Technical Papers: MV/LV transformer substations: theory and examples of short-circuit calculation

An electrical transformer substation consists of a whole set of devices (conductors, measuring and control apparatus and electric machines) dedicated to transforming the voltage supplied by the medium voltage distribution grid (e.g. 5kV or 20kV), into voltage values suitable for supplying low voltage lines with power (400V - 690V).

The electrical substations can be divided into public substations and private substations:

public substations: these belong to the electricity utility and supply private users in alternating single-phase or three-phase current (typical values of the voltage for the two types of power supply can be 230V and 400V). In turn, these are divided into urban or rural type substations, consisting of a single reduced-size power transformer.

Urban substations are usually built using bricks, whereasrural ones are often installed externally directly on the MV pylon.

private substations: these can often be considered as terminal type substations, i.e. substations where the MV line ends at the point of installation of the substation itself.

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They belong to the user and can supply both civil users (schools, hospitals, etc.) with power and industrial users with supply from the public MV grid. These substations are mostly located in the same rooms of the factory they supply and basically consist of three distinct rooms:

v  Delivery room: where the switching apparatus of the utility is installed. This room must be of a size to allow any construction of the in-feed/output system which the utility has the right to realise even at a later time to satisfy its new requirements. The take-up point is found in the delivery room, which represents the border and connection between the public grid and the user plant.

v  Instrument room: where the measuring units are  located.

Both these rooms must have public road access to allow intervention by authorized personnel whether  the user is present or not.

v  User room: destined to contain the transformer and the MV and LV switching apparatus which are the concern of the user. This room must normally be adjacent to the other two rooms.