An incremental transformer model for the study of harmonic overvoltages in weak AC/DC systems

An incremental transformer model suitable for slow transient studies was developed. A typical application is the study of temporary harmonic overvoltages in AC/DC systems caused by transformer saturation. The proposed model represents the nonlinear magnetic circuit of the transformer and is, therefore, more complex that the piecewise-linear models often used to simulate saturation. Despite its complexity, the model formulation is suitable for direct solution at each integration step and this results in reasonable simulation time. The proposed model is particularly suitable for HVDC system studies since only a few elements of the transformer matrix need to be updated at each switching instant to account for the switching action of the converter valves. A study of temporary overvoltages in a weak AC/DC system is presented. Two types of disturbances leading to temporary harmonic overvoltages are simulated using both the piecewise-linear and the proposed incremental transformer model. Comparison of the simulation results indicates that the proposed model can be successfully used for the validation of temporary overvoltage studies conducted with simpler and faster transformer models