Qiongxuan Ge

Pre-charging control strategies of modular multilevel converter

Based on a novel multilevel voltage source converter (VSC) used for electric drive, the pre-charging process of the converter is fully analyzed, so as to find a suitable pre-charging control strategy. For a single converter with redundant modules, the process is divided into two stages, the unlock charging course is in particular analyzed. To ensure the normal operation of the converter, an optimized charging method is presented. For a modular multilevel converter system used as rectifier side and inverter side, the pre-charging control strategy is separately analyzed and proved to be suitable and effective.

High performance controller with effective voltage balance regulation for a cascade STATCOM with star configuration under unbalanced conditions

A high performance controller with effective voltage balance regulation is proposed for a cascade STATtic synchronous COMpensator(STATCOM) with star configuration. Zero-sequence voltages are injected to the output voltages of STATCOM in order to compensate the unblanced load currents, which can also decouple the star-connected three-phase STATCOM system into three single-phase systems. Additionally, an effective voltage balance control method and a robust current tracking technique are proposed to guarantee all capacitor voltages balanced and to ensure good current tracking ability. Moreover, a quick and accurate current extraction method is proposed to obtain the negative and reactive currents of unblanced loads to improve the systems dynamic response. The proposed control method is validated on a 10kV/±1Mvar cascade star-connected STATCOM industrial prototype. All the experimental results verify that with the proposed control methods the STATCOM can compensate unblanced load currents with quick dynamic response while keeping all capacitor voltages balanced.

The optimal control strategy for rectifier side of low switching frequency back-to-back converter

The industry application of High-power mediumvoltage drives converter is an appealing spot in Power electronics technology in recent years. In this paper, an optimal control strategy for rectifier side of three-level NPC back-to-back converter under low switching frequency conditions have been presented. First, the power feed forward is applied for enhancing the dynamic characteristics in the circumstance of bidirectional energy flow. And the demand of three-level DC neutral point control is solved by using a modified asymmetrical regular sampling SVPWM whenever the energy is being used or feed backed. Simulation and experimental results based on a rectifier side of a 75kVA three-level NPC back-to-back experimental prototype converter with a 400Hz device switching frequency constraint show satisfactory performance.

Implementation of Sensorless Control with improved flux integrator for Wound Field Synchronous Motor

In this paper, a sensorless control with improved flux integrator for wound field synchronous motor is presented. This paper analyses the wound field synchronous motors mathematical model. The stator flux can be accurately estimated based on the improved flux integrator with feedback compensator. With the estimated flux and only the q-axis inductance, the position is accurately estimated which is critical for field oriented vector control. The experimental results show that the proposed sensorless algorithm is reliable in a wide speed range above 1Hz.

The optimized strategy for input current harmonic of low switching frequency PWM rectifier

The industry application of High-power medium-voltage drives converter is an appealing spot in Power electronics technology in recent years. This paper presents an optimized control strategy for input current harmonic of rectifier of three-level NPC back-to-back converter under low switching frequency conditions. A modified P-N SVPWM method includes asymmetrical regular sampling, even-order harmonic elimination and natural DC bus balancing is applied for optimize the THD performance of input currents. And a modified proportional sinusoidal signal integrator method based on control scheme in two phase stationary reference frame which is different from ordinary direct-quadrature (DQ) rotating reference frame is proposed. It provides a specific input current harmonic elimination performance especially considerable in low switching frequency circumstance. Simulation and experimental results based on a rectifier of a 75kVA three-level NPC back-to-back converter prototype with a 400Hz device switching frequency constraint show satisfactory performance.

Research on a sensorless SVM-DTC strategy for induction motors based on modified stator model

In sensorless induction motor drives, the stator model is always used to estimate the stator flux because it does not require a speed signal. Combined with the stator model, stator field orientation control strategies are preferred in implementation. In these strategies, direct torque and flux control based on space-vector modulation (SVM-DTC) is the one with good performance. It produces fast torque and flux control with constant switching frequency, and, compared with the conventional DTC, the ripples of torque, flux and current is reduced. However, the stator model suffers from the initial value and DC drift problems of pure integrator, which makes the SVM-DTC strategy not stable when the motor starts or runs in low-speed range. In this paper, a modified stator model is designed to solve the problems. In combination with a simple and practical speed estimation algorithm, a high-performance sensorless SVM-DTC strategy is constructed. The validity of the control strategy is verified through simulation and experiment results.

Hardware-in-loop simulation of linear synchronous motor based on three-level converter

With the development of high power devices, Hardware in loop simulation is widely applied to power electronics to test the performance of control system. This paper describes a simulation method on basis of RT-LAB platform to test the controller of long-stator linear synchronous motor(LLSM). Three-level converter model and the dynamic mathematical model of LLSM are built in MATLAB/Simulink, after compiling, they can be downloaded and executed on target node. Utilizing the peripheral port such as analog I/O, digital I/O, RT-LAB can realize interaction with the actual controller. A sensorless control method based on extended electromotive force is mentioned in this paper, the whole algorithm is developed in IOWorks environment, The experimental results verify the performance of the controller.

Research on voltage balance control of Modular Multilevel Converter for low frequency operation

This paper analyses the reason of module capacitor voltage fluctuation when the Modular Multilevel Convertor (MMC) provide low frequency output voltage to motor. In order to restrain the voltage fluctuation, appropriate values of the arm inductor and the cell capacitor must be chosen. Furthermore, the common voltage is added in output AC phase voltage and the corresponding inner current in the arms is controlled to balance the energy fluctuation of module capacitor due to low frequency AC current.

Improved NP-SVM suitable for multi-sampling used in high power NPC rectifiers without even order harmonics

In order to eliminate the even order harmonic currents of the NPC rectifiers, the modified SVM (short for NP-SVM) is proposed in the past. However, when the NP-SVM is used in the multi-sampling system, there will be direct jumps between N and P which can result in narrower pulse width, higher dv/dt, even the destruction of devices, which is very undesirable. In this paper, the reasons of direct NP jumps generation are briefly analyzed at first, then an improved NP-SVM(INP-SVM) suitable for multi-sampling system is proposed to overcome the problems in high power three level NPC rectifiers. Finally, all these analyses and the effectiveness of the proposed NP-SVM are firstly validated by simulation, and then are verified further in a 75kVA laboratory prototype.

Implementation and Analysis of 3-phase Voltage Sourced Regenerative Rectifier

This paper describes an implementation of a 3-phase voltage sourced regenerative rectifier under direct current control strategy. General mathematical model of the converter is first established. Based on the model, the principle of the direct current control strategy is presented, simulation results are given, and circuit operations in steady state are analyzed. Based on SVPWM, the analysis explains the cause of dc voltage ripple, ac side current harmonics, and over modulation. Finally, parameters of a prototype are listed, and experimental results are presented