Bonding of the Neutral and Ground on transpot generators

[daniej2]

We assemble generators from one meggawatt to two meggawatt using a wye configeration. The voltage can range from 240 volts to 14,400 voltsThey are load tested through a delta to delta transformer then to a resistive and reactive loadbank.
There hasn't been an issue until one day, the video monitor for the test cell was fluctuating more than normal. In trying to locate the problem, I noticed that that there was large amount of current to ground. Further investigation showed that we are bonding the neutral of the generator to the generator housing ground. This was directed by UL and CSA (Canadian Standard Association). The generator was properly grounded for ground protection.
I immediately stopped operations and ohmed the generator, transformer, switchgear and loadbank. Did not find the problem. If I presumed to be correct, is that when we bonded the neutral to ground the ground then became a current carrying conductor. The neutral was not connected to anything but the ground at this point. The procedure was immediately changed to disconnect the bond and test the generator. I was concerned of the potental voltage could have caused personell safety along with equipment.
My question is why does UL specify bonding the neutral with the ground and should this be done in the field verus production. Second, If testing utilizing the neutral has to be done, what meathod should I use. Was there other ways to resolve this other than what I did? I know my recomendations but would very much recieved different views. daniel_jim@cat.com

 

[don_resqcapt19]

Re: Bonding of the Neutral and Ground on transpot generators

Daniel,
Just the fact that the neutral is bonded to the generator frame should not cause any current to flow. You need to look for some other problem such as a partial ground fault on one of the phases.
Don

 

[bennie]

Re: Bonding of the Neutral and Ground on transpot generators

OK Don, you are always catching and correcting me, and you are usually right.

Tell me, what is a partial ground fault?

 

[don_resqcapt19]

Re: Bonding of the Neutral and Ground on transpot generators

Bennie,
It?s not a real term. It is a ground fault that has a high enough impedance that there is no obvious problem.

 

[bennie]

Re: Bonding of the Neutral and Ground on transpot generators

That makes sense, it is a low level fault under high impedance conditions.

I thought it may be like; " a little bit pregnant".

 

[daniej2]

Re: Bonding of the Neutral and Ground on transpot generators

Don,
Thanks for the reply. If the neutral is bonded to ground without neutral being in service, wouldn"t this create a potential in voltage. This would seem to me being like getting 120 from a 240 volt system using a ground as a conductor like my great grandfather did.

 

[advan]

Re: Bonding of the Neutral and Ground on transpot generators

Obviously if testing phase to neutral loads there is going to be high neutral currents. Is this type of test included in your company's test proceedure or maybee is your test equipment failing. Or if this is not the case and the generators are soly for phase to phase use. Then try using a high-impedance grounded neutral system 250.36.

 

[daniej2]

Re: Bonding of the Neutral and Ground on transpot generators

Our testing system monitors and records phase to phase voltage and amperage which I think that we have what I call a floating neutral. I think that if our system was a WYE system so the neutrals will be connected, then testing of the system with the neutral and ground bonded could safely happened. Instead we have a Delta system which the bond is removed and the test is performed then reattached. There is ground fault protection on the system if a short occurs.

 

[Ed MacLaren]

Re: Bonding of the Neutral and Ground on transpot generators

We assemble generators from one meggawatt to two meggawatt using a wye configeration

I think that if our system was a WYE system ------Instead we have a Delta system

Can you clarify whether the generator windings are connected wye or delta?

Ed

 

[daniej2]

Re: Bonding of the Neutral and Ground on transpot generators

All of the generators have the WYE configeration. The customer dictakes wether to use a four or five wire system on the installation.

 

[don_resqcapt19]

Re: Bonding of the Neutral and Ground on transpot generators

Dan,
While bonding the center of the wye to a grounding electrode will create a potential voltage between the phases and the grounding system, there should be no current flowing on the grounding system. Even with line to neutral loads there should be no current on the grounding system.
The only reason that your grandfather could get 120 from a 240 volt feed by using a grounding electrode is because the supply system had a grounded conductor and he was using the earth as a neutral conductor for the 120 volt loads.
Don

 

[daniej2]

Re: Bonding of the Neutral and Ground on transpot generators

Don,Yes I agree and he was using an old jack as a ground rod. Thats' old technology but if the ground had some sort of resistance due to a faulty connection or the ground plane wasn't sufficient to support the it, wouldn't the the person or oject that made continuity between the generator and ground become the conductor?
Thanks Jim