PHENOMENON OF SYMPATHETIC INTERACTION BETWEEN TRANSFORMERS CAUSED BY INRUSH TRANSIENTS

In power systems with appreciable resistance, the transformers already connected to the supply system can experience unexpected saturation during the inrush transient of an incoming transformer. This saturation, which is established by the asymmetrical voltage drop across the system resistance caused by the inrush current, demands offset magnetising currents of high magnitude in the already connected transformers. This generates a transient interaction between the transformers, a 'sympathetic interaction', that affects the magnitude and duration of the inrush current. As a consequence, problems in the operation of the system, such as false operation of transformer differential relays and prolonged temporary harmonic overvoltages, can occur. The sympathetic interaction between the transformers is analysed in the paper. System configurations with transformers in parallel and in series are investigated. The transformers and system models have been validated using small transformers.