28.01.2021 | Solar plants cause voltage fluctuations
Switzerland must increasingly rely on decentralised solar plants to produce electricity. Photovoltaics provides renewable, CO2-free energy – but the plants also increasingly cause voltage fluctuations in the grid. Researchers at Zurich University of Applied Sciences (ZHAW) have pointed out that this problem can be best solved by intervening directly in the PV system.
In the future, Switzerland must produce more power from decentralised photovoltaic systems. If the power is not consumed at the generation site, part or all of it is fed into the local low-voltage grid and this impacts voltage.
However, these impacts may not be too extreme because limits have been set for voltage: In Switzerland and European electric sockets the current has a voltage of about 230 volts. Depending on power production and consumption this value can vary – it can be higher when power production surpasses consumption, or lower when consumption is higher than production. Deviations may not exceed ten per cent – plus or minus. If the voltage is not in a range between 207 and 253 volts, connected electrical devices or computer systems can be damaged.
ZHAW researchers investigated how the expansion of photovoltaics affects low-voltage grids and what measures could be instituted to prevent voltage fluctuations. They found an optimal test environment in Dettighofen near Schaffhausen. The village in Germany currently covers 45 per cent of its energy requirements with photovoltaic systems.
The measurements indicated that voltage fluctuations up to 7 per cent occur in the grid owing to the numerous solar plants. Power companies can counteract these fluctuations with various measures. Strong, efficient power grids with reserves are key here – however, expanding the grid expansion because of the increasing feed-in of photovoltaic power can result in high costs. Alternative, less expensive solutions would be controlling voltage at substations or on the inverters of PV systems. It is also possible to stabilise voltage through targeted connection and disconnection of consumers or by means of storage batteries. This is the case in a project for the Elektrizitätswerke Jona-Rapperswil, which was realised by Axpo and CKW.
The ZHAW researchers examined various measures in terms of efficiency and costs. They come to the conclusion that voltage stabilisation measures create high costs primarily in remote regions. Best and least expensive is to intervene directly in the PV systems when voltage fluctuations occur and to control the reactive and active power (see box) similar to the way voltage stability occurs on higher voltage levels through the targeted feed-in and off-take of reactive power.
Active, reactive and apparent power control
Electric current is categorised in reactive, active and apparent power and measured in watts. When electric current flows through power lines, as much as possible must reach the consumer. The power that the consumer can use is called active power. Reactive power refers to power that is required to transmit active power to the consumer. In order for electricity to flow through the grid, generators, motors or transformers create a magnetic field with the help of reactive power. Together active and reactive power result in apparent power.
A power grid can only transport a certain quantity of energy. The higher the undesired reactive power, the lower the active power. Power companies must continuously control the reactive power volume and keep it as low as possible. However, if reactive power is too low, the voltage drops and the power can no longer flow properly.
The ZHAW researchers point out that it is possible to make use of this factor in PV inverters that transform direct current from the PV modules into alternating current: "Controlling reactive power can reduce the voltage in the grid without having to reduce the feed-in of the photovoltaic plant. However, if the active power is controlled, the photovoltaic plant will invariably feed less power into the grid."