Negative watts on one phase of a three phase wye system

[crawfelect]

We have a 4 section MCC, fed with 480 Volt, three phase, wye power. The MCC has a 480 - 240/120 volt, single phase transformer tapped off the MCC buss, (L1 and L2) to feed a lighting panel that feeds a couple receptacles and an RTU panel, (a minimal load). When the larger loads , the two pump motors are not running we see a negative 230 Watts on L2. All wiring has been checked, the orientation of the CTs have been checked, the power monitor has been changed out, and the system has been run on a generator to see if possibly the utilities had a phase shift or something? Can anyone explain what is going on, why do we see this negative watts on Line 2 of the 480 volt, three phase system?

 

[SG-1]

I think the meter is just seeing the current from L1 flow through the single phase transformer as positive watts. That same current must return to it's source using L2. The meter sees the return current on L2 as negative watts, since it is flowing in the opposite direction.

 

[rcwilson]

What type of meter and meter connection do you have: 4-wire, three element? 2-1/2 element? or 3-wire two element?

Some meter configurations read three phase power correctly but the display of per phase power shows a negative value on one phase. This confused us when connecting a Dranetz power analyzer to a circuit with three CT's and two VT's with VT's connected in broken delta. One phase would read negative power but the sum of the three phases was correct. The meter is looking at phase currents but phase-phase voltages. With certain power factors the math works out to showing a negative power on one phase.

 

[crawfelect]

Negative Watts on one phase of a three phase wye system

Negative Watts on one phase of a three phase wye system

Bob,

The power monitor is a AB 3000, which utilizes CTs on all phases including the neutral.

Thanks,
BC

 

[Besoeker]

Bob,

The power monitor is a AB 3000, which utilizes CTs on all phases including the neutral.

Thanks,
BC

Could it be a CT round the wrong way?

 

[ATSman]

Could it be a CT round the wrong way?

I think you mean wound but the possibility is unlikely.
Rec he double checks CT and voltage connection polarity.

 

[crawfelect]

I have checked the polarity of the CTs more than once and also the wiring. I suppose it could be possible the CT is miss marked? All current and voltage readings are accurate on all phases. When one or both pumps run, the negative watts goes away, and when we de-energize the lighting transformer the system becomes balanced, same watts on all phases, same amps and volts on all phases. Once you re-energize that lighting transformer the system becomes unbalance and we see a negative watts on L2 when both pumps are off line.

 

[SG-1]

The negative watts goes away when the pumps run because the -200 watt is over powered by the pump watts. Move the single phase connection from L2 to L3. This will prove if a CT is backwards on phase 2. I expect the -200W will move also.

I would expect if CT polarity were incorrect you would see -W on Phase 2 all the time.

 

[crawfelect]

Good suggestion, I'll try that,
thanks for the responce.

 

[T.M.Haja Sahib]

The negative watts goes away when the pumps run because the -200 watt is over powered by the pump watts. Move the single phase connection from L2 to L3. This will prove if a CT is backwards on phase 2. I expect the -200W will move also.

I would expect if CT polarity were incorrect you would see -W on Phase 2 all the time.

I want to bring to your kind notice one detail:when a motor is started through an auto transformer,during initial low voltage,the second watt meter reads negative many a time.I think there may be a link between the two.

 

[SG-1]

I want to bring to your kind notice one detail:when a motor is started through an auto transformer,during initial low voltage,the second watt meter reads negative many a time.I think there may be a link between the two.

That is very interesting. I hope to experiment with some diferent watt meters here in the near future. Usually I do not monitor the watt readings while checking the meter or a relay with watt functions. I usually just check each phase of current & voltage by single phasing the device, then check the 3-phase watt reading to ensure the current & voltage are in phase with each other.

Now I am curious about what happens with the single phase watt readings as I cycle through the various settings. It may be a few days before that happens.

 

[ggunn]

I have checked the polarity of the CTs more than once and also the wiring. I suppose it could be possible the CT is miss marked? All current and voltage readings are accurate on all phases. When one or both pumps run, the negative watts goes away, and when we de-energize the lighting transformer the system becomes balanced, same watts on all phases, same amps and volts on all phases. Once you re-energize that lighting transformer the system becomes unbalance and we see a negative watts on L2 when both pumps are off line.

Swap a couple of the CT's and see if the anomaly follows the CT.

 

[crawfelect]

Everyone,

To begin with , there is minimal amount of load on the system when the pumps are not running, just the RTU cabinet, and the power monitor reads as such: L1 =758.5 Watts, L2 = (-)241.9 Watts, L3 = 0 Watts, L1L2L3 = 516.6 Watts, the true power factor readings are L1 =(-)75.1, L2 = 25.2, L3 = 0, L1L2L3 = (-)24.4,

so I swapped the leads of the lighting panel transformer from L1-L2 to L1-L3. The readings now became L1 = (-)125.2 Watts, L2 = 0 Watts, L3 =650.9 Watts, L1L2L3 = 520.5 Watts, the true power factor readings became, L1 =14.02, L2 = 0, L3 = (-)75.1,
L1L2L3 = (-)30,

I then swapped the leads of the lighting panel transformer from L1-L3 to L2-L3. The readings now became L1 = 128.4 Watts, L2 = 644.8 Watts, L3 = (-)230.1 Watts, L1L2L3 = 542.2 Watts, the true power factor readings read L1 = 87.4, L2 = (-)68.6, L3 = 28.0, L1L2L3 = (-)28.01,

In conclusion; I have ruled out the possibility that the CTs are the wrong polarity, and I would say that there is a diffinate role in the connection of the lighting panel transformer, when the lighting panel transformer is de-energized the wattage readings are L1 =129.1 Watts, L2 = 0 Watts, L3 = 0 Watts, L1L2L3 = 131.0 Watts,

I can't say that I have payed much attention to the wattage on a power monitor in the past, but is this normal? Could there be anything wrong with the lighting panel transformer, something in the windings, or possibly something wrong with the panel or am I just seeing a phase shift. We are going from a 480 volt, 3 phase wye system, tapping off to a 480 - 240/120 single phase transformer, which must be playing a roll in a phase shift resulting in a negative wattage reading.

Most importantly, throughout this whole experiment or troubleshooting the amps and the volts have been right on exactly the same as what I read with my ampmeter and voltmeter.

Any thoughts or comments?

Thanks,
BC

 

[jim dungar]

If the meter is using the Neutral as its reference point for its calculations, then based on your loading the negative watts readings are believable.
The meter is calculating based on the math of Vln = -Vnl. But effectively you have a single current flowing out of the L1 CT and into the L2 CT, giving the appearance that the current is 180? out of phase with the voltage.

Since you have verified the current and voltage connection are correct, just ignore the negative readings.

 

[ATSman]

Minimum Load Requirement

Minimum Load Requirement

Everyone,

To begin with , there is minimal amount of load on the system when the pumps are not running, just the RTU cabinet, and the power monitor reads as such: L1 =758.5 Watts, L2 = (-)241.9 Watts, L3 = 0 Watts, L1L2L3 = 516.6 Watts, the true power factor readings are L1 =(-)75.1, L2 = 25.2, L3 = 0, L1L2L3 = (-)24.4,

so I swapped the leads of the lighting panel transformer from L1-L2 to L1-L3. The readings now became L1 = (-)125.2 Watts, L2 = 0 Watts, L3 =650.9 Watts, L1L2L3 = 520.5 Watts, the true power factor readings became, L1 =14.02, L2 = 0, L3 = (-)75.1,
L1L2L3 = (-)30,

I then swapped the leads of the lighting panel transformer from L1-L3 to L2-L3. The readings now became L1 = 128.4 Watts, L2 = 644.8 Watts, L3 = (-)230.1 Watts, L1L2L3 = 542.2 Watts, the true power factor readings read L1 = 87.4, L2 = (-)68.6, L3 = 28.0, L1L2L3 = (-)28.01,

In conclusion; I have ruled out the possibility that the CTs are the wrong polarity, and I would say that there is a diffinate role in the connection of the lighting panel transformer, when the lighting panel transformer is de-energized the wattage readings are L1 =129.1 Watts, L2 = 0 Watts, L3 = 0 Watts, L1L2L3 = 131.0 Watts,

I can't say that I have payed much attention to the wattage on a power monitor in the past, but is this normal? Could there be anything wrong with the lighting panel transformer, something in the windings, or possibly something wrong with the panel or am I just seeing a phase shift. We are going from a 480 volt, 3 phase wye system, tapping off to a 480 - 240/120 single phase transformer, which must be playing a roll in a phase shift resulting in a negative wattage reading.

Most importantly, throughout this whole experiment or troubleshooting the amps and the volts have been right on exactly the same as what I read with my ampmeter and voltmeter.

Any thoughts or comments?

Thanks,
BC

I recall pulling my hair out on a similar problem with a GE-Zenith MX350 processor on an ATS. It is a controller and power monitor. Once we got the CT polarity corrected which can cause negative power readings, we still had negative readings on one phase of a 3 phase system. We did everything, measured all primary and sec CT currents, replaced micro & display, etc. till we found out that the micro required a min 15% loading before it would display the correct positive values. The solution was to turn on more pumps fed from the ATS and all the readings fell into place. Try adding more load (at least 20-30% of rated.)
What type of power monitor is it? Last resort, talk to manufacturer's applications engineer to confirm if min loading is the problem.

 

[SG-1]

The power meters I have programmed all required that one selects either a 3-phase or 1-phase system. When your only load is that transformer it is trying to measure a single phase system, but is programmed for a 3-phase system. As an experiment re-program the meter for single phase & see if it reads the transformer load correctly.

tonytonon brings up a good point about minimum loading. The bushing type CTs we use must be loaded to about 10% ( I think 10% is correct, without checking ) to reach the published accuracy.

I will try some meters as fortune permits, but I have been busy with customers for the last few days.

 

[crawfelect]

That is exactly what we see. When one of the pumps come on, the power corrects itself and the negative goes away. We'll have to try reprogramming as a single phase monitor just to see what happens.

Thanks,
BC

 

[SG-1]

There may be a setting in the 3-Phase menu to only display watts as positive.

I had a chance today to play with an Eaton PX8000. I could not get it to display negative watts even by single phasing the current inputs. I will try to post some pics later tonight.

 

[SG-1]

Here are the pics. I was unsuccessful trying to recreate your readings with this meter. I will continue to try with other meters as they come to me. Unless you find a setting to always make the watt reading positive you are probably stuck with the negative reading.

 

[crawfelect]

It looks like you have more load than what we have to get these negative watts. I like tonytonons idea about minimal loading and needing more load to get the correct wattage. We're only seeing (-) 230 watts on L2, not much in the grand scheme a of things. It is still abit baffling though.