Novel Fuzzy Controlled Energy Storage for Low-Voltage Distribution Networks with Photovoltaic Systems under Highly Cloudy Conditions

The amount of small-scaled renewable energy sources is anticipated to increase on the low-voltage distribution networks for the improvement of energy security and reduction of greenhouse gas emission. Malaysia has a high solar irradiance level that is ideal for photovoltaic (PV) systems. However, the growth of the PV systems on the low-voltage distribution networks can create a number of technical issues such as voltage rise, voltage unbalance, and reversed power flow. Usually, these issues happen with little fluctuations. However, these issues tend to fluctuate very rapidly in Malaysia because this region has a very low clear sky index. A large amount of clouds often passes over the country, hence making the incident solar irradiance to be highly scattered and fluctuating. Therefore, the power output of the PV systems fluctuates substantially. To ensure an effective operation of the distribution networks with the PV systems, a novel fuzzy control method is developed and implemented to govern the operation of an energy storage system consisting of a bidirectional inverter coupled with lead-acid batteries. The fuzzy controlled energy storage system is able to mitigate the fluctuating voltage rises and voltage unbalances on the networks by actively manipulating the flow of real power between the networks and the batteries. To verify the effectiveness of this fuzzy control method, an experimental network integrated with 7.2-kW photovoltaic systems was set up. A number of case studies are performed to show that the proposed fuzzy control system is able to mitigate fluctuating voltage rises and voltage unbalances under the highly intermittent power output of the PV systems.