Modified offset keying apparatus for phase comparison relaying

Phase comparison relaying apparatus for protecting an alternating current power transmission line including at least one conductor interconnecting first and second busses, said apparatus comprising:

• a. first circuit means, cooperatively associated with said first bus, adapted to receive a signal representative of the instantaneous magnitude of current flowing in the conductor, for establishing a first set of constant upper and lower limits of a current security margin, both of said first set limits being established to one side of the effective zero axis of the waveform of the signal to be received and for generating
• i. a first output signal when the instantaneous magnitude of the received signal is between said upper and lower limits of said security margin,
• ii. a second output signal when the instantaneous magnitude of the received signal is above said limits of said security margin, and
• iii. a third output signal when the instantaneous magnitude of the received signal is below said limits of said security margin;
• b. second circuit means, cooperatively associated with said first bus, adapted to receive the signal representative of the instantaneous magnitude of current flowing in the conductor, for establishing a first constant keying threshold limit to the other side of the effective zero axis of the waveform of the received signal and for generating
• i. a first output keying signal when the instantaneous magnitude of the received signal is above said first keying threshold limit, and
• ii. a second output keying signal when the instantaneous magnitude of the received signal is below said first keying threshold limit;
• c. third circuit means, cooperatively associated with said second bus, adapted to receive a signal representative of the instantaneous magnitude of current flowing in the conductor, for establishing a second set of constant upper and lower limits of a current security margin, both of said limits being established to one side of the effective zero axis of the waveform of the received signal and for generating
• i. a first output signal when the instantaneous magnitude of the received signal is between said upper and lower limits of said security margin,
• ii. a second output signal when the instantaneous magnitude of the received signal is above said limits of said security margin, and
• iii. a third output signal at the other of its said outputs when the instantaneous magnitude of the received signal is below said limits of said security margin;
• and
• d. fourth circuit means, adapted to receive the signal representative of the instantaneous magnitude of current flowing in the conductor, for establishing a second constant keying threshold limit to the other side of the effective zero axis of the waveform of the received signal and for generating
• i. a first output keying signal when the instantaneous magnitude of the received signal is above said second keying threshold limit, and
• ii. a second output keying signal when the instantaneous magnitude of the received signal is below said second keying threshold limit;
• and
• wherein said first and second keying threshold limits are established to be of different magnitudes.