Method and apparatus for detecting faults using current unbalance

A fault detection system for line-phases of electric power transmission systems, comprising:

• a plurality of line current monitors, each line current monitor comprising:
• a controlled oscillator, to produce a reference signal;
• a phase detector having a second order generalized integrator (SOGI) circuit to couple to a line-phase and to provide in-phase and quadrature alternating current signals proportional to current in the line-phase and relative to the reference signal produced by the controlled oscillator, and a Park Transformation block coupled to the SOGI circuit to transform the in-phase and quadrature alternating current signals to in-phase and quadrature signals in a rotating frame, with the frame rotating at a nominal line frequency defined by the controlled oscillator;
• and
• a loop filter coupling the phase detector and the controlled oscillator in a phase locked loop;
• one or more fault detection modules, to detect error or fault in the line-phases of the electric power transmission system based on the plurality of line current monitors;
• and
• one or more inter-phase communication modules to provide electrical isolation and couple to the one or more fault detection modules and the plurality of line current monitors.

A method of fault detection, performed by a fault detection system, comprising:

• determining in-phase and quadrature signals proportional to current in a line-phase of an electric power transmission system and relative to a reference signal generated by a controlled oscillator in a phase locked loop, in each of a plurality of line current monitors;
• transforming the in-phase and quadrature signals to in-phase and quadrature signals in a rotating frame, the frame rotating at a line frequency according to the controlled oscillator, in each of the plurality of line current monitors;
• communicating output of the plurality of line current monitors, through one or more inter-phase communication modules coupled to the plurality of line current monitors with electrical isolation, to one or more fault detection modules;
• and
• detecting and indicating error of one or more line-phases of the electric power transmission system, through the one or more fault detection modules, based on the output of the plurality of line current monitors communicated through the one or more inter-phase communication modules to the one or more fault detection modules.