Key technologies for medium and low voltage DC distribution system

Development of the medium and low voltage DC distribution system is of great significance to a regional transmission of electric energy,increasing a penetration rate of new energy,and enhancing a safety of the operation of the AC/DC interconnected grid.This paper first summarizes the medium and low voltage DC distribution system schemes and plans put forward by many countries,and then elaborate status of under-construction medium and low voltage DC distribution system project cases in China.Based on these project cases,this paper analyzes key issues involved in the medium and low voltage DC distribution system topologies,equipment,operation control technologies and DC fault protections,in order to provide theoretical and technical reference for future medium and low voltage DC distribution system-related projects.Finally,this paper combines a current China research status to summarize and give a prediction about the future research direction of medium and low voltage DC distribution system,which can provide reference for the research of medium and low voltage DC distribution system.

Studies of commutation failures in hybrid LCC/MMC HVDC systems

A hybrid of line commutated converters(LCCs)and modular multi-level converters(MMCs)can provide the advantages of both the technologies.However,the commutation failure still exists if the LCC operates as an inverter in a hybrid LCC/MMC system.In this paper,the system behavior during a commutation failure is investigated.Both halfbridge and full-bridge MMCs are considered.Control strategies are examined through simulations conducted in PSCAD/EMTDC.Additionally,commutation failure protection strategies for multi-terminal hybrid LCC/MMC systems with AC and DC circuit breakers are studied.This paper can contribute to the protection design of future hybrid LCC/MMC systems against commutation failures.

Optimal Allocation of Fault Current Limiter in MMC-HVDC Grid Based on Transient Energy Flow

Economical and reliable protection of DC-side shortcircuit faults has become a key technology for promoting the development of module multilevel converters based on the high voltage direct current grid(MMC-HVDC-Grid).The fault current limiter(FCL)can effectively suppress the rapid development of the fault current and reduce the current breaking capacity of the circuit breaker.In this paper,a method based on transient energy flow(TEF)analysis is proposed to optimize the allocation of a resistive and inductive FCL in the MMC-HVDC-Grid.In the proposed method,the electromagnetic TEF is measured first,and then,the TEF suppression rate and suppression efficiency are defined as optimization objectives,and the installation location of the FCL and its impedance parameters as optimization variables.To test the proposed method,two-terminal and four-terminal bipolar MMC-HVDC-Grids with single-pole-to-ground DC faults are modeled in the PSCAD/EMTDC so that the TEF data can be acquired.The optimal FCLs’location and parameter values are determined through investigating the evolution paradigm of TEF along with changes of the FCL position and parameters.The results prove that the selected parameters can effectively slow down the DC fault current rising rate,thus reducing the requirements on tripping current of the DC breakers.

DC fault analysis for modular multilevel converter-based system

DC fault protection is the key technique for the development of the DC distribution and transmission system. This paper analyzes the transient characteristics of DC faults in a modular multilevel converter(MMC) based DC system combining with the numerical method. Meanwhile,lots of simulation tests based on MATLAB/Simulink are carried out to verify the correctness of the theoretical analysis. Finally, the technological difficulties of and requirements for the protection and isolation are discussed to provide the theoretical foundation for the design of dc fault protection strategy.