| Distributed generation technology has evolved over time,demanding higher standards for system economics,load compatibility and capacity,among which parallel operation of energy storage DC/AC converter is an important and effective means.The parallel system often has load imbalance during operation,and the ability to effectively respond to changes in operating conditions is an important basis for evaluating the compatibility of parallel systems with load.Therefore,this dissertation will study the grid-connection/island operation control of energy storage DC/AC converters under unbalanced load conditions.First,in order to making the energy storage DC/AC converter capable of operating with unbalanced loads,a three-phase four-wire diode-clamped inverter topology is used in this thesis.Under the unbalanced load condition,the output voltage unbalance is analyzed according to the symmetric component method,and the Proportional Credit(PI)regulator combined with the quasi-resonant controller is used to weaken the negative-sequence component interference;the three Dimensions Vector Pulse Width Modulation(3DSVPWM)modulation technique combines zero-axis control with DC bus equalization capacitor midpoint potential control to weaken the zero-sequence component interference,eliminating the need for an external circuit to balance the midpoint potential and making the system simpler and more efficient.Then,an improved dividing-sequence droop control strategy based on voltage frequency recovery is proposed for parallel energy storage DC/AC converters operating with unbalanced loads,the control relies on the positive sequence component to build a basic droop control loop and thus provide an accurate voltage amplitude and frequency reference for each converters;the second-order generalized integrator(SOGI)is also used to construct a dividing-sequence virtual impedance to regulate the problem of unreasonable output power distribution due to the difference in impedance of each converter connection.To address the disadvantage of voltage reduction at the common coupling point caused by the adoption of virtual impedance.In this paper,voltage and frequency recovery control is proposed,by compensating the voltage and frequency to increase the output voltage of the energy storage converter and eliminate the difference of frequency between converter modules,it also adopts pre-synchronous control to realize parallel grid-connected operation of energy storage DC/AC converter,which improves the integrity of parallel system operation mode.Finally,a simulation model is built based on MATLAB/Simulink software for the improved dividing-sequence droop control proposed in this paper,and it is verified that the voltage at the common coupling point can be recovered to near 311 V under this control scheme,at the same time,the experiments of reasonable distribution of output power of energy storage DC/AC converters under 1:1 and 2:1 capacity ratio,switching unbalanced load,voltage and frequency recovery experiment and parallel grid-connected experiment were conducted by semi-physical platform combined with Digital Signal Process(DSP)controller,all of which verified the feasibility and effectiveness of the control strategy proposed in this paper. |
PDF Full Text Request