Stability Analysis of Unbalanced Distribution Systems With Synchronous Machine and DFIG Based Distributed Generators

There are many technical aspects and challenges in distributed generation (DG) that have not been properly understood and addressed so far. Distribution systems cannot be considered as balanced three-phase systems, because these are inherently unbalanced in steady-state operation. A full characterization of the unbalanced system with respect to system stability allows a better understanding of the dynamic behavior of such systems. This paper presents a comprehensive investigation of the effects of system unbalance on the stability of the distribution systems with synchronous generator (SG) and doubly-fed induction generator (DFIG) based DG units at different loading levels. Detailed steady-state and dynamic analyses of the system are performed. Based on classical voltage, small-perturbation and transient stability studies, it is demonstrated that system unbalance can significantly affect the distribution system dynamic performance, in ways that have not been discussed in the technical literature so far. A simple and effective control strategy based on an Unbalanced Voltage Stabilizer (UVS) is also proposed to improve the system control and the stability of unbalanced distribution systems with SG and DFIG. Eigenvalue analyses and time-domain simulations (TDS) demonstrate the effectiveness of the proposed UVS for unbalance conditions.