Testing of Capacitor Bank

ANSI, IEEE, NEMA or IEC standard is used for testing a power capacitor bank.
There are three types of test performed on capacitor banks. They are

1. Design Tests or Type Tests.

2. Production Test or Routine Tests.

3. Field Tests or Pre commissioning Tests.

Design Tests or Type Tests of Capacitor Bank

When a new design of power capacitor is launched by a manufacturer, it to be tested whether the new batch of capacitor comply the standard or not. Design tests or type tests are not performed on individual capacitor rather they are performed on some randomly selected capacitors to ensure compliance of the standard.

During launching of new design, once these design tests are performed, there is no need of repeating these tests for any further batch of production until the design is changed. Type tests or design tests are normally destructive and expensive.
The type tests Performed on Capacitor Bank are –

1. High Voltage Impulse Withstand Test.

2. Bushing Test.

3. Thermal Stability Test.

4. Radio Influence Voltage (RIV) Test.

5. Voltage Decay Test.

6. Short Circuit Discharge Test.

High Voltage Impulse Withstand Test

This test ensures the withstand capability of insulation used in capacitor unit. Insulation provided on capacitor unit should be capable of withstanding high voltage ensures during transient over voltage condition.
There are three types of capacitor units.

1. 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.
   

2. 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.
   

3. 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 is performed to see how much the capacitor unit is thermally stable. For this test the test unit is mounted between two dummy capacitor units. The dummy capacitor units must have same dimensions as test unit.
The dummy units and test unit are to be mounted in same manner as these would be practically mounted on the capacitor bank structure.
To reduce air circulation all three capacitors are kept inside a closed enclosure. The dummy units may be of same rated capacitor units as test unit or these are resistor model of test unit. Resistor model means instead of capacitor elements, resistors are placed inside the capacitor casing to generate same thermal effect as original capacitor unit for same unit power. The air inside the enclosure must not be forced circulated. All three samples i.e. test capacitor and two dummy capacitors are energized by a test voltage which is calculated by formula given below,

Where,
VT is test voltage,
VR is rated voltage of test unit,
WM is maximum allowable power loss,
WA is actual power loss.
Although the test voltage is calculated from above formula but the test voltage should be limited up to that value which produces maximum 144 % of rated KVAR of the capacitor unit. The voltage once calculated or estimated and applied, it must be maintained with in ± 2 % though out 24 hours of the test period.

Radio Influence Voltage Test

This test is done at rated frequency and 115 % of rated rms voltage of capacitor. This test is only performed on the unit having more than one bushing. Because single bushing unit has casing connected direct with capacitor elements. During the test the casing of multi bushing unit must be properly earthed. The test capacitor should be kept at room temperature and its bushing should be dry and clean. The unit should be mounted on its recommended position. During measurement at 1 MHz, the radio frequency voltage must not exceed 250 µv.

Voltage Decay Test

Here the capacitor unit is changed with the direct voltage whose value is equal to the peak value of rated alternating voltage of the unit. After charging the unit, let it be discharged by some means and decay of voltage is measured. If the voltage comes down to less than 50 V with in 5 min in the case of capacitor unit rated higher than 600 V (rms), then the unit is considered as passed in the voltage decay test. This voltage decay should be within 1 min in the case of capacitor unit rated with lower than 600 V (rms).

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 test are also referred as production tests. These tests should be performed on each capacitor unit of a production batch to ensure performance parameter of individual.

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 25o C then the meandered result should be calculated according to 25o 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 is used to measure the capacitance of the bank as whole to ensure the connection of the bank is as per specification. If the measured value is not as calculated, there must be some wrong connection in the bank which to be rectified. For measuring capacitance of a bank, we need not to apply full rated voltage, instead only 10 % of rated voltage to determine the capacitance of the unit. The formula of capacitance is

Where, V is the applied voltage to the bank,
I is the supply current and
ω = 377.7 which is a constant quality.

High Voltage Insulation Test

High voltage insulation test can be performed in accordance with NBMA CP-1

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