Jiaxin Yuan

Modeling and Harmonic Optimization of a Two-Stage Saturable Magnetically Controlled Reactor for an Arc Suppression Coil

Magnetically controlled reactors (MCRs) are usually used as three-phase shunt reactors. They have low harmonic distortion independent of the third harmonic current because most three-phase MCRs are delta connected. However, as arc suppression coils, MCRs are operated in the single-phase mode, and the harmonics can be much higher than those of three-phase MCRs. In this paper, the structure and the mathematical model of a two-stage saturable MCR (TSMCR) are proposed. There are two stages with different lengths and areas in the iron cores. The stages saturate at different times when the TSMCR outputs reactive current. The current harmonics of the first saturated stage can be compensated for when the second stage begins to saturate, to reduce the total harmonics of the output current. The mathematical model that reveals the distribution characteristics of the current harmonics for the TSMCR is also presented. A study of the mathematical model indicates that there are two key factors that affect the total current harmonics of the TSMCR. One is the parameter k, which represents the area ratio of the second stage to the first stage. The other one is the parameter m, which represents the ratio of the length of the first stage to the total length of the magnetic valve in the iron core. The simulations and experiments show that the maximum current harmonics of the novel MCR can be limited to 3.61% of the rated output current when k and m are chosen according to the theoretical mathematical model.

An Immune-Algorithm-Based Space-Vector PWM Control Strategy in a Three-Phase Inverter

In this paper, an immune algorithm (IA) is developed for optimization of the harmonic performance of a three-phase inverter under the space-vector pulsewidth-modulation (SVPWM) control strategy. The presented algorithm employs the immune approach as the search method for finding the best optimal control vectors and action time of the three-phase inverter. As a result, the optimal control vectors and action time are calculated to minimize the objective function of the weighted total harmonic distortion of the output voltage waveforms. In addition, an experimental platform based on DSP and field-programmable gate array is built. This paper provides a detailed performance analysis of the method with comparison to the conventional SVPWM. The simulation and experimental results verify the superiority of the best control sequences generated by IA compared with the existing conventional control strategies.

Selective Harmonic Elimination With Groebner Bases and Symmetric Polynomials

Selective harmonic elimination (SHE) technology has been widely used in many medium- and high-power converters which operates at very low switching frequency; however, it is still a challenging work to solve the switching angles from a group of nonlinear transcendental equations, especially for the multilevel converters. Based on the Groebner bases and symmetric polynomial theory, an algebraic method is proposed for SHE. The SHE equations are transformed to an equivalent canonical system which consists of a univariate high-order equations and a group of univariate linear equations, thus the solving procedure is simplified dramatically. In order to solve the final solutions from the definition of the elementary symmetric polynomials, a univariate polynomial equation is constructed according to the intermediate solutions and two criteria are given to check whether the results are true or not. Unlike the commonly used numerical and random searching methods, this method has no requirement on choosing initial values and can find all the solutions. Compared with the existing algebraic methods, such as the resultant elimination method, the calculation efficiency is improved, and the maximum solvable switching angles is nine. Experiments on three-phase two-level and 13-level inverters verify the correctness of the switching angles solved by the proposed method.

A Groebner Bases Theory-Based Method for Selective Harmonic Elimination

An algebraic method is proposed for selective harmonic elimination PWM (SHEPWM). By computing its Groebner bases under the pure lexicographic monomial order, the nonlinear high-order SHE equations are converted to an equivalent triangular form, and then a recursive algorithm is used to solve the triangular equations one by one. Based on the proposed method, a user-friendly software package has been developed and some computation results are given. Unlike the commonly used numerical and intelligent methods, this method does not need to choose the initial values and can find all the solutions. Also, this method can give a definite answer to the question of whether the SHE equations have solutions or not, and the accuracy of the solved switching angles are much higher than that of the reference method. Compared with the existing algebraic methods, such as the resultant elimination method, the calculation efficiency is improved. Experimental verification is also shown in this paper.

An Immune-Algorithm-Based Dead-Time Elimination PWM Control Strategy in a Single-Phase Inverter

In this paper, an immune algorithm is developed for optimization of the harmonic performance of a single-phase inverter under a novel dead-time elimination PWM control strategy. The proposed scheme is based on the division of reference current and constraining switching states, and executes the conventional dead-time elimination in most of the reference current fundamental period but switches to the novel dead-time elimination around the zero crossing point. Meanwhile, the presented algorithm employs the immune approach as the search method for finding the best optimal control sequence to minimize the objective function (OB) of the Total Harmonic Distortion (THD) of the output voltage waveforms. Additionally, an experimental platform based on DSP and FPGA is built. The simulation and experimental results verify the best dead-time elimination control sequences generated by IA compared with the existing conventional control strategies, not only effectively and safely eliminate the effect of dead-time, but also significantly reduce the output waveforms of THD and increase the amplitude of the fundamental voltage.

A Hybrid Electrical Magnetic Power Quality Compensation System With Minimum Active Compensation Capacity for V/V Cophase Railway Power Supply System

To solve the power quality problem in a V/V cophase railway power supply system, a hybrid electrical magnetic power quality compensator (HEMPQC) based on magnetic static var compensator (MSVC) and hybrid power quality compensator (HPQC) is proposed in this paper. Compared with conventional HPQC, the proposed HEMPQC could keep the active compensation capacity minimum to any load condition. The output current of MSVC is conducted to make the active compensation capacity minimum. The coupling branch impedance optimum design procedures are deduced. Based on the instantaneous current detecting and reactive current distribution method, the collaboration control strategy is proposed for HEMPQC to achieve the dynamic tracking of the reference signals. Finally, simulation and experimental results have verified the proposed hybrid compensation system and compensation method effectively.

Analysis and Suppression of Circulating Harmonic Currents in a Modular Multilevel Converter Considering the Impact of Dead Time

This paper focuses on analysis and suppression of circulating harmonic currents in a modular multilevel converter (MMC) considering the impact of dead time in medium-voltage applications. A continuous equivalent model of the MMC containing two ideal transformer models is presented. Using this model, the impact of a harmonic voltage upon the dc side is analyzed and the production mechanism of circulating harmonic currents is elucidated. At the same time, the impact of dead time and insulated gate bipolar transistor (IGBT) voltage drop (DTVD) is studied, which indicates that capacitor voltages, output harmonics, and circulating harmonic currents are influenced. Based on this analysis, an open-loop control strategy to suppressing circulating harmonic currents caused by the output current and DTVD is presented. Finally, all these conclusions are verified using a simulation platform with 14 modules per arm fed by a 14-kV dc voltage source and a downscaled experimental platform with four modules per arm fed by a 560-V dc voltage source.

An electrical-magnetic hybrid power quality compensation strategy for V/V traction power supply system

To compensate negative sequence current in highspeed railway power supply system, a novel compensation strategy based on railway power conditioner (RPC) and magnetic controlled reactor (MCR) based static var compensator (SVC) is proposed in this paper. As the traditional RPC compensation needs a big capacity, a combination of RPC and magnetic static var compensation (MSVC) is put forward. The electrical models based on fundamental and harmonic domain are analyzed. Furthermore, this paper comes up with the principle of negative current compensation and harmonic current suppression. The control strategy is proposed. The simulation results confirm the correctness of the proposed method.

Performance Investigation of a Novel Permanent Magnet-Biased Fault-Current Limiter

The fault current limiter (FCL) is one of the most useful measures for limiting fault currents in power systems. This paper introduces a new structure for a parallel-type permanent magnet (PM)-biased saturation-based FCL (PPFCL). The PPFCL consists of two magnetic devices connected in parallel and in magnetic counter opposite to each other. The operating principle and the core inductances in different states were analyzed using an electromagnetic equivalent circuit. In addition, a 220 V/10 A experimental prototype was designed and tested in a simple system and compared with Maxwell-3-D simulation and analytical methods. Finally, a 10 kV case-study model was created and compared with a series-type FCL, which verified the effectiveness of the limiter in high-voltage, large-capacity application. Experimental and simulation results have demonstrated that the PPFCL exhibits fast response and low impedance during normal operations, as well as high bias capability and low possibility of demagnetization of the PM during fault conditions.

Parameter Design and Performance Investigation of a Novel Bridge-Type Saturated Core Fault Current Limiter

A saturated core fault current limiter (SCFCL) is a promising device for limiting the fault currents effectively, and improving the safety and reliability of power systems. Due to the core structure and the coupling effect between the ac and dc windings, traditional SCFCLs suffer the drawbacks of insufficient clipping performance and large size. Moreover, theoretical analysis of fault clipping performance has received little attention. Therefore, this paper proposes a novel bridge-type SCFCL (BSFCL). Compared with the traditional type, the BSFCL effectively reduces size and cost by using a bridge-type structure. In addition, the proposed additional limiting inductor improves the fault clipping performance of SCFCL. This paper first presents the principle and characteristics of BSFCL. Then, a theoretical analysis of clipping performance and parameter design method of BSFCL are presented in detail. Finally, the principle and performance of BSFCL are validated through electromagnetic simulation and experiments. The simulation and experimental results demonstrate the effectiveness of the theoretical analysis and the structure. The influence of different BSFCL parameters on fault clipping performance is also discussed.