| Frequency is an important index of power quality to characterize the safety and stability of the power grid.It reflects the active power imbalance between generation and load.As the last defense line of power system,under frequency load shedding(UFLS)is the measure to tackle with accidents that may cause extremely low frequency and even system collapse.In order to arrest the frequency drop and restore the frequency in time,the excess load must be cut off.In recent years,the frequency regulation ability of power system has reduced as the large number of conventional energy generation gradually replaced by new energy generation.However,due to the construction and operation of large-capacity long-distance transmission projects,the danger of serious disturbances with active power imbalance still exists in power system.Therefore,it is of great significance to study the UFLS scheme with higher adaptability and reliability.With the development of smart grids,the condition of controllable loads represented by temperature-controlled loads and electric vehicles participating in frequency control has been maturing.Therefore,some loads can be controlled continuously and it is possible to improve the UFSL with continuous loads.A load shedding strategy proportional to frequency deviation is proposed in this article based on the study of the traditional schemes.The tuning algorithm and implementation methods of the strategy are also formulated in this paper.Cases are demonstrated to validate the proposed scheme.The main research work and results are as follows:(1)A load shedding strategy proportional to frequency deviation is proposed for the problems caused by load shedding stages of traditional schemes.The discontinuity of total load tripping amount is the major factor causing under-shedding or over-shedding.To solve this problem,an UFLS strategy with controllable load is proposed in which the amount of shedding load is proportional to frequency deviation according to shedding factor KLS.The calculation results of the established single-machine continuous load shedding model verified that it is no risk of over-shedding in the implementation of proposed strategy,and the amount of load shedding is increasing with disturbances and KLS.By studying the influence of frequency threshold,time delay,and spinning reserve on the control performance of proposed scheme,a complete continuous UFLS model nonlinear links is established in a single-machine system based on the proposed strategy.(2)An optimization tuning algorithm of shedding factor KLS for UFLS control requirements and an implementation of the proposed scheme in multi-machine system are proposed.According to the frequency control characteristics of the proposed scheme,the KLS optimization tuning model is established with the constraint condition of UFLS requirements and the goal of minimum the amount of load shedding.The model calculation results show that it is easy to find of the critical KLS value of the constraint as the optimization result when spinning reserve reaches its minima and generation deficiency reaches its maxima.By analyzing the space-time distribution features of dynamic frequency responses in multi-machine systems,a specific implementation of continuous UFLS scheme in multi-machine systems is proposed.(3)The adaptability of the proposed scheme is verified by simulation analysis,and the scheme can be applied to different systems by outgoing lines control and additional stage separation.A large number of simulations are performed by the power system simulation software on the IEEE 3 9-bus system.Simulations show that the proposed scheme can well perform under various possible circumstances.The proposed scheme is highly adaptable with different types of faults,load levels,active power imbalance and spinning reserve.And then,an improved implementation by outgoing control is proposed for systems without controllable load.An improved control of additional wheels is proposed for systems with different requirements of steady-state frequency. |
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