PM8240 Negative Values of Power and Energy

[usman_baig_cnse]

HI,

We have installed the PM8240 Energy Meter at one of our project. Few meters are displaying Negative values of Following parameters although Current and Voltage values are in positive. Meter Pics are also attached for reference.

1: Energy Delivered-Received

2: Power Phase-A

3: Power Phase-B

4: Total Power

Kindly advice for the rectification of this issue.

[Charles_Murison]

Hello @usman_baig_cnse ,

Moved the topic to the metering forum as this is a metering question. A few items to start with. First, voltage and current will never be negative. The power and energy "negative" is used to describe the direction of power flow where positive is defined as from source to load, and negative is from load to source.

If not expecting a negative power, there could be a few possible reasons, first, the CT orientation is reversed, or wires reversed. A second possibility is phase mismatch between voltage and currents. For example CT around conductor connected to Phase A voltage input is connected to Phase B current inputs I21, I22. 

A way to troubleshoot this would be to look at the phasor diagram of the meter.

Regards,

Charles

[N4th4n13L]

Hello Usman,

Are you able to get a picture of the phasor diagrams? In the Alt display mode it's under the power quality section.

If it is expected that all power should be delivered in this installation. It sounds like some CTs or currents could be wired backwards, or phases aren't aligned. 

Seeing the phase relationship between phases will really help narrow down the issue. If you see the current are close to 180 degrees from voltage the current's are likely reverse. Then narrowing down is CT is installed backwards or wired backwards. If physically re-wiring isn't an option or can't be done safely. You can invert current polarity internally in the meter.

If it looks like voltages and currents are closer to 120 degrees apart the phases are likely not wired correctly. Like C phase current on A phase current inputs. 

The PF is kind of all over the place which makes it hard to narrow down without a picture of the phasor diagram.

[DanL]

The per-phase real and reactive power values are sufficient to compute an approximate phasor diagram (assuming low THD and quasi-stationary signals).

The most important details still missing are:

1. what is the actual load and it's expected characteristics? Why is there so much load unbalance?

2. what "Volts Mode" is configured in the meter? (default is "4W-WYE")

3. what wiring diagram was used (ie. no PTs and 3 CTs)?

4. what is the measured THD for voltage and current?

5. what firmware and/or framework version is on the meter? (serial number can also help)

The power values show:

element A in quadrant 3 (exported/received power with inductive/lagging PF)

element B in quadrant 2 (exported/received power with capacitive/leading PF)

element C in quadrant 1 (imported/delivered power with inductive/lagging PF)

For ABC rotation, the current phasor vectors are all bunched together in a narrow 20 degree region, so there is not a single simple change to make.
For ACB rotation, the I1 and I2 current phasor vectors appear swapped

[Charles_Murison]

Hello @usman_baig_cnse ,

The phasor diagram from the meter would be a great place to start with troubleshooting. This would also help determine if the meter is ABC rotation for ACB rotation. Part of why this is important is that you would get very different results when trying to build a phasor diagram from the information provided. 

For example if V2 is 120 degrees, and V3 240 degrees (ACB rotation), it looks like I1 and I2 are phase swapped. No CT inverting needed.

Again this would just be a guess as we would need to know more about the load in question, Leading or lagging, generating or consuming power. There are multiple different changes that could be made to give different possible phasor diagrams. One to match what the actual system is would be the important part.

Regards,

Charles

[usman_baig_cnse]

Hi

I have attached the  phasor diagram of Two meters on which we are having negative values.

[Charles_Murison]

Hello @usman_baig_cnse ,

What are the Ratings of the CT in question. the meter is showing units for current as kA, suggesting that the CT ratio is at least 1000:[something]. Many CTs require a minimum current for the CT to provide accurate signals, a general rule of thumb is 10% of rated current, for example a 1000:5 CT may require a minimum current of 100A.

Meter 2 phasor diagram has next to no load and does not tell us anything. Would need to see this meter under load. The same goes doe Meter 1 picture 2. Meter 1 picture 1 has potential. 

There are some assumptions I am currently making, if these assumptions are not correct then what I say below will not be correct.
Assumptions:
1) CTs have enough current to give a good signal. If this is not true then any phasor information could be inaccurate

2) System is expected to be a lagging system. (Quadrant 1) 
3) System is a 4 wire Wye

Corrective actions:

Actions should be done while system is in de-energized state.

CT connected to I11, I12, needs to be swapped with I21,I22. Either swapping the conduits the CTs are connected to or swapping the wires I11 <-> I21, I12 <-> I22.

[usman_baig_cnse]

Hi @Charles_Murison 

Energy Analyzer is installed on incoming Electrical Panel with 5000 ACB Breaker and CT setting has been configured same.
I have attached the Volts and CT setting for review.
Further both meters has no load while pics were taken. I will also get the phasor diagram while load is active.

[Charles_Murison]

Hello @usman_baig_cnse 

56A on a 5000:5 CT is barely 1% of rating. Either you have fantastic CTs or there is not enough load to make any conclusions.

Regards,

Charles

[usman_baig_cnse]

Hi @Charles_Murison 
I have attached the updated pictures of Meter #2 with load.

[Charles_Murison · Accepted solution]

Hello @usman_baig_cnse 

The phasor diagram shows that I1 and I2 need to be swapped. I11<-> I21, I12<->I22.

Regards,

Charles

[usman_baig_cnse]

@Charles_Murison  Is there any document or knowledge base to understand and read the phasor diagram for PM8000 meters as I need to adjust few more meters so want to understand the basis of these phasor angle and what their value actually meant.

Thanks in advance 

[Charles_Murison]

Hello @usman_baig_cnse ,

This is a loaded question. Each system could have a slightly different phaser diagram depending on the system. Leading, verses lagging power factor, generating verses receiving power, volts mode, amount of power factor, etc.

A good starting point could be the ION7650/ION7550 install manual, https://www.se.com/ww/en/download/document/70002-0247/ 

Starting on page 25.

Regards,

Charles