Page 40 - Basic PD Theory

PD Investigations

6.1.1 Long-term Trend

Trend Analysis Plot Purpose: To determine whether there is a
notable long-term trend over many years that is
200 Mach. Qm+ Mach. Qm- Noise Qm+ Noise Qm- changing.
175 Requirements: Linear regression comparison to
150
125 the “best” measurements from all relevant test
100 dates within the database for a specific asset.
75Y Axis Note that tests older than the winding itself
50 should be excluded and data collected within
25 the first 12-18 months of operation may be
0
Dec Jan 2002 Feb Mar Apr May Jun Jul Aug Sep Oct Nov excluded if they highly influence the results.
2001 X Axis The latter is due to the common occurrence
Figure 37: Trend Plot that PD behaviour is often erratic and may

show a gradual reduction for new windings as the winding settles-in and the bonding resin continues to cure.
Caution: Erratic PD can cause wide swings in trends that may be misleading. Do not interpret these in
isolation. Variations of some percent, say +/- 25%, are normal [11].
When a trend line is established for PD tests taken over a period, it will be obvious that most show an up and down variation
between successive tests [Figure 37]. However, as an insulation system ages, there will be an easily discernible overall upward
movement of PD with time if the PD is measured at approximately the same operating conditions. Ageing is normally a very
slow process and sudden changes are not expected in the PD test results. Though the condition of the stator winding insulation
can be assessed, time to failure cannot be predicted. The actual failure is normally the result of an unusual source of insulation
stress such as lightning, out-of-phase synchronization or severe overheating. Maintenance can often be done on a machine to
lower the PD activity. Examples of maintenance that have been known to successfully reduce PD are re-wedging, cleaning, dip
and bake, and repairs to the voltage stress coatings. If the source of the PD is within the bulk of the insulation, then repairs may

Trend of results for same
machine
Current PD
Levels

PD increase No No specific
> 150%? problems – monitor
in 6 months

Ye

Further
investigation A
required

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