Power Factor Across Transformer

[adamscb]

Hi All,

Technical question here - I work at an industrial plant in which we have a power factor measurement coming into our computer network. I asked the electrical utility for their measurements of power factor, just to see if ours are accurate. It turns out that our power factor readings were consistently 7-8% higher than the utility's. I believe this is due to the fact that our metering is on the 69kV side of the transformer, while the utility measures at the low side (13.8kV). My belief is that the power loss in the transformer is why our power factor is slightly lower on the load side of the transformer. Has anyone experienced this? Thoughts? Thanks in advance.

 

[publicgood]

My belief is that the power loss in the transformer is why our power factor is slightly lower on the load side of the transformer.

Yes, exactly right. There will be kvar losses; hence, change in power factor. You may calculate this if you know the impedance and % loading.

 

[Ingenieur]

S = P + jQ
pf = cos(arctan Q/P)
assume a primary pf = 0.9 lagging or inductive
positive P
negative Q
the xfmr will absorb reactive power Q or go more negative
that will increase the angle between P and Q
as angle increases the cos decreases
thus decreasing the pf

so 2 factors
depends on the xfmr x/r and pu Z

 

[Sahib]

If your utility does not care about power loss in the transformer, then their power factor reading on low side of the transformer may not be a disadvantage to you really. But hold on, the power factor on high side will always be lower than that on low side, contrary to what you are stating!

 

[topgone]

If your utility does not care about power loss in the transformer, then their power factor reading on low side of the transformer may not be a disadvantage to you really. But hold on, the power factor on high side will always be lower than that on low side, contrary to what you are stating!

The OP didn't say his readings are lower than the utilities. Read again.

 

[gar]

170627-2514 EDT

If we assume the load looks like a series inductance and resistance with a moderately high power factor (meaning approaching 1) (this also means the inductance is small compared to resistance), and a transformer is connected before the load input, then we need to look at the LR series equivalent circuit of the transformer in relation to the load LR to determine which way the power factor will go at the the transformer input.

My guess is that input power factor to the transformer will be less than the power factor into the load.

.

 

[Sahib]

The OP didn't say his readings are lower than the utilities. Read again.

I did not say he did either (although in last part of his OP, he really did).

 

[Julius Right]

I agree with Sahib. I think the Utility measured the power factor at primary and the client at secondary in his main switchgear of 13.8 kV.

 

[Smart $]

I agree with Sahib. I think the Utility measured the power factor at primary and the client at secondary in his main switchgear of 13.8 kV.

But the OP says he's measuring 69kV side 7-8% higher than POCO's measurement on 13.8kV side. I read his post before anyone answered and I'm still uncertain which side is primary. :blink:

 

[rian0201]

But the OP says he's measuring 69kV side 7-8% higher than POCO's measurement on 13.8kV side. I read his post before anyone answered and I'm still uncertain which side is primary. :blink:

i agree, and its unusual for me that they are comparing readings at two different points which obviously results to different answers..


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[publicgood]

Who owns the transformer?

 

[Jraef]

Who owns the transformer?

This speaks to the issue of "Who cares?"

If the utility owns the transformer and is measuring the PF on the load side (presuming that is the 13.8kV), then that is all you, as the end user, should care about. The PF on the line side of that transformer is the utility's problem, not yours. There would be no reason for you to bother measuring it.

If on the other hand YOU own the transformer and the utility is billing you at 69kV, then the PF at the 69kV side is indeed your problem. But I find it odd that the utility would measure the PF on the load side of YOUR transformer because if you own it, THEY don't care.

 

[Ingenieur]

He said higher on the 69
lower on the 13.8
assume the 13.8 to be the load?
as expected

and x/r > 10 usually so inductive reactance is the main loss factor

 

[adamscb]

I guess I should clarify some things. This step-down transformer is owned by us, and the high-side voltage is 69kV, and the low-side voltage is 13.8kV. The local utility measures on the low side (why I don't know), and then applies a correction factor to include the transformer losses (this factor is above 1.0). We have metering on the 69kV side (this is just for us to know how much of a load we're drawing), and the power factor we show on the high side is consistently higher than on the low-side. This still baffles me, because I would think the power factor would increase on the low side of the transformer, because the kVAR's are decreasing.

Why we care about our power factor measurement, is that we're doing a cost-saving analysis on how much money we would save by commissioning our new substation, rather than relying on our old one.

 

[Jraef]

I guess I should clarify some things. This step-down transformer is owned by us, and the high-side voltage is 69kV, and the low-side voltage is 13.8kV. The local utility measures on the low side (why I don't know), and then applies a correction factor to include the transformer losses (this factor is above 1.0). We have metering on the 69kV side (this is just for us to know how much of a load we're drawing), and the power factor we show on the high side is consistently higher than on the low-side. This still baffles me, because I would think the power factor would increase on the low side of the transformer, because the kVAR's are decreasing.

Why we care about our power factor measurement, is that we're doing a cost-saving analysis on how much money we would save by commissioning our new substation, rather than relying on our old one.

OK, so if you own the transformer, then yes you would be concerned with the high side PF.

If I had to guess at the reason, I'd think it likely that there is some distortion power factor on the 13.8kV side caused by harmonics, and the harmonic current is circulating in the delta primary windings of that transformer, so it isn't showing up as distortion PF any more on the HV side, it just shows up as additional thermal losses.

 

[Ingenieur]

Does utility charge a penalty based on pf?
if so they want to start as low as possible lol

 

[Jraef]

Does utility charge a penalty based on pf?
if so they want to start as low as possible lol

What? Do you mean to imply that a utility might try to manipulate their revenue stream by carefully planning where they place their meters? Unheard of!

:lol:

 

[topgone]

I guess I should clarify some things. This step-down transformer is owned by us, and the high-side voltage is 69kV, and the low-side voltage is 13.8kV. The local utility measures on the low side (why I don't know), and then applies a correction factor to include the transformer losses (this factor is above 1.0). We have metering on the 69kV side (this is just for us to know how much of a load we're drawing), and the power factor we show on the high side is consistently higher than on the low-side. This still baffles me, because I would think the power factor would increase on the low side of the transformer, because the kVAR's are decreasing.

Why we care about our power factor measurement, is that we're doing a cost-saving analysis on how much money we would save by commissioning our new substation, rather than relying on our old one.

Normally, the utilities' meter should be at the primary if they don't own the transformer! They should not care what's done beyond that point. Yours is a special case, I think.

 

[Sahib]

If I had to guess at the reason, I'd think it likely that there is some distortion power factor on the 13.8kV side caused by harmonics, and the harmonic current is circulating in the delta primary windings of that transformer, so it isn't showing up as distortion PF any more on the HV side, it just shows up as additional thermal losses.

Most unlikely, as POCO usually measure and bill for regular displacement power factor only. My guess is POCO/OP meter is defective and a calibration check may be done.

 

[mivey]

I guess I should clarify some things. This step-down transformer is owned by us, and the high-side voltage is 69kV, and the low-side voltage is 13.8kV. The local utility measures on the low side (why I don't know), and then applies a correction factor to include the transformer losses (this factor is above 1.0). We have metering on the 69kV side (this is just for us to know how much of a load we're drawing), and the power factor we show on the high side is consistently higher than on the low-side. This still baffles me, because I would think the power factor would increase on the low side of the transformer, because the kVAR's are decreasing.

Why we care about our power factor measurement, is that we're doing a cost-saving analysis on how much money we would save by commissioning our new substation, rather than relying on our old one.

Your thinking is correct for normal loads.

The power factor on the 69 kV side should be lower and depends on transformer losses and impedance, loading, and load power factor. For a higher amount of per unit load, I would expect it to be in the neighborhood of a percent lower or less but no more than 2-3 percent for most any normal case except extremely light loading.

So your readings are not what you would expect for routine loads.

What size station and what type and size load?