How voltage affects solar PV
Posted by James Page on 13 May 2013 at 9:40 am
As most people will know mains voltage in the UK is - nominally - 230V (it was 240V until we compromised with the European 220V supply). But why does it change in both time and place even within mainland Britain?
The answer to both stems from Ohm’s law. The voltage drop in the cable to your house is proportional to the current being taken at the time. And of course your neighbours will have an impact on this too, since you most likely share a cable to a substation. The variation of the voltage is quite wide, but if it exceeds 253V you can request the electricity network operator to reduce the voltage at the substation. They may be reluctant to comply however, lest it creates a low voltage problem at other times and places (further away from the substation).
Installing solar PV sends energy the other way up the street and the voltage drop to your house becomes a voltage rise. So inverters are required to switch off automatically if the voltage exceeds certain limits. This is also one of the reasons why large PV systems require prior approval. Otherwise the network operator would be constantly responding to complaints of high voltage caused by generation in neighbouring properties. Much better that such voltage issues are identified before the PV is installed, so far as they can be. Revenue will be lost if the inverter is switching off regularly. Also note that the network operator is only responsible for the voltage at the incoming connection point for the building. The voltage at the inverter could be several volts higher, even in a correctly designed system.
Wiring regulations for solar, and for that matter for electrical wiring in general, were not set with energy losses in mind. They exist to prevent cables overheating and to maintain the correct supply voltages. Even if energy loss was better taken in to account standards would rapidly become out of date. Look how often Part L building regulations for insulation are updated. Even if a house is built to high standards it will rapidly fall behind ‘state of the art.’ With our current knowledge of carbon emissions and rising electricity prices a quick calculation will show that oversizing cables will pay off very quickly. The bonus will be fewer overvoltage problems.
So how do you - or your installer - check the voltage if a problem does arise ? Plug-in meters are readily available (for example from Clas Ohlson) but ideally the voltage needs to be logged over a few days. Monitors used by electricity companies are expensive to buy or hire. Smart meters, and sometimes the inverter itself, could be used for online voltage measurements but they are not normally set up to do so. For the time being I have adapted other devices to fulfil the need. I can see that the voltage in our office ranged from 238V and 254V today. Otherwise enjoy the sun, and keep half an eye out for a red light.
About the author: James Page is a chartered engineer and is head of engineering at Joju Solar. All views expressed are his own. He stores solar energy under the kitchen floor.
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