3.5 MW motor starting problems continue... 7

[ters]

A while ago, I started a thread

http://eng-tips.com/viewthread.cfm?qid=235885&page=1

which many of you contributed to, but that story has not ended yet...

So the issue is that a 3.5 MW motor driving a large fan has problems starting. The starting method is the soft starter. Rotational inertia is high.

We discussed several causes, such as inadequate motor, inadequate starter, week and inadequate power system, etc. But calculations show that although things are marginal this still should work, but it does not.

Today we managed for the first time to put two identical 13/20MVA transformer in parallel. Their impedance is 10.5% at 20MVA but each transformer also has a current limiting reactor 6%. However, 20+20MVA system still would no start a 4MVA motor...

System: two 13/20 MVA transformers in parallel, + about 5 MW of additional load shared between two transformers.

Motor: asynchronous, TECO-Westinghouse 3.5MW, In = 385Amp, 6kV, 50Hz, 1485 rmp, locked rotor current 2300Amp.

Fan: Large rotational inertia. The fan has no load at the present (no air, dampers closed). The Impeller weight is 15,000 lbs, and the Impeller moment of inertia (WR^2) is 66,000 lb-ft^2.)

Starter: constant current, voltage ramp ~10 sec, current limit 470%, starting time 50 sec, after which bypass contactor closes (when then speed is about 80%).

We had some luck starting and running the motor using one 13/20 MVA dedicated transformer but now with 2 transformers in parallel it would not work. We tried three times and none of the starts was successful. Each time the soft starter was manually shut down after about 20 sec since motor was creating too much vibrations and was not accelerating any more.

Problems still could be electrical, but for a change we started to suspect that there may be some mechanical problems, perhaps related to the motor rotor axial movement. If the rotor indeed does try to move, either it may not have enough space to move to the electrical center or the shaft is moving back and forth around its electrical center for some unknown reason and hitting the trust.

The attached charts provide more info. During the voltage rump time, which was about 10 sec, motor runs smooth and with two transformers in parallel the initial voltage drop is rather modest - falls from 6.4kV to 6kV. This is good news, with one transfomer we had much larger initial voltage drops. However, the bad news is that after the voltage is ramped up high enough that the current reached the preset limit (470% In), some vibrations start.

As you can see, the current and voltage outline forms on the charts start at some point having "teeth" like a saw. Once stabilized there is about 3 - 4 “teeth”/second. Just judging by the ear, the frequency of mechanical vibrations is in the same range, meaning current spikes likely correspond with frequency of mechanical vibrations.

Any idea what might be happening?

 

[HienN]

I am the one who still confuse what is you really want.

1. For the switchgear: Normally, the control power is VDC or 110VAC fed from the external source. For example, the UPS, which is energized from another transformer, let's say, even your factory use medium voltage for big motor. It also must have another transformer for lighting and general purpose. 22/0.4kV transformer and it is separated from the medium voltage one. When the short circuit occurs all over the medium voltage system, the protection devices are still alive and able to isolate the faults.

2. You mean that, when using a breaker instead of vacuum contactor, how you could wire a motor differential protection? You mean is about power circuit or control circuit?
3. Why don’t you choose the fuse with a higher thermal withstanding capability?
4. The fuse was melted during sort start starting or via by-pass? Just look for the another fuses which have the higher operating current, not the one that used to protect motor. This fuse, use for only one purpose, protect the system from short circuit only.
5. For soft starter, the by-pass contactor does not need fuses (even the manufacturer offers you or not, you are customer, you will be responsible for what you choose). If the inline is a breaker, do not need fuse. Simple...

 

[ters]

OK, HienN, thank you. We both seem to be confused to some extent . You keep indicating that fuses could be eliminated, and I’m on “your side”, but several gurus here said no, and I cannot be ignorant to that, so I'm trying to understand why they say no and thus providing more specific details.

I our case, everything is already there, installed. Fully featured ABB UniGear switchgear fed by 220VDC (battery) and and fully featured ABB soft starter with optional GE motor protection fed by 120AC (UPS). Nothing to choose, everything is there, just to optimize as much as possible given the circumstances.

 

[ters]

Forgot to add, we did not specify fuses and any decision to change fuses type or eliminate then may have some warranty implication. The fuse size was determined by the soft starter manufacturer. We recently asked them to verify the adequacy of 24R fuse, they said they did it and found fuses adequate. But when we presented results of the thermal imaging, they realized that there was some problem.

Fuses go on the restart in bypass mode. The soft starter mode is not working, but it does not really matter, the current would be about the same (maybe 5% less in the soft start mode, as lower currents cause modulation phenomena which was initial subject of this thread).

Is inline a breaker? I would say yes. The entire 6kV circuit is like this:

motor switchgear incoming breaker- > bus-> motor feeder breaker -> 15m of cable -> soft starter switch -> fuses-> inline contactor -> bypass contactor -> 15 of cable -> motor

 

[LionelHutz]

Go here and read page L13.

http://us.ferrazshawmut.com/oem/resources/literature/Advisor Circuit Protection Applguide.pdf

I know you don't have Ferraz fuses but they are still 'R' rated fuses.

The point I was trying to make is that you can't just say "I've got a breaker with a protection relay and lot's of equipment is supplied with a breaker and protection relay so this application should not require fuses". You'd better ensure that the fault energy allowed by the breaker and protection relay doesn't exceed the fault energy that the downstream equipment can handle.

I did not state the only way to not have fuses is to use Power Buss -> Vacuum Breaker -> motor. There are posts here stating that many motors don't use fuses. Those motors operate as Power Buss -> Vacuum Breaker -> motor with no vacuum contactor (or any other power component in the circuit for that matter). Arguing that motors are connected Power Buss -> Vacuum Breaker -> Motor as proof that the fuses can be eliminated in this application is an invalid arguement.

 

[ters]

Thank you very much LionelHutz. My idea was not really to argue anything. I just had to make sure that simple reasons such as CPT voltage dies with the fault or there may not be 50 element on the protection relay or switchgear has no controls voltage when a fault occurs, and the like don’t apply in our case. But any other reason that fuses should exist may as well still apply and that is why I’m asking for an advice.

Based on the link you left above (Ferraz), 24R fuses appear inadequate by far for our motor (FLA 385 Amp). They would seem to be good for 330 Amp FLA and full voltage starts lasting 10 sec or less. With a large inertia load and using a reduced voltage start (and we have a reduced voltage start anyway, even in bypass mode – motor voltage is about 90%), the start lasts much longer in which case 24R would need to be de-rated to 75%, making it good for only 250 Amp FLA, according to Ferraz. Even 36R would seem to be marginally adequate for our case.

But as I said, we did not select fuses, the soft starter manufacturer did and they still claim they are adequate. Or, the problem could be that a large fuse, 36R, would properly coordinate with the motor load but would be too big to provide adequate protection for the soft starter contactors and other internal elements...

Speaking of the interrupting capacity fuse vs. breaker + instantaneous, in our case it is not so easy to conclude at what kA exactly the fuse becomes clearly more advantageous since the Bussmann fuse curves are a bit crude. VD4 breaker opening time is, ABB says 40-60 ms. Minimum protection operating time is 15ms. Adding other delays, seems that the fault would be cleared by the breaker at no more than 100 ms. Fuse 24R, on the other hand, based on availabe curves appears faster than 100ms for fault currents above ~ 12kA.

 

[LionelHutz]

OK, good you now see how the fuses are too small and you also can see how the fuses will clear quicker than the breaker.

Honesly, my concern is that a bad decision to remove the fuses could end up killing some poor innocent operator or site electrician.

The soft-starter problems seem to have dragged well into the it's never going to work territory so at this point I'd say your best bet is to move the motor leads and motor protection relay to the circuit breaker and use it as a full-voltage starter.

 

[ters]

We have already given some consideration to your proposal (just to wire motor directly to breaker). However, it would be much more complicated than it seems. Automation people created various interfaces to the soft starter, so all those interfaces should now be rewired to the switchgear. Also, motor protection, Multilin and its CTS are part of the soft starter. Multilin deals with motor RTDs, motor and fan vibration sensors, motor differential, etc and it also interfaces with the PLC for various automation, measurement control and monitoring reasons via a communication protocol. If the soft starter is not to be used, Multilin would have to be somehow fitted on the switchgear along with all mentioned wiring. That would be another project by itself.

May still be simple if we fix the heat issue so that fuses stop blowing and we continue using the soft starter as a regular starter. However, we also have dust problems, so we would have to pressurize existing IP54 enclosures with something like Vortex cooler if we want to use soft starters long time, as sensitive components inside don’t seem to like dust, so all together, as you said, not have these soft starters at all (except for the motor protection which is part of it) would be the best.

As for removing fuses, you are surely making a valid warning, nor we are looking forward to remove them, but I think that we deal with a border line case here. The SC level of 17kA on the motor terminals I stated above is the maximum one. Depending on the operational scheme of the upstream utility the range of SC level varies significantly. With a lower SC level than maximum, the fuse will be just marginally faster than breaker. I know that we have to factor in the worst case, but probability is still somewhat reduced.

 

[MikeHalloran]

The discussions of the system behavior here make it sound like a relaxation oscillator on a grand scale.

... which is a possibility, if some of the multitude of control/ monitor/ supervisory circuits attached to the soft starter are interacting with each other or with the soft starter in some unintended way.

Since you have apparently tried everything else already, may I humbly suggest carefully labeling and disconnecting _everything_ from the soft starter except that which is absolutely necessary for its basic operation, and attempting a start. If the system then works as intended, you know where to look.



Mike Halloran
Pembroke Pines, FL, USA

 

[ters]

Thank you for the post Mike. We tried already to strip down most of the external connections, no change. PLC wired interfaces are of discrete nature where nothing could even theoretically modulate like start, stop, ready, emergency contacts. Re analog signals, PLC interfaces with Multilin protection only where, via a communication link, it gathers info about motor current, temperatures, vibrations and the like.

But we don’t worry about it that much any more. The soft starter enters into that modulation phenomena if currents are below 450%. I would think that it is designed to have fairly good measurement accuracy and stable control loops if currents are in the area which still can be called a soft start. Say up to 400% at most. Beyond that, its accuracy and stability is questionable.

However, if the load characteristic is such that you need more than 400% LRA, what sort of soft start is that anyway? It then becomes a marginal case where use of a soft starter is not worth the hassle. One should then use direct online starts and live with marginally higher currents, or if that is unacceptable, invest in something which is way much more expensive, but does provide a real soft start, such as VFDs or mechanical devices like fluidrives.

 

[HenryOhm]

Probably insignificant point, but you said the motor is TECO-Westinghouse. Then you mentioned a lack of communication because of Taiwan.

TECO-Westinghouse I thought was based out of Round Rock, Texas versus just TECO being based out of Taiwan. I also had the impression the big custom stuff like your dealing with was Round Rock's bread and butter whereas smaller higher production runs were TECO's. See here:

http://www.tecowestinghouse.com/company/profile.aspx

http://www.teco.com.tw/fa/about.htm

 

[electricpete]

Good point. We bought a 4000P motor out of teco Round Rock and got absolutely outstanding support for some problems upon installation.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[ters]

Our 3.5 MW motors came from Taiwan, but for some reason the procurement went through a TECO office which was neither in Taiwan nor USA and they were our point of contact. Three other MV TECO motors of 900kW on the same project are also Taiwanese. Possibly they don’t do MV 50Hz specs in USA. But it is behind us, motors generally had no issues, except the above discussed burn marks which apparently were not very harmful but it has never been clearly explained why they occurred, there were some issues with the steel plates inside heat exchangers (cracks) and as mentioned above manuals were very generic but getting more info was sometimes slow and difficult.

 

[ters]

I started this thread in April, but for those who might some day suffer with similar issues, here is another update of this never ending story. Could be the last one, but may not be, who knows.

Since April we were using soft starters as just regular direct online starters (DOL) in the bypass mode as the soft start mode was rather much more harsh and came with severe current modulations, where the 50Hz was a “carrier” being modulated with something like 2-5 Hz depending on the speed during acceleration.

Soft start mode caused voltage swings and other oscillations, so it was basically unusable. We tried many things to tune it up, but nothing helped, so we used DOL mode.

At the time when reduced voltage soft starters were delivered, they came with two possible soft start modes:

- Voltage ramp/current limit mode. Meaning, it initially ramps the voltage from the starting preset level using preset steps until the motor current reaches a preset limit. Then keeps the current constant until the motor accelerates or until the bypass contactor closes, if set so.

- Current ramp/current limit mode. Similar as above, except initially ramps current until it reaches preset limit.

Neither of them worked in our case without electrical modulations and oscillations or occasionally complete motor stalling. Commissopning and tune ups were done with manufacturer's acceptance.

Recently, the soft starter manufacturer finally decided that they should do something about this and came up with a new control option, which they call kW mode, whereas during both, ramp and reduced voltage run, the soft starter is attempting to control power delivered to the motor, rather than current.

In essence, all three ramp/control modes do a similar thing - they all increase/control voltage on motor terminals. The difference between the three seems to be linearity of voltage increase during the ramp and what feedback information is used in the control loop, only current (I), or both voltage and current, being power (kW).

So one would hope that selection of a particular ramp/control mode shouldn't mean it does not work at all, but rather to help with tune up and optimization. But in our case, only kW mode, which became available 8 months after the equipment was delivered, does the trick and the motor accelerates with no current modulation and voltage swings.

The manufacturer indicated that two original control modes were purely hardware controlled, while the new kW mode was a firmware built in feature.

OK, that solves one of several design deficiencies these soft starters seem to have, while remaining, namely problems with equipment overheating during normal motor operating (fuse and protection cabinet overheating), frequent MV fuse blow on restart (fuses appear to be undersized) and dust issues are still to be dealt with.

Once all is solved, these RVSS units will be a sort of what they were expected to be when specified two years ago.

But we still cannot change laws of physics; RVSS are clearly a suboptimal starting method for such high rotational inertia fans, offering, compared with DOL starts, marginally softer but much longer starts. Our present options are:

- DOL start 26 sec, current in the range of 1800 Amp, or

Some sort of semi-soft start:
Ramp kW portion with average current 1300 Amp during 10 sec
Reduced voltage portion at constant kW with current in the range of 1600 Amp for next 10 sec
Full voltage portion (bypass closed) current in the range of 1700 Amp for ~ 17 sec.
Total start time ~ 37 sec.

 

[itsmoked]

Wow. Big wallowing, muddy, pig u g l y.

Great demonstration for the need of shutters. I would guess that shutters would have allowed the SS to work with any mode without instability.

They'd also allow things like a pony motor to get it all moving first.

I have a hard time seeing why kW would improve any dynamics associated with starting over the original modes. Perhaps they actually mess with the dynamics of the system with a control algorithm now. Or, the delays in the control loop provided new stability.

Thanks for the report.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[ters]

Thank you for the post itsmoked. I’m not sure I understand your comments well, if by “shutters” you mean air intake damper, it has been there from day one, closed during the start, but made no difference. There is also a guillotine type air outlet shutter, but closing it as well during the start made no difference either. Sorry if I misunderstood your comment.

I cannot explain any specifics as what makes the big difference between original hardware controlled control loop and new hybrid controlled loop, which is a combination of hardware and software control (part of firmware).

On the modulation subject, I could publish a novel with initial correspondence with the manufacturer, and where they were insisting that the modulation phenomena was caused by 110/6kV transformers, impedances, cables, motors, and the like. We then did a major 110/6kV substation upgrade, substantially reconfigured 6kV system, etc, but the modulation phenomena did not go away. Actually it got worse -. So if we were to seriously take their explanations of the modulation problem, our net step in solving that phenomena was probably supposed to be to upgrade the entire national power generation and transmission system only to suite RVSS original control modes ; even though there is a 1000 MW power plant next door to this facility.

At the end, the modulation problem was solved just by revising RVSS controller firmware and introducing an improved more stable control loop. Normally, one would expect a stable control loop to be part of our RVSS from day one, however, the fix arrived about 8 months after the equipment was delivered.

Who knows why it all went like this. These are fairly large units, in theory rated to 7.2kV 2400 Amp for 30 sec, and since it is a USA manufacturer, but the equipment is installed in the 50 Hz world, we are unsure how many of similar applications they had experience with before, if any. They also indicated that their new control mode, kW, was not available before and was employed for the very first time in our case

 

[LionelHutz]

Some sort of semi-soft start: Ramp kW portion with average current 1300 Amp during 10 sec Reduced voltage portion at constant kW with current in the range of 1600 Amp for next 10 secFull voltage portion (bypass closed) current in the range of 1700 Amp for ~ 17 sec. Total start time ~ 37 sec.

I'm just going to be completely blunt. This is still a completely useless piece of garbage acceleration profile. Motortronics has still not fixed the issue and your soft-starters are not working as soft-starters. If you were my customer, the head engineer who designed the soft-starter controller would be on your site until the controller worked as intended.

The fact that the bypass closes before the motor reaches full speed makes the soft-starter completely useless. The soft-starter should not ramp the motor for a time and then just bypass and full-voltage it. Completely useless....

Just don't judge future soft-starter performace based on the crappy operation of that piece of junk. It deserves to be pulled out and smashed or used as target practice for giving you so many issues and such a bad impression of soft-starters.

It appears to me that a properly working soft-starter would work well on this application. You use about 470% current and 26 sec. with a full-voltage start and, from what I calculated before, a 450% current would require 43 seconds to start. This indicates that your data which I used was very conservative and, most likely, the 350% current limit start would work very well.

 

[ters]

LionelHutz , I can only hope that someone from Motortronics reads this thread as due to political reasons I cannot communicate to them using exactly the same tone you just did.

Strange situation and a strange business practice. The Client is basically at Motortronics mercy when it comes to solving any issues as the equipment was paid long ago, so remaining options are perhaps some legal actions, while Motortronics apparently have enough work and do not seem to care that this case might possibly be damaging to their commercial reputation.

Maybe their approach is such since this is all happening in a Balkans country, a market, both of them, ABB and Motortronics appear totally uninterested in when it comes to the RVSS equipment. However, still strange, as this industrial facility is owned by a USA company, one of the largest steel manufactures in the world, a prime customer for many big equipment suppliers, while, we, owners engineers on the project, also come from a large Canadian consulting company (8000 staff), so we are occasionally in the business of recommending someone’s equipment or not... Apparently, the fact that equipment was made and serviced by Motortronics, but sold with an ABB label doesn’t seem to help too.