Transformer Rated MVA NOT Actual Output Capacity
Transformer Rated MVA NOT Actual Output Capacity
(OP)
Hi all,
In industrial systems involving secondary selective systems (two transformers), we normally size the one transformer to be able to take up all the load on the entire bus with the bus tie breaker closed and other incoming transformer out of service. In addition, we apply a 10% spare capacity on the transformer OA (base rating).
I was informed by one manufacturer that the way transformers are designed on their end is based on the rated current and rated voltage. So if we specify transformer MVA and voltage ratings, they will calculate the associate current and design the windings and insulation (voltage based). However, I also got informed that transformer losses and voltage drop (impedance) is also inclusive. Therefore if I change my tap changer (and change transformation ratio and associated voltage rating), there is a case were the current may exceed the rated current on the primary. Hence, in actual design and actual operating conditions, to maintain MVA loading on the primary windings, the seconday MVA loading should be reduced (and hence not an ideal equal primary and secondary MVA rating due to load power factor and current rating).
Can someone make comment or better explain this as I have been made to believe that for say a transformer with 10/13.33MVA OA/FA rating, I am guaranteed of 13.33 MVA capacity.
In industrial systems involving secondary selective systems (two transformers), we normally size the one transformer to be able to take up all the load on the entire bus with the bus tie breaker closed and other incoming transformer out of service. In addition, we apply a 10% spare capacity on the transformer OA (base rating).
I was informed by one manufacturer that the way transformers are designed on their end is based on the rated current and rated voltage. So if we specify transformer MVA and voltage ratings, they will calculate the associate current and design the windings and insulation (voltage based). However, I also got informed that transformer losses and voltage drop (impedance) is also inclusive. Therefore if I change my tap changer (and change transformation ratio and associated voltage rating), there is a case were the current may exceed the rated current on the primary. Hence, in actual design and actual operating conditions, to maintain MVA loading on the primary windings, the seconday MVA loading should be reduced (and hence not an ideal equal primary and secondary MVA rating due to load power factor and current rating).
Can someone make comment or better explain this as I have been made to believe that for say a transformer with 10/13.33MVA OA/FA rating, I am guaranteed of 13.33 MVA capacity.






RE: Transformer Rated MVA NOT Actual Output Capacity
The current limit is based on heating of the windings and the cooling system.
The voltage is the applied voltage and if the voltage changes the MVA changes.
I have had similar issues with generators. A supplier sold one of my customers a diesel generator with the KVA based on 240 Volts. Unfortunately the load was 120/208 Volts. Replacing the set was not an option. We had to drop off some load when on generator power.
In your case, check with the manufacturer. The primary winding winding may be able to carry the increased current associated with full load and reduced primary voltage.
Bill
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"Why not the best?"
Jimmy Carter
RE: Transformer Rated MVA NOT Actual Output Capacity
We typically size double-ended substations such that one transformer can carry the expected maximum demand load using forced cooling with minimal loss of transformer life.
dpc
RE: Transformer Rated MVA NOT Actual Output Capacity
If you really need the full specified MVA under all conditions, put it in your specification that the transformer must be capable of delivering rated MVA even on the lowest tap setting.
It is non-standard, and you will likely pay more, but you are the customer.
RE: Transformer Rated MVA NOT Actual Output Capacity
Whenever a transformer is provided with taps from a winding for de-energized operation, they shall be full-capacity taps. Transformers with load tap-changing equipment may have reduced capacity taps, unless specified otherwise, for taps below rated winding voltage. When specified, other capacity taps may be provided. In all cases, the capacity shall be stated on the nameplate.
RE: Transformer Rated MVA NOT Actual Output Capacity
As already pointed out, the reality is that the transformer does not have a winding current limit where it lives a good healthy life below that current and fails immediately above that current. The limitations of the transformer depend on a number of variables and how much you're willing to trade some of the expected tranformer life for the output you require.
Spec the transformer to be capable of rated power on all voltage taps to help avoid this.
RE: Transformer Rated MVA NOT Actual Output Capacity
As he said, you cannot get 1250kva out of a 1250 kva tranformer!
This is because the rated Kva is determined by the product of rated no load voltage and the rated line current.
As on full load the voltage reduces, the rated line current stays the same, the permissible kva reduces since temperature rise (as specified in the relevant standard) depends on the current, not the voltage.
Simple arithmetic.
I have been in power generation for 40+ years and never realised this
RE: Transformer Rated MVA NOT Actual Output Capacity
As per IEC standards, rated MVA of a transformer is the input power.As per ANSI/IEEE standard, rated MVA is the out put power from transformer.So efficiency calculations also vary accordingly.
In auto transformers it is possible to have a loading of 100/100/25 MVA simultaneously, provided tertiary load is capacitive or inductive.Eventhough primary rating will not exceed 100 MVA ,one of the windings(series or common winding) will carry more current than when tertiary load is zero.
As per standards, full MVA tappings are required up to minus tap of 10to15%.Most of the customer specifications do demand full rating tappings at ALL taps,irrespective of range in the negative zone.