Power Transformer Sizing

[glalrodax]

I have to size a 19/6.3kV power transformer which feeds the auxiliary loads of a power plant (some medium voltage motors, some distributions transformers, an excitation transformer and an isolation transformer for a static starter). The transformer should be around 20 to 25MVA considering the total loads that it feeds during the unit start up. Does anyone know a dimensioning/sizing guide? Are there any points that I must consider? (e.g. impedance, start of a motor, peak load value of the isolation transformer, e.t.c.)

 

[davidbeach]

Sounds like there are all sorts of ways to get into trouble with this one. I would suggest hiring a consultant familiar with these installations and getting good help.

Eng-Tips is just that, a source of tips not a source of free engineering.

 

[HVDragon]

You need few more data before start:
1. Power S?
2. how much phases
3. frequency
4. Primary voltage U1 , secondary voltage U2 and taps?
- what is the tollerance for ratio on main tap and the other taps
5. System for voltage regulation
6. Connection
7. Short circuit voltage or impedance?????????? ?? ???? ?????????? ?? ???????? ???????
8. Loses: Pk
Po
Io


Somethin terrible is stalking between the high voltage potentials

 

[rcw retired EE]

Check the turbine supplier's specifications for minimum short circuit level at the static starter. Low SC levels lead to high harmonics during turbine starting. That criteria is usually the controlling factor for the auxiliary trasnformer KVA rating.

There are lots of other specification items to look at also- tap range, impedance, voltage withstand, etc.

 

[sslobodan]

Well I might help you with this issue.
In our company (we do projects for power plants) we are always using safety principle. What that means. First you can not set 1 transformer because of fault (and that plant needs to work even if transformer fails or it loses more money than 1 transformer price until they replace that only one, so you go on 2 transformers). Each one must withstand entire load of subsystems That means nominal power (S) of transformer must take nominal power of entire plant. Now there are many issues about that but you need to take in consideration the maximum engaged power at one time (There are many dual pumps working and reserve one so you need to know the exact scheme of substation) When you do that consider that for light P factor is 1 and for motors is 0.8 so you need to get 1 on lightning and to calculate S for motors (P=S*0.8) sum those two and get first above. Now you have to know that dry transformers can run with 25% of overload if you get the adequate heat dissipation so make sure you have good fan in transformer substation and you don't have to worry about small miscalculation you can make if you are not experienced enough in this.
This is especially important if you continue with 1 transformer bank (although I would never recommend that). If you get 2 banks load share is not the problem and you need to make 2 distribution rails with section switch, and share load on them. Usual configuration is to have 2 sections (each for one transformer) and one of them divided in 2 subsections one which has power plant necessary systems for reboot that requires non stop power (Emergency systems) and that section is supplied from transformer bank and additional bank of emergency power.
Now I would be able to draft entire substation on this I hope that you would be able to do the same