# Coordinated Control Strategy of CU-MTDC under Abnormal Conditions Considering Power Supply Security
## 1 Introduction  
With the rapidly increasing urban power load, the security of coastal urban power (CUP) supply is facing challenges [1] because of the shortage of transmission corridors and limitation of short circuit capacity [2; 3]. Meanwhile, the CUP grid security has further deteriorated as a result of the large scale integration of offshore wind power [4; 5]. High voltage direct current based on voltage source converter (VSC-HVDC) technology is recognized as an advanced approach for efficient and stable operation in future CUP grid [2], due to their high transmission capability [6; 7], the flexibility of power dispatch and no short circuit current limitation et al. [2; 8]. Up to now, several point-to-point VSC-HVDC projects [2; 3; 9] have been in operation or planned for the CUP grid. With the enlargement of the CUP grid, more than one point-to-point VSC-HVDC systems in different areas will be put into operation in the same CUP grid. To improve system flexibility and reliability, VSC-HVDC systems can be interconnected and developed into a coastal urban multi-terminal DC (CU-MTDC) system. When MTDC is applied in the CUP grid, the main control objective is to relieve the transmission stress of the critical AC lines and improve the power supply security. Meanwhile, to realize the local absorption, offshore wind power further affects the security of the power supply to the CUP grid. Therefore, taking the power supply security into account, coordinated control strategies of the CU-MTDC are very important. In recent years, as a new research field, some researchers have begun to pay attention to the application of VSC-HVDC in the urban power grid. In [10], the authors proposeda locating and sizing method of interconnected converter stations. According to the method, the optimum location and capacity of the converter station are well determined. However, the control strategy of the DC system is not involved. On the basis of the [10], the detailed control strategy of a back-to-back VSC-HVDC interconnected two load areas of the urban power grid is proposed in [11]. In this paper, the proposed method includes load-balanced control, emergency support control and static voltage control. The results verify the effectiveness of the proposed control strategy. In [12], a coordinative control model that combines the load transfer capability of the HVDN and flexible and precise control of VSC is proposed to alleviate the urban transmission congestion. Although the above two papers have studied the control of VSC-HVDC in the urban power grid, the research project is the two-terminal DC system, and the multi-terminal DC system is not mentioned. Reference [13] focuses on a multi-voltage urban DC system and proposes a comprehensive control strategy that included free operation mode, limit operation mode and layer operation mode. Based on [13], a coordinated operation strategy for DC grids applied to the multi-regional urban power grid is studied in reference [14], and the coordination between multiple DC subsystems is analyzed. However, the influence of the AC system is not considered in the proposed coordinated control strategy, especially the power supply security. Based on the above analysis, the research on urban MTDC needs further attention. Compared with the research on urban MTDC, considering the influence of the AC system, the coordinated operation mode and control strategy of MTDC in large power systems have been extensively studied, which can be used for reference. In [15], the paper considers the OPF of a meshed AC/DC system with MTDC networks. The OPF problem is formulated to minimize the transmission loss of the whole AC/DC network. In [16], an extended OPF model with a detailed MTDC is proposed. And then, a cost-benefit analysis approach using the model is proposed to determine the economy of the VSC project. In [17], the paper focuses on the frequency security of the AC system after DC fault and proposes a distribution method of DC unbalanced power.