Ground Generator ->Step Up Transformer ->Step Down Transformer Questio 1

[baumboy]

Hello all,

I am a newbie, so I will try not to make too much of a fool out of myself, but I need some help.

First of all, the problem: I don’t know how to properly ground a Generator -> Step Up Transformer -> Step Down Transformer.

We have a 27kva generator wired for three phase 216v Y with the center neutral earth grounded. This powers various motors and battery chargers at 216v and 125v, we are not using any three phase equipment at this time.

We also have a run of wires about ½ mile that powers another building, but we are getting too much line lose on this run. This is why I am trying to install the step up / step down Transformers.

The step up transformer is a 75kva 216v (depending on the taps) Y <-> 480v Delta transformer and the step down transformer is a 100kva 216v (depending on the taps) Y <-> 480v Delta. The step up transformer’s Y side is wired to the generator, and the Delta is wired to the Delta side of the step down transformer. I need to use the output of the step down transformer to power unbalanced 125v and 216v loads so I will need a neutral line.

I would think that the proper grounding would be to wire the center/neutral of both of the Y sides of the transformers to earth ground and to the generator center/neutral as well.

However, I have heard that this will cause problems if unbalanced phases are used on the Step-Down, the neutral could wander from the zero position on the Generator side of the Step-Up transformer. This could cause an increase or decrease (cause instability) between the neutral and the different legs of the Y (generator side). As I will be using unbalanced loads on both sides of the transformer I want to try to avoid any instability.

It was recommended to me to leave the neutral/ground off of the Y side of the step up transformer, and instead connect the neutral/ground to phase #2 of the delta side of the transformer. Then connect the neutral/ground to the Y side of the step down transformer. I do NOT like the sound of this advice as it seems counter intuitive to have a grounded phase #2 on the delta side of the step down transformer and at the same time have the center/neutral of the Y side grounded as well.

Any advice or help that you could offer would be great!!

 

[TurbineGen]

Are both transformers going to be connected to the same grounding system? I drew this up on a piece of paper and am getting acceptable circulating currents in the Delta windings in even a severely imbalanced load with all neutrals grounded. At 27 kVA with half a mile run, my instinct tells me that neutral wandering shouldn't be that great of a problem, but I could be wrong. I have heard of corner ground deltas, but can offer no advice as I have no experience with them.

My experiences with these systems is limited. Perhaps there is someone else out there that can offer more?


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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[waross]

#1, Don't ground the neutral of the step-up transformer. Don't connect the neutral of the step-up transformer to the generator neutral. Leave the neutral floating.
#2, The secondary step down transformer is a separate system. Ground the neutral.
#3, You may leave the 480 volt floating under some codes.
From the size of this system I imagine that the budget may not include grounding transformers or high resistance grounding equipment. You may ground one phase of the 480 volt delta system. Be aware that any faults to ground will now be 480 volts rather than 277 volts. Shock hazard to ground will also rise from 277 V to 480 V.
Note; You may experience switching surges when you energize the step up transformer. These voltage surges may be up to 173% of normal voltage. Damage may be avoided by;
A> Energizing the step up transformer BEFORE any breakers are closed on the secondary loads on the step down transformer. This protects your equipment but the step down transformer will still experience switching surges.
B> Close the breaker to the step-up transformer before the generator is started. Let the voltage on the transformer rise as the generator output rises. This is the preferred method. This method will also eliminate transformer inrush. Actually this may be the only way that you can successfully energize a 75 KVA transformer from a 27 KVA generator.
C> Close a single pole switch that connects the neutral of the step up transformer to the neutral of the generator. This will eliminate switching surges. Close the breaker to energize the transformer. Open the neutral switch. This will avoid circulating currents.

Bill
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"Why not the best?"
Jimmy Carter

 

[sobeys81]

Sorry baumboy in interjecting in your question.

Hi Waross,

Interesting comments, the last point (B) and (C) seems to be mixed up.

We already have the generator energized and then we close the single pole switch and then close the breaker.
Durinng operation Generator neutral and step up transformer neutral will not be connected. How switching surges play role in neutral.
Could you kindly drop a word.

 

[waross]

Sorry for any confusion. Please read my post as:
Option "A"
OR
Option "B"
OR
Option "C".

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[baumboy]

Thanks to all who have replied especial Bill, the information you provided.

I was not aware that we are going to have a start up voltage surge, as we will be starting and stopping this generator twice a day. Will there be any kind of shut down surge?

Option B or C sounds like it will work. However, option B will be a little rough on the generator motor, as it is not a good idea to put it under full load right as it starts…

This is being installed in a third world African country, so codes do not apply. Is there any reason not to corner ground the Delta? Other wise I think I will corner ground it.

Thanks to all again,

Benjamin Baum

 

[itsmoked]

Hi baumboy.

B would not be rough on the generator. Is is the easiest on the generator. If you're concerned about loading a cold generator, just close the breakers on the load side of the step down transformer after the generator has achieved whatever warm up you desire.

Yes there is good reason NOT to corner ground the delta. As Bill noted ground faults become 460V!! They can also be more perplexing to find.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

The surges depend on both the point on the sine wave that the current was interrupted and the resulting residual magnetism and the point on the sine wave that the voltage is applied. Given the wrong combination, one of the wye connected transformers may saturate and exhibit almost zero impedance. This can impose almost full line to line voltage across one of the other phases rather than line to neutral voltage. You may get both a voltage surge and a current surge. Even without a voltage surge you can expect severe current surges energizing a transformer. A Keith says, "Starting the generator with the transformer connected will probably be the easiest on the equipment."
Out of time, will comment on grounding later.

Bill
--------------------
"Why not the best?"
Jimmy Carter