Autonomous converter control in a multi-terminal HVDC system

With the development and increasing availability from multiple vendors of Voltage Source Converter (VSC) technology the prospect for multi-terminal HVDC links, commonly referred to as ´HVDC Grids´, composed of multiple converters has become a realistic possibility. Moreover, in such HVDC Grids the power flow into, or out of, each converter can be dynamically changed without any reconfiguration of the HVDC grid. With multiple, parallel converters supplied by different vendors a control scheme is required which provides the required power flow at each converter. One method which has been proposed involves one master station controlling the DC voltage on the link and all other converters operating in current control. However, in the event of disturbances to or even the complete loss of the master station very fast inter-station communications and control is required in order to transfer voltage control to another converter station. Such a solution becomes reliant on inter-station telecommunications. In this paper an alternative approach will be presented, utilising a slope characteristic to allow parallel converters to operate independently on a common DC link, similar to the manner already employed with AC power generators, without the need for inter-station communications.