Switching transformer neutral to earth link ?

[cesgibis]

Hi,
I have a 33kV distribution system with three bus bars and two bus couplers (oeprating normally closed). Each bus bar is supplied from a transformer and in normal operation the three transformers operate in paralllel.
The client specifies the NER to limit earth fault current to 400A.
Th earth fault curent limitation can have then 3 values (400, 800 and 1200A) depending on how many transformers are paralleled.
Is there any advantage in implementing an NER switching logic in order to have one NER in service at any given time with a fixed 400A limitation ?
Otherwise is there an issue with protecting the system with these multiple earthing points ?
(the syetm may be less sensitive since the earth fault current may spilt between the three NERs?)
Thanks

 

[davidbeach]

Switches fail, ungrounded systems are far worse than impedance grounded systems, therefore no switching. Shouldn't be that difficult to work with the differing fault currents, just let the relays know how many transformers and torque control different elements based on known maximum fault current.

 

[waross]

Can you consider connecting all three transformers to a transformer neutral bus and then grounding the neutral bus through a single NGR?

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

 

[davidbeach]

That could work too, good one Bill.

 

[slavag]

Hi.
It's a big problem!!!
Last week we config and check two such systems with NER switching, in one case it was must, second case like to your case, but with 2 transformers.
It's work now, but I don't like this solution.
Are it will be auto switch or manual?

Second, I think possible work with 3 NER in parallel, but design biggest size of NER and add REF ( 87N) functionality.
In additional set protection for minimum ground fault level. With newer relays possible coordinate with current 100A or less.

Bill, IMHO, your option is good one, but have some problem with maintanance and one fail point ( only one NER in the system)

Best Regards.
Slava

 

[cesgibis]

If switch option is selected it will be automatic through a control system.
But I start thinking like Bill and David that this is not the good option.
The problem is that this switchgear will supply an industrial plant, but will also supply an overhead line.
There is a compromise to find between:
- A too low NER value with problems of capacitive currents for insulated lines and imepedant faults on the overhead line
- too high a fault current, which is detrimnetal to thermal withstand of equipment.
I think I will be going with 600A for one NER with one transformer on line (and a maximum of 3 x 600A when the three transformers are paralleled)
Thanks