How might the proposed changes to UK mains voltage limits affect older electrical infrastructure?

Question

Hi all, 
 
 I&rsquo;m looking for some expert insight into how proposed changes to UK mains voltage limits, particularly any shift away from the nominal 230 to 240 volt range, might affect older electrical infrastructure. 
 
 Our site is fed directly from a DNO transformer, although only two of its output connections are currently in use and we&rsquo;re unsure how many it has in total. The infrastructure was originally designed and installed in the 1980s and includes legacy three-phase motors and analogue control systems that were built to operate on 240 volts single-phase and 440 volts three-phase. 
 
 We also have a step-down transformer that is fed by a three-phase 125 amp supply and provides 110 volts at 50 hertz. This is used to meet extra-low voltage requirements in our training environment. In addition, there is a separate system currently locked out that runs on 115 volts at 60 hertz, which is temporarily offline due to a failed frequency converter. This is scheduled for replacement shortly. 
 
 I&rsquo;m particularly interested in understanding how any changes to mains voltage might affect the transformers themselves, both the DNO-fed unit and our internal step-down transformers. I would also appreciate any thoughts on the risks to analogue control systems calibrated for 240 and 440 volts, potential safety or compliance concerns from undervoltage or harmonics introduced by modern supply variations, and any practical mitigation strategies or retrofit options for mixed-voltage and mixed-frequency setups. 
 
 Any advice, experiences, or references to standards would be greatly appreciated. 
 
 Thanks in advance

mapj1 · Accepted Answer

Well currently the voltage can be anywhere from 216 to 253, and the proposal is only to declare the 5% below (207 to 216 ) as no longer out of spec. The intention is to allow it to droop a bit more when EVs are charging and maybe set the taps one click lower so it has the freedom to surge a bit more without going out of limits on sites where that is an issue when embedded generation kicks in. In some ways actually this proposal is only acknowledgement of what is already happening... I will be very surprised if DNOs rush around shifting any transformer settings except where there are persistent reports/ complaints of things being too low or too high, and this really allows them to say they don't need to do so just yet in a few edge cases. If you don't already have voltage problems, I suspect you won't see any changes at all, until additional solar panels or EV chargers are added at your site, and then only if the additions force a change of transformer configurations. 
 At that point whole site step up or step down may be worth a look, but very few things are that critical. If as it sounds, you are the only load on the transformer, then it is not uncommon to ask the DNO to set it a bit high or a bit low as your local needs require, and normally they can oblige. (typically 2.5% steps ) Mike

Sergio Fernandez · Answer

As with lots of things the answer is it depends. EG. 6amp rcbo trip time at 207v vs 230v vs 240v vs 253v 
 
 The primary effect of different voltages relates to the current that can flow for a given fault or load (Ohm's Law: $I=V/R$). A lower voltage (207V) will result in a slightly lower current than a higher voltage (240V) for the exact same circuit resistance: For any given current level above 6A, the trip time is determined by the fixed trip curve (eg Type B or Type C).The only practical difference is that a specific fault resistance might draw a slightly lower current at 207V compared to 240V, potentially moving the fault to a different point on the time-current curve and resulting in a marginally different&mdash;perhaps slightly longer&mdash;trip time 
 
 The above being said a pump or a motor at the low end of the voltage spectrum might start to seem underpowered or run slower which could be an issue for some that have the pump set to a run time rather than a volume or sensor on and sensor off. 
 
 Caveat time. 
 240v in the UK became 230v nominal ages ago. (about 1994) but in Yorkshire (other regions are available) 250v and above are quite regular. The DNO/DSO/NG are probably in no rush to change the transformer tap to 207v but they reserve to use the right of 207v IF it is needed espcially for single phase. This is probably being lead by the amount of re-newable energy be ProSumed/Generated

mapj1 · Answer

Its also worth noting that it would be a most odd situation where you saw the full variation from 253 off load to 207 on load, partly due to the heat losses in cables becoming more than 20% of that in the load, and partly because you would have problems to arrange circuit protection safely. A consequence of droop of 20% under load is that the PSSC is only 5 times the full load current - and given that most fuses will carry 100% overload i.e. 200% of nominal rating, for ages, and at a multiple of less than 5 times a B type breaker may not prompt trip and a C type is guaranteed by design, not to, then unusual measures (well earth fault relays and shunt trips) are needed. Also by the time you add on the permitted voltage drop in the final installation the socket in the garage at the end of the garden in the house at the end of the street main is looking very poorly ;-)
You would only really expect to see both extremes when at some time of day local generation reverses the slope of the voltage drop, and then at other times heavy loads take the full quota.
I'd expect this concession to be used by the DNOs as a reason not to adjust things, rather than as a basis for re-setting transformers where no problems are reported, and perhaps allow slightly longer runs on new housing estates where in the past 2 transformers would have been used but were only just required.
Mike.

How might the proposed changes to UK mains voltage limits affect older electrical infrastructure?

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