Research On Reactive Power Equalization Strategy Of Multiple Parallel Inverters For AC Microgrid

In recent years,the scale of distributed generation units on the grid continues to increase,and the penetration rate of the grid continues to climb,while the safe operation of the grid is seriously challenged due to the randomness and volatility of distributed power sources.To improve the stability of distributed generation system power supply,microgrids are widely used.Currently,sag control,as the primary control strategy mainly used in microgrids,can achieve plug-and-play availability of microgrid inverters.However,the line impedance mismatch and different loads in a multiple parallel inverter system can cause problems such as uneven system power distribution.Therefore,overcoming the influence of line impedance and achieving proportional distribution of reactive power is an urgent problem for islanded microgrids.In response to the above problems,this paper adopts the sequencing control and hierarchical control methods aiming to improve the degree of reactive power equalization among the shunt inverters.Using the method of sequencing control,the voltage and current signals output in the abc three-phase stationary coordinate system are first extracted and separated and sequenced by the second-order generalized integrator,and the fundamental positive sequence reactive power,fundamental negative sequence reactive power and 5th,7th and 9th harmonic negative sequence reactive power are obtained by power calculation.The fundamental negative sequence reactive power and the 5th,7th and 9th harmonic negative sequence reactive power are used to construct an adaptive virtual impedance to achieve the equalization of the negative sequence reactive power and to compensate for the voltage drop at the common coupling point.In order to verify the impact of the designed control method on the stability of the microgrid,a local small-signal model is established to determine the optimal range of the virtual impedance coefficients.A simulation model of two inverters operating in parallel is built in MATLAB\Simulink environment,and simulation experiments are conducted to compare with the conventional sag control to verify the feasibility of the proposed strategy.The hierarchical control method is used to further subdivide the system reactive power into two parts:fundamental reactive power and harmonic reactive power.The conventional sag control is used as the primary control,and the consistency control algorithm is used as the secondary control to compensate for the power deviation generated by the primary control,thus realizing the equalization of fundamental reactive power;for harmonic reactive power,the harmonic adaptive harmonic virtual impedance is constructed by extracting the 3rd,5th,7th,and 9th odd harmonics,and the equalization of harmonic reactive power is realized by proportional integral adjustment;in addition,in the voltage and current dualloop control,the multi-module quasi-resonant controller is used as the controller of the voltage outer loop to enhance the stability of the system,and the proposed algorithm is subjected to Lyapunov stability analysis.Finally,based on the topology of the islanded microgrid,a simulation model of multiple inverters operating in parallel is established in the environment to verify the effectiveness of the strategy on the equalization effect of fundamental and harmonic reactive power.