A large DC earth current will have a negative impact on Henan power grid

when ±800 kV Tianshan-Zhongzhou（Tianzhong） UHVDC transmission project operates in ground-return mode or asymmetric operation mode. Therefore

based on the practical operation of Henan power grid

a simulation model to analyze the DC bias was established. According to the measured results on site

the DC bias classification protection of Henan power grid was made. The capacitive-type DC suppression device with reliable return system was developed and operated in Henan power grid. The results show that the magnitude of the DC bias current is mainly affected by the surface potential rising distribution

also affected by the distance between the substations and the number of outlets. Special attention should be paid to the DC bias of the station which is far away from other station and have less outlets. The principle of classified protection is that the station of which DC bias exceeds the standard needs to be suppressed

and substation with significant DC bias needs to be further monitored. The capacitive-type DC suppression device which has a bypass protection will reliably operate when the control system is de-energized. When this capacitive-type DC suppression device is adopted in the 220 kV Minghe substation

the DC bias is effectively suppressed

and the safe and reliable operation of Henan power grid is ensured. Therefore

the influence of DC bias to the transformer of Henan power grid can be effectively solved after Tianzhong UHVDC transmission project commissioning.