Impact of low load power factor on transformer capability

[Distribution73]

Hello,
Whilst going through the transformer standard requirements stated in IEEE C57.12.00 I have found the following statements related to their capability:
"Transformer shall be capable of [...] operating above rated volts or below frequency, at maximum rated kVA" when:
- The secondary voltage and volts/hertz do not exceed 105% of rated value.
- Frequency is at least 95% of rated value.
- Load Power factor is 80% of higher.

I can understand the impact on the transformer capability of an increase in V or a reduction in frequency. However it is not that clear to me how a low load power factor has an impact on the capability of the transformer, considering that it is rated in kVA and not in kW.
Has it got to do with its effect on internal voltage drop due to winding resistance / leakage inductance, resulting in higher core excitation levels?
Your help is much appreciated.
Thanks F.

 

[waross]

Compare the transformers rated regulation with the %impedance voltage.
Rated %regulation at a stated power factor is less than the % impedance voltage.
As the power factor drops below stated power factor, the actual % regulation drops from the rated % regulation towards the % impedance voltage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RRaghunath]

IEEE indicates transformer secondary voltage with transformer on load (unlike the IEC where no-load voltage is the transformer rated secondary voltage).
Hence, it is necessary to stipulate the load power factor.
Under poor power factor conditions, the transformer regulation will be higher than estimated during design and thus will affect the transformer secondary terminal voltage and consequently the output kVA.

 

[Distribution73]

Thank you very much Waross and RRagunath.
I think that I can see what you indicate. The lower power factor will result in an increase on transformer voltage drop (increase in regulation) due to the predominantly inductive nature of the XFMR series resistance.
I deduce then that the actual physical limitation on the ability of the transformer to operate in these conditions, will be derived from the fact that the transformer must be able to work a 105% rated secondary V at max rated kVA.
Hence, when we have low power factor loads, the HV side will be subject to excessive voltage that in turn will result in core overheating due to overexcitation. Is that in line with your view?
On a similar note, as such I haven´t found any requirement in terms of maximum regulation limits detailed in the IEEE standards ( I am no expert at all and I might have missed that though). Only guidelines on % voltage impedance voltage that is ultimately related to regulation (as waross has pointed out).
Thank you

 

[waross]

It often doesn't matter.
But, be aware that when a transformer is heavily loaded or overloaded with a poor PF load, the terminal voltage may be less than you anticipate.
Example:
Regulation = 3%
Impedance voltage = 5%
Motor full load = 80 KVA
Transformer size = 100 KVA (125%)
The transformer is loaded to 80%.
No load terminal voltage = 600 Volts.
Terminal voltage with motor running = 600V -(600V x 3% x 80%) = 585.6 Volts.
Terminal voltage when motor starts based on 3% regulation and 600% of 80 KVA = 600 = (600 x 3% x 80% x 600%) = 513.6 Volts.
Realistic estimation of terminal voltage based on the low power factor of a starting motor and a %imp of 5%.
600 - (600 x 5% x 80% x 600%) = 456 Volts.
The motor doesn't much care, but if you are calculating the voltage supplied to other equipment on the same transformer during motor starting, we see a terminal voltage that is 513.6V - 456 V - 57.6 Volts lower than anticipated.
That may be the most important issue when considering heavy loads with poor PF.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Hence, when we have low power factor loads, the HV side will be subject to excessive voltage that in turn will result in core overheating due to overexcitation. Is that in line with your view?

Close, but the effect is to push the seoondary voltage lower, not push the primary voltage higher.

the transformer must be able to work a 105% rated secondary V at max rated kVA.

Close again.
The full statement should be: " the transformer must be able to work a 105% rated secondary V at max rated kVA, "at a stated power factor"
When the actual power factor is less than the stated power factor, the transformer will not meet this condition.
The limit is the safe full load current.
At rated full load current and 105% terminal voltage, the transformer will deliver 105% of rated KVA.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Distribution73]

Thank you very much for the clarifications!