Multi-Functional Inverter Based Microgrid for Power Quality Improvement with Smooth Transient Performance

Abstract

The increasing utilisation of renewable energy sources (RESs) plays an important role in the modern power system. The high penetration of the RESs in the electrical power network leads to challenges to the system’s stability. Therefore, the microgrid concept is proposed to solve such problems due to its flexibility and practicality. The hybrid AC/DC microgrid configuration is compatible with AC power supplies, DC power supplies, AC loads and DC loads which is viewed as the most practical choice in the power system application. The microgrid utilises an interlinking converter for the system’s control and grid connection. In this case, the microgrid implementation provides the possibility to achieve the other power electronic device functions such as a static compensator (STATCOM) or an active power filter (APF) in addition to dealing with the RES problems. In this PhD thesis, a grid-connected-mode-based control strategy for the microgrid is proposed to regulate the power generation as well as to reduce the grid current harmonics and neutral current. A virtual synchronous generator (VSG) based control strategy is introduced for microgrid’s smooth transient performance purpose. The first contribution of this research is the combination of the notch filter and traditional grid-connected microgrid control strategy. The notch filter is introduced into a three-phase system for current harmonics detection. The obtained harmonics signals are converted into reference signals for a revised constant power control strategy so that the microgrid is able to achieve the power regulation and harmonics reduction simultaneously. The interlinking converter is designed with three transistor-based bridges and a split-capacitor-based bridge so that the system is decoupled from hardware without any internal signals interference. The second contribution of this research is the design for grid neutral current compensation. The interlinking converter is designed with another independent neutral module bridge for the neutral current compensation purpose and a controller is developed based on this. In the context, the microgrid is implemented as a traditional microgrid with STATCOM-APF functions embedded. The third contribution of this research is the introduction of an improved VSG controller with its specific pre-synchronisation module. The designed method conducts smooth transient performances for a microgrid under different scenarios. An exclusive pre-synchronisation block is designed for the improved VSG controller for the microgrid to reconnect to the utility network smoothly when there is a requirement. The final contribution of this research is the establishment of the microgrid system benchtop. The hardware setup achieves the traditional power regulation function as well as the power-sharing between two microgrids. The proposed STATCOM-APF-based grid-connected microgrid controller is also tested under different scenarios. The effectiveness is validated through experiment. Various case studies are conducted in this research to investigate the microgrid possibilities. The proposed methods are validated through simulation first. The hardware prototype also verifies the correctness of the proposed grid-connected microgrid control method. The microgrid setup finally realises the power regulation, the harmonics reduction, the neutral current compensation and the cooperative operation functions.