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Arrestor Connection

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nickoliver

nickoliver

Electrical
Aug 20, 2004
27
I am looking for some input on these pictures.


This is the 12KV secondary of one of two step up transformers on a rush project. I did the terminations last week prior to the transformers being delivered so that when they arrived the men on the job could just bolt them up. (I am on a job 100 miles away) They sent me these pictures today because the surge arrestors you see in the photos were not expected to be there. The concern is two fold.

1. The #6 connecting the arrestor to the HV bushing was factory installed. As you may be able to see it is a 600V THHN insulated conductor. What I have seen in the past is unshielded 15KV cable making this connection inside equipment.

2. The arrestors are mounted directly under the HV bushings and had to be loosened and pushed aside to get the power conductors landed. This moves the top terminal closer to the tank.

My suggestion, since they are owner supplied transformers, was to notify the owner/engineers of or concerns and we are awaiting there direction.

Does everyone else here see this problem??

- Nick
 
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Looks like bad news to me. First off, I've never seen lightning arrestors in pad-mount transformers - why aren't they at the riser pole. Second, for all the good the insulation is going to do, those #6 conductors might as well be bare, a total misapplication of that type of insulation; your unshielded 15kV conductor sounds much more appropriate. Third, #6 seems rather small; that unshielded 15kV cable wouldn't be less than #2.

I don't know what affect the changed spacing will have on the situation, but I wouldn't want to be anywhere near that transformer if there is ever enough overvoltage to cause the arresters to operate.
 
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Would also try to put the arresters between the transformer and the source - as close as possible to the transformer.
 
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I have a different take on the situation. We see lightning arresters routinely installed in padmount transformers. I believe it is for good reason but that is another discussion.
The #6 wire is a little small but the 600V insulation would not concern me unless the clearance between any portion of it and ground or another phase is inadequate. It does not seem to be the case here. We actually see this type installation fail fairly often when the arresters are mounted lower than in this situation and the grounded outer jacket of the phase conductor(s) is postioned too close to the bare terminal on top of the arrester. By the way, based on the number of skirts on the arresters, there is a good possibility they are rated too high to be of much value on what appears to be a 12kV solidly grounded system.
 
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I would be concerned about the clearance between the arrester and the tank and between the arrester and the skirts of the cable termination. The top skirts of the arrester will be at about line potential and the bottom skirts of the cable termination will be at about ground potential. The arrester manufacturer may have some guidance on required clearance.

You need to make sure that the air gap has the equivalent BIL of the installation (probably 110 kV). The impulse insulation level of an air gap depends on the configuration, but one source I have squirreled away (unknown origin) gives the level of a 5 inch gap as 106-114 kV. Unless you can get better information, I would treat this as a minimum for the spacing between the top skirts of the arrester and either the tank or the bottom skirts of the cable termination.
 
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For pad mounted transformer surge protection, elbow arrester is the proper application.
images



For MV insulated underground cable protection, consider installing surge arrester at the riser pole

 
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I don't see too much wrong with that installation. It would be nice to have more room between the cables and top of the arresters...can you push out the bottom of the cables and have them "bend" in more at the termination?

You didn't need to use skirted terminations in this case. Indoor unskirted would have worked and given more clearance.

Insulating red board could be used between the phases, but it is not required unless the phase seperation is not adequate.

JTK

 
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I agree that elbows are usually used to make these connections. And arresters should be placed at both ends of any underground run. For whatever reason, elbows were not chosen. As jghrist pointed out, there appears to be inadaquate clearance between termination and arrester skirts. Get some longer brackets for the arresters to bring them out away from the cables. Arrester leads should always be as short as possible. Is that the concentric neutral termination on the bottom of the H1 arrester? I'd bond concentrics together with arrester grounds and H0 bushing with copper sized for the available fault current and clearing time, and not rely on the tank alone to complete the path. Phase side of arresters should be similarly sized.
 
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Seems the whole transformer, arrestor, etc. would have to have been tested to apply the appropriate rating to the equipment. Moving equipment around would generally null and void any testing which was done on the complete package. I would go back to the vendor and ask for these test certificates as well as have some discussions on their selection of cable, not only the size (biggest concern) but also the insulation. I guess if they produce some test results which identify all these wires and locations and such then there is not much more to talk about.
All in all, my concerns would be the size of the wire to the arrestor (not really the insulation - although this will come into play some since you moved the arrestors)and the issue with moving the hot side of the arrestors closer to a grounded metal surface.
Stevenals comments about combining the grounds at the bottom of each arrestor are valid ones. Keep the current out of the inside/shell of the transformer.
 
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Thank you for all the input. It's much appreciated. BTW-The power cable is EPR with a copper tape shields, not concentric neutral. I agree with not relying on the tank for a ground path. Also, most manufacturer literature I have read suggests using skirted terminations in outdoor gear even if it is inside the enclosure. (S & C is one in particular) There is a longer creepage path with the skirted terms to help when the elements inevitably take there toll.
 
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Nick,

I have seen installations just like that before. In every case we yanked them out and installed them on the pole as they should be. I don't see the clearance as a problem at 12kv. The problem is with those mov type arrestors, when they blow..they blow. One blown arrestor in that confined space can take out multiple cables.

Your terminations look fine. It's just the factory set up that is junk.

HP
 
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re the thhn;
I see quite a bit of it in Central America. No Codes and no learning curve. Some contractors use THHN for transformer primary connections on 13,800 Volt systems and 34.500 volt systems.
After a few years of tropical sunshine, the outer covering splits off and starts to fall away. It hangs off the cable in streamers 3,4, or 5 feet long. Scares the ---- out of me.
 
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