Mr. Phil Corso!

Sir,We have an old transformer of 20MVA. We had sent it to a private company for testing. The company issued test report that "Voltage applied (3phase) to HT is 1082.4V with LT short circuited and the current passed through HT is 1050 amp in each phase. % Z 9.542 at tape three". Could you please explain how did he determine in above test that %Z is 9.542.

Transformer specifications are
Stepup power transformer
20MVA
HT: 11KV
LT: 6.6KV
%Z: 9.542

Regards
Shaheed

 

Shaheed... two questions:

1) How many taps are there?

2) What are Nameplate Volts & Ampere listed for Tap 3?

Regards, Phil Corso

 

Sir

Unfortunately there is no name plate but the test report also shows that
Transformer 20MVA, 11KV/6.3KV
Measurement of voltage ratio and vector group.
Vector group detected: YNd11<pre>
Tape# Phase A, Phase B, Phase C, Calculated Ratio
AB-an BC-bn CA-cn
1 1.835 1.835 1.835 1.8333
2 1.787 1.787 1.787 1.7896
3 1.748 1.748 1.748 1.7460
4 1.699 1.699 1.699 1.7023
5 1.661 1.661 1.661 1.6587</pre>
Regards
Shaheed

 

Shaheed... something is unclear.

The %Z value provided by the Testing Company for Tap #3, is certainly in the ballpark! At least for the rated parameters of the Xfmr, which are: 20,000 kVA, 11kV, and A = kVA /[sqt(3)x kV] !

But if you want greater accuracy I need to know Tap #3 V and A values! With these specific values I can apply a correction factor that will yield %Z for the Xfmr's nominal rating!

Is it possible to post the Test Report to this forum, or forward it to me at:

[email protected]

Regards, Phil

 

Shaheed... the test vaues, i.e., 1,082.4 Volts applied to the primary terminals that caused 1,050 Amps to flow in the shorted secondary yield a %Z of 9.84% (1082.4/11,000) I don’t know how the Test Company determined it was 9.542%! Perhaps, correction factors were used because of significant temperature differences between those at the time of test and the Xfmr's rated-temperature!

A second concern is that a 3-limb xfmr will have significantly different power-loss values between the three phases, AB, BC, and CA! But without review of test data I can only speculate on how the resistance %R was determined!

Furthermore, the tabulated phase-ratio data indicate voltage ratio’s, and are not the Transformer-Turns-Ratio (TTR)!

Finally, a few words about Taps: Taps 1 and 2 are used when supply voltage is higher than nominal voltage; Tap 3 is the nominal or normal voltage, i.e., 11.0 kV; and Taps 4 and 5 are used when supply voltage is lower than nominal!

Regards, Phil

 

Shaheed... further to my earlier post coverng Taps:

Tap '1' approx +5.0%

Tap '2' approx +2.5%

Tap '3' Nomimal

Tap '4' approx -2.5%

Tap '5' approx -5.0%

Phil

 

Sir,

I am feeling that the test report is not authentic but not sure. I am sending test report by email to you.

Regards
Shaheed

 

Shaheed ... thank you for your prompt response to my request for the Test Report!

I have to agree with your "feeling" about the test! I believe you are entitled to ask the following questions:

1) The Stator primary and secondary winding DC-Resistance-Test values have been listed but where is the correction for operating temperature?

2) The Short-Circuit-Test reveals V and I magnitudes, but where is the P value?

3) Where are the values of V, I, and P for the Open-Circuit-Test?

4) Finally, where are the calculations?

Phil

 

Mr.Phil Corso

Sir, I appreciate your valued information's that make me to able to come to the conclusion.

Best regards
Shaheed