An efficient simulation approach for long-term stability assessment of electric power systems

Abstract Long-term stability assessments of power systems are traditionally analyzed using root mean square simulation models, whose dynamic behavior is represented by a set of nonlinear electromechanical and controller differential equations. This article presents a simplified root mean square simulation approach derived from the commonly applied root mean square simulation method. This efficient approach is structured based on quasi-stationary assumption of electrical state variables and their associated controllers, which are comparatively faster than mechanical ones. This results in unavoidable algebraic loops at the terminals of the simplified models of active devices, which are solved by integrating their Thevenin equivalent circuits into the Newton-Raphson based power flow calculation method, which is solved in every time step of the dynamic simulation. The assumptions mentioned above lead to a significantly lower model complexity and thus to a considerably lower parametrization effort in large-scale system studies. Furthermore, they enable higher simulation time steps as well as lower computational time and effort. In order to showcase the impact of the underlying simplifications, both methods are programmed in MATLAB and compared with focus on their frequency behavior in the event of a frequency drop. It is shown that the quantities analyzed in the case studies in both simulation approaches (i.e., the center of inertia frequency, terminal active powers, voltages and rotational speeds) exhibit negligible deviations within the very fast transients and especially after they have subsided. This confirms that the simplifications associated with neglecting small electrical time constants are valid for the investigated frequency event.