Medium Voltage Variable Speed Drive Harmonics

Appreciate your reply, the bearings have been inspected and they are all good, on this shaft we have 17 composite bearings and not with a ball bearing raceway. They are similar to phospher bronze bearings.

The failure of the ESP string was not motor related, but the input cable connection, I suspect water seeped in. The EDM on the shaft under the bearings however indicates another problem maybe with the VSD output.

Once again thanks to you and Rolf!!

Great answer.

not quite.. here are a few photos I took

Motor shaft

In this photo above it show the shaft sleeve on the right and the bearing on the left. The bearing fits into the stator inbetween the rotors with the sleeve keys onto the shaft.

The sleeve when removed of the shaft revealed the EDM under each shaft sleeve on the entire length of the shaft (about 20ft ±)

The motor is filled with di-electric oil and this oil acts as lubricant, insulation and in conjunction with the protector, it equalizes the pressure inside the motor, so at depths of 10,000ft plus in well fluid, we don't get stator squeeze on the rotors.

Good white paper.....thanks.

I never realised it could be a problem....

In addition to my earlier reply.. I've just read the white paper, many thanks. GA!

317 days continuous running at 44Hz

Is the motor supply cable properly shielded? Is it properly terminated at both ends?

If so, I would suggest finding a way to ground the shaft to the bearing housings or at least make sure the motor shaft and the bearing housing are always at the same potential so that no discharge can occur between the two.

If you further insulate the bearing housing from the bearing and the shaft then the induced voltage in the shaft may go even higher in magnitude and cause increased damage instead of solving the issue.

Wow, you stole all my thunder!

Succinct and to the point.

Awesome answer.... many thanks.

You have given clarity to my research, you have confirmed some of my findings in your answer and answered further questions as well, regarding the motor construction.

I have already suggested a filter for the output, BUT as with all vendors, they are stating that their drive does NOT need it. So it will be hard work I feel for the customer to get any satisfaction out of the vendor!

I will keep you posted on the progress

JRaef,

As always, I find your expertise most helpful, especially the capacitive circuit concept.

Your comprehensive VFD information is not only interesting, it enables me to better perform my job.

Thank you.

Just a few considerations,

Pump is 13,000 ft below the surface, this means the supply leads between VFD and motor are also 13,000 ft. Long cables between motor and VFD cause problems.

What is the carrier frequency set on the VFD ? A low carrier frequency (2 - 3 kHz) makes a noisy motor but give less problems with harmonics.

Bearing damage from EDM (Electric Discharge Machining) is normally seen on the raceway in the bearing. The place where de balls are running in the groves.

I'm not familiar with this type of bearing, but according to the photo, it seems to me that the problem is situated between the bearing inner ring and the shaft.

Rudy.. many thanks for your comments

the bearings are designed for the ESP motor application, while I know and understand the reason for the EDM, my research and info from the supplier of the VSD are in contradiction.

I'm looking for, and have received in the comments (including yours) information to confirm my theories, which I intend to use, as I now have more ammunition to use.

The carrier Hz.... they are not saying!! So there is a little bit of smoke and mirrors happening between vendor and customer, and I work for the customer.

Very frustrating to say the least!!

Do you know what harmonic frequencies are going into the cable & their amplitude?

4000 metres is a long cable. My rough estimate is a quarter wavelength at 15 to 20 kHz. Radio engineers use 1/4 wave lines as transformers, low volts one end - high volts the other. Over about 1/10 wavelength has to be treated as a transmission line, long 50 Hz open wire HV power lines get "transmission line" effects.

What are the harmonic voltages at the motor end??

No one knows - and the drive supplier says no filter is needed.

The point about "snubbers" is vital, L & C can just move resonance frequencies without damping them - resistance at the right place to limit magnification and remove HF energy is needed.

Since the bearings are "grounded", it is probably the "earth return" currents and frequencies you should measure.

67model

I will ask regarding the frequencies, as for the measurements, the motor is at the bottom of an oil well drilled into the sea floor.

The power supply is the rig supply, diesel generators at 60Hz with an operating frequency for the drive set at 44Hz

I guess you have a sheathed and armoured cable from the deep. I would measure on the bond between the gland plate and the drive on the drilling platform, hopefully that is the return leg of the common mode currents which disappear down the three wires running to the motor. This would help to define what needs keeping out of the motor feed.

Impress on the drive supplier how lucky he is that the motor got pulled out for other reasons before the bearings failed. After all, they are electronics engineers and should have thought more carefully about the implications for their reputation of trouble with a motor 4000m down a hole with removal and down-time costs that make the cost of a filter look small. If I understand you right, they said a filter was unnecessary.

It is to their advantage to be a drive supplier who knows the problems for motors on their drives and can provide the customer with what he needs to avoid in-service motor failures.

You need to get the motor and drive engineers in the same room.

the ESP and drive are supplied by the same company, and yes you would think they would know what they are doing. But they don't, because they would rather fudge the real data and info hoping that, as is the case here, the ESP fails for another reason.

I must add that I am not looking to fix the problem, only report to the operator & customer of the pump and drive supplier, what I have found and give a substantial reason for the findings, that is, what caused the EDM. It is way above my pay grade and my overall involvement to offering words of advice and suggestions on the construction of the ESP motor or the MVD.

I report my findings to the operator, they batter the supplier round the head with it, and hopefully the supplier will come up with the right answer. But I'm not holding my breath.

I would like to thank everyone who has replied to my question, as I said in my original post I needed clarity to my research and info received off the Drive supplier.

All I do now is report my findings of EDM, the reason why it happened, which as been confirmed by everyone's comments. Job done! Thanks Guys!!

Is that a SigGen or some form of Radio as your Avatar, its too small to examone?

I have to agree with you on the smoke and mirrors. - There is always a lot of smoke blown around.

I was recently asked to verify the installation of new VFD drives (PWM) on half a dozen existing municipal effluent treatment pumps - about 700HP each. The "old" drive had a significant output filter and the waveform at the motor at about 200ft was essentially a pure sine wave. The supplier of the new VFD said filters were not required. After just 200ft the motor now saw the PWM carrier frequency and additionally there was a spike on the leading and trailing edge of the PWM waveform of over 1000volts and had a damped ringing for about 3 cycles at approx 1mega Hz. Heavy power cables do not make a good high frequency transmission line and with a miss-matched terminating load (the motor) there were significant problems.

End result - OEM of drive was forced to supply line filters. They opted for line termination filters - and they also initially ran well above their rated temperature and had to be beefed up.

As a side note - with the new drives the pumping room was now really annoying to be in. The wine of the PWM carrier on the motors was very loud! You can easily tell which pumps are energized now - they sing loudly. Before they just rumbled along.

Carrier frequency on an MV drive will be very low, in the 1.2-3kHz range most likely. It's a valid issue with LV drives because the higher CFs used on small motors to make them run quite do indeed increase the risks of bearing (and winding) damage. But they cant make the MV IGBTs turn on faster than about 3kHz.

Your good - it was indeed a 600vac system!

Another good presentation on this subject, from a friend of mine in Sweden, is in the link below. At the end he is hawking a small measuring device he fabricated for looking at the bearing currents, but I'm not sure he still makes them, is is semi-retired now. It's still a great explanation of the causes and effects from an industry expert who spent his career at Siemens and ABB before becoming an independent consultant on large motors and drives. Sorry the link is not activated, I'm on my IPad and the Apple OS in almost incompatible with the software for this forum, so I don't get automatic parsing and linking features. You will have to copy and paste it into your browser.

http://www.gke.org/presentationer/files/EDM%20Physics%20and%20Reality%201.pdf

If you want an expert to come and take a look at it, he will go anywhere in the world. He speaks something like 9 languages fluently. One of the best guys you will ever meet.

The pdf is very interesting....thanks.

This problem is not isolated to PWM drives. It also occurs in DC SCR (thyristor) drives. I have seen numerous EDM damaged bearings in DC machines. It just seems to happen faster in PWM AC machines.

Phase controlled rectifiers can also have high dV/dt voltages in the motor leads.

I am suspicious also that the very arrangement of machine and motors can induce currents from potential differences in the total machine. I have seen a paper machine while running discharge sparks a foot long. That must go somewhere, and I suspect bearings also see current / voltage from those potential differences.

Along those lines, another factor to consider is any arc welding done on the rig.

Good point to bring up (the welding issue). In my early days, that was the only time I ever saw this phenomenon, working for a company that made large "tunnel washers" for research animal cages. They were 40-50' long, all stainless steel, all welded construction. When we commissioned them, and there was problem with the chain conveyor that needed welding, we learned the hard way that if the welder forgot to move his ground clamp from his work table to the machine and struck an arc, it would work, but was grounding through the motor bearings! Tore them up in seconds.

Wouldn't be applicable to Brich's problem, unless he has tiny little down-hole welders working on something under water!

Those despicable Minions get into all sorts of places!

I had this mental picture, based on post#19, of a rig with a drill pipe and a motor at the bottom of the hole, all common to each other electrically by means of equipment grounds.

"... The drive is on an offshore rig, the ESP is 13,000ft below the surface, ..."

It's an acronym familiarity issue. ESP = Electrical Submersible Pump, meaning the motor is at the bottom of the 13,000ft hole!

Paper machines are notorious for such high voltages, especially with low levels of humidity. We had a fairly recent blog about this as well here.....

The big (paper) printers I worked on many, many years ago, both impact and laser, had to have very finely tuned anti static systems to keep these voltages as low as possible.

Winter in Germany used to bring out the worst in static discharges as the humidity would drop dramatically, testing both the A/C systems ability to replace the missing moisture and the various methods of reducing those voltages....

Van de Graaff machines were invented many years ago to produce high voltages from a belt made basically of rubber and paper if I remember correctly.....and are still used today:-

Van_de_Graaff_generator

Dear Mr.brich,

You have referred about Harmonics effect. But you have not mentioned about the BEARINGS used in the pump - whether the bearing insulation is provided or the Bearing is INSULATED BEARING.

The shaft voltage will induce a shaft current and circulate to the stator of the motor through the bearing.

When the Harmonics issue is present - the Bearings Arrangement for the motor will be of one of the following.

1. Insulated bearing on one side. - As the name implies bearing itself will be of insulated type.

2. Bearing Insulation - The bearing will be conventional, but insulation sheet will be provided.

The above arrangement will totally eliminate the shaft current circulation.

Pl. check this aspect and inform this forum - how i was solved.

DHAYANANDHAN.S

Hello D. That wasn't off topic BUT it has already been discussed in previous posts.

This has been one of the most informative blogs on CR4 for sometime to my mind, with very valid infos from a couple of great posters.....many, many thanks from me personally.

I never ever DREAMED of such problem areas with regard to VFDs.....

There have also been a few "badly/late informed" posts, but thats normal here.....

Andy...

Have to agree.. yes very informative and the problem with VSD/VFD's has been around since they were first used, the only thing, no one realised how much they would affect everything else.

Stating that the drive is IEEE 519 is a bit of a smoke screen, as in the case of this MVD I refer to it is compliant only on the input side, so no or very little harmonic are placed back on the utilities power grid, if the drive was on land. But the output harmonics and that was the reason for my question to clarify my research and the infomation given by the drive manufacturer is a totally different thing. It is NOT IEEE 519 compliant as they have stated, or strongly suggested, but offered no real supporting evidence.

So filters are needed, the absence of filters and the long length of cable give the results as shown in the photos.

If we had a vote, I would vote the whole blog as the single best blog of 2015!!

Definitely caught my attention too.

By using the variable speed drives on the control system, it proves the stability and control accuracy of the whole system. The good quality VSDs often have low harmonic rates, so there is little affects to power grid. To learn more about medium voltage drives.