Abstract：A Z-source inverter is a topology that can replace a two-stage inverter. It can increase the voltage of the DC link by adding a Z-source network and shoot-through vector. The shoot-through current generated by the shoot-through vector is usually large, which increases the switching device losses of the Z-source inverter. In recent studies, some modulation strategies to reduce switching losses, such as DPWM, have been proposed. However, these modulation strategies seldom consider the variation of load power factor. When the load power factor changes, the current phase through the switching device changes, and the switching device losses caused by the original modulation strategy are not optimal. This paper proposes a generalized ZSVM2 modulation strategy. By moving the relative position between the shoot-through current and the AC output current of the switching device, the switching losses are optimized when the load power factor changes. First, the current passing through the switching device using the ZSVM2 modulation strategy is analyzed. According to the phase relationship between the AC output current and the shoot-through current, the relationship between the average current of the device and the power factor is obtained. Through formula derivation, when the power factor is 1, the phase relationship between the AC output current and the shoot-through current can achieve the lowest average switching current. Secondly, an improved modulation strategy is proposed. During a switching period, the ZSVM2 strategy has a bridge arm that does not use the shoot-through vector. The proposed strategy exploits this to move the shoot-through current relative to the AC output current. When the power factor is less than 1, the AC output current lags the voltage angle φ, and at this time, the shoot-through vector follows the AC output current and also lags the angle φ. In this way, the position of the shoot-through vector can be determined according to the output current of the AC side, and the optimal effect of switching losses can be achieved. The proposed improved modulation strategy does not change parameters such as the boost factor of the original modulation strategy, so the switching losses can be compared by the average current of the device. Through the derivation of the formula, it can be known that when the power factor is lower, the switching loss optimization of the proposed modulation strategy is better. The proposed modulation strategy optimizes the average current through the device and thus reduces conduction losses. Finally, due to the redistribution of the bridge arm with the shoot-through vector, the maximum value of the inductance ripple of the proposed modulation is no longer generated at φ =π/6. However, it must be re-determined according to the actual operating power factor and modulation parameters. The improved modulation does not affect the use of the inductor ripple optimized modulation algorithm. The proposed modulation strategy can also be applied to the ZSVM1 modulation strategy. The loss optimization effect is verified by Matlab/Simulink combined with PLECS simulation. The results show that when the power factor is 0.966 and 0.866, the proposed modulation strategy can reduce the switching losses by 3.3% and 9.7%, and the conduction losses by 1.6% and 3.2%, respectively. Simulations show that the proposed modulation strategy slightly increases the total harmonic content THD of the AC output current. A low-power test platform is built to verify the loss optimization effect. The thermal imager shows that when the power factor is 0.866, compared with the ZSVM2 modulation strategy, the proposed ZSVM2 modulation reduces the temperature of the switching device by 1-2 degrees Celsius.
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2023, Vol. 38 
