Power measurements help needed

[Denan]

Hi,

I am on commissioning, and have troubles finding correct wiring for current measurements. I have tried many different combinations, and this attached is "the best". Active power follows currents, but reactive power is opposite, while in the AVR, I see both of them positive (meaning inductive reactive is positive).

I don't have any more ideas, and any help is very appreciated.

Brg,
Denan

 

[Skogsgurra]

Is this a motor or a generator? That may have an impact on definitions. Also, synchronous or asynchronous may be an issue.

If you have all transducers correctly installed and follow the manual, you shall get correct readings. Trying out connections blindly until you get what you *think* is correct will not get you anywhere - except in trouble, probably.

Good to see that your instrument uses the (somewhat discussed, but correct) unit for reactive power.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Denan]

Hei Gunnar, thanx for replay.

It is a genset (synch. generator), 690V.
I can trend all the values, but chose only these for this purpose.
Governor "sees" the same P, but I am not happy with Q. PF is 0,98I (I can see that in the AVR).

 

[davidbeach]

Power factor is what it is; your happiness doesn't much enter the equation. Absent other confirming information your VArs (sorry Gunnar) they could be flowing in either direction.

 

[Skogsgurra]

In hoc signum vincit!

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Denan]

Readings in the AVR and the measuring/protection unit are different. Those two units get the voltage from the same VT, but current from two different CTs.

PF in the AVR is what I would expect to have, while in the other unit, it is different. Therefore I trust the AVR readings.
Please correct me if I am wrong.

What I am looking for in this thread, is a hint how to measure current direction with available equipment: multimeter and/or Osciloscope (and those trendings from AVR and the other unit).

Attached is measurements picture from the AVR, taken sometimes during trending in the picture above

Thanks in advance for any suggestion!

 

[dpc]

If you can compare the voltage and current from the same phase to determine the phase angle between them, that would be a good way to verify "direction" of current, if I understand your question correctly.

Unfortunately there is not a universally agreed upon definition of positive and negative vars.

Per IEEE, if current is lagging the voltage, the machine has a lagging power factor, regardless of the direction of power flow. So a motor consuming power and vars and generator producing power and vars are both considered to be operating in the lagging region. When power and vars go in opposite directions, the machine is operating at a leading power factor.

But I'm not actually sure I really understand your question.

David Castor

http://www.cvoes.com

 

[rbulsara]

while in the AVR, I see both of them positive (meaning inductive reactive is positive).

Your AVR picture is showing a leading power factor (overexcited) and appears consistent with the positive Q, which is normally associated to capacitive, not inductive, reactive power.

Also the PQ trend has two periods, one with average kW @350 and kvar@-350 and other with average kW@100 and kvar@-100, a PF of about -0.7, during each period.

This is no where near being close to what the AVR snap shot is showing, namely 422 kW and 97 kvar!

Were they taken at the same time?

You need to provide more info on the system and loads to pass on any comments. As hinted before, start with comparing the actual wiring to the respective device's manual!

What I am looking for in this thread, is a hint how to measure current direction with available equipment: multimeter and/or Osciloscope (and those trendings from AVR and the other unit).

.

If the PQ meter is capable, look up the phasors (vectors) of V and I on the PQ meter screen along with phase rotation.

or look at the waveforms not trending of V and I on the oscilloscope. (these again require correct wiring).

Or connect a known type of load and see the meters are reading it correctly.

As stated before, you need to verify that each device is connected correctly, especially the CT polarities.

Rafiq Bulsara

http://www.srengineersct.com

 

[ScottyUK]

Hi Rafiq,

For a generator (in the IEC world) an over-excited machine exporting vars is a lagging machine, not a leading machine. Leading operation operates in the under-excited region. Is IEEE different?


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If we learn from our mistakes I'm getting a great education!

 

[rbulsara]

Jon:

No. I goofed up on signage conventions. Exporting vars in gen is a lagging machine. So I stand corrected.

The readings still do not match though.
Thanks

Rafiq Bulsara

http://www.srengineersct.com

 

[Denan]

Thank you all for the comments. The readings do not match because the first picture shows very unstable readings, while in the AVR they are more stable, so I wonder if the CT might be bad or there is some noise causing problems on metering.

I tried to measure both current and voltage at the same time with osciloscope, as dpc suggested, but current reading there is pretty bad, I will try to upload a picture of that later today (in the evening, EU time).

 

[ScottyUK]

"The readings still do not match though."

Agreed, something is certainly wrong.

Denan, if you are confident about the instrument then look for a CT or VT group connected in delta when star is expected (or vice-versa). The 30[°] phase shift will cause a major error in the values calculated by the instrument. You can also get some weird results if you reverse some of CT and / or VT polarities so they no longer form a symmetrically distributed 3-phase group (e.g. instead of a 0[°] / 120[°] / 240[°] group, you end up with 0[°], 60[°], 120[°])


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If we learn from our mistakes I'm getting a great education!

 

[waross]

If you can adjust the excitation up or down a little and watch the VARs at the same time, that should give you an idea of which direction the VARs are flowing.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Denan]

Hi again,

Today we were going to do completely new check of all voltages and currents (polarities etc.), but the generator breaker stopped working before we could do it.
We'll try again tomorrow, hopefully.

The system is connected to the grid, so it is not possible to connect to a known load, but those trendings are done during 5-20% load of the engine.
Part of the problem is that some of the equipment is old and part is new, making it more difficult to know what is correct. The generator had a service done few months ago, and it looks like some rewiring was be done (around CTs), but I don't have documentation showing what is done.

On the other hand, one voltage is taken directly from the generator measuring point, while the other is taken from the switchboard.
Generator side is U-V-W, and the power cables are connected to the grid U-W-V. On the generator there is a plate showing field is running W-V-U (generator rotation is CCW).
CT for measuring and protection is taken from the generator CT (3 phase), and AVR CT is mounted on phase V.

So what we wanted to check is if wiring is correct, meaning voltage U-current U, voltage V-current V and voltage W-current W.

To have the same phase rotation on both sides, we removed zero point from the generator and closed the breaker, which showed some difference in phase rotation. We followed the grid rotation, and matched wiring from the generator. Doing that, we should have the current-phases following voltage phases, right?

I suppose that there is my error (current phases not following voltage phases). But would it maybe be more correct to follow generator phase rotation, and match bus/grid VT according to that?

I took some pictures showing voltage and current measured with osciloscope, one is attached. I experience a lot noise (or what it is) on the current reading. Thin sinus line is voltage... the noise is the same in all phases.

 

[Denan]

Now we have found it, the problem was caused by bad wiring inside the generator. Phase that is supposed to be V was connected as phase W (not in the middle), so the AVR had problems to regulate. Once we found that the rest was ok.

Brg,
Denan

 

[rbulsara]

I am glad that all worked out. Thank you for the feed back.


Rafiq Bulsara

http://www.srengineersct.com