VFD High DC Bus Voltage

[controlnovice]

Have a 480Vac, 200HP well pump (motor and pump in well) driven by VFD.

Distance between VFD and motor is 1300 ft using buried armoured cable. Sorry, don't have cable size yet.

We have a bypass contactor around the drive, and the motor/pump starts and runs without problem using it.

Original cable was same, armoured cable, but a larger size (again, don't have sizes yet), and the VFD ran the motor without issues (or without tripping). BUT, we did recently get a ground fault on the cable and had to change it. We happen to have the smaller cable (again, no size yet) running/installed parallel with the larger one (for a previously smaller well pump/motor) that we found was large enough for the larger motor. So we used it instead of installing a new cable.

We do have reactor on load side of VFD.

Ever since using the smaller cable, we've been experiencing high DC bus voltage on the VFD on start.

Any ideas why it would work on larger cable, and not smaller cable, even though smaller cable is sized for the amp load?
Length of cable vs size of cable?
Armoured cable creating choke?
Length of cable creating larger reflective wave?

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[controlnovice]

Looking at some other postings, I see there are some cap bank related issues that could be causing the problem.

We do have cap banks on our 4160kV incoming service that stay on, they do not switch. We do have a reactor on the line side of the VFD also.

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[controlnovice]

Murphy's Law:

Just got a call from the electrician. I had suggested selecting a lower carrier frequency.

Motor started no problem.

Still curious as to why we would see the problem in one cable, but not another.

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[DickDV]

The capacitance between the phase leads or maybe between the leads and ground are likely different. At such long lead lengths, that can be the difference.

 

[rockman7892]

DickDV

I follow what you are saying about the cable capacitance being different but how could this cause the DC bus overvoltage problem in the drive?

 

[jraef]

It was reflecting back into the drive, where all it could do was charge up the DC bus.

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[DickDV]

A key piece of information is missing here. Is the incoming AC power balanced to ground, floating to ground, or deliberately offset to ground as in 120/208 or 460V corner-grounded delta.

Floating or unbalanced input power can cause the output side capacitance to charge up the DC bus. I've seen it happen more than once.

I don't think I could explain exactly how it happens.

 

[resqcapt19]

I am the original installing contractor for this. The supply is solidly grounded 480 volt wye system.

The VFD was installed about 10-12 years ago as part of a project to upgrade the well pump. The original installation was 150 hp across the line start submersible pump fed with 3C 350kcmil MC cable listed for direct burial. As part of the upgrade 3C 500kcmil MC cable was installed between the new VFD and the well head. The original 350 was abandoned in place.

Recently the 500 kcmil cable failed and an attempt was made to use the 350 to run the motor from the VFD. The motor would run fine in bypass mode but would trip on high DC bus voltage when connected to the VFD output.

There was some recent construction activity in the area where the underground cable is located. We have testing company with fault locating equipment scheduled to locate the fault. If there is evidence of localized physical damage the cable will be repaired. If there is no evidence of physical damage or if there are multiple fault locations I will recommend that the cable be replaced.
If the cable is to be replaced, I assume it should be the shielded VFD type cable and not the standard MC cable that is now in the ground?

 

[xj25]

To identify more clearly the cause I suggest:

-You say that lowering switching frequency it works. Try to increase freq. to confirm/discard that there is some resonance problem.

-bypass the output filter just to confirm that it can not cause resonance problems with stray capacitances of 350 cable.

- measure voltage with an scope after output inductor filter and (if you can) at motor terminals, both in normal freq. and lower freq. Better with a differential probe.

About DickV comment regarding earthing of supply side,
i.e. if it is used a star supply with neutral point to earth, it is a very bad idea to connect in anyway VFD DC bus negative to earth, as there will be a big potential difference between transformer neutral and DC bus negative, i will cause big earthing currents and probably DC bus capacitor charging currents that will trip the VFD.

Regards