Transformer sizing

[REEngineer]

There is a load list summary report for this plant and based on it, I'm going to size substation transformer. But why so many terms in the report.

Plant has three kind of loads: Normal, Essential, Vital. All loads can be categorized into Continuous, Intermittent and standby. For every load we have two power data, Mechanical(The shaft power) and Rated(Installed power).
Cyclic Factor, Power Factor, Efficiency and Load Factor for each load is determined (or predetermined).

My procedure was like this:
1) Absorbed Power = (Mechanical Power/Efficiency) * Cyclic factor.
2) Absorbed KVAR = Absorbed Power * TAN(ACOS(Power Factor)).
3) Capacity = Sum(Absorbed KVAR) * 1.15
And the next recommended value in IEC37 was chosen for each transformer.

These are the questions:
1) Should I try to size transformers based on Rated power instead of Absorbed power? This way, larger sizes would be estimated.
2) If calculation shows 200KVA transformer is needed for one bus bar and 400KVA transformer for the other, isn't it better to select both as 400KVA? Of course the two mentioned bus bars are not going to be paralleled. I just have this feeling that it's better to have exactly same transformers in one substation for a specific voltage rating and it may be better for maintenance.

 

[petersonra]

I think it depends on a lot of other factors you have not mentioned, like where these things are going to be located (what country, for instance). If this is a utility application, they have a lot of leeway in sizing things, so if these things are under their control, you can size things a lot smaller, based more on what the real loads are likely to be.

I don't think there is any real advantage in transformers of this size in making one of them twice the size it needs to be just to make them the same, unless you are required to provide a hot or warm standby unit.

 

[kingpb]

There are certainly advantages to having the same size, but with that said, it can cost more, and you have to be able to reasonably justify that it is really needed (now or later).

IMO, at that size, I would install two identical set-ups, and arrange them so you could possibly install a tie at a later date.

 

[REEngineer]

It's going to be a petrochemical plant with mostly asynchronous motor loads.

I rephrase my question. This is what really disturbs me:
Is it safe to size transformer based on absorbed power? Assume an asynchronous motor absorbs 22KW, but the nameplate is 45KW. Isn't it better to choose transformer to supply 45KW instead of 22KW? Besides, which power should be considered when sizing cables and breakers? Rated or absorbed?

 

[petersonra]

It's going to be a petrochemical plant with mostly asynchronous motor loads.

I rephrase my question. This is what really disturbs me:
Is it safe to size transformer based on absorbed power? Assume an asynchronous motor absorbs 22KW, but the nameplate is 45KW. Isn't it better to choose transformer to supply 45KW instead of 22KW? Besides, which power should be considered when sizing cables and breakers? Rated or absorbed?

Motor feeders are sized according to code which will tell you what minimum size the wire and the breakers have to be.

 

[eric stromberg]

It's going to be a petrochemical plant with mostly asynchronous motor loads.

I rephrase my question. This is what really disturbs me:
Is it safe to size transformer based on absorbed power? Assume an asynchronous motor absorbs 22KW, but the nameplate is 45KW. Isn't it better to choose transformer to supply 45KW instead of 22KW? Besides, which power should be considered when sizing cables and breakers? Rated or absorbed?

I'd size the transformer (and the rest of the system, for that matter) based on the connected load, not the running load.

 

[kingpb]

Knowing that it is a petrochemical plant tells me you should go with connected load and oversize. There will be plenty of opportunities down the road, as processes change to add more loads to the bus. The extra capacity will be welcomed.