Power sharing control of parallel connected inverter systems

Abstract

Microgrid is an emerging cutting-edge technology that can effectively improve the safety and reliability of the power system and promote the access and local consumption of renewable energy. It is also a key enabler for improving the utilization efficiency of renewable energy and the energy efficiency of the power distribution network. However, a standalone microgrid generally has shortcomings such as limited working capacity and weak anti-disturbance ability. These are normally coupled with the intermittent output power of renewable energy and the variability of the load. Some of these challenges can be overcome if the microgrid can be connected to the main grid. With a high penetration rate of renewable energy, many technical problems in the coordinated control of power need to be solved in order to improve the power supply quality and reliability. Parallel operation of inverter-based distributed generation systems, in the two modes of islanded microgrid operation and grid-connected operation, brings many control challenges to the microgrid including load sharing, stability, amplitude and phase synchronization, and circulating current. Therefore, it is necessary to study the parallel control strategies for the inverters in microgrids and to develop novel solutions to the associated problems. This thesis conducts research into the islanded and grid-connected operation characteristics of microgrid inverters and also the switching between the two operation modes. The thesis covers the following: (1) Islanded and grid-connected operation characteristics of microgrid converters. The different output droop characteristics of the microgrid converter are discussed considering different output impedances. The mathematical model of a microgrid with inverter-based generation is established and the full system is simulated in the islanded and grid-connected operation modes using MATLAB/Simulink platform. The simulation is used to verify the proposed control scheme which aims at improving the power-sharing ability between parallel-connected inverters, allowing the output power of each inverter to be based on its own capacity and improving immunity to power grid fluctuations. (2) Power sharing control of parallel inverters with different line impedances. In an actual electricity distribution system, the distance between the power generation units in the microgrid and the main grid access point may be different, which causes the power output of each distributed power source to be unable to reach equal distribution or to be distributed in proportion to the capacity. Some distributed power sources may likely operate at full power, while other sources may not be able to provide sufficient effective power output. In this thesis, a hierarchical control strategy for power distribution is proposed, and its effectiveness is verified by theoretical proof and MATLAB/Simulink simulation. (3) Distributed and schedulable power droop control method. The traditional droop control cannot realize the tracking of a given active power reference signal, which makes it impossible for the distributed power supply operating in droop mode to perform accurate power dispatch control. Based on the hierarchical control architecture, this thesis proposes a schedulable distributed power droop control method, which retains the characteristics of traditional droop control, and realizes distributed power by modifying the power reference value in droop control, with accurate tracking of active reference signals. In addition, this method can keep the system frequency stable. In summary, the improved droop control proposed in this thesis enables the inverter-based power generation system to operate smoothly under different system parameters and operating modes, thus solving the problem of not being able to distribute the output power according to the inverter capacity when multiple inverters are connected in parallel.