A controlled islanding strategy should be carefully designed to prevent power systems from overall collapse caused by extremely serious faults, which is of high computational complexity for large-scale AC/DC power systems. An efficient controlled islanding strategy for large-scale AC/DC power systems is proposed in this paper. The original power system is first reduced to decrease the computational complexity. The reduced power system is then partitioned into a number of dispatch areas by using the breadth-first search algorithm. Next, the reduced power system is recovered to obtain the detailed information of the dispatch areas. The spectral clustering algorithm is used to cluster the large number of dispatch areas into a small number of islanding areas. Finally, the controlled islanding strategy is verified by assessing the steady and dynamic performance of the power system. The feasibility, efficacy, and adaptability of the controlled islanding strategy are demonstrated by the case study of a large-scale multi-infeed AC/DC power system in China.

A DC power support strategy utilizes the flexibility of a High-voltage direct-current (HVDC) system in power modulation to optimize the operating point or compensate the power imbalance caused by a disturbance. The major impediment to the strategy is the difficulty in maintaining DC voltage values at converter stations during the process of DC power support. To overcome the difficulty, a coordinated DC power support strategy for multi-infeed HVDC systems is proposed in this paper. Synchronous condensers are employed to provide dynamic reactive power compensation in sustaining DC voltage values at converter stations. Models are built for the optimal leading phase operation and adjusting excitation voltage reference value of synchronous condensers. Multiple HVDC links are coordinated to participate by using the DC power support factor to rank and select the links. Optimal DC power support values of the participating HVDC links are obtained with a comprehensive stability margin index that accounts for transient stability of the sending-end systems and frequency security of the receiving-end systems. An optimal load shedding model is used to ensure the frequency security of receiving-end systems. Case study results of a provincial power system in China demonstrate the effectiveness and performance of the proposed DC power support strategy.

This paper discusses the zero sequence power directional unit performance of multi-circuit transmission lines on the same tower with different voltage levels in different type faults. The analysis results show that the zero-sequence mutual induction between transmission lines will impact the performance of zero sequence power directional unit and in some conditions, the zero sequence power directional unit will mal operate. The analysis results was proven using RTDS (Real-time Digital Simulator) in 1000kV/500kV quadruple circuit transmission line considering different fault cases.