Short-Circuit Current Calculation Method Based On Doubly-Fed Induction Generator

In recent years wind power gets rapid development in the world, and the proportion of wind power in the electric energy is higher and higher. Both grid size and single machine capacity are greatly improved, so the impact of wind power system for the grid has can’t be ignored. When the interconnection capacity of wind power reaches a certain value, it needs to consider the short-circuit current to the grid provided by wind turbines. Therefore, it’s necessary to study the steady-state characteristic, fault-transient characteristic and short-circuit current calculation method of wind turbines in wind farms. When choosing and calibration equipments of the grid or setting and configuration the relay protection, the short-circuit current provided by wind turbines should be considered.At present, the variable speed constant frequency (VSCF) wind power generation system that composes of doubly-fed induction generator (DFIG) has become the development trend of wind power. Firstly, the structure composition and operation principle of the VSCF wind power generation system were introduced in this paper. This paper set up mathematic models of different parts of the doubly-fed wind power generator, derived of steady-state and transient-state mathematical equations of the DFIG under different coordinate system and selected equivalent simplified method to analysis of short-circuit calculation. Rotor side and network side control sections of the DFIG were studied by using the steady-state mathematical equations, and control strategies based on PWM converter control system were studied. Under the grid short-circuit fault, the paper detailed the working principle and implementation method of the protection of rotor side, stator side and dc side.Based on the analysis of factors influencing the transient-state characteristics of the DFIG, the electromagnetic transient mechanism of stator and rotor flux, voltage and current under symmetrical and asymmetrical short-circuit were further researched by using the transient-state mathematical equations. Expressions of short circuit current of the DFIG under different fault conditions were proposed, which were expressed by stator and rotor flux, attenuation coefficient. The initial value of stator flux was calculated, considering the influence of stator resistance, and the rotor flux equation under steady-state condition was used to calculate the initial value of rotor flux. Considering the influence of the crowbar protection circuit, transient expressions of stator and rotor flux were analyzed under symmetrical and asymmetrical short-circuit, then detailed equations of the stator short-circuit current of the DFIG could be obtained.Secondly, operational curves method is used to simplify the stator short-circuit current expression of the DFIG It led to get short-circuit current calculation method, which met the needs of engineering calculation. This method simplified and equaled power system containing wind turbines and made the power system into two branch network by generator branch and system branch. The paper illustrated calculation method of different components (power frequency component, rotor frequency component and dc component) in the short-circuit current, drew the operation curve of wind turbines short-circuit current power frequency component under different fault types and proved the effectiveness of the method by actual example.Thirdly, Dig SILENT/Power Factory was used to establish wind farm model and we could got short-circuit current curve of simulation results. MATLAB was used to implement programming calculation and we could get calculation results. By comparing the maximum short-circuit current and the maximum moment, it showed that it was efficiently to use expressions of short-circuit current proposed by this paper to calculate the short-circuit current of the DFIG The effectiveness of the expression was proved by simulation and calculation example.