Assessment and compensation of dynamic instabilities in voltage source converters connected to weak AC grids and microgrids

This thesis addresses the integration of three-phase dc/ac voltage source converter (VSC)-interfaced dc sources and loads into the three-phase ac grids or microgrids under very weak conditions. VSCs are the dominant topologies in high-power electronics due to their excellent performance and their common control method is voltage-oriented control (VOC). Motivated by the VSC industrial popularity on one hand and the issues yielding from dynamic interactions between the VOC-based VSC and the weak grid (WG) or weak microgrid (WMG) impedances, four situations are considered: VSC connected to a very weak grid (VSC-WG) in the inversion mode; VSC-WG in the rectification mode; passive load (PL) connected to a very weak microgrid (PL-WMG); and VSC connected to very weak microgrid (VSC-WMG). The dynamic model of the VSC-WG system is derived in the direct-quadrature reference frame (dq-RF) which serves as the basis for the study. Using the VSC-WG dynamics, the small-signal model of the system is developed in the standard state-space form which makes it possible to obtain the eigenvalue spectrum of the system. Using the eigenvalue analysis, the small-signal stability of the system is investigated under different operating points, grid impedance values, and control parameters variations. It is found that complex unstable modes are present in the eigenvalue spectrum of the system under the nominal conditions of the VSC-WG in both inversion and rectification modes as well as WMG in islanded mode. Linear active compensators are proposed that utilize the VSC output variables as inputs and integrate them into the primary control system of the VSC without adding extra measurement sensors or interfering with the VSC original control design. Moreover, these active compensation methods are lossless and do not change the VSC steady-state values. The design of the compensators is provided in detail using the state-space approach. It is shown that with proper design of the linear compensators, the instabilities are completely resolved under nominal conditions and the system dynamic performance under different conditions is improved. Computer simulations and real-time simulation (using hardware-in-the-loop platform) results are provided to verify the proposed methods and validate the theoretical findings