Testing a 240V, 3ph, 100 KVAR cap

[bbaumer]

I was asked today about the proper way to test a 40 year old, 100KVAR, 240V 3 phase capacitor bank to see if it is still working properly.

I could not answer.

I was told current measurements were taken and the following values were obtained on the capacitor feeder: A - 67A, B - 45A and C- 39A.

I do not know the load on the system but going from memory it is a 2000A service and likely has at least 800A on it. Just guessing. This service feeds nearly all motor loads. It is a 240V ungrounded delta system (ground fault indicator lights and all.)

I have not looked into this first hand yet. Thought I would ask for thoughts on here before going over. I have seen the service at this building before but it has been awhile.


thanks.

 

[gar]

090402-0735 EST

MY first reaction is that there is substantial unbalance of the capacitor load. Assuming that the voltages are nearly the same this means the capacitors are not equal.

I do not know that it really matters, but is the capacitor bank wired as a delta or Y?

.

 

[zog]

Visual and Mechanical Inspection
1. Inspect physical and mechanical condition.
2. Inspect anchorage, alignment, grounding, and required clearances.
3. Prior to cleaning the unit, perform as-found tests.
4. Clean the unit.
5. Verify that capacitors are electrically connected in their specified configuration.
6. Inspect bolted electrical connections for high resistance using one of the following methods:
1. Use of low-resistance ohmmeter in accordance with Section 7.20.1.2.
2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer's published data or Table 10.12.
3. Perform thermographic survey in accordance with Section 9.
7. Perform as-left tests.
2. Electrical Tests
1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable. See Section 7.20.1.1.
2. Perform insulation-resistance tests from phase terminal(s) to case for one minute. Test voltage and minimum resistance shall be in accordance with manufacturer's published data or Table 10.1.
3. Measure the capacitance of all terminal combinations.
4. Measure resistance of internal discharge resistors.


3. Test Values
1. Compare bolted connection resistances to values of similar connections.
2. Bolt-torque levels should be in accordance with Table 10.12 unless otherwise specified by manufacturer.
3. Microhm or millivolt drop values shall not exceed the high levels of the normal range as indicated in the manufacturer's published data. If manufacturer's data is not available, investigate any values which deviate from similar connections by more than 50 percent of the lowest value.
4. Insulation-resistance values should be in accordance with Table 10.1.
5. Investigate capacitance values differing from manufacturer's published data.
6. Investigate discharge resistor values differing from manufacturer's published data. In accordance with NEC Article 460, residual voltage of a capacitor shall be reduced to 50 volts in the following time intervals after being disconnected from the source of supply:

Rated Voltage
 600 volts
> 600 volts Discharge Time
1 minute
5 minutes