High and extra-high voltage substations - Metal-clad substations (PSEM)

Gas-insulated metal-clad switchgear (GIS) can be used to build all types of high and extra-high voltage substations, the role and structure of which are defined in another section. It differs from conventional substations in that each phase is electrically insulated from ground, and between phases in the case of three-phase switchgear.

The main advantage of this technique is its compactness, achieved by replacing atmospheric air with a gas with high dielectric strength. The gas universally used today is sulfur hexafluoride (SF ₆ ). This is kept under pressure, enabling a significant reduction in the insulation distances required to meet the permanent and temporary voltages likely to occur in operation. SF ₆ provides insulation between phases and earth, between phases and between open contacts on circuit breakers and disconnectors.

To achieve this, the devices and their connections are enclosed in watertight enclosures in which the SF ₆ is confined under pressure. Each phase can be enclosed in an enclosure, or all three phases can be grouped together in a single enclosure, mainly for lower voltage levels. These enclosures, electrically connected to earth, ensure the safety of people with regard to the risk of electrical proximity. This also means that dielectric strength is insensitive to climatic conditions and industrial or marine pollution.

Its great installation flexibility means it can be used to extend an open substation where space is limited. Some 420 kV substations have been installed in caves or on platforms several meters above ground.

Its small size means it can be installed in buildings, and even on upper floors or in basements. Some substations can even be installed completely underground. In these conditions, a SEM substation connected to the power grid by buried cables is particularly discreet in an urban environment.

However, when there are no installation or environmental constraints, air-insulated equipment remains a more economical solution, and easier to troubleshoot in the event of damage, particularly as it allows the use of live working techniques that are not applicable to SEM equipment.

The article High and extra-high voltage substations is the subject of several booklets:

• Role and structure ;

• Control and monitoring systems ;

• Constructive measures ;

• Station construction and component installation...