Election of phase-to-phase distance protections for extra high voltage cables;

Algorithm realization based on the load characteristics of beam-pumping unit and phase-failure protection circuit;

Builds the relation between the three phase symmetrical short circuit current and phase angle,gives the new method of deciding on character for amplitude- phase protec- tion and proposes sure realizable form of amplitude- phase protection.

In order to insure electric-motor running safely,and to be immune to burn-up while lost its phase,besides of general safe considerations,some other protections should be taken into account such as lack-phase protection,so the rest of this paper will depict why an electric-motor lost its phase and how to deal with the problem.