Grounding & Protection Scheme for Parallel Generators

[andytang]

Dear Folks,
We have three 3-MW, 4160 V, 3 phase geneators. I would like to know the proper grounding and protection scheme for parallelling those generators. We are experiencing high circulating current (appr. 300A) when two generators are running in parallel and the load is very small (appr. 50A). Does it sound like a grounding problem or something else?
Andy

 

[dpc]

The problem you are describing sounds like reactive power circulating because the two voltage regulators are fighting each other trying to maintain voltage setpoint. When the generators are operated in parallel, the voltage regulators need to be designed and configured to share the reactive power load. This is typically done in a reactive power droop mode using paralleling CTs on each generator. Check the Basler website

http://www.basler.com

for more information on parallel operation.

As far as grounding goes, some type of resistance grounding should be used. The exact configuration depends on your system. The brute force approach is to put a grounding resistor on each generator. This increases the ground fault current when multiple generators are operating and increases potential problems with harmonic currents. An alternate approach is to install switching such that only one resistor is in use at one time.

If you have a grounded transformer on the bus that will always be energized, you can put the grounding resistor there, and leave the generators ungrounded.

Hope that helps a little.

 

[jbartos]

Suggestion: I just would like to add a bit to the previous posting. One generator is preferred to have system grounding. It could be high-resistance or medium resistance system grounding. The other generator neutrals are not connected to the neutral bus but they may have a circuit breaker plug in arrangement on stand-by just in case needed when the one generator with the system grounding is being out of service.

 

[jbartos]

Small encore: Visit

http://www.azpsoasis.com/oasis/unsecure/docs/PSEI/PSE Intercon.pdf

for Governor Requirements

http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/55-511/5113ch.pdf

etc. for more info

 

[busbar]

Possibly IEEE C62.92.2-1989 Grounding of Synchronous Generator Systems ISBN 0-7381-0933-9

And, IEEE C37.101-1993 – Guide for Generator Ground Protection ISBN 0-7381-0901-0