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partial discharge testing: a progress report - Iris Power Engineering

partial discharge testing: a progress report - Iris Power Engineering

off-line tests and/or a

off-line tests and/or a visual inspection would be prudent. Continuous PD monitors should have their alarm levels set to the 90% level. 4 HUMIDITY EFFECT Environmental conditions may have a very noticeable impact, especially if the surface contamination becomes partly conductive when damp, so it should be recorded from one test to the next. It is appreciated that at times it may be very difficult to duplicate test conditions, particularly temperatures; however, the emphasis should always be on trying to achieve as uniform conditions as possible. This means taking tests when units are hot and the temperature has stabilized, while accurately recording both the operating and ambient conditions. Perhaps the most influential, yet unpredictable, impacts of ambient conditions to PD are the effects of variances in ambient humidity on air-cooled machines. Humidity can cause the electrical breakdown stress of the air to decrease, and therefore cause an increase in surface PD. On the other hand, humidity can also impact the electrical tracking of surface PD, and therefore cause a decrease in PD. Thus, in some circumstances, the PD may go up (direct effect), while in others it would go down (inverse effect) with increases in relative humidity. In some circumstances, the humidity effect has been known to cause the PD to fluctuate by as much as 300%. Since each scenario is different, it is extremely important the ambient humidity be recorded at test time so trends can be properly evaluated and decisions are not made about upward trends that occur because of humidity effects and not changes in winding condition. Present experience indicates humidity only affects surface activity. 4.1 DATABASE ANALYSIS Several different analyses were done on the Iris database to determine whether there was a distinctive pattern between PD levels and humidity. Since relative humidity is temperature-dependent, the examinations were done using relative humidity, dew point and moisture content (kg/m 3 of water). There was no discernible pattern for any of the investigations, as shown below with the PD versus Ambient Relative Humidity. The data was first separated by type of sensor and PD measuring device, that is, using only data collected from 80pF sensors with a high-noise reduction instrument, such as PDA-IV, TGA or Guard. Then the data was further separated by gas coolant (air-cooled versus hydrogen) cooled, and finally by voltage class. 4.1.1 Air-Cooled Machines with 80pF Couplers 3000 2500 2000 1500 1000 500 0 6-9kV Air-Cooled with 80pF Capacitors 100 80 60 40 20 0 Qm - Qm + Rel Humidity The goal was to determine if there was a pattern of inverse or direct correlation with humidity. As shown in the Charts for 6-9kV and 13-15kV air-cooled machines, there was neither. All other voltage classes were similar without any indication of correlation. Speculation as to the lack of correlation leads to the following possibilities: 3500 3000 2500 2000 1500 1000 500 13-15kV Air-Cooled with 80pF Capacitors 100 80 60 40 20 Qm - Qm + Rel Humidity a. That the majority of the problems within the winding are internal or within the slot and therefore not impacted by humidity like surface PD could be b. That though the humidity outside of the housing may be variable, the ambient temperature and humidity within the housing does not vary significantly 0 0 PD Progress Report 4 of 13 IRMC 2012

4.1.2 Hydrogen-Cooled Machines with 80pF Couplers 3500 3000 2500 2000 1500 1000 500 0 13-15kV Hydrogen-cooled with 80pF Capacitors 100 80 60 40 20 0 Qm - Qm + Rel Humidity There should NOT be any impact to PD due to varying about humidity levels in hydrogencooled machines, since the humidity in the hydrogen is not recorded. This may not be true if the hydrogen dryers are malfunctioning, and there is “wet” hydrogen. However, when PD sensors are installed outside of the hydrogen environment at the machine’s terminals, then PD sources in the air-environment may be impacted by humidity. 3500 3000 2500 2000 1500 1000 500 0 16-18kV Hydrogen-cooled with 80pF Capacitors 100 80 60 40 20 0 Qm- Qm + Rel Humidity Though the 13-15kV may show some direct correlation, it is not sufficient at this time to formulate a conclusion. Like with the air-cooled machines, there is no distinguishable correlation between humidity and PD levels. Therefore, it can be assumed that humidity effects are rare. However, should they be detected on hydrogen-cooled machines, the problem is outside of the hydrogen-environment from PD sources within the air environment. 4.2 CASE STUDIES 4.2.1 Case Study 1 (Voltage Stress Coatings) Figure 1 shows the effect of dew point measurement in the plant environment on the peak PD magnitude (Qm) over a six-year cycle for a 20 kV, 450 MVA pump storage generator. It is clear from the figure that there is an inverse effect, that is, the PD activity increases in drier (lower dew point) air. Visual inspections of the stator winding found surface PD occurring between the semi-conductive and stress control coatings. [18. Figure 1. 20kV Pump-storage Unit (6 years) 80pF sensors measured with a PDA-IV PD Progress Report 5 of 13 IRMC 2012

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