Medium-voltage switching techniques

From the generation plants, electrical energy is routed to points of consumption via an electrical network made up of a mesh or tree structure of links, lines or cables, as shown in figure 1 .

It is essential to be able to cut power at any point on the network for operating and maintenance purposes, or to protect the network in the event of a fault. It is also essential to be able to restore power in various normal or fault situations. To achieve this, disconnecting devices are used, the functions of which are as follows

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The choice depends on the nature of the currents to be cut and the field of application (Table A).

• These currents can be classified into three categories:

  • load currents, in principle less than or equal to the rated current I _r ; the rated current I _r is the rms value of the current that the equipment must be able to withstand indefinitely under prescribed conditions of use and operation;

  • overload current, when the current exceeds its rated value ;

  • short-circuit current, in the event of a network fault, the value of which depends on the power of the source, the type of fault and the upstream impedances of the circuit.

• In addition, all these devices are subject to stresses, whether on opening, closing or continuous operation:

  • dielectric (voltage) ;

  • thermal (normal and fault currents) ;

  • electrodynamic (fault current) ;

  • mechanical.

  The most significant...