摘要:
The previous thyristor rectifier based DC ice-melting equipment consumes too much reactive power and brings about characteristic harmonics,which further affects the AC system stability.The full-bridge modular multilevel converter (MMC),with its ability of outputting bidirectional DC voltage and DC current,can meet the demand of DC melting on operation conditions of converters.This paper focuses on the control strategies for DC ice-melting equipment for full-bridge MMC.First,the operation principle of the main circuit topology of DC ice-melting equipment based on full-bridge MMC is described,and the modulation ratio definition applied in full bridge MMC is given.By taking advantage of the full-bridge sub-module (FBSM) ability of outputting positive,zero and negative capacitor voltages,the zero-initial voltage and current boosting of the fullbridge MMC is analyzed to satisfy the de-ice demands of different types and lengths of AC conductors.At the same time,the AC side voltage of full-bridge MMC should be made to remain satisfactory and almost sinusoidal even when the DC voltage is very low.Considering the capacities for var compensation,facility utilization and element redundance,the topological structure of the ice-melting equipment in the reactive compensation mode is determined.Finally,an electromagnetic transient model of DC ice-melting equipment is developed on PSCAD/EMTDC and the results have verified the effectiveness of the proposed control strategies.%现有直流融冰装置采用了晶闸管可控整流技术,在运行中均会消耗一定的无功功率,产生特征次谐波,对交流系统带来一定影响.全桥型模块化多电平换流器(MMC)具有直流电压和直流电流双向运行能力,可满足直流融冰对换流器运行工况的要求.文中提出了适用于全桥型MMC的直流融冰装置控制策略.首先,对全桥型MMC直流融冰装置的主回路拓扑的运行原理进行了介绍,并给出了适用于全桥型MMC的调制比定义.其次,充分利用全桥型子模块可输出正、零、负模块电压的特点,分析如何实现零起升压/升流,以满足不同类型、长度线路的融冰需求,使得直流电压很低时,交流侧仍然保持很高的电能质量.然后,基于无功补偿容量、设备利用率和器件裕度等对装置的无功补偿进行研究,确定了装置无功补偿模式的拓扑结构.最后,在PSCAD/EMTDC中搭建了直流融冰模型,仿真结果验证了所提控制策略的有效性.