Negative sequence network having a frequency compensated filter

A sequence network adapted to be coupled to each phase of a three phase transmission line by coupling means which develops three signals A, B, and C respectively representative in magnitude and phase of the transmission line currents or of the transmission line voltages, said sequence network being adapted to detect one phase sequence component and reject another phase sequence component of the transmission line currents or voltages, said sequence network having an operating frequency equal to the rated frequency of the transmission line and comprising:

• (a) a first phase shift network having applied thereto said signal A and developing a first output signal, said first phase shift network comprising two legs each contributing substantially equal phase shifts but each leg contributing opposite variation in phase shift in response to frequency changes such that the total phase shift of the first phase shift network does not vary substantially in response to frequency changes within a predetermined limited range encompassing said operating frequency,
• (b) a second phase shift network having applied thereto said signal B and developing a second output signal, said second phase shift network providing a phase shift opposite to that provided for signal A, said second phase shift network comprising two legs each contributing substantially equal phase shifts but each leg contributing opposite phase shift variations in response to frequency changes such that the total phase shift of the second phase shift network does not vary substantially in response to frequency changes within said predetermined limited range,
• (c) first means for summing said first and second output signals so as to obtain a resultant signal that is representative of the vector sum of said first and second output signals,
• (d) an additional network with no substantial phase shift properties having applied thereto said signal C for developing a third output signal in phase with signal C and having a magnitude equal to said resultant signal for the particular phase sequence component to be rejected, and
• (e) means for summing said third output signal with said resultant signal and developing a fourth output signal representative of the particular phase sequence component to be detected.