Generator with ground and netrual problems in ?

[papa smurf]

Need help in Iraq. I'm inspecting an electrical substation that the main power is coming from 5 large Diesel Generators. The Generators A.C. output is 3phase 400 volts A.c. The Generator has a 3 PHASE 1600 amp 3 pole breaker mounted on the generator and then feeds a y - delta step up transformer. Steps up to 11k volts. The Neutral from the Gen. and the Neutral from the y 400v side are going to a ground rod grounding grid. Not sure if that is correct. The secondary 11k delta side is going inside to the 11k switchgear breaker and has no neutral, but this cable is armored and they are grounding the armor at the generator to it frame which is grounded to a ground rod grid system. The other end coming in at the switch gear is grounded to the gear frame. That equip. ground on the gear goes outside to another grounding grid with 2 -70mm wires to a ground rod grounding grid. Which actually means both grounding grids are tech. grounded together. The 11k leaves again with 3 wire armoured cable by means of another 11k breaker and goes to a step down transformer, 400v on the secondary, Primary is 11k delta - 400volt - y secondary. The armor again is grounded on both ends.The frame on the transformer is grounded in a 400volt outside panel to the ground bar and then that bar is grounded to another grounding grid consisting of 4 ground rods located 6ft apart. I believe the neutral on the sec. should be tied to the grounding nut on top of the transformer also. This outside panel has a main and 5 breakers that feed 5 building. The Neutral and ground are not tied together in this panel, and not sure if this is correct. The wire feeding the building is only 4 wire with no ground feeding a 3phase panel. If I'm correct shouldn't there be a ground going to the building also. They have another grounding grid at the building with a ground coming from the panel but like i said no ground feeding the panel from the first means of disconnect located at the step down transformer. They have no way to run another ground to the building panel. Would it be OK to run a ground from the first means located at the transformer to the grounding grid located at the building and meet the ground that was run from the building panel. Your expertise is well needed at this time please. Tried to explain the best i could and hope this helps. THANKS.

 

[chaterpilar]

What are the problems you are facing?

Your explanation of the field status is good,

We have 16 generators in parallel each of 1.275 Mw 380 volts 60 hz stepped up to 13.8 kv.

We have used dYn11 step up transformer.Please check the vector group again as it is recommended to have delta on the LV side.

Generator output Neutral should be bonded to ground at any one generator and all generator's body should be grounded.

The neutral from Y side of stepup transformer need not be grounded.

The neutral of stepdown transformer should be grounded.

There are some pundits on this forum who will have more inputs when they chime in..

Cheers.

 

[chaterpilar]

This is a good link Papa..

http://www.cumminspower.com/www/literature/technicalpapers/PT-6006-GroundingAC-2.pdf

Cheers.

 

[templdl]

If I am to unbderstand the installation correctly the primary of the transformer is a 11kv delta, the secondary a 400Y/231. You are feeding the 400Y with the generator stepping up to 11kv.
It is of my opinion that the X0 of the 400Y should not be grounded, that all that is needed from the generator of the L1-L2-L3 connected to the X1-X2-X3 and bringing the EGC from the generator and bonding it to the enclosure of the transformer. You and drive as many ground rods as you would like for the EGC but don't bond that X0 to the EGC.

Should a transformer winding fail to ground you will have a fault current returning to the source through the EGC such that the GF pick-up of the 1600 breaker will detect and trip the breaker.
Bring out a neutral from the generator to the X0 just because you have an X0 doesn't imply that it requires a connection to it. Which often happens is that you will have undersirable current flowing back to the generator though it. In addition if the neural were to be brought out the second error would be to groung the neutral more than once.
My advice is to leave the X0 insulated and isolated with no connections made to it at all and save the cost of running an additional conductor from the generator.
On the 11kv side since its a delta have you given thought to fult protection for the step up transformer to the step down? Being that it's 3-wire and the 400v is 3-wire the 1600a breaker will provided the protection.

 

[RUWired]

The armor again is grounded on both ends.The frame on the transformer is grounded in a 400volt outside panel to the ground bar and then that bar is grounded to another grounding grid consisting of 4 ground rods located 6ft apart. I believe the neutral on the sec. should be tied to the grounding nut on top of the transformer also.

The neutral ( with the step down transformer) should be tied to the frame of the transformer and to the ground grid.

This outside panel has a main and 5 breakers that feed 5 building. The Neutral and ground are not tied together in this panel, and not sure if this is correct.

The neutral should be tied to the frame and to the ground grid.

The wire feeding the building is only 4 wire with no ground feeding a 3phase panel. If I'm correct shouldn't there be a ground going to the building also. They have another grounding grid at the building with a ground coming from the panel but like i said no ground feeding the panel from the first means of disconnect located at the step down transformer. .

By 2008 nec standards, you would need a seperate ground wire with the feeder to the building. In your case, in desperation of not being able to get a ground wire with the feeder to the panel in the building, i would bond the neutral to the ground at the panel in the building asap to clear any faults and to keep the system balanced. as of now, it sounds as if you have a floating neutral in the building and no ground paths. The ground wire from the outdoor panel to the buildings ground grid won't be of any use clearing any faults with feeder.It would be good for equal potential if in close proximity of other outdoor equipment.
Rick