Identification and Classification of

Spark Erosion with EMI Diagnostics

James E. Timperley Doble Engineering Columbus, Ohio

jtimperley@doble.com Abstract This paper discusses the detection, identification and classification of spark erosion or the “vibration sparking” deterioration of generator stator bars. A theory for the unique EMI pattern developed is discussed. Validation of stator repairs is also provided.

Typical spark erosion deterioration of a stator bar.

FIGURE 1

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Background A few designs of modern generators have suffered stator failures within the first five years of service due to a condition called spark erosion by the OEM and referred to as vibration sparking by others. The reason behind this rare failure mode is the subject of some debate but in general it is felt the conductivity of the stator bar slot coating is too high. This carbon based coating is applied to prevent high voltage partial discharges in the slots. If the coating is too conductive circulating currents through the core laminations may develop. At the bore of a large generator, several hundred volts are induced axially from one end of the stator bar to the other. Circulating currents from this voltage are prevented in the core by insulation on each of the thousands of steel core laminations. These laminations are however electrically connected to each other and the frame at the back of the core. If a stator bar surface is too conductive, a loop is formed with the core frame and high circulating currents can result. This current is only limited by the resistance of the stator bar coating.

This can be a serious problem with some generator designs and suppliers with more than 75% of those inspected having the deterioration. Most of these machines are less than ten years old.

Electromagnetic Interference (EMI) Diagnostics has been developed since 1980 as a system wide surveillance technique that can detect and identify electrical defects in motors, generators, isolated phase bus, transformers and associated electrical systems. EMI is a frequency domain technique that scans a wide frequency range for meaningful information. Partial discharge analysis (PDA) is a time domain technique that only looks at selected parts of the spectrum. The purpose of this paper is to identify the EMI signature for a spark erosion failure mode. Four generators with different stages of deterioration were tested and the results are presented in this report.

EMI EMI is generated by energy conversion from the power frequency to radio frequencies at the site of a defect. This activity can originate from a high voltage low current discharge (partial discharge) inside or on the surface of insulation. Another EMI source is from a low voltage high current arc such as with broken conductors, loose connections or shaft currents through a bearing rub. This ability to detect arcing defects in conductors is a major advantage of the EMI technique over PDA where only the insulation is monitored. There are many mechanical sources of arcing including vibration sparking. The Generators Tested Three of the four generators tested had been rewound. The fourth stator rewind was the last one to be scheduled before a failure developed. One of the stators had received enhanced repairs after the stator rewind and two machines had been rewound only. These four air cooled generators are rated 226 MVA, 18 kV, 3600 r/min. The generator load during data collection varied from 145 to 160 MVA depending on the machine. Operation varies with the time of year but these machines usually run 24/7.

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EMI signature of Unit 1. FIGURE 2

The EMI signature for Unit 1 in Figure 2 is typical of what would be expected for a generator in good condition. This generator stator had been replaced about six months before this data was collected. Exciter diode noise dominates the lower frequencies. Local radio transmitters, power line carriers (PLC), AM and FM stations all have amplitudes higher than the generator related activity. Minor generator related PD was noted. There is some PD activity on the 345 kV transmission line that can be detected above 10 MHz.

EMI signature of Unit 2. FIGURE 3

Unit 2 has the original stator winding and was scheduled for rewinding. Spark erosion of the stator bars was advanced. High random noise patterns were noted across the spectrum. No typical corona or PD activity was noted. A high random noise pattern at the low frequencies was unusual.

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EMI signature of Unit 3. FIGURE 4

This stator has been rewound and operated about a year when this data was collected. Local radio stations were measured with high amplitudes. Stator related random noise was detected at several frequencies. The 345 kV transmission line PD activity was detected above 10 MHz.

EMI signature of Unit 4. FIGURE 5

This stator had been rewound and operated several years when this data was collected. Additional enhancements have been made to the rewound stator. Side ripple springs were installed about a year before this data was collected. Stator related EMI was at very low levels. This validates that the additional stator repairs were successful in reducing the spark erosion activity.

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EMI signature comparison. FIGURE 6

There are major differences in the low frequency part of the spectrum, and again above 2 MHz and also around 30 MHz. A very cluttered graph results when trying to compare all four generators. The OEM has developed a ranking scale of from 1 (good condition) to 8 (a failed stator). It can be seen at the lower frequencies there is a ranking with CT4 in the best condition and CT2 with the most deterioration. CT3 has slightly more deterioration than CT1 and it has operated for about a year longer.

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Unit 2 & Unit 4 EMI signature comparison. FIGURE 7

Unit 2 had the highest EMI levels and Unit 4 had the lowest. No significant stator related PD activity was noted with Unit 4. Unit 2 had a pattern of random noise and low level arcing across the spectrum. Random noise is usually associated with the accumulation of conductive contamination on a stator winding. It is unusual for random noise to be at low frequencies and at a high level.

Unit 2 EMI pattern @ 56 kHz. Unit 4 EMI pattern @ 56 kHz. FIGURE 9 FIGURE 8

The pattern at 56 kHz should be dominated by exciter diode transients but these are almost completely hidden by the high level random noise. Several hundred generators have been tested and none have had this pattern of extreme random noise at these low frequencies.

The exciter pattern for Unit 4 is as expected. When exciter diodes stop conducting transients are produced and can be measured by their EMI pattern. There are no problems with this exciter, no loose connections, no open circuits or weak diodes are present.

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Unit 2 EMI pattern @ 6.7 MHz. FIGURE 10

The EMI pattern noted at 6.7 MHz is typical for this part of the EMI signature for Unit 2. There is no corona or partial discharge spikes present. The random noise is mixed with low level arcing. There appears to be little synchronization with the power frequency. This pattern made a “sizzling” sound. It appears that a combination of low level arcing and random noise are the EMI patterns generated by vibration sparking or spark erosion. No corona or PD activity was noted. Random Noise Sources Every electrical conductor produces an irregularly varying voltage across its terminals as a result of the random motion of the free electrons in the conductor caused by thermal action. This is often called Johnson-Nyquist noise and is continuous across the radio frequency spectrum with uniform amplitude. This voltage is however usually very small. Thermal agitation noise does not depend on applied voltage or current flow through the conductor. The voltages are small and EMI is not generated in most cases. Noise voltage also arises from the small random variations in current flow through a conductor. The amplitude of this random noise is proportional to the temperature, resistance and applied voltage. It also depends on the conductor material. In the electronics industry it is well known that resistors composed of carbon granules generate high levels of electric noise when current flows through them. This noise voltage results from fluctuations in contact resistances between adjacent particulate carbon granules, a consequence of the heterogeneous structure. The noise voltage generated is due to that variation and is proportional to the amount of current flowing. Even small vibrations of a stator bar and the conductive carbon layer with respect to the grounded core would generate “noise”. This property was used for the first telephone microphones where sound vibrations changed the contact resistance between carbon granules and produced current flow related to sound vibrations. It has been reported the contact resistance between the stator bar and the core varies dramatically with pressure.

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Random noise is one of the EMI patterns documented along with gap discharges, corona and arcing. In the past this pattern has been associated with the accumulation of conductive contamination such as coal dust on a motor stator winding. Typically the EMI signature is a peak of random noise developing around 1 MHz. This peak increases as the deterioration increases. This has been a useful characteristic to plan maintenance and schedule 4 kV motors in need of cleaning. Slot Coating Deterioration Spark erosion of the conductive layer on a stator bar is different from the typical partial discharge erosion. Figure 11 shows the typical deterioration that develops. Bars at any location in the winding, both line and neutral, have developed the same distinctive “pock marked” pattern. Deterioration starts where the bar is in contact with the core teeth and is less at the air vents. These pits are very similar to what develops on Babbitt bearings resulting from shaft currents.

Slot coating deterioration resulting from vibration sparking. FIGURE 11

There is little bleaching of the materials in the sample in Figure 11, this would indicate the sample is not a high voltage bar. Burning of the carbon coating and resin as well as melting of the glass tapes indicate very high temperatures were developed. This also implies very high currents are also present. With this generator design there is around 800 volts axially from one end of the core to the other. The conductive slot coating shorts out this voltage and current flows along the bar surface. Arcing type activity is the result of the bar conductive coating making and breaking contact with the core. Sparking activity is greatest during the peaks of vibration. Random noise results from current flow through this long “carbon resistor”. If a high voltage bar is involved then conventional PD develops as the slot coating is removed. If a low voltage bar is involved there is no PD and no ozone bleaching develops.

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Ozone deterioration of a stator bar. FIGURE 12

Classic bleaching of the conductive coating is present when corona and PD are the only deterioration mechanisms. There was no stator bar vibration with this generator and there are no core lamination imprints on the side of the bar. The glass cloth backing for the carbon based conductive coating is bleached white from the PD generated ozone. There is no melting of the glass and no burning of the carbon or resin.

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Side-packing deterioration from spark erosion looks like burning. FIGURE 13

Side-packing deterioration from corona and PD is totally different. FIGURE 14

As with stator bar deterioration there are marked differences in the deterioration of the conductive side packing used to hold stator bars tight in the slots. Flat side packing, not side ripple springs are shown in the two examples above. Spark erosion deterioration often burns the conductive packing resin and carbon filler. PD will only destroy the carbon filler and resin binder, leaving the clean bright glass cloth.

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CONCLUSION EMI Diagnostics is able to detect and classify the deterioration from spark erosion or “vibration sparking” of stator bars. This type of deterioration is unique and a unique signature is developed. Classic stator bar PD or corona is a high voltage driven ozone deterioration. Spark erosion is a low voltage high current burning of the stator bar conductive coating. Trending of the EMI signature and patterns can provide information on deterioration rates. The verification of stator repairs can also be determined by a second diagnostic test after the repairs are implemented. This on line evaluation can be used to supplement the off-line borescopic inspections that are now used to determine stator bar condition. Spark erosion will be added to the 30 other generator system defects and types of deterioration that EMI Diagnostics can detected and identified. BIOGRAPHY

James E. Timperley (BSEE, 1968 Oklahoma State University) began working in the utility industry with American Electric Power. He was involved with station engineering the R&D laboratory and rotating machinery. He retired from AEP after 38 years of service and joined Doble Engineering in 2007. Jim has published over 70 technical papers on operating, maintaining, testing, advanced insulation materials and repairing rotating electrical machinery. Other activities include maintaining high current isolated phase bus, equipment root cause failure analysis, and the development and application of EMI Diagnostics. Mr. Timperley is an IEEE Fellow and was presented the 2006 Dakin Award by the IEEE Dielectric & Insulation Society for the development of EMI Diagnostics. He is active in several standards groups and is a registered professional engineer in the state of Ohio.

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Turbine Generator Conditions Detected with EMI Diagnostics

1. Slot discharges resulting from side packing deterioration 2. Slot discharged resulting from stator bar coating deterioration 3. Loose endwindings (broken ties) 4. Loose stator bars (loose wedging) 5. Loose phase rings (circuit rings) 6. Verify maintenance corrected all winding defects 7. Loose flux shield ground 8. Broken stator bar sub-conductors (strands) 9. Foreign metal objects on endwindings 10 .Shaft oil/hydrogen seal rub 11. Arcing shaft grounding brush 12. Shaft currents through bearings 13. Verify shaft ground maintenance eliminated bearing currents 14. Contaminated windings (dirt, water, oil) cleaning needed 15. Contamination in insulation (wet insulation) 16. No contamination present (no maintenance necessary) 17. Arcing alternator exciter or main field slip-ring brushes 18. Verify field ground was not present 19. Defective alternator exciter diodes present 20. Loose brushless exciter components 21. Loose static exciter power circuits 22. Open exciter diode fuses 23. Defective voltage regulator and / or control settings 24. Stator Bar Spark erosion (vibration sparking)