Study On The Seamless Transfer Between Islanded And Grid-Connected Operation Mode Of Three-Level-Inverter

In recent years, the technology of distributed generation based on renewable energy has been developed rapidly. The key technology of rational and efficient use of distributed power supply is integrating the distributed power supply with the grid to realize grid-connected generation. Smooth switching of interface inverter in distributed generation system between independent operation mode and grid-connected operation mode is the key to realize the combined system of power grid and distributed generation, and this is exactly the research content of this paper.First, the working principle of diode clamped three level inverter and SVPWM control technology was expounded in detail in this paper. In view of the sector transition defect resulted by starting from zero, continuous and repetitive timing mechanism of switching period in the digital implementation of SVPWM during actual projects, an improved timing method of switch period was put forward.In order to meet the requirements of dual mode operation and smooth switching of inverter, the overall control strategy of dual mode inverter based on P-ω、Q-V droop control was proposed in this paper. Compared with the traditional droop control, the effect of inconsistent line impedance on reactive load distribution of inverters during parallel operation can be reduced by the control scheme combining P-ω、Q-V droop control and virtual impedance. According to the characteristics of P-ω、Q-V droop control, the frequency co and rate of amplitude V recovery control was proposed to ensure that the inverter output voltage frequency was the rated value and amplitude remained unchanged in the steady state.The active synchronous control strategy suitable for P-ω Q-V droop control was designed in this paper. This strategy only needed to consider the phase angle and amplitude synchronization and therefore the problem that frequency and phase synchronization is difficult to achieve at the same time when using the traditional droop control was solved. The transient impact of inverter when connected to the grid was affected by the instantaneous voltage deviation and system impedance. Differential items were added into P-ω、Q-V droop control to increase the damping of the system. Together with the active synchronous control strategy, a smooth transition of the inverter output power in the process of connecting to the grid can be obtained.During grid-connected operation, the traditional droop control leads to two problems: one is that the output reactive power of inverter is out of control; the other one is that the output active power of inverter will deviate from its nominal value during the fluctuation of grid voltage frequency. The active and reactive power was kept in a controlled state due to the P-ω、Q-V droop control adopted in this paper. In addition, the grid voltage angular frequency was set as the angular frequency set point of P-ω droop control ω0 to solve the active power control problem during the fluctuation of grid voltage frequency in this paper.A 5kW inverter model was designed and built in MATLAB/simulink in this paper. The control scheme of double mode inverter proposed was verified by simulation. An experimental platform of three level grid-connected inverter using DSP+FPGA as the main control circuit has been built. The correctness of the proposed control scheme was proved by experimental results.