The　impact　on　operation　of　backup　protection　in　medium　voltage　side　caused　by　phase　to　phase　short　circuit　fault　in　low　voltage　10　kV　bus　side　of　110　kV　transformer　is　analyzed.　Then,　corresponding　solutions　such　as　fitting　backup　protection　time,　changing　operation　mode　of　system,　etc,　are　proposed　for　reducing　influence　of　low　voltage　side　phase　to　phase　fault　on　backup　protection　in　medium　voltage　side.　Through　failure　model　simulation　and　theoretical　calculation,　the　feasibility　of　previous　solutions　is　verified,　which　may　offer　a　reference　for　the　setting　calculation　of　power　system.　©　2012　State　Grid　Electric　Power　Research　Institute　Press.