Page 890 - Industrial Power Engineering and Applications Handbook

Protection, maintenance and testing of capacitor units 26/841
capacitance of the test unit may be measured before the flashover should be observed. The variation in the
discharge test and after the voltage test. The capacitance capacitance value as measured before and at the end of
values thus measured should not be exceeded by more the test should not vary by more than 3%. The capacitor
than 2% or breakdown of an element or blowing of an unit should withstand the voltage test between the
internal fuse. terminals and the container as stated in test (4) above
and the scaling test as in test (6) under routine tests.
(7) Partial discharge test (applicable to HT units)
There are invariably voids that will appear in an insulating (IO) Self-healing test
system as a matter of process, irrespective of the type This test is applicable to LT capacitors only. which are
and procedure of impregnation adopted. as discussed in the self-healing type. The capacitor is subjected to a
Section 9.6.1. For all practical purposes, a dielectric system voltage of 2.15 V,, for 10 seconds. If fewer than five
may therefore be considered to be an imperfect capacitor. breakdowns occur during this time. the voltage may be
All such voids may cause internal discharges and lead to increased slowly until it reaches 3.5 times its rated value,
erosion and ultimate failure of the dielectric under the or five breakdowns, whichever happens first. If five
effect of the applied voltage. The applied voltage may breakdowns have still not occurred, the test is continued
be fluctuating too. aggravating the situation. This test is until they do. The capacitance measured before and after
conducted to check the condition and suitability of the the test should show no significant change.
dielectric used to perform the required duties in service
without causing an undue discharge (ionization). It is a (1 I) Destruction test
means of detecting defects in the insulating system of a
capacitor unit and is complementary to a dielectric test. This test is also applicable to LT capacitor units only.
To perform this test, the rated voltage may be applied The basic objective is to establish that the failure of a
long enough for thermal equilibrium to be achieved. Then unit is within a safe zone and is not accompanied by the
a test voltage of twice the rated voltage is applied only risk of a violent explosion or fire.
once for one second. The voltage is then reduced to 1.2
times the rated voltage and maintained for 10 minutes These are only brief descriptions of the recommended
and then it is raised again to I .5 times the rated voltage tests and their procedures on a completed capacitor unit.
for 30 minutes. During the last 10 minutes, the discharge For more details refer to the table of Standards provided
under such conditions should be less than SO pc (pico- at the end of the chapter.
coulombs). The capacitance should be measured both
before and after the test. The change in capacitance should 26.3.3 Checking field operating conditions
not exceed 2%. On larger installations, particularly. it is recommended
One will appreciate that such tests are no guarantee of
the minimum life of a capacitor unit. Moreover, it is not that the actual operating conditions he checked and
compared with the assumed conditions for which thc
expected that such tests can predict the future of the capacitor units were designed to ensure that thcy do not
capacitor units. They can, at best, suggest the compliance differ by more than the permissible limits. This is because
of the test requirements which should ensure a reasonably a capacitor is highly susceptible to some operating
prolonged operating life of the capacitors, as envisaged. conditions such as fluctuations in voltage and harmonic
These tests do provide feedback to the manufacturer on contents in the system. An excess of any of these or both
the quality of the dielectric and the process of insulation V and V, may shorten the life of the capacitor units and
adopted and if any improvements are required.
cause frequent failures of individual units. It may also
lead to overvoltages across the units and incrcasc harmonic
(8) Endurance test (process test for HT capacitors) contents. All this may cause failure of the capacitor units,
This is an alternative to the ageing test and is applicable leading to even a cascade failure. increasing the unbalance
for shunt capacitors of 1000 V and above. It is an in- in voltage and kVAr across each phase and destabilize
house process test and is carried out on the capacitor the cntirc capacitor nctwork.
elements before they are assembled into a capacitor unit The effect of voltage and voltage fluctuations can he
to ascertain their dielectric stability against repeated over- checked through the capacitor line current. I,, which is a
voltages. For the test procedure refer to IEC 6087 1-2. measure of the capacitance C of the capacitor units. The
capacitor current must correspond to the designed (rated)
(9) Ageing test (applicable to LT units) current plus the permissible variation. A higher current
than this would mean a voltage variation or the presence
This test is conducted to estimate the working life of a of harmonics in the system or both. While the voltage
capacitor. Briefly. the test can be conducted in three stages may be measured through a voltmeter and the current
as follows: through an ammeter in the three lines, the harmonics
(a) The capacitor is energized at 1.25 times the rated content may be checked by an oscilloscope. If the actual
voltage for 750 hours. operating conditions differ from the designed parameters
(b) It is then subjected to 1000 discharges. by more than is permissible. suitable measures will be
(c) After test (b). the test in (a) is repeated. essential to restore the operating conditions within the
permissible limits. say, by providing series reactors or
On conclusion of the test no permanent breakdown or filter circuits in the system as discussed alrcady.