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Point on Wave Switching for Capacitor Banks

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daryl64

Electrical
Sep 18, 2006
1
Can anyone tell me the benefits of point on wave (POW) swicthing for 90MVAR cap banks shunt connected in utility applications at 132kV. The untis are typically installed with high series reactance (ie. 6.5%), so would POW be justified?
 
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POW controls can get complicated. They only work if you have single pole operators on your circuit breakers that can close and open each phase pole in sequence. Most 132 kV breakers are three phase, using a common operating mechanism for all three phases.

Theoretically, the POW can be programmed to minimize inrush current on closing and avoid trapped charges on the capacitors after breaker opening. We have only used them on 230 kV and above systems to minimize transformer and line charging curents. (The alternative was to use a closing resistor but it was not available on GIS equipment.)

We do not have experience using POW with cap banks.
 
ABB did some work on this. I think on 110 kV or 220 kV lines. Three-pole breakers with a common shaft. They adjusted the poles so that they closed in sequence and triggered "just in time" to get minimum transients. There was even a "learning function" that adjusted the timing to minimize current - learning from switching to switching. Took care about thermal effects on grease and also aging. I have searched the ABB sites, but no luck so far.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
We have one of our 80 MVA @ 115 kV cap banks switched by a POW breaker with 3 indepentant phases. An ATP/EMTP study indicated that there would be a voltage magnification problem at customer capacitor banks in the industrial area the substation serves. The reactors we have are fairly large at 5 mH.

Unfortunately, we never did any field measurements to verify that the study correctly predicted the system response.

Another thing to consider is how often they will be switched. Ours are switched in in the fall and taken back out of service in the spring. If they were switched multiple times a day, there would be a much greater opportunity for the transients to cause harm.
 
Sorry I am late in on this one, haven't been in the forum for some time.

POW switching would be for energising transients on a cap bank/filter bank.

The initial voltage collapse is given by dV=Ls/(Ls+Lf); where Ls = source inductance and Lf = filter inductance.

For a large bank such as b4l's above and assuming a fault level of say 2000MVA at 50Hz we would have :
dV = 21mH/(21mH + 5mH) = 0,80p.u. or 80%, which is serious.

For a smaller bank, say 25MVAr tuned at 4,8p.u. Lf would be about 76mH which would only give a collapse of about 22% which is acceptable (this would further be reduced by system damping).
 
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