Power transformers are critical components of the electricity networks. It is common knowledge that transformer energisation causes a number of slow-front inrush current transient problems. With the increase of distributed generations with inherent high intermittency resulting in more switching events, the transformers in service are increasingly vulnerable to electrical transients. This thesis is concerned with transformer energisation inrush, particularly focussing on the sympathetic inrush in supply networks with parallel-connected transformers. To perform the investigation, transformer models are developed using MATLAB/Simulink, and simulations are also carried out using PSCAD/EMTDC software. The study is initiated with the modelling using the classic Steinmetz model. Subsequently, in order to improve the accuracy, the Jiles-Atherton model is considered whereby its parameters are based on magnetic quantities and computed via a series of differential evaluation algorithms. The results demonstrated a significant improvement of accuracy of the transformer model. The findings obtained from simulation are then validated with laboratory experiments. One, two, and/or three single-phase 16 kVA, 11kV/250V oil-immersed distribution transformers are tested to examine inrush transients under different energisation cases. Special focus is placed on the first inrush peak, to determine whether or not the level of the incoming inrush can be predicted. This thesis also analyses the sympathetic inrush prolonging effects on voltage sags. The impacts of flux density and system resistance as well as the number of simultaneously energised transformers are also investigated. The contributions of this research include the development of an improved transformer model for simulating the sympathetic inrush using a combination of the classic model and Jiles-Atherton hysteresis model, the proposal of measurement methodology to extract model parameters for transformers with only nameplate data, the design and implementation of a point-on-wave switch, the laboratory works on mitigating parallel-connected inrush transients, the assessment of the effects and characteristics of sympathetic inrush through observing waveform patterns and the prediction of the incoming peak inrush, and the analysis of voltage sagging. Also, this research examines the growing Australia’s wind energy development and demonstrates the emerging problem of sympathetic inrush in parallel-connected wind turbine transformers. All the above research contributions are achieved with the completion of this thesis.