Neutral Earthing (Grounding) Resistor for Parallel Generators

[jimmy2times]

We have a ship with 6.6kV system and dissimilar rated generators, one 30MVA GT and two 10MVA DGs. Each earthed via 3A resistor, not switched, only manual disconnector for maintenance purpose. I have info the GT alternator is 5/6 pitch, but still waiting on info of DG alternator winding design.

From what I understand there would be possibility of circulating current no matter the pitch design of the DG alternators, basically because the 3rd harmonic voltages in each machine will not be identical (load sharing propotional to machine rating, or asymetrical load sharing can be achieved in operation). This then allows driving force of 3rd harmonic current. Is this correct?

I have read few varying papers on subject with differing views, many on here seem to suggest this is no issue and probably best design rather than complexity of switching resistors. I've scanned over C62.92 series and that seems to advocate grounding transformer at the switchboard or neutral switching. So read some conflicting advice on this.

We don't necessary have an issue at the moment, just I was asked what purpose of disconnect was. That question arose for GT alternator supplier and subsequent correspondence with them is that they are of view that neutral resistors should be controlled to switch only one in (they are not the overall system designer, so that's just their view). There is some corrosion issue with a skid of their system and they were suggesting that possibility could be linked to 3rd harmonics.

Interested in your views as always or pointers to further reading on the subject.

 

[Parchie]

To do away with circulating currents once generators are paralleled, the best option would be to match all generators pitches. If not, there are "dissimilar pitch neutral limiter devices on the market. The idea is to minimize the circulating neutral currents by insertion of some impedance in the common neutral.

Good luck.

 

[RRaghunath]

3A NER means a lot of resistance in the neutral. There is no concern of circulating currents with such a large resistance in the neutral circuit.
I read in a text book some time back that if the NER is 100A or lower, there is no need to worry about circulating currents and thus the NERs of all machines can remain connected all the time.

 

[dpc]

Probably not a problem. But the third harmonic voltage difference could potentially create some circulating current. The grounding resistors may have only a 10 second rating at their design current. So it's a bit difficult to know what the continuous rating might be for 3rd harmonic current. I'd probably try to measure the current while operating. If the resistors have a continuous rating, then just compare any circulatng current with this.

You can use only one of the grounding resistors (there must be a grounding transformers associated with the resistors?) as long you as remember to do correct switching.

 

[jimmy2times]

Each generator has 1270ohm resistor connected to neutral point of winding. Disconnect switch in the circuit to allow isolation of machine for maintenance testing. There is no grounding transformer used neither is there automated switching such that only one resistor in circuit at a time. I.e if all three machines in parallel on network, then all three NERs are in circuit also.