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Parallel operation of diesel generators

Hsien Fong Lee

Hi everyone, 

I am faced with a technical problem related to the synchronization of diesel generators. 
My customer had 2 gensets that are operating in parallel to provide emergency power source to the financial institute installations. 

Recently, a third unit of genset was added and the design was to have all these 3 units to be supplying power to a common genset supply switchboard which the gensets shall be synchronized. 

G1 and G1 (genset 1 & 2) are the 2 existing units, and they could sync up well. 
With the new G3 unit added, site test was done to test the synchronization. G3 could close it supply breaker to the common bus with either of the older units (G1 or G2). When a dummy load test was initiated, the G3 will get its earth fault relay to be activated. No tripping will occur when G3 and G1 are sync-up and running at no load condition. This is similar when G3 and G2 are sync-up and running at no-load condition. 

Earth fault relay activates when the dummy load (3 wire system resistive load bank) was activated. Despite increasing the setting of the Earth fault protection for G3 to 20% of the rated output, the EF relay still had picked up. 

What shall be checked for such case?

a.   Common Neutral-earth grounding? (only single point connection of the Neutral common bus to the earth?)
b.   Does the G3 genset configuration (controls / parameter settings) plays a part for such nuisance tripping?

  • 0

    My first guess would be that the new generator has an unwanted N-PE link - but others here may have better ideas...

       - Andy.

  • 0 in reply to AJJewsbury

    How large are the gensets - are they floor mounted and fixed installation ? 
    If large (hundreds of KVA) the earth fault tripping would normally be the kind with a separate current transformer and breaker, and would sample the wiring after the point of junction - i.e. looking for load-side earth faults while NOT measuring  out of balance currents circulating between the individual gensets. (to some degree one generator will always pick up more of the load than the other, and when very lightly loaded the lazy one may actually motor, rather than generate, and be pushed round by the harder working one. This is more noticeable when the generators are not identical models. However if this amounts to a current imbalance of more than a small percentage of the full load, then there is a problem with the controller or its settings.)
    Where are the N-E links in the system and where exactly is the earth fault relay measuring to make its decision  ? (it should be on the 'S' side of your TNS)
    Do the other gensets have earth fault protection relays, and where are they doing their sensing to make a  decision to trip ?

    Can you upload a sketch showing the windings and their linking.
    Mike.

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