Search

For the high-voltage (HV) DC power supply feeding X-ray generator, LCC resonant converter with primary-parallel-secondary-series (PPSS) configuration is commonly adopted due to low switching losses and high output capability. In this paper, an analytical state-space model of the PPSS-LCC is derived with the diode-rectified output capacitor filter taken into consideration. Based on extended describing function (EDF) method, the high-order non-linear system is linearized by small-signal approximation for frequency control, and steady-state working point is solved. Simulation and experiments on a practical 100kV-50kW digital controlled PPSS-LCC serving an X-ray generator are carried out. The operation of the PPSS-LCC system on varying loads verifies the effectiveness of proposed model and control scheme.

The modular cascaded power electronic transformers (PETs) designed for interfacing AC and DC grids are mostly composed of active front end (AFE) and dual active bridges (DABs). Due to the remarkable difference between primary side and secondary side capacitor voltages of DAB at start-up, high input-inrush current will occur if the phase-shift-control (PSC) of DAB is directly employed, which can pose damage to power devices. This paper proposes a novel soft start-up method for DABs in PET with fast pre-charge process. The duty cycle of primary bridge is dynamically adjusted to precisely limit the peak of input-inrush current, and the regulating range is also enlarged.

Modular multilevel converters (MMCs) become popular in high-voltage dc transmission systems; however, their applications in medium-voltage (MV) motor drives are still limited due to the technical challenge of low-frequency voltage fluctuation. This paper proposes a new solution to address this problem. Full-bridge submodules (SMs) are used in the place of half-bridge SMs in the MMC to significantly reduce the low-frequency pulsating power in the SM capacitors in startup or low-speed motor drives. Furthermore, quasi-resonant controllers are adopted to acquire improved tracking performance for injected circulating current. Simulation and experimental results are presented and have proved the validity of the proposed method. Comparisons of efficiency and cost between the conventional and the proposed solutions further demonstrate the potential of proposed method for MV drive applications.

Power electronic transformer (PET) is being widely used in place of line frequency transformer (LFT) in modern power systems. For PETs' circuit in high-power and high-voltage applications, cascaded H-bridge or modular multilevel converter is preferred due to the advantages of modular structure. For such topologies, the low-frequency ripple powers stored in dc-link capacitors pose passive effects on power density. In this paper, a power channel based power electronic transformer (PC-PET) as well as power decoupling control is proposed for three-phase AC - DC conversion. Based on cascaded architecture, the ripple powers introduced by each AC input are counteracted without flowing into dc link. Therefore the dc capacitance can be reduced so as to achieve higher power density and lower cost. Simulation results prove the validity of proposed topology and decoupling control strategy. It is expected to be applied in the interconnection of three-phase AC and DC grid as smart energy router.

Modular multilevel converter (MMC) has become one of the most promising converter topologies for future high-power applications. A challenging issue of the MMC is the voltage balancing among arm capacitors. A good overall control system is also vital for the MMC, which should be based on sound mathematical model, readily adaptable for different applications, and capable of high performance. This paper presents a general control structure for MMC inverters, which is suitable for both voltage-based and energy-based control methods, and includes voltage balancing between the upper and lower arms. A new method for voltage balancing among arm capacitors, which is based on an improved pulse-width modulation, is also presented. The proposed method avoids some major disadvantages found in present voltage balancing methods, such as dependence on computation-intensive voltage sorting algorithms, extra switching actions, interference with output voltage, etc. Furthermore, all switching actions are evenly distributed among power devices. The proposed control system as a whole can serve as a promising solution for practical applications, especially when the number of submodules is fairly high. Simulation and experimental results verify the effectiveness of the proposed methods.

In a modular multilevel converter (MMC), the interaction between switching actions and fluctuating capacitor voltages of the submodules results in second- and other even-order harmonics in the circulating currents. These harmonic currents will introduce extra power loss, increase current stress of power devices, and even cause instability during transients. Traditional methods for circulating current harmonic suppression have problems such as limited harmonic rejection capability, limited application area, and complex implementation. This paper presents a plug-in repetitive control scheme to solve the problem. It combines the high dynamics of PI controller and good steady-state harmonic suppression of the repetitive controller, and minimizes the interference between the two controllers. It is suitable for multiple harmonic suppression, easy to implement, and applicable for both single-phase and three-phase MMCs. Simulation and experimental results on a single-phase MMC inverter proved the validity of the proposed control method.

Single-phase pulsewidth modulation rectifiers suffer from ripple power pulsating at twice the line frequency. The ripple power is usually filtered by a bulky capacitor bank or an LC branch, resulting in lower power density. The alternative way is active power decoupling, which uses an active circuit to direct the pulsating power into another energy-storage component. The main dc-link filter capacitor can, therefore, be reduced substantially. This paper proposed a new scheme of active power decoupling. The circuit consists of a third leg, an energy-storage capacitor and a smoothing inductor. The topology combined the advantages of high energy-storage efficiency and low requirement on control bandwidth. Both the pulsating power from the ac source and the reactive power of the smoothing inductors are taken into consideration when deriving the power decoupling scheme. The active power filter's (APF) capacitor voltage control system consists of inner loop pole-placement control and outer loop proportional-resonant control. To enhance the steady-state performance, the capacitor voltage reference is modified in a closed-loop manner. Simulation and experimental results show that the proposed APF scheme has good power decoupling performance and is more suited for high-power applications where switching frequency is limited.

Due to the low-frequency voltage fluctuation problem, application of modular multilevel converters (MMC) in wide-speed-range medium-voltage (MV) drives is still difficult. This paper proposes a new approach to this problem. Firstly, full-bridge submodules (FB-SM) are used in place of the traditional half-bridge submodules (HB-SM). The former can significantly reduce the low-frequency pulsating power in the SM capacitors. Secondly, an improved “square wave plus third-order harmonic injection” with quasi-resonant (QR) control scheme is proposed to transfer the low-frequency ripple power into high-frequency ones. Simulation and experimental results prove the validity of proposed method. A comparison between the FB-SM based and HB-SM based MMCs showed good potential of the former in MV drive applications.

When a modular multilevel converter (MMC) is operated at low frequencies, the submodule (SM) capacitors will suffer from huge voltage fluctuations. This problem must be overcome before MMCs find wider applications in medium voltage (MV) drives that require a wide speed range. The established solution for suppressing the voltage fluctuation have been injecting high frequency common-mode voltage and circulating current into each phase leg. This inevitably increases current stress of the power devices, which is not welcome in high power applications. Based on the fact that the pulsating powers of the upper- and lower arms are in opposite phase, this paper aims to eliminate the voltage fluctuation via added power channels between the upper- and lower arms of each phase. These power channels can also be used for active power decoupling, which can greatly reduce the SM capacitance.

Modular multilevel converter becomes one of the most promising high power converters thanks to their modular structure design, distributed dc capacitors, improved power quality, and no separate dc voltage sources required. However, capacitor voltage balancing remains one major challenge. This paper proposes a new voltage balancing control associated with an improved pulsewidth modulation (PWM) method. Specifically, a new mathematical model and control structure with multi-loops including a new balancing control loop between the upper and lower arms are presented. The capacitor voltages can be well balanced without adding unnecessary switching to any SMs and voltage sorting algorithm. Moreover, the control and modulation are more accurate and the ac power quality can be improved. Simulations and experimental results have verified the effectiveness of proposed methods.

A single-stage single-switch (S4) converter integrating a buck-type power factor correction (PFC) cell and a buck-type dc-dc output cell is presented in this paper. The most attractive feature of the proposed S4 converter is that, its input power factor (PF) always keeps in constant even when the input voltage and the load change. Besides, the proposed converter also preserves the advantages of those existent S4 converters, such as the low voltage stress across the intermediate capacitor, low current stress of the switch and high step-down output voltage. These advantages make it cost-efficient and highly suitable for the low power and non-isolated applications. In this paper, the operational modes and circuit design are given, and a prototype is also built to show the validity of the analysis.

Modular multilevel converters (MMC) are intensively researched and developed in high power, high/medium voltage applications. However, the interaction between switching action and fundamental-frequency pulsation in capacitor voltage of submodules will induce 2nd-order and other even-harmonic harmonics in circulating current. These harmonic currents will introduce extra power loss, increase current stress of power devices and even jeopardize safe operation of MMC. Traditional methods are usually designed under single harmonic frequency, resulting in increased complexity for an integrated solution. This paper presents a repetitive control scheme based on unified model of circulating currents to solve the problem. It is simultaneously effective for multi-harmonic suppression, easy to implement and can be widely applied to single-phase or three-phase MMC. Simulation and experimental results of a MMC operating at inverter mode prove the feasibility of proposed methodology.

Conventional single-phase inverters exhibit double line frequency power pulsating, which affects dc sources such as photovoltaic performance and battery lifetime. Bulky dc-link electrolytic capacitors are typically employed as transient energy buffer to decouple the pulsating ac power from constant dc power, but such passive components suffer from temperature and aging concerns. For high reliability and high power density, active power decoupling approach is preferred. This paper presents a novel active power decoupling method by using a six-switch single-phase inverter topology. A small film capacitor is used as pulsating energy buffer in ac side, which not only improves the reliability but also the efficiency. A novel vector PWM for this topology is also proposed to maximize the dc voltage utilization, and to achieve the independent controls of the inverter output power and power decoupling. The simulation results have verified the proposed power decoupling method.

This paper presents a novel indirect current control technique applied to a Single-Phase PWM rectifier to increase power factor and reduce harmonic current. It is applied to railway traction converters with low switching frequency, high voltage and high power. To get better stability and easier realization, indirect current control scheme is used in the inner loop control. Much computing such as extraction of square root and arc tangent affects the switching time of power devices and reduces the control accuracy. A new PWM control technique based on volt-seconds equalization is adopted. Simulation and experiment results proved the proposed scheme.