Research On The Droop Control Method In Microgrid Including Motor Load

Microgrid has two operating states:interconnection and island. During the island operation, the important control objective of microgrid is the rational allocation of power and improvement of power quality. Traditional droop control method, restricted by itself and affected by the motor load, the line impedance and the local load, has some problems like the drop of inverter terminal voltage and imprecise allocation of reactive power.Currently, the study of these problems is mostly around with constant impedance load (constant power load) of microgrid. The proposed solutions were mostly required to design operating parameters of the microgrid many times, and can’t solve the problem of reactive power distribution and voltage recovery simultaneously. When the microgrid system based on the droop control contains the motor load, due to the large starting current and reactive power absorption of the motor load, it will lead to a short-time overload of distributed power and a voltage drop of the inverter terminal, which seriously affects the microgrid power quality and the normal operation of the microgrid load.Therefore, the research object of this thesis is the low voltage microgrid including motor loads on the island operation, and the research on the reactive power distribution and voltage recovery method of microgrid based on droop control is carried out.First of all, the inverter control mode was analyzed in this thesis. The mathematical models of the voltage and current loop controller and power droop controller in the microgrid were established, and the simulation verified that the microgrid droop control method was steady.Then, the influence of motor load, line impedance and local load on microgrid reactive power distribution was analyzed by this thesis. The proportional distribution conditions of microgrid reactive power were drew, which were those the line impedance was inversely proportional to the reactive power capacity of distributed power and local load was proportional to the reactive power capacity of distributed power. In the case of these conditions were not met, the use of reactive droop factor correction method for changing the voltage output characteristics of the inverter has achieved the optimization of reactive power allocation in the microgrid including motor load and bassed on droop control. The simulation results verified that the optimization method of the reactive power allocation was effective.Finally, an improved reactive power-voltage difference (Q-△U) method was put forward by this thesis. The inverter terminal voltage of the microgrid including motor load has been recovered by introducing voltage recovery mechanisms. This method was expanded to active power-frequency droop control. The active power-frequency difference (P-△f) method was used to restore the microgrid system frequency. The simulation verified that using P-△f, Q-△U droop control method to achieve the recovery of voltage and frequency was correct and effective.In this thesis, the theoretical analysis and simulation results showed that the reactive droop factor correction method can effectively optimize the reactive power distribution of microgrid including motor load. Using P-△f, Q-△U droop control method can effectively solve the decline of frequency and voltage in the microgrid containing motor load, and achieve voltage and frequency recovery. The results of this thesis have some reference value on the control of the microgrid based on droop control and including motor load.