Research On Single Stage High-frequency Isolated Modular Cascaded Converter

Nowadays,natural gas,oil and other fossil energy are gradually exhausting,while renewable energy such as solar energy has gradually became a potential substitute for fossil fuels with rapid development.At present,centralized photovoltaic system is mainly connected to medium-voltage ac grid through voltage source converter and line-frequency step-up transformer.However the line-frequency transformer has some inherent problems such as large volume,heavy weight and poor controllability,which are not conducive to the transportation,installation and maintenance of photovoltaic power stations in remote areas.In this paper,an isolated modular cascaded converter(I-MCC)with high frequency isolation is proposed for PV MVAC grid-connected system without line-frequency transformer.I-MCC belongs to single-stage power conversion and eliminates the dc-link capacitors in the high voltage side,which can significantly reducing the volume,simplifing the voltage balance control between dc-link capacitors and improving the power density and efficiency at the same time.Firstly,the topology,working principle,phase-shifted modulation strategy and multi-module carrier-phase-shifted modulation strategy of I-MCC proposed in this paper are described in detail.Then,a DC component suppression strategy based on DC modulation ratio compensation is proposed for the DC offset of output voltage in I-MCC.Then,the working modes of I-MCC under phase-shifted modulation is analyzed.Secondly,for the multi-module I-MCC system controlled by carrier-phase-shifted strategy,the inductor current ripple in AC side is analyzed and summarized.On this basis,the parameters of filter,high frequency transformer and voltage clamping circuit of I-MCC are optimized and designed,which provides a basis for the experimental platform.Then,aiming at Y-connected three-phase I-MCC photovoltaic grid-connected system,a PQ control strategy for Y-connected three-phase I-MCC PV grid-connected system is proposed based on the establishment of I-MCC mathematical model.The control strategy includes two parts: power outer-loop control and current inner-loop control,and the correctness of the PQ grid-connected control strategy is verified by simulation.Finally,a 3kW single-phase I-MCC hardware experimental platform is built,and the feasibility and effectiveness of the proposed I-MCC topology and its modulation as well as control strategy are verified by steady-state experiments and transient experiments.The experimental results are basically consistent with the theoretical analysis,which proves that I-MCC can achieve high-efficiency power conversion from LVDC to MVAC and it can be applied to PV MVAC grid-connected.