Page 39 - Basic PD Theory

PD Investigations

6
Chapter

6 PD Investigations

Basic tips on how to investigate PD test results
Y
ou have installed the sensors and collected a lot of data, now what? The collection or interrogation of data should be
repeated at least every six months for machines rated 6kV and up. If a problem develops, then the frequency of
collection may need to be increased. Due to the short-time frame between detection and failure for motors rated less
than 4kV, it is recommended that PD analysis be done more frequently on these machines. By using the summary
variables (see Section 4.2), trending of data is straightforward and useful. Collection of data at different operating
parameters may also help in determining the condition of the stator winding [Section 5.1.3].
6.1 Trend Analysis
The first step of analysis is to compare the results of the current test with any previous test results. Trending of the Q m and
NQN values can give you an indication of the progression of the aging mechanisms. A doubling of PD activity (Q m values twice
that of the previous test) every twelve months is a strong indication of a rapidly developing failure mechanism. [12] See Section
4.2 for explanation of NQN and Q m values. For our purposes, Q m values are derived from the test result and are valuable for
comparisons amongst machines or trending over time. There are three significant trends, the short-term, long-term and operating
condition trend. See Section 6.1.3 for more information about the Operating Condition trend.
If the unit operating parameters – voltage, load, winding temperature and gas pressure – are similar to those of the previous test,
then a direct comparison can be made between the two test results. In order to compare tests it is best to maintain operating
conditions within certain ranges, such as:
PD increases with voltage, so variances in voltage can lead to
Voltage ± 2.5%
fluctuations in PD levels.
Load ± 10% If a machine has PD that is mechanically dependent, such as
Real power ± 10% loose coils or connections, then it is likely the PD levels will
Reactive power ± 10% change with load.
Operating temperature affects the volume of internal voids,
Stator temperature ± 10°C which changes PD magnitudes.
± 5 psi-g
Hydrogen pressure ± 35 kPa Gas pressure impacts the electrical breakdown stress and
therefore levels of PD.
± 0.35 bar
Humidity can affect surface PD, especially PD in the endwinding
Humidity Unknown
or ring busses

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