Power System Frequency Stability Mechanism And Coordination Control With Large Frequency Deviation Scenario

It is of significance to maintain power system frequency within an acceptable range for the system security and stability operation.A number of power system blackout events at home and broad have shown that the regional power grids may be decomposed into isolated regions with unbalanced power generation and load due to large-scale wind power and photovoltaic off-grid or cascading failures.This situation may result in large frequency deviations or even lead to system frequency collapse in isolated power grids.The failure of the large interconnection power grid also leads to serious active power disturbance.With the gradual increasing of the proportion of the renewable energy generating units and HVDC transmission participating in power system,the form and structure of the power network is undergoing a process of the change from quantity to quality.The characteristics of the low inertia,the low controllability and the potertial large active power disturbance lead to prominent system frequency stability problems.Driven by the development of power electronics power supply and HVDC transmission,the dynamic frequency response of power grid becomes more complex and intense,which increases the risk of large frequency deviations,frequency instability and collapse and brings new challenges to power system frequency security and stability analysis and control of.Most of the research work related to the security and stability of power system frequency mainly focuses on obtaining system frequency response curve.The conventional frequency response model usually assumes that the frequency deviations are not large and the effects of frequency deviations on boiler thermal state and the output of the auxiliaries are ignored.However,large frequency deviaitons directly affect the output of feedwater pump,induced draft fan and the other auxiliaries,which will further affect power system frequency characteristics.At present,few studies involve the quantitative assessment of frequency stability and the mechanism analysis of frequency stability is unclear,so it is unable to know the stability degree of the system.Generally,the frequency security and frequency stability of power system are not differentiated strictly in the study and frequency stability analysis is mostly based on frequency deviation security assessment.Frequency stability assessment often uses qualitative methods or approximately uses transient frequency deviation assessment to instead of the authentic frequency stability margin.The connotation of system frequency security and stability is lack of insightful analysis,as well as the differentiation and elaboration of their essence.The mechanism of system frequency stability and its quantitative analysis are still a relative weak link.The paper studies the frequency response modeling,frequency stability mechanism and its quantitative analysis and optimal control strategy problems under frequency wide-range fluctuations scenario.The main innovative achievements of this thesis are as follows.(1)Aiming at the problems that the conventional frequency response model is not accurate enough to simulate the frequency characteristics in the case of large frequency deviation,a detailed active power and frequency response extended model is constructed firstly,considering the effects of frequency deviations on the boiler thermal state and the output of the feedwater system,which improves the adaptability of the conventional frequency response models.Secondly,the frequency deviations affecting the output of the boiler and auxiliaries and then resulting in system frequency instability mechanism is revealed systematacially based on the extended frequency response model.Then,the extended frequency response model of thermal power unit is established and programming realization for PSS/E that is widely used in power system simulation program in the world.The frequency dynamic simulation and analysis function of PSS/E when power system experienceslarge frequency deviationis extended and the calculation accuracy after large disturbance is improved.(2)Aiming at the problems that the frequency security and stability is not differentiated clearly and frequency stability mechanism is lack of effective analysis method,the static power-frequency characteristic curves of the generating unit and load having two intersections is revealed within large frequency variation range firsly.Secondly,the stability properties of the two equilibrium points are systematically analyzed theoretically and the frequency static unstable equilibrium point is defined.Then,the static frequency stability margin index is propsed and the quantitative assessment methods of power system frequency stability are given.Furthermore,the essence and connotation of system frequency security and frequency stability are expounded.Frequency security and frequency stability are clearly distinguished from physical mechanism,which enriches the throretical system of power system stability analysis.(3)Aiming at the problems of frequency stability coordination control of the sending-end system under the large disturbance,based on frequency security and frequency stability margin index,a mathematical model of frequency stability coordination control is estabilished.In order to simplify the solution process of the model,a decomposition-synthesis coordination optimization strategy is proposed for the total generator tripping quantity submodel of the first round and the optimized setting submodel of generator tripping quantity of each basic round.The solution is based on the trajectory sensitivity to improve the adptability of the decision scheme.A method combining the electric distance between the generating unit and DC drop point,the output of the generating unit and the greedy algorithm model to solve the knapsack problem is used to optimize the generator tripping combination scheme.In this paper,an extended model of the active power and frequency response of thermal power unit is established considering the power-frequency characteristics of feedwater system,which improves the adaptability of the conventional system frequency response model.The static frequency stability margin index and quantitative assessment method are proposed,which provides quantitative information for frequency stability coordination control and comprehensively evaluates the frequency security and stability level of power grid under give large disturbance.Based on frequency security and frequency stability margin index,a mathematical model of frequency coordination control in over-frequency scenario is established and a decomposition-synthesis coordination optimization strategy is further proposed to simplify the solution,which improves the adaptability of the decision control scheme.System frequency stability research is from frequency response calculation,frequency stability analysis and frequency stability coordination control,which enriches the theory of frequency analysis and control,improves system frequency response model and the level of power system frequency response numerical simulation,enriches power system stability analysis theories and promotes the progress of the frequency control technology.It provides a useful theoretical support for the construction of a new type power system.