Overvoltages due to system grounding 2

[Power0020]

I got a bit confused with some manufacturers application notes stating that no overvoltages occur in systems with neutral grounding resistors...I understand that from phasor diagrams the neutral point will shift during line-to-ground faults, for both low resistance and high resistance grounding....correct me if I am wrong!

It seems that they meant by overvoltages the dangerous transient oscillations due to arcing ground faults, where system inductance and capacitance inter-exchange energy (this happens in IT systems). However, the NGR systems will have a line-to-line voltages appearing on healthy phases to ground.

Any clue?

 

[Parchie]

Neutral point potential will be at ground potential if "adequately grounded". If one line gets grounded, its line to ground voltage will go down to zero, other phases won't. That's how it is.

 

[davidbeach]

If there is any resistance between the neutral and ground there will be a neutral shift and there will be increased voltage to ground on the unfaulted phases. Ohm's Law leaves no other option. The neutral voltage to ground will be determined by the division of voltage between the conductor to the fault point, the fault resistance, the ground return path, and the neutral resistor. It doesn't take much neutral resistance to be the dominant component in that voltage divider.

As for the transient oscillations due to arcing ground faults; any value of neutral grounding resistance that allows more fault current than the charging current will stop that escalating voltage buildup.

 

[Power0020]

Thanks davidbeach, this is what exactly I know, I wonder why some application notes say that no overvoltage occurs, they must mean transient escalating voltage.

Is there any clue about the selection of resistor value? I have read somewhere that the resistance value should checked against the power loss during faults? I couldn't get that as the power loss during faults will be Vph^2/R, or I^2.R, i.e. for a 33 kV transformer and about 500 A fault current, the power loss during faults will be 9.5 MW, this can be multiples of transformer rating but the fault acts for very short time....

any clue?

 

[RRaghunath]

While sizing the neutral earthing resistor, we need to ensure that the resistive current exceeds the total capacitive current in the system.
This is explained through an example in IEEE C62.92.3.