Generator earth

[lyledunn]

What is the reasoning behind the 10ohm maximum impedance specification (low voltage generators in UK )for a gen set connection of its neutral point to earth? I trust I understand the reason for the connection but I would be interested to know just why the figure of 10 ohms was insisted upon. Regards,

Lyledunn

 

[RajT]

On low voltage system (ie 380-660V)it is necessary to ensure that in the event of a through earth fault, there is sufficient fault current for the gen standby e/f (51G) to operate. The low NER value as stated will ensure that this condition is met as having a higher imped of say 100ohms will limit the value of the fault current to such a low level that it would not be possible to discriminate between the 51G on the gen CB and the largest outgoing feeder on the switcboard.
As per IEC the fault current seen by the gen 51G relay must be atleast 3 times the max e/f current seen by the largest outgoing feeder setting or the fuse size. This is to ensure that the gen relay has adequate margin for it to operate.
You do not say how big the gen is but I suspect that the 10ohms will limit the e/f current to atleast the gen FLC if not a bit higher.

 

[lyledunn]

Thank you Rajt, however I dont think that this is the reason. Afterall, the earth conductor could have an impedance much the same as the phase. Consider a simple gen set offering an alternative supply to a building, no such discrimination requirements are required. Regards,

Lyledunn

 

[RajT]

I would agree to disagree.

 

[sngpl]

Well if there is no neutral transformer and only a resistor, then theoratically the resistance of this grounding resistor cannot be very high because the ground currents are normally very low and are even to be detected by the most sensitive ground fault relays.

This low resistance is sort of a compromise between no resistance at all and high resistances.

 

[lyledunn]

Rajt,
I see where you are coming from, my apologies, I didnt explain fully. Take a typical say 20Kva 400/230v gen set. It has 3xphase, 1xneutral and a separate dedicate earth conductor, (no earth fault relay). The current generated due to any fault to earth will be carried by that dedicated earth conductor. In this case the required connection of the neutral point of the set to earth then has little to do with limiting fault current for the purposes of discrimination as you desribe (I take it that you were assuming no separate dedicated earth conductor when you made your response).The purpose of the connection of the star point to earth is to establish a reference and ensure that the neutral conductor remains at or about earth potential. I was just wondering why 10 ohms was set as an upper limit. Regards,

Lyledunn

 

[RajT]

I had assumed in the early reply that you had a NER in the N-Earth connection but this is not so and that the gen set is directly connected to earth.
The requirement for an upper limit of 10ohms is to ensure that the gen can diliver a maximum amount of earth fault current in the event of a e/f.

 

[jbartos]

Suggestion: If there happen to be a current transformer or shunt resistor in the neutral, the 10 Ohm ground resistance would produce, for example, Vline-line/10=380/10=38 Amps max. and with Vline-ground/(sqrt3 x 10)=380/(sqrt3 x 10)=22 Amps max. The 22Amp would give, over 0.1 Ohm shunt, 2.2 Watts available for a relay to operate. The 2.2 Watt relay coil is considered on a smaller side.