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?

 

[QBplanner]

Hi Ters:
Have not got on this site for almost a month.
It seems the voltage drop is 11% which is close to what I estimated 12% voltage drop in my previous post with cross line direct start. Not sure how many transformers you use single or double. At least you can start the motor. congratulations!

Fan vibrations problems is another area which may triggered by mechnical or electrical or both. I disagree with some words like "chased and exaggerated" there are no such things. There are always the causes. people should provide some professional comments.

In your case, soft starter is not useful since the system the motor connected is pretty strong with 220kV substation closeby.
Good luck with your 3.5MVA motor. Keep us posted once you solve the fan problems. AT least you ruled out the soft starter.

 

[ters]

Thanks QBPlannner. Here is a happy resolution of vibration problems... After running out of the options, it was decided that in the nearby large city there could be some relevant expertise. So a university professor (mechanical) was hired and he fixed vibration problems in an unbelievable efficient way. He came, did some measurements with an old analog meter and said that impeller needed to be balanced. Nobody believed that was the case since supposedly it was properly balanced in the factory... Then he welded some weigh on the impeller at a spot which seemed to be randomly chosen (that old meter could not tell him where to weld it) and vibrations went down from 25 mm/s to 15 mm/sec. Then he calculated something and welded another weight at a different spot and vibrations went down to 2 mm/sec...

 

[QBplanner]

Hi Ters:

Good to know you fixed the problems. As I said problem has its causes.

Know I can conclude that you are the expert of motor starting now after this 3.5MVA motor case.

Best Wishes to you.

 

[ters]

For those who were interested in this never ending story, just to let you know that the saga is still ongoing...

We meanwhile commissioned two units, but none still works in the soft start mode. Both have some hardware problems and we use DOL start. The manufacturer’s rep was here, but looks like he will have to pay another visit.

The question I would like to ask you experts is related to MV fuses this time. They are in the incoming cell of the soft starter. Bussmann type JCR-A-24R 7.2kV, double arrangement. Several have already blown. We have all kind protections before fuses which should trip much faster in case of a fault, but since the protection is not operating, then the only explanation is that fuses go because of extensive heat.

Fuses blow on motor restart, after motor has been running for some extended hours. Starting current 1800Amp and fuses tend to blow after 10-15 ec.

We just did thermal imaging and the results are: fuse temperature about 120 DEG C (~250F), while fuse cabinet air temperature is about 65-70 DEG C (~ 150-155F). The ambient temperature at the time of measurement was 32 DEG C (~90F).

The question is: do these temperatures appear to you being well outside normal operating range for similar applications? The fuse compartment dissipates like a furnace of several kW.

 

[LionelHutz]

A single 24R fuse per phase? Those fuses are too small considering the starting current and starting duration.

You indicated an approximate 30 second start earlier.

30 seconds minimum melt =~ 2200A
2200A x 0.75 = 1650A

So, the starting current should be under 1650A for the duration of the start. The fuses might be acceptable if the start time is shorter and/or they were kept cooler.

R fuses should not open on motor starting. The motor overload relay should trip before the fuses open. R fuse are for short circuit protection. 24R fuses should never operate if the current is below 2000A.

I'd have to look up the UL file to see what the maximum temperature allowed is. The fuse may be within it's ratings but 120C is still hot. The fuse is allowed to run with a continuous 450A in a 40C ambient but the ambient is getting up to 70C. The fuses need some cooling or need to be replaced with larger fuses.

 

[edison123]

The bigger question would be why were the protections not acting before the fuses blew up ? Or were the fuses designed for soft start and not for DOL ?

You mention that "fuse cabinet air temperature is about 65-70 DEG C" and yet you claim "The fuse compartment dissipates like a furnace of several kW."

The whole system sounds screwy and the data integrity seems suspect.

 

[electricpete]

I can’t say I have ever heard of fuses used to protect motors that big. Don’t they have CT’s feeding relays to trip the breaker (for instantaneous etc) ? Why not just use a suitable time overcurrent relay instead of fuses?

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[ters]

There are really no any real faults in this case and I’m unsure why fuse are there at all.

As for other protections, we have plenty. Feeder protection before the soft starter, ABB 542Plus, which is set to trip much faster, something like I>> = 100 ms at 2500 Amp, and I> = 500 Amp for 50 sec (to allow motor start). I also set I>>> to something like several ms at 3000 Amp. Then, we have Multilin 469 motor protection just one meter before fuses (and some 10 meters after 542 Plus) which also has adequate setting for both, I>> and overload set so that is coordinates with the upstream protection, 87 works fine, etc. All protection elements are set and tested and definitely operating if and when needed.

But since none of the protection elements ever operated, the fuses simply blow for now reason, just due to extensive heat.

So I was suggesting to eliminate fuses as they seem to be rather redundant and unnecessary and just maintenance nightmare. The manufacturer elected not to comment on this proposal, silent like fish, while the Client is for now too scared to agree with such modification...

As for the fuses, they are double arrangement, and according to manufacturer's curves, minimum melt time @ 2000 Amp is about 70 sec, while total clearing time @ 2000 Amp is in theory indefinitely. Since manufacturer's curves are declared for a normal ambient temperature range, I suppose that fuse characteristic is severely altered by very elevated fuse and fuse cabinet temperature.

As Edison indicated the system seems to be screwy, but not the whole system, just the soft starter part...

 

[ters]

Forgot to mention, my thinking was in like with LionelHutz, expect maybe I don’t quite understand manufactures ratings for 24R (which seem that that nominal temperature, total clearing time will be indefinitely @ 2000Amp) and that was a reason for posting this question.

Yes, the fuses seem to be inadequate for the enclosure type (NEMA 12) and size and cooling is very poor, none. However, the manufacturer blames it all on the way how we use the soft starter and said that we need to cool down fuses for one hour after each motor restart... But, sometimes we have no way of doing it as the process must work, so there is some cooling time always but may not be longer than 30 minutes.

 

[LionelHutz]

Yes, it appears you don't understand the ratings.

The fuse is rated;
450A maximum continuous at 40C
1910Arms minimum interrupting rating

This is not a full range fuse. There is a range of currents where it is not safe for the fuse to operate. If you allow the fuse to operate in this range the fuse might actually burn or rupture. The fuse should never be allowed to clear an over current or fault current that is above 450A and below 1910A. Above this, I'd believe the 450A rating would be lower when the fuse is in an ambient above 40C.

I believe the fuses should be 36R rated. However, depending on the other components in the starter you could lose your fault rating with a 36R fuse. Still, the 36R fuses is your best bet to keep the fault rating of the starter.

Without the fuses a vacuum contactor could rupture. You'd have to do a co-ordination study between the failure curve of the contactor and your protection to ensure that fuseless operation is safe.

Also, it sounds like the fuse compartment should be ventilated. I'd expect that a few 4" grills top and bottom of that compartment would be all it would require.

 

[electricpete]

So these are current limiting fuses used for faults above the breaker interrupting rating?

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[electricpete]

Or are you saying the fuses protect a time/current region in between the contactor overload and the beraker isntantaneous... i.e the fuses protect some region higher currents / lower time than the contactor and lower currents/higher time than the breaker?

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[LionelHutz]

Not really. The fuses might be co-ordinated with the bypass and/or inline vacuum contactor in the soft-starter to give the package a UL347 - Class E2 type of rating. The package will no longer have the factory fault rating without the fuses.

Why is this so hard? The soft-starter is shipped with class R fuses and called, say, 50kA fault rated. If you put jumper bars in place of the fuses then you have modified the origional package and it will no longer have a 50kA fault rating.

If you decide to remove the fuses, then you would need to look at the fault withstand ratings of the components in the soft-starter and "take ownership" of ensuring there won't be a catastropic explosion the first time there is a failure after the fuses (likely a motor or wiring failure).

 

[electricpete]

Thanks. I certainly was not suggesting anything be removed without a review. I was just curious to understand the purpose of those fuses - (we don’t have any soft starters or fused large motors).

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[HienN]

Hi all,

Contribute to this never-end story, the power fuses are there to give an extra protection for the switchgear (contactor type...). For our case, normally,the motor control center (contactor type)is provided with (51 and 51N) protection relays.

Depends on the types, so we decide the power fuses. If you dont like power fuses, you can easily provide (50,51, 51N...) in the protection scheme. This case, power fuses of course are eliminated.

Due to extremely fact acting with instantaneous over current (naturally), so power fuse still can be an option. I think.

Furthermore, you can see in low voltage motors, some of them are also equiped with fuse above starters over MCCBs... Anyway, it really depends on which type of startes used.

 

[ters]

LioneHutz, thank you very much for your explanation. I have mostly consulting background, and in theory consulting engineers are supposed to know everything about anything . In the reality, though, that is usually not the case so my knowledge of MV fuse theory is more basic, especially when it comes to the de-rating math based on very elevated (abnormal) fuse compartment temperature.

Speaking of interrupting capacity, yes, it appears that the soft starter package was designed to provide its own high interrupting capacity since the manufacturer cannot normally anticipate for every single case what current interrupting device exists upstream. In our case, however, we have a brand new switchgear with a vacuum breaker feeding the soft starter (rated 40Ka 1 sec) and an adequate protection just 10 meters away from the soft starter.

The protection is settable to operate at very low time delay, such as 30ms (has 4 current elements,I>, I>>, I>>> and IDMT). So 30ms + breaker opening time, that all should not exceed 120ms - is this far inferir to the fuse clearing time at say 20kA? 20kA is the short circuit level on the swithgear bus.

HienN, thank you for participating.

 

[LionelHutz]

Well, I'm not spending time to do any type of co-ordination study and that is what is required to determine what protection is suitable. That's why I posted that you will have to take ownership of that. If you can't do it then you'll have to pay someone who can.

If you want a factory supported solution that works then you're more than welcome to purchase a replacement starter from us. We compete with Motortronics in the North American marketplace.

 

[oldnotbold]

Does your Soft starter use IGBT's or thyristors?
Power Converters may develop short circuits or faults and the resultant fault currents must be cleared quickly. Fast acting fuses are normally used to protect the semiconductor devices. As the fault current increases, the fuse opens and clears the fault current in a few milliseconds, (less than 1/4 cycle). Fuse manufacturers recommend placing a fuse in series with each semiconductor device. The individual protection allows better co-ordination between a device and its fuse and protects from shoot-through-faults. In selecting a fuse, it is necessary to estimate the fault current and then to satisfy the following requirements:
1) The fuse must carry continuously the semiconductor device rated current.
2) The I^2t let-through value of the fuse before the fault current is cleared must be less than than the rated I^2t of the device to be protected.
3) The fuse must be able to withstand the voltage after arc extinction.
4)The peak arc voltage must be less than the peak rating of the semiconductor device.
Thyristors have more overcurrent capability than transistors, as a result it is more difficult to protect transistors than thyristors. I don't know what your SoftStarter is fitted with.

From previous entries it appears that you may degrade the fuses, either through using too long a starting current, or by not giving the fuses time to cool before attempting subsequent restarts. They thus become degraded but not blown. If you need to perform a series of tests or starts on an already well established circuit, remove the fuses, replace with shorts and hopefully rely on your upstream fast high-set O/C protection. You can change/replace fuses after test completion.

 

[ters]

Thank you LionHutz again. We are able to do a small coordination study and have tools to do so, but we need to collect some more data. We already calculated short circuit level at the motor switchgear and concluded that, depending on the substation operational scheme upstream, 15-18kA, which I called 20kA above.

Re taking the ownership of that decision (to eliminate fuses), we don’t have problems with that, however, we believe that this equipment simply has some design flaws. As I said, the fuse compartment typical air temperature (when the motor is operating) is 70C (155F). This is high. Also, motor protection (Multilin 469) panel temperature is about the same – the protection panel is somehow indented (sunken) in the fuse compartment, it is very close to fuses, and its temperature is no different. We are going to consult GE about this, but it is unlikely that they will answer that Multlin 469 normal operating temperature can be 70C. It should be much less, it is a sensitive electronic device.

As for the new soft starter, I’m not sure that is practical to replace it now, but who knows, if we face other problems, we may have to research that option too. This is a 50Hz world, however, not N. America.

Oldnotbold, thank you very much for reply. Our soft starters use SCRs (thyristor). The fuse characteristic is attached (page 8, JCR type, exact model JCR-A-24R). This appears to be motor circuit protection fuse, and does not seem to be also an ultra fast SCR protection type. From its characteristic, it seems that total clearing time at 15 kA is about 30 msec. These fuse in in the incmoning cell, one per phase.

However, the soft starter is not being uses as a soft starter yet, SCRs are still in the circuit but turned off and the motor is being started through the bypass contactor only (DOL).

Instantaneous protections are set at low current and short time (3000Amp; 0.1 sec) but none ever operated. We wiull shorten the time furher but there appears to be no shorts on the motor or the soft starter circuits since the same phenomena (fuses blow on motor restart) occurred on two different units under the same circumstances. It is just that during normal operation, fuses operate at high temperature with no ventilation and cooling and occasionally blow on the motor restart after a time delay of 10-15 sec.

And while we could “buy” the explanation offered by the manufacturer that fuses need to be cooled down before each motor restart, there are two problems with that: the process sometimes cannot wait an hour or two just for that trivial reason (to cool down fuses) and the manufacturer never took an exception that such cooling would be required, and, as I mentioned above, in no way the fuse compartment and protection compartment normal operating air temperature should be 70C (155F).

According to the curves, fuses should never blow during the motor restart, 1800 Amp, 30 sec. But curves are not given for the temperature we measured (fuse temperature 120C), so we have to determine what is the total clearing time at 1800Amp if fuses are already preheated to 120C (250F).

 

[LionelHutz]

however, we believe that this equipment simply has some design flaws

Yes, there does appear to be some issues.

We do sell some products in South America, China, Middle East, Africa, etc, etc. 50Hz or 60Hz makes no difference.