inciple bas been applied to line, busbar, transformer and generator successfully [9][11].

RCS-931 ultra high voltage series protection employed DPFC current differential protection with adaptive floating threshold, which can withstand system unbalance and disturbance. The measuring element can operate with ultra high speed and high security. The pickup element based the same principle has high sensitivity, and yet it will not pick up frequently under normal condition even with unbalance and disturbance thanks to floating threshold.

Inter-turn fault is the main kind of fault in electric power transformer. It is meaningful that employing sensitive and secure protection to identify inter-turn fault inside power transformer. In RCS-978 series power transformer protection, DPFC percentage restraint differential protection makes use of DPFC in phase current on each side of transformer and DPFC in differential current, with a high setting of percentage restraint factor. The unwanted operation caused by TA saturation during external fault can be prevented. The sensitivity during internal fault is not affected by pre-fault load current and is high enough to identify internal slight fault, such as phase-to-ground fault near neutral point and low percentage inter-turn fault on the same phase under heavy load.

When slight fault occurs in generator or power transformer, traditional current differential protection cannot identify such fault for the sake of sensitivity limited by normal load current. RCS-985 Series generator-transformer unit protection is equipped with transformer DPFC percentage restraint differential protection and generator DPFC percentage restraint differential protection. Comparing with traditional percentage restraint differential protection, the DPFC percentage restraint differential protection can identify the slight fault with higher sensitivity, because they are not influenced by normal load current at all and are affected by fault transient resistance slightly. Higher restraint percentage is selected to deal with TA saturation; floating threshold prevents unwanted operation during system swing and frequency shifting.

  

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