circuit breaker and capacitor discharge

[royclh]

hi, due to a requirement to discharge the capacitor in between operations, the capacitor is not possible to re-use it for reactive VAR control for a period of 10 mins after the first switching in and out. Hence, the power utility claims that it does not comply to the continuous voltage control. To reduce the discharge time of capacitors to 10seconds, a discharged VT has been initially suggested to be connected along with the steps. However, my discussion with several engineers seems to suggest that the period of 10 mins "DEAD TIME" delay is NOT actually the consequence of the capacitor discharge but the spring charging of the circuit breaker. The capacitor is designed to withstand out of phase switching. Also, whether the capacitor is fully charged or discharged, it does not affect much on its VAR generation. I'm getting confused.

Can anyone enlighten me this? Is there any detailed information about the "spring charging" of the circuit breaker on the web? If this is the limitation of the circuit breaker and apparently if it's nothing we could do about it, I'm just curious how one can comply to a continuous voltage control?

Look forward to your reply. Many thanks in advance.

 

[rasevskii]

The spring charging time of most CBs is like around 20 seconds or less.

Your questions are far too vague for anyone to answer. Why not enclose a one line dwg and some info on the setup that you have and what sort of plant it is.

regards, rasevskii

 

[royclh]

thanks rasevskii. my question is actually referring to an offshore wind farm project equipped with the mechanically switched capacitors and STATCOM. In order to comply with the grid code, a continuous voltage control must be ensured. However, the capacitor is not possible to re-use for a period of 10 mins after the first switching in and out.

To reduce the discharge time of capacitors to 10seconds, a discharged VT has been initially suggested to be connected along with the steps. However, my discussion with several engineers seems to suggest that the limitation of multiple sequential switching of capacitors are limited by spring charging of the circuit breaker. They also mentioned that the capacitor should be designed to withstand out of phase switching. It is possible to reclose the circuit even when the capacitor is still fully charged to provide VAR although switching transients could be a problem. My understanding is that capacitor discharge has always been a common problem for automatic capacitor banks. This is why a shunt reactor, discharge coil or VT has been used to fasten the discharge time from 10mins to 10s. From what you told me, it takes about 20s to discharge the circuit breaker before the circuit breaker can be reused again. Is it necessary to have a VT to discharge the capacitor then?

I actually do not have the single line diagram with me. It's a question raised by one of my colleagues. Is there any information or catalogue/brochure that talks about the "spring charging" of the circuit breaker?

Appreciate your advice. Many thanks.

 

[Marmite]

I think you are getting confused with the issues. The requirement to avoid switching a capacitor until it has discharged is due to circuit breaker limitations. Inrush currents to capacitor banks can be high in magnitude and frequency. The rate of rise of current can exceed the rate of rise in contact pressure, which causes burning of the contacts. If you close onto a charged capacitor you can potentially double the inrush current. The inrush current (and frequency)can be limited using series reactors. Capacitor banks can be fitted with discharge resistors to dissipate the stored charge over a few minutes. This has nothing to do with spring charging time, but everything to do with circuit breaker duty. The number of close/open cycles the breaker can safely carry within a certain time period at a certain current is important. This will be specified in the approriate standard for the switchgear you are using.
Regards
Marmite

 

[cranky108]

What about the use of a grounding switch to discharge the capacitors? Not that I have used them. I just wait the discharge time.
However in a wind farm application the capacitors may be required to come on and off rather quickly depending on the wind.

Have you consitered the use of several capacitor banks?

 

[KJLRSA]

Is an active system such as a SVC not better suited to the voltage control problem.

Some people I have worked with have suggested resistively grounding the capacitors to discharge quicker. This is practical in LV applications but due to cost of switching devices we found that in MV applications it isnt cost effective.

Most MV capacitors are manufactured with internal resistors to reduce the voltage to under 50V within 5 minutes and not 10 min. I think this is some sort of IEC spec.