5kV motor starter failure 2

[rockman7892]

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?

 

[rockman7892]

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.)

 

[rockman7892]

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.

 

[Laplacian]

What type of Multilin do you have? Have you pulled the event report? Also, do you have any upstream electronic relays that could have captured an oscillogram or other event data during the fault; or adjacent buses to capture magnitude of voltage drop?

 

[rockman7892]

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.

 

[LionelHutz]

Was there evidence of arcing inside the cubicle after the contactor?

Do you check your contactors for loss of vacuum?

 

[rockman7892]

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?

 

[Ceast]

FYI Here's a paragraph from a problem and resolution on one of our customer's 4160Vac (3) motors on a common bus.

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.

 

[rockman7892]

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.