Power Factor metering 1

[SilverArc]

Hi All,

Here are few numbers which I got from a client metering on the secondary side of two 44KV/4160 V transformers T1 and T2.

T1; P= 3 MW
Q= -5 MVAR
S= 6 MVAR
P.F= 52.2 %

T2 P= 2.2 MW
Q= .836 MVAR
S= 2.3 MVA
P.F= -94 %

T1 means, 5 MVAR is the reactive power being absorbed from utility.

T2 means 839 KVAR is being fed back to the utility.
This is a huge mine and full of induction machines and T2 has a 1000 KVAR cap. bank.

I think the meter is not wired properly. I spoke to ION metering tech support and they advised that PF sign -ve means lagging and +ve means leading as per meter conventions.

If I follow the sign conventions that means 5 MVAR is being fed back to utiliy.

How would you guys interpret this meter reading.

Regards

 

[unclebob]

May be (I hope) it is not wired properly. But, if it is, you are correct. You 're producing 5 MVAR!

 

[stevenal]

Look at the per phase values. The per phase pfs should be about the same. IONs with displays can show the phasors. Look for roughly 120 degree displacement for the three voltages. Same for currents.

 

[stevenal]

Note: If configured for a three wire system, the per phase pf, kw and kvar quantities are not available. You can also use the symmetrical components to check your wiring. If negative or zero sequence quantities exceed the positive, you have a wiring issue.

 

[dpc]

By definition, if watts are going one direction and vars are going the other, power factor is leading. Per IEEE definitions for metering, power flow and var flow from source to load is positive, so the kW readings seem correct, but the kVAR readings seem wrong.

Agree with stevenal that your best bet is to look at the phasors or at least magnitudes and angles on the ION meter. High negative sequence current is a good indicator of a wiring problem. There are about 27 ways to connect the CTs and PTs to a three-phase meter, but only one is correct. Be prepared to possibly swap CTs phases as well as polarity.

 

[jghrist]

Check the meter instruction manual. I suspect that ION technical support is wrong. Usually, a negative pf number means leading.

 

[stevenal]

Negative is lagging in the Power Measurement Ltd meters. Now part of Schneider.

 

[JBinCA]

Some meters will allow you to define which quadrants are displayed as positive and which are negative for kW and kVAR. Is such the case with your meter? I can't recall if the Ion meters I've worked with have this setting or not.

 

[stevenal]

The ION allows each measured quantity to be "normal" or "inverted," so individual polarities can be adjusted. Okay for wholesale quadrant change, but a kinda messy way to correct a single CT polarity problem. Better to go back and make sure the drawings were correctly followed. Could use the feature for a quick test, though.

 

[JBinCA]

The IONs I've worked with allowed me to invert each CT or PT input individually if necessary (They were 7350's, I think). Is this what you're referring to? I always assumed that this was to allow for troubleshooting a reversed phase. Changing all current input polarities would have the effect of reversing the definged direction of positive kW though.

 

[waross]

Reality check.
Given that T2 has 1000 KVAR of capacity, and you are showing a leading power factor, subtract 1000 KVAR.
That would give you the load on T2 drawing 0.161 MVAR and running at a power factor of 0.997%.
This does not seem reasonable for an industrial environment.
Now consider that the power factor is still lagging.
The total reactive draw of the load would be 1.836 MVARs.
The uncorrected power factor would be 76.8%
This seems reasonable for an industrial environment.
I think that you have something reversed on T2.
Can you talk to accounting and correlate your totals with the utility readings? My first question when dealing with KWs and KVARs is
"Please ask the accounting department to bring to us the power bills for the last 6 months. The last 12 months will be much better if they have time".
respectfully

 

[SilverArc]

Thanks everybody for their comments. This is an ACM3720 meter model and the tech support guys from ION asnwered my question after carefully looking at the manual.

How can I find out that there is an issue with wiring by examining the negative sequence or zero sequence component of current.
How can wiring connection lead to an increased magnitude display.
Could you kindly elaborate.

Regards,

 

[JBinCA]

SilverArc,

I'll give you some real world advice from a field guy.

1. There is typicall only one right combination of meter set-up, programming, and wiring for a given system. Unfortunately there are typically dozens of wrong combinations (I'm still adding to my list!)

2. In a relatively balanced system, you'd expect very high positive sequence voltages and currents, and very low negative and zero sequence voltages and currents.

2a. If you had a roll in voltage or current inputs, your indication of this might be high positive sequence voltages (PT's wired correctly) with high negative sequence currents (two phases of current inputs are swapped).

2b. Any other problem that resulted in unbalanced voltage and current phasors shoul show up in the positive and negative sequence calculations.

3. As a general rule of thumb, I just assume that wiring or settings problems can cause an error in any calculated quantity in a mircoprocessor based meter. Differences in algorithms for calculations used in each meter account for this. Some assume a particular phase rotation. Most won't calculate per phase values on delta systems, some do. Some let you define PT/CT polarity. Some let you define positive and negative Watt / VAR directions. Some will even let you define PT and CT ratios per phase. All of this results in a multitude of opportunities to make a mistake.

Hope that helps.

JB

 

[LionelHutz]

per Waross's post if a negative power factor means lagging then the T2 readings seem corrrect.

This means the T1 readings are indicating a large capacitive load. The only time I've seen reading like that is with the CT's or PT's improperly connected.

If the T1 readings are correct then there must be a lot of unloaded motor load on T1.

 

[stevenal]

The 3710 (and 3720 also I assume) has the per phase values available from the front panel. See that kvar has the same sign and approximately the same magnitude on each phase. Same for kW. Four wire wye system is assumed and properly set in meter. (you can help us here).

As far as what the result of each wiring error? Too many possibilities, and too little time. The per phase checks and symmetrical component checks let you know if you have a wiring problem or not. Proceed from there.