Different types of MV power supply
From Electrical Installation Guide
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The following power supply methods may be used as appropriate for the type of medium-voltage network.
Connection to an MV radial network: Single-line service
The substation is supplied by a tee-off from the MV radial network (overhead or cable), also known as a spur network. This type of network supports a single supply for loads (see Fig. B1).
The substation usually consists of an incoming panel, and overall protection is provided by a load-break switch and fuses with earthing switches as shown in Figure B1.
In some countries, the “substation” comprises a pole-mounted transformer without a load-break switch or fuses (installed on the pole). This type of distribution is very common in rural areas. Protection and switching devices are located remotely from the transformer. These usually control a main overhead line to which secondary overhead lines are connected.
Fig. B1: Single-line service (single supply)
Connection to an MV loop: Ring-main service
The power supply for the substation is connected in series to the power line of the medium-voltage distribution network to form a loop(1). This allows the line current to pass through a busbar, making it possible for loads to have two different power supplies (see Fig. B2).
The substation has three medium-voltage modular units or an integrated ring-main unit supporting the following functions:
- 2 incoming panels, each with a load-break switch. These are part of the loop and are connected to a busbar.
- 1 transformer feeder connected to the busbar. General protection is provided by load-break switches, a combined load-break/isolating switch or a circuit breaker.
All these types of switchgear are fitted with earthing switches.
All switches and earthing switches have a making capacity which enables them to close at the network’s short-circuit current. Under this arrangement, the user benefits from a reliable power supply based on two MV feeders, with downtime kept to a minimum in the event of faults or work on the supplier network(1).
This method is used for the underground MV distribution networks found in urban areas.
Fig. B2: Ring-main service (double supply). The transformer is protected, in accordance with the applicable standards, by a circuit breaker or load-break switch as shown in Figure B1.
Connection to two parallel MV cables: Parallel feeders service
If two parallel underground cables can be used to supply a substation, an MV switchboard similar to that of a ring-main station can be used (see Fig. B3).
The main difference to the ring-main station is that both load-break switches are interlocked. This means that only one of them can be closed at any one time (if one is closed, the other must be open).
In the event of the loss of supply, the associated incoming load-break switch must be open and the interlocking system must enable the switch which was open to close. This sequence can be implemented either manually or automatically.
This method is used for networks in some densely-populated or expanding urban areas supplied by underground cables.
Fig. B3: Parallel feeders service (double supply). The transformer is protected, in accordance with local standards, by a circuit breaker or load-break switch as shown in Figure B1.
