Transformer sizing- size at 100% or more? 1

[tumbleweed1]

From what I've seen in the CEC there doesnt seem to be an indication as to how much larger to size a transformer than the load. The code makes great mention of sizing the OCPD or conductors, but not the TX itself.

Particularly, if I have a transformer for a stand alone load bank, with no future possibility of expansion, and this load bank is purely resistive at 500kW, would sizing a transformer at 500kVa be adequate? Or is there a standard practice to add 10%-25%?

Thanks.

 

[X49]

By code (I am assuming Canadian, not Californian), a 500kVA transformer is adequate. However, with a what I assume is a continuous load that could be running for hours on end, you may want to size the transformer larger for reliability reasons. How long do you want it to last?

You can either use a transformer with a larger rating (750kVA), or use one with a 500kVA rating, but with a low temperature rise rating. For example, by using a dry type transformer with a temperature rise of 80 or 115 degrees C instead of the standard 150 degrees, you are reducing the amount of heat in the windings and extending insulation life.

Personally, I would use a 500kVA transformer with a temperature rise of 80 or 115 degrees C (assuming dry-type), depending on the duty cycle and required life of the transformer.

 

[tumbleweed1]

That's a valid point, i'll look into the temp rise of the TX.

After I wrote this I thought about how often the load bank will be running at max, and quite frankly, it wont be often. It will be controlled to apply load to generators, and when the real load is switched in, the load bank will be dumped. This will help maintain frequency excursions by exchanging "temp load" with real load.

So that said, I realistically cant see this thing running at max for very long, which should help our case of sizing the TX to the same rating as the load bank.

And yes, it was Canadian Elec. Code.

 

[waross]

I agree that there does not seem to be a specific rule for transformer sizes, but several rules may have an effect on the sizing of transformers.
2-024 Use of approved equipment (see Appendix A)
Electrical equipment used in electrical installations within the jurisdiction of the inspection department shall
be approved and shall be of a kind or type and rating approved for the specific purpose for which it is to be
employed.
And-
It will be hard to comply with Section 8, Circuit Loading and Demand factors, specifically 8-104 Maximum circuit loading (see Appendix B)
size of conductors and breakers.
And with rule 26-260 Transformer continuous load (see Appendix B)
With a transformer that is rated less than the calculated load.


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

 

[X49]

Bill,

I don't think tumbleweed is expecting to exceed the transformer rating, but to run it at exactly its rating.

In practice, the more heavily loaded the transformer is, the lower the secondary voltage will be due to voltage regulation, and the load bank will end up running at at ~5% less than nominal voltage, ~10% less than rated power. I don't think there is any real chance of overloading a 500kVA transformer in this case.