ABSTRACT
Current differential relaying is a method of extending the benefits of differential protection as applied to generators and transformers to the protection of transmission lines. The simplicity of comparing current flowing into a power system component to the current flowing out of that component is very attractive, with the important advantage of requiring no voltage transformers. With the power system substations being widely separated, the pilot wires between the two may be subjected to the introduction of extraneous voltages. Zero-sequence mutual effects between nearby power system circuits and the pilot wires interconnecting the pilot wire relays may be severe. One successful scheme that has been used widely to supervise the integrity of the continuous metallic pilot wires that are used in the HCB or HCB-1 scheme involves the introduction of 1 mA of dc circulating current. The filter output is generated in the HCB and HCB-1 relays with mutual reactors and resistors.
