Study On Active Compensation Of Electrified Railway Based On Four-switch Three-phase Converter

With the widely use of the high-power AC-DC electric locomotive and AC-DC-AC electric locomotive, the power quality problems such as harmonic component, reactive power and negative sequence are especially outstanding. Using active power filter (APF) to compensate a plurality of power quality problems of electrified railway traction substation, is a significant approach to ensure the stability, safety and high-efficient of the electrified railway, and it is also an ideal solution to control the harmonic pollution of electrified railway. Therefore, it is great significant to research APF to improve the performance of electrified railway power quality. The main contents are as follows:(1) First, based on the typical topology of APF and this report provides a four-switch three-phase converter as the main circuit of shunt APF. Additional, analyzing the working principle of four-switch three-phase converter and basic mathematical model can provide the theoretical principle for it to remove harmonic current components, compensate reactive components, and restrain negative sequence components. After that, the following chapters focus on the theoretical basis for the selection and design of control strategy.(2) Harmonic detection algorithm is a key factor in affecting performance of APF. In this report, after analyzing and comparing harmonic characteristics of two types of locomotives, an effective harmonic detection algorithm based on synchronous rotating reference coordinates is selected, which can achieve high-accuracy detection even with non-sinusoidal and unbalanced voltage. Moreover, this method does not require phase locked loop, the speed and accuracy of harmonic detection can be improved.(3) In order to improve the compensation ability of APF, two types of high tracing precision harmonic current control strategies are proposed based on internal model principle including both vector proportional integral (VPI) controller and repetitive controller. As the selective harmonic compensation technique, VPI controller can enhance the flexibility and adaptability of the APF. It also reduces the computational process of the controllers. Therefore, because of those unique properties, a new non-harmonic detection technology is proposed. This technology not only omits compensation current detection and complex harmonic current calculation, but also achieves a lower cost and improves the dynamic performance of the whole system. Additional, to simplify the controllers, another current tracking control strategy based on repetitive controller is also provided in this report. Thesis demonstrates the design method of the controller and analyzes the dynamic response and steady-state characteristic. After that, uses the Matlab to verify two types of controllers' superiority.(4) When APF is used in medium-voltage high-power domain such as electrified railway, the multilevel technique can remarkably improve the compensation performance of APF. Modular multilevel converter (MMC) seems to be the most feasible topology for many reasons. It not only simplifies hardware manufacturability and makes the system flexible, but also requires less number of main power components.After analyzing the working principle and the basic mathematical model of MMC-APF and the basic principle of carrier phase shifted-SPWM (CPS-SPWM) technique, the superiority of MMC-APF is verified by simulation.