Operation and Control of a Microgrid with Distributed Generation Systems

Abstract

The energy has always played a crucial role in the development and progress of human society. People have long been aware of the drawbacks of traditional fossil energy, such as the limited resources, resulting in environmental pollution and other defects. However, due to the needs of social development and the constraints of backward technology, people have to use fossil energy as the main energy source. In recent years, with the rapid development of science and technology, how to effectively use renewable energy to generate electricity has become the focus of attention in many countries. Because of its unique advantages in the use of new energy, microgrids have received more and more research and development. The distributed power generation system based on microgrid technology is an important way to develop renewable energy, increase the reliability of power supply, and expand the capacity of the power supply system. The power supply of the distributed power system can be formed by a variety of energy sources through power conversion. The power supply units of the distributed power system are distributed and are all connected to the AC grid bus. The power supply unit of distributed generation micro-power system is generally a parallel inverter, and there are many parallel modes of inverter and the parallel mode of inverter power without interconnection line is especially suitable for distributed power generation system with grid-connected inverter. The ideal distributed generation microgrid system includes parallel DG inverter power modules, output line impedance, AC bus and loads connected to the AC bus. The DG inverter is the core of the distributed power generation system, which is responsible for transforming the distributed energy into electric energy and realizing the parallel network operation of the system.This thesis studies the droop controlled distributed generation inverters power decoupling and the restoration of frequency and voltage under resistive and inductive impedance microgrid environment. Summarized the research background, definition and characteristics of microgrid. Summarizes the existing control structure of the microgrid. The classification, comparison and analysis of control methods for power electronic converters,vi especially distributed generation inverters in microgrids are focused on. The topology of the distributed generation inverter main circuit and the filter circuit was chosen and filter parameters were designed. Then the mathematical model of distributed generation inverter in different coordinate were established. Since the output voltage strategy and output impedance of an distributed generation inverter always have an important influence on the DG inverter parallel system and power distribution. The instantaneous voltage closed-loop control in three-phase stationary coordinate and the inverter output voltage decoupling control strategy in dq rotating coordinate were analysized, in order to reduce variable numbers, while ensuring the DG inverter output voltage tracking with no difference to the reference voltage, the DG inverter output voltage control strategy based PI controller in dq coordinate is implemented and the influence of the controller parameters on closed-loop transfer function of output voltage and inverter equivalent output impedance were analysized. The droop control is widely employed when multiple distributed generation inverters operate in parallel. However, due to inconsistent line impedance and the local load, there exists power sharing errors when the droop control is adopted, thereby reducing the efficiency of the system. In addition, there is a coupling between the active power and reactive power with the direct droop control, which affects the stability of the system. Though the traditional power decoupling control is able to realize power decoupling, the actual real power and reactive power cannot be shared equally. To deal with the power sharing and power coupling problem, this chapter explicitly analyzes the causes of the power sharing error and power coupling with the direct droop control respectively, quantizes the power sharing error and the extent of power coupling and also gives the basic solution to reducing the power sharing error and solving the problem of power coupling. To solve the inaccurate power sharing problem of the direct droop control, virtual inductance is adopted. By adding the virtual inductance, can decouple the active and reactive power, but also achieve accurate power sharing. The simulation results verify the accuracy and effectiveness of the adopted control scheme. Using direct droop control, the active power and reactive power can be decoupled when the line impedance is mainly inductive. However, it is not applicable to the microgrid with low voltage when the line impedance is resistive. As a result, thevii active power and reactive power will be coupled and errors in preset ratio of power sharing will arise. Aiming at solving the problem about the inapplicability of direct droop control in low voltage micrigrid, this chapter implements reverse droop control. The influence of transmission impedance of distributed generation inverter to public load on power distribution, introduces virtual resistor and then uses reverse droop control strategy to distribute load in low voltage distributed power generation system. Analyzes the conditions that need to be met to accurately share the load according to the ratio of rated capacity for inverter power supply. In the actual distributed generation system, due to the distributed location of the distributed inverter power supply, the impedance of the line is uncertain and the traditional reverse droop control has certain limitations. The simulation model of DG inverter parallel operation is built under the matlab/simulink environment, the reverse droop control and the improved power allocation strategy using virtual resistance are simulated and compared, the correctness and validity of the adopted improved strategy are validated. The traditional centralized control method cannot solve the problem of the various modes of microgrid operation, for example, the probem of controlling the microgrid systems induced by hard to collect information signals and low controllable. But the distributed secondary control method based on direct and reverse droop control has obvious advantage to solve the problem of parallel connected DG inverters operation. Aiming at the problem of voltage and frequency differences caused by the direct and reverse droop control and considering the actual situation of inductive and resistive line impedance mismatch, this thesis proposes a distributed secondary control. The simulation verifies that the proposed distributed secondary control method can guarantee the voltage amplitude and frequency recovery.