Relay Acting

dear,

If neutral displacement relay is acted without any seemingly fault,

it is mainly due to un balanced current flowing in the neutral. Most probably, Normally this kind of fault occures with single phasing in supply lines / un balanced voltages / or un balanced currents.

Assuming the un balanced currents generated by load side are nil, or within limits,

Source you are encountering with un wanted un balanced voltages from source side .

This may be due to Faulty capacitor Bank in the 11kv side, as you may have suspected. You may also need to check all the capacitor banks - wether they are drawing balanced currents. If not, you need to correct .

For more information we are using VCB to connect the capacitor bank.

dear,

what is the voltages - line to line - are they equal? if not, neutral un balance will occur.

what is the current - line - on secondary side. are they equal ? if not, neutral un balance will occur.

If voltage un balance, then you may need to check capacitors, part by part, if the source supply voltages on incoming side are balanced.

Ok so We are going to charge single capacitor of 117KVAR each in 3 phase to verify faulty capacitor.

YES.

We found no problem with any of the capacitor and we found doubt the problem is with reactor or PT connected to the bank

Was there a phase fault or disturbance anywhere on the 11kV system to which the cap bank is connected, that could have caused an unbalance? Any (nearly) simultaneous trips on the 11kV when the cap bank tripped? The cap bank relay may just have been seeing it feeding a disturbance on the rest of the system.

We could not figure out the problem.The capacitor,limiting reactor,PT and lightning arrestor is found to be healthy. We doubt the relay and associated VCB. Now the work is in the middle and we will check that tomorrow morning. The load is always on and we need to fix it soon.

No, I was asking if there was a disturbance further out on the 11kV system (utility supply?) that could have caused an unbalance that your cap bank relay sensed. Without the larger picture, sometimes we narrow our search too much, too early. Typical feeder protection relays (usually overcurrent) would not have seen the disturbance, but since the sensitive cap bank relay is looking for an imbalance, it might have sensed it.

So you are telling either from the 110KV incoming or in any of the feeder there may be an imbalance between phase which cant be sensed by any of the relay. Let me explain, we have 3 electric arc furnace with 5MVA YNd step down transformer for each. The AC arc furnace is having a feedback control to melt the iron. Now a days only 1 furnace work at a time so i dont think it could create a reasonable imbalance. If your assumption is correct how do we check that imbalance? The problem is not yet solved and we found a burn out cable in VCB.

hi.. based from the discussions above, the problem was not clearly defined.. as i was reading, i saw secondary lines.. was there a secondary line from the capacitor?.. it is shocking..

how was the capacitor connected?.. is it a capacitor bank?.. how the relays connected in the bank?.. when you are talking about neutral displacement, are you trying to mean voltage unbalance?.. if so, where is the pt connected, how was it connected?..

if you may, cite again the problem, with enough details.. so that fellow readers could understand and suggest solutions clearer..

thanks.

There is no secondary line.In 3 phase AC the star connected capacitor is in series with current limiting reactor.The neutral of capacitor and PT is shorted in-order to measure total potential in capacitor.I believe neutral displacement relay is connected to this PT.

As we know an LA is connected in parallel to protect the whole arrangement.

VCB is used to make and break the circuit and when we close VCB the neutral displacement relay act after a couple of seconds.

Except neutral displacement relay we have over current relay(2),earth leakage relay and under voltage relay.

I hope its clear a bit now.

Aha, I wondered about arc furnaces. Even AC, they do not always operate balanced. That could have been source of disturbance that cap bank relay sensed.

What kind of burned out cable on your VCB? Control cable or power cable? In what circuit? You need to provide more data.

As I said before, I am willing to bet that you have no other relays in your plant that would sense a voltage imbalance like this, since you have essentially all load. Thus the capacitor bank relay would be the only one to operate for an imbalance or disturbance that is severe enough for it to detect, but wouldn't cause something like an overcurrent relay to operate.

If you don't get much farther on your own, you need to bring in someone who is qualified in troubleshooting HV systems.

Mr. Peter arc furnaces worked within the range of relay set current before also. I am looking for the possible conditions of the fault that happened here. It will be very helpful if you list the proper trouble shooting procedure. The cable is replaced with a new one.