Patent ID: 7199991

Claim:
A protection relay for a power line system where current on a bus is provided to one or more power lines for receiving and generating power, and where associated with each of said power lines is a current transformer for providing a line signal representative of current flowing through a respective power line, and wherein said protection relay is operative for generating a fault trip signal in response to a detected power line fault, the protection relay comprising: a signal processor responsive to said line signals for generating a fault trip signal in response to a detected fault on any one of the power lines; means for summing said line signals, separately for each power line phase, for all power lines coupled to said bus and producing, a first signal indicative of the phasor sum of all line signals, and a second signal representative of the magnitude sum of said line signals; means for deriving, a first time derivative signal representative of the first derivative of said first signal, and a second time derivative signal representative of the first derivative of said second signal; means for deriving, a first harmonic signal representative of the a selected harmonic of said first time derivative signal, and a second harmonic signal representative of a selected harmonic of said second time derivative signal; means for determining the harmonic phase difference between said first and second harmonic signals; phase comparator means responsive to said harmonic phase difference for selectively generating a blocking signal for blocking said fault trip signal if said harmonic phase difference satisfies a first condition; and a trip signal generator means responsive to said first and second signals for generating a fault trip signal upon said first and second signals being beyond a defined bus differential characteristic conditioned upon the status of said blocking signal.