To solve the problems of the inherent defects of inaccurate distance regulation and unstable flashover voltage for the current separately installed rod-rod gap for 110 kV and 220 kV transformer neutral protection, a new composite gap is developed, which is supported by the composite insulators, and the electrodes in the horn form at high and low voltages are bond on these insulators.

The calculation results of DC currents through transformer neutrals of HVDC system operated in monopole-ground return mode with the influence of sea considered are evidently different from those calculated by terrestrial soil model.

Holding that there exists to a certain extent coupling between DC and AC earth electrodes,the authors analyze the transformer DC magnetic biasing in the grid,and present the network algorithm for the DC component of transformer neutral point.

By analyzing the phase\'s relationship between transformer neutral zero sequence current and zero sequence current of the protection,a novel direction element based on zero sequence current of the transformer neutral point is proposed to overcome the above shortcoming.

There are several factors affecting neutral grounded transformers,among then,ground potential rise is most important.