Rotor Angle Stability of Multiconverter Based Autonomous Microgrid with 100% VISMA Control (Band 84)

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Autonomous microgrids are known to lack appropriate inertia and damping for grid stabilization. Due to this, a virtual synchronous machine (VISMA) has been introduced to provide necessary ancillary services through the control of power converters. In a multi-VISMA (n-VISMA) microgrid, relative rotor angle stability of the power system is dependent on the active power balance after a small perturbation . Using relevant analytical models is an essential issue for microgrid stability analysis. In this PhD dissertation, a comprehensive small-signal stability analysis to study the inherent electromechanical oscillations in the virtual rotors is presented. The subsystems of the microgrid consisting of VISMAs, network, loads and the outer power controller were all modelled in Synchronously-rotating Reference Frame. The small-signal model was tested on IEEE-9 bus system with VISMA replacing the electromechanical synchronous machines on the network. To validate the developed numerical analytics, dynamic responses of the small-signal model are compared with those of the nonlinear system dynamics and the results reveal that the developed linearized small-signal model is sufficient to accurately characterize behaviour of the VISMA microgrid when operated in autonomous mode. Eigenvalues analysis and parameter sensitivities of the critical modes were investigated. Oscillatory participations of the VISMAs and steady state stability limit of the microgrid have also been investigated.
However, before starting the stability analysis of the multiconverter based power system with VISMA control, it is necessary to obtain the steady-state operating points (SSOPs) of all dynamic nodes in the network. Modified traditional iterative schemes using the concept of droop bus technique in an islanded microgrid are not feasible for load flow analysis of VISMA microgrid incorporating non-control dynamics. This dissertation thus proposes a closed-form steady-state, fundamental-frequency models for autonomous/islanded VISMA microgrid using the concept of virtual swing bus. In this technique, the virtual internal buses of all VISMAs in the network are governed by the swing equation. The voltage at all buses is variable except the virtual buses in which the pole wheel voltages are prespecified.