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nebiyou

neutral connection in generator synchronizing

Hello

I am planning to connect the neutral of the entire three synchronized cat 608 kva similar generator set  on the common ground bus bar (the ground will also be connected to the bus bar) without any neutral-  grounding contactor (this is because I am using 3 pole contactors for the main synchronization). I have tried to gather information about this idea on the internet and I have seen different ideas posted which contradicts. Will this create problem?

Please advise me from practical point of view.

If it is possible advice me, any ways which could avoids the possible risks without using the neutral contactor

Thanks in advance 
New member
boopathy86

Re: neutral connection in generator synchronizing

(81) why a neutral grounding contactor is needed in diseal generator? ■There would not be any current flow in neutral if DG is loaded equally in 3 phases , if there any fault(earth fault or over load) in any one of the phase ,then there will be un balanced load in DG . at that time heavy current flow through the neutral ,it is sensed by CT and trips the DG. so neutral in grounded to give low resistance path to fault current. ■An electrical system consisting of more than two low voltage Diesel Generator sets intended for parallel operation shall meet the following conditions: ■(i) neutral of only one generator needs to be earthed to avoid the flow of zero sequence current. ■(ii) during independent operation, neutrals of both generators are required in low voltage switchboard to obtain three phase, 4 wire system including phase to neutral voltage. ■(iii) required to achieve restricted earth fault protection (REF) for both the generators whilst in operation. Solution: ■Considering the requirement of earthing neutral of only one generator, a contactor of suitable rating shall be provided in neutral to earth circuit of each generator. This contactor can be termed as “neutral contactor”. ■Neutral contactors shall be interlocked in such a way that only one contactor shall remain closed during parallel operation of generators. During independent operation of any generator its neutral contactor shall be closed. ■Operation of neutral contactors shall be preferably made automatic using breaker auxiliary contacts.

New member
esantos

Re: neutral connection in generator synchronizing

Hi nebiyou.

The reason for the use of ground contactor is to avoid circulating high currents among gensets. The high current will depend on lot of things as the kind of load, installation, exciter system, etc. In our projects, we rarely use the ground relay. We have projects with six gensets running for about 3 years without that. I recommend to swap the 3 pole contactor for a breaker with LSIG protection. So the system will be improved in case of a short circuit.

Regards
E. Santos
Cat Dealer
72

Re: neutral connection in generator synchronizing

there is problem when you use 3pole contactor in synchronizing system,especially when harmonic/unbalanceload occur there will be no protection.

 

solution :use 4pole contanctor or neutral isollator contactor,

 

 

regards

Contributor
see-deif

Re: neutral connection in generator synchronizing

There are many "rules of thumb" you can listen to, but the only way to know for sure is to do a power system study. Anytime changes are done to a power system, a power system study should be done to find 'hidden' potential problems, to possibly change protective relay settings, etc. I don't do PSS, the company I work for does not do them, so I have no vested interest. Just trying to help.

 

That being said, you did not say if you were going to connect more than 1 neutral - to - ground bonding connection? If you have more than 1 ground connection, you DEFINATELY will have problems. You also did not say how close these gensets would be to each other/how much impedance there might be between them. Last is the type of load: balanced 3 phase (like transformers and motors) or unbalanced single phase (like lighting or single phase transformers.) 

 

It is possible for circulating currents if the generators are not exactly identical. Compare the actual test voltages, not the rated or nameplate values, looking carefully at the winding pitch and voltage balance between phases. AVRs and KVAR/PR regulators will adjust for average voltage differences between machines, but they cannot do anything for voltage balance between phases. Generators are not perfectly identical, but with modern quality control they could be close enough.

 

People parallel generators all the time, so this will probably work. If the voltage balance is not good and there is not much impedance between them, they may not work well unloaded. Expect unstability and higher-than-normal circulating currents. But when you apply some load, the internal voltage drop should reduce/stop the unstability and allow this with no problems. If they are physically far apart or have other sources of system impedances between them, this will be easier. For example, if they have step up or isolation transformers between them or they have many feet of cabling or bus work they will have higher system impedances, which actually helps when running lightly/un-loaded.

 

To fix that, you may need to do 'load-dependent start/stop' of the gensets...only starting as many gensets as needed to supply the actual load KW. Start one gen, load it up to 80%, start the next one, load them both up to 80%, start the next one....you get the idea. Starting and paralleling 3 gens to 1 bus with low system impedances and un-balanced generators will most probably have problems. Unfortunately this is normally the first thing customers want to see!

 

Good luck, write back if you have any more questions,

Steve