An algorithm for accurate switching representation in fixed-step simulation of power electronics

Fixed-step integration methods are widely used in simulation of large power systems. Unfortunately they have some limitations, especially in the presence of hard nonlinearity as produced by switching of power electronic devices. Switching occurs most frequently between time steps and rarely coincide with them. This introduces an error on the switching instant. Without corrections, this error causes jitter and noncharacteristic harmonics. This article presents a simple yet effective method to achieve a very accurate switching representation in fixed-step simulations of power systems. A piecewise integration is performed and interpolated to the exact switching instant. From that point on, a second integration is performed with the usual fixed step using the final conditions present before the discontinuity as initial conditions. Finally, a second interpolation yields the current step results. The method has given very satisfactory results in various configurations such as HVDC converters, PWM inverters and STATCOM devices. Some of these results are presented and compared with those obtained with a precise simulation software based on variable-step integration methods.