LOW VOLTAGE

[PUNGLU]

We have a client metered on 11kV. From the breaker, the client has a breaker which switches on a 1.5MVA transformer on to a 415V bus.

The client has a power factor correction capacitor bank of 500KVA and the loading is mostly inductive.

The client has been complaining of high voltages between 4p.m and 8 p.m. The client offloads most of the industrial loads at 5 p.m.

The transformer is on the lowest tap.

What could be the problem?!

 

[Marmite]

1. The utility volts are high. Check on the metering VT connections, and if possible attach a datalogger so the results can be presented to the utility if necessary.
2. The power factor correction control is faulty. Try switching the pfc off and see what happens to the volts.
3. The transformer is on the wrong tap. Sometimes people get confused. The taps are on the HV winding, so +5% on the tapchanger means to get normal LV volts you need +5% on the HV, which ends up as a reduction in LV volts. You can understand why people get confused.

Regards
Marmite

 

[PUNGLU]

Thank you. I thought the PFC could be injecting more VARs. I have to liase with the client to carry out the above checks. I will give you feedback soon.

 

[alehman]

500 kVA seems like a lot. It's certainly believable that the voltage would be high during light load times if that much capacitance remains on the system continuously. What time of day does the load begin to curtail? Is the voltage OK during high load times?

It may be necessary to switch the capacitor off during light load periods or use voltage control.