Capacitor banks neutral grounding

[pwrengrds]

There are three cap banks (7800kVAR, 130 amps, 34.5kV) at a outdoor substation. The 34.5kV is hard neutral grounded at the transformer. They have an neutral unbalance relay that trips them off with an unbalance if the individual caps blow fuses.

Each cap bank is grounded through a PT (175/1), with the secondary of the PT's all paralleled inside the substation building going to a common over voltage relay.

The settings on the voltage relay are 8 volt to alarm and 15 volts trip with a 20-30 second delay.

My problem with this is that since the secondaries are all in parallel, wouldn't it take a lot of current on the secondary of the PT's to make the primary neutral match up on all three banks? There are no fuses on this.

Is this the proper way to do it or should there be a voltage imbalance relay for each?

 

[PHovnanian]

Your capacitor banks neutrals are floating for all practical purposes.

The PTs only monitor the cap neutral point voltage difference from system neutral. They do not provide a substantial path to ground. However, tying all three PTs secondaries together will produce some interesting results should one bank open one phase fuse. That bank's neutral point will head toward the center point of the two good phases. The other two banks' neutrals will stay close to the primary system neutral. The resulting PT secondaries will see high circulating current due to this imbalance. The average secondary voltage (given an open cap bank primary phase) should be sufficient to cause a trip in 20 to 30 seconds. This may fall within the PTs short time rating. Or perhaps not. Some analysis will be required.

The other case to consider is an imbalance between banks that will cause a similar PT secondary circulating current but not sufficient to cause a protection trip. Can the PTs withstand this current continuously?

 

[waross]

A series connection may serve better.

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

 

[pwrengrds]

I don't know, I am having ESA model it for me. The engineer of record swears that it's like that all over the place and has always worked in the past. My concern is the secondary currents in the PT's, if you blow a 20 amp can (each of the caps is actually 6 cans in parallel) it seems to me that it would be more current than the couple of amps that the PT can handle, before the 20 seconds it takes to trip.

 

[PHovnanian]

If you 'blow a 20 amp can', the PT itself will see nothing more than the voltage swing of the cap bank common point. A PT rated to tolerate line to line voltage will be OK. The PTs do not provide a path for the cap's fault current to ground. If a cap goes shorted between its line and neutral side (not to its grounded can), the bank will just sit there with the remaining legs running at line to line voltage. For some time in all likelihood, until they degrade due to the over voltage.

Its one PT secondary bucking another that might* over-duty them (the PTs). And as Bill pointed out, putting the secondaries in series will eliminate that problem. Separate OV relays will do so as well (for a few more $$).

 

[waross]

Let's see: For the sake of argument let's say that under normal conditions the PTs output zero Volts. One cap fails and that PT goes to 20 Volts. But it is in parallel with two PTs at zero volts.
Two results;
1> Relatively heavy circulating currents, limited by the PT impedances.
2> One PT at 20 Volts in parallel with two PTs at zero volts. I would expect a resultant voltage around 7 Volts until the circulating currents blow a PT.

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

 

[LionelHutz]

It certainly doesn't look right to me. I would do as suggested, either put the secondary windings in series or install 3 voltage relays.

The engineer is possibly confusing parallel CT secondary windings with parallel VT secondary windings.

 

[cranky108]

If the concern is total bus voltage unbalance, then this is correct. The unbalance current won't be that much on the secendary because of the PT impedance.

 

[pwrengrds]

It turns out that ESA does not do imbalance calculations for something like this, just fault studies.

What is the impedance of a PT anyway? The trip voltage primary is 2625 volts, the other two cap banks neutral would need to be pushed off center by that same amount, while accounting for the difference in the PT impedance.

 

[pwrengrds]

We energized the capacitor bank yesterday. I tested the imbalance portion of the circuit by removing one fuse. What I found was that on a 7680KVAR bank with one of eight fuses blown it would have an imbalance of 5.08 volts secondary (889 primary). When the second capacitor bank was put on line with no fuses removed the voltage dropped to 2.4 volts and 1.2 amps of PT secondary current.

I concluded that the PT's have a high impedance and will not cause a fire. I also conclude that the proper way of doing this is to put the PT's in series.

I am working on getting it changed. Thank you for your advice.