In some application circumstances, both positive and negative sequence reactive power compensation are desired from the star-connected cascaded H-bridge static synchronous compensator (CHB STATCOM). However, the imbalanced phase currents/voltages will cause the uneven active power flow among three clusters, which makes the cluster voltage balance more challenging. In this scenario, only zero sequence voltage is available to balance the cluster voltages. Although the zero sequence voltage can be calculated in all operating conditions based on the equilibrium of the active power for each phase cluster, the calculation is definitely complicated due to the zero sequence voltage expressed in the form of trigonometric function in the abc or dq0 frame. In this paper, the zero sequence voltage is represented in a DC form in the dq frame through introducing a virtually orthogonal zero sequence voltage. As a result, a linear relationship between the active powers and the zero sequence voltage is established, based on which, a simple zero sequence voltage calculation method is proposed with a small calculation burden. Furthermore, the accurate compensation region for the negative sequence current injection is identified by establishing the relationship between the maximum converter voltage and the negative sequence currents taking into consideration the initial phase angle of the negative sequence currents, while still keeping the cluster voltage balanced by using zero sequence voltage control. The effectiveness of the proposed control and the compensation region are verified by the simulation and experimental results on a 400V/±7.5kVAr star-connected CHB STATCOM.

In this article, a hybrid connected unified power quality conditioner integrating distributed generation (HCUPQC-DG) is proposed. Two dc ports are created at the dc link of the HCUPQC-DG, in which one low-voltage (LV) dc port is directly connected to distributed generation (DG), and the other high-voltage (HV) dc port is indirectly connected to DG through the front-end dc-dc converter. With the hybrid connected configuration, the HV is designed to ensure the dc-ac voltage conversion capability, whereas the LV, i.e., the voltage of DG, can be relatively low and vary in a wide range. Besides, most active power can be directly transferred from DG to the ac load or grid through the direct power flow path. Hence, the conversion efficiency is enhanced, and the power capacity of the front-end dc-dc converter is significantly reduced. The operating principle and the control and modulation strategies of the proposed HCUPQC-DG are discussed. Moreover, the power flow through the HCUPQC-DG is analyzed in detail to understand the system operation. Experimental results with a 3-kVA prototype are provided to verify the feasibility and effectiveness of the proposed HCUPQC-DG.

A novel quasi-two-stage multi-functional inverter (QMFI) for photovoltaic (PV) applications is proposed in this paper. With the help of the quasi-two-stage architecture, part of active power can be directly transferred from PV arrays to the grid or load within a single power conversion stage and hence improve the efficiency. In addition to active power transfer, the realization of maximum power point tracking (MPPT) and compensation of non-active current (i.e., reactive, harmonic, or unbalanced current) of the QMFI are both considered. A mathematical model of the QMFI is presented to guide the control parameters design. Meanwhile, an analysis model based on the rotating frame is proposed to derive the space vector PWM strategy of the QMFI and also offer straightforward insight into the influence of non-active current compensation on quasi-two-stage active power flow. Compared with traditional solutions, the QMFI can realize active power delivery with higher efficiency, and also keep functions of achieving MPPT and enhancing power quality. The feasibility and effectiveness of the proposed solution are verified with a 3-kVA prototype.

A hybrid three-phase AC/DC power system, which is composed of a diode-bridge rectifier and a two-stage AC-DC converter, is proposed for low frequency pulsed load applications. With the proposed hybrid power system, the low frequency pulsed load power is decoupled with the average power of the AC source, while the high frequency harmonics induced by the diode-bridge rectifier is compensated by the AC-DC converter. Fault-tolerant operation is achieved with four possible operation modes of the proposed hybrid power system. Therefore, high reliability is achieved with these redundant operation modes. Operation principles, control methods and design consideration of the hybrid AC/DC power system are analyzed in detail. An experimental prototype is built and tested to verify the effectiveness and feasibility of the proposed hybrid AC/DC circuit and its control strategy.

A dual-dc output three-phase three-level ac–dc converter is proposed for low-frequency pulsed power decoupling applications. One of the dc output is connected to the dc pulsed load, whereas the other one is used as power decoupling port and connected to the decoupling capacitor. The voltage of the power decoupling dc port can vary in a wide range to reduce the value and volume of the power decoupling capacitor. A buck/boost converter is employed to interface the power decoupling port with the dc pulsed load. A modified space vector pulsewidth modulation strategy is proposed for the ac–dc converter, based on which independent voltage and power regulation of the two dc outputs is achieved. The steady-state power is directly fed to the dc load with single power conversion stage, and the low frequency pulse power is fed to the power decoupling port. As a result, high-power quality and efficient power decoupling with reduced power conversion stages are achieved. Operation principles, modulation, and control strategies of the ac–dc converter are analyzed in detail. A prototype is built and tested to verify the effectiveness and feasibility of the proposed ac–dc converter for pulsed load.

Pulsed load poses a significant challenge to the safe operation of the airborne power systems. This article proposes a semi-two-stage ac dc converter with power decoupling function for the airborne power system with pulsed load. The semi-two-stage converter provides two dc ports, in which one dc port with lower voltage is the output port connected to the pulsed load, while the other dc port is the decoupling port and interfaced with the output via a buck converter. A power flow path with single conversion stage from the ac input to the load exists, and the steady-state power can be directly fed to the load. Hence, higher efficiency can be achieved comparing to the conventional two-stage solution. After analyzing the effect of the zero-sequence components in modulation strategy on power flow control, a simple and effective carrier-based pulsewidth modulation (CBPWM) with adjustable zero-sequence injection and variable carriers is proposed to achieve the power regulation of the two dc ports. The detailed operation principle and control scheme of the semi-two-stage ac dc converter are presented, and the effectiveness and feasibility of the proposed solution for pulsed load are validated by simulation and experimental results.