Researches On Power Decoupling Problem Of Single-phase Converter

With the quick development of full controllable power switches, a type of converter Pulse Width Modulation (PWM) rectifier, in which power can be transferred bidirectionally, is becoming more and more popular. Its merits include no harmonic pollution to the grid, adjustable power factor, and bidirectional power transfer. With merits such as simple construction and high reliability, PWM rectifier is widely use in power electronics system such as rail way driving system, uninterruptable power supply(UPS), photovoltaic(PV) inverter, AC-motor drive, cascade multilevel converters, and aerospace power supply etc.However, in single-phase PWM rectifier, there is inherent second-order ripple power, which will lead to second-order ripple voltage on the DC-link. A bulk capacitor or LC resonant branch is usually connected on DC-link to eliminate this second-order ripple voltage, but these passive methods will increase the volume and cost of the converter. For solving this problem, some scholars proposed active power filter (APF) methods. All the existing single-phase second-order ripple power APF circuits can reduce or even eliminate second-order voltage on the DC-link and reduce the volume of the converter. However, additional switches and other auxiliary components are needed in all the existing circuits, causing adverse impact on the efficiency, cost and reliability of the converter system.For solving the problem above, a novel single-phase PWM rectifier topology with power decoupling function is proposed in this paper. In the circuit, the input inductor of the conventional H-bridge circuit is divided into two parts, and a storage capacitor is added to absorb second-order ripple power. The capacitor is connected between one input AC-terminal and the negative DC-terminal. The proposed circuit has the same merits as existing circuit, such as high power density and reduction or even elimination of second-ripple voltage on DC-link, but not only that, compared with other APF circuits, the proposed circuit no longer needs auxiliary additional switches. As a result, it can obtain higher efficiency and reliability. For the proposed circuit, the control method working as unit power factor rectifier is proposed. In the paper, first, with the analysis of the modulation strategy of the proposed circuit, a working principle is obtained for the circuit; then after analysis of current loops in the circuit and based on the principle obtained before, we get the conclusion that the current of decoupling capacitor can’t be controlled directly, but the currents of the two inductors can be controller directly, as a result, the current of decoupling capacitor can be controller indirectly by controlling the two inductors’ current; finally, the control scheme of the proposed circuit working as unit power factor rectifier is presented.In software PSIM9.0, the model of the proposed circuit is built to demonstrate its feasibility. Meanwhile, after comparison with other power decoupling circuits, in which the decoupling capacitor is working in DC-mode, we obtain the conclusion: even though the capacitance used in the proposed circuit is larger than existing APF circuit, the maximum working voltage of the capacitor is lower. So the proposed circuit is more suitable for higher DC-link voltage.A prototype converter with low power is built for experiment to demonstrate the feasibility of the proposed circuit. It is proved that the capacitance used in the circuit can be reduced significantly without reduction of the efficiency. As a result, the cost and volume of the converter can be reduced correspondingly.