5kV motor starter failure
5kV motor starter failure
(OP)
We had an interesting series of events happen when one of our motor starters failed that has left me scratching my head. I wanted to see what others thought of these events and see if they had any input.
This morning we lost power to the plant when the main utility circuit switcher opened. Upon investigating loads in the plant we came to realize that we had a fault on a 4.16kV starter when trying to start a 1500hp motor.
The contactor used to start this motor is a 4.16kV Siemens vaccum contactor used in their 8100 series switchgear. After pulling the starter out we found that the fault was signifiganct enough to blow apart both of the vaccum contactor bottles on tha A & B phases of the contactor. Both of the R type motor fuses on the contactor were also blown on the A & B phases as well.
After going to our main switchgear lineup we found that we had an 87 differential trip on the differential relay protecting the cable betwwen the utility transformer and main switchgear lineup. I am puzzled as to why a fault downstream of the main swtichgear lineup would cause a differential trip on a zone which was upstream?
After isolating the faulted starter we restored power to the plant. Going back to the starter and motor we found that the source of the fault appeared to be powerfactor capacitors that were located on the load side of the motor contactor. The cap bank had two 200kVar caps in parallel and had the A&C phases blown on each cap. After isolating the cap it appeared that the cap was shorted as well.
So in trying to piece this all together I am trying to answer a couple of questions.
1) If there was indeed a short caused by the cap bank why would the contactor bottles explode? Shouldn't the R fuses have blown and protected damgage to any equipment? These fuses did indeed blow as well so if they blew i'm thinking they blew as a result of the arcing falult caused when the bottles blew
2) Why would this fault on a bus downstream of the main bus cause a differential relay to pickup on an upstream zone?






RE: 5kV motor starter failure
RE: 5kV motor starter failure
The fault current seen by the motor starter may have exceed the interrupting capacity of the motor starter.
Are you able to interrogate your differential relay for fault event data? This may provide clues. Possbilities for the differential relay tripping are numerous.
RE: 5kV motor starter failure
CT saturation is a likely reason for the differential trip.
RE: 5kV motor starter failure
The differential relay is an older Siemens 70UT relay which I am not familiar with. I'm looking for the manual now to become more familiar with it and possibly try to find event information.
The CT saturation seems to make sense. I guess every CT is different and therfore has a differnet saturation point. It sounds like in this case the two CT's may have had slightly different saturation points and therefore one saturated and the other didn't. The CT's in reference are 3000:5
I am looking through the coordination study now to try to find what the avaliable fault current is at this starter as well as find coordination of the OCPD's. I am looking for the contactor rating as well.
Are most PF capacitors connected in wye or delta between the phases? My electricans are telling me that these capacitors failed open and are now currently reading open. I want to make sure that they are testing the capacitors correctly with their meters by letting them charge and discharge and looking at resistance values. Are there any red herrings to watch out fore when testing these caps depending on how they are connected?
RE: 5kV motor starter failure
As LionelHutz poined out, if the contactor doesn't make up solid, you rapidly build up voltage on the capacitors until something fails. Although vacuum switches are often used for capacitor switching, a vacuum contactor is not the same as a definite purpose capacitor switch.
On motor starting, before the shaft starts turning, the circuit looks mostly like a capacitor switch.
RE: 5kV motor starter failure
Attached is what I have found from the coordination study. The fuse used on the contactor is an 18R fuse. It appears that the avaliable fault current at the contactor is about 27kA.
I went and looked at the interrupting ratings of the contactor itself. At 4.0kV it had an unfused interrupting capacity of 5kA. With fuses it had a symetrical MVA of 350MVA. I'm not quite sure how to interperet these ratings however I would think that the contactor alone without fuses only has an interrupting rating of 5kA, and by adding the fuses this interrupting rating is brought up to 350MVA = 48kA.
When looking through the motor relay I noticed that the Multilin had a short circuit trip setting used. This setting was set for 8 x FLC (205A)= 1640A with a delay of .5s. I have heard and have read in our power study that these Siemens 97H35 contactors were not capable of interrupting short circuit faults and thefore should not have a short circuit setting in the relay. Tha attached TCC does not show an instantaneous setting on the relay however in the field someone must have set this setting.
Also the dropout time for the contactor I found to be listed as >340ms.
Is it possible from this information that a fault occured somewhere (possibly at the cap) and the multilin saw this fault current as an instantaneous value and tried to clear the fault with the contactor. Because this fault may have exceeded the 5kA capacity of the contactor alone it caused the mentioned damage to the contactor which in turn caused another fault which lead to the two 18R fuses being blown? I do not have the cap fuse curve to see where these fit into the coordination of everything.
I'm not sure I understand LionelHutz's comment about the contactor needing to hold during the fault. Is this basically saying what I mentioned above about the contactor trying to interrupt the fualt via the multilin short circuit setting. Should the contactor stay closed during the fault and rely on the fuses for interruption since the contactor itself is not rated high enough?
Where is a good place to look for the contactors capacitor rated switching capability. I could not seem to find this in the application guide.
RE: 5kV motor starter failure
Some newer contactors have a built-in time delay on dropout to give the fuses time to clear.
RE: 5kV motor starter failure
RE: 5kV motor starter failure
Because the 3000:5 CT's used on the differental relay most likely saturated is it safe to say that the fault that occured was a magnitude of at least 3000A or greater. I'm trying to find at what percent over their rated value CT's saturate at. With this said, I'm assuming that its possible that the initial fault could have been greater than 3000A, and since the S/C on the contactor is set for 1640A it would have tried to interrupt it and possbily caused further damage. The one thing that is puzzling me about the CT saturation is that if the 3000:5 CT's saturated why didn't the 1200:5 CT's on downstream breakers saturate and cause them to trip instantaneously.
We have had multiple cases of failures related to capacitors recently. All of these failures occur during our plant shutdowns and occur when shutting motors down or when starting motors up after a shutdown. (None of these failures seem to occur with plant in steady state) With that said I am starting to think that we may have capacitor switching issues or back to back capacitor switching issues occuring as others have mentioned. I was abl to find the instruction guide for the Siemens contactor and it said it was rated for capacitor switching in paragraph format, but did not give any specific values.
Just so I understand the phenomenom of capacitor switching I want to summarize what I have learned through research on the issue. When switching on a capacitor since the current leads the volate by 90deg the current tries to jump instantaneously. Because the current cannot change instaneously it changes rapidly and overshoots its peak value. From there it osilates at a high frequency unitl it settles down to steady state. This initial overshoot of current causes the voltage to be pulled to zero followed by an voltage overshoot and oscillation until the voltage settles down to steady state. These voltage and current oscillations can cause damage to switching devices as well as the capacitors themselves.
Back to back switching occurs when one capacitor in proximity to the other is energized. This appears as a short circuit to the energized capacitor and therefore this capacitor discharges into the capacitor being energized causing hish current and voltage transients. These high current and voltage transients can damage equipment as well as the caps themselves.
Does my understanding sound correct?
RE: 5kV motor starter failure
RE: 5kV motor starter failure
Davidbeach
These starters are electrically held similar to LV starters. The control voltage is 120V AC which is then converted to DC for the actual contactor coil.
The control voltage does come from a CPT which is connected L-L on phases B&C on the 4.16kV side. Since phases A&B fuses blew, phase B connetcted to the CPT was one of the effected phases.
The contactor operating data lists the dropout voltage to be Hot-50% rated and Cold-40% rated. This tells me that the contactor would drop out somewhere between 50-60V.
I like this possibility that you are presenting. Are you saying that the fault possibly at the capacitor caused the voltage to drop in the circuit thus forcing the control volage low and causing the contactor to drop out?
RE: 5kV motor starter failure
If the 120AC was converted to DC, does the circuitry of the rectifier have caps? I know DC coils drop out slower than AC coils, especially if rectifer have caps. 0.5 seconds could be more than enough for the fault to pull down the voltage below dropout level.
Davidbeach's scenario is a great possibility. Contactors trying to break short circuit currents usually blow out and get damaged.
RE: 5kV motor starter failure
Was is the capacitor fuse size?
What is the capacitor lead wire size?
What is the capacitor configuration; 3 individual cans, or one 3 phase unit?
Was the fault phase-phase or phase-ground?
What is your maximum ground fault current; resistor limited?
What is your source transformer impedance and size?
What is the maximum phase-phase fault current on its primary?
27kA is a decent amount of fault current; how much fault current would have to flow to cause your contactor to drop out and for how long? I would think the capacitor lead wire would limit the current below this value. Maybe a fault somewhere else and the capacitor fuse was already blown.
Connect a variac to a similar starter test circuit and drop the voltage down to where the contactor chatters. Does it actually chatter, or does it seal itself out? Or does your contactor have two wire start/stop control where an external relay seals the contactor in?
I've had a contactor chatter problem with a different manufacturer and discovered the seal-in contact needed adjustment as it was breaking too long after the main contacts had separated.
RE: 5kV motor starter failure
So, as I hinted at before - if the fault current was higher than 5kA the contactor needs to stay closed. There are 2 main things that could cause it to open. The protection relay on the motor could trip and begin to open it. The control voltage could drop and allow it to open. Considering you had a short that may have pulled L-L voltage down close to 0V, the protection relay and the control voltage could have both been working to open the contactor.
FYI, it is common to see the protection relays running on UPS or DC power to avoid keep them powered during a fault. However, it is not common for the contactor to be on UPS power. As you have possibly seen, this can still cause problems.
RE: 5kV motor starter failure
The one thing that I can not seem to make sense of, is why a deferential relay would saturate before a downstream 1200A breaker would open?
As I mentioned the differential realays use 3000:5 CT's for the differential scheme. Downstream of this main incoming breaker there is a 1200A breaker with 1200:5 CT's feeder breaker which serves the bus which the faulted starter is located.
So if we had a fault that was large enough to saturate one of the 3000:5 CT's wouldn't it have saturated the 1200:5 CT's as well? I believe the instantaneous setting on the 1200A breaker is aprox 24,000A. I would think that this breaker should have opened if the 1200:5 relay saturated or saw a fault current high enough to saturate the 3000:5 CT's?
RE: 5kV motor starter failure
FYI... the PFR is dependant on the gage of wire used.
RE: 5kV motor starter failure
RE: 5kV motor starter failure
RE: 5kV motor starter failure
Maybe the fault sequece was:
1- During the motor start, a short-circuit at the capacitors is detected by the Multilin.
2- The Multilin orders the contactor to open, but owing to the high short-circuit current (not enough yet to blow out the fuses), during the open maneuver the contactor suffer the current chopping phenomenon (increased by the capacitor bank) and causing the bottles blown.
3- As the contactor couldn't clear the fault, the motor fuses blow; meanwhile the upstream differential protection fires beause of the CT's saturation.
The current chopping phenomenon of vacuum breakers could explain your problems with the capacitor banks.
RE: 5kV motor starter failure
PolyMike
I see what you are saying about the vacuum contactor/cap bank combination as others have mentioned as well. The operating manual for the vaccum contactor does state in a brief sentence that the contactor is capable of capacitor switching but doesn't say much more, or go into detail about capacitance values it is capable of switching. It also goes on to say that these contactors dont need any sure protection at motor due to the chopping phenomenam that has been discussed in other threads. I am going to however contact Siemens and speak with them about this application with capacitors.
I tend to agree with your series of events that you proposed. The caps shorted during the start and the contactor tried to interrupt the short either from the short circuit setting in the Multilin or the voltage drop as Davidbeach proposed. After the contactor could not clear the fault the fuses blew.
The two questions that I have to investigate are why the cap fuses didn't blow to clear the short before it caused the contactor to try to clear the fault (cap fuses did blow at some point, I'll have to check fuse curves)and why the 3000:5 CT possibly saturated but nothing happened with the associated 1200:5 CT or breaker.
Laplacian
I'm away from the plant right now but here is the info I recall off the top of my head.
Was is the capacitor fuse size? (Cooper 50C fuse rated for 50A)
What is the capacitor lead wire size? unsure
What is the capacitor configuration; 3 individual cans, or one 3 phase unit? (This is a three phase unit.)
Was the fault phase-phase or phase-ground? I believe it was phase to phase.
What is your maximum ground fault current; resistor limited?
(400A HRG limited)
What is your source transformer impedance and size? (15/20/25MVA source xfmr @ 6.5% impedance)
What is the maximum phase-phase fault current on its primary? (I'm not sure exactly but I believe its around 38kA.)
RE: 5kV motor starter failure
This morning I was able to plot the attached TCC's for the cap fuse, relay setting, and contactor fuses. As you can see from the plot, the 50C cap fuses appear to be well in front of both the relay S/C setting and contactor fuses. Because these cap fuses are well in front of the multilin s/c setting I think we can eliminate the s/c setting being a possibl cause of dropping out the relay (Assuming cap fuses are operaing properly)
I then started to look at the cap fuse curve to try to determine the possibility of what Davidbeach suggested about the voltage drop dropping out the contactor. It appears that even at 1kA this cap fuse should blow almost instantaneously thus limiting the fault current to below 1kA. So with that said, the theory of the contactor dropping out on voltage drop and interrupting a fault greater than its rated 5kA does not make sense with the fact that the fuses should have blown a 1kA or below and thus limited the fault to below around 1kA. Do you guys agree with this?
How can you figure out how quickly the volate would drop at current values to the left of the cap fuse curve?
PolyMike
I do not yet have access to these IEEE articles is there anywhere I can find a pdf version to read about this topic.
RE: 5kV motor starter failure
RE: 5kV motor starter failure
The multilin that is used is a Multilin 269. I dont believe the Multilin even saw the fault becasue the cause for the last trip it gives as an overload. Becasue there is no timestamp it is uncertain when this last trip actually occured and could actually be the overload trip that happened the night prior.
Unfortunatley most of the other relays are older relays that I'm not really familiar with. I'm looking throught the manuals to try and see if there is a way of capturing a waveform, but as of now I am uncertain.
The adjacent buses do not meter voltage which is a poor design which I have been unhappy with for some time now.
RE: 5kV motor starter failure
Do you check your contactors for loss of vacuum?
RE: 5kV motor starter failure
After replacing the contactor and removing the damaged pf caps from the circuit, and checking the motor and cable we went ahead and tried to start the motor again. We had some trobule getting the motor started (overloads) but eventually got it started.
I know the theory behind pf caps and how they are used to provide reactive power to the motor, however is there any other benefit to having them in the circuit during starting such as exciting the motor or any other reason. It may have just been a coincidence or something else going on, but I was wondering if us having trouble getting the motor started was because we are now starting it without the caps in the circuit. The caps were connected on the line side of the motor relay CT's so no FLA current adjustment was needed to the relay.
I am also looking into a possible cap transient switching issue in the plant. Can these cap transient switching or back-back switching be modeled in SKM in any sort of way?
RE: 5kV motor starter failure
Two significant voltage transients were recorded on starter #400 (See wave form captures on pages 23 thru 27). These transients reached as high as 21,208 volts on the C-A phase. These transients were associated with starts on the adjacent starter. These voltage transients are caused by the closing of the vacuum contactor for the motor starter. The phenomenon is known as bottle clipping. This transient can be controlled by the application of surge / lightning arresters and surge capacitors. The surge / lightning arresters are applied at the 4,160 volt bus. The surge capacitors are applied at or near the motor. Surge capacitors should not be applied without the surge / lightning arresters.
The first step should be the installation of the surge / lightning arresters. We recommend Intermediate or Station Class arresters from Cooper Power Systems. Information in this report and an electrical one-line diagram of the systems should be used to properly size the arresters.
RE: 5kV motor starter failure
After doing a little more investigating into the situation I found the motor datasheet for this motors which states a max capacitor kVAR of 390. As I mentioned in an my OP the caps that are used for this motor are 400kVAR. What would going over this recommended kVAR do to the motor. I know it can over excite the windings but what are these values based off of?
I've seen some referneces that state that you never want the cap kVAR to be any larger than the reactive current for the particular motor. A rough way for gauging the reactive current was to use the unloaded motor current if a reactive current was not specifically given.