Ground Relay Coordination

[kibz]

Hi Guys,
I have encountered a relaying problem. Can you help me on this.
I am doing a coordination study of an industrial plant with a self generation from a single unit 3.5 Mw, 13.8 kv generator which is grounded thru a neutral grounding resistor of 100 Amp. From the generator to the load there are 3 mv VCBs with SEl Relays. At the load end is a 3 MVA Delta-wye grounded 13.8/0.48 kv transformer. The ground fault current at the 13.8 kv load end is 94 Amps. The 51G relay at the generator neutral is set at 10% or 10 Amps, with trip time of 1 second. My question is can I set the 51N unit of the 3 SEL relays in series at 10% same as generator 51G and staggered the time say 0.3, 0.6, 0.8 sec to coordinate with the Generator 51G? I am worried that the 51N pick up setting of 10 Amp & 0.3 sec at the 3MVA HV side might operate due to unbalance loading & during inrush. The plant owner do not want the setting of 51G Generator Relay changed, because the generator is still on warranty.
Pls advise me on what to do. Your advise is greatly appreciated...Thanks Kibz

 

[collies99]

It is likely the 3 MV VCBs are not in series and radially fed from a bus with the gen breaker acting as main and each breaker acts as a feeder bresker. Therefore each feeder ground protection should be coordinated with the gen ground protection.

In a nutshell under these assumptions, you cannot Series the SEL ground elements.

 

[kibz]

Collies 99,
Thanks for your time.
Yap you are correct its radial with a generator breaker with ground relay connected to the bus, then from the bus a main feeder breaker with SEL 351A, connected is a 100 meters power cable, at the end of the power cable is a another feeder breaker with SEL 451, from SEL 451 is a 200 meters overhead line terminating in a another breaker with SEL 551 serving a 3 mva power transformer.
From the generator bus there is only 1 feeder with 3 breakers up to the transformer load. In between these 3 breakers
are cables & overhead line.
The objective is to coordinate the ground relays, the SEL 551 ground relay be coordinated to SEL 451, SEL 451 coordinated to SEL 351, SEl 351 be coordinated to Generator Ground Relay. The only source of ground fault is generator 100 Amps. The installation is quite redundant.
Can I coordinate the SEL ground relays using a setting of 10 Amps & definite time on all the SEL relays? Say 0.3 sec for SEl 551, 0.6sec for SEL 451, 0.8sec for SEL 351 to coordinate with the generator ground relay set at 10 Amps, 1.0 sec? Will they work?
I am apprehensive the SEL relays might operate during unbalance loading & inrush current of the 3MVA.
I find using inverse time hard to coordinate with almost same level of ground fault current of 100A at source & 94 A at load end.
Please give me your advise.
Thanks..Kbz

 

[collies99]

The staggered delay time is doable, time difference is tight by utility standard.

Since ground setting is set above maximum unbalance, this should not cause tripping due to unbalance conditions. Load inrush decays quickly, cold load is much slower to decay ... Unless you are using ground instantaneous, time setting is still set above max unbalance for these conditions.

Ground coordination is best using same curve type, so changing the Time Dial to get your coordination time between curve should be easy if pick-up setting is identical.

I do not believe transformer inrush typically affect ground elements, just phase elements. You can test and initiate event record on the 551 using 52a/b next time you have a chance, but it will be different every event due to remnant flux. You may get an idea what ground / neutral trace you get and see the decay to corroborate your .3 time. My guess is that it will be okay.

Just my thoughts ... Keep us posted.

 

[jghrist]

Are the ground relays fed by ground (core balance) CTs or by the residual of phase CTs? If ground CTs are used, then they will see ground fault current only, not unbalance. There will be no ground current in ungrounded transformer winding inrush current. Phase CTs could have different errors in each phase causing an apparent unbalance. This is usually associated with one or more phases saturating under a heavy multi-phase fault.

 

[kibz]

Thank you guys,
The ground relays on the feeders are fed by the residual of phase CTs. The generator ground relay is fed from the CT at the Neutral Ground Resistor. The apparent unbalance caused by different errors of phase CTs during transformer energization is what I am worried at,
that might cause false trip due to 10% setting of the ground fault current.