A　novel　impedance　monitoring　scheme　is　proposed　in　this　paper　in　order　to　detect　faults　occurring　in　the　ground　electrode　lines　of　ultra　high　voltage　DC　(UHVDC)　transmission　systems　reliably　and　rapidly.　Firstly,　the　theoretical　analysis　of　the　full-band　impedance　vs.　distance　characteristic　of　the　ground　electrode　line　is　carried　out.　Because　a　high　frequency　signal　injection-based　method　is　used　in　the　proposed　scheme,　the　principle　of　determining　the　frequency　of　the　injection　signal　is　discussed　and　the　criterion　of　fault　alarm　are　given.　The　study　shows　that　the　imaginary　part　of　the　measured　impedance　has　a　periodic　variation　with　the　distance　of　the　bolted　fault　when　the　length　of　the　ground　electrode　line　is　larger　than　half　the　injection　signal＇s　wave　length,　which　increases　the　difficulty　of　setting　the　criterion.　Therefore　the　selected　frequency　of　the　injected　signal　should　ensure　that　in　case　of　a　bolted　fault　the　line　impedance　changes　monotonically　with　the　fault　distance,　and　the　modulus　value　of　the　maximum　fault　impedance　should　be　less　than　that　of　a　normal　operation　case　with　a　certain　margin.　Some　PSCAD　based　simulations　are　performed　to　verify　the　proposed　scheme.　The　results　show　that　the　faults　occur　in　the　ground　electrode　line　can　be　detected　rapidly　and　reliably,　and　the　proposed　scheme　has　good　practice　perspective.　©　2016,　The　editorial　office　of　Transaction　of　China　Electrotechnical　Society.　All　right　reserved.