% impedance of 3 phase 15 KVA transformer - normal values 5

[edison123]

480 V Delta / 400 V Wye, 60 Hz, 15 KVA 3-phase transformer.

The OEM says %impedance will be about 1.375.

Is this normal value for such transformers?

Muthu

http://www.edison.co.in

 

[davidbeach]

Small transformers can have very low impedances.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[RRaghunath]

Lower the %Z better for the transformer regulation, when there is no necessity to reduce short circuit current magnitude at the secondary terminals of the transformer.
So,
* if there is no problem for the transformer designer and
* if there is no problem with the secondary switchboard fault rating,
there is no issue with the indicated low value %Z, I think.

 

[edison123]

Thank you, David and Raghunath.

One more question. The transformer is going for an off-shore rig with 60 Hz supply. The transformer is being made and tested at 50 Hz with 400 V, 50 Hz primary voltage for open circuit test and 5.5 V, 50 Hz primary voltage for short circuit test => %impedance of 5.5x100/400V = 1.375.

Will the %impedance go up by 60/50 at 60 Hz due to proportional increase in inductive impedance? (though the V/Hz is constant)

Muthu

http://www.edison.co.in

 

[waross]

A little. How much depends on the X/R ratio.
The inductive reactance will increase in the ratio of 6/5.
There will be a slight increase in the effective resistance due to increased skin effect, but probably less than the PU increase in inductive reactance.
I tasked a class of students with verifying the nameplate %imp of some 2 KVA transformers.
The students found noticeable differences in the measured %imp between room temperature tests and tests on transformers at operating temperature.
Considering that the predominance of inductive reactance and the quadrature relationship between inductive reactance and resistance, I suggest that the 1.375 figure is usable but not 100& accurate.
I suggest that the difference due to skin effect will be less than the difference in actual impedance between a hot day and a cold day.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

Bill

Agreed the winding resistance increase due to frequency increase will be negligible.

On further thought, while the inductive impedance will rise at 60 Hz proportionately, since the voltage is also being raised to 480 V, the net effect on %imp will be zero. Am I correct?

Muthu

http://www.edison.co.in

 

[waross]

Yes, good catch.
The inductive component of the %imp will remain the same. The resistive component of the %imp will drop by about 5/6.
With an X/R ratio of 8:1 I estimate an error of 0.235% if you ignore the resistance.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

Thanks for the confirmation, Bill.

Muthu

http://www.edison.co.in

 

[RRaghunath]

% impedance will be lower at 60Hz hen compared to that at 50Hz.
The transformer impedance (read inductive reactance) is increasing by 1.2 times whereas, the base impedance increases by square of 1.2. Hence, %impedance will be lower by a factor of 1.2.

 

[prc]

1)Minimum impedance for this rating is 4% as per IEC standard 600076; 4.5 % as per IS 1180, but 1 % as per IEEE C57.12.20 -2017 Table 12. IEC world insists for minimum 4 % impedance to limit winding stresses under short circuit current flow. IEEE allows lower impedance probably due to the fact that in US this rage of transformers are invariably made with wound core ( shell type) having inherently higher short circuit withstand strength.

2) YEs. Your estimate is correct.
% X= k x If/V where I = rated current V= applied terminal voltage F= frequency.
Please see Annexure B of C57.12.90-2015 for corrections when 60 Hz transformer is tested at 50HZ.
You got 5.5 V on 480 V winding to get rated flow in shorted 400 V winding. Then % impedance at 60 Hz= (5.5/480)x1.2= 1.375

 

[waross]

I don't follow, (edit) RRaghunath.
The rated Amps and the test Amps remain the same.
The inductive reactance increases by a factor of 1.2.
The rated voltage increases by a factor of 1.2.
At the higher impedance the test voltage to force rated current through a short circuit is 1.2 times higer.
%imp is test Volts/rated Volts, which becomes test Volts x 1.2/rated Volts x 1.2

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

Thanks, prc for confirming the %imp stays the same due to same V/F . This is a core wound transformer for lighting duty for an off-shore rig.

Bill, isn't prc saying the same thing?


Muthu

http://www.edison.co.in

 

[RRaghunath]

I agree with prc, provided the kVA rating of transformer (@60Hz) is also increasing by 1.2 factor.

 

[waross]

I meant to reply to RRaghunath.
I agree with RRaghunath's latest post, and I now understand the basis for his earlier comment. The increase in KVA capacity is inherent in the increase in voltage whether the nameplate rating is increased or not. The maximum allowable current remains the same. If you are basing your maximum allowable current on nameplate ratings, then you will use the nameplate rated voltage of 400 Volts.
The 15 KVA transformer becomes an 18 KVA transformer at 60 Hz.
When calculating loading, it may be best to use current rather than KVA or kW for loading. You may then calculate the transformer loading to the nameplate current without regard to frequency, or base KVA adjustments.
Edison123, prc, RRaghunath, I know that you all understand this, but other readers may appreciate some additiona explanation.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

The transformer is rated 15 KVA, 60 Hz, but being tested in 50 Hz world. The short circuit test was done at rated 60 Hz secondary current of 22 A. The maximum winding temperature during SC test was 57 deg C with an ambient of 30 deg C.

Muthu

http://www.edison.co.in

 

[prc]

1) Leakage inductance of a transformer depends on geometric dimensions of windings + number of turns. So, it is not affected by frequency. Impedance in ohms= constant x frequency x inductance.

Percentage or per unit impedance= impedance in ohms/base impedance ; base impedance= (Rated kV)²/MVA

2) When you try to use a 50 HZ designed transformer on 60 Hz system, there will be no increase in voltage or KVA. The flux density in the core will decrease by 1.2 and % impedance will go up by approx 1.2.

3) When you use a 60 Hz designed transformer on 50 Hz, then also voltage or current will not be an issue. The core flux density will go to saturation if the designed flux density at 60 Hz is not 20 % below the saturation flux density. Hence you can say a 50 Hz transformer can be used at 60 Hz, but the reverse may not be feasible.

4) Here edison's issue is - he made a transformer for 60 HZ, but testing at 50 Hz. He wants to extrapolate % impedance for 60 Hz operation from 50 Hz measured data. A caution note- in such small transformers impedance may go up by 1.3 instead of 1.2. But in large units (10 MVA and above) 1.2 is correct.

5) Hope waross and Raghu will agree with me. It is not clear to me how KVA or kV will go up with frequency.

 

[waross]

Limit #1, Current: The maximum allowable current is determined by heating and is relatively unchanged by frequency changes.
Limit #2, Maximum voltage: The maximum voltage is limited more by by saturation than by insulation, and saturation is directly related to frequency.
Single phase example:
Hence maximum safe KVA = "Amps" times "Test frequency (F1) voltage times (test frequency/frequency of interest (F2)).
Or Maximum KVA = I x E x(F2/F1)
Or
KVA at 50 Hz = 22 Amps x 400 Volts/1000 = 8.8 KVA (50Hz/50Hz) = 1
KVA at 60 Hz = 22 Amps x 480 Volts/1000 = 10.56 KVA (60Hz/50Hz) = 1.2
8.8 KVA x 1.2 = 10.56 KVA
400 Volts x 1.2 = 480 Volts.
I have done similar calculations many times, in both directions for motor horsepower and occasionally for control circuit transformers, when imported 50Hz equipment was being converted for 60 Hz operation.
Impedance voltage by the test definition is the percent of rated voltage required to drive full load current through a short circuit secondary.
The target current is 22 Amps for both frequencies.
For 5%Imp, the voltage required at 50 Hz will be 5% of 400 Volts.
For 5%Imp, the voltage required at 60 Hz will be 5% of 480 Volts.
The only time you need to involve KVA in impedance PU calculations, when the rated full load current is not readily available. In that instance you will use rated KVA and rrated voltage to calculate rated Amperes.
You may need to know the KVA for loading calculations,but you have no further need of KVA for %imp voltage calculations or conversions.
Motor calculations, on the other hand, are generally to determine HP at a different frequency. HP is roughly analogous to KVA, and we are not interested in the %imp values, which in any case, change with motor loading.
The same formula is used, but the formula is transcribed to find a different unknown variable.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

The short answer is that the KVA and allowable voltage go up to maintain the V/HZ relationship.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[prc]

Bill, please reconsider.
The voltage applied to transformer- fixed, nothing to do with frequency. Then the condition is fixed by the following transformer formula:
V/N = 4.44xfxBmx A where V/N =volts per turn; F=frequency Hz; Bm= maximum flux density in core T; A= effective area of core in square meter.
Since all other parameters are fixed, the only variable in formula is flux density in the core. So when you connect a 60 HZ designed transformer to 50 Hz supply, core flux density will go up by 1.2,but primary and secondary voltages will remain the same. If the B selected for 60 Hz is high ( more than 1.4 T), then you have to reduce supply voltage to avoid core saturation.( your example for that situation) But then purpose of transformer is defeated.

But if you use a 50 Hz designed unit in 60 Hz, above concerns are not required as flux density will come down with higher frequency.

 

[waross]

Motors and transformers are both subject to Volts per Hertz limits. There is roughly a 5:6 ratio between standard 50Hz voltages and standard 60Hz voltages. A 380-400 Volt 50 Hz motor will be run on 460-480 Volts at 60 Hz because that's what is available, and vice versa.

The voltage applied to transformer- fixed, nothing to do with frequency.

The common procedure for re-rating transformers and motors between 60 Hz and 50 Hz is to adjust the rated voltage and the the rated KVA or HP.
Your statement is not valid when considering frequency changes of actual equipment in the real world.
The current is fixed due to heating. The safe voltage varies with the frequency.
If you energize a 480 Volt, 60 Hz transformer with 50 Hz, you have converted it to 50 Hz and it becomes a 400 Volt transformer.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!