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partial discharge testing: a progress report - Iris Power Engineering

partial discharge testing: a progress report - Iris Power Engineering

P u l s e M a g n i t u

P u l s e M a g n i t u d e [ m V ] 4.2.5 Case Study 5 (Voltage Stress Coatings) A 13.8kV air-cooled turbine generator with bus couplers has been periodically tested for PD activity over the past year. Though the levels are relatively low compared to the Iris database, it is apparent that there are some fluctuations that appear to track the relative humidity values. At present, the PD levels on B-phase are low and inversely track with humidity. Figure 5. Case Study 5 Trend 1 5 0 1 0 0 50 0 - 5 0 - 1 0 0 B ip o la r M a c h in e P D 1 to 3 .1 6 p p s 3 .1 6 to 1 0 p p s 1 0 to 3 1 .6 p p s 3 1 .6 to 1 0 0 p p s 1 0 0 to 3 1 6 p p s 3 1 6 to 1 0 0 0 p p s > 1 0 0 0 p p s S u b s e t 8 1 5 0 1 0 0 50 0 - 5 0 - 1 0 0 Though not as prevalent with Case Study 3, there is a noticeable “hook” in the negative PD clusters for B-phase. This combined with the clusters of PD near 90°/270° suggests and the inverse humidity dependence suggests that this may be the onset of early deterioration of the voltage stress coatings or perhaps phase-to-phase contamination either within the winding or perhaps at the machine’s terminals. - 1 5 0 - 1 5 0 0 45 90 1 3 5 1 8 0 2 2 5 2 7 0 3 1 5 3 6 0 P h a s e A n g l e [ d e g ] B-phase (clusters/hook) 5 CONCLUSION Based on the database analysis, it is unusual to find situations where the PD directly or inversely tracks with the relative humidity. This is likely due to two issues: one, the fact that there is minimal variability of humidity within the machine housing, and two, the majority of machines probably have PD activity that is within the slot section of the winding. Therefore, if humidity dependence is noted, then it is highly recommended that the investigation begin at the machine’s terminals and then proceed into the machine housing. This is especially the case when PD sensors for hydrogen-cooled windings are installed outside of the hydrogen environment at the terminals, and where only PD sources at the terminals would be impacted by humidity changes. Results that track with humidity are from surface PD activity, such as would occur with deterioration of voltage stress coatings, contamination or insufficient spacing between high voltage components. Though it is always recommended that you trend the results for one machine over time and thus monitor the rate of degradation of the stator winding, it is also possible to compare results from similar machines. If the test instrument is a TGA, PDA-IV, Trac or Guard and the sensors are either 80pF capacitors, or stator slot couplers, then the tables contained within the appendix can be used to ascertain whether a machine warrants further tests and inspections or is operating within reasonable limits. Red flags should only be raised if the PD levels on a specific machine are doubling over a six-month interval, or if they are above the 90 th percentile and steadily rising. In all cases, raising the red flag means increasing the frequency of PD testing to determine the rate of deterioration and when possible, conduct specialized tests, inspections and repairs as required. PD is a symptom of a failure mechanism; action should be based on the severity of the failure mechanism detected by the PD, not the PD results. PD levels exceeding threshold alarms are warnings for further investigation to determine the cause of the high PD; however, be aware that PD levels can fluctuate with ambient and operating conditions. PD Progress Report 8 of 13 IRMC 2012

Maintenance should be based on the cause of the PD, not the overall levels. Continuous PD monitors should have their alarm levels set to the 90% level. The time of winding failure is normally the result of a deteriorated winding being subjected to an extreme stress such as a lightning strike, out-of-phase synchronization, excessive starts, or system imbalance. As these are unpredictable, it is impossible to forecast when a failure will occur. However, by monitoring the PD characteristics of a stator winding, it is often possible to determine which machines are more susceptible to failure, and therefore which require maintenance. 6 REFERENCES 1. J.F. Lyles, T.E. Goodeve, and G.C. Stone, “Using Diagnostic Technology for Identifying Generator Winding Maintenance Needs,” Hydro Review Magazine, June 1993, pp. 59-67. 2. V. Warren, “How Much PD is Too Much PD?” Proc. Iris Rotating Machine Conference, Dallas, TX, March 1998. 3. IEEE 1434-2000 “IEEE Guide to the Measurement of Partial Discharges in Rotating Machinery.” 4. V. Warren, G.C. Stone, “Recent Developments in Diagnostic Testing of Stator Windings,” IEEE Electrical Insulation Magazine, September 1998. 5. V. Warren, “Further Analysis of PD Test Results” Proc. Iris Rotating Machine Conference, Scottsdale, AZ, March 1999. 6. V. Warren, “Partial Discharge Testing – A Progress Report” Proc. Iris Rotating Machine Conference, New Orleans, LA, June 2000. 7. V. Warren, “Partial Discharge Testing – A Progress Report” Proc. Iris Rotating Machine Conference, Washington, D.C., June 2001. 8. V. Warren, “Partial Discharge Testing – A Progress Report” Proc. Iris Rotating Machine Conference, San Antonio, TX, June 2002. 9. V. Warren, “Partial Discharge Testing – A Progress Report” Proc. Iris Rotating Machine Conference, Santa Monica, CA, June 2003. 10. V. Warren, “Partial Discharge Testing – A Progress Report” Proc. Iris Rotating Machine Conference, New Orleans, LA, June 2004. 11. V. Warren, “Partial Discharge Testing – A Progress Report” Proc. Iris Rotating Machine Conference, Scottsdale, AZ, June 2005. 12. V. Warren, “Partial Discharge Testing – A Progress Report” Proc. Iris Rotating Machine Conference, Toronto, ON, June 2006. 13. V. Warren, “Partial Discharge Testing – A Progress Report” Proc. Iris Rotating Machine Conference, San Antonio, TX, June 2007. 14. V. Warren, “Partial Discharge Testing – A Progress Report” Proc. Iris Rotating Machine Conference, Long Beach, CA, June 2008. 15. V. Warren, “Partial Discharge Testing – A Progress Report” Proc. Iris Rotating Machine Conference, New Orleans, LA, June 2009. 16. V. Warren, “Partial Discharge Testing – A Progress Report” Proc. Iris Rotating Machine Conference, Orlando, FL, June 2010. 17. V. Warren, “Partial Discharge Testing – A Progress Report” Proc. Iris Rotating Machine Conference, San Antonio, TX, June 2011. 18. M. Fenger and G. Stone, “Investigations into the Effect of Humidity on Stator Winding Partial Discharges,” IEEE Transactions on Dielectrics and Electrical Insulation Vol. 12, No. 2; April 2005, pp. 341-365. 19. V. Warren, G. C. Stone and M. Fenger, "Advancements in Partial Discharge Analysis to Diagnose Stator Winding Problems", IEEE Intcrn. Sympos. Electr. Insul., pp. 497-500, 2000. PD Progress Report 9 of 13 IRMC 2012

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