UNGROUNDED SYSTEM GENERATOR 13.8 KV

[ajlimuran]

Hi,

To all,

We have 2.1 MW Diesel Power plant supply the small grid , generator rated voltage of 13,8KV and ungrounded or floating nuetral . Our problem is the generator potential transformer wye connected primary and wye connected secondary and broken delta connected secondary , primary PR fuse blown 3 times.
What is the problem why PT primary fuse blow?

Please help!

AJ

 

[7anoter4]

If the primary PT is Y connected and grounded check if one of the generator phases is
grounded too. Then the ground potential will rise from 0 to this phase potential.

 

[slavag]

usually problem is Ferroresonance.
Firstly check damping resistor are installed ( if installed ) in the Brocken delta circuit.
size must be about 30-40Ohm and 400-500W

 

[ajlimuran]

Sir, The generator nuetral is not grounded.

 

[ajlimuran]

Sir Slavag, there is no installed damping resistor in the secondary of the broken delta which is connected to the relay input. What is the ferro-resonance ? This is cause PT fuse blown?

 

[slavag]

Yep, it must be installed, w/o any suggestions. Please ask PT mnf for recommended size and type of PT.
or possible order antyferroresonace VT.
Yes, ferroresonance is first reason for damages of PT.

In this case: ferroresonance is resonance between capacity of system and inductivity of PT.
reason of wye PT. "Standard" problem in the ungrounded or compensated networks.
If you need more information, I will found some theoretical documents and will attach here.

 

[slavag]

sorry: recommended size and type od dumping resistor

 

[7anoter4]

The ungrounded system- generator and the supplied cables, transformer, motors and other-has to be
provided with grounding fault detector. One of the ungrounded system phases could be accidentally
grounded and then the grounding potential would rise up to this phase potential. The primary winding
of the potential transformer voltage –between Y point and the terminal of the other phase [not grounded]
would rise 1.73 times.

 

[ajlimuran]

To all,

What will be the best solution so that i can run the unit in ungrounded system?
It is okay to change the generator PT from wye connected primary to open delta? What is the effect?
Thanks all you Sir.

 

[ajlimuran]

Sir Zanoter we are connected directly to grid. Assuming 90% all faults is cause of ground faults due to sub-trasmission lines.
In case there is a line to ground fault in the grid, since my generator is ungrounded or floating nuetral , the effect is the unfaulted phases increase twice or trice ? Right?

 

[slavag]

What type of grid grounding?

 

[slavag]

Do you have another power plants in the grid?

 

[ajlimuran]

Yes Sir, there are many power plant connected in the GRID. I am looking for a solution to operate the plant in ungrounded system what to do because fuse blown twice.
First move, we replace the withdrawble PT and fus, then thesame fuse blown.

 

[ajlimuran]

Im confuse also because, generator potential transformer fuse is always experience blowing fuse but the unit is synchronized to the GRID then why is my bus PT is not affected and its configuration is the same, wye connected primary and wye secondary and broken delta.

 

[slavag]

Where is generator PT is installed, in the SWG or near to generator?.
Do you have cable link from switchgear to generator?.
All the small details, cleaning, trace are influenced on the ferroresonance phenomena.

ok, this is a grid, what is a grounding system of grid?

 

[ajlimuran]

The PT is installed in the switchgear. Yes power cable is connected from generator to switchgear. What do you mean the small details ?
Why the bus PT limiting fuse not blown but generator PT blown?

 

[slavag]

From my point of view, you have ferroresinance between a short cable ( generator - switchgear ) and generator PT. PT buses is connected with lot of other cables to bus.
this is difference between both PTs, problem is started before synchronizing.

 

[slavag]

Please add dumping resistors to both PTs.
its all, simple and standard practic

 

[ajlimuran]

Dear All,

I will raise this concern because , recently after we tried to back the unit online after replacing the fuse, explosion occurred again but now fuse are okay. During last explosion I observed that there was arcing evidence originated from the VCB. The relay indicates current differential and over-current and voltage restraint.
Then, I will check all VCB replaced after 3 times explosion happened.
After inspection and checked of all VCB, it was found out that there was a flash-over or insulation failure of the load side terminal phase B of the VCB.
My question now is what causes the VCB insulation failed?
Why only phase B insulation failed only then phase A & C are okay.
Specs of the VCB VD4 ABB 17.5 KV/630 Amps. , 25 KA
Here is the pictures.
Help me Sir.

Thanks,

 

[ScottyUK]

Silly question: is this breaker actually rated for continuous service on an ungrounded 13.8kV system?

If you have an earth fault on one phase then the other phases will rise to full line voltage with respect to earth. If this breaker is designed for use on an impedance-earthed system then it may not have adequate insulation for continuous service with one phase earthed. If so then I think you have identified the weakest point in the insulation system.