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Tony S

Voltage optimising transformers

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Tony S

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.



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