Relay Protection

[Jerry123]

Hello everyone,
I was hoping to get input on a situation my site encountered last week.

System - VCP-W Breaker with ABB TPU relay. Distributes 13.2KV directly to 5 Oil Switches and a line up of MV Switches. Each of the Oil Switches feed to 1000KVA D-Y oil filled transformer. The MV switches have a Main switch on it. The Main feeds 4 feeder MV switches. 3 of the MV switches feed 500KVA D-Y transformers and 1 feeds some rectifiers. I hope that makes sense.

We had some maintenance performed on the switches and had the above circuit de-energized.

We went to energize the VCP-W breaker with all downstream switches closed and the TPU relay tripped on a 51N (time overcurrent on the neutral phase).

51N setting: PU 50amps and 1s I2T.

Fault information in the relays history was that the relay tripped after 5seconds at 72amps.

We closed the breaker again and the same thing happened.

After several hours we found that the Phase A fuse in the Main MV switch was not put back in.


Questions.
Is this proper protection? If a fuse opened downstream by a fault or if someone forgot to put it back in after maintenance would I want the relay to trip off the breaker causing an outage on the entire circuit? Is the 51N setting to low?

Let me know if there are any questions and thanks.

 

[rasevskii]

for jerry123:

Yes, you were lucky that the relay had tripped on 51N after 5 seconds. The circuit would have been energized at single phase only, with possible dire consequences for 3 phase loads downstream, such as motors unable to start. A proper protection would have been a voltage unbalance protection at some point downstream (aka fuse failure protection) which would trip the incoming breaker after a time lag.

rasevskii

 

[rbulsara]

51N picks up unbalance. Sounds like a trip event on along the expected lines, given the cause. As for the settings and the design setup, have a professional take a look.

Fuses are economical, but they do come with sacrifice in sophistication. Level of protection is a form of insurance, you invest based on the probable risk/benefit analysis and your risk tolerance.

Rafiq Bulsara

http://www.srengineersct.com

 

[GenSwgrEng]

Yes, it was proper operation of the breaker to trip on 51N. You don't want to single-phase your loads. No, it is not ideal to have all 5 circuits dropped to clear a problem on 1. Ideally you would have a breaker, or at least a switch that could open to clear the fault as locally as possible. Given the configuration of your system, the protective circuit worked properly.

Engineers are always honest in matters of technology and human relationships. That's why it's a good idea to keep engineers away from customers, romantic interests, and other people who can't handle the truth.

 

[jghrist]

It may have been a fortunate operation of the 51N, but it was not proper. An open fuse should not result in any zero-sequence current if all of the load is connected to Delta-Wye transformers.

 

[Kiribanda]

Jerry123,
At the first place,as jghrist pointed out,how did 51N function operate without a path for zero seq currents,when there are unbalnace voltages. Is there any motor connected in WYE grounded or is there any transformer primary connected in WYE grounded?

 

[Jair5400]

Hi list:
In my opinion, Best option for back up protection schema is deploy 67, 67N relays; don´t care be a multy feeded smart grid?, agree you with me?
I´d be very interested to read your opinions.
All the best to everyone.

Jair5400.