The high zero-sequence inrush will be caused when the high-voltage built-in high-impedance transformer performs no-load closing, which may penetrate into the 20 kV distribution network with a grounding small resistance through the adjacent transformers, leading to the maloperation of zero-sequence overcurrent protection of distribution lines. This will result in the power loss of users. To solve this, the amplitude characteristics of high-voltage built-in high-impedance transformers are analyzed based on zero-sequence inrush formula. Then, the validity of the model and the possibility of protection maloperation are studied based on PSCAD/EMTDC simulation platform. At last, the calculation procedure for assessing risks of zero-sequence over-current protection in distribution networks is produced by MATLAB, where the actual parameter is used to quantitatively analyze the maloperation probability of zero-sequence overcurrent protection when a high-voltage built-in high-impedance transformer performs no-load energize. The experienmental results indicate protection operation situations. A protection blocking strategy based on 5G wireless communication technology is proposed and its feasibility is analyzed.