## Transformer Impedance Data

## Transformer Impedance Data

(OP)

Here is one of the downsides of deregulation. I'm a steam guy that needs to provide data to our Independant System Operator on GSU Transformers. There used to be a whole department of knowledgeable electrical engineers across the hall, but with FERC 888 and 889, we can't talk.

Here is my problem. I need to provide impedance data (R and X) at 100 MVA Base for GSUs ranging fron 5MVA to 700MVA. I have the test reports so I can get the following:

Nameplate Voltage, H & X

Load loss watts

Test MVA

% impedance

I think that is all I need to get nominal resistance and reactance, but I could be wrong. If that data is sufficient could someone show me the formulas needed?

Here is my problem. I need to provide impedance data (R and X) at 100 MVA Base for GSUs ranging fron 5MVA to 700MVA. I have the test reports so I can get the following:

Nameplate Voltage, H & X

Load loss watts

Test MVA

% impedance

I think that is all I need to get nominal resistance and reactance, but I could be wrong. If that data is sufficient could someone show me the formulas needed?

## RE: Transformer Impedance Data

%R = 100*(Load Loss)/(MVA of test)

note: the test MVA will usually be the base or OA rating of the transformer.

%X = sqrt(%Z**2 - %R**2)

The values of %R and %Z are on the transformer's base MVA. To convert multiple by the 100 MVA and divide by the transformer's base MVA.

Example

20 MVA transformer

Impedance 10.24%

Load Loss 75800 watts

%R = 100*(75.8 kW)/20,000 kVA = 0.379%

%X = sqrt(10.24*10.24 - 0.379*0.379) = 10.23%

Change to 100 MVA base

%R (on 100 MVA base) = 0.379*100/20 = 1.895%

%X (on 100 MVA base) = 10.23*100/20 = 51.15%

If the transformer's not on its rated tap or does not match the system's nominal voltage, additional adjustments must be made. I would suggest talking to one of those EEs over lunch to verify you have what you need.

## RE: Transformer Impedance Data

Call % resistance: %R

Call % reactance: %X

Call load loss watts: LL

Call transformer MVA: TMVA

Call base MVA: BMVA

Call per unit resistance at base MVA: R

Call per unit reactance at base MVA: X

%R=100*LL/(TMVA*1000000)

%X=sqrt(%Z^2-%R^2)

R=(%R/100)*BMVA/TMVA

Likewise for X.

## RE: Transformer Impedance Data

Jack

## RE: Transformer Impedance Data

Jnims, how do you adjust for the off rated tap situation?

## RE: Transformer Impedance Data

## RE: Transformer Impedance Data

Taps Impedance

169400:13090 12.26

161700:13090 12.27

154000:13090 12.51

141900:11000 7.58

135300:11000 7.59

128700:11000 7.85

However, this is not alway true. I can easily find units where this is not true. I would use the best data available (from the nameplate or test report) then correct it to the system base voltage.

## RE: Transformer Impedance Data

In system studies, we usually define a base MVA, but not a base kV. All voltages are stated in per unit of the particular voltage level - a transformer on the nominal tap will have 1.0 pu voltage on both sides and will have the same per unit impedance at both voltage levels.

Impedances are rationalized to the base MVA by the relationship Z (pu) = Z(Ohms)/Z(base Ohms), where Z(base Ohms) = (kV^2)/MVA(base). This puts all voltage levels on a common basis for simplified analysis.

If the nominal voltage level varies from 1.0 pu, say because of tapchanger position, then the impedance at that voltage level must be adjusted accordingly.