The substation AC transformer near a high voltage DC converter station is often intruded by DC in its neutrals. This paper proposes a method to block this DC in order to reduce its negative effect.In this paper

the reasons why DC magnetic bias exists in AC power transformer are analyzed

and the DC magnetic bias principle and its hazard to transformer are also discussed.Furthermore

a new DC blocking method is developed

in which a main capacitor with a bypass circuit is connected in series with transformer neutral to block the fault current.The influence of the method on blocking DC in neutral and relay protection are analyzed.The simulation test verifies its validity for neutral grounded transformer.