MV network diagrams for feeding secondary switchboards and MV/LV transformers

MV network diagrams

Let’s take a look at the main MV network diagrams and structures used to feed secondary switchboards and MV/LV transformers. The complexity of the diagrams differs, depending on the level of power supply security required. Note that there are many other variations of presented medium voltage network diagrams, we shall focus on the five most common.

The following MV network supply arrangements are the ones most commonly adopted.

1. Single fed radial network
2. Dual fed radial network with no coupler
3. Dual fed radial network with coupler
4. Loop systems:
   1. Open loop
   2. Closed loop
5. MV parallel feeder

1. Single fed radial network

Network characteristics:

• The main switchboard is fed by 2 sources with coupler.
• Switchboards 1 and 2 are fed by a single source, and there is no emergency back-up supply.

This structure should be used when service continuity is not a vital requirement and it is often adopted for cement works networks.

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2. Dual fed radial network with no coupler

Network characteristics:

• The main switchboard is fed by two sources with coupler.
• Switchboards 1 and 2 are fed by two sources with no coupler, the one backing up the other.
• Service continuity is good. The fact that there is no source coupler for switchboards 1 and 2 means that the network is less flexible to use.

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3. Dual fed radial network with coupler

Network characteristics:

• The main switchboard is fed by two sources with coupler.
• Switchboards 1 and 2 are fed by 2 sources with coupler. During normal operation, the bus coupler circuit-breakers are open.
• Each bus section can be backed up and fed by one or other of the sources.

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4. Loop system

This system should be used for widespread networks with large future extensions. There are two types depending on whether the loop is open or closed during normal operation.

Open loop (see Figure 1-20a)

Network characteristics:

• The main switchboard is fed by two sources with coupler.
• The loop heads in A and B are fitted with circuit-breakers.
• Switchboards 1, 2 and 3 are fitted with switches.
• During normal operation, the loop is open (in the figure it is normally open at switchboard 2).
• The switchboards can be fed by one or other of the sources.
• Reconfiguration of the loop enables the supply to be restored upon occurrence of a fault or loss of a source.
  (The loop heads at A1 and B1 are fitted with circuit-breakers, and phase and earth fault protection devices. The substations are fitted with switches. Fault detectors are installed on each switch. They indicate the passage of a fault current.)

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Closed loop

Network characteristics:

• The main switchboard is fed by two sources with coupler.
• All the loop switching devices are circuit-breakers.
• During normal operation, the loop is closed.
• The protection system ensures against power cuts caused by a fault.
  (Each link end is fitted with a circuit-breaker. During normal operation, the loop is closed. The protection systems will allow power cuts to be avoided when a fault occurs.).

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MV parallel feeder

Network characteristics:

• Switchboards 1, 2 and 3 can be backed up and fed by one or other of the sources independently.
• The main switchboard is fed by two sources with coupler.

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Reference // Protection of Electrical Networks by Christophe Prévé