Close NDS at Generator side or Transformer side? 1

[ciy]

There is a power system, one Generator connected to 13.8kV Bus and one Step-up transformer 69/13.8kV primary side connected to the power grid, secondary connect to 13.8kV Bus.
Both generator and transformer have equipped with NGR for forming a low resistance grounding system.
Choose one of two NDS that be closed for grounding.
In general, the NDS of Generator side shall be closed rather than transformer side.

My question is

Is there anything have to take into account if close NDS of transformer side rather than Generator side.

THANKS!

 

[dpc]

I really try to avoid the need to switch the grounding resistor. But with the NGR at the transformer, your generator is probably ungrounded until the generator breaker is closed - depending on the location of the generator breaker.

 

[ScottyUK]

What is the transformer winding configuration? Normally an HV generator has a high resistance grounding system, possibly using a grounding transformer, to limit damage to the stator core in the event of an earth fault. Ideally the generator would feed a delta winding.

 

[ciy]

ScottyUK,The said power system scheme roughly is shown on the attached for reference.

dpc, thanks for your advices!!
From generator protective view, keep NDS closing with generator is good solution.

 

[odlanor]

you must install both NGR.
NGR transformer is necessary to keep BUS 13.8kV 2500A 40kA system grounded, when generator is desconnected.
NGR generator is necessary to keep generator grounded, when generator is desconnected but excited with speed no load.

 

[Kiribanda]

ciy,
Just for curiosity I am asking this question.
It looks like your Utility had allowed to use a DELTA for the GST on 69 kV side.When there is a ground fault on your 69 kV network the 69 kV line
will trip and isolate from its own ground fault fault protection at the upstream sub.But the generator will continue to feed to the
69 kV ground fault because your GST is having a DELTA connection at 69 kV.How do you trip the generator under that faulty condition? Do you have
an intertrip by a communication line?

 

[ciy]

Kiribanda,
In our practice, GST 69kV side is Delta connection and the Utility side is Wye connection.
If transmission line has a SLG fault then the utility trip the circuit at substation.
The incoming CB shall be tripped by undervoltage Relay(27)to separate from the Grid.
At this time, my power system to be a island system to power continuously.
To prevent the Grid to be a ungrounding system, 59Vo Relay is necesssary to reserve.