NER(Neutral Earth Resistor) Sizing

[maqhawe]

Good day,

I need help, with sizing the NER to limit the fault current, there are a couple of things I am not sure of.

1. selecting the current that will flow through the NER, I understand is the most critical stage, since after that you will be able to use ohms law and calculate the resistor size that will limit the current in 10 sec.
2. Must the current be limited to 10-20% of full load current of the genset?
3. I understand the current must be more or equals to the charging current of the genset, how do we calculate the charging current?

Example:

if I have a 10MVA, 11KV, 0.8p.f standby power station, the NER Size, according to my understanding:

the full load current will be 524.86A,
Then 10% of that will be about 52A,
so the NER size will be a 60A,
R=V/I
=11000/(1.732X60)
=105.85ohms

That's how I understand it, I need to confirm if I am correct or not, and please clarify what could go wrong if I have a small or much bigger size of NER with the whole protection network?

 

[waross]

Charging current: This is the losses to ground of the system supplied by the gen-set or transformer under consideration.
All energized conductors in a system will have resistance to ground and capacitance to ground, however slight. In a large system the resulting current may be several Amps. If your system is in use, you may be able to measure the charging current directly. With due regard for safety use a clamp type Ammeter to measure the current in the system grounding conductor. Some codes require one and only one system grounding connection. If that is true for your installation then the current through the conductor which makes the one connection to ground is the one to measure.
One of the reasons for impedance grounding is to reduce arcing damage in the event of an arcing ground fault. The less current the better.
5 Amps is a good starting value if the charging current is less than 5 Amps.
Consider grounding through the primary of a small distribution transformer and putting the resistor in the secondary circuit.
Such a resistor will be much more robust and less prone to damage due to breakage or corrosion. The lower voltage will be safer.
If the NGR current is less than the charging current you will have difficulty discriminating ground fault current from charging current.
The higher the NGR current the more arcing damage may occur before a fault is cleared.
Arcing ground faults and high voltages;
When ungrounded delta systems were popular, some plants experienced a high frequency, high voltage superimposed on the plant wiring. This would lead to random insulation failure and burn out of motors through-out the plant.
The cause was found to be an arcing fault in a motor or transformer winding. The combination of system capacitance, winding induction and the arc would form a spark gap radio transmitter that would generate high frequencies. The faulted winding could act as an auto transformer and raise the high frequency voltage to several thousand Volts.
Grounding protects against this effect. Too little NGR current may not provide full protection.

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

 

[Parchie]

@maghawe,
It's not about arbitrarily choosing an earth fault current. You need to determine which level of fault current your available equipment withstand. Since most equipment are standardized, choose the fault level that will be handled by readily available standard equipment. What @waross said, 5A is a good start. Get yourself a copy of IEEE Std 142 and be guided.

 

[maqhawe]

Thanks Waross and Parchie, much appreciated, I will get the copy of IEEE Std 142.