sampling, chemical analysis and interpretation of the generated data. Accordingly, the success of DGA is governed by the proper execuation of each step. A knowledge of cable operating history relating to repairs, fluid leaks, make-up fluid, initial pipe fluid quality and presence of any hotspots is important for data interpretation. Improper interpretation can lead to wrong decisions that can be expensive. The various aspects on these three steps have been addressed in a comprehensive fashion in previous chapters; tables, illustrations and photographs are an integral part of this coverage. The entire process of DGA is summarized in a flow-chart, which covers the sampling procedures on splices, terminations, static versus forced-cooled cables, and frequency of sampling, Figure 6-1 The approaches to locate the source of problem in splices, terminations and cable runs through fluid drainage have been included in the flow-chart. While this flow-chart contains all the important details in a sequential fashion, the user should refer to the report for backup information to make sound decisions on cable system condition and follow-up action(s). The flow-chart is fully applicable to HPFF cable systems, both static and forced-cooled. It holds equally for splices and cable runs of SCLF cables, with the exception of SCLF terminations. Unlike HPFF terminations, the design of SCLF terminations complicates the coverage of the entire termination length through fluid drainage from the top as the termination fluid and the hollow core conductor fluid are in contact at this location. However, if a bottom valve is provided for a SCLF termination, fluid drainage is possible. Because of the limited fluid supply in SCLF cables, it is recommended to perform only two fluid drainages for SCLF terminations, if a bottom valve is available. It should be ensured that the connector needle valve is open when fluid is drained from the bottom valve. The flowchart applies to the splices and terminations of HPGF cables. However, the process of gas drainage to locate the problem source at a distance is not applicable to HPGF cables due to the high diffusion rate of the generated gases along the pipe in such cables, as opposed to HPFF cable systems. 6-1 Flow-Chart on Sampling Procedure, Problem Location and DGA Interpretation Start Identify cable system (static or forced-cooled, no. of splices & pumping plant(s); cable history, etc) Yes Forced-cooled ? No Sample at a single location (circulating pump) Valve on all splices ? No Install valves Yes Sample splices (without cable shutdown) Take two samples at each location Deenergize cable for termination sampling Bottom valve available ? No 1st sample from top valve without drainage, 2nd and 3rd samples from top valve with 2 - 4 ga. drainage Yes 1st sample from bottom valve without drainage, 2nd and 3rd samples from top valve with 2 - 4 ga. drainage Continued on next page Figure 6-1 Flow-Chart for Fluid Sampling and DGA Interpretation 6-2 Flow-Chart on Sampling Procedure, Problem Location and DGA Interpretation From previous page Perform DGA Results for splices Results for terminations No Gases Acceptable ? Gases Acceptable ? No Yes Yes Locate gas source by fluid movement Repeat sampling in 2 to 4 years Repeat sampling in 2 to 4 years Sample trifurcator Concentration decreases with drainage No Problem in termination Yes Yes Gases Acceptable ? No Gas source in splice Gas source in cable run Problem is the in raiser or further away in the pipe Consult with experts Move fluid to localize source Consult with experts Figure 6-1 Flow-Chart for Fluid Sampling and DGA Interpretation (cont.) 6-3 7 CONCLUSIONS This guide is based on laboratory and field data generated by Detroit Edison since 1986. While the former is important to understand the role of the electrical and thermal stresses involved in gas generation from fluid and cellulosic paper, it is the field data that serves as the foundation for the guidelines to interpret DGA data. Since the vast majority of the field data has been generated in the U.S., it involves predominantly HPFF cable systems. Accordingly, the guidelines provided for SCLF cable systems, particularly the splices, should be used with care. As more field data becomes available, it will be prudent to update this document, as is regularly done for transformers. Nevertheless the present document covers its intended function, providing the user with sampling and analysis procedures along with guidelines to interpret the field data. The potential of DGA is being recognized and U.S. utilities are increasingly applying DGA to assess the condition of paper-insulated transmission cable systems in service. DGA can readily distinguish between a satisfactory and a problem cable system. While it can assess the severity of the problem, DGA data cannot, like any other diagnostic technique, assess the immediate consequences of the problem, particularly problems associated with cable runs, which fortunately are not common. 7-1 A GLOSSARY Alkylbenzene dielectric fluids: Synthetic fluids containing a paraffinic hydrocarbon (alkyl) group attached to a benzene ring. The alkyl group can be either linear or branched. Linear groups can more easily be degraded by micro-organisms and, therefore, are preferred over branched groups. Examples of alkylbenzenes used in cables are dodecylbenzenes and tridecylbenzenes. Alkylbenzene fluids are characterized by low viscosity. DGA: Dissolved gas analysis, this term refers to the identification and quantification of gaseous components in a dielectric fluid sample, usually through gas chromatography. Degassing: Removal of gases from a fluid by exposure to vacuum and heat. Elevated temperatures are applied to reduce the fluid viscosity and moisture content. A lower fluid viscosity facilitates the degassing process. Dissolved gas: This term indicates that the gas forms a liquid solution in the complete absence of a gas phase (bubbles). A gas will form a liquid solution only if the pressure applied to the solution is greater than its vapor pressure. The total amount of gas that can dissolve in a fluid depends on the pressure and temperature. For most gases, solubility increases linearly with pressure. According to the nature of the gas, solubility can increase, decrease or remain unaffected by temperature. EDOSS: Abbreviation for EPRI Disposable Oil Sampling System. EDOSS Method: DGA performed with EDOSS vials by headspace chromatography. EPOSS: abbreviation for EPRI Pressurized Oil Sampling System. EPOSS Method: DGA performed with EPOSS cells by headspace chromatography. Gas chromatograph (GC): Device utilized to separate mixtures of gases or vapors. Briefly, this consists of a controlled temperature oven to support the separation column(s), a carrier gas & its flow control system, a sample injection system and a detection system. In the case of gas samples, the sample is introduced into a fixed volume loop contained in a remotely actuated multi-port valve. Upon the start of the analysis, the sample is forced through the separation column(s) with an inert carrier gas. The retention time of a gas in the separation column is governed by the nature of the gas and its affinity to the column-packing material. Thus gases are identified by their retention times. The separated gaseous components exit the column and enter the detection system. Calibration of the system is made with certified gas blends containing the components of interest. A-1 Glossary Gas in Liquid Solubility: Ability or tendency of a gas to dissolve uniformly in a liquid. The dissolved gas does not behave as a gas but as a liquid. It has, therefore, no buoyancy and will not accumulate in terminations or other elevated locations along a cable system. Bubble formation can only occur when the solution becomes over-saturated as the system pressure drops below the saturated solution pressure. Headspace Chromatography: In this technique, the gas phase components are related to the gases and their concentrations in the liquid phase. This enables the determination of the levels of gases in the original liquid. HPFF: Abbreviation for High-Pressure Fluid-Filled Cables. It is a cable system having three impregnated paper-insulated conductors, corresponding to the three phases, placed in a common steel pipe. A high pressure (200 psi) fluid is utilized to fill the pipe. Because of the large volume of fluid involved, large storage tank and pressure control units with pumps and relief valves are needed to accommodate fluid expansion and contraction. HPGF: Abbreviation for High-Pressure Gas-Filled cables. This is a variation of HPFF cables where high-pressure nitrogen (200 psi), instead of a fluid (a liquid dielectric), is utilized to fill the steel pipe. These cables do not require permanently connected gas reservoirs or pumping plants; nitrogen is supplied to maintain the pressure as needed. Hydrocarbons: Organic compounds consisting exclusively of the elements of carbon and hydrogen. These can be organized into three broad classes: aliphatics (paraffins, olefins, acetylenes), alicyclic (naphthenes) and aromatics. Over-saturated gas solution: If the vapor pressure is larger than the pressure applied over the solution, the solution becomes over-saturated and a gas phase will separate in the form of bubbles. Peroxides: Peroxides are organic compounds containing an -O-O- group in their structure. In hydrocarbon fluids, the formation of peroxides starts with the break up of the hydrocarbon backbone (R - R' ! R• + •R'), followed by radical combination of the broken piece with O2 (R• + O2 ! ROO•). Subtraction of H from a neighboring molecule can lead to the formation of a hydroperoxide (ROO• + •H ! ROOH), or reaction with another R' fraction can lead to a peroxide (ROO• + •R' ! ROOR'). The stability of these compounds strongly depends on the structure of R• and/or •R'. Polybutenes dielectric fluids: Synthetic polyolefin resulting from the polymerization of isobutylene gas. The polybutene structure contains a straight chain with alternating (CH3)2C groups and a double bond at the end of the chain. Also known as polyisobutylene or polyisobutene fluids. ppm: Abbreviation for parts per million. This expression is the ratio of the volume of a solute (gas in this case) to the total volume of the solution or the weight of the solute to the total weight of solution, multiplied by 1x106. A-2 Glossary Saturated gas solution: If the pressure over the solution equals the vapor pressure of the gas, the solution is said to be saturated. A saturated solution will not uptake more gas without forming bubbles. Saturated hydrocarbons: Hydrocarbons without double bonds or cycles in their structure. Straight-chain paraffins are typical saturated compounds SCLF: Abbreviation for Self-Contained Liquid-Filled cables. It represents the earliest impregnated paper-insulated transmission cable, which was developed in the early 1920s. SCLF cable is comprised of a single hollow conductor impregnated paper-insulated cable with a lead or aluminum sheath serving as moisture barrier and mechanical protection. In present day practice, the sheath is covered with a polymeric jacket. SCLF cables are generally installed in ducts but can be directly buried. Fluid pressure is maintained through the hollow conductor by means of pressurized tanks, which also accommodate the expansion and contraction of the fluid. Submarine SCLF cables frequently require pumping plants at both ends. Unsaturated hydrocarbons: Hydrocarbons that contain cycles and/or double or triple bonds between their carbon atoms, for example, acetylene, ethylene and propylene. Mineral Oils: Liquid petroleum derivatives containing complex mixtures of paraffinic, naphthenic and aromatic hydrocarbons with a specified range of boiling points and viscosities. A-3 Blank page Target: Underground Transmission About EPRI EPRI creates science and technology solutions for the global energy and energy services industry. U.S. electric utilities established the Electric Power Research Institute in 1973 as a nonprofit research consortium for the benefit of utility members, their customers, and society. Now known simply as EPRI, the company provides a wide range of innovative products and services to more than 1000 energyrelated organizations in 40 countries. EPRI’s multidisciplinary team of scientists and engineers draws on a worldwide network of technical and business expertise to help solve today’s toughest energy and environmental problems. EPRI. Electrify the World © 2000 Electric Power Research Institute (EPRI), Inc. All rights reserved. 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