phase angle displacement and undervoltage 2

[enverd]

I am faced with an undervoltage problem at work where some Air Handling and Exhaust Fan unit, while being fed through VFDs, are tripping breakers. The monitored voltage indicates an ongoing AB = 415V, BC = 417V and CA = 475V.
I am thinking that there might be issues with the VFD, control power transformers or IGBT's that could have an effect on the voltage. Comparing the read voltage levels it also seems as if phase B has some type of a fault.

One of the engineers at work suggested that I look into phase angle displacement that could create an undervoltage problem?
However, even though this should possibly be already clear to me, I am having a hard time putting together a phase angle displacement effect on undervoltage?

Is there a relationship and how would it affect it?

Also just in general any suggestions what the problem might be?

Thanks in advance.

 

[ScottyUK]

Where did you make the measurements - line side or load side of the VFD? If the latter, be aware that VFD outputs are notoriously difficult to make measurements on without using something more sophisticated than the average multimeter. Best bet is to use an oscilloscope with isolated probes. If the measurement was on the supply side of the VFD then you have something desperately wrong with your supply.

 

[Skogsgurra]

Are those VFDs connected to three phases or two?

Anyhow, as Scotty says, there's something wrong with your supply.

Phase angle and undervoltage? Yes, perhaps, if your transformer has issues. What should the nominal voltage be? 480 V? Do you have a PQ analyzer available?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[enverd]

Thank you both for the reply.

ScottyUK, as much as I know about this problem is that there was some type of monitoring system at the VFD that has repeatedly reported undervoltage problem, however I have not been told if it was on line or load side.

Skogsgurra, the VFD's are 3phase serving 3phase load. Phase angle and undervoltage? that was exactly my reaction as the distribution set up as it was explained to me didn't include any immediate transformers. From a distribution board there is a feed going into the VFD and the VFD in turn feeds the AHU's motor or EF's motor.
When I was asked about the phase shift, I was confused as I just never heard of a phase angle displacement that happens between, what is supposed to be a "balanced" load. And that is what this is supposed to be, an AH or EF, "balanced" load fed from 480V, 3phase feed.
I am not sure if they do have a PQ analyzer.

 

[waross]

With input voltages like that, one phase will be supplying most of the power. The input current will be proportionally higher on that phase and may be high enough to cause a breaker trip.
What is your nominal voltage?
Voltages like that are common when there is a combination of unbalanced single phase loads and single phase voltage regulators on a long distribution circuit.
When unbalanced line to neutral loads cause a noticeable voltage drop on distribution circuits, the voltage drop on the neutral conductor may be equal to or greater than the voltage drop on the phase conductor(s). The voltage drop on the neutral conductor shifts the center of the star point so that the phase to phase voltages are no longer equal and the phase angles are no longer equal. The addition of single phase voltage regulators tends to increase the neutral current and the resulting phase displacement. When the line to neutral voltages are corrected with single phase regulators the line to line unbalance becomes worse and the phase angle error becomes worse.
A fix is to use a star delta isolating transformer. Use a three wire primary connection and leave the primary star point floating. Feed the VFDs from the delta secondary. Arrange your switching so that the transformer is always energized unloaded as there are often severe over-voltage transients when this arrangement is energized. The delta secondary may be grounded by any of several methods.
DO NOT CONNECT THE PRIMARY STAR POINT to the system neutral or to ground.
The delta voltages will be inherently equal. The primary wye point will shift off center as required so that the resulting line to star point voltages on the transformer primary are equal allowing the delta to be balanced without circulating currents.

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

 

[enverd]

Thanks waross,

That is another great point, however I have requested that we investigate the entire setup, from utility to the load, to understand the system.

This entire problem comes from a project that is not even mine. However I was thrown off when asked if there is a phase shift creating a problem. The only phase shift that I would think of is if a transformer is involved. I also would be considering the power factor at the load.

But you bring up an interesting point which I didn't think about, when you say:
[highlight #8AE234]When unbalanced line to neutral loads cause a noticeable voltage drop on distribution circuits, the voltage drop on the neutral conductor may be equal to or greater than the voltage drop on the phase conductor(s). The voltage drop on the neutral conductor shifts the center of the star point so that the phase to phase voltages are no longer equal and the phase angles are no longer equal.[/highlight]

How does that happen in theory? How do I calculate the new phase to phase voltages? And what is my new star point?

Thanks,

 

[waross]

Calculation is difficult.
Visualization is easier.
Any current on the neutral conductor will result in a voltage drop on the neutral conductor, however small.
Represent that voltage drop by a vector originating at the perfect neutral point and the actual neutral point has now moved as described by the vector. That move will result in phase angle errors and line to neutral voltage unbalances, even though the line to line voltages may be well balanced.
Add to that the voltage drop on the phase conductor carrying the unbalanced current that is causing the neutral issue and you add line to line voltage unbalances.
Now if the utility installs single phase voltage regulators as is common on some rural distribution lines, the line to neutral voltages are again well balanced but the phase angle error and the line to line unbalances get worse.
Calculations? Sorry. Never done them. I have recognized the condition a number of times and I have successfully mitigated the worst effects. Any calculations were estimates of the size of correction transformers required.
For your application I would expect complete correction with a wye:delta transformer with the primary wye point floating.
The transformer is sized for the load just as any isolation transformer would be.
Your load is now fed from an ungrounded delta system. You may corner ground it, (NOT recommended), you may use a four wire delta connection and ground the center point of one phase, or you may use a second wye:delta transformer and ground the wye point. (Or a zig-zag transformer).
The grounding transformer must have a withstand capability that will safely trip the protection of the larger wye:delta transformer, or be of sufficient capacity to supply fault current to a grounding impedance. This may be 1 KVA or less.

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