Generator Neutral Grounding 1

[cod]

An odd arrangement of five generators; 2-3500 kW gas turbines, 2-800 kVA gas engines and 1-800 kVA diesel unit, all rated at 11 kV and different rotational speeds. What is the best way to handle the neutral grounding of these different units?
1) Use a medium voltage switch or circuit breaker from the star point of each, then through the resistor and then to ground, or
2) Tie all star points together onto a common bus then to the resistor then to ground, with or without individual switches in each neutral lead, or
3) Connect the generator neutral to a fourth pole on the normal generator circuit breaker in the switchgear lineup, then to a neutral bus, then resistor, then ground, assuming one can find a MV circuit breaker manufactured with a fourth pole. This looks like an economical choice if the CB is available.
Any comments on safety or other ramifications of the different connections would be appreciated. Thanks to all.

 

[dpc]

I think we need a little more information on your installation and your desired degree of reliability versus your desired level of protection. There are always trade-offs.

Are all of these generators to be connected to the same 11 kV bus? Are there other loads on the bus?

 

[cod]

dpc - thanks for your reply;

The power plant has a very large turndown in load, varying from about 3300 kW down to 200 kW requiring unusual operating routines.

The generators are all tied to a bus that is split with the loads more or less evenly divided between the two sections. The bus tie breaker will be either open or closed depending upon the particular load - see next.

At the highest load, one GT will be on the bus with the bus tie closed. This will be used until the loads fall to about 2400 kW when the gas engine generators (and possibly the diesel) will be synchronized and brought online and the GT retired. The second GT is always used as a standby, only synchronized with he first when transferring load.

The bus tie may be opened if the loads are more or less equal to the generator capacities, otherwise, left closed.

At some lesser load, the diesel will be retired (if on) and the two gas engines continuing and the bus tie normally open. It is intended that the diesel be used rarely to keep downtime for maintenance as low as possible. It is used primarily for starting the GTs and in emergencies.

At the lowest load, one gas engine is used alone with the bus tie closed. The whole process is reversed when loads increase although more synchronizing is obviously required. Incidently, the loads vary seasonally so these routines are not repeated very often.

One more quirk; it may be possible, under the smallest loads, to synchronize the plant with an incoming local utility line and then drop all generators with the bus tie closed. The line is reportedly unreliable and would be used sparingly.

Yes, about 75% of the loads are critical and are served with dual MV feeders connected to duplicate transformers with a LV secondary selective arranement. Some large motor loads are similiary served at MV but are not thought to come online when the smaller generators are being used.

The job is now being bid so can't say too much more but if there are other questions, please ask.

 

[dpc]

Well, I'm sure you will get a lot of different opinions on this, but if I were designing this system, I would ground each generator through individual grounding resistors, limiting the fault current to about 100 to 200 A through any one resistor. If your 11kv transformer winding is grounded, it needs to be grounded through as resistor as well.

This will allow you to bring generators on and off line without worrying about grounding and will also allow you to selectively coordinate to avoid tripping all generators in event of 11 kV ground fault.

The drawbacks to this approach are possibility of circulating harmonic currents and the increased fault current when multiple generators are operating. But all the other approaches have even more problems. You will need to make sure the grounding resistors have a continuous rating adequate to deal with expected circulating harmonic currents.

Just my $0.02 - hope it helps.