Failure of the DC system itself to withdraw from operation or failure of the AC system causing blocking of the DC system may result in a tidal shift to overload the AC line. Unreasonable tripping of the AC line's distance III backup protection based on local information drives the development of chain tripping incidents and can lead to significant outages. For this reason, a method based on conservation and control synergy is proposed, using DC control information to optimise AC distance III backup protection action behaviour. When the tidal shift triggered by the DC phase change failure blocking makes the distance protection section III act unreasonably, the DC control information is used as the activation condition for the backup protection action range adjustment to improve reliability and prevent the backup protection from acting unreasonably. Finally, the effectiveness of the above strategy is verified by PSCAD software simulation.

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