PDA

View Full Version : switching 24.9 urd.



foreman755
03-28-2008, 08:51 AM
Does any body else out here having problems with switching hot 24.9 elbows blowing up.??

loodvig
03-28-2008, 09:14 AM
What brand elbows were they? I hope you had a really long stick!

500 KVA
03-28-2008, 09:19 AM
The problem, we have discovered, is called Partial Vacuum discharge.

We would have elbows flash when we pulled them off of the bushings. Even when there was no load what so ever.

Your bushings and elbows are creating a vacuum when put together. When you try to seperate them that vacuum built up inside of the elbow does not let the load break portion of the probe function, and the result is an arc that flows to the grounded bushing as well as the grounded surface of the elbow. The arc uses the vacuum environment to travel beyond the probe. FLASH!

Since this environment isn't held within a piece of equipment such as a vacuum breaker canister it is allowed to travel beyond the confines of the elbow and bushing to a grounded surface. Remember the closeness of the area we are talking about here.

We went to a bushing that has a ribbed rim were the elbow lip seats. This doesn't let a vacuum build up and we haven't had any problems since.

Hope this helps.

lewy
03-28-2008, 03:49 PM
We also had the same problem on our 27.6/16 kv system & yes it was because of the vacum effect
Some of the solutions we have come up with are
1 All of our new transformers are switchable so most times the elbow can be isolated first
2 All transformers come with Elastimold vented bushings (part# 2701A41CS854BG 15.2kv phase-to-ground and 26.3kv phase-to-phase) & we also have a plan to change existing transformers
If the transformers do not have a vented bushing & the temperature is below freezing we will not switch at that particular transformer.

reppy007
10-12-2011, 11:37 AM
ive never worked on any 27.6/16 kv,but have worked with alot of 12kv and 34.5 ,so there shouldnt be much of a difference,what i know about switching 34.5,is that its way more sensitive,in good dry weather,switching is no problem,being gentle helps,on the other hand if its really humid,lots of moisture in the air,dont try it chances are high that you will blow the fuse,just take the loop out.another thing about 34.5 is that its not only the weather your dealing with,now it maybe loose probes,not tightened ,loose bushings,sensitive arresters,and terminaters.all things that might take out a fuse.another thing is the bending just below the elbow,seen some guys making too much. long time ago while working on a 12kv loop,we found a bad span,isolated it .when going to,and opening the switching transformer,the elbow that was ,attached ,had a bend under the elbow that was adding stress when load was put on it,well we picked up that side making the loop hot again,while walking away i told him,that if he ever gets that loop out,that it would be the first place too look.the moment we walked across the street,back to the truck we hear something blow,since it was night time it was obvious that it now was the other side that went out,i looked at joe,the other worker and i swear i could read his mind,i said to him,that this would be a good place to look,since we already were there.went back to the back yard ,opened the pot,sure enough,right below the elbow it was blown out.makes you think.,,,, never heard of the vacume discharge in all these years,thats some good info,thanks alot.

T-Man
10-12-2011, 01:07 PM
Our company worked with Cooper Power and they came up with an elbow for 24.9 that we called "enhanced" they call them "Posi-Break" elbows. Our expierience was having flashes on 24.9 during cold temperatures so we started heating the elbows with a torpedo heater for a certian amount of time according to how cold it was and for how many hours. it bacame a real pain in the keester because a dispatcher had little idea when writting a switching job of which units had these and which didn't. It would push back the outage times a bunch till all the elbows got changed out. . .

Like stated earlier the flash occurs form inside the elbow or dust cover and follows the bushing right to the grounded case.

The new enhanced elbows, dust covers or bushing caps are marked with a dot at the top of the elbow near the pulling ring and they also have a yellow band around the base or in the case of a dust cover around the main body. Here is the site at Cooper.

http://www.cooperindustries.com/content/public/en/power_systems/products/underground_distributionequipment/loadbreak_connectors/connectors/200_a_25_kv_classposi-breakloadbreakelbowconnectorwithoptionalin.html

reppy007
10-12-2011, 02:21 PM
im in texas the houston area,been around a little,here we mostly have single phase urd,except for downtown where the big buildings,ect.mainly the urd circuits,where the 3 phase urd department usually takes that kind of work.but as far as residential its usually just single phase,you might have a few loops with an opened-delta bank,but usually not.while in flordia,it wasnt this way,all we worked there was 3-phase urd,circuits and residential, seen alot of material there that i never seen here.now here when we splice residential urd ,we dont tape over the splice,seems to be goo enough,if its a proper and clean splice,there in fla.they taped everything,,,,,,also here when we rotate a 3 phase service its mainly at the meter location,thats important.there these guys did it at the bank ,now i say that ,scratching my head cause ,while i was new there,near the truck ,i over hear the lead ,saying his rotater is broke,2 broken rotators in 2 weeks,i observe the guy and this is what i saw,he is trying to rotate the phases in an open-delta bank,thats fine but he is in the power transformer,that was new to me,so i informed him ,well you know,but i was just surprised to see him trying to do that.i was impressed with the equipment fpl had,impressed ,with the way the made up urd looked,wow to me it was like a candy store,just when i thought ive seen most,i learn about all this extra new stuff,that other utilities have,residential loops that are almost all 3 phase,is there any other things that yall might think of?i gotta say this,its either now or never,kind of a joke, have you ever met a person that is from new york,you know the way they talk,we had one at this other co. and he would always be sure that we all new that he was from NEW YUK,LOL

T-Man
10-12-2011, 03:27 PM
We have all sorts of URD situations, some small subdivisions with one single phase loop, some bigger subdivisions with three phase loop, single phase homes some on each phase then maybe a three phase tr for a well pump for the area. We also have industrial parks with three phase and single phase some 120/208 4wire and some 240 or 480 3 wire.

As far as tapping joints or splices I believe they use either heat shrink sleeves or the older self shrinking style (I think those are gone now) and they used to tape all the splices years ago.

reppy007
10-12-2011, 05:00 PM
We have all sorts of URD situations, some small subdivisions with one single phase loop, some bigger subdivisions with three phase loop, single phase homes some on each phase then maybe a three phase tr for a well pump for the area. We also have industrial parks with three phase and single phase some 120/208 4wire and some 240 or 480 3 wire.

As far as tapping joints or splices I believe they use either heat shrink sleeves or the older self shrinking style (I think those are gone now) and they used to tape all the splices years ago.

funny thing,how some areas are so different,the proceedures and all,while in flordia,i soon learned that they had alot of three phase loops feeding subdivisions,get this with no well pumps involved,well i cant say all because i havent been on too many there,enough that its seemed way different.lamberts were in very small printed letters,ect.you would be on what i thought was a single phase loop,and the transformer to the left might be feeding off c phase,transformer to the right might be feeding off b phase,and the transformer that you were at might be off a phase ,remember this is basically a field with no houses yet,just the new transformers,and the new wire installed.some temp-services,some homes being built,some homes ,with just the forms,well the wire sometimes was in the same trenchline,then some phases would take off another way,it was new to me,that was down in the naples area,richy types that dont ,and i mean dont like seeing poles.to me the loops were really long,never ,well maybe once could you see the feed from the houses that were going to be built.and alots of circuits underground,all types of swichgear and all of that ,i dont claim to know all switch gear ,but basically they do the same things,i figure.all i know is that what i seen down there,is something nasa couldnt even build,some switches had switches inside ,that after opening or closing you would use your stick to flip the damn thing over 360 degrees,kind of neat to see,who ever built ,or designed that stuff,id be glad to see his drug test info,cause the guy had to be on something from far far away.i live in houston where we have way more urd,and stuff but not nearly the different types of hardware,elbows,splices,damn everything to the socket that you use to open the transformers,everything different than here.