Research On Coordination Control Strategy For A Typical Hybrid HVDC Transmission System

Line Commutated Converter High Voltage Direct Current(LCC-HVDC) has been widely used in China since 1950s. As HVDC has the advantages of large capacity, long distance, and low power loss, it plays an important role in our gird. In recent years, Voltage Source Converter based on High Voltage Direct Current (VSC-HVDC) with full-control led devices has developed rapidly. It can control the power flexibly and connect to passive network, becomes an important part of multi-terminal transmission direct current and DC grid. As most projects put into operation adopt ordinary thyristor converter vavel, the DC grid constructed on the base will involve LCC and VSC hybrid problem, then it is necessary to make a research on hybrid HVDC system.Firstly, the topology of the hybrid HVDC system is studied, and the electromagnetic transient simulation platform of hybrid HVDC system is developed based on the PSCAD/EMTDC software environment. Then five different kinds operation modes’ starting and steady running characteristic is studied. For the operation mode 2 with four terminal bipolar hybrid whose rectifier is MMC and inverter is LCC, the power flow distribution, calculation and optimization algorithm are analyzed, and a distributed voltage control strategy is proposed based on the optimization algorithm. Finally, aimed at operation mode 2 whose inverter connected to weak ac system is easy to occur commutation failure, this paper designs a new coordinated control method. A Voltage Dependent Voltage Order Limiter (VDVOL) is designed based on the constant DC voltage control at the rectifier side, and a Power Dependent Voltage Order Limiter (VDPOL) is designed based on the constant power control at the inverter side. It is concluded that, the proposed control method has the ability to effectively reduce the probability of commutation failure.