APT Power Technology Co.,Ltd, Xi’an April,2017 Application of EL CID Test and Ring Flux Test on a Stator Core by Guo Qiang APT Power Technology Co.,Ltd, Xi’an Abstract: Generators are subjected to high electrical, thermal and mechanical stresses over the course of their service life. Continuous extreme thermo-mechanical stresses and thermal cycles can have a particularly severe impact on the stator core. Over time, this can lead to deficiencies in the core insulation. A damaged insulation can cause the formation of larger eddy currents and local hot spots, which spread out during time. Therefore, it is recommended to regularly check the insulation condition, to avoid possible consequential damage such as core burning or damage of the insulation. The flux test is used to measure the condition of the stator core insulation and to detect possible local insulation damage. Local hot spots caused by insulation failure between several sheets of the core during magnetization will be detected by the infrared imaging instruments. For evaluating the condition of generator stator cores, three test methods exist: Electromagnetic core imperfection detection (EL-CID) 50-60 Hz ring flux testing 500 Hz ring flux testing The information contained in this article is a brief discussion of the EL-CID test technique, 50-60 Hz ring flux testing and 500 Hz ring flux testing. Key words: EL-CID, stator core, ring flux testing, Insulation Tester, DC Hipot test sets 1. Introduction Generator which is the key component of a power plant must be safe and reliable. To ensure this, the condition of the generator must be checked regularly and inspections planned predictively. The stator core of the generator is constantly under a load during operation and ages as the result of mechanical, thermal, and electrical stresses. In particular, the insulation between the many thousands of individual laminations may be damaged. Damaged insulation can result in higher eddy currents and local hot spots, which gradually spread to a core burning or interturn faults around the stator core, even to complete failure of the machine. Fig. 1 Possible forms of damage and traces of burning to the stator core 1 APT Power Technology Co.,Ltd, Xi’an April,2017 Three measurement methods are in use globally for assessing the condition of the stator core, each of which will be studied and compared in this paper. Two of these methods deliberately generate hysteresis and eddy current losses in the stator core in order to create hot spots at defective points and detect them by means of thermal imaging. 2. The Chinese National Standard and International Standard The Chinese National Standard GB/T 20835-2016 Guides for magnetization test of generator stator core shows the test procedure of the generator stator core by the ring flux testing method, and EL-CID is mentioned in the appendix. The international standard IEEE Std 56-2016 Insulation Maintenance of Electric Machines and IEEE 56-1997 Guide for Insulation Maintenance of Large Alternating-Current Rotating Machinery (10,000 kVA and Larger) also show the ring flux test of the generator stator core during the maintenance. 3. Electromagnetic Core Imperfection Detection (EL-CID) 3.1. Fundamental of EL-CID EL-CID is the abbreviation for “Electromagnetic Core Imperfection Detection” Starting at the early 1980s, the EL-CID test has been developed as an alternative to the ring flux test. The technique is based on the detection of core faults by measuring the magnetic flux resulting from the current flowing in the fault area (Fig.2), at only 3 to 4% of rated flux in the core. Furthermore the test usually requires only two or even one man to complete (using the latest version) in less than one-eight-hour shift. Fig. 2 Currents induced through damage In the EL-CID measurement, only very low magnetic flux densities are applied. This makes the test setup very simple and it is easier to comply with safety requirements. The fault currents can be detected by a Chattock coil (Fig.3), which is routed along each slot of the stator core. The output signal is proportional to the magnetic potential difference between the two contact points on the stator core surface. Fig.3 Chattock coil spanning one slot and two teeth 2 APT Power Technology Co.,Ltd, Xi’an April,2017 Examination of each slot together with its adjacent teeth results in overlapping that provides additional information about the defective points. The measurement signal is split by a signal processor into one component in phase with the excitation field and one component caused by the fault current. To split the signal, a phase reference is required, which can, for example, be determined from the current of the excitation winding. 3.2. EL-CID Test set up The EL-CID equipment is set up as shown in Fig.4. A CT is placed around the excitation winding to reference the supply signal. The digital equipment uses a laptop computer to store the axial traces. The sensor head (Chattock potentiometer) is pulled axially along the core and always bridging two stator teeth. The fault current signal then will be read from the computer. Fig.4 EL-CID equipment set up 3.3. Representative equipment The EL-CID (Electromagnetic-Core Imperfection Detection) technique was originally designed as portable test equipment for inspection and repair of rotating electric machine stator cores. It was designed as a low excitation power alternative to the high power level stator core flux test, for looking for stator core inter-laminar insulation problems. One of the Representative EL-CID instruments is shown in Fig. 5, and a Robotic Inspection Vehicle is optional (Fig.6). Fig. 5 the “Iris Power EL CID Evolution” Fig.6 the vehicle of Chattock Potentiometer 3.4. Data display of EL-CID The “hot spots” can be Easily identified via a color map display shown in Fig.7. A signal interpretation of surface and deep faults is given in Fig.8 by the EL-CID experience. Fig.7 the “hot spots” color map display Fig.8 EL-CID signal interpretation of surface and deep faults 3 APT Power Technology Co.,Ltd, Xi’an April,2017 4. 50-60 Hz ring flux test method Traditionally stator core inter-laminar insulation testing has been done using the “ring” or “loop” flux test method, in which rated or near-rated flux is induced in the stator core yoke. This corresponds to a magnetic induction of up to 1.3 Tesla or approximately 100V/m of induced longitudinal voltage. Fig. 9 shows the stator magnetic flux during operating and during inspection. The induced flux in turn induces circulating currents from the faulted area usually to the back of the core. These circulating currents cause excessive heating in areas where the stator iron is damaged. The heat produced is generally detected and quantified using established infrared techniques. Fig.10 is the thermal imaging hotspot analysis. This method has been proved to be successful over the years, but it requires a large power source and considerable time, manpower and resources to complete. Fig.11 shows the basic setup of a rated flux test. Fig. 9Magnetic flux during operation (left) and during inspection (right) Fig. 10 Generator thermal imaging 6. References Fig. 11 Basic setup of a flux test 4 APT Power Technology Co.,Ltd, Xi’an April,2017 5. 500Hz ring flux test Electrical testing at elevated frequency of Stator Cores is developed by companies such as Siemens. That is the 500 Hz ring flux test, which is developed since 2002 and patented internationally in 2005. It is a further test method that works by exciting the object under test at a higher frequency (500Hz) and induces voltage levels comparable with those of a high-flux ring test – roughly 100 V/m. 50Hz versus 500 Hz ring flux testing: In the flux test at a frequency of 500 Hz, considerably lower magnetic flux densities must be applied to generate voltages that are comparable with those induced with 50-Hz testing. To avoid overheating, induced longitudinal voltages of 80% to 90% are aimed at during testing. The features of 500Hz ring flux testing: Magnetization of the stator core with 0.12 T to 0.15 T, equivalent to 10% of the rated induction. The induced longitudinal voltage about 100V/m is of the same level of magnitude as for the test at operating frequency. Production of core losses to detect hot spots in the fault locations by eddy currents. Smaller loss and apparent power than 50Hz. Connectivity to the 400V 3phase AC power net. EL-CID and both test methods involving a stator core temperature rise are compared below (Table 1 and Table 2): Item EL-CID 50Hz 500Hz Corresponds to 4% of Corresponds to approx. 85% of Induction is reduced inversely Magnetic flux the flux density during the flux density during operation, proportionally to the frequency density rate operation (approx. smaller flux densities of up to (0.1 to 0.15 Tesla) B 60 mT) 1Tesla are also common Corresponds to the induced Corresponds to the induced voltage in the 50 Hz test, the Induced longitudinal voltage during rated operation, Approx. 5V/m insulation is stressed with the voltage smaller flux densities result in same electrical voltage as slightly lower voltages during rated operation In the stator yoke comparable Higher frequency causes Magnetization Approx. 1kW to 1.5kW to magnetizing losses under rather high losses at lower flux losses operation densities Detection of defective Defects in stator core can be The hot spots have comparable By evaluation of the test points in the stator core detected by hot spots intensity to the 50 Hz test High power of up to approx. 3 Lower power of up to approx. Measurement easy to Test effort MVA and high voltage 250 kVA using a 400V set up connection required connection sufficient Table 1 Comparison of existing test methods Supplier Tooth top Slot ground Tooth top Slot ground recommended Item horizontal horizontal vertical vertical limit values for position position position position fault detection 100 mA at 5 V/m EL-CID test 35 mA 40 mA 60 mA 65 mA excitation Ring flux test at 10 K: permanent rated frequency Temperature rise Temperature rise Temperature rise Temperature rise monitoring B=1.3 Tesla 22 K 52 K 26 K 58 K 20 K: repair (105 V/m) 500Hz Ring flux test at Temperature rise Temperature rise Temperature rise Assessment: Temperature rise rated frequency 11 K 15 K 9K >10 K: repair 10 K B=1.3 Tesla (105 V/m) Table 2: Comparison of limit values for fault detection 5 APT Power Technology Co.,Ltd, Xi’an April,2017 6. Conclusion The EL-CID test provides rapid testing of the machine, often less than one work shift for turbo generators and motors. The Ring Flux test typically takes 3 work shifts. The typical labor is reduced than the Ring Flux test, and rapid setup to retest after any repairs ensure quick turnaround. It is safe for core and operator, unlike Ring Flux test which is potentially damaging to the uncooled core. Both 50 Hz flux test and testing at higher frequencies of 500 Hz are suitable methods for detecting defects in stator cores. The stator core test using the EL-CID method is, however, less suitable because here the stator core cannot be tested under realistic operation conditions and the measurement result has to be interpreted. Defects may then be overlooked and incorrect assessments made. The 500-Hz test produces results comparable with the established test at 50 Hz, but is considerably easier to implement due to the lower power losses and considerably lower apparent powers. Its advantages include, in particular, the considerably smaller test setup, the concomitant simpler transport and possibility of connecting it to the 400-V grid. Fig. 12 shows the 500-Hz setup for use in a power plant, its power comes from the high frequency generator with control panel which connecting to the 400-V grid. Fig. 13 and Fig. 14 show the thermal imaging and close-up of hotspots of a stator core. Fig.12 500 Hz test setup for use in a power plant Fig.13 Thermal imaging of a stator Fig.14 Close-up of hot spots 7. References [1] OPERATION AND MAINTENANCE OF LARGE TURBO-GENERATORS-IEEE Press Editorial Board [2] Testing stator cores of turbo generators using the ring flux method-by Claus-Georg Richter, Jürgen R. Weidner Siemens AG Energy Sector Service Division 2013 [3] GB/T 20835-2016 Guides for magnetization test of generator stator core [4]IEEE 56-1997 Guide for Insulation Maintenance of Large Alternating-Current Rotating Machinery (10,000 kVA and Larger) Copyrights Reserved By APT Power. www.xaapt.com Tel:0086-29-88897866 Fax:0086-29-82526199 E-mail:
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