Research On Several Kev Techniques Of Large-Capacity Converter Based On H-bridge Topology

The large-scale development of renewable energy、electric vehicle and electric locomotive can alleviate the growing shortage of fossil energy and the environment deterioration caused by the consumption of traditional energy. The large-capacity converter is a core component of the large-scale renewable energy system、the electric vehicle drive system and the electric locomotive traction drive system. The key technical issues associated with the large-capacity converter include power quality problems、circuit topologies、parallel technology and multi-level technology. Focused on these key technical issues, and based on the overview of the existing studies and technologies, this paper has made thorough research on the following aspects:the operation method for the parallel inverter system (PIS) with common dc link, the topology and operation method of the input-series-output-parallel modular AC-DC-DC converter, H-bridge dead-time elimination sinusoidal pulse width modulation (SPWM), the topology and dead-time elimination space vector pulse width modulation (SVPWM) of six half-bridge (3H bridge) inverter, et al.Firstly, the ideal parallel operation method with no circulating current among parallel units is presented for PIS with common dc link. The PIS with common dc link is suitable for the large-capacity grid-connected inverter or uninterruptible power supply (UPS). The mathematical model of the output port is developed for the PIS which is used as UPS, then, two double loop control methods which are composed of an outer voltage loop and an inner current loop are designed. In addition, the circulating currents caused by the gate control signals with time delay are described. At the same time, the corresponding solution is put forward to eliminate circulating current.Secondly, the relationship between output current sharing and input voltage sharing is discussed for the input-series-output-parallel modular AC-DC-DC converter which can convert the high AC voltage into the low voltage DC voltage. The experimental results indicate that the theoretical analysis of the relationship between the input/output voltage sharing of the former cascade PWM rectifier and the DC output equivalent resistances is correct. Then, the scheme which can simultaneously achieve the former input/output voltage sharing and the later output current sharing is proposed. Besides, the control methods for the cascade PWM rectifier and the DC-DC parallel system are proposed in this paper.Thirdly, the novel switching strategy in one sampling period for H bridge is proposed to avoid that the PWM signals for upper and lower side switches are with180°phase shift. Based on the novel switching strategy, a novel dead-time elimination SPWM is proposed. In addition, how to accurately determine the polarity of the inverter output current is introduced in detail. The dead-time elimination SPWM can be used in the PIS with common dc link and the cascade PWM rectifier.Finally, drawing on the advantages of single-phase full-bridge inverter with respect to the half-bridge inverter, the three-phase three-leg inverter can be transformed into three-phase3H-bridge inverter. Under the condition of the same DC bus voltage, the output fundamental voltage range of the three-phase3H-bridge inverter is twice times that of three-phase three-leg inverter although their voltage/current stress is the same. A dead-time elimination SVPWM is proposed for the three-phase3H-bridge inverter which can feed load directly. Besides, the isolated transformers can be used to block the flow of third-order harmonic currents in the output port circuit.