transformer bank

[tem1234]

Hi guy,

here's the problem,

We have 3 single phase transformer 150 kVA 14.4kV/600V Z=2% connected wye-delta

one transformer burned.

We cannot find a 150 kVA transformer, but can find a 167 kVA easily.

From my point of view, there's no problem to add a 167 kVA if we adjust the impedance.

Z(167 kva) = 167/150 * 2% = 2.23 %, the impedance must be close to this at 167 kVA (±10%), and we protect the bank as a 3x150=450 kVA transformer.

The transformer vendor said we cannot do this. I Am pretty sure we can. What do you think?

And a other thing, the new tranformer will probably have two tap at the secondary, so it can be 14.4kV/347V or 14.4kV/600V. The impedance that the manufacturer give is for the 600 V or 347 V? I'm not sure it's very clear on the nameplate.

Thank for your help

 

[magoo2]

I don't see a problem with having one unit different or larger. We routinely use banks of single phase transformers with X/120-240 V connections, where X could be 7.2 kV or 7.62 kV. It is common to have a larger transformer for the lighting load (120/240 V). The other 2 transformers will just have 240 V load.

With your voltage ratings, it sounds like you'll wind up with the capability to have either a 600 V delta or a 347 V delta. Is that what you want?

 

[tem1234]

thank magoo,

The bank is presently connected as the picture attached.

We don't need 347 V, but all the transformer available in the next few days have a 347 and 600 V tap. So the 347 V tap will be floating. Since the transformer must have the same impedance as the other 2, i was wondering if the impedance on the nameplate is for the 600 V tap.

 

[magoo2]

Normally I'd expect the impedance is for the full winding, or 600 V in your case. You should be OK.

 

[dpc]

I don't see a problem. We have clients that have a lot of mismatched three-phase banks.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[waross]

I have never seen a transformer of that size with 600/347V taps. I have seen a lot of transformers with an output of 347V, suitable for use on a 600V system. That is probably what you are looking at.
BTW I know where you can find another similar transformer that was ordered by the General Manager without having the order vetted by anyone knowledgeable in transformer ratings.
It's been sitting unused for years because it's not worth paying the return freight to the US from an island in Central America. 347V suitable for 600V wye systems. Needed 600V
If the voltage IS correct, the impedance is probably acceptable.

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

 

[tem1234]

Yeah, You're right waross

After talking back to the vendor, the transformer is 14.4kV/347 V without 600 V tap.

it won't work on the system since the 2 other transformers are really 14.4kV/600 V, without possibility of 347 V.

Since we have a hard time finding 14.4kV/600 V transformer quickly, we'll probably be force to change the complete bank for 3 transformer 14.4kV/347V

Thanks

 

[BJC]

"if we adjust the impedance." What does that mean? Your load will determine if the impedance imballance is tollerable.
While your checking you load see if you can get by with a Y-open Delta. You only need two transformers for the open delta.

 

[dpc]

"if we adjust the impedance." What does that mean?

Putting both transformer impedances on the same kVA base, I assumed.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[tem1234]

BJC,

Adjust the impedance means to calculate the impedance of the 150 kVA tranformer on a 167 KVA basis (2% on a 150kVA basis equal 2.23 % on a 167kVA basis).

Operate in OPEN-Delta allow only 57.7% of the initial rating of the bank, it isn't enought for the load.

 

[waross]

I much prefer a wye installation. Though the Canadian code does not require a 347/600 Volt system to be grounded if there is no provision to connect 347V loads, I recommend that you consider grounding the wye point. With no 347V loads you may use high impedance grounding.

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

 

[rcw retired EE]

tem1234 - Is your concern about impedance because you want to match the other two units? Impedance differences will not cause much of a problem, maybe a little imbalance in voltage at high loads and a slight change in short circuit current on one phase.

Impedance matching is a concern when paralleling transformers, but these won't be in parallel.