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Found 4 results

  1. Voltage and current flow theory. It's a beautiful day today South of Edinburgh but I know it's all Gona change soon...... Anyway, I'm installing 12v led strip lighting(for a pretty effect) in renovated buildings. At the first fix stage just now. The led strip lights can be supplied with single insulated 2 core flex from wholesalers but I'm running twin an earth from a central location within each flat. The plan is for the 12v led transformers to be located in a central location with the cables radiating out to the light switches and led strips. Wrt step up/step down transformer theory etc is there any problem with eg 12v and 10A flowing from transformer to said led tape/strip light via 1.0mm twin an earth? I assume current carrying capacity tables based on 230v. All cables run withing thermal insulation. Hope above makes sense and any learned help much appreciated.
  2. Voltage optimising transformers: No not the VPhase domestic units, industrial units up in to the MVA range. So the declared voltage in the UK is 400/230V, as we all know it is around 424/245V (average voltage from DNO statistics based on the distance from the transformer. The standard distribution transformer output is 433/250V). Lowering the distribution voltage is feasible but it carries penalties. I tried it on a 40MVA system using the 33/11kV tap changers, it caused nothing but trouble. Motors on overload, discharge lighting failing and I was getting severe earache from the plant operators. I got the voltage down to 400V, I soon put it back to 433V before I was lynched. The DNO’s regulate the LV system voltage as high as they can without going over the maximum statutory limit of 230V +10%. The higher the voltage the more efficient the system becomes enabling each cable to handle more power. Much of the system uses old PILCSTA cables. A 0.3in² 3½c will handle 455A for resistive loads, at 433V that is 337kW, at 400V it drops to 311kW. To add to the problems inductive loads will draw more current the lower the voltage so the power per cable drops in an exponential manner. Having a rummage around on the web I came across a UK made voltage optimiser transformer using cutting edge technology. I found the same set up in a 1930’s copy of Stubbs Electrical Encyclopedia, so much for cutting edge technology. It is basically a buck/boost transformer with tertiary windings. A 3Ø YY or autotransformer is almost transparent to harmonics, the tertiary winding quells the harmonics. Something that has to be bourn in mind is the load current only passes through the primary part of the winding this makes the kVA rating a little complicated. The rating is based on the voltage difference and line current. A single phase transformer rated at 10kVA to change the voltage from 245V to 230V would have an output FLC of 45.5A. To get the same output from a buck transformer, 0.75 kVA would suffice. Normally there are two primary windings, depending on how they are connected gives either a finer voltage control at reduced current or higher current with coarse voltage control. High current. Low current. Please remember these transformers are rarely used in the UK and there are restrictions on the types of supply they can be used. The UK being in the main Yn for LV supplies they’re no problem The only time I’ve used anything like this was for a ¾ of a mile string of 40W MBFU lighting poles. Due to volt drop the MBFU’s at the remote end were failing to strike, or if they did strike it was like a disco. One of the mates went rummaging around in stores and found 300 yards of SWA apparently not booked to a job so I made out a chit and “borrowed” it on a permanent basis. It got part way along the string so it was paralleled with the original feed as far as it would go. End of problem as far as we were concerned. Unfortunately the lights went in to a different area of the works and one of their electricians decided to add a 400W MBFU at the far end. Back to square one with the disco lights. I’d had my arse kicked for the “borrowed” cable so forget that option. Another raid on stores and an old 5kVA 250/110V compound filled transformer was “borrowed” as it had tappings for 250/240/230 on the primary. As an autotransformer it did the job. It was hidden under a dry stone wall for years, officially it didn’t exist even though “der management” knew it was there. Hidden behind a screen of bureaucracy and weeds.
  3. Tony S

    Polyphase transformers

    Polyphase transformers Three phase transformers (and a history lesson). Now we get to where you need more information on phase angles. This is simply down to the fact that every time the supply is stepped down from the grid supply and a ∆Y transformer is involved the phase angle gets altered. Normally not a problem until you get a phase mismatch. This was a hell of a problem as the National Grid developed. A while ago I lived in Mansfield and being the curious type (nosey) I’d found out the house I was living in had originally been supplied by the Mansfield Corporation Electricity Supply Co. The supply was generated at the “Destructor”, a waste incinerator in the town that generated the 650V DC for the trams. This got me nosing around and I found a web site by a retired EMEB engineer Leslie Marriot. (Unfortunately he died and his son shut the site down). I should contact the IET to find out more about him. Leslie Marriot operating a Ferguson Pallin 3.3KV DMO OCB and also at the main 33KV control board. It covered the change over from DC to AC and the developing National Grid. Mansfield was tied in to the grid fed from Castle Donnington and later Doncaster and Newark. (There’s a brilliant recollection by Leslie of converting a chip shop from DC to AC. He misread the speed of the potato peeler motor. The end result was the room, the chip shop owner and himself covered in mashed potato.) To add to the confusion the local pits wanted their private powerhouses to be tied in to the system. Tying some of them in was a lost cause, 12.5 and 25Hz tend to get a bit upset being connected to 50Hz. (There were ways around this but leave that for now). Voltages were all over the place, transformers fixed that. Some had reversed phase rotation, simply reverse the phases at the intake sub. The place I served my apprenticeship at had both rotations on different plants. In the town, some 433V supplies were stepped down from 11KV direct, others went through the 3.3KV system first. So you could have a street with an underground link box where the phases were 30° out from each other. They did stick a lump of wood in to warn not to parallel the feeds. When Doncaster (I think) was linked in the 132KV was out of phase. So you can see phase angles do make a difference. Particularly to an industrial electrician where the site has an internal distribution system. To give you some idea, an 11/.433KV transformer can have 12 different vector diagrams and phase angles. Dyn1 = 30° Yyn2 = 60° Dyn3 = 90° Yyn4 = 120° Dyn5 = 150° Yyn6 = 180° Dyn7 = 210° Yyn8 = 240° Dyn9 = 270° Yyn10 = 300° Dyn11 = 330° Yyn12 = 360° Believe it or not you can get all twelve from just two basic transformers. You just have to do a bit of knitting with the HV links. You will be familiar with this diagram even though I’ve corrected the rotation to anticlockwise. Now if we correct the angle to delta red at 360° you will see that star red is at 330°, or the 11 o’clock position. Dyn11 @ 330° Is this rotation malarkey becoming clear? If we were to feed a second ∆Y transformer No.2 from the first then a second shift occurs. This time I’ve turned delta red to the position of star red in transformer No.1. No.2 transformer is now at 300° or 10 o’clock. Dyn11 @ 300° One of the tricks of winding a transformer is the primary and secondary can be reversed on each other. This is used to create a 180° shift in a three-phase transformer. This shows what I mean using a single-phase transformer. This is used in six of the twelve configurations. So far we can create 4 vectors by using either star or delta and 0° or 180°. Next comes the crafty bit. Three tins of paint required, “who says that’s the red phase, quick dob of paint, it’s yellow now!” 4 vectors X three tins of paint = twelve possibilities. Keep the tins in the right order though, it gets confusing otherwise! In reality the HV links are swapped around in the tank. You couldn’t really charge £xxK more for a bespoke transformer if you’ve just done a quick paint job, or could you ? ? ? ? If we have a 11KV board feeding two transformers, one 11/3.3KV and the other 11/.433KV, both Dyn11. Now someone decides to add a 3.3/.433KV Dyn11 transformer to the 3.3KV board to be a 2nd feed to the LV board. You can see that the bus-section can be closed only so long as one side of the board is dead first. In the right hand side drawing the bus-section could be closed on load if needed. (I wouldn’t.) Many moons ago we were making major modifications to several plants including changing the supply voltage. Some kit had to remain on the original voltage so we had a couple of these made. Yyn12 with a territory winding to control harmonics © Tony S
  4. Voltage optimising transformers: No not the VPhase domestic units, industrial units up in to the MVA range. So the declared voltage in the UK is 400/230V, as we all know it is around 424/245V (average voltage from DNO statistics based on the distance from the transformer. The standard distribution transformer output is 433/250V). Lowering the distribution voltage is feasible but it carries penalties. I tried it on a 40MVA system using the 33/11kV tap changers, it caused nothing but trouble. Motors on overload, discharge lighting failing and I was getting severe earache from the plant operators. I got the voltage down to 400V, I soon put it back to 433V before I was lynched. The DNO’s regulate the LV system voltage as high as they can without going over the maximum statutory limit of 230V +10%. The higher the voltage the more efficient the system becomes enabling each cable to handle more power. Much of the system uses old PILCSTA cables. A 0.3in² 3½c will handle 455A for resistive loads, at 433V that is 337kW, at 400V it drops to 311kW. To add to the problems inductive loads will draw more current the lower the voltage so the power per cable drops in an exponential manner. Having a rummage around on the web I came across a UK made voltage optimiser transformer using cutting edge technology. I found the same set up in a 1930’s copy of Stubbs Electrical Encyclopedia, so much for cutting edge technology. It is basically a buck/boost transformer with tertiary windings. A 3Ø YY or autotransformer is almost transparent to harmonics, the tertiary winding quells the harmonics. Something that has to be bourn in mind is the load current only passes through the primary part of the winding this makes the kVA rating a little complicated. The rating is based on the voltage difference and line current. A single phase transformer rated at 10kVA to change the voltage from 245V to 230V would have an output FLC of 45.5A. To get the same output from a buck transformer, 0.75 kVA would suffice. Normally there are two primary windings, depending on how they are connected gives either a finer voltage control at reduced current or higher current with coarse voltage control. High current. Low current. Please remember these transformers are rarely used in the UK and there are restrictions on the types of supply they can be used. The UK being in the main Yn for LV supplies they’re no problem The only time I’ve used anything like this was for a ¾ of a mile string of 40W MBFU lighting poles. Due to volt drop the MBFU’s at the remote end were failing to strike, or if they did strike it was like a disco. One of the mates went rummaging around in stores and found 300 yards of SWA apparently not booked to a job so I made out a chit and “borrowed” it on a permanent basis. It got part way along the string so it was paralleled with the original feed as far as it would go. End of problem as far as we were concerned. Unfortunately the lights went in to a different area of the works and one of their electricians decided to add a 400W MBFU at the far end. Back to square one with the disco lights. I’d had my arse kicked for the “borrowed” cable so forget that option. Another raid on stores and an old 5kVA 250/110V compound filled transformer was “borrowed” as it had tappings for 250/240/230 on the primary. As an autotransformer it did the job. It was hidden under a dry stone wall for years, officially it didn’t exist even though “der management” knew it was there. Hidden behind a screen of bureaucracy and weeds. View full knowledgebase
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