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I live in area where many high buildings are being built. Each building is more than 10 floors. A special room is made next to the main entrance or the garage. The room is designed for installing a power transformer inside it. The room also provides good ventilation. The primary coil (winding) of The transformer is 11 KV and the secondary coil is 380/220 V.

Why do we need to install a transformer for each building?

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    \$\begingroup\$ I wonder what your idea would be for doing instead... \$\endgroup\$
    – PlasmaHH
    Nov 27, 2015 at 11:15

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Well, if you didn't have a transformer how would you convert the relatively very, very lethal 11 kV into the definitely less lethal (but still rather lethal) 380/220V?

If, on the other hand, you meant that somewhere down-town you converted 11 kV to the relatively-safer lower voltage and fed this to all the high rise buildings then, the cable losses would be massive because the currents would be 50 times higher.

Or maybe you're thinking that consumers convert all their appliances to 11 kV - that just aint gonna happen for reasons of safety (irrespective of the massive global cost).

Another half-reason is that the 11 kV network is transmitted as 3 phase balanced i.e. it doesn't have a neutral connection: -

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It's transmitted as 3 phase due to economies of making 3 phase generators in the hundreds of mega watt range. But, households in the UK require a single phase and neutral and this is from a historical safety standpoint (in the UK and Europe at least). To create a neutral wire requires a transformer (delta to star configuration) so there is also the cost of extending the neutral wire if only one transformer fed several high-rise buildings.

Also, one fault in one building might trip the whole building out but it wouldn't trip a whole precinct of buildings out. Same is true from a maintenance standpoint - it's less disuptive to isolate a single building than a whole precinct.

I would also imagine, with high rise buildings (subject to lightning strikes) that having a local transformer for each building will be less disruptive to other buildings not hit by a strike.

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There's no need to do this, it's just that above a certain amount of power, it's cheaper to distribute/supply the power at a higher voltage than 380/220.

It's just the difference between one smaller transformer in each block vs one bigger transformer shared between the blocks. In the former case you might spend more on transformers (though manufacturing volumes don't make this a certainty), and in the latter you spend more on losses and LV cable.

You also don't necessarily want your LV distribution system to be too massive, because the fault levels get very high.

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    \$\begingroup\$ The heat from the transformer would be easier to harvest when the transformer is in the building. \$\endgroup\$
    – Autistic
    Nov 27, 2015 at 11:04
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You have to have the transformer from 11 kV to 380V somewhere. If it's far away from your buildings, you have substantially increased transmission losses. If you had one big transformer close by all the buildings, those losses would be substantially mitigated. But there are still a few disadvantages.

One, transformer secondaries are often neutral-grounded to building steel, and each building can have a slightly different ground potential. With one big transformer, you'd have one ground, which would be different from the steel in each building. This could cause grounding issues.

Another (just to make up some numbers), moving ten 1 MVA transformers around is probably easier than moving one 10 MVA transformer, and the transformers themselves may be cheaper as well. Economies of scale and all that.

Third, the single huge transformer can't just sit outside. It has to be enclosed in something. Why build a purpose-built structure or expensive outdoor-rated enclosure, when you have perfectly good buildings available?

Fourth, maintenance becomes problematic. If your transformer ever has to be replaced for any reason (admittedly a rare occurrence), now you have to shut down all ten buildings instead of just one. And since each building still needs its own disconnects from the grid, you still need a lot of room in the building for breakers and switchgear.

Fifth, having separate transformers helps isolate each building from line disturbances in all the other buildings. If someone sticks a fork in the light socket in building A, or everyone in building B turns on their vacuum cleaners simultaneously, building C will see all that line noise if they're on one big transformer. Separate transformers help filter a lot of high frequency noise from passing between buildings.

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220V circuits require 50x more current for the same power drawn from 11KV and cable sizes would need to be prohibitively large to supply so many individual installations at 220V

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Based on Ohm's law there are a reverse relation between current and voltage,for distributing a large electrical power(P=V.I) it is more economic that increasing V parameter instead of increasing I parameter(because we need bigger cables for distributing bigger electrical current and bigger cables are more expensive , while increasing voltage does not need this, but it has it's problems and dangers too.Then Electrical organizations approve a special amount of this parameters in generation,high voltage distribution,urban area distribution and etc.

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You're not required (except for cases when local codes do pose some restrictions) to have a separate transformer per building or a single transformer for multiple buildings. I've seen all of these configurations:

  1. one huge transformer per huge apartment building
  2. one huge transformer per several apartment buildings
  3. one smaller transformer per single small house
  4. one medium size transformer per street with several dozen small houses

Transformers can be installed inside buildings they power or inside standalone structures - steel kiosks, concrete structures, even on poles.

Here are some considerations...

Having a transformer in a separate structure or on a pole is rather ugly and requires extra land. Even if a pole doesn't take much land local codes may require that a fence surrounds the pole to provide a guard band around the transformer. Those faceless standalone structures often look poorly and attract graffiti. Underground substations are quite rare - most likely because of ground water issues.

Having a transformer inside the building may pose serious fire risks. Power equipment burns and smokes pretty well so a lot of effort must be put into making the installation safe.

Having a transformer inside a separate structure makes maintenance easier - the utility company has the keys and when they come over and open up the substation - noone cares. Having a transformer inside the building implies that the utility company will have keys from some doors of the building and will sometimes open up those doors and do something in the building. Various liability, security and trust issues may arise.

Having a transformer inside the building may make it easier to prevent external ("temporary") consumers from being connected to the same transformer and basically reducing the power available to the building. It also makes power distribution more transparent - building owners can see all the lower voltage cables and know that power is indeed used for their building and by electrical equipment that they can examine.

Having multiple consumers spread over a large area and connected to the same transformer also poses voltage regulation problems. Suppose you have a transformer and two consumers - A and B. A is one hundred meters from the transformer and B is seven hundred meters from the transformer. Voltage drop for B will be larger than voltage drop for A. It may so happen that you'll be unable to regulate the transformer such that both consumers receive voltage acceptable by the standard - either A will receive unacceptably high voltage or B will receive unacceptably low voltage. This gets worse for cases when there' a long street of small houses with a transformer on one end. As distance to the transformer increases voltage decreases and starting at some point consumers get undervoltage at all times. You have to either install a booster or divide the lower voltage line into segments and connect each segment to a separate transformer.

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It's an economic decision. Lower voltages like 220V have much larger transmission losses over long distances, unless you use very expensive fat wires to carry them.**

By using transformers they can punch the transmission voltage up much higher. Transmission losses are proportional to the voltage change squared. 220V is 1/50 of 11,000 volts, so transmission losses would be 50*50 or 2500 times worse for the same wire. (in practice the higher voltage lets them use thinner, cheaper wire).

You don't want to haul 220V any farther than you have to.

In a perfect world, they would put a transformer on every floor. However, that would be a considerable expense in transformers. Ultimately, economics decide. They compromise among

  • the cost of many transformers,
  • the cost of heavy wires, and
  • the cost of transmission losses.

One per 10-story building is about right.


** absolute worst case, the poor sap trying to carry 12 volts from his windmill or micro-hydro 200 metres away. They don't make wire big enough for that to work.

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Many high rises have a network feed, meaning multiple transformers being fed from different primary circuits. The secondary side is tied together so if any primary circuit has a fault, the customer will still be energized from the other circuits.

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    \$\begingroup\$ Welcome to EE.SE, Randall. Your second sentence is incorrect. Transformer secondaries would not be connected together as (among other reasons) in the event of the primary circuit fault the working transformer would backfeed onto the failed network through another transformer and would be overloaded. \$\endgroup\$
    – Transistor
    Sep 11, 2016 at 13:14
  • \$\begingroup\$ What is common over here is for an area to be fed by a medium voltage ring main (11kV typically) with each transformer having a tap into the ring that can 1: Isolate the transformer, 2: open the connection to either leg of the ring. The idea is that you can cut a transformer out of circuit for maintenance and can also isolate a faulty section of the ring by opening the switches on either side of it without impacting any users, it is single fault resilient at the 11kV level. As most of our medium voltage distro is underground in cities being able to survive a cable fault is important. \$\endgroup\$
    – Dan Mills
    Jul 7, 2017 at 18:22

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