on-line testing of High Voltage equipment

1. Introduction
Discharge (PD), as it's name would suggest, is an electrical discharge that occurs
across a portion of the insulation between two conducting electrodes, without
completely bridging the gap. Partial Discharges can occur in voids in solid insulation
(paper, polymer etc), gas bubbles in liquid insulation or around an electrode in a gas
(corona). Once PD occurs, insulation will degrade and eventually lead to equipment
breakdown.
Partial Discharge testing is a predictive maintenance tool for any high voltage
equipment. It is the only on-line electrical test recommended by IEEE that correctly
identifies and warns of insulation defects such as contamination, thermal
deterioration, poor manufacture, and more!
Why test for Partial Discharge?
Failure of High Voltage insulation is the No. 1 cause of High voltage
system failures with IEEE statistics indicating that electrical insulation
deterioration causes up to 90% of electrical failures of certain high
voltage equipment. On-line PD testing of HV plant gives advance
warning of pending insulation failure thus allowing the plant owner to
take remedial action during planned outages. Unlike off-line testing, on-
line PD testing and monitoring gives an accurate picture of the plants
health and performance under service conditions.

2. Benefits of Partial Discharge Testing
Partial Discharge testing WILL provide users a huge benefit in the form of one or
more of the following:
It is truly a predictive test, indicating insulation degradation in advance of
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the failure.
It need not use any over-voltages, thereby not exposing the tested
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equipment to higher voltage stresses than those encountered under
normal operating conditions. Avoid unexpected in-service failures of the
equipment and extend up-time between outages.
Avoid unnecessary servicing and repairs of older equipment by
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maximizing the operating hours.
Find a problem and remedy it before it has a chance to fail the
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equipment.
Find problems on new equipments which may still be under warranty.
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Assess the quality of maintenance repairs and/or rewinds with before and
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after testing.
Compare results from similar equipment to focus maintenance on those
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with higher levels of PD.
Identify specific failure mechanisms in the equipment to allow for
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corrective action prior to an outage.
Improve the overall reliability of the equipment.
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Accomplish all this while the machine remains in operation (On-Line).
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3. Why choose PD testing over Hipot testing?
HI-POT (DC,AC)-Pressure Test Partial Discharge (PD) Test
Destructive Test Non Destructive Test
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- Based on Pass/Fail criteria. - Does not test to fail equipment and
does not affect insulation at all.
- Weaken insulation due to overvoltage applied
thus unnecessarily reducing equipment life.
Not recommended to be done on aged
equipment
Can hardly detect voids, cuts and Ability to detect voids, cuts and
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workmanship induced defects which may workmanship induced defects.
develop into serious defects when put in
service.
Required de-energised state Can be carried out in energized and de-
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- Potential of damage due to inappropriate energized state.
Switching. -No loss of revenue.
- Loss of revenue.
Intrusive form of testing Non intrusive form of testing and very safe
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to carry out.
-Some degree of safety risk.
Bulk testing concept Localized testing concept
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- Only tell there is a defect somewhere but - Able to locate the source and location of
cannot locate the source of defect the defect.
- Servicing time will be longer and higher -Cut down on servicing cost.
servicing cost incurred
Degradation of insulation cannot be Degradation of insulation can be
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predicted predicted by trending over time.
-Proactive maintenance can be carried out
- Have to service equipment once defect is
detected.
- No flexibility in planning maintenance
schedules.
Results are less conclusive Results are more conclusive.
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-Testing is carried out in non operating -Testing is carried out “live” and really
normal condition. stimulates a true normal operating
condition.

4. Note:
It is not our usual practice to recommend PD testing with the intention to replace other
forms of electrical testing as most are good tests to perform when given the chance.
However, why take an operating equipment out of service or extend an outage to do these
tests when totally non destructive on-line Partial Discharge testing can do the same job
and with more detailed inferences while “Live”. Hoestar does recommend that PD testing
be used as the first line of testing to identify any possible insulation defects before
performing any off-line tests (Corona, hipot or megger test) as the benefits of PD testing
are much more worthwhile. Offline test can then be carried out to confirm the problem
existence if necessary.
Commonly asked questions:
1) Is Partial Discharge (PD) accurate? What standard does it conforms to?
PD Testing is an internationally recognized testing technique used in more than 28
countries for detecting insulation defects. According to IEEE 400, the only proven
and effective way to establish an electrical preventative/ predictive maintenance
program is using Partial Discharge Testing.
Many grids are turning to this test as a first line test to check for early insulation
defects as it is totally non destructive in nature compared to the usual hipot test.
2) If Partial Discharge testing is so good and highly rated, why is it not many people are
adopting
this testing method?
There are three reasons behind this:
PD testing is a fairly new technology compared to the usual Hipot (Megger)
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test. PD testing has matured over the years and today it has already been
proven to be much more powerful indicator for insulation defects compared to
offline test (like hipot, Megger etc).
PD testing may be the most powerful indicator for detecting insulation defect
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but it is also the most sophisticated test to be carried out. The interpretations of
the results are extremely challenging and require very well trained personnel to
do the analysis. Once results are correctly interpreted, PD testing will give the
best detailed diagnosis of the insulation condition of equipment compared to any
Hipot. Megger test etc.
As PD testing is sophisticated, the cost of inspection is generally much higher
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than usual Hipot testing. However, the cost of PD testing is much lower
nowadays.