Incorrect CT Phase Angles leading to wrong power readings 3

[rockman7892]

We are bringing a new plant power distribution system online which is the reason for alot of my posts lately. I am a fairly new engineer who is trying to learn a lot of this stuff as it happens. I appreciate all of the help from this forumn.

On our main breaker we have a Multilin 750 protective relay, and a Multilin PQMII power quality meter. When looking at the currents on the Multilin, they are showing up strange for they are blinking between some current value and zero. I'm not sure if this is because I have a bad relay, or because of what I am seeing on my PQM meter which is as follows:

When looking at the PQMII meter I notice that I am getting negative kW power readings. I find this strange, so when I look at the current phasors they look strange as well. I know this negative kW reading is incorrect because I have another PQMII meter downstream measuring the exacty same load giving the exact same readings except the numbers are positive. This is why I am thinking that I may have a CT or a PT out of phase on the PQMII and ML 750 on the main breaker, which are both looking at the same CT's. Here are the readings I took off of the PQMII.

Voltge Phasors:

Vab = 4137 @ 0 deg LAG
Vbc = 4181 @ 120 deg LAG
Vca = 4172 @ 300 deg LAG

Current Phasors:

Ia = 27A @ 255 deg LAG
Ib = 27A @ 18 deg LAG
Ic = 27A @ 130 deg LAG

Three phase real power = -152 kW
Three phase Reactive Power = -130 kVAR
Three phase Apparant Power = 195 kVA
Three phase power factor = .74 LAG

I'm guessing that the CT's are out of phase with the PT's. Does this make sense?

 

[dpc]

Probably the CT polarity and/or phasing is incorrect. There about 27 ways to hook these up and only one is correct.

Get the instruction manual and look at the wiring diagram VERY closely, paying particular attention to the power flow sign convention and the CT polarities shown. If you are using doughnut CTs, make sure they are all mounted in the same orientation. It makes a difference if you flip one.

The PQM may be able to show you a phasor diagram, which can be helpful in troubleshooting connections.

 

[davidbeach]

Personally, I'd start with the VT connections. Vbc and Vca being 180[°] apart is not a good sign. The CTs at least exhibit about 120[°] between phases. Too bad you don't have line to neutral/ground voltages to work with. It also looks like you have an ACB rotation instead of the more common ABC rotation.

 

[dpc]

Good point - I didn't look at the data too closely.

 

[slavag]

Yes, it seems as Vcb instead Vca.

 

[rockman7892]

When looking at this problem I have been getting hung up with the representation of the different voltage and current phasors. I have drawn the two phasor diagram scenarios I've been confused between in the attached document. Is the correct way for representing the voltage phasor diagram shown in Case I, or Case II of the attachment.

Which diagram would depict the phasors that I have listed above? I am thinking that it is the left diagram in Case II. Is this correct? When looking at these diagrams which diagram is considered positive sequence and which is considered negative sequence rotation?

When looking at a phasor such as Vbc, is this measuring the angle of b with respect to c which in this case is lagging?

(I know the phasors above have an angle of 300deg however for my diagrams I am using an hypothetical 240deg)

 

[rockman7892]

Sorry forgot the attachment!

 

[dpc]

The standard convention is that phase rotation is counterclockwise. All three-phase power meters will be based on this assumption. This will be shown on the meter wiring diagram that I referenced previously. If for some reason, your system has negative phase sequence, this must be configured in the meter setup.

If you get the meter hooked up correctly, it should work.

You just need to draw out the connection diagram including all CT and PT polarity dots to verify that you have everything connected per the meter's wiring diagram.

 

[sibeen]

rockman, the first diagram you have on your PDF, top left, which you have as ABC rotation - clockwise, is actually the correct diagram although you have it called clockwise incorrectly.

In this diagram, if you have the whole lot rotating anti-clockwise, the A phase will pass through the 0 degrees point first, then followed by the B phase - 120 degrees later, then followed by pahse C, lagging phase A by 240 degrees and phase B by 120 degrees.

This is the standard convention. If you looked at the three phases on an oscdilloscope this will give a A-B-C across the screen.

 

[sibeen]

A picture paints a thousnad word - hopefully

See attachment.

 

[rockman7892]

Sibeen

Thanks for the picture. Your right a picture is worth 1000 words, I now understand the concept of counter clockwise rotation.

The only part I have left to question is labeling the phase angles. Ignoring the voltage vectors for a minute when you are labeling the polar coordinates for the vector representation do you start with the positive x-axis and lebel the degrees clocwise or counter clocwise? In other words which of the following cases below represents the proper phase angles for your drawing assuming the vector magnitude is 120?

Rotation is ABC

Case I: A phasor = 120@ 0deg
B phasor = 120@ 240deg (-120deg)
C phasor = 120@ 120deg (-240deg)

or

Case II: A phasor = 120@ 0deg
B phasor = 120@ 120deg (-240deg)
C phasor = 120@ 240deg (-120deg)

 

[stevenal]

Case I

 

[rockman7892]

Ok Thanks I see it now!

So with the phasors in my origonal post, the phase rotation is ACB. The only issue here is the 300deg on phase C instead of 240deg (possibly a PT issue)

The thing that I have to investigate more is that on my Multilin 750 relay there is a phase sequence monitor that indicates a positive sequence voltage for my voltage phasors. Looking at the relay program the phase sequence in the relay is set up for ABC. I would assume the realy would indicate a positive sequence if my phase rotation was indeed ABC, however since my phase sequence is actually ACB then I would expect the realy to read a negative sequence phase rotation.

I'm going to go re-check this setting, but does anyone know why I'm getting this positive sequence indication?

 

[slavag]

Hi Rockman.
about 300deg "problem".
Please pay attention again on the David's post.
300deg of Vca it's 180deg from the Vbc, that mean your wiring is Vcb.
Firstly, please check it with simple Fluke on the terminals of your device..
Regards.
Slava

 

[rockman7892]

Slava

What should I be looking for with a fluke meter when checking these terminations on the device?

How do you properlay state the term Vca?

Does this read the voltage at C with respect to A?

How about Vbc? Does this read the voltage at B with respect to C?

 

[dpc]

I hate to sound like a broken record, but none of this data means much if the meter is not connected to the CTs and PTs correctly.

Unless you have strong understanding of voltage and current phasors and the conventions used in defining them, my suggestion is to trace out the PT and CT wiring very carefully, verifying all polarities and phases. Then compare this to the meter wiring diagram provided by the manufacturer.

I have seen hours and hours wasted by trying to guess where the problem was, swapping wires, changing polarities and phases, etc and ending up totally flummoxed.

OK, back in my hole...

Dave

 

[slavag]

Hi Rockman.
It's very old and very simple voltage wiring test.
Assume voltages terminal of your device are:
1 Va
2 Vb
3 Vb
4 Vc
5 Vc
6 Va
Now with Fluke you chek voltage between:
1-2 110V
1-3 110V
1-4 110V
1-5 110V
1-6 0V
2-3 0V
2-4 110V
2-5 110V
2-6 110V
I assume you see 2-6 "0V", that mean some mistake in wiring.
Regards.
Good Luck
Slava
And after this you can check rotation setting

 

[rockman7892]

Slavag

The PT's are configured in an Open Delta Connection. On the secondary side Vb input to the relay is gounded and also jumpered to the common input.

I measured voltages and found.

Va-Vb=120V
Va-Vc=120V
Vb-Vc=120V
Vb-Vcom=0V

Everything appeard to be corrct here but there may be a wiring issue I have to trace and match up with the Open Delta VT connection to the realy per the wiring diagram as dpc mentioned.

 

[dpc]

You may need to tell the meter via configuration settings what the PT configuration is. I believe the PQM is pretty flexible in this regard. I have run into meters that simply will not work with open delta PT connections. But it needs to know if the PTs are line-to-line or line-neutral as a minimum.

 

[slavag]

Hi Rochman.
Start again.
Please open your PT connection drw. and meter manual.
As Dave said, you have check connection of PT at the manual and drw.
In the modern meters you have several options for the PT connection and it's selecteble in the setting of meter.
Good Luck.
Slava