Testing of Capacitor Bank

IEEE Std 18, with related ANSI, NEMA or IEC rules, is used to test a power capacitor bank.
Three classes of test are used on capacitor banks. They are

Design Tests or Type Tests.
Production Test or Routine Tests.
Field Tests or Pre commissioning Tests.

Design Tests or Type Tests of Capacitor Bank

When a manufacturer launches a new power-capacitor design, sample units are tested to show that the new capacitor meets the standard. Design or type tests are not run on every unit. They are run on randomly selected samples.

After a design has passed, those tests are not repeated on later production lots unless the design changes. Type or design tests are usually destructive and costly.
The type tests performed on a Capacitor Bank are

High Voltage Impulse Withstand Test.
Bushing Test.
Thermal Stability Test.
Radio Influence Voltage (RIV) Test.
Voltage Decay Test.
Short Circuit Discharge Test.

High Voltage Impulse Withstand Test

This test checks that the unit insulation can stand the high voltage of a lightning-impulse transient. IEEE 18 uses a 1.2/50 µs wave for that check.
There are three types of capacitor units.

Single Bushing Capacitor Unit

Here, one terminal of capacitor element comes out from the casting through a bushing and other terminal of capacitor element is directly connected to the cashing itself. Here cashing of capacitor unit serves as one terminal of capacitor unit serves as one terminal of capacitor unit is connected to the bushing stand through the capacitor elements high voltage impulse with stand test cannot be performed in this unit.
Double Bushing Capacitor Unit

Here two ends of capacitor element are terminated on the cashing via two separate bushing. Here the cashing is entirely isolated from the cashing body.
Three Bushing Capacitor Unit

In three phase capacitor unit, the line terminal of each phase of three phase capacitor elements come out from cashing via three separate bushings.

This test is performed only on multi bushing capacitor unit. All the bushing stands to be short circuited by a high conductive wire before applying high voltage impulse. The body of the cashing should be properly earthed.
If more than one unit of some BIL or Basic Insulation Level rating to be tested, then all bushings of the batches to be shorted together.
In this test standard impulse cover voltage is applied to each of the bushing stand. The recommended impulse over voltage is 1.2/50 µsec. If the capacitor unit has two different BIL bushing then the impulse voltage applied is based on low BIL bushing. If there is no flash over in the bushing for three successive applications of rated impulse voltage, the unit is considered to be passed in the test.

Bushing Test

If there is no flash over in the previous impulse test, there is no need of separate bushing test. But if there is a flash over in first three successive applications of impulse over voltage, then other three successive over voltage are applied further. If no additional flash over occurs in the bushing, then the bushing is considered as passed in the test.

Thermal Stability Test of Power Capacitor

This test checks whether the capacitor unit stays thermally stable. The test unit sits between two dummy units of the same size.
Mount the dummy units and the test unit as they would sit on the capacitor bank rack.
Place all three in a closed enclosure so air does not circulate freely. The dummy units may be live units of the same rating or resistor models. In a resistor model, resistors inside the case produce the same heat as the real unit at the same power. Do not force-circulate the air. Energise the test capacitor and the two dummy units at the test voltage from the formula below,

Where,
V_T is test voltage,
V_R is rated voltage of the test unit,
W_M is maximum allowable power loss,
W_A is actual power loss.
IEEE 18 limits that test voltage so the capacitor does not exceed 144% of rated kvar. Hold the applied voltage within ± 2% through the last 24 hours of the test.

Radio Influence Voltage Test

IEEE 18 runs this test at rated frequency and 115% of rated rms voltage. It applies only to units with more than one bushing, because a single-bushing unit has the case tied to one capacitor terminal. Earth the case of a multi-bushing unit. Keep the test capacitor at room temperature with dry, clean bushings, mounted as recommended. At 1 MHz the radio-influence voltage must not exceed 250 µV.

Voltage Decay Test

Charge the unit with a direct voltage equal to the peak of its rated alternating voltage. Then let it discharge and measure the decay. IEEE 18 passes the unit if residual voltage falls to 50 V or less within 5 min for a unit rated above 600 V (rms), or within 1 min for a unit rated 600 V or less.

Short Circuit Discharge Test

This test is performed to verify the tightness of all internal connection of a capacitor unit. Not only tightness it also verifies the size of conductors and their electrical properties are selected and designed properly or not, in a capacitor unit. In this test the capacitor units are charged up to 2.5 times of its rated rms voltage. Then the capacitor unit is discharged. This charging and discharging should be done at least 5 times. The capacitance of the capacitor unit is measured before applying the charging voltage and also after fifth discharge of the unit. The difference between initial and final capacitance is recorded and it should not be more then the capacitance difference of the unit when one capacitor elements is shorted or one fuse element in operated.
That means,
(Initially measured capacitance – capacitance measured after fifth discharge) < (capacitance of the unit with all elements and fuse element- capacitance with one capacitor element shorted or one fuse element operated)

Routine Test of Capacitor Bank

Routine tests are the production tests. IEEE 18 requires them on every capacitor unit in a production lot to check that unit’s performance.

Short Time Over Voltage Test

In this test, a direct voltage of 4.3 times of rated rms voltage or alternating voltage of 2 times of rated rms voltage is applied to the bushing stands of capacitor unit. The capacitor limit should with stand either of these voltages at least for 10 seconds. The temperature of the unit during test should be maintained at 25 ± 5 Degree. In case of three phase capacitor unit, if the three phase capacitor elements are connected in star with neutral connected through a fourth bushing or through casing, the voltage applied between phase terminals, would be √3 times of above mentioned voltages. Same voltage as above would be applied across phase terminal and neutral terminal.
For three phase delta connected unit the rated voltage is phase to phase voltage.
The capacitance is to be measured before and after the test voltage is applied. The change in capacitance should be less than 2% of original measured capacitance or that caused by failure of single capacitive element or fuse element whichever is less.

Terminal to Case Voltage Test

This test is only applicable where internal capacitor elements of a unit are isolated from its casing. This test ensures the withstand capability of over voltage of the insulation provided between metal casing and capacitor elements. The test voltage is applied between casing and bushing stand for 10 seconds. For the capacitor unit having bushings of different BIL, this test is done on the basis of lower BIL bushing.

Capacitance Test

This test is done to ensure that each of the capacitor unit in a batch or lot should give not more than 110 % of its rated VAR during normal operating condition that is charging application of rated voltage and frequency to the unit within possible temperature limit which is considered as Degree C. If the measurement is done at any temperature other than 25^o C then the meandered result should be calculated according to 25^o C.

Leakage Test of Capacitor Units

This test is done to ensure that the limit is free from any leakage. In this test the test unit is heated by an external oven, to force the insulating liquid to come out from the casing if there is any leakage point. This test ensures that all the joints are sealed and tightened properly.

Discharge Resistor Test

This test is done on each capacitor unit to ensure that internal discharge device or resistor is capable enough to discharge the capacitor unit from its initial residual voltage to 50 V or less with in specified time limit. Initial residual voltage may be √2 times of rated rms voltage of the capacitor.

Loss Determination Test

This test is performed on each capacitor unit to demonstrate, the loss occurs in the unit during operation is less than the maximum allowable loss of the unit.

Fuse Capability Test of Internal Fused Capacitor Unit

In this test the capacitor unit is first charged with direct voltage (DC) up to 1.7 times of the rated rms voltage of the capacitor unit. Then this unit is allowed to discharge through a gap situated as closely as possible without any additional impedance to the discharge circuit.
The capacitance of capacitor should be measured before application of charging voltage and also after discharging the unit. The difference of these two measurements must be less than the difference of capacitance when an internal fuse element is operated.

Pre commissioning or Installation Test of Capacitor Bank

When a capacitor bank is practically installed at site, there must be some specific tests to be performed to ensure the connection of each unit and the bank as a whole are in order and as per specifications.

Capacitance Measurement

A sensitive capacitance meter measures the bank as a whole to check that the connections match the specification. If the reading does not match the calculated value, a connection is wrong and must be corrected. The bank does not need full rated voltage for this check; about 10% of rated voltage is enough. Capacitance follows

where V is the applied voltage to the bank,
I is the supply current and
ω = 2πf (about 377 rad/s at 60 Hz, 314 rad/s at 50 Hz), not a fixed universal number.