Research On Operational Characteristics And Control Strategies Of Full-bridge MMC Based DC Ice-melting Equipment

Higher voltage and longer distance overhead line has been widely used worldwide.However,the ice disaster has caused serious harm in the easy-icing areas.Due to the high frequency of the ice disasters,DC ice-melting device has become common equipment in the easy-icing areas.The previous thyristor rectifier based DC ice-melting device consumes too much reactive power and outputs characteristic harmonics,which further affected the system stability when connected to AC system.Full bridge modular multilevel converter(MMC)has the ability of outputting bidirectional DC voltage and DCcurrent.This dissertation focuses on the control strategies of full bridge MMC for DC ice-melting applications.This dissertation firstly introduces the modulation ratio definition which applied in full bridge MMC,besides the models of dq coordinate and switching function are founded in order to provide reference for the design of the control system.Then the ability of the full bridge sub-module(FBSM)which can output positive,zero and negative capacitor voltages is applied to make full bridge MMC output widely ranged and continuous-tuning(zero-initial boosting)DC voltage and current to de-ice the AC conductors of different varieties and length.At the same time,the AC side voltages of full bridge MMC remain regulated and almost sinusoidal even when the DC voltage is quite small.And the device can be used as a reactive power compensator.The topology structure of static synchronous compensator(STATCOM)is selected as delta-connected considering these factors: capacities for var compensation,facility utilization and elements redundance.Finally electromagnetic transient models of the full bridge MMC based DC ice-melting device are simulated on PSCAD/EMTDC and RT-LAB.The simulation results show full bridge MMC's DC voltage and current can be widely ranged which illustrates the device can de-ice the AC conductors of different varieties and length.The results have been compared and proved the effectiveness of the proposed control and modeling methods.