High-voltage switchgear

High voltage switchgear is any switchgear used to connect or disconnect a part of a high-voltage power system. This equipment is essential for the protection and safe operation, without interruption, of a high voltage power system, and is important because it is directly linked to the quality of the electricity supply. The term "high voltage" covers the former medium voltage (MV) and the former high voltage (HV), and therefore refers to equipment with a rated voltage of over 1,000 V in the case of alternating current, and over 1,500 V in the case of direct current. The industrial applications of high voltage circuit breakers are for the moment limited to alternating current because they are more economical, there are however high voltage disconnectors for direct current connections. High voltage switchgear was invented at the end of the 19th century for operating motors and others electric machines. The technology has been improved over time and can be used with voltages up to 1,100 kV. Disconnectors and earthing switches are safety devices used to open or to close a circuit when there is no current through them. They are used to isolate a part of a circuit, a machine, a part of an overhead line or an underground line so that maintenance can be safely conducted. The opening of the line isolator or busbar section isolator is necessary for safety, but not sufficient. Grounding must be conducted at both the upstream and downstream sections of the device under maintenance. This is accomplished by earthing switches. In principle, disconnecting switches do not have to interrupt currents, as they are designed for use on de-energized circuits. In practice, some are capable of interrupting currents (as much as 1,600 ampere under 300 V but only if current is drawn via a same circuit half breaker bypass system), and some earthing switches must interrupt induced currents which are generated in a non-current-carrying line by inductive and capacitive coupling with nearby lines (up to 160 A under 20 kV).