The Inverter Port Control Strategy In Islanded Microgrid

With the consumption of non-renewable energy, such as coal and petroleum, energy crisis has become a serious problem for human beings. However, renewable energy, such as solar and wind energy, is developing rapidly, which provide us a new way to solve this problem. Due to the intermittent and randomness of the new energy sources, new problems arise, when connecting the distributed generation to the traditional grid. To solve this problem, some scientists proposed the conception of microgrid, which integrates the renewable energy with the main grid and is taken to be an organic whole for common control. Without reasonable control strategy, the microgrid can’t operate well. So it is necessary to have a deep research on it.The traditional generator port and inverter port are two basic interfaces connecting with main grid. Since the inverter port is now dominant in microgrid, the main research of this paper is on the control method for micro sources of inverter port and their parallel system. The basic control method of inverter port includes constant power control, constant voltage with constant frequency control and droop control.As microgrid based on droop control can achieve automatic adjustment of voltage and frequency without resorting to high requirements for communication, this paper mainly focuses on the research of droop control strategy. Besides, the constant voltage with constant frequency control, which plays an important role in stabilizing voltage and frequency, will be studied and simulated as well.Using conventional droop control, the active power and reactive power can be decoupled and correct control 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 conventional droop control is lack of reasonable analysis and consideration about line impedance. As a result, the active power and reactive power will be coupled. And errors in preset ratio of power sharing will arise, due to the difference of line voltage droop, which will lead to the hyper-capacity and severe overcurrent of the micro source, causing damage to the micro source. Aiming at solving the problem about the inapplicability of traditional droop control in low voltage micrigrid, this paper firstly deduces a linear relationship between the voltage in the load and the distribution of reactive power, based on the reactive distribution theory in pertinent literature, then propose an improved droop control. It puts forward such kind of control model that taking into the characteristics, which the line impedance is equivalent to the inner impedance of the inverter, and the voltage in the load is adopted as the parameter of double closed-loop of voltage and current. The virtual inductor is added to the improved droop control to regulate the property of line impedance and decouple the active and reactive power. At last, the power control module, the virtual impedance module and the double closed-loop module of voltage and current under this control model are analyzed and designed carefully in the thesis.The results of simulation in Matlab/Simulink environment demonstrate the good performance of the proposed control approach in the low-voltage microgrid.