This study presents a new control strategy for three-level neutral point-clamped pulse-width modulated (PWM) rectifier. The relationship between instantaneous power and voltage vectors is derived mathematically from the model of three-phase three-level voltage source PWM rectifier, and a direct power control (DPC) scheme for three-level PWM rectifier is presented. An optimal switching table is established, from which appropriate voltage vectors can be selected to control active and reactive power directly. Principle for vector switching to avoid excessive ΔV in phase and line-to-line voltages and balance strategy for the neutral point potential are also discussed. A digital-signal-processor-based experimental system is developed to implement a series of experiments. The obtained results have verified the theoretical approach, and the threelevel PWM rectifier with proposed DPC exhibits advantages of simple algorithm and good dynamic performance, compared with conventional voltage-oriented control.

To obtain a high step-up gain with high efficiency in distributed power sources application, such as photovoltaic (PV) and fuel cells, a passive absorption circuit with voltage step-up character is inserted in the interleaved flyback-forward converter for leakage energy recycle and voltage gain extension. By applying the high step-up passive-clamp circuit, the voltage spike on the metal-oxide semiconductor field-effect transistor is clamped and the energy stored inside the leakage inductance of the coupled inductor can be recycled to get high-output voltage. Moreover, it offers another design freedom to obtain higher voltage-conversion ratio. The operational principle and characteristics of the improved flyback-forward converter are presented, and verified experimentally with a 500 W prototype converter. The proposed high step-up passive-clamp can be widely used in high step-up DC-DC converter applications, and a new high step-up topology is deduced to illustrate the theoretical analysis.

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This paper proposes a novel virtual impedance method for uninterruptible power system (UPS) inverters with parallel connected capability. To achieve wireless control, the droop control method is adopted. However, the power sharing accuracy is highly sensitive to the output impedance of inverter. So, a virtual output impedance is usually added to alleviate the unbalance of output impedance between each inverter and then, improve power sharing accuracy. The virtual impedance is implemented as the time derivative of the inverter output current, the differential progress will amplify the noise presented in output current, result in high sensitivity to the noise for the virtual impedance. To solve this problem,a novel virtual impedance by combining with a low-pass filter is proposed, which not only balances the output impedance of inverter, but also achieves better output voltage THD when sharing nonlinear loads. The double loop control method for inverter provids a good tracking of output voltage reference and high quality output waveform. Simulation results are presented from a parallel connected UPS system to validate this method.

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