High voltage transmission lines, especially high-voltage direct-current (HVDC) lines, have been built around the world linking the populated load centers with major green energy resources at a far distance or offshore. Any major faults on these long-distance point-to-point connections may lead to large power unbalances and cause frequency security problems in the regional grid at the energy receiving end. In this work, an emergence frequency control strategy is proposed, utilizing available frequency response resources at the receiving end, including synchronous generators, local wind farms, battery-based energy storage systems, pumped hydro storages, etc. First, a system frequency response model consisting of multiple resources is formulated, including the secondary frequency drop caused by wind farms. Then, the order of the model is reduced to derive the analytical expressions of the frequency nadirs and the quasi-steady-state frequency. Finally, the emergency frequency control decision method is proposed with the consideration of the system frequency security constraints. The proposed strategy is studied and analyzed with simulations on a generalized model of Huadong regional grid in eastern China.

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, effective 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 efficiency 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 effectiveness 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 effectively to maintain the security and stability of systems.

The shortage of fossil fuels, wind power generation and energy storage equipment are developing so rapidly that make the large-scale energy storage equipment become an trend in the future grid. Seeing the wind-storage system as a whole, the transmission network planning will be inevitably affected by its output mode change. In this paper, a transmission network planning model which considers the influence of the wind-storage system will be established. An optimal operation mode of the wind-storage system will be calculated by the wind plant data from the Eastern Mongolia (according to the peak valley electricity price), and maximizing the energy conversion efficiency. Artificial Fish Swarm Algorithm (ASFA) and the improved Garver 6 node system are used to verify the transmission planning model which is included the influence of the wind-storage system. When the unit price of the energy storage equipment is determined, the corresponding optimal plan of different capacity coefficients is obtained, and the optimal capacity coefficient and the transmission planning under the different unit price are given when the unit price of the energy storage equipment is uncertain. The example shows that it is feasible to add energy storage equipment within a certain price range to effectively improve the economy of transmission network planning.