Neutral Grounding Reactors

[genhead]

Resistance/reactance grounding of low voltage generators is new to me.
A number of 400v generators are supplying a stand-alone (island) 400v 4-wire system, a lot of which is single-phase load. Normally the neutral bus would be solidly grounded at one point, but the plant owner has asked for neutral-grounding reactors on each generator. All generators are identical, and circulating current is not a problem, neither is the ground-fault-level.
My question is, could single-phase loads be connected to the system if reactors were added, and if not why not?

 

[ScottyUK]

If they are single phase loads connected line-line then I don't see a problem other than the generator won't like the imbalance, but that would apply regardless of neutral grounding method. I am assuming that '4-wire' is 3P+E with no neutral conductor.

It is down to local codes whether an impedance-earthed LV system is acceptable - some allow it, some don't. where are you - that would assist in giving an answer.


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If we learn from our mistakes I'm getting a great education!

 

[rbulsara]

Scotty:

But if you bypass the grounding reactor then you just negated the a major purpose of it. The L-N fault current will not be limited and only true L-G faults will be limited. Probably one of the reasons, I believe single phase loads are not recommended (even prohibited, I need to check on it) to a impedance grounded system in ANSI world.

 

[rbulsara]

NEC 250.36 does not permit single phase loads on impedance grounded systems of 480 to 1000V

 

[rbulsara]

and NEC 250.186 says the same thing for systems of 1kV and above

 

[davidbeach]

The reason that single phase (L-N) loads are prohibited on impedance grounded systems is that during a ground fault the voltage to ground on the unfaulted phases moves toward L-L voltage and will damage the L-N loads on those phases. Single phase L-L is perfectly fine on impedance grounded systems.

 

[rbulsara]

Interestingly, NEC leaves out systems between 260 and 479V!!

Thanks davidbeach.

 

[davidbeach]

250.36 gives permission for high impedance grounded systems, sets a minimum nominal voltage for those systems, and goes on to describe how such a system is to be constructed and used. No where is the possibility of a low impedance grounded system less that 1000V allowed.

250.20(B) requires a solid ground for all systems 50V-1000V that use the "neutral" as a circuit conductor. 250.20(B) would also require a solid ground for any 208V wye system regardless of whether the neutral is used or not.

Once above 1000V you can use low or high impedance grounding, but still can not serve L-N loads due to voltage stability issues on the unfaulted phases during a line to ground fault.