Leading PF

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ATSman

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
I was on a job as a rep for the ATS manufacturer along with the reps for the genset and UPS. Contractor was cutting over a second MSB and we were to each go thru the senarios during an outage and explain what should happen and what to do if things go wrong.
I noticed that the KW reading on the MX350 controller (GE Zenith) was negative, the KVA & KVARs were both positive values. First thing comes to mind is a reversed CT or reversed sec connection. I believe all 3 currents were reading positive but I am not positive:grin: The power factor was 0.8 leading. After discussion with the gen & UPS guys we concluded that the leading PF was caused by the lightly loaded UPS: 60KVA out of a total of 400kva (15%.) Total load #'s were:
-96 KW
123 KVA
77 KVAR
+0.8 PF leading
on top of this, the UPS load is the majority of the total building load which we believe is causing the high current THD of 16%.
Question: Can the leading PF cause the negative KW reading and if so, why?
I can't see that it would because that would mean we were co-generating and that was not the case. GE is checking into it, also.
Any input would be appreciated:)
I will also post this in another category.

Tony
 
mivey

mivey

Senior Member
kVA will always be positive. It is sqrt(kW^2 + kvar^2). The kvar has nothing to do with the power being positive or negative as it is on the Y-axis and power is on the X-axis.

The kW might be negative because you had reverse power flow. Was the UPS pushing power back through the metering point? Do you have a diagram or sketch of the layout?

add: If the voltage and current have the same sign, power is being received by the load. If the voltage and current have different signs, power is being delivered by the load.
 
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ATSman

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
mivey said:
kVA will always be positive. It is sqrt(kW^2 + kvar^2). The kvar has nothing to do with the power being positive or negative as it is on the Y-axis and power is on the X-axis.

The kW might be negative because you had reverse power flow. Was the UPS pushing power back through the metering point? Do you have a diagram or sketch of the layout?

add: If the voltage and current have the same sign, power is being received by the load. If the voltage and current have different signs, power is being delivered by the load.
Click to expand...

Mivey,
The 3 currents were positive, as I mentioned which makes me believe no reverse power flow existed (generator 32 relay would trip if this occured)
For a reverse power condition, one of the phase currents has to be negative(reversed.), correct?
Based on this, I thing it is a wiring error (it is a new install)

Tony
 
Smart $

Smart $

Esteemed Member
Location
Ohio
tonytonon said:
Mivey,
The 3 currents were positive, as I mentioned which makes me believe no reverse power flow existed (generator 32 relay would trip if this occured)
For a reverse power condition, one of the phase currents has to be negative(reversed.), correct?
Based on this, I thing it is a wiring error (it is a new install)

Tony
Click to expand...
I think it is a controller firmware error (i.e. internal calculation). First off, there is no such thing as a positive/negative power factor (IIRC, only values from 1 to theoretically 0... and power inversion would occur at less than 0.5). If a sign is applied to indicate leading or lagging power factor, and cascaded to the power calculation (in error), it very well could errantly show as a power inversion.
 
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C

Cold Fusion

Senior Member
Location
way north
Smart $ said:
...First off, there is no such thing as a positive/negative power factor (IIRC, only values from 1 to theoretically 0....
Click to expand...
I would have said that power factor goes from -0 (load is fully capacitive reactive) to +0 (load is fully inductive reactive) with +1 = -1 = fully real power.

Smart $ said:
......and power inversion would occur at less than 0.5). ...
Click to expand...
I don't get that one at all. pf = .5 occurs at E leads I by 60deg An example is an unloaded motor running free.

Or, I leads E by 60 deg, say a plant where the pf correction caps have been switched on, but the load has not yet been brought up.

Or, I don't know the definition of "power inversion"

cf
 
mivey

mivey

Senior Member
tonytonon said:
Mivey,
The 3 currents were positive, as I mentioned which makes me believe no reverse power flow existed (generator 32 relay would trip if this occured)
For a reverse power condition, one of the phase currents has to be negative(reversed.), correct?
Based on this, I thing it is a wiring error (it is a new install)

Tony
Click to expand...
Positive relative to what? Were these instantaneous readings? Readings referenced to a voltage angle? Readings from a scope? You will not get a positive negative indicator on a normal current meter for AC.

What is the configuration you are talking about? Do you have a sketch of the layout?
 
Smart $

Smart $

Esteemed Member
Location
Ohio
Cold Fusion said:
I would have said that power factor goes from -0 (load is fully capacitive reactive) to +0 (load is fully inductive reactive) with +1 = -1 = fully real power.


I don't get that one at all. pf = .5 occurs at E leads I by 60deg An example is an unloaded motor running free.

Or, I leads E by 60 deg, say a plant where the pf correction caps have been switched on, but the load has not yet been brought up.

Or, I don't know the definition of "power inversion"

cf
Click to expand...
Ooops!...my bad.

Power factor has the range of -1 to +1, the limits of the trigonometric cosine function. Power inversion (power flow changes direction) occurs when the power factor is less than 0. Yet the positive and negative indicators have no bearing on leading or lagging of voltage to current.

PS: There is no "signed" 0 value resulting from the cosine function.
 
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mivey

mivey

Senior Member
tonytonon said:
...generator 32 relay would trip if this occured
Click to expand...
Were you running on the generator or utility? Was it synched or running in island mode?

If the only thing tied was a single generator feeding the supply to a UPS load, and no other sources are involved, then it sounds like a polarity issue.

As for your first post, why would a negative kW indicate cogeneration? Where are the CT's located? They would probably only be metering power to the load. The generator could pump power to the utility all day and vice-versa and the CTs would never see it. Is there a transformer at the generator?
 
C

Cold Fusion

Senior Member
Location
way north
Cold Fusion said:
I would have said that power factor goes from -0 (load is fully capacitive reactive) to +0 (load is fully inductive reactive) with +1 = -1 = fully real power. ...
Click to expand...

Smart $ said:
...Power factor has the range of -1 to +1, the limits of the trigonometric cosine function. Power inversion (power flow changes direction) occurs when the power factor is less than 0. Yet the positive and negative indicators have no bearing on leading or lagging of voltage to current.

PS: There is no "signed" 0 value resulting from the cosine function.
Click to expand...

You're right, for all three cases. However is is extremely rare that one sets up the model such that the voltage/current has one in the first/fourth quadtant and the other in the second /third quadrant, which is what it would take to get the cos negative. I'm sure someone has done that - I've just never seen it. I think this is what you had in mind with, "(power flow changes direction) occurs when the power factor is less than 0."

Whoops, there was one. It was on a switchboard CB set up for a feeder, and connected to a generator. The numbers on the readout looked wierd, until you paid attention to it. But that is not a model that one would intentionally use.

But that wasn't the point.

Rather that 0 leading is the opposite end from 0 lagging. While .999 leading is very close to .999 lagging. Or one might say 1 leading is the same number as 1 lagging.

As an idiomatic expression, one often hears a leading pf refered to as negative - the vars are going from the load to the gen.

And I am thinking you knew all of this.:)

cf
 
Smart $

Smart $

Esteemed Member
Location
Ohio
Cold Fusion said:
...

And I am thinking you knew all of this.:)
Click to expand...
...and it all started because I believe someone in the programming stages of the OP controller, set the firmware up for a controller readout exactly as you mentioned... with a minus (–) to indicate a leading pf (though I've always wished it were the other way :D).

If the sign didn't get removed prior to "handshaking" the value onto the power stage, the power calculation could very well see the minus a as truly negative pf and calculate it as a power inversion.

Who knows, perhaps two different programmers worked on it, each there own stage of the calcs. Definitely hard to know for sure from here. Its the only viable explanation in my mind so far, and thus haven't considered any others... much ;)
 
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