HV And MV Switch Disconnectors And Isolators in Power System

Switch Disconnectors

Switching operations can be performed through switch-disconnectors, equipments that are able to be operated on-load and can withstand and extinguish electric arc that is produced when the rated current of the installation is interrupted.

Switch-disconnectors are nowadays used only in MV installations, most commonly in MV switchgears and in MV overhead lines (see Figure 1) and they have usually an arcing chamber (air or SF6 insulated) to extinguish the arc, but they also perform that extinction in the air (Air Break Switch – ABS). 

Characteristics of Switch Disconnectors

Main characteristics of switch-disconnectors are:

Rated voltage: 2 kV; 12 kV; 17.5 kV; 24 kV; 36 kV

Rated current: 400 A; 630 A; 1,250 A

Rated short-time withstand current (3 s): 5 kA; 16 kA; 20 kA; 25 kA 

Isolators

Isolators, also known as disconnectors, are used in HV and MV installations to isolate a circuit or parts of a circuit and to ensure, for safety reasons, to the operators and maintenance teams a visible interruption of the circuit.

Similar to MV switch-disconnectors, MV isolators are installed in MV switchgears and in MV overhead lines. HV isolators are generally installed outdoors, in AIS substations (Air Insulated Substation), seated in metallic structures, or indoors, in GIS substations (Gas Insulated Substation).

Isolators are not able to be withstand and to extinguish electric arc and they can only be operated off-load. If an isolator is operated on load an arc blast and an explosion can occur and provoke damages to persons and equipments.

Characteristics of Isolators

Main characteristics of isolators are:

Rated voltage

MV: 2 kV; 12 kV; 17.5 kV; 24 kV; 36 kV

HV: 5 kV; 123 kV; 170 kV; 245 kV, 300 kV; 420 kV; 550 kV; 800 kV

Rated current

MV: 400 A; 630 A; 1,250 A

HV: 2,000 A; 2,500 A; 3,150 A; 4,000 A; 5,000 A

Rated short-time withstand current (3 s)

MV: 5 kA; 16 kA; 20 kA; 25 kA

HV: 50 kA; 63 kA; 80 kA