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]

Some more words, considering of eliminating the power fuses, this practice should be very careful. Fuse is natural protection device while relays are only mechanical and electronic one. Hence, the malfunction, mis-coordination, and damage could occur on such equipments. Extra protection by power fuses are not unnecessary especially for medium voltage where one prolonged fault could lead to another disaster like shutting down the whole power system.

Although coordination is carefully studied, application and actual condition are very different.

Old problem, it happened one a rainy day, there was “unlucky” rat found his shelter by hiding himself inside 6.6kV Switchgear supplying power to 640kW motor. He touched one leg to one of the phase. “BANG”, the whole system shut down, 6.6kV power fuse, 51, 51N Protection relays, 22kV switchgear at incoming of 22/6.6kV transformer, and the recloser at 22kV incoming power pole…

HienN

 

[oldnotbold]

Ters, this project seems to be a veritable minefield and will probably take years off your life! I would seriously recommend some sort of design review, or Site Acceptance Test, together with the Softstart/Control panel manufacturer. Is this ABB? If it is, I'm a bit surprised that they haven't given you more useful advice or help. I used to work with ABB Drives in Baden, Switzerland, back in the 1980's. We had several people who's only job was customer service, troubleshooting/answering customer questions etc. Those days seem to have gone. Still, your system doesn't appear to be fit-for-purpose because of a questionable soft-starter application and a heat dissipation/retention problem. As a consultant you should probably reject the whole installation until ABB prove that it's fit for the purpose in hand. How did the commissioning proceed? are ABB responsible for commissioning the soft starter? There seems to be a project-management problem in co-ordinating all the resources necessary for satisfactory project completion, I expect your client is trying to do too many things on-the-cheap, however there is a limit one can go in this regard. Do you need another consultant/project manager?

 

[electricpete]

fuse is natural protection device while relays are only mechanical and electronic one. Hence, the malfunction, mis-coordination, and damage could occur on such equipments. Extra protection by power fuses are not unnecessary especially for medium voltage where one prolonged fault could lead to another disaster like shutting down the whole power system.

It sounds like you are saying that all large motors need fuse for protection? That would be surprising to me since we have running at this moment approx 120 MW worth of large motors without fuses. The largest are 13.2kv 8000hp.


=====================================
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[HienN]

Electricpete:

I agree with you for this case. I did mean by saying that all large motors need fuses. But would like add up by saying a little bit more that: For big motors (Medium voltage motors) that use contactor as switching equipment.

Fused vacuum contactor is the cheapest one in comparison with any other switching equipment. Let's take an example: We provide 50,51, and 51N protection relays for the switching device (Circuit beaker type, this will get a faster tripping as a result in comparison with contactor). However, there is always the mater that should be though about: Price, and while circuit breaker type seems to be not fit with normally ON and OFF switching. That is why the switchgear manufacturer prefers fused contactor type with 51,51N protection relay for overload and ground fault current.

Otherwise, in most of brochure and manual, you will see that many manufacturer mentions fused contactors as the most convenient device (easy operation, replacing, and coordination).

Could you describe a little bit about your motor starter (protection and switching type).

I am working on a project that has motor more than 800kW. I will also discuss this issue with the manufacturer and see how they answer.

Regards,

HienN

 

[HienN]

I like Oldnotbold post. Gave him a star.

 

[ters]

Thank you all for contributing to this useful discussion.

I still tend to believe those fuses are there being part of the standard package and with adequate protections can be omitted. The inline contactor is rated 4.8kA without fuses and 36kA with fuses. Our short circuit level will never exceed 20kA. It is more like 17kA most of the time.

Our protections are redundant, there is even 3 devices fed from different power sources, so the likelihood of a malfunction is not that high. As electricpete underlined, most large MV motor starting circuits don’t seem to employ fuses.

But fuses apparently go due to extensive heat in the cabinet and we are trying to settle the issue down with the manufacturer. Whatever, the source of the heat is, it is too extensive, a design flaw, and they should deal with it.

Two major sources of the heat in the NEMA12/IP54 non ventilated fuse compartment are fuses (24R) and inline contactor (800 Amp, vacuum), operating at ~ 400Amp 6kV. Could someone comment what would be typical heat dissipation for those two items? Various sources indicate various numbers, one of them being that a 600Amp vacuum contactor at the mentioned current could be dissipating ~ 1kW. Is this realistic?

Oldnotbold... You deserve several stars. Well, the ABB you are talking about might still exist. Possibly somewhere in Europe. But this is ABB USA selling equipment which they actually don't manufacture, to a country where they actually weren't allowed to travel until very recently and moreover not using any help of the local ABB office, even though a sizeable one exists... So does it get any worse than this ? Or maybe we are both just a bit too old to understand some new trends and business practices...

I do admit that using the soft starter for this application is suboptimal (load too large), and using this particular make and model is rather waste of time since the only way they can operate seems that starting current has to be more or less the same as DOL (start in bypass mode), which is 470%. Lower current, 450% or lower results in the above discussed modulation phenomena, making starts much more harsh than DOL...

But OK, the system is strong enough, can take the DOL start abuse, while workable alternatives would be ~ 10 times more expensive (VFD) or ~ 4 times more expensive (fluidrive). So you get what you pay for. However, the problem is that these units have overheating problems so working as ordinary starters in the DOL mode is rather unreliable too, and also using them as soft starters even at that high current limit setting (470%) is not working yet. ABB tried to commission them, but both units had some hardware problems and they have to come back again.

 

[Laplacian]

You state that available fault current is 17kA and that max contactor interrupting current is 4.8kA, but that you don't need fuses? I disagree. The only medium voltage motor starters that don't have fuses are circuit breakers in NEMA world. All contactors have fuses or some other short circuit protection device.

As for the heat - have you ductored (low resistance ohm meter) the bus and fuse clips? Have you performed infrared thermography? Have you taken apart the burnt fuses to see if all links are melted indicating short circuit, or some links are melted indicating thermal failure? Have you verified adequate bus bar cross sectional area for the rated current?

 

[Laplacian]

A few posts up, you state that instantaneous is set for 3kA @ 0.1s. Is this how you justify not using fuses? If so, this presumption is incorrect. Power circuit breakers aren't even fast enough to clamp current - contactors are much slower.

 

[ters]

No, 3000 Amp 0.1sec, that is present I>> settings with fuses still in the circuit. If fuses are to be eliminated permanently, we would need revise it and lower the time to zero or use an inverse curve.

I'm not sure what you mean by some other protection devices (other than fuse). If that means overcurrent protection devices, we have several multifunction relays in the same circuit.

I agree that at 17kA the fuse might blow faster than breaker opening time, but at the same time, if breakers are too slow, then fuses would be used for just about anything which is not the case. Why, for example, fuses would be a must in motor circuits but not for transformers of the same size?

I have no desire to argue, I’m just looking for help and solutions, but this starter came with the incoming switch + fuse cell, however , some other soft starter manufacturers seem to offers the incoming switch + fuse cell as an option:

http://www.opalstarters.com/Technical Manuals/MV Brochure.pdf.

;

http://www.solcon.com/Index.asp?CategoryID=79&ArticleID=23

Does this mean that in these cases fuses would still have to be somewhere else in the circuit, even though there is a switchgear with motor protection before the starter, and if yes, where that somewhere else would be? I worked on some 15MW motor starters which had no fuses.

We did some thermal imaging and found that fuses operate at about 120C, but we will do some more to confirm (hard to do it when the unit is running, cannot access, door cannot be opened).

No, we have not opened fuses to see what is inside yet. But since this is happening on two different units, and only on motor restart and only occasionally and on hot days and none of other protections even ever pick up, I think it is waste of time to look for short circuits. There isn’t any, it is just poor design with no cooling and ventilation and fuses are hot (cannot touch them)

The busbar appears to be adequate for 400Amp, actually the unit is rated 600Amp, (contactor 800Amp) and it is a standard desing, not a prototype, supposedly they made many of these.

 

[HienN]

Not only overcurrent protection. it should better be added with Short circuit (instantaneous) current protection. In this case, we can eliminate the power fuses.

For most of contactor switching, equipped with fuses may be standardized. The manufacturer offers you fused compartment as an option means this switchgear is already protected from short circuit protection.

For transformer, in most ANSI books, transformer protected by fuse with disconnector is available. The important thing, I think, depends on the cost.

 

[ters]

OK HienN, agreed, but when I said overcurrent protection, since the switchgear and starters are new, I meant new digital multifunction protection devices which includes several overcurrent elements, and even if you want, I don't think that now days you can buy a multifunction feeder relay which does not have a current element where the time delay can be set to (or very near) zero. I did mentioned somewhere up that on the switchgear, protection is ABB REF 542PLUS. The switchgear has two sections, each section feeds one motor and two small 6/0.4 transformers, therefore one of the fast elements (I>> and/or I>>>) of the incoming cell can also be set similarly to instantaneous of the motor feeder. Also, motor protection Multilin 469 (physically residing on the soft starter but wired to trip the switchgear motor feeder breaker) has a fast current element which can be set to almost no time delay. Breakers are now vacuum VD4.

So seems fuses could be eliminated, but that is not what we are recommending. Since there is no spare any more, fuses might be replaced with bus jumpers, not all, but one by one as they go, only as some sort of emergency solution until a solution for their failures is found, i.e., some sort of panel ventilation. The supplier just sent a small fan, something like 50W, which will circulate air from one sealed cabinet (where the fuses and the inline contactor are) to another very small adjacent sealed cabinet where Multilin protection is, but we do not seem to understand how is that going to help. Will try...

 

[HienN]

Ters:

Some more words, for big motors (hundreds of kW up), "starting regime" is required by manufacturer. (Like 01 hot, 02 cold…) For example, it can only be restarted after several minutes for cooling down (cooling the motors down only). At the same time, power fuses could also have time to rest. After an unsuccessful starting, it normally takes longer than the required starting time of the motor. Heat then accumulated, inside motor and inside the power fuses.

We go on with another starting. It draws a longer time. As a result, the fuse is melted up because of heat accumulation. It is not like short-circuit problem, because if it is a short circuit case, the fuse not only melt but also shatter the tube. Of course, this depends on the type of fuse (with strike pin or not).

For safety, should better check the cable and busbar… for sure of any abnormal conditions. Recheck the starting regime of the motor (this can only be sent by its own manufacturer and seems it is impossible for you in this case).

No, it does not seem. It is actually that if you are provided with 51 relay. The power fuse of course can be eliminated. I am not sure about your protection relay (it should be better confirmed ABB directly). I just guess that, it just has time-delay overcurrent protection device and not instantaneous protection. That is why the fuses are there. (That is why we need to confirm).

For my air compressor, after starting and operation, then shutdown, we have to wait for next 5 minutes for restarting. Note: At this, my motor does not take any load. Load will be applied gradually on its shaft when it is in full operation or something like that…

Hope it helps.

 

[ters]

Thank you HienN. It certainly helps.

Re motor thermal capacity, our motor is a 400Amp unit, but it has an 800Amp frame, so it is a bit more massive and we can squeeze two starts without waiting that much. However, minimum waiting time is still ~ 20 min, that is how long motor/fan takes to get to the full stop once de-energized. Thermal capacity is monitored by Multilin motor protection, which is a fairly advanced unit. Typical waiting time is usually longer for other reasons.

As for the ABB protection, we know its characteristics in details. It is a general trend among newer digital protection devices than traditional definition of 50 and 51 is getting obsolete. For example, this relay (ABB REF 542PLUS) has one IDMT + three definite time current elements, called OC low, OC high and OC instantaneous. But time ranges are:

OC low, 20 ms – 300 sec
OC high, 20ms – 300 sec
OC instantaneous, 15 ms – 30 sec

So the difference between so called instantaneous element and those which are like not is 5ms (15 vs. 20 ms minimum operating time). Practically all three definite time elements are about the same, it just depends how you want to use them.

 

[LionelHutz]

You have 17kA fault current on site with a contactor rated 4.8kA without fuses or 36kA with fuses yet you still don't understand why the fuses are necessary?

Have you ever seen a major vacuum contactor bottle failure. That is what will occur without properly protecting it.

Why can't you understand that the sytems without fuses go like this;

Power Buss -> Vacuum Breaker -> motor

The vacuum breaker can handle opening a fault without using fuses.

If you have a circuit with a vacuum contactor then it would begin to open when a fault occurs - it's coil will be powered by a transformer connected to the main power which has just been shorted out due to the fault - and if the bottles can't handle the energy it's trying to open then the bottles will fail. This isn't rocket science.

 

[HienN]

LionelHuzt:

It could be understood. However, I think, just base on the safety aspect. The main coil as well as protection relays shall be energized from the external sources (like batteries or UPS).

So, there will be time to cut off the main contacts during the short circuit problem.

For reference, some other times, the manufacture produces the switchgear withstanding a very high current. That is to bear the short circuit case if it is available downward the outlet of the switchgear. These times, during switch on, the main contacts are closed with the current much higher than the actual short circuit current in the system.

 

[ters]

If you have a circuit with a vacuum contactor then it would begin to open when a fault occurs - it's coil will be powered by a transformer connected to the main power which has just been shorted out due to the fault

LioneHutz, although I write a lot, it is hard to mention all details. One which I probably haven’t mentioned before is that both contactors (inline and bypass) and other soft starter controls were originally powered from an internal CPT. This, however, has been modified and it is now fed from an external UPS. So a collapsing motor voltage will not collapse the control voltage.

Also, motor protection (any current element, including motor differential) is wired or set in that way that it trips the upstream breaker, not the contactor.

Therefore, neither inline or bypass contactor should open on a fault. We recognize that a difference still exists and that instantaneous protection operating time + breaker operating time is significantly longer that the fuse blowing time at 17kA, so we will eliminate fuses only if we have to, until we find the solution for cabinet overheating.

The equipment is still under warranty and we are not doing anything contrary to normal practice to make it operating at abnormally high temperatures. The manufacturer have finally realized that mentioned operating temperatures are too high and decided that we should install a small fan which blows air from the adjacent very small protection control cabinet (sealed, IP54) to the fuse cabinet (also sealed, IP54), claiming that this will make fuses much cooler. They say: “the fan will increase skin effect of the enclosures and stop the issue.” That remains to be seen. I don’t see how is that possible based on only internal air circulation.

 

[ters]

Also, LionelHuzt, would you elaborate on something else. Based on your comments, and the Benshaw web site, your medium voltage soft starter packages seem to be similar to one we have, swith + fuses are included.

However, some other manufacturers appear offering switch + fuse cell as an option. Since you stated that the only way how an instantaneous overcurrent motor protection can operate without fuses is to use the combination: Power Buss -> Vacuum Breaker -> motor (no MV contactors in the circuit), does this mean that that those offering medium voltage soft starters and similar equipment (such as VFDs) without fuses, assume that the upstream switchgear is already equipped with fuses, rather than breaker? If so, where in such a system I will wire my motor differential protection, which, if exists, should trip a breaker somewhere?

 

[HienN]

I think this case will depend on the type of Breaker you intend to use (Motorized or mechanical). As a matter of fact that the 02 cases will use a typical "Tripping circuit", where the trip coil is energized via external signals (via reset, emergency, or via protection relays contacts...). The trip coil will trip the breaker and then the breaker in its turn open the main contacts by mechanical energy.

Even the switchgear comes with breaker (if not equiped with relays like the ones you have). It is no longer a breakers. It is common sense.

 

[davidbeach]

If the contactor is there, without fuses, it will at some point open when it "shouldn't". Removing the fuses is just asking for trouble sometime in the future. The fuses have to stay or the contactors have to be replaced with something that can interrupt the available fault current. Sure, the UPS helps, but they fail too. Murphy looks to it that the probability of simultaneous motor fault and UPS failure will be higher than the probability or either alone.

 

[ters]

Davidbeach, OK, we don’t dispute that fuses provide more adequate protection and that we should not remove them. And we are not considering it for the sake of trying to be smarter than those who elected to use fuses, but because we have problems. Fuses blow for no real (short circuit) reason, they very are expensive to buy and not easy to find (not available locally), and since they fail for reason other that the actual short circuits (overheating), if we don’t fix the heat issue, we still may have to compare benefits of having and not having fuses.

We have a bit more redundancy there. UPS feeds only the soft starter circuits, including motor protection, Multilin (but its tripping circuits are DC). Feeder protection, where instantaneous is located, is fed from a 220V DC battery. Feeder breaker is some 15 meters from the soft starter. Incoming breaker instantaneous can be set similarly. We hoped then two upstream breakers + two 50 elements set to minimum would be something adequate to deal with available fault currents, if we have to use it that way. We hope not.

HeinN, I’ m not sure I understand your comment about motorized and mechanical breakers. You seem to be referring to a different situation. Could you reword it please. There could be many theoretical cases out there, but we have a specific case to deal with. Our switchgear is ABB UniGear (ABB says it is a toughest switchgear in the world ), normal motorized vacuum breakers VD4, with multifunction protective devices.

I’m still puzzled with the fact that some other manufacturers appear to offer soft starters which also have contactors and where fuse are an option, unless it is trictly for a case where fuses are upstream.