Control of PV-STATCOM: a synchronverter approach

Abstract

As the use of renewable energy sources like solar PV and Wind Power in the power system is increasing and becoming dominant, the inertia of the power system is tending to reduce. The reason is that the renewable energy sources (wind and solar) have very less or no rotating mass with them like synchronous machine in conventional power generation. Due to that whenever there is a mismatch between power supply and demand, the variation in the system frequency will be faster and larger. Therefore, it is more difficult to keep the frequency in the normal range. It may result in cascade tripping and in the worst-case, blackouts. Inertia in the power system is important for the power system stability and it is one of the important factors in maintaining the power balance between supply and demand. Moreover, it slows down the rate of change of frequency during the disturbance. The increased penetration of the inverter based distributed generators (DGs) will diminish the power system inertia. To mitigate this challenge, a new concept called “virtual inertia” is becoming popular now-a-days. Based on this concept, inverter based DGs are supposed to emulate the synchronous generator rotor dynamics and supposed to provide virtual inertia support. This type of system is known as the “Virtual Synchronous Generator (VSG)”. VSG have many different realizations and the “Synchronverter” is one of many realizations of VSG. This thesis is focused on the synchronverter application. Other than that, voltage regulation at the Point of Common Coupling (PCC) is also a challenging issue for the grid connected DG system. As the conventional voltage regulators like OLTC and shunt capacitors are not useful in the distribution system with DGs, STATCOM can be used for voltage regulation purpose. STATCOM which is called Static Synchronous Compensator can act as a synchronous condenser for the regulation of the PCC (Point of Common Coupling) voltage by injecting or absorbing appropriate amount of reactive power. Also, the inverter, based on solar energy, does not run on its full VA capacity throughout the day. After the active power generation if the inverter’s remaining VA capacity can be utilized for the PCC voltage regulation, then the inverter VA capacity can be fully utilized and also it improves the grid integration of the DGs by providing good voltage regulation. With the above objectives, in this thesis, Synchronverter control and STATCOM (AC voltage control) both have been implemented as a single system for a grid-connected inverter with a photovoltaic source. In this thesis, all associated system components and its control design have been discussed. The performance of the proposed system is simulated using the Real Time Digital Simulator RTDS. It is observed that this approach of using a grid-connected inverter for multiple purposes (for Active and Reactive power control, providing the “Virtual Inertia” and PCC voltage regulation), is really helpful for connecting more and more inverter-based renewable energy sources to the grid which may ensure better grid operations.