Lightning surges at distribution transformer secondary

The 400/230 V low voltage systems are usually affected by lightning mainly due to surges coming through the distribution transformers. The lightning surges appear at the transformer primary due to direct strikes or induced voltages in the MV lines and then transfer to the secondary side and to the LV system. This paper presents a broad investigation of the lightning surges transferring through a 33/0.4 kV distribution transformer in Sri Lanka. In this work, a simple high frequency model for a distribution transformer was proposed. The parameters affecting surge voltage and frequency at the secondary was investigated for a 100 kVA distribution transformer and the proposed model was validated with experimental results. It was found that the leakage reactance and a transformer winding resistance have a negligible effect on the surge outputs. The analysis was extended to obtain the surges appearing on common distribution transformers in Sri Lanka such as 100, 160, 250, 400, 630, 800 and 1000 kVA.