Research On Soft Switching Three-Phase High-Frequency Link Matrix Inverter And Parallel Operation Technology Under Unbalanced Condition

Distributed generation systems usually need to use inverters for power transmission between systems and large power grids and loads.Its large-scale application should not only meet the requirements of miniaturization,lightweight and sufficient redundancy,but also consider that the system can maintain good output performance under unbalanced or nonlinear conditions to adapt to harsh conditions.High Frequency Link Matrix Inverter(HFL-MI)is widely used in new energy power generation,especially photovoltaic power generation,due to its advantages of small size,high power density and bidirectional power flow.This essay mainly focused on the three-phase HFL-MI,and studies the realization of soft switching of a single system,the power matching and circulation suppression strategy between parallel units under unbalanced conditions.Firstly,in order to suppress the voltage spikes at both ends of the switch tube,a push-pull forward three-phase three-leg HFL-MI topology with clamping capacitor is proposed.The clamping capacitor in this topology can absorb and store the excess energy of the leakage inductance,thereby suppressing the bias phenomenon of the high-frequency transformer and clamping the voltage at both ends of the switch tube to within 2 times of the input voltage.Based on the Sinusoidal Pulse Width Modulation(SPWM)strategy,the working mode and soft switching process of the system are analyzed theoretically.The proposed topology and modulation strategy can not only realize the safe commutation of the matrix converter,but also realize the zero-voltage soft-switching(ZVS)of the switch tube of the post-stage matrix converter.Secondly,in order to cope with unbalanced conditions,the fourth bridge arm is added to the proposed topology.The single system is modeled based on the idea of de-re-coupling,and the transfer functions of each part and the whole system are derived.The first three bridge arms adopt the traditional voltage and current double closed-loop control strategy based on dq rotating coordinate system,in which the voltage loop adopts the proportional integral resonant(PIR)controller to eliminate the influence of the double frequency pulsation component.The fourth bridge arm adopts the independent control of the optimized inductor current integral,which can avoid the saturation phenomenon during the integral operation.Then,based on the control strategy of Virtual Synchronous Generator(VSG),this essay studies the parallel three-phase four-leg HFL-MI system.The key parameters are analyzed and tuned,and the proportional relationship of each parameter when each inverter in the parallel system outputs at a set proportion is discussed.In order to reduce the power impact at the parallel time,suppress the circulating current and improve the output impedance of the system,the virtual impedance(VI)is introduced in the αβcoordinate system,and the influence of the change of virtual resistance and virtual inductance parameters on the output impedance of the system is analyzed.Considering that when the line impedance parameters are inconsistent,the inverters no longer divide the output according to the set proportion,a voltage compensation scheme is studied to compensate the voltage drop on the line impedance.Finally,the experimental platform of the two-machine parallel system is made.The experimental results verify the feasibility and stability of the proposed topology and control method under unbalanced conditions.