Abstract：The single-phase transformerless three-leg UPQC can achieve AC voltage regulation and reactive power compensation at the grid connection point. However, traditional control methods always result in high DC-link voltage and significant voltage fluctuations, which incur substantial capacitor costs and affect the equipment’s safety and reliability. Although an additional auxiliary device can be installed on the DC bus to suppress voltage fluctuations actively, it adds additional costs. This paper proposes a control method to achieve flexible reduction and active ripple suppression for DC-link voltage. From the modulation perspective, the constraint range of DC-link voltage is analyzed. A mathematical model is derived for DC-link voltage fluctuations with complex factors, such as grid power factor, load power factor, series compensation voltage phase, and reference DC-link voltage. With the objective of the lowest bus voltage and the lowest voltage fluctuation, the optimal solution of the compensation voltage phase and reactive power compensation coefficient are calculated, and the corresponding DC bus voltage reference is obtained. Finally, all parameters are sent to the voltage-current closed-loop controller of UPQC. The proposed method can effectively reduce the DC side voltage value and suppress the DC voltage fluctuation magnitude. The experimental results show that the proposed method does not affect the function of UPQC. Regardless of whether the grid voltage per-unit value is 0.8(pu) or 1.2(pu), the proposed method can stabilize the load side voltage at 1.0(pu). In the instant of voltage jumps, no obvious fluctuation exists in the load voltage. The equipment can ensure stable operation, and the DC-link voltage meets the regular changes of the proposed control strategy. In addition, compared with the traditional same-phase load voltage compensation method, the proposed method significantly affects the DC bus voltage suppression. The DC-link voltage fluctuation amplitude is 50% and 35% of the traditional method when the grid-side power factor is required to be 1.0 and no less than 0.9, respectively. The DC-link voltage fluctuation amplitude is 20% of the traditional method when the grid-side power factor is limited. The following conclusions can be drawn. (1) The setting of the DC side bus voltage should consider factors such as the grid voltage, compensation voltage, compensated load voltage, and DC side voltage fluctuation. The proposed DC side voltage flexible adjustment method reduces the bus voltage to near the phase voltage peak, decreasing the DC side voltage size and the cost of DC bus capacitance. (2) The DC bus voltage fluctuation is affected by the load capacity, load power and grid power factors, and load voltage phase-shift angle after compensation. The proposed method reduces DC bus voltage fluctuation by adjusting the phase shift angle of load voltage after compensation and reactive power compensation coefficient, which significantly reduces the DC-link’s voltage fluctuation and ensures the equipment’s safe and stable operation.
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2024, Vol. 39 
