Research Of Compensator With Cascade H-bridge Inverter Based On High Frequency Isolated Converter

As the special load working on electrified railway, electric locomotive is driven by single phase power supply in power frequency. Because of its strong impacting and volatile character, a large number of negative-sequence current, reactive current and harmonic current inject into power system. They lower the power quality, destroy the stable running of power system and affect the power users. At present, dynamic compensation device is the most useful method to improve power quality.By analyzing the structure characters of three-port bi-directional DC-DC converter and H-bridge inverter, A compensator with cascade H-bridge inverter based on high frequency isolated converter is proposed in this paper. Comparing with traditional compensator based on Y-connected cascade inverter used in three-phase three-wire high voltage system, the new topology conquers the difficulty of compensating negative-sequence current by bringing in a three-port bi-directional converter with a three windings high frequency transformer to energy storage capacitors. The special structure allows active power to flow between phase and phase, weakens DC-link voltage pulsation. The compensator can be applied in both three-phase four-wire and three-wire system.Further, voltage balancing strategy of three-port bi-directional DC-DC converter is presented. Different kind of methods to detect and control compensating current are compared. Carrier phase shifting control used in cascade H-bridge structure is analyzed. By simulating the control strategy chosen for the structure in MATLAB, the feasibility of the topology is verified. Process of putting up the prototype, selecting of devices, designing of main circuit and control circuit, optimizing the structure is shown in the paper. Negative-sequence current balancing, reactive current compensation and harmonic current elimination are realized by the combination control of DSP and FPGA. Experimental results prove advantage and limitation of the topology, provide evidence for reforming.