Key Technology Research And Application Of Modern Power System Short-Circuit Current Practical Calculation

China's economy has been developing rapidly.The social electricity demand,the power grid installed capacity and the transmission volume keeps increasing.These factors cause the power system short-circuit current level to get higher and the operational circuit breaker breaking capacity margin less.Besides,the reactance to resistance ratio in power system is also getting larger because of the requirement of energy efficiency.All these bring new attention to the practical calculation method of short-circuit current in power system for the enhanced requirements of more accurate and effective calculation of short-circuit current,including periodic component and its attenuation,DC component and its attenuation.This paper first summarizes and compares the short-circuit current practical calculation methods and standards,pointing out their shortcomings,e.g.the traditional calculation methods mainly emphasize the calculation of the initial rms of the periodic component of short-circuit current,paying less attention to the attenuation calculation of the periodic component,having no effective and yet simple approaches for the attenuation calculation of DC component.Then,a multi-machine decoupling method and a potential attenuation method for calculating the decay of short-circuit current periodic components are proposed.The frequency domain equivalence method for calculating DC component attenuation is also proposed based on Thevenin's impedance and transfer impedance respectively.In addition,in view of the practices of neglecting the different parallel branches of the power system equivalent circuit in different mainstream power system analysis softwares,the conditions for applying the classical hypothesis to calculate the short circuit current are revealed.On this basis,it is proposed that the complete set of classical hypothesis,not just part of it,should be applied in the practical calculation of short-circuit current.Moreover,the potentials of power sources rather than the working voltage of the fault point before the short circuit fault occurs should be applied along with the classical hypothesis.Finally,the calculation of short circuit current periodic component attenuation and DC component attenuation in Shanghai power grid is completed,and the calculation results are compared with the EMTP results and the real short circuit fault recording results.Based on the results,the adaptiveness of related short-circuit current calculation standard and circuit breaker standard are evaluated and the suggestions is proposed to improve the related work.The short circuit current limiting effect of series reactors in Xingjiang Wubei power grid is also calculated and assessed by the proposed method,and related suggestions is given based on the assessment.The innovative points of the paper mainly manifest in the following aspects:(1)Based on the unified algebraic model(instead of differential model)of the rotary machine(generator and dynamic load),both the multi-machine decoupling method and the electric potential decay mothed for analyzing the attenuation of short-circuit current periodic components with the actual power system structures and parameters considered are proposed.(2)From theory to engineering calculation practice,a frequency-domain equivalence method for calculating DC component time constant of multi-machine power system short circuit current is devised and realized by using AC Thevenin impedance or AC transfer impedances in order to provide the DC percentage and the like to support the circuit breaker breaking capability verification by total short circuit current.(3)The 120 path of DC component in power system short circuit current is analyzed by using the instantaneous symmetrical component theory.Combined with the frequency-domain equivalent of the 120 path,the method for calculating the time constant of unsymmetrical short circuit current DC component is constructed by using the interconnected symmetrical sequence network for the first time.This make the frequency-domain equivalence method successfully cover various unsymmetrical fault.(4)The calculation results of the propsed Thevenin impedance method and transfer impedance method are compared in diffirent application scenario of morden power system.It concludes that the transfer impedance method is more adaptive for three phase short circuit current calculation.But in the case of unsymmetrical fault,the Thevenin equivalent impedance method can achieve the same calculation effect as the transfer impedance method.(5)The propsed short circuit current calculation is implemented using the real metropolitan grid structure and parameters of Shanghai,and the results are proved by that of EMTP simulation and the real fault recording data.Based on the results of the calculation and the result of the fault recording,the adaptiveness of the short circuit current calculation standard and the circuit breaker standard for the morden power grid application is evaluated and the suggestions for improvement are put forward.