Negative Power Factor 2

[RSTEng1]

Observing power factor on PQM at one of the sewage pumping station we noticed that power factor become negative (≈-0.85) when one of the pumps is running.
Assuming that some capacitive load in the VFD motor circuitry exist, we can accept this situation.
When there is no pump running pf is 0.98 with 3Φ 6A current and 600V which is correct.
But when the pump is running in bypass mode, bypassing VFD, pf become negative, again (-0.97).
Can you, please, try to explain this to me? Thank you.

 

[davidbeach]

Power factor, by definition, is always positive. So you’re going to need to understand what the meter means. A capture of the phasors would be useful.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[oldfieldguy]

I too would like to see vectors.

In the normal world most VFD's exhibit near unity power factor, so you'd expect that when the majority of your load is on VFD, the power factor would be near unity, and when you bypass the VFD, you're back in the world of induction motors, where a power factor of 0.85 would be pretty common.

I'd venture a guess that either your instrument transformers are incorrectly connected of you have an error in your configuration of the PQM.

old field guy

 

[7anoter4]

In my opinion you have 150 degrees between voltage and current measured.
Actually, the R current at 0.866 power factor and B voltage will present a 150 degrees angle that means -0.866 power factor.

 

[xnuke]

There are two defined power factor sign conventions, IEC and IEEE, and many meters allow selection of a convention to attach a sign to the power factor. This link shows the difference.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[davidbeach]

Wow. And here I’ve made it through nearly an entire career believing what I learned working on my Bachelors Degree - power factor is the ratio of the magnitude of real power to the magnitude of apparent power. Since both magnitudes are, definitionally, positive, it seems likely that the result will be positive.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[RSTEng1]

Thank you, everyone.
I was on site yesterday and this is what I've captured from the PQMII screen:
Van= 357V at 0 Lag
Vbn= 353V at 240 Lag
Vac= 355V at 120 Lag
Ia = 14A at 334 Lag
Ib = 13A at 207 Lag
Ic = 12A at 95 Lag
Thank you.

 

[waross]

You may have your CT connections reversed.
A leading power factor with no load and a lagging power factor when loaded makes more sense.
What do you have that will push the PF leading at no load?
By the way, 334 degrees lagging is close to 26 degrees leading, for what it's worth.

Power factor, by definition, is always positive.

No argument there. But on a practical level, VARs are directed.
If VARs and kW are used to calculate the PF, then PF does become directed.
I suspect that showing PF as positive or negative is an indication of a leading or lagging PF.
The manufacturer is saving space on the display of the Power Quality Monitor.
Please take this as a friendly suggestion David. No disrespect is intended.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RSTEng1]

Thank you, waross.
In fact, leading power factor is with load.
The captured values are with pump running.
Unfortunately I haven't captured exact power factor number, but it was leading.
Thanks again.

 

[davidbeach]

Leading and lagging seem less ambiguous. Another place where IEC and IEEE disagree for no real reason. Didn’t know those definitions, but I think I like the IEC one better since it is the “power” factor. There is a similar reaction factor and if it’s going to be signed it should then match the sign of the vars.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

In fact, leading power factor is with load.

If a CT is reversed, that would be;
"In fact, indicated leading power factor is with load."
If you have a switchable capacitor anywhere on the system, switch it and see how the indicated PF reacts.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[itsmoked]

I suspect that showing PF as positive or negative is an indication of a leading or lagging PF.

Like your thinking Bill.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[xnuke]

Wow. And here I’ve made it through nearly an entire career believing what I learned working on my Bachelors Degree - power factor is the ratio of the magnitude of real power to the magnitude of apparent power. Since both magnitudes are, definitionally, positive, it seems likely that the result will be positive.

IEEE Standard 100 has this as the definition of arithmetic power factor and definition (B) of phasor power factor. It's pretty much the basis of the IEC power factor sign convention based on the fundamental frequency. However, remember that real power isn't always positive -- it's defined to be negative per passive sign convention when the device is a source. The IEC power factor sign convention takes this into account, so loads always have a positive power factor sign as they have positive real power and sources always have a negative power factor sign as they have negative real power. This sign convention agrees with textbooks that define power factor as the cosine of the load impedance angle, since in Quadrants I and IV, which both have positive real power, the cosine of any angle is positive. In Quadrants II or III, which both have negative real power, the cosine of any angle is negative.

The IEEE power factor sign convention seems to be based on the relationship between the fundamental voltage phasor and fundamental current phasor. IEEE convention appears to use IEEE Standard 100 definition (A) of phasor power factor to determine the power factor sign, which is "defined by the cosine of the phasor angle between the fundamental sinusoidal phase voltage and the fundamental sinusoidal phase current." If the current phasor lags the voltage phasor (an inductive/lagging load), the angular difference of current with respect to voltage is defined to be negative, and the IEEE convention states the power factor is negative. If the current phasor leads the voltage phasor (a capacitive/leading load), the angular difference is positive, and the IEEE convention states the power factor is positive.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Palletjack]

I would think if a rope CT is backwards that would be caught when the initial install is done.
You should always check the phasors before you walk away and are recording anyway.
It would be great to actually see the display and the phasors.