isolation transformer

[zaza123]

In my plant there are two substations which have 480 V system, due to some mistake the upstream MCC bus was made 3p+E & the downstream 3P+E+N , now to correct the system isolation transformer (delta\star) are used in the design.What are the Critical Points related to isolation transformers which should be taken care of

thanks in advance
ZAZA

 

[ScottyUK]

Rating
Impedance
Neutral earthing

All of which have an impact on downstream fault levels and protection settings. watch for unexpectedly high volt-drops at point of use due to the additional transformer impedance.


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If we learn from our mistakes I'm getting a great education!

 

[jghrist]

The delta-star transformer will shift the voltages by 30°.

What do you mean by upstream and downstream?

 

[waross]

Are your substations near a river?
The isolation transformer will create a separately derived system. The isolating transformer neutral must be earthed (or not earthed) in accordance with local codes.

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

 

[13450901941]

I am not so clearly of your question.Pls explain more.

 

[zaza123]

Friends i will try to make myself clear.
Substation 1 has 480 V MCC (3P+E NO NEUTRAL),from the same substation supply is directly going to 480V MCC in substation 2(3P+Earth+Neutral).But we can not run the Loads as we dont have NEUTRAL.So now in the Modification isolation transformers are required.The rating is 650KVA,DYN5.Percenatge impedance i dont know.The grounding system in the substation1 is high resistive grounding.The grounding system in substation 2 will be now made solid grounig type(Neutral directly earthed)

My plant is not near river, upstream & downstream were used for system

thanks

 

[jghrist]

My question was because I didn't know that Sub 2 was served from Sub 1. ScottyUK's and waross' responses cover it, except that you cannot tie any loads from Sub 2 to loads served directly from Sub 1 because of the phase shift in the isolation transformer.

You could save the cost of a neutral by putting the isolation transformer in Sub 2.

 

[zaza123]

jghirst , you are right, there are no changes in sub 1 , only the isolation transformer will be put in sub 2 with the neutal grounded.with this the loads from sub 2 will get neutral connection

thanks for your support

 

[rbulsara]

Hopefully you are not suggesting a isolation transformer for the entire MCC-2 load. If it is an MCC, why do you really need a neutral? If it is only for the control circuits you can either provide:

1. A smaller common control transformer for the entire MCC or
2. Control transformers at each starter cubicle.

 

[alehman]

If there are only a few loads in the MCC that need a neutral, consider separating them and using a smaller transformer just for those loads. Could save a lot of money.