Transformer Impedance and Operating Temperature 4

[wesles24]

Hello,

Can anyone tell me why a mobile substation transformer can operate at higher temperatures than pad mounted (up to 95°)?

Can anyone tell me why a mobile substation transformer can have impedance as high as 18%?

Thanks for any help!

 

[ScottyUK]

What do you mean by 'mobile substation transformer'?

Higher temperatures generally mean either shorter insulation life for a given insulation type. Most pad-type transformers are expected to have a 40 year life so the operating temperatures reflect this. Some dry type transformers can run at Class H limits which is blistering hot, far above where an oil-filled design would operate, but they have their own problems.

The impedance is a factor of the winding and core design. 18% is fairly high and would normally seen be on a medium-to-large transformer equipped with a tapchanger to regulate the secondary voltage. Impedance is usually chosen to limit the fault current to within values the switchgear and other equipment can handle.

 

[wesles24]

As in a transformer that is mounted on a trailer along with breaker/switchgear that can be used for temporary outages/construction.

Thanks for the information, I am trying to understand why it would be higher for this application. Maybe because of the shorter time period it is in service?

 

[waross]

Another factor is that transformers may have the KVA rating increased by rating them for higher temperatures.
If a transformer with 12% impedance has the KVA rating increased 50% by allowing higher operating temperatures, the impedance doesn't change (much) but the per unit impedance will increase to 18%.

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

 

[davidbeach]

Many compromises are made when designing a mobile substation, it does have to fit highway clearances. Typically it will be an FOA transformer with a single MVA rating. That transformer in a decent sized OA housing would probably have a much lower MVA rating. That smaller transformer, with an impedance at that lower rating, will have a higher impedance if given a higher rating. The standard substation transformer would be expected to carry load for 40-50 years or more. That mobile substation will be quite old at 40 years but in those 40 years it may have only been energized for a year or two total. Things that would unacceptably life shortening in a normal substation will be accepted in a mobile without a second thought.

 

[bacon4life]

The higher rated temperature is due to using alternative insulating materials such as nomex instead of kraft paper.

 

[prc]

Mobile transformers are used as short time replacement for any failed transformer or to meet the short period peak load occurring, say, once in a year.In mobile transformer, the target is to reduce the over all weight so that total trailer carrying capacity is reduced. When the impedance is at upper band, transformer becomes a copper machine- ie less weight of core, more copper, high load loss, less core loss and minimum overall weight.

Since the unit will be working for short periods only, higher current density is used (with consequent higher winding temperature rise) to reduce the copper weight. Of course it will shorten paper life, but any way it is running only for short periods.Energy efficiency is not important for such short period loadings. Many times nomex of higher thermal class is used as conductor insulation for such trfs.