Differential relay for protecting an electrical power transmission system

Claim 1

A differential relay arrangement for protecting an electrical feeder in an electrical power transmission system, comprising: (A) a first apparatus at a first monitoring point on the feeder, including (a) a first clock means for generating a series of first timing intervals, (b) first sampling means for sampling an electrical quantity at the first monitoring point at each of said first timing intervals to generate first electrical quantity samples, (c) means for generating forst digital data representative of the first electrical quantity samples; (B) a second apparatus at a second monitoring point on the feeder remote from the first monitoring point, including (a) a second clock means for generating a series of second timing intervals independently of the first clock means, (b) second sampling means for sampling the electrical quantity at the second monitoring point at each of said second timing intervals to generate second electrical quantity samples, (c) means for generating second digital data representative of the second electrical quantity samples; (C) a digital data communication channel linking the first and the second monitoring points; (D) polling means in the first apparatus, for polling the second apparatus by transmitting a polling message from the first apparatus in one direction along the communication channel to the second apparatus; (E) return means in the second apparatus and responsive to the polling message, for generating and returning a return data message in he opposite direction along the communication channel to the first apparatus, said return data message containing one of the second digital data representative of one of the second electrical quantity samples, as well as an indication of the second timing interval at which the sample represented by said one of the second digital data was obtained; (F) synchronizing means in the first apparatus and responsive to the return data message, for time aligning the sample represented by said one of the second digital data and the sample represented by said one of the first digital data to substantially the same instant of time; (G) comparison means for comparing said time-aligned samples, and for generating a protection signal when the difference between said one of the second digital data and said one of the first digital data exceeds a predetermined threshold value; and (H) relay means responsive to the protection signal, for disconnecting the feeder from the power transmission system upon generation of the protection signal.