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Found 16 results

  1. Tony S

    Neutral earth connections #1

    Neutral earth connections: Under normal conditions the LV neutral is securely connected to earth via a test link. There are stipulations regarding LV N→E links, mainly that they should be <20Ω. A separation of 8m from the MV earth is required. If however <1Ω is achieved a link between the MV and LV earths is fitted. I’ve had UKPN try to pull a fast one with this. I asked about the <1Ω rule and was told don’t worry about that, then the penny dropped, they wanted our earth nest connected to their MV system. For OH systems the same 8m rule applies. This shows the reason for the separation of the electrodes. It reduces the possibility of crossover of a MV fault in to the LV system. As sometimes happens in rural areas the feed to a house is direct from the OH transformer with no intermediate pole neutral/earth rod, therefore a PNB system is used. Neutral is earthed 8m away from the transformer pole but the connection is at the customers installation. A friend from Scotland sent me this picture. The earth is as big as the concentric cable, apparently it is a quite common practice in the wilds of Scotland. There are a few exceptions to solid bonding. For LV with a delta secondary if it requires earth reference can be connected to a center earthed high impedance zigzag transformer. The top two drawings are for LV secondary, the center point of the zigzag is taken to either a choke or metal plate resistors to regulate the fault current. The bottom drawing is for a HV secondary where the fault current is regulated by internal chokes. For mining transformers earth fault current has to be limited to a maximum of 5A so a choke is used. All loads on these transformers are phase to phase with no neutral. As you can see these transformers have a 3.3KV OCB (VCB now) built in to the unit. As there is no neutral earth fault detection is by a CT and relay on the star point tap. This relay trips the 3.3kV there being no switchgear on the LV side. Somehow the quarry had acquired two of these units, I was told “get them working, we're opening a mine”. IT system. Impedance terra, For HV the earth fault current needs to be limited. For a domestic supply you may be dealing with a couple of KA as the maximum. With a 11/3.3KV transformer the secondary output voltage to earth is 1905V, the earth fault current shoots way up in the KA range. If you consider most systems use the armored cable as the CPC (I’m not starting that argument again). I don’t know about a CPC, it’s going to be more like a fuse. The current is usually limited to between 500 to 1000A. Limited to 500A it’s still 952KW of heat going somewhere. This is for a 33/11KV 20MVA Dyn11 transformer. The star point is usually brought out to an external insulated terminal. From there to the N/E test switch followed by the protection C/T’s and finally the Neutral Earth resistor (NER). This resistor has to be able to absorb most of the heat generated until the fault is cleared by remote substations local E/F protection. If the fault persists the transformers own protection steps in and cuts the incoming supply. The NER can either be metal grids or an electrolyte filled tank with a central electrode. The protection relays are for Restricted Earth Fault (R.E.F.), this detects an earth fault in the transformer secondary and tails only. The inverse EF. has the same function as an S type RCD. O/L protects the transformer O/P from believe it of not, overload. These need a certain amount of maintenance such as topping up the electrolyte level with distilled water. I found one where the connection to the electrode had rotted through, the connection also supports the electrode. It had fallen off and was sat in the bottom of the tank! Now that’s what I call a floating neutral fault! We had four 33/11KV 20MVA and four 11/3.3KV 4MVA transformers with liquid NER’s at the foundry. Just to give an idea of the size of these things. Metal grid resistors are also used. © Tony S
  2. Tony S

    Transformer configuration

    Transformer configuration These four drawings show the internal connections of the main types, Dyn11 is the most common. As you can see all the different configurations are made by changing the HV links or coil orientation. Sometimes when it is known that loads are going to be badly out or balance a Zigzag secondary transformer is used. The foundry I worked at used these because the electric furnaces work only on 2 phases each off the 11KV system. We had five furnaces, no mater how you swap them around you’re not going to balance the loading. These do use approximately 30% more copper in the windings. The transformer I’ve shown is Yzyn11 @330° so in theory it’s the same vectors as a Dyn11. I wouldn’t like to try paralleling them though the impedance would stop them working together. Now if we take this shuffling of phases around another step we can get all 12 phases out of one ∆Y transformer. These would be used for large industrial rectified DC supplies where the star point forms part of the DC system. By playing with the secondary windings we can make a Zigzag with 12 phases. This is one branch of a rectifier 12 phase secondary winding. Now as a complete secondary. It shows the phase angles or have I got it mixed up with the story board for Custer’s Last Stand. Grid distribution transformers: Economics really start to come in to things now. It would be nice to separate the EHV from the HV but that would need extra windings. Instead the cheap option is an autotransformer (1). This acts as a double Y transformer. The problem is they are almost transparent to harmonics and fault let through current is horrendous. To counteract this a tertiary ∆ winding (2) is used to limit the fault current and the harmonics. As you can see in drawing (2) it doesn’t do anything useful. By bringing out the tappings an on site auxiliary supply can be provided, usually at around 11KV. The rest of the gubbins shown will be dealt with when we come to earthing. Scott T transformers: These were mainly used in America and are a hangover from the first Edison polyphase distribution system. (Some out of the way places still use it. An American friend sent me a copy of the PoCo’s tariff for his farm.) Don’t get them mixed up with center tapped split phase. I just found out this afternoon, some parts of Pennsylvania still use 90° two phase. Where the original system used two phase five wire centre tapped it got adapted to three wire, both at 90°. To create it now from three phase AC a Scott transformer is used, its output being two phase 3 wire or 5 wire at 90°. Split an equilateral triangle with a perpendicular from the apex to the base. You then have two right angle triangles. Hypotenuse = the line-to-line primary voltage. Base = ½ the line-to-line primary voltage The resultant perpendicular gives the tapping for the horizontal winding of 86.6% or √3/2. Therefore the manufacturers only make one set of coils each with 0%, 50%, 86% and 100% taps. Neutral current is given by In=√(Ia²-Ib²) It seems UKPN has inherited Scott transformers around Croydon and Bristol. Both originally had a 250-0-250V DC distribution system. Rather than scrap the three core cables the electricity board of time used 250-0-250V Scott transformers. LeBlanc transformers: When I first saw these to be honest, I couldn’t see an advantage over the Scott T. I glibly said in the first write up “someone had to reinvent the wheel and add a few more spokes". Then I looked at the vector diagrams and found the load on the secondary could be balanced evenly over three phases, something that can’t be done with the Scott T. The calculation for the neutral current is the same In=√(Ia²-Ib²) © Tony S
  3. Tony S

    Tap changers

    Tap changers: The HV supply voltage may not always be correct so a means of changing it is required. This is an Off Load Tap Changer, only to be operated with the transformer isolated. They will normally be padlocked in one position. Where I served my time the 11KV supply was actually 11.4KV. So the tap changer would be set to +2.5%, which would be position 2. Under no circumstances must these tap changers be operated on load. The settings are for 11KV nominal: On load tap changer: The required tap is automatically selected and then the change over takes place. A dead short is prevented by the resistors carrying the current during change over from tap to tap. If you’ve ever been stood next to a tap changer transformer when it operates the sound is horrendous. It’s like it’s being violently sick due to the partial shorting of the windings. Our company accountant had read or been told of the economic advantages of reducing the LV from the standard 433V to 400V. The works engineer sent me a memo telling me to give it a try the following weekend, he then promptly vanished in to the sunset on his canal barge. I think the idea was that I would try it on one of the intake transformers. I put all four 20MVA units in to leader and follower mode the leader being in manual. After they had all settled on the leader value I gradually lowered the voltage. I had fitters in various substations radioing back the LV meter readings, I only had the 11KV meters to go by. 433V no problems 415V some drives taking higher load (this isn’t boding well.) 408V drives on higher load and lights failing 400V drives tripping on O/L and I’m getting severe earache. I gave up! All units back in to independent auto and let them sort themselves out. Four units sounding like they are all being sick at the same time isn’t a sound I want to hear again. I wasn’t available for the Monday morning arse kicking party, I would have loved to have been there to see the accountant squirm. Auto tap changer settings: Where multiple transformers are used they can be configured several ways. Each has its own voltage trim relay that monitors the secondary output. Using selector switches several modes are available. Independent auto (normal for most situations) Leader auto (I’ve only used this for paralleling feeders) Manual Manual leader (the accountants little “experiment”) Follower (do what the leader says) There been several nasty accidents due to faulty or incorrectly operated tap changers this is just one. http://www.healthandsafetyatwork.com/hsw/john-higgins If anyone’s interested this is what went in to just one animation. © Tony S
  4. Tony S

    Neutral earth connections #2

    Thompson strap: Nothing to do with my somewhat peculiar proclivities what so ever. As you go further up the electricity supply chain fault currents are a real problem. Sometimes to get around the problem a double secondary transformer is used. As far as I know the biggest 11kV breaker is 2000A or 30MVA, splitting the O/P from a 60MVA transformer gets fault levels down. An additional problem is correcting vectors to make the transformer output compatible with other systems so some weird and wonderful configurations are used. I’ve shown a Yddz 132/11kV transformer. ∆ windings have no earth reference for fault detection. Therefore a high impedance Y Zig-Zag transformer is used to provide the reference point. Two outputs equals two expensive earthing transformers. A cheaper way around the problem is the “Thompson Strap”. Connect the two blue L3 O/P’s together and use just one high impedance Y Zig-Zag transformer for both outgoing circuits. By convention the two blue L3 phases are linked by the Thompson Strap and O/P “B” always hosts the earthing transformer. For the national grid 400/275/132kV transformers are usually Yy with the neutrals solidly earthed. The same principal can be applied to star connected secondaries. Although NER’s (Neutral Earth Resistor) are cheaper than the earthing transformer they are still an expense, a lump of copper is even cheaper. BTW. I haven’t a clue who Thompson is, all I get on a Google search is a strap for the Thompson machine gun. © Tony S
  5. Hi all, Another 1st project after i did all the lights in the lounge, hall and bedrooms - thanks to those who helped on that post. Bit confused with this as firstly, the new fittings have no indication anywhere of what is the neutral wire and what is the live wire in the unit. Does it not matter when wiring to the ceiling? Surely i do not have to undo the sealed unit to see if they are coloured? The new unit does not come with a transformer, so should i remove the transformer from the existing fittings, or use the fittings that are in place and just add them to the surrounds? Next comes the issue with the current fittings (see 2nd set of pics). I have started to pull them out, and there appears to be no slack on the wire for me to access anything? I can see the screws, but not sure I can access them? Am i missing something? Any guidance would be greatly appreciated. Thank you. Martin.
  6. Tony S

    Loss of 1Ph to a 3Ph transformer:

    Loss of 1Ph to a 3Ph transformer: So what happens if a 3 phase transformer tries to run on 2 phases? I’ve drawn a 11/.433kV Dyn11 transformer showing the actual phase relationship between primary and secondary windings. The LV outputs being: l1→l2 = 433V l2→l3 = 433V l3→l1 = 433V l1→N = 250V l2→N = 250V l3→N = 250V Loose one of the MV primary phases (in this case red) both L3→L1 and L1→L2 become series windings The offload LV outputs now being: l1→l2 = 0V l2→l3 = 375V l3→l1 = 375V l1→N = 125V l2→N = 125V l3→N = 250V ©Tony S
  7. Tony S

    Single Wire Earth Return:

    Single Wire Earth Return: I thought this system was only used in the Australian outback and stopped being used many moons ago, it seems it is still being installed in some countries NZ, Australia, Canada, India, Brazil, Africa and Asia for sparsely populated rural areas. As the system relies on an efficient earth return deep driven/multiple rods are required. Typically two rods 6m deep, 6m apart at 0.3m below ground level. Step voltage can be a problem especially in rural areas where cattle are likely to be present. India sets a maximum of 20V voltage rise at 1m, usually <5V is achieved. Obviously the higher the phase voltage the lower the earth return current. Namibia for example uses 19.1kV and so the insulators are the same as 33kV 3Ph. (19.1x√3=33) Basically they use what can be bought of the shelf. Surge diverters are extensively used throughout the system. An auto-reclose breaker is fitted at the source transformer. SWER Advantages: Simplicity Low capital cost Lower maintenance costs Reliability SWER Disadvantages: Low power transfer Single phase only This shows the single phase conductor, expulsion fuse and surge diverter along with the three LV conductors. As a variation on a theme, MV single phase and neutral courtesy of the US of A. OK the neutral insulators and only one fuse are cheaper but they’re the only advantages I can see. © Tony S
  8. Cooling and ancillary equipment: Oil filled transformers have four basic forms of cooling ONAN the most common set up. Natural convection for both oil and air. ONAF Natural convection for the oil, air is forced through the radiators by a fan. OFAN The oil is pumped though the radiators, cooling air is by natural convection. OFAF Both oil and water are pumped/blown. As per usual, there’s the odd ball, I was asked to go to S Africa to look at a 2nd hand transformer with a view to buying. I didn’t go, it was off a tunnel boring machine, totally unsuitable for what we needed. OFWF The cooling was through an oil/water heat exchanger. Oil conservators Basically a header tank to allow the oil and metalwork in the main tank to expand and contract with temperature at normal atmospheric pressure. The vent pipe has an air filter/desiccant breather to stop moist air being drawn in to the main tank. The pipe to the main tank via the buchholtz unit is extended in to the conservator to prevent oil sludge going back to the main tank. Drain and filler are pretty obvious. The level sight glass will have a “cold oil level” mark. Often totally ignored until the buchholtz unit wakes up. One essential (to my mind) for oil cooled transformers over 500KVA, is the Buchholtz unit. It sits in the pipework between the transformer tank and conservator (oil header tank). There are plenty of cutaway pictures on the net but they aren’t too easy to see how it works. Fault conditions within a transformer produce gases such as carbon monoxide, hydrogen and a range of hydrocarbons. A small fault produces a small volume of gas that is deliberately trapped in the gas collection chamber connected to the relay. Typically, as the oil is displaced a float falls and a switch operates - normally to send an alarm. A sight glass in the side of the top chamber allows the amount of gas to be measured, I’ve never come across one I could see through. The petcock allows a sample of gas to be sent for analysis. A Buchholz relay will detect: Gas produced within the transformer. A complete loss of oil from the conservator (very low oil level). An oil surge from the tank to the conservator. A large fault produces a large volume of gas which drives a surge of oil towards the conservator. This surge moves the flap in the unit to operate a switch and send a trip signal. A severe reduction in the oil level will also result in this float falling. The floats are normally arranged in two stages, alarm (top) followed by trip (bottom). Conservator air breathers need a bit of TLC. Unlike the ones I found at a ceramics works, they really shouldn’t have moss growing on them. Every 3 or 6 months we would change the silica gel, clean out and top up the oil bath. It’s not an onerous task, normally the mates did the rounds. Some of our remote substations were in scenic locations, a nature ramble along the banks of the River Wye and getting paid for it. The silica gel we recycled by gently drying out the old gel in a 5 gallon drum sat near the heaters in stores. One of the mates would put the dried gel though a riddle to get rid of the dust before use. Perlised silica gel doesn’t give off dust and therefore is safer. Tank pressure relief: The older bursting disks gave no trouble, they are either burst or they aren’t. Safety valve types are a pain in the bum. Because of the very low working pressure the slightest disturbance and they leak. The first you know about it, the transformer Buchholz unit has tripped the MV supply on low oil. We tried reseating them to no avail. To work on them the tank oil level has to be dropped, the valve replaced and the tank and conservator refilled. Then it was time for the environmental clean up crew. Oil temperature thermostat: Not a lot to say about them, the oil gets hot the MV supply trips. OK until you’re running a transformer overloaded due to failures, then additional cooling is required. Some interesting articles on transformer cooling. There's been some strange things used in the past. http://www.geindustrial.com/publibrary/checkout/Dielectric?TNR=White%20Papers|Dielectric|generic http://www.siemens.com/innovation/en/news/2013/e_inno_1321_2.htm http://www.cargill.com/products/industrial/dielectric-ester-fluids/envirotemp-fr3/index.jsp They’re a bit more environmentally friendly than me then. I must have poured 100’s of gallons of transformer oil down the drains at work. Don’t worry all our drains went in to the process water and was eventually disposed of at near 1700°C in a cement kiln. Transformer oil over the years picks up various nasties that in time will cause it or the transformer windings to fail usually by flash over. Regular testing of oil samples will indicate a trend in contaminants. Allowed to build up and there is going to be a very expensive firework display. Not testing the oil is liable to bring about an unexpected display so it’s worth doing. Two options: Change the oil. A small 250KVA unit will have 100 gallons in it and it is an “off line” operation. Clean and filter the oil. The impurities and moisture are removed while the transformer remains on line. It’s environmentally friendly, no loss in production. Initially more expensive than an oil change but what will lost production cost? BTW, dielectric testing oil is fun if you like watching dials measuring KV. I called GEC in to test the oil in an OCB they had refurbished. Before commissioning I dropped the tank to see what they had done, the bottom of the tank had carbon deposits. GEC came out with a portable test rig which we set up in the workshop. The oil is allowed to settle for 20 minutes and then DC voltage applied at a rate rise of 1kV per second. 30kV being the minimum acceptable. The guy was quite smug as the voltage went on to 40kV. It seemed that the OCB had been cleaned but the 70 mile road trip to return it had sloshed the oil around cleaning every last bit of carbon out of the top of the tank. Painting and cooling. I’ve never known any of our transformers needing a lick of paint other than cover scratches during commissioning. All ours were bog standard battleship grey (BS18B25) and placed out of direct sunlight. A three sided compound with a roof to keep the atmospheric fall out to a minimum, an important factor with lime, cement and iron manufacturing. Trials carried out on transformers in the US show the colour and location of a transformer make a difference to the working temperature. OK it seems fairly obvious when you think about it but do you actually think about it when ordering a transformer? Trials on 2 x 25KVA ONAN transformers in a room at 26°C Top oil temp rise Tank surface temp rise Black Aluminum Black Aluminum Tx 1 37.2°C 46.3°C 32.5°C 41.0°C Tx 2 36.7°C 47.6°C 32.0°C 41.6°C Average 37.0°C 47.0°C 32.3°C 41.3°C Relative % 78.5 100 78.5 100 For outdoors it gets a bit more complicated, the sun mucks things up. Some practical trials were run but were unreliable. So this table is based on solar radiation absorption factors White lead paint coating 0.25* Light cream paint coating 0.35 Aluminum paint coating 0.55 Gray paint coating 0.75 Mat black paint coating 0.97. *Not available for environmental reasons, for comparison only. © Tony S
  9. (VT). These are odd beasts. Single phase will have the LV end tapped to earth. So two MV and one LV fuse. Three phase is nearly always open ∆ form but the LV yellow phase isn’t fused, it’s tied to earth. So you will have three MV fuses but only two LV. Open ∆ is to keep the KVA metering and protection relay voltage inputs in phase with the Current Transformer inputs. A word of warning, many switchgear manufactures will say the metering transformers can be isolated while on line. The isolation contacts are DMO (Dependant Manual Operation). Draw an arc and then its god help you. I’ve withdrawn a 3.3KV VT live, I wasn’t happy about it and wouldn’t do it again. A VT can also be Yy, again with one leg earthed. The early Ferguson Palin 11KV and Whipp & Bourne 3.3KV gear I worked on used Yy. Later gear from GEC and A Reyrolle were open ∆∆. Fine with the old protection relays. Now with new all singing all dancing electronic protection relays Yy are back in favour. Voltage isn’t used in protection relays very often, current as you would expect being the prime consideration. I’ve only installed three OCB’s that needed VT’s for functions other than metering, they were an absolute pain in the backside. I fitted them, I took them out. VT’s are all well and good up to about 33kV but what do you do with higher voltages? A conventional VT with its insulating bushings is going to be massive. H. W. Clothier of A. Reyrolle & Co. Ltd came up with this. A series of capacitors built in to an oil or gas filled insulator post with a tap off near the earthed base. It serves two purposes it supports the conductor and at the same time supplies the metering voltage. The system is still the standard method used today. PS, I've tipped it on its side to fit it in © Tony S
  10. Tony S

    Neutral earth connections #3

    Parallel NER’s (Neutral Earth Resistors) Like generators, transformers can be run in parallel but it’s not good practice. At times needs must and once again prospective fault currents become a problem. Ignoring line to line faults for now, line to earth can be controlled. As shown in neutral earth connections #1 the NER is effective at limiting current for a single (islanded) unit. Close the secondary bus-section switch you not only parallel the transformer phase outputs but also the NER’s, reducing the total fault limiting resistance by a half and therefore doubling the prospective fault current. See where this is going? bigger and better bangs if things go wrong. A system working normally at a primary substation will have the 11kV bus-section open and each NER looking after its own transformer and outgoing network. In transformer overloading I mentioned the limits imposed on DNO transformers depending on cyclic loading. Sometimes a transformer has to be taken out of service for maintenance and so its partner has to take up the load. In exceptional circumstances transformers will be run in parallel. One NER is switched out of service keeping the prospective earth fault currents within design levels. (I’ve seen a 11kV joint fail to earth and I’m glad I wasn’t near it.)
  11. Tony S

    Basic Phase Angles

    Transformer phase angles: Sorry to start with this but if we get it out of the way to start with it will save confusion later. Phase or vector angles is the relationship of current flow in the primary and secondary winding. Or as in the case of an autotransformer the one and only coil. Autotransformer: As you can see, the current has to be the same direction as a section the winding is both primary and secondary. (An advantage of the autotransformer is if a RCD supplies the primary tapping it also protects the secondary. OK it was an advantage to me in my darkroom.) Basic single phase transformer: You can see that this time any one of the two windings can be turned up side down, reversing the output current flow. So far it hasn’t made any practical difference, but next we come to the centre tapped or split phase transformer. Centre tapped transformer: There is a long held myth about these transformers where people believe the secondary windings are at 180° to each other. Hope this dispels that myth. If you put a two channel oscilloscope between L1→N and L2→N you will see this Change the leads to L1→N and N→L2 or just go between L1→L2 you will see this. Confusing isn’t it? If you look at the secondary of each transformer you will see the winding is in two halves. The left hand shows the voltages adding to each other giving 230/460V outputs, the neutral current is like three phase where the neutral current in one line cancels out the current from the other. In=Ia-Ib With the right hand side the two voltages are not added and as you can see the output would be 230V/0V If you tried to use this as a distribution transformer it wouldn’t be of much use. Neutral also is a problem, instead of canceling out they add, so you would need a neutral twice the size of the line conductors. In=Ia+Ib This now leads us on to our old friend the Site Transformer. The major difference is the earthing arrangement where the earth is imported with the supply. The winding direction makes no odds until you get the silly pillock I worked with. He tried to connect two of different makes together, I hate the smell of burnt insulation but found it funny watching him. Fixed Low Voltage: The ones I’ve had dealings with were mainly at the first place I worked where we worked to M&Q regs. Therefore they had to be fixed installations, 1KVA maximum. Power tools were a maximum of 25V to earth. You’re pistol drill had to be 50V, soldering irons and hand lamps 25V. Not the cheapest things in the world, and getting hard to source. As for something like a jigsaw you had no chance, no one would manufacture one. To get 25V we used the centre tap as the neutral. A pain in the backside when you’re dragging a couple of extension leads and a 700W Black & Decker drill around. Start the drill and you had to wait for it to build up speed the volt drop was that bad. After I’d been there for about 10 years the regs changed and we were allowed 110V tools. But the transformer still had to be fixed. To give an idea of well made the 50V transformer were I converted one to a DC welding set, changing the input tapping was the current regulation. © Tony S
  12. Loss of 1Ph to a 3Ph transformer: So what happens if a 3 phase transformer tries to run on 2 phases? I’ve drawn a 11/.433kV Dyn11 transformer showing the actual phase relationship between primary and secondary windings. The LV outputs being: l1→l2 = 433V l2→l3 = 433V l3→l1 = 433V l1→N = 250V l2→N = 250V l3→N = 250V Loose one of the MV primary phases (in this case red) both L3→L1 and L1→L2 become series windings The offload LV outputs now being: l1→l2 = 0V l2→l3 = 375V l3→l1 = 375V l1→N = 125V l2→N = 125V l3→N = 250V ©Tony S View full knowledgebase
  13. Parallel NER’s (Neutral Earth Resistors) Like generators, transformers can be run in parallel but it’s not good practice. At times needs must and once again prospective fault currents become a problem. Ignoring line to line faults for now, line to earth can be controlled. As shown in neutral earth connections #1 the NER is effective at limiting current for a single (islanded) unit. Close the secondary bus-section switch you not only parallel the transformer phase outputs but also the NER’s, reducing the total fault limiting resistance by a half and therefore doubling the prospective fault current. See where this is going? bigger and better bangs if things go wrong. A system working normally at a primary substation will have the 11kV bus-section open and each NER looking after its own transformer and outgoing network. In transformer overloading I mentioned the limits imposed on DNO transformers depending on cyclic loading. Sometimes a transformer has to be taken out of service for maintenance and so its partner has to take up the load. In exceptional circumstances transformers will be run in parallel. One NER is switched out of service keeping the prospective earth fault currents within design levels. (I’ve seen a 11kV joint fail to earth and I’m glad I wasn’t near it.) View full knowledgebase
  14. Thompson strap: Nothing to do with my somewhat peculiar proclivities what so ever. As you go further up the electricity supply chain fault currents are a real problem. Sometimes to get around the problem a double secondary transformer is used. As far as I know the biggest 11kV breaker is 2000A or 30MVA, splitting the O/P from a 60MVA transformer gets fault levels down. An additional problem is correcting vectors to make the transformer output compatible with other systems so some weird and wonderful configurations are used. I’ve shown a Yddz 132/11kV transformer. ∆ windings have no earth reference for fault detection. Therefore a high impedance Y Zig-Zag transformer is used to provide the reference point. Two outputs equals two expensive earthing transformers. A cheaper way around the problem is the “Thompson Strap”. Connect the two blue L3 O/P’s together and use just one high impedance Y Zig-Zag transformer for both outgoing circuits. By convention the two blue L3 phases are linked by the Thompson Strap and O/P “B” always hosts the earthing transformer. For the national grid 400/275/132kV transformers are usually Yy with the neutrals solidly earthed. The same principal can be applied to star connected secondaries. Although NER’s (Neutral Earth Resistor) are cheaper than the earthing transformer they are still an expense, a lump of copper is even cheaper. BTW. I haven’t a clue who Thompson is, all I get on a Google search is a strap for the Thompson machine gun. © Tony S View full knowledgebase
  15. Single Wire Earth Return: I thought this system was only used in the Australian outback and stopped being used many moons ago, it seems it is still being installed in some countries NZ, Australia, Canada, India, Brazil, Africa and Asia for sparsely populated rural areas. As the system relies on an efficient earth return deep driven/multiple rods are required. Typically two rods 6m deep, 6m apart at 0.3m below ground level. Step voltage can be a problem especially in rural areas where cattle are likely to be present. India sets a maximum of 20V voltage rise at 1m, usually <5V is achieved. Obviously the higher the phase voltage the lower the earth return current. Namibia for example uses 19.1kV and so the insulators are the same as 33kV 3Ph. (19.1x√3=33) Basically they use what can be bought of the shelf. Surge diverters are extensively used throughout the system. An auto-reclose breaker is fitted at the source transformer. SWER Advantages: Simplicity Low capital cost Lower maintenance costs Reliability SWER Disadvantages: Low power transfer Single phase only This shows the single phase conductor, expulsion fuse and surge diverter along with the three LV conductors. As a variation on a theme, MV single phase and neutral courtesy of the US of A. OK the neutral insulators and only one fuse are cheaper but they’re the only advantages I can see. © Tony S View full knowledgebase
  16. Transformer phase angles: Sorry to start with this but if we get it out of the way to start with it will save confusion later. Phase or vector angles is the relationship of current flow in the primary and secondary winding. Or as in the case of an autotransformer the one and only coil. Autotransformer: As you can see, the current has to be the same direction as a section the winding is both primary and secondary. (An advantage of the autotransformer is if a RCD supplies the primary tapping it also protects the secondary. OK it was an advantage to me in my darkroom.) Basic single phase transformer: You can see that this time any one of the two windings can be turned up side down, reversing the output current flow. So far it hasn’t made any practical difference, but next we come to the centre tapped or split phase transformer. Centre tapped transformer: There is a long held myth about these transformers where people believe the secondary windings are at 180° to each other. Hope this dispels that myth. If you put a two channel oscilloscope between L1→N and L2→N you will see this Change the leads to L1→N and N→L2 or just go between L1→L2 you will see this. Confusing isn’t it? If you look at the secondary of each transformer you will see the winding is in two halves. The left hand shows the voltages adding to each other giving 230/460V outputs, the neutral current is like three phase where the neutral current in one line cancels out the current from the other. In=Ia-Ib With the right hand side the two voltages are not added and as you can see the output would be 230V/0V If you tried to use this as a distribution transformer it wouldn’t be of much use. Neutral also is a problem, instead of canceling out they add, so you would need a neutral twice the size of the line conductors. In=Ia+Ib This now leads us on to our old friend the Site Transformer. The major difference is the earthing arrangement where the earth is imported with the supply. The winding direction makes no odds until you get the silly pillock I worked with. He tried to connect two of different makes together, I hate the smell of burnt insulation but found it funny watching him. Fixed Low Voltage: The ones I’ve had dealings with were mainly at the first place I worked where we worked to M&Q regs. Therefore they had to be fixed installations, 1KVA maximum. Power tools were a maximum of 25V to earth. You’re pistol drill had to be 50V, soldering irons and hand lamps 25V. Not the cheapest things in the world, and getting hard to source. As for something like a jigsaw you had no chance, no one would manufacture one. To get 25V we used the centre tap as the neutral. A pain in the backside when you’re dragging a couple of extension leads and a 700W Black & Decker drill around. Start the drill and you had to wait for it to build up speed the volt drop was that bad. After I’d been there for about 10 years the regs changed and we were allowed 110V tools. But the transformer still had to be fixed. To give an idea of well made the 50V transformer were I converted one to a DC welding set, changing the input tapping was the current regulation. © Tony S View full knowledgebase
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