Reactive power control strategies for renewable energy systems

In recent years, photovoltaic (PV) systems have increasingly been used to generate power. Connecting these units to existing power grids allows users to sell power back to the utility, but many grid-connected PV systems can cause voltage fluctuations and instabilities. Policies for dealing with these problems exist, but they vary from one country to another and may not be uniformly effective in dealing with issues of reliability.

In a recent paper, the range of ways instabilities are handled when several PV sources are connected to the grid was reviewed. The authors specifically looked at the use of low voltage ride through (LVRT) capabilities in power networks. LVRT and grid support requirements vary across countries, and some policies lead to the generation of harmonic oscillations in the network. In addition, large numbers of PV units connected to the grid can cause the PV power to exceed the load demand, resulting in a reverse power flow, overheating and other problems, such as voltage flickers, power outages and blackouts.

The authors review three reactive power control strategies used to deal with voltage disturbances. One strategy keeps the active power and reactive power at the same level when voltage drops occur. Another uses active power control where the power generated by the PV sources is kept equal to the active power’s mean. A third strategy keeps the reactive power constant.

The authors of this review propose combining two or more control strategies and provide several calculations to show when the outcome would probably be favorable. They also suggest a multimode control technique, where the control strategy is selected based on the depth of the voltage decrease.

Source: “A comprehensive review of reactive power control strategies for three phase grid connected photovoltaic systems with low voltage ride through capability,” by M. Sufyan, N.A. Rahim, B. Eid and S.R.S. Raihan, Journal of Renewable and Sustainable Energy (2019) The article can be accessed at https://doi.org/10.1063/1.5095185 .