In order to solve such safety hazard problem as aging of wire joint and serious partial temperature rise of partial distribution transformer in the distribution grid

which is occurred after long term operation and and is not found in time

a high-accuracy calculation model for the contact impedance of the three-phase four-wire system of distribution transformers is set up

and a data-driven early warning method for potential safety hazards of distribution transformers has been formed. The equivalent circuit model of the transformer in the three-phase four-wire system is used to construct the equivalent equation set for calculating the contact resistance. Since the number of variables of the equation set are greater than the number of equation set

the equation set model is specially converted into an optimized model for seeking the minimum value and the BFGS optimization algorithm is used to optimize the model iteratively. The long-term heating model under the maximum load current is used to set up the equivalent relationship between contact resistance and temperature

forming a hierarchical warning system of contact resistance. The effective-ness of the proposed method is verified by the field operating data of two actual stations in Jiangxi power grid of China.