| Presently,with the rapid development of photovoltaic inverter market,inverters usually include miniature type,centralized type and string type.The centralized inverter cannot track the maximum power of the partial module.The load of string inverter is high and the efficiency is low.The power level of micro-inverter is low and the cost is high.Therefore,an improved series micro-converter in the connected grid system is researched in this paper in order to overcome these shortcomings.The multiple DC bus of micro-converter are connected in series in this system.Maximum power of photovoltaic module is tracked by each independent micro-inverter,and the output efficiency of the system can be effectively improved.Moreover,the converter power level is higher,and the cost of the system is reduced,and the size is smaller.Firstly,the series-connection of single-phase micro-inverter system is researched in this paper.And the disturbance observation method is used to track maximum power of single micro-converter system.Meanwhile,the pre-MPPT control method is used to implement conversion of isolated boost converter with active clamp.The stability of the DC bus is realized by the control algorithm of voltage and current double closed-loop at the inverter grid side.What’s more,the series-connection of DC bus is implement by independent control of front level and rear level.Secondly,an improved isolated boost converter with active clamp is proposed in this paper in order to complete the topology of the system.The control method of converter is simple and feasible,dynamic response is fast,precision voltage regulator is high,and the harmonic distortion of grid current is small.This converter is applied to the series-connection micro-converter which can realize the electrical isolation of the system and improve the safety of the system.What’s more,the control algorithm of the system is improved,and an improved grid side control algorithm is studied.Based on comparing and analyzing of the traditional PI control,the quasi-PR control and the repetitive control algorithm,and the compensation of repetitive controller with quasi-PR is proposed.The dynamic response time of the controll system is improved by the quasi-PR control module.Furthermore,when the grid voltage and current is stable,periodic error of current is modulated by the repetitive controller.And the system is good stability.Finally,the grid simulation model of series-connected micro-converter is built in MATLAB.The simulation results show that the inverter efficiency is more than92%,and the grid connected harmonic is 3.83% which meets the requirements of theconnection.Then,the design of the hardware parameters of the series-connected micro-converter is designed in this paper.And the hardware platform of series connection micro-converter of grid system is built.The input power of single solar panel is stabilized at about 200 W.The experimental results show that the output power of the system is about 353 W which the series-connection of two DC bus of micro-converters is realized. |
