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partial discharge testing: a progress report - Iris Power Engineering

partial discharge testing: a progress report - Iris Power Engineering

4.2.2 Case Study 2

4.2.2 Case Study 2 (Contamination) A 4.1 kV cooling water motor in a nuclear power plant was equipped with a continuous PD monitor employing 80pF capacitive sensors. At the same time, a portable relative humidity indicator monitored the humidity of the air in the motor environment. Figure 2 shows that over a 48 h period there is an excellent inverse effect between the PD activity, expressed by Qm and NQN (IEEE Std 1434-2000), and relative humidity. That is as the relative humidity increases from 32% to 54%, the peak PD magnitude (Qm) drops from 200 mV to 100 mV, over a 5 h period. Figure 2. 4.1kV Motor (24 hrs) 80pF sensors with continuous PD monitor Analysis of the data revealed that the PD was occurring in the end-winding area between phase A and B, since the PD was phase shifted +30° in A phase and -30" in B phase from the normal phase position in the ac voltage cycle for PD occurring in the stator slot [19. ]. Since the endwinding area of machines is specifically designed to be away from ground planes, PD that occurs due to contamination in the endwinding is normally phase-to-phase voltage dependent and will be 30 phase shifted from the classic positions of 45 and 225 . The direction of the shift is dependent on the location of the contamination and the voltages of the affected coils and is therefore unpredictable. Though there may be some effect of temperature or load changes on PD activity from contamination, it is difficult to predict the expected results. However, environmental conditions such as humidity may have a very noticeable effect, especially if the surface contamination becomes partly conductive when damp. In a later visual inspection of the winding by plant staff, the end-windings of this vertical motor were found to be soaked in oil from an upper bearing oil leak. In addition, a heavy layer of dead insects covered the end-winding. Such contamination is conductive and known to produce electrical tracking in the endwindings of coils connected to the phase terminals. Thus for this motor it appears that lower humidity causes the end-winding electrical tracking to accelerate. [18. PD Progress Report 6 of 13 IRMC 2012

P u l s e M a g n i t u d e [ m V ] P u l s e M a g n i t u d e [ m V ] P u l s e M a g n i t u d e [ m V ] 4.2.3 Case Study 3 (Multiple effects) A 21kV air-cooled steam 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. Note that most of the Qm levels are directly correlated; whereas, the C-Qm+ is inversely correlated between the PD levels and the relative humidity levels. Figure 3. Case Study 3 Trend The pulse phase patterns for this unit reveal a distinctive “hook” pattern or clusters of activity at the peak positions of 90°/270° for A-phase and B-phase activity that is indicative of problems at the semicon-gradient interface. In C-phase the activity is a combination of phase-to-phase and internal activity. Because there appears to be a direct correlation with humidity, it is highly likely that some of this activity is occurring at or near the machine terminals. B ip o la r M a c h in e P D B ip o la r M a c h in e P D 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 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 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 75 50 25 0 - 2 5 - 5 0 - 7 5 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 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 ] 75 50 25 0 - 2 5 - 5 0 - 7 5 75 50 25 0 - 2 5 - 5 0 - 7 5 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 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 ] 75 50 25 0 - 2 5 - 5 0 - 7 5 75 50 25 0 - 2 5 - 5 0 - 7 5 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 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 ] A-phase (clusters at 90°/270°) B-phase (clusters/hook at 90°/270°) C-phase (combination of phase-to-phase and internal sources) 75 50 25 0 - 2 5 - 5 0 - 7 5 4.2.4 Case Study 4 (Contamination) A 13.8kV air-cooled steam 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 are negligible and this inverse tracking with humidity suggests the possibility of minor contamination. Figure 4. Case Study 4 Trend PD Progress Report 7 of 13 IRMC 2012

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