Transformer vs power factor.

[Hameedulla-Ekhlas]

Greeting all,

I had a discussion with my friend regarding the effect of transformer on power factor.

He was insisting " Transformer effects the power factor because transformer does not have 100% efficiency and because the effect is too low that's why it is always ignored"

I was agree with him but I found in a book the below formula which shows it does not effect at all.

Any good comment and explanation.

 

[charlie b]

I think you are familiar enough with the topic that I need not try to explain. So let me simply point out the significant, though well-hidden, change in terminology that takes place within the book?s formula. In the first formula, they use the word ?power.? In the second formula, they use the word ?power? and the phrase ?volt amperes.?

Efficiency has nothing to do with power factor. It is all about energy in and energy out. Power factor is all about the relationship between what they call ?power,? what is more precisely called ?real power,? and VA.

 

[rcwilson]

A transformer has kw and kvar losses when it is under load so power factor in does not equal power factor out.

A 100 KVA transformer with Z=5% will have about 5 kvar losses at full load.

(Kvar loss = I^2 x X. In per unit, at 1.0 load, I = 1.0, X = .05, Kvar = (1x1) x0.05 =0.05 per unit. 0.05 x 100 KVA = 5 kVAR).

Typical losses for a 100 kVA transformer is 2 kW at full load.

Assume the transformer is feeding a 100 kW load, power factor = 1.0. (100% loaded)

Power into the transformer will be 102 kW and 5 KVAR to cover the losses. (102.12 KVA)

That's power factor = 0.9988. So the transformer appears to change the load's power factor.

This effect gets more pronounced at high voltages where a transformer may have an effective impedance of 18%-20%. I have had many generating plant operators get suprised when they run the generators at 0.9 power factor but the power delivered to the utility is only 0.92 or 0.95 power factor due to transformer MVAR losses.

 

[winnie]

I believe that Charlie B nailed the issue: the equations are written in terms of output power and efficiency, but just end up expressing the definition of power factor.

First equation 15, which writes efficiency as 'output power / (output power plus losses)'...well 'output power plus losses' is simply another way of saying 'input power'.

Similarly, equation 16 writes power factor as '(output power plus losses)/ input volt-amperes', which is simply another way of saying 'input power/input volt-amperes'.

The equivalent circuit of a transformer plus load may be approximated by placing a series resistance and a shunt inductance between the supply and the load. For a normal distribution transformer, the transformer equivalent resistance will be quite a bit lower than the load resistance, and the 'magnetizing impedance' of that shunt inductance would be quite high...for normal loading the input power factor will be very similar to the load power factor. On the other hand, during abnormal circumstances such as a short circuit, the transformer impedance and power factor might dominate.

-Jon