# Coordinated Control Strategy of CU-MTDC under Abnormal Conditions Considering Power Supply Security
## 1 Introduction
The simulation results show the method can effectively reduce the frequency fluctuation. The above research optimized the operation mode of MTDC through different control objectives. However, the power supply security of the AC system has not been given special attention and is usually introduced into the optimization process only as an inequality constraint. Therefore, considering the power supply security, a better-optimized operation mode of MTDC is obtained by the security-constrained optimal power flow (SCOPF) method. In [18], improved corrective security-constrained OPF for a meshed AC/DC power grid is proposed. The security constraints of N\(-\)1 contingencies for both AC and DC networks are fully considered. The results show that this method can be applied to middle and long-term schedules. In [19], a hierarchical SCOPF model is proposed for an AC/DC system. The MTDC system could provide support for the AC system by redistributing power flow across the entire grid. Through the above methods, the power supply security of the AC/DC system is fully considered. However, due to the large-scale power grid and too many optimization factors, this kind of method cannot be applied in real-time control because of the complicated calculation process. Therefore, a more direct calculation method is proposed for real-time control in [20]. In this paper, active power sensitivity is used to calculate the redistribution of DC unbalanced power after DC fault. The methods can guarantee power supply security by distributing the DC unbalanced power among all the normal converters. However, the influence area of DC fault on the AC system is expanded. In addition, the detailed coordinated control strategy is not proposed to cooperate with the calculation method, and parameter design of controllers is not involved. In general, the drawbacks of the existing approaches are described as follows:  
1. The DC unbalanced power distribution method usually adopts an optimized calculation method using global data. There are generally many optimization factors in this kind of calculation method, and the calculation process is very complicated. Therefore, it is difficult to apply the existing approaches to real-time control;2. The DC unbalanced power distribution strategies usually adopt all the normal converters to participate in the regulation, which inevitably leads to the expansion of the influence range of a DC fault on the AC system. To solve the problems in the exiting approaches, a novel coordinated control strategy of CU-MTDC under abnormal conditions is proposed in this paper. Compared with traditional methods, the proposed method offers the following features and improvements: (1) Based on the priority control idea, the control thresholds of different priorities are determined by the magnitude of the DC unbalanced power value and the maximum regulation capacity of the converter. The calculation and control process is very simple, which is easy to realize real-time control; (2) The number of the regulation converters is dynamically scheduled according to the DC unbalanced power value, instead of all the converters involved in control. With the proposed method, the influence range of DC faults on the urban power grid can be limited in a sufficiently small area; (3) Moreover, the parameter design of controllers is proposed to cooperate with the distribution strategy, which is very friendly to practical engineering realization. The rest of this paper is organized as follows: Section 2 describes the basic system structure of CU-MTDC. In Section 3, the coordinated control strategy of CU-MTDC under abnormal conditions is proposed. The results of the simulations are performed in Section 4. Finally, Section 5 concludes the paper.