Research On Power System Frequency Stability Control Strategies Under Major Disturbance

Due to the rapid development of renewable energy generations(REGs)and ultra high voltage direct currents(UHVDCs),the traditional thermal generators are being replaced,and the characteristics of power systems are changing gradually.The frequency dynamic response becomes more drastic but the controllable resources are reduced,which poses huge challenges on frequency stability control.On one hand,the uncertainty and complexity of power systems increase,and the severity and probability of underlying active power deficits deteriorate.On the other hand,the power system equivalent inertia declines.Moreover,the power reserve of primary frequency regulation(PFR)decreases,and the influence of thermal states and other factors on PFR capability becomes more prominent.Additionally,the new polices on power system accidents reduce the redundancy of load shedding,which further brings down the controllable resources for frequency stability.In order to deal with the aforementioned challenges,ensure the frequency stability of power systems,and sustain the continuous accommodation and safe operation of REGs and UHVDCs,the study of control strategy for frequency stability under large power deficits is of great significance.In this context,the dissertation focuses on enhancing the frequency control of generators and the load shedding for frequency stability.More specially,the dissertation carries out studies on the assessment of PFR capability,the frequency control of generators,and the load shedding for frequency stability.Furthermore,the main contents and contributions of the dissertation are summarized as follows:(1)Aiming at the assessment of PFR capability,an assessment approach for PFR capability considering boiler thermal states is proposed.Firstly,an extended system frequency response(ESFR)model incorporating thermal states is established,on which a linear and low-order time-domain expression in the time scale of PFR is derived to depict the frequency dynamic response under large power deficits.Secondly,the index of equivalent speed droop coefficient(ESDC)is constructed to quantify the PFR capability with boiler thermal states.From perspectives of the instantaneous and cumulative characteristics of frequency dynamic response,a quantitative framework is provided to analyze the influence of thermal states on PFR capability.Then,based on the ESFR model,an assessment approach for PFR capability is presented.The PFR capability is promptly estimated via the ESFR model and wide area measurement systems(WAMSs).Furthermore,based on the ESDC index,an optimal adjustment approach for PFR capability is presented.An optimization model for the coordination between PFR and under-frequency load shedding(UFLS)is established using the ESDC index.The case study analysis results indicate that the proposed assessment approach is effective to reduce the estimation errors of PFR capability and consolidate the foundation for frequency stability analysis and control.And the proposed optimization approach is effective to coordinate PFR and UFLS,and avoid the unanticipated activation of UFLS.(2)Aiming at the frequency control of generators under large power deficits,an event-driven power advanced control(EPAC)approach is proposed.Firstly,an EPAC control framework is established and a multi-level control scheme for the output power of generators is designed.The contingency is detected to activate the control strategy at a specific level,and the output power of generators is regulated using the information from WAMSs.Then,based on the model prediction control(MPC),the first-level EPAC strategy is presented.With the objective of intercepting frequency decline and substituting the passive UFLS,the receding horizon optimization of total EPAC power is carried out via MPC,and a multi-step optimization model is built to distribute the EPAC command to each generator.Furthermore,based on the frequency trajectory sensitivity(FTS),the second-level EPAC strategy is presented.With the coordination of EPAC and UFLS to intercept frequency decline,the total EPAC power is optimized via FTS and optimally distributed to each generator.The case study analysis results indicate that the proposed EPAC approach is effective to make full use of power reserve,and coordinate with UFLS to improve frequency dynamic response,and reduce the amount of load shedding.(3)Aiming at the coordinated load shedding for frequency stability under large power deficits,an event-characteristic-driven load shedding(ECLS)approach is proposed.Firstly,a quantitative framework combining the frequency and voltage dynamic response is established.The dynamic response pattern of receiving-end power grid under UHVDC blocking is analyzed,from which the typical event characteristic is extracted and derived theoretically.Then,a load shedding scheme of the emergency stage for ECLS is designed to coordinate with the emergency load shedding(ELS)and UFLS,and make up for the underlying blank interval of frequency stability control.The gray correlation analysis(GCA)method is utilized to capture the information of frequency and voltage dynamic response,and the k-nearest neighbor(KNN)method is adopted to identify the disturbance scenarios.And the emergency stage is set to be activated promptly once the UHVDC blocking is detected.Furthermore,based on the FTS,a decision-making approach for the emergency stage is presented.An optimization model of multi-type and multi-level load shedding is established and solved by FTS to determine the control strategy.The case study analysis results indicate that the proposed ECLS approach is effective to coordinate with the ELS and UFLS to improve frequency dynamic response,and reduce the amount of passive load shedding.