Whenever anyone has checked the mains supply voltage at the property it’s been higher than the expected 240V (and hence much higher than the nominal 230 V). While this isn’t a major problem in itself, the commissioning of the Solar PV installation (and its real-time monitoring) highlighted just how high the voltage can get and introduced the possibility of the Solar PV inverter tripping offline if the voltage gets too high.

Carbon Legacy (who installed the Solar PV system) were seeing the monitoring data from the SolarEdge inverter reporting up to 257 V and contacted Western Power Distribution (the local electricity Distribution Network Operator, DNO) to ask for the voltage to be adjusted and that work was completed today.

I’ve been consistently impressed by the team from Western Power Distribution and today was no different. Everyone involved has always been thoroughly professional and completely reasonable about the work to be done and the fees involved. Today’s work was free.

For those with a more technical interest in the subject, read on…

The electricity supply to the property comes from overhead 11 kV distribution lines via a dedicated, single-phase, pole-mounted transformer which reduces the voltage down to a nominal 230 V. The resistance of the cables causes a voltage drop on both the high- and low-voltage lines, with the result that the transformer is expected to see a bit less than 11 kV on the HV side and needs to deliver a bit more than 230 V on the LV side (to compensate for the voltage drop on the cables to the house). The exact details depend on the length and resistance of the two sets of cables, which in turn depends on the placement of the transformer within the distribution network and on the cable run from the transformer to the house. Due to this variability most transformers have a handful of different settings to switch in slightly different numbers of turns of transformer coils to change the ratios between the input and output voltages. In the case of “my” transformer (a 25 kVA Brentford Electric model dating from 1990) there’s a rotary switch with 5 settings ranging from -5% to +5% and it had been on the middle setting. The setting can only be changed with the transformer isolated.

In reality it’s all a bit more complex since the voltage drop varies with current (V = I x R and all that) so the actual voltages float up and down depending on the load on the network.  When consumption is high (e.g. 10 kW coming into the house) the voltage drop will be high – especially if other properties are also putting a high load on the network and tending to reduce the 11 kV on the distribution grid.

To make matters worse, adding on-site generation reverses the direction of current flow – and also the direction of the voltage drop. When generation is high (e.g. 5 kW going into the grid) the voltage will be raised at the house. This is especially true with Solar PV generation since all of the solar panels in one area tend to come on and go off together and so the voltage on the distribution grid will tend to rise and fall too. The voltage in the house will generally be highest when the Solar PV generation is at its maximum.

This is all well understood by the electrical engineering community and all of the installations are specified in such a way as to keep the voltage within acceptable limits and eliminate troublesome issues like lights flickering when big loads switch on and off (the worst culprits typically being big motors – like those in heat pumps, which is why the distribution network operator needs to be consulted about such installations). For the UK the limits are defined in national legislation and allow the voltage to vary between -6% and +10% of the nominal 230V, giving a minimum of 216 V and a maximum of 253 V. (The UK limits used to be -6% and +6% and the nominal voltage used to be 240 V but that’s a different story.)

One other item worth mentioning is that – in general – it’s better to have a supply at the bottom end of the permissible voltage range than the top end. Voltage optimiser technology works on the principle of monitoring the voltage and actively reducing it to the bottom of the permissible range – typically something like 220 V. For some types of electrical loads this can provide energy savings, but the general view seems to be that the savings are not large enough to warrant the installation of a specialist device for this purpose – especially not in a domestic property.

Update 2017-05-18

With the transformer switched to its lowest setting, the voltage reported by the Solar PV inverter is has reduced to about 240 V. The Immersun unit which diverts excess solar generation capacity to heat the hot water tank monitors the mains voltage and is reporting a minimum of 229 V and a maximum of 247 V.