Conversion of RMS into Instantaneous Transformer Saturation Characteristics – Implementation in MATLAB/SPS-ST

The paper presents a reduced form of the algorithm for converting a measured RMS transformer characteristic into the corresponding peak instantaneous transformer saturation curve. The peak instantaneous saturation curve is an essential input for stationary and electromagnetic transient simulations of power systems such as harmonics, subharmonics, inrush currents, ferroresonance, load rejections and similar. The conversion algorithm was implemented in Saturable Transformer Block inside MATLAB/SPS-ST software. The existing transformer model in SPS-ST implies the need of knowing the saturation characteristic, which is generally absent in practice. Consequently, the RMS transformer characteristic was offered as an alternative to the actual core saturation characteristic in Saturable Transformer Block. A novel Saturable Transformer Block design tool, consisting of a graphical user interface, allows for a choice of either old saturation or novel RMS saturation characteristic. The developed transformer model with embedded conversion procedure was tested on an application example of transient and steady-state single-phase transformer simulation. It was exhibited that a novel Saturable Transformer Block produces identical simulation results to an actual Saturable Transformer Block. The new SPS-ST transformer model was further validated by laboratory measurements. The results of the transformer inrush current and steady-state current simulation and measurement showed a very good matching ratio. The developed algorithm for conversion of RMS into peak saturation characteristic applies to modeling and simulation of steady-state and transients in electrical systems, where voltage and current transformer, iron-cored reactors, nonlinear coil and electromagnetic ballast are the dominant elements.