Research On Load Shedding Equipment For Under-Frequency

Large power grids, large capacity generators and extra-high voltages are important characteristics of modern power system. Its reliability and economical efficiency have been significantly increased. However, the danger of frequency collapse always exists, and has a great impact for power system running. At present, one of the measures adopted to prevent frequency collapse of power system is loading shedding for under-frequency equipment(LSUF) and separation devices. The concept of "three-defense-lines" has been proposed, and this enhances the status of LSUF equipments. China has used the LSUF equipment since the 1950s. In recent years, along with the development of power system and computer technology, computer- LSUF equipments have been used in a wider range, among which microprocessor-based LSUF equipments are more rapidly developed, and have a good prospective market.This paper studies the impact of the frequency characteristics and frequency offset of power system. The active power and frequency have an interactional relationship, so frequency deviation from a reasonable range will cause a very serious result to the power system and the users. LSUF devices are used to cut the active load to achieve the purpose of stable frequency.In this paper, a centralized LSUF device based on DSP and MCU is designed, with a detailed description of the hardware and software projects and LSUF control logic. The frequency calculation is the core of the device. In this paper, the zero-crossing frequency measurement is adopted. Because the sampling interval is fixed, and therefore the calculation of the phasors will inevitably be affected by the frequency changes. In order to eliminate this impact, this paper presents an adaptive phasor algorithm, and a Matlab simulation is conducted. Simulation results show that the algorithm can meet project needs.The device in this paper have passed the static simulation tests in the NCQTR and RTDS simulation tests in NCEPRI, and the device has been widely used in practical applications.