Suppression Strategy Of Current Circulation For Two T-type Three-level Parallel Inverters In The Grid-connected Photovoltaic Power Generation System

Facing outstanding conflict between energy sources and environmental pollution problem,the importance of renewable and green energies in the future is highlighted and the proportion of electric energy generated by photovoltaic(PV)grid-connected generation system in the whole power system is also increasing.In recent years,with the increasing of installed capacity of the PV grid-connected inverter system,in terms of the cost,reliability and flexibility,the multi-parallel inverter system can better meet customer’s demand than one inverter system.The current circulation between the parallel inverters in operation will be produced.How to suppress the current circulation is the focus of this paper.Firstly,the factors causing current circulation for parallel inverters are qualitatively analyzed by means of the mathematical model and average model for each switching cycle.Secondly,single T-type three-level inverter is studied.Its working principle is described,mathematical model is built,and the circuit parameters are designed.The SPWM strategy of dual modulation waves with the same phase and different magnitude shifts and hysteresis control for midpoint voltage balance are discussed.The phase-locked loop and grid-connected control algorithm of one T-type three-level inverter in the balanced and unbalanced grid are designed.Finally,circulation suppression strategy with two inverters in parallel as an example is studied,i.e.control algorithm based on virtual impedance is used to reduce current circulation generated by line impedance difference;Synchronous control technique based on zero crossing of grid voltage and synchronous PWM carrier are proposed to suppress the circulation caused by asynchornous operation of the switching devices;Zero-sequence current feedforward solution is utilized to lessen the circulation produced by integrated parameters such as hardware circuit,control parameter.Simulation model based on and the above control strategies is established to verify the system design by means of PSIM simulation software.An experimental platform of two T-type three-level parallel grid-connected inverters based on control core of STM32F407 ZG is built.The feasibility and effectiveness of and the control strategies are confirmed by simulation and experimental results.