Calculating Percent Regulation From Transformer Parameters 1

[111R]

If I know the percent impedance and load loss (kW) for a given three phase transformer, how can I calculate the percent regulation as shown on the test report? I'm getting fairly close, but there's still an obvious oversight I'm not recognizing.

I'm calculating percent real impedance by taking the load losses divided by the base MVA rating of the transformer and multiplying by 100 to get percent. Is this correct for a three phase transformer?

I'm then taking the per unit nominal voltage of 1.0 and subtracting (real per unit impedance * per unit real current) and (reactive per unit impedance * per unit reactive current).

Thanks

 

[davidbeach]

How are you getting the reactive per unit impedance?

 

[pwrengrds]

My understanding has the percent regulation is the impedance. Primary voltage it takes to go to full load with the secondary shorted / full primary voltage is the impedance.

That is the same voltage drop going from 0-100% load.

 

[davidbeach]

Nope, impedance and regulation are different; it least if your load has a reasonable power factor. The voltage drop of the real component of the load is across the resistive component of the impedance and the reactive component across the reactive impedance. Typical the majority of the load current produces a small drop across a small impedance. Fault current on the other hand is predominantly reactive.

 

[jghrist]

See thread238-221175

 

[prc]

%impedance = Z % R = load loss in kW/rating in KVA x100 % X = root of ( Z square -R square)
% Voltage regulation at full load = %R cos phi + %Xsinphi where cos phi is the PF of the load

 

[jghrist]

Actually, it is a bit more complicated.

Regulation is defined as the change in output (secondary) voltage which occurs when the load is
reduced from rated kVA to zero, with the applied (primary) voltage maintained constant and is
expressed as a percentage of the full-load secondary voltage.

Example:
Z = 7.94%
R = 51.192/12000 (load loss divided by rating)
R = 0.427 %
X := sqrt(Z² − R²)
X = 7.929 %
pf := 80% φ := acos(pf ) φ = 36.87deg

Formula from "ABB Transformer Tests, Their Purpose and Importance":

Reg = R⋅cos (φ) + X⋅sin(φ) + [(X⋅cos (φ) − R⋅sin(φ)]²/2

Reg = 5.283649%