Innovative compensation methods for large voltage drop effects in very long HVDC transmission systems

This paper discusses the impacts of large voltage drops on very long HVDC transmission lines, especially when operating in a monopolar mode. The effect of using dedicated metallic return (DMR) conductors is discussed. The main focus is on the potential rise at the neutral point of the converters and the need for an increased range for the converter transformer tap changers. Two grounding schemes for multiterminals transnational HVDC systems are presented: single point low impedance grounding and high impedance multi-point grounding. The advantages of the high impedance scheme are discussed. An innovative compensation method for voltage drop effects using a series HVDC converter is presented along with simulation results. The novel series converter can address both issues of neutral voltage rise and the impractical tap changer range. To put the ideas into the perspective, a realistic example of a transnational HVDC connecting China, central Asia, Russia and West Europe is presented.