Research On Hybrid Active Power Filter For Electrified Railway

Due to a higher energy utilization ratio, the electrical railway is becoming an important means to alleviate the shortage of transport capacity. Most of the traction loads are phased rectifiers with the features of asymmetric structure, low power factor, high harmonic contents and big negative sequence currents, causing serious damages to the electric power system. In the scheme of LC Series Resonant Injection Hybrid Active Power Filter (SRIHAPF), the inverter just needs to withstand very small fundamental voltage and harmonic current, which can significantly improve the compensation capacity with the same cost and manufacturing process. This thesis focuses on its topology and principle, parametric design, current detecting and tracking, and integrative compensation scheme for traction substations, which has theoretical significance and practical value for improving the power quality of electrical railway. The Phase-Locked Loop (PLL) and Low-Pass Filter (LPF) bring errors to the fundamental active currents detection in ip-iq/method. An improved current detection method for single-phase circuit is proposed, based on the frequency selective characteristics of integral signal amplitude. Both of PLL and LPF are omitted, and it can accurately extract the positive or negative sequence fundamental current or a specific harmonic, without complex coordinate transformation. The MATLAB simulation experiment validates that the improved algorithm has higher rapidity, accuracy and stability.In the traditional scheme of capacitor voltage control in DC side, the phase errors included in adjustments pass on to the fundamental current detection, cause capacitor voltage fluctuation, and do harm to the stable operation of APF. An optimization scheme is proposed in the thesis. By modifying the adjustments flow and separately controlling its amplitude and phase, the transmission channel of the phase errors is cut off, which make capacitor voltage controlling more stable.The separate single-phase compensation towards the low-voltage side of traction substation can not reduce the harm of negative sequence currents in its high-voltage side. With comprehensive consideration of both arms, a new scheme using SRIHAPF to compensate the harmonic, reactive power and big negative sequence currents is proposed in the thesis. To make the three-phase currents be positive sequence fundamental active currents is the compensation target of the scheme. Firstly, two separate sets of passive filter are installed to filter out most of the harmonic in each arm. Then the public current detection and distribution unit calculates the directive compensation currents for three-phase windings, according to the corresponding transfer coefficient of traction transformer. The directive compensation currents for each power arm are distributed according to its inverse transformation. At last, the single-phase APF generates the compensation current respectively. The MATLAB simulation system built on actual surveys validates the feasibility of the above compensation scheme.