Power Factor Testing
Why Perform Power Factor Testing?
Most failures of electrical equipment are the result of a
breakdown in the insulation system. Power factor testing
is an effective way to detect defective electrical
equipment insulation. Power factor is the ratio of the
capacitive or ‘charging’ current (measured in voltamperes)
to resistive or ‘leakage’ current (measured in
watts). It is a means of measuring insulation integrity
since the smaller the power factor, the better the
insulator.
Insulation may be deteriorating, however, the equipment
may not fail for some time. Although this condition
represents a serious hazard, if deteriorated insulation is
detected before failure, it can often be reconditioned
instead of being replaced. This allows corrective action
to be taken during a planned outage before extensive
equipment damage results from insulation failure.
Power factor testing also makes it possible to detect
equipment insulation problems without making an
internal visual inspection. The result is a significant
decrease in maintenance cost and downtime. Power
factor testing is one of the best testing techniques for
predictive and preventive maintenance.
What are the Test Procedures?
Once the equipment is taken out of service, an AC
voltage at or below the normal rating is placed on the
equipment conductors. At the same time, the dielectric
losses to ground are measured. After correcting the
field measurements to standard conditions, the test
results are then compared to guides which list proper
values for specific types of equipment and insulations.
These guides are contained in an extensive data library
compiled by testing thousands of similar types of
equipment in the field and at the factory.
If an abnormal condition is indicated, a number of special
tests may be used to localize the problem. These include
hot guard tests, hot collar tests, ungrounded specimen
tests, cross check tests, oil tests, and dielectric loss tests
on wood members.
What Types of Equipment are Tested?
The most frequent applications of power factor testing
include medium and high voltage transformers and
circuit breakers. Other equipment such as bushings, reclosers, switches, cables, capacitors, lightning
arrestors, liquid insulation, potheads, rotating
machinery, and voltage regulators may also be power
factor tested.
Power factor testing is used as a factory test and in the
field for acceptance testing, preventive maintenance,
and emergency diagnostic testing.
What are the Advantages?
Power factor tests can detect the presence of “poor”
insulation even if there is a layer of “good” insulation in
series. However, DC tests usually show a high insulation
resistance under this condition.
At a given voltage, the charging current and the dielectric
loss increase as the quantity of insulation increases.
However, the ratio, power factor, remains the same no
matter how much insulation is present. This makes it
possible to establish simplified test values for particular
types of insulation.
Power factor testing provides a measure and evaluation
of the overall condition of medium and high voltage AC
apparatus insulation under simulated normal-frequency
operating conditions. This is independent of the duration
of the test. DC tests not only require more time, they
also do not simulate normal insulation operating
conditions. Furthermore, the results are affected by
the duration of the test.
Power factor tests provide a benchmark which may be
compared over periods of time in an organized manner
and are used to identify any trends on apparatus
insulation. It provides a more complete picture of overall
insulation conditions.
Used in conjunction with capacitance and AC
resistance, it provides comprehensive data of the
nature of the insulation.
adapted from www.hvmcorp.com