An Improved coordinated control strategy of VSC-MTDC Distribution Network

The rapid development of high voltage direct current (HVDC) and the increasing amount of distributed renewable energy sources are stimulating engineers and researchers to think towards the future of DC grids. Unfortunately, the integration of renewable energy source raised stability concerns amongst others issues [1]. Currently DC technology based on VSC has been recognized as the key technology to construct the future DC power grid[2],[3]. Similar to frequency control in the AC system, DC voltage control is certainly one of the most important aspects to guarantee operation and stability of a VSC MTDC grid. A large amount of research has been done on the control issue of MTDC system. Proposed control strategies present their own advantages and disadvantages. In a master-slave control only one converter station known as master or main converter is configured to operate as constant DC voltage mode, while the others called slaves terminals are set to operate in constant power mode. The implementation of master control strategy is quite simple but an outage of the master converter results in over-voltage or under voltage and subsequently total collapse of the entire MTDC network[4],[5],[6]. Another technique called voltage margin control can be recognized as an improved constant dc voltage control, because it has more than one station in voltage control mode. Once the main dc control VSC terminal is offline or exceeds its limits, another VSC converter will take over the duty of dc voltage regulation at a new value[7], [8] [9]. Unfortunately, this strategy still has some deficiencies, particularly in a large system, because only one converter possesses the duty of controlling dc voltage at a time. Voltage droop control is more reliable because it allows different converter stations to jointly maintain power balance and regulate the dc voltage at the same time; which do not give rise to voltage oscillations associated with the voltage margin method caused by the transition of voltage regulation duty [10], [11], [12]. In [13] an adaptive droop control is proposed based on the dc system operating conditions. In [14] an hybrid control strategy presenting the features of both droop and voltage margin control is proposed, where the main converter is set as constant dc voltage control, and two others are configured as voltage droop and voltage margin control respectively.

Up to now, VSC based DC technology applied in the transmission network has been used a lot throughout the world both theoretically and practically[15], [16]. However, research on MTDC distribution network is far from enough yet all over the world. Structures of distribution network are far more complicated than that of transmission network, which poses great challenge to practice of MTDC distribution network[17]. Current trends indicate that DC distribution network plays an important role in improving the reliability, quality and efficiency of power system in future smart grids and facilitating the integration of distributed energy sources into the grid well[18], [19]. The VSC-MTDC distribution system has small inertia, large power disturbance and many controllable terminals; it is necessary to design a coordinated control strategy with high reliability[20], [21].

In this paper an improved coordinated control strategy of voltage-source-converters multi-terminals dc systems (VSC-MTDC) in distribution network is proposed by combining the properties of droop control, modified double stage voltage margin and undead-band voltage droop control strategies. This strategy is helpful to restrain large power and voltage variations and guarantee the stability of the DC system. Finally simulations under normal and abnormal conditions and comparison to voltage margin control and voltage droop control verify the effectiveness of the proposed approach. To the best of the author’s knowledge, this type of improved coordinated control has never been implemented previously in VSC-MTDC distribution network.