Impact of Internal Algebraic Variable Treatment on Transient Stability Simulation Performance

It is a general notion that, in transient stability simulations, reducing the number of algebraic variables for the differential-algebraic equations (DAE) can improve the simulation performance. Many simulation programs split algebraic variables internal to a dynamic model from the full DAE and evaluate them outside each iterative step, using results from the previous iteration. The updated internal variables are then treated as constants when solving for the current iteration. This letter discusses how such a split formulation can impact simulation performance. Case studies using various systems with synchronous generator and converter models demonstrate the impact of the split on the convergence pattern and simulation performance.