arrester failures 2

[alehman]

We experienced failure of the arresters I mentioned in thread238-213060. 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?

 

[MxGxK]

Embarrassing. I should have read the post more carefully.

 

[opmgr1]

Nothing to be embarrassed about!

 

[alehman]

I appreciate all the feedback.

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.

 

[opmgr1]

Yes, Voltage rating of the arrester. What type of damage(s) did the switch gear sustained?
Anything from the utility regarding the EF?

 

[alehman]

Damage was due to arcing that occurred and residue. One cubicle door was blown open and bent. Arcing damage to barriers and doors. Nothing yet from the utility.

 

[opmgr1]

For a station class arrester you may want to go with pressure relief of 80kA min. Look through the hubbell arrester catalogue for additional info.

 

[LionelHutz]

How would the utility measure the ground fault current on a floating system?

 

[burnt2x]

How would the utility measure the ground fault current on a floating system?

A Zero Sequence CT can detect the difference (current passing to the ground). Normally, the vector sum of the three phase currents is zero. ZCT does not register current. With ground faults, additional current on the grounded phase, though highly capacitive, appears in the ZCT.

 

[LionelHutz]

A Zero Sequence CT can detect the difference (current passing to the ground). Normally, the vector sum of the three phase currents is zero. ZCT does not register current. With ground faults, additional current on the grounded phase, though highly capacitive, appears in the ZCT.

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.

 

[burnt2x]

LionelHutz,
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.

 

[alehman]

The utility finally reports that there were no recorded anomalies leading up to our failures. So the arresters are going back to the manufacturer for analysis.

 

[magoo2]

With no recorded anomalies, I assume this rules out ferroresonance, resonance, switching transients and lightning.

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.

 

[alehman]

Magoo,
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...

 

[opmgr1]

I would not pressure the utility too much on this, being a utility person myself you may find that you ran into a dead-end and there is no information from them to support the theory of an over-voltage. The fact of the matter is that based on the exposure of the lines, utility customers will be subjected to voltage sags and swells.
Alehman, I would still explore specifying a minimum pressure relief of 80kA without modifying the arrester duty cycle.

 

[alehman]

The manufacturer examined the arresters and said (as I suspected), that one on each service failed internally and the others sustained external arcing damage. They said that the arresters were "misapplied", in spite of the fact that they were selected for the system voltage in accordance with their published literature.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[racobb]

Is the utility quoting you the substation regulated output voltage? Does your utility service transformer have primary taps and if so where are they set? If you said so I missed it, but do you monitor incoming voltage?

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

 

[alehman]

The utility was quoting substation bus voltage. Voltage at our transformer secondaries is also about +3% with on-nominal tap. I have no primary meter.

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
----
"It’s always fun to do the impossible." - Walt Disney