WAMS-based Under-frequency Load Shedding Research

Under-frequency load shedding(UFLS) ,one of the most simple and effective ways to prevent power system from frequency collapse after disturbances causing dangerous imbalance between load and generation, is widely adopted in present power systems all over the world. Nowadays, in modern power systems, larger generation unit installation and larger capacity of power transmission over longer distance are becoming the main features of power systems. All these bring a new challenge to power systems to maintain frequency stability. With the implementation of PMU-based wide-area measurement system (WAMS) in power systems, further study on frequency stability control using system-wide information is becoming an important subject in power systems.In this thesis, firstly the significance and progress of UFLS are introduced, and at the same time the application of PMU in power systems is presented.Secondly, the theories, including the concept of power system frequency, power system frequency static and dynamic responses, the process of frequency collapse and the relation between frequency and voltage, are discussed comprehensively, which give a solid foundation for UFLS design.Thirdly, two typical UFLS schemes, which are triggered respectively by the frequency measurement and/or the rate of change of frequency, are introduced in detail. Subsequently, a constructive comment on these two UFLS schemes is made to provide some useful idea for WAMS-based UFLS design.Finally, a novel WAMS-based UFLS scheme is proposed in detail. The UFLS scheme is based on the understanding of the magnitude and localization of disturbances. Through the real-time estimation of the magnitude of disturbances that occurred at power system, a steady-state frequency is estimated, and subsequently the amount of load shedding is calculated with the constraint placed by target frequency. By compensating for the generation unit of which the frequency dynamic response is the most intense, the localization of load shedding is determined.