Research On Short-circuit Calculation Method For Inverter Interfaced Distributed Generator Integrated Into Power System

Distributed generators(DGs) in a distribution network change the power flow and short-circuit current. Protection and reclosing devices are affected by the short-circuit current generated by DGs. With the development of the renewable energy technologies, different forms of distributed generator begin to be embedded in utility grid, because of the fast and flexible control provided by their power electronic devices, inverter- interfaced distributed genera tors(IIDGs) are widely utilized in DG. The basic control modes of IIDGs can be divided into voltage and current control according to the different tracking variables in the inner- loop control. The fault current of IIDG is determined by its control strategy under grid fault. However, the traditional equivalent model of IIDG doesn’t consider the effect of ride-through control and causes limitations on short circuit calculation of power system with IIDG. Therefore, according to the studies of the fault characteristics of current- and voltage-controlled three-phase IIDG, the current source model considered the ride-through control of IIDG is established for short circuit current calculation. And an iterative algorithm based on symmetrical components to determine the sequence components of short circuit current for multiple IIDGs in power system is proposed. The main content of this paper is as follows:(1) According to the ride through control of current-controlled IIDG during the unsymmetrical fault, the current source model based on symmetrical components of IIDG is established for short circuit current calculation. Considering the inverter capacity constraint during low- voltage ride-through(LVRT), the current reference of IIDG is derived. Then according to the sequence currents control and reactive power support of IIDG during the fault, the current source model based on symmetrical components of IIDG is established for short circuit current calculation. The current and terminal voltage responses of IIDGs under different control conditions are compared to investigate the relationship of the short circuit currents with power system and the ride through control. Finally, a model for short circuit calculation for multiple IIDGs in power system was proposed.(2) As for voltage-controlled IIDG, the short circuit calculation model is established according to the transient characteristics under the fault. According to the fault characteristics of voltage-controlled three-phase IIDG and the relatedness between three-phase power and sequence power during asymmetrical fault, a sequence components model for short-circuit current calculation of voltage-controlled IIDG is established. Base on the interaction between IIDG and the sequence network, to derive the variation rule of fault current, and summarize the change trend of IIDG fault current during the transient process. And then a model to determine the fault current for voltage-controlled IIDG in power system is proposed.(3) An iterative algorithm considered the ride through time and the sequence current control to determine the sequence components of short circuit current for multiple IIDGs in power system is proposed. And the correctness of proposed method is verified by PSCAD/EMTDC simulation software. According to the German E.ON and China SGCC grid codes, the influence of ride-through time on short circuit of IIDG under different fault types and connection capabilities are analyzed. And then the calculation method of ride-through time sequence is proposed. Based on the analysis of the relationship of the short circuit currents with power system and the ride through time, according to the model to determine the fault current for IIDG in the previous paper and the method to achieve the node impedance matrix of the unbalanced system, an iterative algorithm based on symmetrical components to determine the sequence components of short circuit current for multiple IIDGs in power system is proposed. Finally, the established model and proposed practical algorithm are applied to balanced and unbalanced distribution networks with multi-IIDG, and the correctness are verified by using PSCAD/EMTDC simulation software.