Different types of MV power supply
From Electrical Installation Guide
The following methods may be used for the connection of an electrical installation to a MV utility distribution network.
Connection to an MV radial network: Single-line service
The substation is supplied by a tee-off from the MV radial network (overhead line or underground cable), also known as a spur network.
This method provides only one supply for loads (see Fig. B6, A and B). It is widely used for installations including a single MV/LV transformer with LV metering. It can also be used without any restriction for installations with MV metering including either several MV/LV transformers or even an internal MV distribution network.
The connection is made by means of a single load break switch associated to a earthing switch dedicated to overhead line or underground cable grounding. This principle can be the first step of the two other methods of connection (ring main and dual parallel feeders), the upgrading of the substation being generally performed during an extension of the installation or required by the adjunction of loads asking a higher level of supply continuity.
Generally, the pole-mounted transformers in rural areas are connected to the overhead lines according to this principle without load break switch nor fuses. Protection of the line and associated switching devices are located in the remote substations supplying the over-head distribution network.
Connection to an MV loop: Ring-main service
The substation is connected to a loop (see Fig. B6, C) of the medium voltage distribution network. The line current passes through the substation which gives the possibility to supply the substation by two different ways. With this arrangement, the user benefits of a reliable power supply based on two redundant MV feeders.
The connection is made by means of two independent load break switches, each associated to an earthing switch for underground cables earthing.
This method is mainly used for the underground MV distribution networks found in urban areas.
Connection to two dual MV cables: Parallel feeders service
Two parallel underground cables are used to supply the substation. Each cable is connected to the substation by means of a load-break switch. (see Fig. B6, D).
As mentioned for single and ring main service cable grounding is performed by means of earthing switches associated to the load break switches. The two load break switches are interlocked, meaning that only one load break switch is closed at a time.
This principle gives the possibility to supply the substation by two independent sources giving a full redundancy.
In the event of the loss of supply, the load-break switch supplying the installation before the loss of supply must be open and the second must be closed.
This sequence can be performed either manually or automatically.
This method is used to supply very sensitive installation such as hospitals for example. It is also often used for densely-populated urban areas supplied by underground cables.