Xiaoqiong He

Voltage Balancing Approaches for Diode-Clamped Multilevel Converters Using Auxiliary Capacitor-Based Circuits

An auxiliary capacitor-based balancing approach is adopted in this paper to equalize the dc-link capacitor voltages of a diode-clamped multilevel converter (DCMC). Four balancing circuits, including the generalized, one-level-capacitor, one-capacitor, and simplified one-capacitor-based configurations are discussed for a five-level DCMC system. These configurations are different in connection of the auxiliary circuits and numbers of the capacitors and switches, but they work on the same principle called ping-pong operation by utilizing the auxiliary capacitor as an equalizer between the capacitors of different voltages. The unbalance phenomenon, ping-pong principle, circuit configurations, and their switching schemes are analyzed, respectively. The superiorities of the proposed approach include balancing operation regardless of load power factor and modulation index, control simplicity, and independence from main circuits when compared to the traditional approaches. Simulations and experimental results verified the performances using the proposed balancing circuits and control strategies.

Advanced Cophase Traction Power Supply System Based on Three-Phase to Single-Phase Converter

An advanced cophase traction power supply system is proposed to solve the power quality problems of the traditional traction power supply system, such as unbalance, reactive power, and harmonics to three-phase industrial grid. The three-phase to single-phase converter-based substation is adopted in this system, which can transfer active power from three-phase grid to single-phase catenary and compensate reactive power and harmonics of the locomotives. One catenary section could be utilized in the advanced cophase system instead of the multiple split sections in traditional system. The neutral sections and problems caused by them in traditional system could be avoided. In this paper, the characteristics of the advanced cophase system and the automatic current-sharing control algorithm of three-phase to single-phase converter are studied and analyzed. The simulation and experimental results verify the viability and effectiveness of the proposed system.

Predictive Harmonic Control and Its Optimal Digital Implementation for MMC-Based Active Power Filter

An optimal active power filter (APF) based on modular multilevel converter (MMC) without load current and lower arm current sensors is proposed. Its control algorithm mainly includes predictive harmonic current control, fundamental frequency control, and dc voltage control. Comparing to direct current control, the first two subcontrollers transfer the harmonic and fundamental active/reactive power current to voltage references, which are suitable to generate the modulation signals to MMCs as well as two-level converters. Their full digital optimization structures combined with the MMC voltage and current control are detail discussed and implemented on field programmable gate array. The synthesis result indicates a low hardware resource utilization and calculation delay of the designed controller. The experimental results on an MMC-based APF prototype verified the static and dynamic performances of the proposed algorithm.

Diode-clamped three-level multi-module cascaded converter based power electronic traction transformer

A diode-clamped three-level multi-module based power electronic transformer is proposed in application of locomotive traction for single-phase 25kV/50Hz railway. Each module is composed of three-level H-bridge AC-DC rectifier and half-bridge bidirectional DC-DC converter. Compared to traditional two-level H-bridge structure, the three-level module can improve interface voltage and power scale. Meanwhile, the number of cascaded modules and medium frequency transformers is decreased by half. The AC-DC rectifier is controlled by direct current control loop with dc-link voltage equalization algorithm. A minimum RMS current control algorithm is adopted in DC-DC converter to improve system efficiency. Finally, experimental results on three-module cascaded prototype verified the performances of the developed topology and control under different operating conditions.

The modulation techniques in 3L-NPC cascade converter

The multilevel converter has been quite popular for the high-voltage and large-power applications. And many modulation techniques have been applied to the multilevel converter topologies. All the modulation techniques are pursuing high-power quality and minimum switching frequency. In this paper, the modulation techniques are analyzed deeply in a three-phase to single-phase 3L-NPC cascade converter. The phase-disposition PWM (PD-PWM) and phase-shifted PWM (PS-PWM) are discussed in this system and compared with phase opposition disposition PWM (POD-PWM) and alternative phase opposition disposition PWM (APOD-PWM). Finally, the PD-SPWM and PS-SPWM can better meet the power quality requirements of system by simulation and experiment.

Output-transformerless traction substation based on three-phase to single-phase cascaded converter

As an ideal power supply, the advanced traction power supply system (ATPSS) is approved to totally remove the neutral sections and promote the power quality, which is a new direction development for traction power supply system. The existing ATPSS based on three-phase to single-phase converter needs to match the step-up transformer. The output current of this system is so heavy that it requires parallel structure to share current, which makes high requirements for manufacturing facilities. Whats more, circulation suppression in the parallel system must be taken into consideration. In this paper, a new ATPSS based on a three-phase to single-phase cascaded converter without output transformer is been studied. The structure of three-phase to single-phase converter and its control system are deeply analyzed. Then the cascade structure implementation of the converter is studied. Finally, a simulation model for this system is built. The simulation results show that the new system built in this paper can remove the output transformer, not only meets the voltage of the traction network and capacity requirements, but also improves power quality. The results verify the correctness and feasibility of main circuit structure and control strategy.

N-D SVPWM With DC Voltage Balancing and Vector Smooth Transition Algorithm for a Cascaded Multilevel Converter

Direct current voltages unbalance of respective power cells is an inherent and critical problem in a cascaded multilevel converter, which is widely used in the solid-state transformer. In this paper, an n -dimension space vector pulse width modulation is presented to address this issue in a three-level H-bridge-based cascaded multilevel converter. This algorithm utilizes the redundant switching pairs of the inner cell and the redundant vectors of mutual cells to balance capacitors’ and cells’ voltages. Meanwhile, a smooth vector transition strategy is included in this algorithm to avoid the level-skip phenomenon. The calculation processes are analyzed in one-cell, two-cells, and three-cells modes, respectively. It is simple, calculable, and flexible to extend in n-cells mode. Finally, the voltage balancing capacity of the inner cell and mutual cells with smooth level transition are verified by simulations and experiments not only at startup but also with unbalanced loads.

Fault diagnosis and system reconfiguration strategy of single-phase cascaded inverter

Three level neutral-point-clamped(NPC) single-phase cascaded inverter is used in advanced cophase traction power supply system. Because a lot of power modules and switches are used, reliability of this system could be decreased. To diagnose fault of cascaded inverter, different fault types of individual module are analyzed. A fault diagnostic method is proposed to diagnose open circuit fault of single-phase inverter. It only uses output voltage of each individual bridge arm. The fault features including harmonic amplitude and phase were extracted from these signals by spectrum analysis. Then results of fault diagnosis can be achieved by neural network algorithm. Based on fault diagnosis, three level single-phase cascaded inverter can operate under faulty condition and keep good harmonic performance. With this fault diagnosis and reconfiguration method, which is verified by simulation results, reliability of cascaded inverter can be improved.

Control strategy of single-phase 3LNPC-CR

Single-phase 3-level neutral point clamped cascaded rectifier (3LNPC-CR) has been successfully made its way into a traction drive system as a high-voltage traction converter. In this passage, the control issue of the 3LNPC-CR is considered. A transient current control strategy, combined with PI-based controllers, is adopted to achieve unity power factor, satisfactory sinusoidal grid current, regulated overall dc voltage, and even efficient voltage balance between each module. Besides, with regard to the instinct voltage fluctuation problem among dc-link capacitors in one 3-level neutral point clamped (3LNPC) rectifier module, a phase shift carrier space vector pulse width modulation (PSC-SVPWM) worked along with a reasonable redundancy selection scheme is addressed. In addition, two auxiliary balancing circuits for a 3LNPC is proposed. The voltage balancing capacity of these internal-module balancing schemes are analyzed and compared. Finally, the control performance of these proposed strategies are verified by simulations and experiments.

Current-sharing performance of an advanced co-phase traction power substation system based on cascade-parallel converter

An advanced co-phase traction power substation system based on the cascade- parallel converter was proposed in this paper, for addressing the power capacity problems of traction power substation and the problems of electrification phase, reactive power, negative sequence and harmonic in railway. The 3-level neutral point clamped (NPC) converter was used in power substation, while the converter was cascaded and paralleled to approve the capacity of system. The system can transfer active power directly between power gird and traction network, the power factor of which is 1, and the neutral-section passing device is canceled completely, which is an ideal co-phase traction power supply system. In this paper, current sharing performance of traction network based on this system is analyzed. The impedance performance of traction network and circulating current characteristic of the system are presented in detail. And then the circulating-current restraining strategy is analyzed, the droop control is proved to achieve a system of parallel. The system is proved having droop characteristic and can parallel operate with multiple converters by using drooping control, achieving current sharing. The simulation results verified the theoretical analysis and that the system achieves current sharing. The control strategy and modulation strategy is verified by experiment.