High-speed grounding switches for suppressing the secondary current in EHV and UHV transmission lines

In EHV and UHV transmission lines without shunt reactors, the secondary current (SC) can be suppressed by the use of high-speed grounding switches. This is achieved from the suppression of the electrostatic component of the SC through the three grounding pathways formed from the closed grounding switches together with the arc canal. With the lightning wires on all towers, the earthing resistance of a transmission line becomes equal to the equivalent resistance value of the parallel resistances of all towers including the grounding switches, and thus is too small to be used in current suppression. This paper shows that although ineffective in current suppression, the arc resistance can effectively enhance the shunting action of the grounding switches. The grounding switches and the line inductances form a balanced bridge circuit, leading to the fact that the zero-sequence current from the load transformer of the remote end passes only through the bridge arms. As a result, the electromagnetic component of the SC passing through the galvanometer circuit is suppressed. It is further pointed out that grounding switches should be installed on both line sides, because in one-side installation the magnetic induced current becomes significantly large.Finally, the formula for computing the maximun switching circuit is derived.