# Impacts of MT-HVDC Systems on Enhancing the Power Transmission Capability
## 1 Introduction  
Recent researches proved that voltage source converter (VSC) based multi-terminal high voltage direct current (MT-HVDC) has a great potential to deal with the challenges of future transmission networks. Along with many advantages, MT-HVDC grids have some promising applications [1, 2]. One of the main applications is integration of offshore wind farms (WFs) to mainland alternating current grids [3] as suggested for European super-grid [4]. Also, the North Sea wind farms can be interconnected with Mediterranean solar and Scandinavian hydropower plants [2]. No charging current exists in MT-HVDC grids as seen in long AC lines. In 2013, Rongxin Power Electronics (RXPE) commissioned the world's first Nan'ao multi-terminal flexible HVDC project in China [5]. In the near future, a lot of new challenges will be faced by power system operators (PSOs) with the provision of flexible and robust controlled infrastructures of MT-HVDC [6]. The consumer-ends' demands like high degree of reliability, security and good power quality will also be the part of those challenges. The system planning and operation is highly affected by the fitful nature associated with the renewable generation, especially with wind power and solar power plants [1]. Hence, the power electronics and power system companies are compelled to design such control schemes to enhance the system security, reliability and efficiency [7]. In [8], a novel transmission system topology of MT-HVDC grids for offshore wind farms is presented. Major hindrance in development of MT-HVDC systems is lack of protection scheme to interrupt DC faults [9; 10]. However, numerous studies have been reported to develop appropriate models and control techniques [11; 12; 13]. The requirements of HVDC circuit breaker parameters and analysis of different types of faults in DC networks are discussed in [14]. Fault ride-through capacity of hybrid line commutated converter (LCC)-VSC based MT-HVDC transmission system is investigated in [15]. A general mathematically derived model, suitable for any topology of DC network, is discussed in [16]. [PERSON] et al. discussed the interaction between MT-HVDC grid and multi machine AC systems in [17]. [PERSON] et al. proposed a novel dc/dc converter for interconnecting multiple DC grids with different voltage levels as well as controlling the power flow between them [18]. In [19], a methodology for droop control in MT-HVDC systems is proposed and the impact of variable droop control technique for independent power sharing during transient conditions is analyzed. [PERSON] et al. proposed a control scheme for MT-HVDC gird by providing a methodology to design coefficients for the DC droop-control [20]. During the large and small disturbances, impacts of the MT-HVDC grid control parameters are discussed by [PERSON] et al. in [21]. In order to provide additional damping to onshore VSC stations, impacts of installing power system stabilizer (PSS) is discussed in [22]. The optimal power flow (OPF) of a combined AC/DC system is explored with a nonlinear model in [23] and [24]. Security constrained OPF (SC-OPF) in an AC only grid is proposed in [25]. [PERSON] et al. presented SC-OPF for two terminal HVDC system with post-contingency control [26]. Calculation of maximum allowable power transfer from generating station to load is done in [27] considering the system stability margins, using particle swarm optimization technique (PSO). A steady-state security region is a combination of active and reactive power injections for which the security constraints and the power flow equations enforced by equipment operating limits are satisfied [28]. [PERSON] et al. presented detailed study regarding definition, model and applications of security regions in power systems [29]. An OPF based control is presented in [30] to minimize the losses in MT-HVDC girds. [PERSON] proposed an algorithm to enhance the linear available power transfer capability by incorporating the reactive power flow [31] but this is limited to AC networks.