Transformer MVA rating 1

[MEUSA]

I'm not an electrical engineer, but looking into a transformer issue. Does MVA rating refer to primary or secondary side of transformer? I know there should be some losses through windings. Thanks!

 

[davidbeach]

MVA is the through rating.

 

[pwrengrds]

Both, the losses are an order of magnitude less. (unless your dealing with a 3 winding TX)

 

[MEUSA]

Thanks guys. Actually, the secondary MVA was found to be 19.067MVA, while primary side was 20.204MVA. That's about 5.6% loss. Which value should I compare to the transformer rating?

 

[ScottyUK]

Sounds like there is something wrong with the transformer, or something wrong with the measurements. If you are continually dissipating a megawatt into a tank of oil you will know about it pretty damned quickly, so I suspect your measurements are in error. What instrument are you using?

 

[MEUSA]

Scotty, actually the client shared an e-tap model showing the following:
Primary side:
72KV
162.2A
Seconday side:
4.092Kv
2693.4A
I looked some formulas online (for 3 phase systems), and calculated Primary and Secondary MVA as I mentioned above. Did I make a mistake in calculation? If not, is that much loss unrealistic for 28MVA size transformers?

 

[burnt2x]

Ahh! You were just fed with data that came from previous computations!
The transformer you are presenting is a 20MVA transformer. Whatever the losses, depends on how much your load is as well as the no-load (core loss) of the transformer.

 

[MEUSA]

Thanks burnt2x. I actually forgot to mention that the voltage and current values I had were from e-tap load flow study (not rated values). The transformer is rated for 28MVA with class FA/FA, and temp rise 65 deg. Thanks!

 

[mgtrp]

To the best of my knowledge, ETAP doesn't take into consideration transformer losses when running load-flow analyses. The "losses" that you are seeing are likely just imaginary (reactive) power, not real (active) power.

 

[cranky108]

To answer one of the first questions. The MVA rating of a two winding transformer is the through rating, as stated above.
I don't believe anything was stated that he is dealing with a two winding. But for more than two winding transformers, each winding will have it's own rating. Most three winding transformers have a third winding for stablizing, which will have a rating, but is normally not used. Which would explain both high and low winding with the same rating.

Just beware there are transformers with different ratings for each winding.

 

[MEUSA]

Thanks everyone. The Transformer is actually a 2-winding one. Based on cranky108's answer, the 28MVA rating is for both windings, and since the prinmary winding sees seconday load plus transformer loss, it should get more attention.

 

[prc]

MEUSA, Strictly speaking, MVA of primary and secondary are not equal.As per ANSI standard, primary MVA will be slightly more adding up losses and reactive drop.As per IEC standard, primary MVA denotes the rated MVA and secondary rating will be less.

 

[veritas]

Actually, HV and LV MVA at any point in time, for a loaded transformer, will hardly be equal – if accurate measurements are taken. Firstly the %Z is a contributing factor, this being mainly inductive generally speaking. Secondly, is the load and its power factor.

Another consideration though, is the action of the tapchanger. By way of example, consider a 33/11kV, 28MVA transformer (sorry I live in the IEC world so ANSI voltage levels do not readily come to mind). Nominal tap = tap7 = 33000 to 11000. Turns ratio is 3.

Maximum tap = tap 17 and corresponding nameplate voltage is 28875kV, i.e. turns ratio is now 2.625.

Now suppose the LV is loaded to rated capacity, i.e 28MVA. ILV = 1470A (assuming the LV voltage is about nominal voltage). IHV = 560A. HV voltage is at system voltage which is 33kV. Thus MVA in HV winding is 1.732 * 560 * 33kV = 32MVA. 4MVA is thus lost in the transfer of power from HV to LV! Admittedly this is a hypothetical question and in reality the actual discrepancy will be determined by %Z, power factor of %Z, tap position, power factor of load, etc. It may not be as much as 4MVA but 1MVA would be quite possible.

Regards.

 

[prc]

Veritas,When the tap is at 17, then the system voltage will be corresponding to that tap ie 28.875 kV in case of CFVV( constant flux voltage regulation, tap changing to keep secondary voltage same during variable primary voltage).But in case you are using in VFVV mode( variable flux voltage regulation to increase secondary voltage for constant primary voltage)then the current on primary will remain same for 33KV. So the reduction in MVA is only due to voltgae regulation due to reactance and losses.

 

[veritas]

Hi prc

I've tested this with two engineering software packages (CAPE and Powertools) a while ago and it confirms what I explained.

The HV (source)voltage is not a function of the tap position but the source. LV voltage (load end) is regulated to within a certain deadband by the AVR. current and subsequently the MVA is a product of the volts and amps on each side, where the amps must differ by the turns ratio.

 

[cranky108]

Not to rain on anyones party (we sure need some rain), there are times when to much information is just confusing.

Yes it is possible to exceed the ratings of the LTC before you exceed the ratings of the transformer. Or if the NLTC is not on the center tap. This is a minor point as most transformer peak loading is less than 4 hours.
Even on generator step-up transformers can operate over ratings for some time before there is a problem, or significent loss of life (Only happens if the generator and turban are also overloaded).

A good maintenance program can extend the life of a transformer by years, which is what should happen if the transformer is at an attended facility. And if not, it is likely a smaller transformer where the cost of a program can't be justified.

 

[prc]

Veritas, I stick to my position based on my experience on transformers at factory and at site. Software will give results based on programming.Please see Annexure B Examples of Specifications for transformers with tappings of IEC 60076-1(2011)Specifications for Power Transformers_General