Different size transformers in the same bank

[wfowfo]

In response to some information I got on this site concerning matching impedances in transformer banks, (thanks to all who participated) we've implemented a +/-10% impedance standard.

Now my problem is that we have an existing bank (3 wire, 3 phase, 480 volt delta with corner ground) that has one 10 Kva pot and two 15 Kva pots (how that happened I don't know).
One of the 15's has failed and the only transformer in stock that fits with the same impedance is a 25 Kva.

The idea of a 10, 15, and 25 in this bank bothers me, although I don't have a good technical reason.

If the impedances match within 10%, can you hang a variety of different sized transformers in one bank of a delta congiuration?

 

[waross]

Is your primary wye or delta and if it is wye, is the neutral connected or floating?
respectfully

 

[wfowfo]

The primary (12.47/7.2 Kv) is connected floating wye.

 

[jghrist]

If the nameplate impedances match, the impedances on a common kVA base will not match for banks with different size transformers. Assuming that the smaller transformer will not be overloaded, the only problem will be a possible phase voltage unbalance if the transformers are heavily loaded. The voltage drop in the 10 kVA transformer will be higher than that in the 25 kVA transformer. If the loading is light, the voltage drops may be small enough that the unbalance is acceptable.

 

[waross]

The X/R ratios allso play a part. We are assuming that the X/R ratios are close enough to be ignored.
Be aware that the percent impedance is dependant on the KVA rating or full load current rating.
If a 10 KVA transformer with 2% imp. is paralleled with a 20 KVA transformer with 2% impedance, they will share the load in the proportion of their KVA ratings.
If a 10 KVA transformer with 2% imp. is paralleled with a 20 KVA transformer with 4% impedance, they will share the load equally.
If transformers are to be paralleled, proper load sharing is important and the percent impedances should match.
If transformers are to be used in a three phase bank, the load will be limited by the smallest transformer. Voltage balance is more important.
I suggest re-rating the transformers before matching percent impedances.
When you rerate a 20 KVA transformer to 10 KVA, the percent impedance is halved.
For a 4% imp., 20 KVA transformer to be rerated to 10 KVA,
Use 0.04 x 20KVA/10 KVA = 2%.
This is, in effect, using the actual impedance, but converting it back to a percentage for ease of comparison.

Summary:
Parallel transformers, match percent impedance for best load sharing.
Three phase transformer banks, match actual impedance for best voltage balance.

Considering that your system was working with the mismatched transformers, you have some forgiveness with you load.
You may be able to find a transformer that is a better match by matching the actual impedances.
Hope this helps.
respectfully

 

[Zogzog]

Thata all good, but how about this. Try a different supplier, this transformer should be easy to find.

 

[waross]

Hi Zogzog;
I understand that wfowfo is not buying a transformer but using one from his own stock.
Hi Zogzog;
My post was restating jghrist's advice in different words.

Short and sweet.
Percent impedance, use to parallel transformers.
Actual impedance, use to bank transformers.
respectfully

 

[wfowfo]

Correct.
A pot burned up and the customer (we're a Coop) was without power. Getting him back on wasn't the problem, whether to leave the service as-is after we got the proper replacement was what I needed to know. Thanks for all the answers. The bank is getting changed out today wiith matching units.