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

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 constraints are related to the transient phenomena that occur during switching operations and fault current arcing. Despite current modeling techniques, the behavior of this arc is difficult to predetermine.

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