Liangcai Shu

A Hybrid ZVZCS Dual-Transformer-Based Full-Bridge Converter Operating in DCM for MVDC Grids

High-power dc–dc converter is the essential component for connecting the renewable energy sources to medium-voltage dc (MVDC) grids. In this paper, a novel zero-voltage zero-current switching converter with two full-bridge cells sharing a bridge leg and connecting the secondary windings of two transformers in series is proposed for MVDC applications. One big feature of the proposed converter is that the required inductance used for energy transmission is reduced remarkably and even can be embedded in the transformer in some cases. Special but simple control strategy adopted by the converter makes it work in discontinuous current mode, which can realize zero-current switching for the main switches and rectifier diodes in the whole load range. Meanwhile, the auxiliary switches with small current rating can realize zero-voltage switching naturally. Hence, the switching loss is reduced, which is very important for high-power applications. The effects of the turns ratio of auxiliary transformer on the total loss and needed inductance are comprehensively analyzed and proved. With a detailed parameters design procedure, a simulation model is established in the software PLECS and the operation principle of the converter is verified. A 120 V–1200 V/1 kW prototype was built to validate the operation principle of the proposed converter.

An Indirect Input-Series Output-Parallel (ISOP) DC-DC Full Bridge Converter System Based on Asymmetric PWM Control Strategy

This paper proposes an indirect input-series output-parallel (I2SOP) dc–dc full-bridge converter system based on asymmetric pulsewidth modulation (APWM) control strategy for high input voltage applications. When a short-circuit fault occurs in a high input voltage dc bus, the proposed I2SOP system can disconnect from the dc bus effectively and avoid the input filter capacitors of constituent modules discharge compared with the traditional ISOP system, facilitating fault handling and protecting the input filter capacitors. Moreover, with the proposed I2SOP structure, it is easy to achieve redundancy design when one or a few constituent modules fail. The APWM control strategy is introduced to the I2SOP system rather than the classical phase-shifted control strategy for the basic full-bridge module to reduce the voltage stress of power switch and the filter requirement. The characteristics of input voltage sharing and output current sharing of the I2SOP system are also analyzed. A three-module I2SOP system prototype was built, and the experimental results verify the correctness and effectiveness of the proposed solution.

A hybrid resonant ZVZCS PWM full-bridge converter for MVDC grids

A novel hybrid resonant zero-voltage zero-current switching PWM full-bridge converter with capacitive output filter is proposed in this paper. The hybrid resonant converter, with two full-bridge cells sharing a bridge leg and connecting the secondary windings of two transformers in series, operates in discontinuous current mode and can achieve zero-current switching for main power switches and rectifier diodes over the whole load range. Meanwhile, the auxiliary switches with small current rating can realize zero-voltage switching naturally. Hence, the switching loss is reduced, which is vital in MVDC applications. The influence of the turns ratio of auxiliary transformer and the resonant capacitance on the peak and turn-off currents is discussed in detail. The performance of the converter is validated by simulation and experimental results.

Modular cascaded converter for MVDC-connected photovoltaic systems

This paper focuses on the relevant content of the MVDC-connected photovoltaic system based on the modular cascaded structure. This paper proposes a new topology for the DC grid-connected photovoltaic system with a novel capacitor voltage balancing control strategy, including the module input power ratio feed forward and module capacitor voltage closed-loop control. The proposed control strategy is independent from the realization of the photovoltaic maximum power point tracking function of the first stage. The operation principles of the proposed converter are analyzed and verified by the simulation and experimental results.

A novel wireless control strategy for input-series output-parallel inverter system

In this paper, a wireless modular control strategy for input-series output-parallel (ISOP) inverter system is proposed to ensure equal sharing of input voltage and output current among the constituent inverters. Compared with existing control strategies for the ISOP inverter system, such as centralized control which restricts the modularity of the system and distributed control which realizes voltage and current sharing by means of connecting constituent inverter modules with several sharing buses susceptible to interference and reduces the reliability of the system, the proposed control strategy based on positive output-voltage-amplitude gradient can achieve fully modular design of the ISOP inverter system. Firstly, the control strategy is deduced according to the droop method, and the principle of the control strategy is analyzed in detail. Then the stability and design of the proposed strategy are also explored. At last, a two-module ISOP inverter system is tested in the laboratory and the experimental results verify the effectiveness of the proposed control strategy.

A high voltage pulsed power supply with reduced device voltage stress for industrial electrostatic precipitators

Compared with the conventional high voltage DC power supply, the high voltage pulse power supply can significantly improve the dust removal efficiency of electrostatic precipitator and reduce the energy consumption. This paper presents a new type of high voltage pulse electrostatic precipitator power supply topology, the symmetric cascaded structure of the low voltage side which can significantly reduce the voltage stress of switching device, rectifier circuit, clamp circuit and resonant capacitor, thereby significantly reducing the cost. This paper analyzes the steady-state operating principle of the high voltage pulse power supply and the working conditions during flashover while the expressions of the output voltage and current of the transformer primary side are both deduced and verified by simulations. Finally, the experimental verification is carried out by a pulse power supply with a peak voltage of 100kV.

A hybrid resonant three-level converter for renewable energy in MVDC collection systems

A hybrid resonant zero-voltage zero-current switching three-level converter is proposed in this paper, which undergoes half of the input voltage for switches. The proposed converter, utilizing two transformers, adopts IGBTs for all six power switches and operates in discontinuous current mode and can achieve zero-current switching for the four main switches and rectifier diodes over the whole load range. Hence, the switching loss is reduced. The influence of the turns ratio of auxiliary transformer and the resonant capacitance on the peak and turn-off currents is discussed in detail. The operation principles of the proposed converter are analyzed and verified by the simulation and experimental results.