ESP/VFD Issues

[Peter Mather]

I have a number of ESP drive trains that are powered from island generation at 400V;50Hz through a 450kVA VFD and a 525kVA step-up transformer to 2600V then down hole cables of varying lengths from 1000m to 2600m to a 225kW pump motor. The VFD has an output filter. The VFD operates as a 6 pulse unit but the diagrams from the vendor indicate a phase shift Tx to operate at 12 pulse. Vendor advises that the transformer is not required.
The VFD exhibits cyclic behaviour on all wells. Cycle period depends on operating output frequency of the VFD.
Take for example the operation at 45Hz. Every 31 seconds there appears to be an anomaly in the VFD that causes a voltage surge on the generators to approximately 417V which quickly recovers to 400V within 2 to 3 seconds. The load doesn't change
The anomaly causes a reduction in VFD output frequency as low as 35Hz which recovers over 10 seconds. The system settles for the next 31 seconds.
Operating at 41Hz increases the cycle time to 52seconds but the effects are the same.
For me this points to a problem in the inverter IGBT control circuit but any thoughts from those on the forum are welcome.

 

[LittleInch]

What's happening at the pump end?

Are you getting any pressure or flow fluctuations? Increasing frequency will increase flow in the pump so is it dragging the water table or whatever down too far, then recovering?

Need to do an end to end assessment and measurement to understand where the issue really lies and not just consequential effects.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[waross]

Sounds like a forbidden frequency issue.
Just as the high frequency component of PWM will cause reflected voltage waves and a buildup of high frequency high voltage, the pressure pulses generated as each pump impeller vane passes the discharge port will be reflected back and will eventually build up to high transient pressures.
The pressure pulses generated by the impeller vanes will reflect back from restrictions and from elbows.
When the time for a pressure pulse to reach a reflection point and return to the pump matches the arrival of a succeeding impeller vane, the next pressure pulse is magnified.
This may continue until physical damage is done.
I have seen a piece of casting about 6 or 7 inches across blown out of the side of a pump housing from operation at forbidden frequencies.
That was on a firm grid supply.
I have no idea how a generator may respond to forbidden frequency operation.
You have filters to prevent reflected electrical waves from building up.
A submersible pump is a prime candidate for forbidden frequency issues.
This may not be the issue, but it may be one of the first things to check.
Most VFDs have provision to avoid extended operation at forbidden frequencies.
Please let us know what you find.
We may learn something here as well as you.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jraef]

I think it odd that the drawings show 12 pulse, and the vendor just declares that is unnecessary? So they get to decide?

A 6 pulse drive can have as much as 80% I-THD, which can put your generator/transformer situation into saturation.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Drivesrock]

The generator voltage rising makes me think that the generator AVR may well be being affected by the VFD 6 pulse input current harmonics causing enough voltage distortion that the AVR is seeing for the voltage feedback. What is the AVR make and model? What is the rating of the generator? Where is the 417 Volts being indicated? Is the VFD the only load on the generator?

The higher the VFD output frequency the higher the load so the input current will be higher and therefore voltage distortion will be greater. And perhaps this explains the difference in behaviour at the 45Hz and 41Hz output frequency references.

Is it possible to switch the AVR to 'Manual' mode where the field current (or voltage) is controlled by hand to achieve the desired generator volts? That would fix the excitation with excitation current or voltage feedback only and turn off the automatic voltage control part that is looking at the generator volts. This could 'steady' the generator voltage.

What is the manufacturer and model of the VFD? There may be some output bridge inhibit when the DC Link voltage goes up with the increased generator voltage. The motor speed will quickly drop. Perhaps the output bridge starts again at 35Hz and ramps up to the 41 or 45Hz and the cycle repeats?

Just some thoughts anyway.