The operation experience of ±500 kV DC transmission system in China shows that the operation of a HVDC transmission system under ground-return mode

partial entering ground current may flow into the AC transformers of which neutral point grounded and cause DC bias

which leads to the local overheating of AC transformer

increasing its vibration and noise.This paper analyzes magnetic field distribution of transformer core under non-load state in different circumstances of DC bias by using the software ansoft.The results show that with the increasing of DC

the saturation of core increased and so did magnetic induction;the fastest increase point is at the magnetic induction of upper and lower iron yoke.