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Found 4 results

  1. Tony S

    Sheathed transformer tails

    Sheathed transformer tails: Each is a single core cable with an earthed outer sheath. PILC (Paper Insulated Lead Covered) PIAS (Paper Insulated Aluminium Sheaved) AWA (PVC Aluminium Wire Armoured) What ever the type the principal is the same, only one end is collectively earthed and by convention it is at the transformer end. The fun and games start when terminating the cables at the switchgear end, PILC and PIAS have to be plumbed to a wiping cone. The papers are then sealed by taping over half lap with resin impregnated silk* followed by filling the cones some form of compound. This would be for a LV switchboad. I suspect these are for 11kV 20 MVA 500mm². You can just make out the insulation at the top of the cone. Whatever they are they are a work of art and a credit to the cable jointer. The inside of a 11kV oil filled terminal box. Note the stress relief where the lead has been terminated. AWA needs the armours terminating in to a gland of some form to prevent damage to the inner serving. Again each gland must be isolated from earth and each other. The reason for insulating one end of the tails is to stop circulating currents in each cable sheath which may be equal to the current carried by the single core. The solidly earthed terminations at the transformer provide the fault path to earth. An advantage of the transformer terminations being earthed is the REF protection relay covers both the transformer windings and the tails. For LV cables a Paxolin gland plate may suffice but given the forces involved during a fault it may not be sufficient to restrain the cables. For PILC and PIAS terminations the separate brass wiping cones are insulated from the iron or steel gland plate and each other by a Tufnell or Paxolin pad. The retaining bolts, washers and nuts also have insulating sleeves and washers so the termination is entirely clear of earth. There is the cheats method of plumbing the PILC and PIAS cable to the brass cone and it is the recommended method for Ellison switchgear. Pack the bottom of the cone with wire wool or asbestos string. A small mirror is handy to see what is happening, There’s now two options. Chop up some lumps of grade D plumbing metal and drop them with some tallow in to the cone. Heat the cone until the metal turns liquid, remove the heat. Melt the grade D plumbing metal in a metal pot and ladle it in to the cone. *As I was writing about the resined silk I could remember the smell and feel of it, it’s soft, sooth, squidgy and fragrant and in a way very sensuous. OK it’s time for a cold bath! © Tony S Photographs of the 11kV terminal box and the transformer earthing arrangement © RoB
  2. Tony S

    Transformer tails

    Transformer Tails This has been written with the help of a few friends. Archy, RoB, Engineer 54 and others. This is the picture that started it. If you note the individual tails are held with nylon straps. One of the problems is that under fault condition there is a violent magnetic reaction between the separate cables. Thanks to youtube here’s some of the nasties that can happen under fault conditions. http://www.youtube.com/watch?feature=player_detailpage&v=HmnrJa7Zk0A#t=84s Each conductor has a magnetic field around it relative to the current carried. The magnetic forces try to repel each other even in the T&E in a domestic install. A bit of the history to this article: Many years back The Electrical Review carried an article about the installation of transformer tails and the inherent dangers they can cause. Which I read not knowing I was due to be dropped in the deep end by being given the task of renovating a substation. The substation: This set out as a relatively simple job, just add the new 1000KVA transformer and its associated MV / LV gear. The substation had 3 1000KVA 11/.433KV and 2 x 1800KVA 11/3.3KV transformers. Just put the new transformer and gear in didn’t seem a problem. As in all tales I set out with a spring in my step thinking “I’m going to enjoy this.” That didn’t last long! All the previously installed transformer tails had just been thrown in the trench. Not a good start Until I’d found the ER article about transformer tails the plan was sling the new in with the rest. Now we would have to put the old and the new tails in to proper cleats and make a job of it. Then the troubles started. We opened the trench with the existing tails in, what tails? No sign of them, the trench had over the years filled with muck. To add to the troubles the plumbers had installed a drain from the lab in to the trench thinking it was a drainage trench. The lads started uncovering the tails to find that it wasn’t just water that had been coming from the lab. It was a lovely concoction of chemicals. The cambric insulation looked like a sponge. Each of the individual tails had to be teased out of the muck in the trench and hung in nylon slings from a scaffold while still live! (A really rotten underhand trick. Only tell the Muppets what they need to know, do not mention getting blown to kingdom come!) Once the tails were clear of the trench was made watertight again and the extension to the new transformer added. The 11kV PILC SWA cables had not been affected at all by the concoction of chemicals so they were left alone (about the only thing that was!) the earthing to the original transformers was uncovered, a single 70mm² looped to all three transformer tanks. This job is just getting better, NOT! BICC came in to advise about cable sheath repairs, only one outcome. All the tails had to be replaced, with the new transformer that came to 21 x 500mm² Cu cambric and 7 x 600mm² PVC 4 sector solidal. Plus we were installing another set of 500mm² singles as an interconnect between the two main LV boards For the main run of tails back to the switchgear we didn’t go for trefoil cleats because of the de-rating factor for cables in close proximity. It was a long run. Instead we used channel iron frames with hardwood separators. Mahogany for god’s sake! What a waste of good wood. (I got a fireplace out of it.) The switching procedure and schedule of work took a fair number of meetings to get what we thought would be right (hope springs eternal they say.) Cobblers to that! It developed in to organized chaos! We couldn’t loose supplies to xxxx so at no time was power off to all transformers. Back feeds here there and everywhere became the norm. The original LV gear was SWS (South Wales Switchgear) with 1600A Whipp & Bourn ACB’s incomers. Both companies had gone to the wall by then so GE System4 gear had to be cobbled in. 5 x GE S4 1600A fault make/ load break isolators used as bus-section and interconnect switches. Two new GE 1600A ACB's were installed one chopped in to the old SWS panels and the other for the new transformer incomer in the new GE System 4 panel. The cleats for the tails in the trench.. In the substation the trenches were deeper and narrow, the 11KV cables had to pass between the LV tails so we had to resort to trefoil cleats. The reason for the cleats being so substantial is due to this little beauty. Ft = (0.17 x Ip²)/S Where Ft = Maximum Force per unit length of cable (N) Ip= Peak short circuit current (KA) (see below). S = Centre to centre distance between neighboring conductors (m) A fault could force the tails to violently throw apart. This had caused the death of an electricians mate when trench covers were thrown in the air due to a fault. The switchgear bus-bars have to be braced to handle a fault of 50KA, the tails should be braced to withstand the magnetic force. Peak short circuit current (KA), this is normally calculated using the infinite source method. Where the Peak short circuit current = (transformer FLA / transformer reactance%)/1000 = KA A 1000KVA transformer has a full load output of 1333.4A at 433V. Its maximum fault current if we assume 4.5% transformer reactance could be up to 29KA Ph→E, 51KA Ph→Ph! All the main boards I installed were built to withstand 50KA. I’ve had the misfortune to see bus-bars bending under magnetic stress, I never want to see or hear them again. The last time I heard busbars singing I’d had to swap over feeders to a MCC panel, no major problem thinks I. No problem until a 250HP DOL motor started at the far end of the MCC, a couple of thousand amps down ½x2½” aluminum bars makes them very unhappy. Now the right way to restrain transformer tails. From RoB I’ve never seen the lead sheath bonded like this before. There are various types of cleats available. Normally trefoil with a separate single cleat for the neutral. As many installs now have to contend with neutral harmonics and therefore quadfoil can be used. Like any cable derating has to be applied. This is why we used the wooden cleats for multiple transformers. Believe it or not there is a use for this magnetic repulsion. Isolating contacts for withdrawable switchgear, the higher the current the tighter the fixed contacts grip the centre isolating contact. There is an insulating pad for the spring to seat on, if current is allowed to pass through the spring the heat will weaken it. © Tony S and © RoB (photodraphs)
  3. Sheathed transformer tails: Each is a single core cable with an earthed outer sheath. PILC (Paper Insulated Lead Covered) PIAS (Paper Insulated Aluminium Sheaved) AWA (PVC Aluminium Wire Armoured) What ever the type the principal is the same, only one end is collectively earthed and by convention it is at the transformer end. The fun and games start when terminating the cables at the switchgear end, PILC and PIAS have to be plumbed to a wiping cone. The papers are then sealed by taping over half lap with resin impregnated silk* followed by filling the cones some form of compound. This would be for a LV switchboad. I suspect these are for 11kV 20 MVA 500mm². You can just make out the insulation at the top of the cone. Whatever they are they are a work of art and a credit to the cable jointer. The inside of a 11kV oil filled terminal box. Note the stress relief where the lead has been terminated. AWA needs the armours terminating in to a gland of some form to prevent damage to the inner serving. Again each gland must be isolated from earth and each other. The reason for insulating one end of the tails is to stop circulating currents in each cable sheath which may be equal to the current carried by the single core. The solidly earthed terminations at the transformer provide the fault path to earth. An advantage of the transformer terminations being earthed is the REF protection relay covers both the transformer windings and the tails. For LV cables a Paxolin gland plate may suffice but given the forces involved during a fault it may not be sufficient to restrain the cables. For PILC and PIAS terminations the separate brass wiping cones are insulated from the iron or steel gland plate and each other by a Tufnell or Paxolin pad. The retaining bolts, washers and nuts also have insulating sleeves and washers so the termination is entirely clear of earth. There is the cheats method of plumbing the PILC and PIAS cable to the brass cone and it is the recommended method for Ellison switchgear. Pack the bottom of the cone with wire wool or asbestos string. A small mirror is handy to see what is happening, There’s now two options. Chop up some lumps of grade D plumbing metal and drop them with some tallow in to the cone. Heat the cone until the metal turns liquid, remove the heat. Melt the grade D plumbing metal in a metal pot and ladle it in to the cone. *As I was writing about the resined silk I could remember the smell and feel of it, it’s soft, sooth, squidgy and fragrant and in a way very sensuous. OK it’s time for a cold bath! © Tony S Photographs of the 11kV terminal box and the transformer earthing arrangement © RoB View full knowledgebase
  4. Transformer Tails This has been written with the help of a few friends. Archy, RoB, Engineer 54 and others. This is the picture that started it. If you note the individual tails are held with nylon straps. One of the problems is that under fault condition there is a violent magnetic reaction between the separate cables. Thanks to youtube here’s some of the nasties that can happen under fault conditions. http://www.youtube.com/watch?feature=player_detailpage&v=HmnrJa7Zk0A#t=84s Each conductor has a magnetic field around it relative to the current carried. The magnetic forces try to repel each other even in the T&E in a domestic install. A bit of the history to this article: Many years back The Electrical Review carried an article about the installation of transformer tails and the inherent dangers they can cause. Which I read not knowing I was due to be dropped in the deep end by being given the task of renovating a substation. The substation: This set out as a relatively simple job, just add the new 1000KVA transformer and its associated MV / LV gear. The substation had 3 1000KVA 11/.433KV and 2 x 1800KVA 11/3.3KV transformers. Just put the new transformer and gear in didn’t seem a problem. As in all tales I set out with a spring in my step thinking “I’m going to enjoy this.” That didn’t last long! All the previously installed transformer tails had just been thrown in the trench. Not a good start Until I’d found the ER article about transformer tails the plan was sling the new in with the rest. Now we would have to put the old and the new tails in to proper cleats and make a job of it. Then the troubles started. We opened the trench with the existing tails in, what tails? No sign of them, the trench had over the years filled with muck. To add to the troubles the plumbers had installed a drain from the lab in to the trench thinking it was a drainage trench. The lads started uncovering the tails to find that it wasn’t just water that had been coming from the lab. It was a lovely concoction of chemicals. The cambric insulation looked like a sponge. Each of the individual tails had to be teased out of the muck in the trench and hung in nylon slings from a scaffold while still live! (A really rotten underhand trick. Only tell the Muppets what they need to know, do not mention getting blown to kingdom come!) Once the tails were clear of the trench was made watertight again and the extension to the new transformer added. The 11kV PILC SWA cables had not been affected at all by the concoction of chemicals so they were left alone (about the only thing that was!) the earthing to the original transformers was uncovered, a single 70mm² looped to all three transformer tanks. This job is just getting better, NOT! BICC came in to advise about cable sheath repairs, only one outcome. All the tails had to be replaced, with the new transformer that came to 21 x 500mm² Cu cambric and 7 x 600mm² PVC 4 sector solidal. Plus we were installing another set of 500mm² singles as an interconnect between the two main LV boards For the main run of tails back to the switchgear we didn’t go for trefoil cleats because of the de-rating factor for cables in close proximity. It was a long run. Instead we used channel iron frames with hardwood separators. Mahogany for god’s sake! What a waste of good wood. (I got a fireplace out of it.) The switching procedure and schedule of work took a fair number of meetings to get what we thought would be right (hope springs eternal they say.) Cobblers to that! It developed in to organized chaos! We couldn’t loose supplies to xxxx so at no time was power off to all transformers. Back feeds here there and everywhere became the norm. The original LV gear was SWS (South Wales Switchgear) with 1600A Whipp & Bourn ACB’s incomers. Both companies had gone to the wall by then so GE System4 gear had to be cobbled in. 5 x GE S4 1600A fault make/ load break isolators used as bus-section and interconnect switches. Two new GE 1600A ACB's were installed one chopped in to the old SWS panels and the other for the new transformer incomer in the new GE System 4 panel. The cleats for the tails in the trench.. In the substation the trenches were deeper and narrow, the 11KV cables had to pass between the LV tails so we had to resort to trefoil cleats. The reason for the cleats being so substantial is due to this little beauty. Ft = (0.17 x Ip²)/S Where Ft = Maximum Force per unit length of cable (N) Ip= Peak short circuit current (KA) (see below). S = Centre to centre distance between neighboring conductors (m) A fault could force the tails to violently throw apart. This had caused the death of an electricians mate when trench covers were thrown in the air due to a fault. The switchgear bus-bars have to be braced to handle a fault of 50KA, the tails should be braced to withstand the magnetic force. Peak short circuit current (KA), this is normally calculated using the infinite source method. Where the Peak short circuit current = (transformer FLA / transformer reactance%)/1000 = KA A 1000KVA transformer has a full load output of 1333.4A at 433V. Its maximum fault current if we assume 4.5% transformer reactance could be up to 29KA Ph→E, 51KA Ph→Ph! All the main boards I installed were built to withstand 50KA. I’ve had the misfortune to see bus-bars bending under magnetic stress, I never want to see or hear them again. The last time I heard busbars singing I’d had to swap over feeders to a MCC panel, no major problem thinks I. No problem until a 250HP DOL motor started at the far end of the MCC, a couple of thousand amps down ½x2½” aluminum bars makes them very unhappy. Now the right way to restrain transformer tails. From RoB I’ve never seen the lead sheath bonded like this before. There are various types of cleats available. Normally trefoil with a separate single cleat for the neutral. As many installs now have to contend with neutral harmonics and therefore quadfoil can be used. Like any cable derating has to be applied. This is why we used the wooden cleats for multiple transformers. Believe it or not there is a use for this magnetic repulsion. Isolating contacts for withdrawable switchgear, the higher the current the tighter the fixed contacts grip the centre isolating contact. There is an insulating pad for the spring to seat on, if current is allowed to pass through the spring the heat will weaken it. © Tony S and © RoB (photodraphs) View full knowledgebase
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