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?

 

[ters]

Thank you for comments LionelHutz. Unfortunately we do not have such a close zoom of what happens with the current within one period or so. Next time when we start the motor, we will try to record it, which is not going to be soon since we are now doing some major substation work and none of the transformers is presently available.

We received an interesting comment from the motor manufacturer (TECO). They seem to believe that the problem can be solved by “fine tuning” the soft starter settings and offered their commissioning engineer to do it.

And while enything is possible, I don’t quite believe that so far we were unable to figure out what would be close to optimal soft starter settings. There were a large group of engineers involved in this, starting from the Client, who has some very good engineers like Mr. Panter, then we also had ABB rep on site for 10 days who is also a good engineer except he had English problems and finally we had even three (3) consulting companies involved in this each having an average of 2 engineers working on this, plus, a great help from several member of this forum, plus some other engineers I personally know worldwide were sending me some comments by e-mail. And so far we all failed to “fine tune” it...

I’m not a hiring authority here, but if I were one, the deal I would be willing to offer to TECO would be: come and fine tune it, and I will pay you $1000/hour; but if you fail, you hourly rate will be $5/hour...

 

[LionelHutz]

Your motor operating rough is odd. A problem with the current loop stability will cause this. I've seen it before in the past with poor code. A bad motor could also cause this.

Your line voltage oscillating during the start is also odd. You should be drawing a steady continuous current which should cause a steady voltage drop. Are there tap changers switching?

Messing around with different transformers and switchgear is most likely not going to fix the problem. You have been using 450% current for many start attempts. You should have had many good starts.

I remember reading about spare motors. Maybe it is time to try another one? I'm not convinced the softstarter is working correctly though. You have enough line capacity according to the voltages you measured during starting, assuming your motor and load data is correct.

 

[ters]

Thank you very much again LionelHutz. No, tap changers are only operated manually - they do not move during the start.

We wanted to try different power system configuration as there were arguments (by soft starter manufacturer) that one 13/20MVA transformer with somewhat higher impedance then typical was barely enough for 2000Amp start. Maybe it was, but two units in parallel should be just enough and impedance goes down as well...

In summary we had 3 scenarios:

1. Initially no start at all (spins but does not reach full speed)

2. Then, occasional successful starts

3. Finally, no start at all again, after the motor have been running for a month or so, ON and OFF

I would say that difference between 1. and 2. was related to the soft starter settings and tuning. But the same is unlikely to apply for difference 2. vs. 3. - the soft starter settings are still the same as they were when starts were possible.

So based on the fact that the motor was able to start the fan (admittedly with difficulties, and randomly successful starts), but now we cannot start it at all any more, one can still make a number of reasonable guesses and among other things say that the difference between 2. and 3. above may be because:

A) We have always had some problems with the soft starter which just got worse

B) We have always had some problems with the motor, such as rotor problems, which just got worse

C) We have always had problems with the soft starter but meanwhile a motor problem has developed as well

D) We have always had problems with the motor, but a soft starter problem occurred meanwhile as well.

Probability that any of the above statements is true still exists. However, soft starter manufacturer says they believe there is nothing wrong with the soft starter while motor manufacturer says there is nothing wrong with the motor...

 

[ters]

And about spare motor... We have spare everything, motors, soft starters, and just installing a spare transformer . Joke - third transformer is being added since we will have three such motors and many other loads, so existing capacity was not enough. But nobody seems to have guts to say, lets try a new soft starter, for example...

System capacitance was also mentioned by the soft starter manufacturer. But I could not identify much of it. Cable between the motor and the soft starter is hardly existing - just abut 15 meters... Cable from the soft starter to the upstream switchgear (which is to feed 3 soft starters) is also very short - maybe 10-20 meters. There is a longer cable, about 300m from the upstream switchgear to upstream-upstream switchgear, which is 3x240mm sq per phase (in the range of 3x500MCM per phase), but that is much before the SCRs, not sure if it can create some sort of resonance phenomena?

 

[electricpete]

On what basis did the motor manufacturer evaluate the motor? As far as I know there is no diagnostic data provided for the motor (correct?). Did they study all of the unsuccessful starts and conclude that this was not enough current to damage the rotor? Just curious how they could reach this conclusion.

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

 

[ters]

Electricpete, the info I post here is very similar to one we provide while talking to manufacturers or some other parties. Sometimes it is literally copy/paste. So what basis TECO used to make the statement, I do not know, but they did not have much more info (from site) than posted here nor we can provide much more for now...

 

[QBplanner]

Ters:

Few requests before I can ofer any useful suggestions:

1:What is the system fault level (Xd") at the 13MVA transformer secondary side with single transformer?

2o you have recorded Toque Vs. Speed Curves? I mean the failed motor+fan starting case?

Thx!

 

[ters]

QBplaner, thank you very much for offering help. I do not have all the info you requested, but speaking of 13/20MVA transformer, the fault level on the secondary side, after the current limiting reactor, is about 200MVA – this is for one transformer only, not for two in parallel (we have an option to parallel two identical transformers). However, the motor is some 400m away for the that bus (from the switchgear after reactors to motor terminals), so the fault level on the soft starter and the motor will be somewhat lower - we will calculate today what it is, and can also provide more info on impedances of all the components.

Unfortunately, we have not recorded torque vs speed curves for any of the failed cases. Nor I'm sure how to do it, we can record speed, voltages and currents, but how do we measure and record the torque in our situation? Could you provide some guidance so that we can try recording it next time when we are in position to start, provided we can find adequate test equipment. We do have standard torque vs. speed curves as provided by the manufactures for both, the motor and the fan, if that can be of some help?

 

[slavag]

Hi Ters.
Are you try shunt this reactors for 50sec?
Are you think about something like to Clip or Is-Limeter for the shunt those reactors?

Best Regards.
Slava

 

[ters]

Hi Slava. Yes, we tried to to bypass the reactor with adequately sized cable. That eliminated almost half of the impedance (reactors are 6%, while transformers are about 8% at 13MVA (name plate says 10.23% at 20MVA). But made no difference... With or without the reactor, the problem with the motor start remains the same. Only voltage drop is lower and voltage is more stable.

 

[ters]

QBplaner, on the motor switchgear, which is just 15 m from the soft starter, the short circuit level is about 175MVA with one transformer, or ~ 350MVA with two transformers in parallel.

 

[LionelHutz]

I really don't have much more to add.

Examining the motor and load data says it will start with the 470% current you have been trying. You recorded voltages on the input and output that showed the soft-starter is reaching the programmed current limit without the SCR's being full-on, meaning you have enough power.

The only issue the system could be causing is that the control voltage dip is messing up the soft-starter controls. The soft-starter should have protection to trip before this happens though. Maybe try a good sinewave output UPS on the soft-starter control circuit? I doubt it would help but you never know unless you try.

Problems not accounting for cable capacitance tends to cause SCR failures, not an unstable start. The capacitors tend to affect the SCR's as they are switching.

Basically, I see these possible problems.
1. The motor manufacturer gave you bad motor data.
2. The load is worse than the data you were given.
3. The soft-starter is bad.
4. The motor is bad.

I'm tending to think it's the starter. The rough starting and line voltage oscillations are pointing to it not being stable. Something in the control loop or SCR firing is not working like it should.

My company could help you figure out how to start the motor for you, but it would cost you new soft-starters because we won't support those units. I doubt you want to go that far.

 

[QBplanner]

Ters:
Thanks!
More questions, the fault level you provided is for Xd".
Do you have the fault level for Xd' and Xd. If you have them it would be helpful.

LionelHutz:
Am I missing sth here since it has a long chain.

1. The motor manufacturer gave you bad motor data.- Could be
2. The load is worse than the data you were given.-I am confused since the fan was not loaded up and Ters mentioned once a time it only has estimated 500kw.
3. The soft-starter is bad.-could be
4. The motor is bad.-Interesting point but what does it mean "bad"
I looked the cases you did for Ters. I don't know which kind of software you were using to simulate the motor starting. But the software you used must be a very rough one with no motor electrical data input. Therefore, it may not correctly representing what really happened in reality.

I agree with you for your analysis:
1: System is too weak to start. Could be and that is the reason I request fault level under different time zones.
2: Soft starter has problems.Could be But I recalled by reading the long chain, it can start the motor without the fan.
3: Motor design problems

 

[QBplanner]

Ters:

"QBplaner, on the motor switchgear, which is just 15 m from the soft starter, the short circuit level is about 175MVA with one transformer, or ~ 350MVA with two transformers in parallel."

I guess even with two transformers in parallel, you may not get 350MVA. you may maximum get 200-250MVA depends on the Trx Leakage impedance and the rest of the system impedance.
More questions: I recalled when you were trying to start this motor. you shut down the other loads in the same plant.Is that right?
Do you have the station one line of your plant or at least a drawing shows how the motor connects to the system and the other loads in the plant? It would be helpful

 

[edison123]

OK. I will stick my neck out and say, screw the soft starter and do it DOL.

 

[QBplanner]

Ters:

I like to understand what happened first before I can offer any helps.

First of all, using IEEE standard induction machine model-simplified one. The opertaing characteristic of the induction motor is such that the internal torque developed by the machine is a function of the rotor resistance and slip at which it occurs.

When starting the motor, R2'/s is small because the s = 1. So the current is high like a short circuit on Trx secondary side.

When trying the increase the voltage and build up the flux and torque(I recalled you use lower voltage starts the motor and increase it step by step), the motor could not speed up to operating one which is about 95%-99% of Synchronous speed but stayed at 15%-30%. It can explain why your upstream CB got tripped. Because if the motor can not speed up, the rotor resistance will still be very small. Ex. if the speed stays at 15% only, then the equivalent motor circuit resistance only increases to 117% compare to the motor with zero speed. Therefore, when the voltage kept increasing, high current will be created until the upstream CB tripped. I estimated the current will be around 2kA.
Let me know if I missunderstood or misinterpreted anything.
Regards,

 

[ters]

LionelHutz, I would bet that we initially had some soft starter problems, and we actually had – at the beginning of the commissioning, ABB rep replaced a couple of boards, but who knows if all was fixed at that time, maybe not… And after that, while testing day and night , a motor problem possibly developed, such as damaged rotor – because the situation is now worse then it used to be – we cannot start at all…

Edison123, I have to agree with you. Using a soft starter with 500% In vs 600% In for DOL start is pure waste of time… What the hell is the difference… But I’m not one driving the show here. Rather one who is trying to plant some common sense here and there whenever I can…

QBPlaner, unfortunately that is all I have, Xd”, but I can possibly get info on Xd’ or Xd or whatever else. If it helps, the fault level on the high voltage side (110kV) is 2600MVA

“More questions: I recalled when you were trying to start this motor. you shut down the other loads in the same plant. Is that right?”

Generally no. The voltage used to drop, but we were experimenting with one transformer with no other load, so nothing else could be shut down. There was one instance when the voltage drop was somehow reflected on the HV side (110kV) as well, but I would ignore it, who knows what was on the grid at that time. I’m not about to speculate (again) that a national grid has problems starting a 3.5MW motor ?

“Do you have the station one line of your plant or at least a drawing shows how the motor connects to the system and the other loads in the plant? It would be helpful.”

Yes here is something, attached. Assume that two 110/6kV 13/20MVA transformers connect to the national grid via two short line. SC level before and after lines 3500 and 2600 MVA. The attached sketch does not show any other load, but there is some at Bus 2 and more at Bus 1, but most of the time during testing, there was not much if any other loads connected – one transformer was dedicated just for this motor.

“I guess even with two transformers in parallel, you may not get 350MVA. you may maximum get 200-250MVA depends on the Trx Leakage impedance and the rest of the system impedance.”

Could be. Lets assume that I’m not getting more than 200MVA, although I do not see where all other 150MVA will vanish. I asked somebody else who does calculations for us to calculate, will confirm. So if I’m getting only 200MVA, is the something what will prevent me to start the motor with such load we have?

“Because if the motor can not speed up, the rotor resistance will still be very small. Ex. if the speed stays at 15% only, then the equivalent motor circuit resistance only increases to 117% compare to the motor with zero speed. Therefore, when the voltage kept increasing, high current will be created until the upstream CB tripped. I estimated the current will be around 2kA.
Let me know if I missunderstood or misinterpreted anything.”

Here is how this starter seems to work. Voltage ramps up quickly until we reach 470% In. Then the voltage floats and can be anything – if the current tend to increase the voltage will drop to bring the current back to 470% and vice versa. Therefore motor may want whatever current it wants, but it will get only what the settings allow, which in our case is 470% In. If motor want more current, the soft starter will simply reduce the voltage what in turn surely will reduce the torque. Forgot some my previous posts, we have all kind of nuisance tripping, but now in general, if the start is say 50 sec, we have say 5 sec to ramp up the voltage to whichever level will push 470% of current and then it just stays there until the current starts dropping (if the start is successful) in which case voltage starts raising. Typically, during the start voltage masured on termainal was not much more than 5kV which is 80% while the current sticks to about 1800Amp or below.

 

[ters]

Forgot to attach the sketch

 

[LionelHutz]

OK. I will stick my neck out and say, screw the soft starter and do it DOL.

It's almost there, the general plan right now seems to be to just keep adding system capacity until the soft-starter can be set to start across-the-line in the hope the motor starts.

I looked the cases you did for Ters. I don't know which kind of software you were using to simulate the motor starting. But the software you used must be a very rough one with no motor electrical data input. Therefore, it may not correctly representing what really happened in reality.

I'm not sure why you conclude that. All the required motor data has been entered. The simulations almost always prove to be conservative compared to the actual motor starting.

The motor and/or the starter is bad. Since I believe that, I have nothing else useful I can add so I'll just be checking back to see if the problem is found.

 

[QBplanner]

Ters:

Thanks for the information and drawing you have provided and you don't need to provided Xd' and Xd fault levels.

I can understand a little bite of your frustration but thing can only be done step by step.

Forgive my different understanding of soft starter. I understand of how soft starter works is it will starting at a lower voltage 50%-70% in order to avoid full in rush current because of the small rotor resistance at zero speed and ramp up later to build up the motor flux and finally stays at its operating point.

The reason of asking the fault level was that I was trying to dismiss the possibility that the failed starting was caused by the lack of enough system strength.

For a single Trx with current limiting reactor (0.21 Ohm mentioned in your previous post) it is about 0.52p.u 100MVA base 6.4kV nominal voltage), you will have approximate 90MVA fault level at the terminal of motor or slightly less considering the cable impedance, and with the 470% current limit, you will have about 18% maximum voltage dips.

If you parallel two Trx and two reactors, it will give you the same fault level-170MVA at the motor terminal as you run a single Trx without current limiting reactor. 170MVA should have no problem to start a 3.5MW motor with 6 X FLC with a direct cross line start without the soft starter.

Therefore, the problem should not be on the system side.

Now, I am not sure the motor status, I saw some posts you mentioned the noise and vibrations happened. I am not sure if the motor has any internal damages.

You may try followings steps:

When you parallel the two transformers, you may try a cross line start without soft starter. It will cause approximate 12% voltage dips. You may have to coordinate P&C settings. If the problem is still there which means the motor can not be started and still stays at a lower speed. Then it means it is not the soft starter problems.

Finally, it is the problems of the motor which is the tough part.

Assuming your motor has no internal damages, then what I can contribute to you from my narrow mind is that, the motor may have 5th or 7th harmonic torque components when starting the motor. (The other people already mentioned lower voltage, lack of starting current, lack of Torques and lack of system strength which I won't mentioned again). With theses torque components combined with the fundamental component, it may be has following scenario:

The sum of the load torque including the load (Fan) and motor loss torques (T Load +T loss) is smaller than the required starting Torque Tst but larger than the minimum Torque T min (During starting period motor will have a minimum Torque), the motor will stay at a lower speed 15-30% and never reach its operating point. If it never reaches the operating point with the slip value of 0.01-0.05, then the motor current will always stay high like 4 or 5 times FLC until it tripped out.
Hopefully it helps.