arrester failures
arrester failures
(OP)
We experienced failure of the arresters I mentioned in thread238-213060: Lightning arrestor short circuit rating. The utility feeding this switchgear is 4800V ungrounded with about 21kA sym available. The circuits are from a common substation bus, ungrounded, and do not have recloser operation. I selected an intermediate class polymer 6kV gapless MOV arrester, rated 40KA pressure relief. Their 10kA, 8/20 crest voltage is 16.7kV. They are installed in separate compartments in metal-enclosed interrupter switchgear, rated 15kV.
This is a main-tie-main setup with the arresters in separate back-to-back compartments. The side was blown out of the C-phase arrester on main #2. About 15 minutes later there was a fault in the arrester compartment protecting main#1 and its C-phase arrester was destroyed. I can't tell if the origin of the second fault was internal to the arrester or external. There was considerable arcing in both arrester compartments, but I don't know exactly when it occurred. Fuses on both mains were found blown, but again I don't know exactly when. The utility reported a ground fault for roughly the time interval between our two events.
The utility engineers and local electricians are suggesting a higher voltage rating. The arresters protect oil-insulated transformers rated 60kV BIL that are about 50 feet away.
Any thoughts on changing the arresters to 9 or 10kV ? Any other thoughts on the failures or replacement arresters type selection?
This is a main-tie-main setup with the arresters in separate back-to-back compartments. The side was blown out of the C-phase arrester on main #2. About 15 minutes later there was a fault in the arrester compartment protecting main#1 and its C-phase arrester was destroyed. I can't tell if the origin of the second fault was internal to the arrester or external. There was considerable arcing in both arrester compartments, but I don't know exactly when it occurred. Fuses on both mains were found blown, but again I don't know exactly when. The utility reported a ground fault for roughly the time interval between our two events.
The utility engineers and local electricians are suggesting a higher voltage rating. The arresters protect oil-insulated transformers rated 60kV BIL that are about 50 feet away.
Any thoughts on changing the arresters to 9 or 10kV ? Any other thoughts on the failures or replacement arresters type selection?






RE: arrester failures
If I have understood your post correctly,
1)Is 6 kV=MOV of the said arresters?
2)How did you install arresters with MOV=6 kV for a 15 kV ungrounded system?
RE: arrester failures
2. The system is 4.8kV ungrounded. The switchgear is rated 15kV.
Thanks,
RE: arrester failures
With voltage regulation, the voltage could be higher than nominal. 5.1/4.8 gives you 1.0625 pu, so you could tolerate up to 6.25% higher voltage.
The polymer arrester failure description with the side blown out sounds like an overvoltage failure. The polymer housing usually splits and you get a power arc external to the blocks.
I'd be inclined to replace it with a 9 kV duty cycle rating (MCOV is 7.65 kV).
The protection characteristics would be roughly 1.5 times the 16.7 kV or 25.1 kV. I estimated this based on the ratio of the arrester ratings (9/6). Even if the surge doubles, you have almost the required 20% margin.
RE: arrester failures
The utility states they maintain +-5% voltage. They are trying to obtain their substation voltage recorder data for the time leading up to the event.
RE: arrester failures
What is the BIL of the entire system? is this more, less than or equal to the arrester BIL?
What is the ground resistance at the point where the arrester is grounded?
RE: arrester failures
It's good to have noble preferences but you're not offering alehman any alternative. Leaving a 6 kV rating in will probably destroy more arresters.
Sure the 9 kV rating is for higher voltage systems, but it will work. You're sacrificing some protective margin, but you should be OK.
System BIL - line should be higher than the transformer and other equipment (if any) should have their own arresters.
I'm not sure the ground resistance would change anything if we're dealing with a sustained overvoltage issue.
RE: arrester failures
Switchgear is 95kV BIL (except for the 6kV arresters), cables are 15kV 133% EPR, transformers are 60kV BIL. There is an extensive grounding system, but I do not know the resistance.
My thought process was that the normal line-ground voltage should be significantly <4.8kV since the utility does not allow ground faults to persist.
A 9kV arrester in the same product line would have 25.0kV 10kA/8x20 crest.
RE: arrester failures
I dont agree with your post. If the problem is one of sustained over-voltage The approach is to find out what is causing the overvoltage (temporary or sustained) and fix that, not putting bandage on the situation by increasing the arrester duty cycle rating.
Alehman, looking at your post re L-G voltage are you saying that this system is an un-grounded wye system and that the C-phase to ground voltage is close to 4.8kV?
RE: arrester failures
RE: arrester failures
9KV MOV may help if the source and mangitude of OV is known such as switccing transient below the ratig of the MOV. On the other hand it may cause greater damage (when operates) or mask something you do not want to mask, if the source of OV is per say restriking ground fault/resonance.
RE: arrester failures
RE: arrester failures
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RE: arrester failures
The utility refers to their service as 4.8kV delta.
mxgxk,
I've gone through several scenarios as to the sequence in my mind. Personnel at the facility reported two audible explosions. After the first explosion loads fed from main#2 were de-energized and there was considerable smoke coming out of the switchgear (I don't know the status of the fuses at that point). The utility says the ground fault current is about 2 to 3 amps. I wouldn't think that would be enough to explain the audible explosion. One possible scenario is there was arcing phase-to-phase after the first failure and one or two fuses blew, leaving a ground fault.
Then after about 15 minutes, there was a 2nd explosion and the remaining loads were off. This could have been caused by the arrester on the 2nd service failing or an external flash-over on the arrester due to residue from the first fault.
RE: arrester failures
RE: arrester failures
The present arresters are applied with an MCOV greater than the line-to-line voltage, but it could be close.
alehman
How much cable are you dealing with between the utility sub and the switchgear? I could envision the utility regulating around 4800 V +/-5% at the sub, but with enough cable, you'll get voltage rise at light load that will raise the voltage unless the regulators/LTC have line drop compensation. So you could be at the ragged edge as far as the 5.1 kV MCOV.
With a ground fault, 2 of the phases will see this overvoltage, as MxGxK points out. From the utility perspective, they might not detect this condition unless they have special relaying since it really doesn't draw much fault current, only some cable charging current. With it undetected, it will subject the 2 unfaulted phases and their arresters to a damaging level of overvoltage.
Maybe alehman can see if I'm off base with these assumptions. Also do you have sensitive ground fault detection in the switchgear or it is just set to alarm? This should help better define the problem.
RE: arrester failures
No matter how many times you replace your arresters, you will get more arresters damaged until you solve the problem of arcing L-G faults in your system. 2 to 3 amps is a typical charging current of ungrounded 4.8kV systems. With the system capacitance highly charged, the threshold voltage could have been reached; re-strikes occurred 'till the arresters gave way!
Try reviewing your line protection and see if you can use the option to trip if L-G fault occurs and not just alarm. Broken-delta ground fault sensing can help.
My 2 cents!
RE: arrester failures
I see your point about voltage regulation. Our secondary voltage runs a few percent high, but I need to verify the transformer tap settings.
I'm still waiting on voltage logs from the utility, but they have said there were no other recent ground fault events.
RE: arrester failures
Note if there is a transient fault there can be a build up of high voltages (up to 6 times nominal). This high transient voltage can initiate a second fault at the weakest insulation point resulting in failure. For this scenario you definately do not want to increase the duty cycle rating of the arrester. As one poster hinted at you may want to invest in SEF protection if this is a financially viable option or it may be cheaper to replace arresters.
RE: arrester failures
RE: arrester failures
RE: arrester failures
opmgr1, by "duty cycle rating" you mean voltage rating?
Any thoughts on the pressure relief rating? We had some damage to the switchgear as a result of this failure. I am concerned about possible future failures causing more extensive damage.
RE: arrester failures
Anything from the utility regarding the EF?
RE: arrester failures
RE: arrester failures
RE: arrester failures
RE: arrester failures
RE: arrester failures
What is the current path from ground back to the transformer side of the CT?? Capacitive coupling in the transformer??
If so, that seems like a very poor ground fault detection scheme to me.
RE: arrester failures
You're probably right! I did have a problem with coordinating ground fault relays with this kind of setup before. The catch is that when you supervise the earth fault trip circuit by a contact from an undervoltage relay taking pickup from a broken delta PT, you can set the EF very low and still detect the ground fault.
RE: arrester failures
RE: arrester failures
I suspect the arrester manufacturer will tell you the arresters saw a TOV condition which exceeded their capability, but let's wait for their response.
Don't let the utility off the hook at this point. They just told you there was nothing that they were doing that was coincident with or just prior to the arrester failures. They still have some useful info that you should get out of them.
- how high did the substation voltage get in the month or so prior to the failures?
- what time of day did they have the highest voltage?
- if you happen to have a just a single-phase to ground fault (SLGF), will the utility relays be able to sense it or is it below their radar screen? This is a critical piece of info.
From your perspective,
- can you calc how much distance and what size 15 kV cable there is between the substation and your switchgear?
- is it just an underground feed from the substation or is there some overhead distribution too in the mix of things?
- what time of day did the failures occur?
- was your load high or low at the time of arrester failure?
I'm still entertaining the possibility that you'd get enough cable rise that you could be close to the MCOV - but this theory requires that you also have a SLGF.
We recently had some termination failures in an arrester cabinet because of the way the incoming cables were arranged in the cabinet. They were almost horizontal and this put the grounded end of 2 terminations close to the adjacent phase. The result was one of the terminations flashed to ground and 2 arresters failed. The initial report was an outage due to arrester failures. On closer examination, we found that the terminations on the incoming cables should have been vertically arranged and no fault would have occurred. These terminations were only a few months old.
There's still a chance to get some additional info from the utility. I'd go after it.
RE: arrester failures
Responding to your questions based on what I currently know...
They said the voltage has been very stable, with some daily variance up to 123 to 124V. Yes, they can detect SLG faults and did see our first fault as an SLG for about 15 minutes.
I may be able to get a rough idea of the distance, but I don't have the utility's maps. They say there's no overhead except bus at the substation. Load was probably on the low side, but there's not much variation.
I think there's a possibility the 2nd failure was a flashover perhaps due to fallout from the first event, but it also appears at that time there may have been a SLG fault resulting from the first event.
We'll pursue additional info from the utility, but I'm doubtful of learning anything meaningful. They have offered to install a power quality analyzer on our transf secondaries, but there wouldn't be any ground reference since they are DY.
Thanks again...
RE: arrester failures
Alehman, I would still explore specifying a minimum pressure relief of 80kA without modifying the arrester duty cycle.
RE: arrester failures
Alan
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"It's always fun to do the impossible." - Walt Disney
RE: arrester failures
I have seen improperly rated arresters fail on 3 wire systems from extended ground faults and the time it will take to failure is proportional to the amount of overvoltage. Appears you are on the edge of the envelope with rated MCOV depending on the actual service voltage.
You could very likely have 6+ percent over nominal voltage on the service.
It is certainly something to investigate, but agree with the other posts with respect to sacrificing protection at the higher ratings....but if it is the arresters already causing failures, it is obviously time up upgrade.
Alan
Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin
RE: arrester failures
The arrester manufacturer and switchgear manufacturer have recommended increasing the voltage rating of the arrester. My inclination is to increase the voltage rating to 9kV and the withstand to 80kA.
Alan
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"It's always fun to do the impossible." - Walt Disney