Capacitor bank switching

[HamidEle]

We have three power factor correction banks in the weak system (generator supply). We didn't have any issue when we were switching the first one on to the system. But we saw a transient overvoltage when we were switching the second one on. We haven't swicthed on the third one. I am afraid we may see a higher transient overvoltage in the system since the capacitors which are already in the system will back charge the third one being switched on. Does a stronger system help to mitigate this kind of overvoltage issue? Any inputs would be appreciated.

 

[waross]

You may be seeing an AVR effect as Avr is unable to instantly reduce the generator excitation in response to a step change in power factor.
If a stronger system implies a larger system, then yes.
Why are you worried about power factor on a generator? Not to imply that there are not valid reasons to consider power factor on a generator, but power factor is often less important on an islanded generator close to the load than on a grid.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HamidEle]

We did see the generator under excitation during the switching of Capaciotor bank. We are trying to see the system response since we are connecting the utility soon.

 

[waross]

Be careful with your generator. Too much capacity can cause the AVR to lose control and the generator to go over voltage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

HamidEle:

What you have is the case switching a capacitor in parallel with one already charged. Without adequate current limiting reactors (or long cables), it will result in what you are seeing. When the second cap is switched on, it has it own charging current, plus the other caps also see it as a short circuit at time t=0+ and discharges through it. You need to install appropriate reactors in series with the caps. Talk to some cap manufacturers and they will help you.

Switching the first bank will not be an issue as you have already experienced.

Rafiq Bulsara

http://www.srengineersct.com

 

[edison123]

About six months back, my client called me about voltage swinging in his stand alone 415 V, 600 KVA DG. I went to the site, checked the usual culprits of loose field wires, bad AVR, bad diodes etc and found nothing wrong. We started the DG and the voltage was steady at 410 V. Then I asked him to throw the load switches one by one and found the voltage was holding steady. Then he switched on a 75 KVAR cap bank (which is normally switched on when he is on utility power since there is a severe penalty for pf < 0.95) and bang, the voltage started hunting. I asked him to switch off the cap bank and DG will voltage was back to normal steady self.

Now I've left standing instruction with the DG operator not to switch on the cap bank when he is on DG power. No probs since then.

Muthu

http://www.edison.co.in

 

[rbulsara]

Generator never likes over capacitive loads. I see the issue is only the 'Voltage' transients and not the high inrush current. My comments apply if you were experiencing high inrush currents as well upon switching the second or third bank.


Rafiq Bulsara

http://www.srengineersct.com

 

[HamidEle]

Rbulsara,

We were not experiencing high inrush currents. we only saw high transient overvoltage upon switching the second.

 

[HamidEle]

For sure, third capacitor bank will further increase both steady state and transient voltage, depending the switching events.We may experience the highest bus voltage in the event of closing the contactor at the peak ( waveform).

 

[waross]

Hi Hamid;
If you are able to monitor the output of the AVR, you may see a voltage drop that somewhat reflects the voltage transient.
If you have an AVR on manual control (not possible with most AVRs that I work with) You would see a steady rise in voltage as the capacitors supply some of the exciting current to the generator. With the AVR in automatic mode, the AVR senses the voltage rise caused by the capacitors and reduces the AVR contribution to excitation accordingly. The transient over voltage that you are seeing is probably an indication of the AVR response to a step increase in terminal voltage, whether caused by load shedding or capacitor connection.

Bill
--------------------
"Why not the best?"
Jimmy Carter