Neutral Switch for Generators at Power Station

can you please explain your DG sets Synchronising procedure and Please attach scanned copy of your ckt diagram in your reply.Then we will get the answer.

Dear,

Very good reply .

Also, for stbility of parallel operation, The neutral of the master dg is switched on.( either manually switch or contactor ). All the fault currents shall pass thru only one neutral , protective instruments shall work more accurately.

Circulating currents in the neutral of dg sets shall be zero. So, stable neutral & zero voltage fluctuations & neusance trippings.

Dear Akihito Shigeno,

Thank you very much of your reply.

Regards

Dear Mr.AkihitoShigeno,

During our college days our Professors taught in the class room, that the 3rd Hormonics is eliminated by Star Connection in the Alternator. 5th Hormonics reduced by short pitching of the windings.

Let the CR4 Forum members give their views or comment on 3rd Hormonics and "What are the standards adopted for 5th, 7th, 11th, 13th Hormonics and heat dissipation Standards on account of the above Hormonics."

DHAYANANDHAN.S

Do you want to ask about neutral contactor????????

Akihito Shigeno is somewhat right. But not only one neutral switch needs to be on.

The problem is given by the ground fault current value and also by the overvoltage value. By alowing a high ground fault current, there will be little concern with overvoltage, but the switching capacity needs to be taken in account. So the grouwnd fault current should not exceed the breaker rating.

On the other hand, if a ground fault occures and the current is low, the result is an overvoltage on the other 2 phases; this is bad for the phase-ground insulation for all the equipment connected in the network.

Engineers usually refer to the Xh/Xd ratio, where Xh is the homopolar reactance equivalent for a particular busbar or power line or DG, and Xd is the positive sequence reactance. Overall, the Xh/Xd ratio shoult be between 1≤Xh/Xd≤3.

1≤Xh/Xd is for the current on one phase ground fault should not be larger than Three phase -ground fault;

Xh/Xd≤3 stands for keeping the overvoltage not larger than 20% of rated voltage.

The Xh/Xd ratio can be modified by switching on/off the neutral switches. To determine how many switches need to be on/ off you have to do some calculation.

Dear msamad,

Thank you very much of your reply.

Best Regards

You're right. I wasn't paying attention at the voltage level. What i said is valid for high voltage, 110 kV and above. Below 35 kV usually the system is able to withstand a 40% overvoltage.

Dear nikolay,

Thank you very much

Regards

Dear Aikhh,

For a solo single DG Alt with Y connection Neutral grounding is OK.

For a Multi DG in parallel, reason not to ground or make common connection for all DG Alternator s is to avoid the circulation current of 3 rd 5th, , , , harmonics generated in Alt Windings from One to Other winding. This causes overheating of winding and this circulating current is not noted in Amp meters. YHIS IS A VERY SERIOUS AND DANGEROUS PHENOMENON

I would advice you to open a chapter on Alternator or transformers running in parallel and STUDY Neutral grounding .. a book by M.G.Say.. etc. or any book on power station design.

Other opinions given by others are valid if you have DG at difft locations, but for one location multi DG one has to carefully do the job else a big difficult position may arise.

Best way u contact the agency, engineer who designed and gave instruction for the plant u have seen.

I wish U dont switch on 2 DG Neutrals simultaneously under any case Best Luck

Dear Guest,

Thank you very much.

Dear Forum members,

I have tried to understand the various explanations shared for the above question and i found it very informative. I had a doubt regarding the internal protection of alternator. I need to protect the stator windings against earth fault and if i have to use restricted earth fault relay and class-px CT to used in phases and neutral for sensing, then doesn't isolating neutral(either by switch or contactor) will cut off the sensing since the unbalance due to earth fault is sensed in the neutral. if restricted earth fault can't be considered in case of isolated neutral, what is the alternative for protection of stator windings?

Please state whether i a correct in the above doubt and kindly share your inputs.

Please consult the Senior Electrical Engineer that wrote the procedure(s) in the first instance. That way a Mentor-Mentee relationship can be established, which will be valuable to all involved going forwards.