Impedance-Based Analysis and Stabilization of Active DC Distribution Systems With Positive Feedback Islanding Detection Schemes

Active dc distribution systems are gaining widespread acceptance inmodern power distribution grids. Islanding detection is very crucial for safety and protection purposes in active distribution systems; therefore, distributed generators (DGs) are usually equipped with active islanding detection methods to detect grid disconnection conditions. The high penetration level of tightly regulated converters to interface both DGs and loads and the poorly damped LC networks structured by the filtering inductors, feeder impedances, and bus capacitors can cause severe stability problems. This paper presents an impedance-based analysis of a grid-connected dc active distribution system, where DGs equipped with active positive feedback islanding detection schemes and a high penetration level of constant power loads (CPLs) are considered. The output impedance of a DG equipped with active islanding detection schemes is derived, and the interactions of the system impedances are discussed to characterize the dynamics of the dc distribution system. Moreover, the performance of multiple DG systems with the islanding detection schemes is investigated and thoroughly addressed. A simple, yet effective, stabilization method is also developed. Detailed time-domain nonlinear simulations and experimental results validate the analytical results.