Grounding and ground fault protection of multiple generators

GROUNDING AND GROUND FAULT PROTECTION OF MULTIPLE GENERATOR INSTALLATIONS ON MEDIUM-VOLTAGE INDUSTRIAL AND COMMERCIAL POWER SYSTEMSPART 4 - CONCLUSION AND BIBLIOGRAPHY An IEEE/IAS Working Group Report [Working Group Members] - Prafulla Pillai (Chair), Alan Pierce, Bruce Bailey, Bruce Douglas, Charles Mozina, Clifford Normand, Daniel Love, David Shipp, Gerald Dalke, James R. Jones, Jay Fischer, Jim Bowen, Lorraine Padden, Louie Powell, Neil Nichols, Ralph Young, Norman T. Stringer Working Group Chair: Prafulla Pillai Kellogg Brown & Root, Inc. Houston, Texas 77002 Abstract - The paper discusses typical grounding practices and ground fault protection methods for medium voltage generator stators, highlighting their merits and drawbacks. Particular attention is given to applications of multiple generators connected to a single bus. The paper also provides an overview of the generator damage mechanism during stator ground faults. Problem areas associated with each type of grounding are identified and solutions are discussed. The paper also provides a list of references on the topic. The paper is intended as a guide to aid engineers in selecting adequate grounding and ground fault protection schemes for medium voltage industrial and commercial generators for new installations, for evaluating existing systems, and for future expansion of facilities, to minimize generator damage from stator ground faults. These topics are presented in four separate parts, Part 1 through Part 4. Part 1 covers scope, introduction, user examples of stator ground failure, and theoretical basis for the problem. Part 2 discusses various grounding methods used in industrial applications. Part 3 describes protection methods for the various types of grounding and Part 4 provides a conclusion and bibliography of additional resource material. I. CONCLUSIONS This paper has shown that the phenomenon that is causing the reported extreme core burning of faulted generator stators is based on two factors: 1) As system complexity has increased, the number of resistor grounds on systems has increased. This has caused the total available ground fault current to increase. As a practical matter, the opening time of generator breakers cannot be made significantly faster than the typical 6 cycles (100 ms for a 60 Hz ystem) value. A major component of the burning is therefore attributable to the magnitude of current that will flow during this tripping time, that is, to the number of ground sources on the system. 2) Current that rises through the neutral of the faulted generator will not be interrupted by tripping the generator breaker, and will persist for several seconds until the field demagnetizes. A considerable amount of burning damage will be done during this time if the generator neutral is lowresistance grounded. Therefore, solutions to this problem must involve several elements: a) The number and ratings of low-resistance grounding resistors on the system should be kept to a minimum. Techniques to accomplish this include designing the system around the concept of “zero-sequence islands” in which the number and rating of transformer ground sources within each island is strictly limited, or the use of a single bus-connected neutral deriving transformer instead of multiple neutral resistors on multiple power transformers. b) The faulted generator should be high-resistance (10 A maximum) grounded, especially during the time after the generator breaker opens and while the field excitation is decaying. If necessary, a hybrid form of generator neutral grounding may be used in which the neutral is both lowresistance grounded and high-resistance grounded. The generator will be low-resistance grounded during normal operation, but a neutral switching device is provided to trip this resistor any time that the generator must be tripped for a stator ground fault. This leaves the generator high-resistance grounded during the ensuing interval as the field flux decays, thereby limiting the fault current to a level that will do significantly less damage. Presented at the 2003 IEEE IAS Pulp and Paper Industry Conference in Charleston, SC: © IEEE 2003 - Personal use of this material is permitted. 4. Books and Technical Reports 1. the first step in identifying a solution for existing systems is to identify what are the expected realistic operating modes of the system. IEEE Guide for AC Generator Protection. II. 7. 1993. ANSI Standard C50. Part II – Grounding of Synchronous Generator Systems. Systems . then it would be necessary to trip both the generator and external source when there is an uncleared downstream fault. 20.13. IEEE Standard C37. IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems (The Green Book). it is important to understand what kinds of outage contingencies must be allowed for. if adequate high-resistance grounding cannot be achieved. IEEE Standard 141. IEEE Standard C62. ANSI C50. Practical solutions on new systems are relatively straightforward and involve application of the above mentioned two factors.92. 11. IEEE Standard 32. it will be necessary to choose between accepting the risk of machine damage and limiting the number of operating contingencies that can be accommodated on the system. BIBLIOGRAPHY A. 1991. Grounding. 10. ANSI Requirements for Salient Pole Synchronous Generators and Generator/Motors for Hydraulic Turbine Applications. 12. 1977. Careful consideration must be given to all potential normal and contingency operating scenarios to permit plant operations to continue in the event of some unexpected component failures. 1995. there is no one set of steps that will suffice for all existing systems. This in turn can lead to a situation where there is a desire to meet the grounding criteria mentioned above for all possible operating arrangements.c) If it can be assured that the generator will never be operated alone without being synchronized to the external power source. IEEE Standard 242.101. IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems. Part I – Introduction IEEE Standard C62. the low-resistance grounding of the external source allows quick tripping of the generator breaker to isolate the fault. 1993 (Reaff 1999). It is important that any solution must be carefully engineered with particular attention in selecting equipment components that are suitably rated for the application. Industrial Power Handbook. For ground faults internal to the generator.92.10. p. Therefore. 6. 15. IEEE Standard C37.21. This grounding method would allow the system to continue to operate with the uncleared high-resistance ground fault present if the condition is alarmed and the fault is located and cleared in a timely manner. it is necessary to consider carefully the architecture of each system to arrive at what will almost certainly be a compromise between the objective of minimizing potential fault damage and certain other operating objectives. while it is certain that the system is expected to operate normally with everything intact. 5. May 1947. 14. ANSI Standard C50. IEEE Standard Requirements. 2. AIEE Standard No.102. Furthermore. IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems.2.12. 1989. That is. Beeman.14. 1999. It is also often the case that there is a perception that this complexity equates with reliability because it affords many different contingency operating modes. ANSI Standard for Rotating Electrical Generators – Cylindrical-Rotor Synchronous Generators. Terminology and Test Procedures for Neutral Grounding Devices. NFPA Standard 70. NEMA Standard for Motors and Generators. However. IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (The Buff Book). National Electrical Code.2. leaving the generator high-resistance grounded. 1982. 1. IEEE Guide for Generator Ground Protection. On existing systems the problem of minimizing fault damage is more challenging. Table 32. 2001. 3. D. ANSI Standard C50. New York: McGraw-Hill. then a good solution would be to employ highresistance grounding of the generator and low-resistance grounding of the external source. NEMA Standard MG1-1998 (Rev. ANSI Requirements for Combustion Gas Turbine Driven Cylindrical Rotor Synchronous Generators. 9. Article 250. American National Standard General Requirements for Synchronous Generators. 1972 (Reaff 1997). IEEE Recommended Practice for Electric Power Distribution for Industrial Plants (The Red Book). thereby eliminating damages. Therefore. 32. In this instance. 1989. IEEE Standard 142. A reality is that it may be impossible to achieve risk mitigation for all practical operating conditions of complex systems. d) Another option would be high-resistance grounding of both the generator and the external sources with the bus being low-resistance grounded via a grounding transformer supplied through a breaker. 1990. 2000). 8. B. Industry Standards Publications 1. 1955. 1989.2. A serious problem with existing systems is that their architecture is often complicated by the manner in which they have evolved over many years. Editor. 13. 72.754-761. “Generator grounding.” in Conf. Peterson." 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