Three phase padmount xformers loop system primary

[quogueelectric]

I over the course of my years have done a few maybe 6- 3 phase padmount xformers. In some large distribution customers the poco feeds the xformers with a loop system where 1 building can be isolated from the primaryfeeds by racking loadbreak elbows. Where is the proper placement for the lightning arrestors in a loop system?? Is it safe to rack live loadbreak elbows to the steel racks in the xformer?? What is the proper layout of the primary distribution??

 

[rcwilson]

Surge Arrestors - install at the overhead to underground cable terminations at the loop's main feed points. Arrestors are also available on or in the padmount transformer and as part of a load break elbow assembly.

Isolating Loadbreak Elbows - When a loop is isolated, portable dead end plugs that look like a transformer bushing without any conductors are placed on the rack. A screw clamp fastens them in place. Then the lineman, wearing suitable protection and using a hot stick, unplugs each cable elbow and plugs it onto the dead end connector. The live voltage is not exposed except when the elbow is moved.

The dead end plugs are also used when the cable/elbow assembly is tested via a high pot. ( DC HV testing is no longer recommended).

 

[quogueelectric]

Surge Arrestors - install at the overhead to underground cable terminations at the loop's main feed points. Arrestors are also available on or in the padmount transformer and as part of a load break elbow assembly.

Isolating Loadbreak Elbows - When a loop is isolated, portable dead end plugs that look like a transformer bushing without any conductors are placed on the rack. A screw clamp fastens them in place. Then the lineman, wearing suitable protection and using a hot stick, unplugs each cable elbow and plugs it onto the dead end connector. The live voltage is not exposed except when the elbow is moved.

The dead end plugs are also used when the cable/elbow assembly is tested via a high pot. ( DC HV testing is no longer recommended).

Thanks for the reply. So the dead end plugs are kind of a high voltage wirenut to safely insulate the racked loadbreaks which are live...??
Now the surge arestors are usually installed on the common loadbreak female receptacles of a radial system are now located in one point where the Isolating loadbreak elbow plugs are racked on the steel racks in a loop system?? Am I making sense?? How is a cable tested without a hypot?? I am very needy this week.... Please bear with me I will snap out of it soon. Thanks again Sherman

 

[Smart $]

... So the dead end plugs are kind of a high voltage wirenut to safely insulate the racked loadbreaks which are live...?? ...

I've only worked on one loop system myself. It is my understanding that the transformers in that system had internal disconnects for each set of connections, operated via lever in the HV terminal compartment. To isolate a transformer, switch out the feed at the adjacent loop transformers. Short of having the internal disconnects, rack the breaks in the two adjacent transformers, or pull the loop side load breaks at the main feed if the first tranny to either side is the one being isolated.

 

[rcwilson]

Thanks for the reply. So the dead end plugs are kind of a high voltage wirenut to safely insulate the racked loadbreaks which are live...??

Correct - They are as safe as the elbow on the transformer bushings. Same hazard.

Now the surge arestors are usually installed on the common loadbreak female receptacles of a radial system are now located in one point where the Isolating loadbreak elbow plugs are racked on the steel racks in a loop system?? Am I making sense?? How is a cable tested without a hypot??

Surge arrestors can be istalled about anywhere on the system- at each transformer (best protection) or at the feed-in points ( least costly) or a combination. If the utility or owner is only worried about lightning surges, then arrestors at the feed-in points make sense. It is hard for lightning to hit an underground cable. If switching surges originating in the loop are a concern, arrestors should be placed on the trasnformers.

Experience now hsows that DC high pot tests deteriorate cables. But teh tests are still used. The preferred test now is a a low frequency AC high voltage test.

My experience in high pot testing URD systems is we were testing the workmanship of the load break elbow installation and not the cable. I never found a cable problem but lots of poorly installed elbows that were tracking. A moderate DC hipot on a newly installed cable will probably not do much damage if the testing time is kept short as possible and theh voltage stays within manufacturer's limits. (No 15 mintue soaks). Installation problems can be identified that way. But no DC hipot maintenance tests.

Search this website. Zog and others have put a lot of good information out on hipot testing. (I'm a testing has been and my information is dated.)

 

[quogueelectric]

Correct - They are as safe as the elbow on the transformer bushings. Same hazard.



Surge arrestors can be istalled about anywhere on the system- at each transformer (best protection) or at the feed-in points ( least costly) or a combination. If the utility or owner is only worried about lightning surges, then arrestors at the feed-in points make sense. It is hard for lightning to hit an underground cable. If switching surges originating in the loop are a concern, arrestors should be placed on the trasnformers.

Experience now hsows that DC high pot tests deteriorate cables. But teh tests are still used. The preferred test now is a a low frequency AC high voltage test.

My experience in high pot testing URD systems is we were testing the workmanship of the load break elbow installation and not the cable. I never found a cable problem but lots of poorly installed elbows that were tracking. A moderate DC hipot on a newly installed cable will probably not do much damage if the testing time is kept short as possible and theh voltage stays within manufacturer's limits. (No 15 mintue soaks). Installation problems can be identified that way. But no DC hipot maintenance tests.

Search this website. Zog and others have put a lot of good information out on hipot testing. (I'm a testing has been and my information is dated.)

My confusion lies in the physical install of the surge arrestors. Most padmounts radial feed have 2 sets of 3 bushings.3 in for feed 3 out for surge arrestors. Now with a loop system there would be no spare bushings for the surge arrestors. There would be 3 bushings A@ B@ C@ in and 3 out bushings A@ B@ C@ out leaving no bushings for the surge arrestors. So where are they placed?? At the last xformer on the loop racking one set of live capped loadbreak feedss to the steel cups to hold unengaged loadbreaks. Or do the xformers come with 3 sets of 3 A@ B@ C@ bushings to accomodate the surge arrestors if ordered for the job that way??

 

[Smart $]

My confusion lies in the physical install of the surge arrestors. Most padmounts radial feed have 2 sets of 3 bushings.3 in for feed 3 out for surge arrestors. Now with a loop system there would be no spare bushings for the surge arrestors. There would be 3 bushings A@ B@ C@ in and 3 out bushings A@ B@ C@ out leaving no bushings for the surge arrestors. So where are they placed?? At the last xformer on the loop racking one set of live capped loadbreak feedss to the steel cups to hold unengaged loadbreaks. Or do the xformers come with 3 sets of 3 A@ B@ C@ bushings to accomodate the surge arrestors if ordered for the job that way??

I believe you may be looking for something like this: http://energy.tycoelectronics.com/getDocument.aspx?PRD_ID=2764

 

[Den]

We never used the transformers as feed thru in case we needed to isolate it. We always used another cabinet for switching purposes. We would always put MOV arresters on the riser blades, in the transformer cabinets, and in the switching cabinets. Always on the dead ends of circuits. Our old fault finders needed to have the arresters out of the circuit to test as it would blow them but our new one did not need to have them isolated. I can't tell you what type of voltage they would put out. Every arrestor has to be MOV type now.
We did not test the cable when we put it in. we just hooked it up up with no load first.

 

[quogueelectric]

I believe you may be looking for something like this: http://energy.tycoelectronics.com/getDocument.aspx?PRD_ID=2764

NICE!! I never saw one of those.. This clears up some confusion. Thank you!

 

[quogueelectric]

We never used the transformers as feed thru in case we needed to isolate it. We always used another cabinet for switching purposes. We would always put MOV arresters on the riser blades, in the transformer cabinets, and in the switching cabinets. Always on the dead ends of circuits. Our old fault finders needed to have the arresters out of the circuit to test as it would blow them but our new one did not need to have them isolated. I can't tell you what type of voltage they would put out. Every arrestor has to be MOV type now.
We did not test the cable when we put it in. we just hooked it up up with no load first.

I thought the whole point of a loop fed system was to be able to feed the loop from both sides so if there was a problem with any xformer it could be easily isolated by disengaging the 2 feed throughs on either side of the problem xformer???? I am back to being confused which is normal.

 

[Smart $]

I thought the whole point of a loop fed system was to be able to feed the loop from both sides so if there was a problem with any xformer it could be easily isolated by disengaging the 2 feed throughs on either side of the problem xformer???? I am back to being confused which is normal.

Well, it's at least half the point. The other half is if you have a cable problem you can isolate the cable for repair or replacement with minimal downtime.

As for doing loops as feed-through or switched taps is a design choice.

 

[Den]

Our switching cabinets were close to the transformers. If you had to change out the trans you would need to de-energize the trans completly and you can't do that on a feed thru so we had our switching cabinets on 2 or 3 way feeds and then we could isolate the trans and short feeds only. Somtimes we would put extra cable to the trans in case we needed them sometime but not hook them up. this keeps the most cable energized especially in industrial areas

 

[Smart $]

Our switching cabinets were close to the transformers. If you had to change out the trans you would need to de-energize the trans completly and you can't do that on a feed thru so we had our switching cabinets on 2 or 3 way feeds and then we could isolate the trans and short feeds only. Somtimes we would put extra cable to the trans in case we needed them sometime but not hook them up. this keeps the most cable energized especially in industrial areas

But you can deenergize a feed through loop-fed transformer in a multi-transformer system completely without deenergizing the others in the loop. You switch out the lines (if tranny is so equipped) and pull the load breaks at the adjacent trannies to isolate any cable(s) and/or one or more other adjacent trannies. The system is then, temporarily, two radial lines rather than one loop.

 

[quogueelectric]

What does MOV type mean??

 

[Smart $]

What does MOV type mean??

The MOV is an acronym for Metal-Oxide Varistor, which is the kind of conductive material used to shunt a surge to ground; a resistance device whose resistive capability deteriorates upon being subjected to higher voltage.

AFAICT, there is no NEC requirement to use the MOV type.

 

[steve66]

We never used the transformers as feed thru in case we needed to isolate it. We always used another cabinet for switching purposes.

I've used transformers with a integral load break switch that has four positions: "A only", "B only", "A and B", and "loop through". If all the transformers on a loop have this, you can still isolate any section. But you have to be careful to get the right style switch - they make another style that I think is just 3 position.

HV isn't my strong point, but I think they also make "load break" elbows, and "non-load break" elbows. That might be something to watch for.

Steve

 

[zog]

I over the course of my years have done a few maybe 6- 3 phase padmount xformers. In some large distribution customers the poco feeds the xformers with a loop system where 1 building can be isolated from the primaryfeeds by racking loadbreak elbows. Where is the proper placement for the lightning arrestors in a loop system?? Is it safe to rack live loadbreak elbows to the steel racks in the xformer?? What is the proper layout of the primary distribution??

Loop fed is a term that often gets misused and can have different meanings to different people, can you provide a drawing of the system so we are all on the same page?

 

[Den]

There are different ways to isolate trans. Our rules were that the disconnected point had to be in site and under our control (lock out-tag out)and this was best done by having a switching cabinet close to the trans. Sometimes our feeds came from a long way or not able to be in sight due to the terrain etc. We never had trans with switching abilities in side. We had some oil switches with 3 feed positions but never used them much. This all goes back to beeing able to physically see an open or switched elbow so there is no error in what was being done. Loop feeds mean that the line feeding a circuit can be connected from 2 or more different directions and circuit has a normal open somewhere that can be closed in and the circuit can then be opened somewhere else without there being an interruption of service providing they are in phase (which is what we tryed to do if possible)We had a lot of places where we could tie 2 subs together if we needed to. Also there is a difference in elbows. The loadbreak type are marked with a stripped band and if an elbow was not marked it was "not" to be opened under load and the hi amperage elbows are solidly bolted to the recepticle and cannot be opened under load. (bolt goes thru center of elbow to receptacle)

 

[jghrist]

Our rules were that the disconnected point had to be in site and under our control (lock out-tag out)and this was best done by having a switching cabinet close to the trans. Sometimes our feeds came from a long way or not able to be in sight due to the terrain etc.

What if you had to work on the switching cabinet? How would you isolate it with an open point within sight?

 

[Varczar]

Hope this reply is not too late...

Hope this reply is not too late...

My day job is with a POCO and I spend most of my time specifying underground (UG) equipment for the Distribution system.

RCWILSON did a good job of explaining the positioning of the Load Break Elbows and the Lighting Arresters in padmounted transformers. I will be attaching a few pictures of a three phase padmount with LA's, a single phase with LA's, a two way feed through bushing which plugs the elbow LA and the UG cable into it, the elbow LA by itself, and then a cut out of the LA that shows the arc follower rod that extinguishes the arc when pulled out of the bushing and then the MOV disks.

I do have to respectfully disagree with Mr. Wilson though on one item, lighting LOVES UG cable. That is the main reason utilities install LA's at normal open point transformers in an UG loop and on riser poles where the overhead turns to UG. There is a difference between direct lighting strikes and induced lighting strikes. The UG distribution system is very susceptible to induced lighting strikes. Lighting hits the ground and can find the UG cable and then hop on the cable looking for an even lower impedance ground path.

The loop UG system provides increased reliability to the customer. A lot of utilities claim that an overhead radial distribution system is the reliabilty equivalent to a looped UG distribution system. This is because the radial overhead and the looped UG will have similar restoration times.


Varczar, P.E.