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MV Power Station - Grounding

MV Power Station - Grounding

MV Power Station - Grounding

(OP)
Hello,

Frist of all my best complements for this forum.

Attached is the single diagram of our power station. The main issue is the grounding system. Inside our team there as been a great discussion about the grounding system. As you can see, in all 6 generators, the neutral isn't grounded. It has a differencial protection and a NEC + Ra, and according to some of my colleagues it's enough. However, i disagree hid this solution and all 6 generators should have the neutral grounded through a resistance. My queston is if the earth fault occur between the generator and the common bus how can we prevent for a major damage for the alternator?

This power station is for a factory, will run always with generators.

RE: MV Power Station - Grounding

What is an NECR? A grounding transformer? If the generators are ungrounded, they can't source zero sequence current. If you have a ground fault between a generator and the bus, where is the ground fault current going to come from? It can't come from the generators if there is no ground return path.



RE: MV Power Station - Grounding

DCarvalho,
In my opinion since there is a Zig-Zag grounding transformer with current limitation (not given) your 11 kV network is "artificially" grounded.
Therefore,for any ground fault on the 11 kV bus you can get an alarm or tripping condition.But where are the CTs for ground fault & overcurrent
protection?
Further,the generator is protected by a 87T scheme for interwinding and ground faults.Per your suggestion, if we ground the gen star point
(with or without a NGR) then you will have two parallel ground paths for the zero seq currents during a 11 kV ground fault.As a result you may
need a very well coordinated ground fault protection scheme.
What is the vector symbol of TX1 trafo?I believe it is Dyn (11kV side is D).

RE: MV Power Station - Grounding

(OP)
dpc:
NEC - Neutral Earth Compensator
______________________________

Kiribanda:

NEC
3Io=360 A, 10 seg
3 phases 11 KV, connections ZN, 50 Hz
Zo=57,0 Ω

TX1
500 KVA, 11/0,4 KV, 50 Hz, Dyn11
Ucc=3,8%

"But where are the CTs for ground fault & overcurrent
protection?"
In every CB, we will have CT's. And the relays are, GE MIF II, only for overcurrent and short circuit. CT's for ground faults don't exist...yet. The Ra isn't installed and the cneutral onnection, between the NEC and Ra, will have a CT for ground faults.

"Per your suggestion, if we ground the gen star point (with or without a NGR) then you will have two parallel ground paths for the zero seq currents during a 11 kV ground fault.As a result you may need a very well coordinated ground fault protection scheme."
If the NEC and the generators will have the same earth point, if we have a ground fault how we will have "two parallel ground paths"?

RE: MV Power Station - Grounding

Apply Kirchoff's current law. Any piece of plant which can source ground fault current must have a matching current return to it.

RE: MV Power Station - Grounding

In my opinion, if the rated zigzag transformer [and NGR] corresponds to the total capacitive current of the System, then a common artificial neutral grounded could be employed.

RE: MV Power Station - Grounding

It's very easy to coordinate 50GN relays.
360/6 = 60 A sets 90A for each 50GN.
With at least 3 generators in operation , a ground fault inside 1 generator , its 50NG will operate with 2*60A > 90 ,
while 2 others 50GN will sending 60A < 90A will not operate.

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