*Article* **Security Assessment and Coordinated Emergency Control Strategy for Power Systems with Multi-Infeed HVDCs**

#### **Qiufang Zhang, Zheng Shi, Ying Wang \*, Jinghan He, Yin Xu and Meng Li**

School of Electrical Engineering, Beijing Jiaotong University, Beijing 100044, China; 16117384@bjtu.edu.cn (Q.Z.); shizheng@bjtu.edu.cn (Z.S.); jhhe@bjtu.edu.cn (J.H.); xuyin@bjtu.edu.cn (Y.X.); mengl@bjtu.edu.cn (M.L.) **\***Correspondence:yingwang1992@bjtu.edu.cn;Tel.:+86-1520-131-5768

Received: 23 May 2020; Accepted: 15 June 2020; Published: 19 June 2020

**Abstract:** Short-circuit faults in a receiving-end power system can lead to blocking events of the feed-in high-voltage direct-current (HVDC) systems, which may further result in system instability. However, security assessment methods based on the transient stability (TS) simulation can hardly catch the fault propagation phenomena between AC and DC subsystems. Moreover, e ffective emergency control strategies are needed to prevent such undesired cascading events. This paper focuses on power systems with multi-infeed HVDCs. An on-line security assessment method based on the electromagnetic transient (EMT)-TS hybrid simulation is proposed. DC and AC subsystems are modeled in EMTDC/PSCAD and PSS/E, respectively. In this way, interactions between AC and DC subsystems can be well reflected. Meanwhile, high computational e fficiency is maintained for the on-line application. In addition, an emergency control strategy is developed, which coordinates multiple control resources, including HVDCs, pumped storages, and interruptible loads, to maintain the security and stability of the receiving-end system. The e ffectiveness of the proposed methods is verified by numerical simulations on two actual power systems in China. The simulation results indicate that the EMT-TS hybrid simulation can accurately reflect the fault propagation phenomena between AC and DC subsystems, and the coordinated emergency control strategy can work e ffectively to maintain the security and stability of systems.

**Keywords:** receiving-end system; multi-infeed HVDCs; security assessment; emergency control strategy; electromagnetic transient (EMT)-transient stability (TS) hybrid simulation
