A Practical Approach for Short-Circuit Current Calculation in IIDG-Integrated Distribution Grid

With an ever-increasing number of inverter interface distributed generations (IIDGs) plugged into the distribution grids, the short-circuit characteristics within the distribution network have dramatically changed. It is, therefore, necessary to perform certain verifications or recalibrations to existing relay settings. The existing short-circuit current calculation methods considering IIDG require iterative solutions, which are incompatible with the non-iterative algorithms embedded in traditional relay setting calculation software. This conflict can give rise to major difficulties in practical industrial applications. Besides, iterative methods may encounter the problem of slow convergence or even no convergence. To break down these barriers, this work proposes a novel approach based on IIDG’s point of common coupling (PCC) voltage estimation to calculate IIDG-integrated short-circuit current within engineering permissible error range from the perspective of industrial application. The introduced method first constructs an IIDG equivalent model considering inverters' control strategy. To facilitate the IIDG model's embedment in relay setting calculation software meanwhile remaining its consistency with traditional synchronous machine's fault equivalent model, branches including IIDG are transformed into equivalent series connection form of constant voltage sources and impedances. The analytical expression of the impedance is also ascertained. Utilizing the equivalent model, a non-iterative short-circuit current calculation is conducted, demonstrating better performance in computation efficiency and convergence speed and its compatibility with existing methods in relay setting calculation software. Finally, a simulation is performed on the IEEE 33-bus system to illustrate the superiority of the proposed method