NER for GTG

Let me just run this through the decoder, and IGBTY....

Not without fewer TLAs.

Some ratios don't sound right.

If the GTG is Y then NER is OK.

I think

My concern on NER rating of Generator

If it isn't then the NER is NBG. WTF happens next is ABG.

IHNFCWAIS

Have you checked your local standards and GTG manual/salesperson to find out if a 10A neutral earth resistor is suitable?

These things (like acronyms) can vary from country to country.

There is no standard in my country, from my previous project, we used 400A NGR at Transformer and GTG, but how to provide discrimination between GTG and Step-up Transfromer if NGR are rated same.

There is something wrong in describing the system.

If I am correct it shall be Y connected GTG connected to Delta Y step up transformer.

If this is the case, then

Full load current of GTG is 1068A.

Rule of thumb is rating of NGR minimum 1/3 of full load current.

Lower than above, Will result in reduced fault current near to neutral and thus sensing of fault.

Higher earth fault will result in more damage to core on earth fault (calculate the fault energy assuming 10% at fault point and balance 90% across NGR and 400A, 80 to 100m sec fault clearing time and you can estimate the energy in fault and subsequent damage in core at location of fault).

10A earth fault is too low even at full voltage, forget fault occurring at partial winding.

Better go for 400A or at least 350A Resistance.

I will still prefer 400A as this will increase fault time withstand capability of resistance, example when comparing 400A 10Sec NGR with 350A 10Sec for the same actual fault current.

Jack Daniel.

There are two systems here - generator voltage system and the 33kV distribution system.

The 400A NER is on 33kV side / star side of Delta/Star Generator transformer.

10A is in generator neutral-to-earth circuit. The 10A NER in this application is preferred considering that this eliminates possibility of any damage to the stator core laminations in the event of generator stator fault. However, if the generator is connected to the transformer through cables, the 10A NER rating may need to be increased (say 15 or 20A but still below 25A) to meet the criteria - resistive current shall be at least 150% of capacitive currents in the system at the time of earth fault.

Generator system is isolated from the 33kV system by the delta in the generator transformer and hence there is no issue of protection coordination for earth faults.

Trust the above is helpful.

Let say the total capcitive current is 50A, could you please advise how to size the NER?, I'm clear about rule of thumb how to estimate capacitive charging current

50A Capacitive current makes the system unsuitable for high resistance earthing (earth fault current not exceeding 25A).

However, the NER shall be 1.5x50 = 75A rated as a minimum (from the point of limiting over voltages). Such a large current can mean the stator core burning risk cannot be eliminated in case of generator stator earth fault.

This is only a hypothetical case I hope.

Could you please elaborate for further understanding

'Resistive current shall be at least 150% of capacitive currents in the system at the time of earth fault.'

1)Do i need to terminate NER Frame earth? if yes, where is the location of termination point? is it at main substation earthing bar or grounding terminal of NER.

2) Neutral Grounding Generator cable is sizing based on full fault current rating without NER consideration or with NER consideration?