How do you determine required transformer LV earth fault current (NER and Earthing Tx sizing)?

[electrical429]

On 33kV and 11kV designs I have worked on standard figure for LV earth fault current was set at 1000A. Neutral Earthing Resistors or High Z Earthing transformers were sized to achieve this level of fault current. However, is there a reason for choosing 1000A? Why not more, or less? I understand that sometimes you need to account for synchronous generator winding fault rating, but when there is no such constraint is there anything else that influences transformer LV side fault level selection?

 

[prdpks2000]

High Resistance Grounding & Low Resistance Grounding.
High Resistance Grounding:
It is generally used in mines, Petrochemical industry, generators with unit transformers to limit the ground
fault current to a value less than 25 Amps.
Low Resistance Grounding:
It is generally adopted to limit the ground fault current from 100 to 1000 Amps and used in the industrial
MV distribution system.
Selection of Limiting Current:
The following factors are to be considered for the selection of the Limiting current for low resistance grounding.
a) The minimum ground fault current shall be more than 10% of the primary current of the biggest CT in the bus. This is to ensure proper relay coordination for the earth fault protection. The earth fault protection setting of any feeder is generally set between 10% and 20% considering the error in the protection class CTs used (5P class). Hence the ground fault current shall be large enough to drive the protection relay to operate for a setting between 10% and 20%. for example full load current of the Transformer secondary is 1925 Amps, the biggest CT in the bus is 2200Amps. Hence, the min. limiting current will be 220A.
b) The maximum ground fault current shall be less than the earth fault current withstand capability of the smallest rotating machine connected to the bus. Normally smaller motors (< 300 kW) in the bus might get damaged during earth fault before the fault is cleared by relays. Hence, the maximum ground fault current shall be limited to a lower value.
With the above considerations the limiting the current of NGR is taken as 300 Amps for 10 sec, and not always 1000A as suggested by You.

Pradeep

 

[electrical429]

Thanks!