Configuration of LV circuits

Main possible configurations (see figures D18 to D25):

• Radial single feeder configuration: This is the reference configuration and the most simple. A load is connected to only one single source. This configuration provides a minimum level of availability, since there is no redundancy in case of power source failure.

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Fig. D18: Radial single feeder configuration

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• Two-pole configuration: The power supply is provided by 2 transformers, connected to the same MV line. When the transformers are close, they are generally connected in parallel to the same MLVS.

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Fig. D19: Two-pole configuration

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• Variant: two-pole with two ½ MLVS: In order to increase the availability in case of failure of the busbars or authorize maintenance on one of the transformers, it is possible to split the MLVS into 2 parts, with a normally open link (NO). This configuration generally requires an Automatic Transfer Switch, (ATS).

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Fig. D20: Two-pole configuration with two ½ MLVS and NO link

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• Shedable switchboard (simple disconnectable attachment): A series of shedable circuits can be connected to a dedicated switchboard. The connection to the MLVS is interrupted when needed (overload, generator operation, etc)

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Fig. D21: Shedable switchboard

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• Interconnected switchboards: If transformers are physically distant from one another, they may be connected by a bus
  bar trunking. A critical load can be supplied by one or other of the transformers. The availability of power is therefore improved, since the load can always be supplied in the case of failure of one of the sources.
  The redundancy can be:

    - Total: each transformer being capable of supplying all of the installation,
    - Partial: each transformer only being able to supply part of the installation. In this case, part of the loads must be disconnected 
      (load-shedding) in the case of one of the transformers failing.

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Fig. D22: Interconnected switchboards

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• Ring configuration: This configuration can be considered as an extension of the configuration with interconnection between switchboards. Typically, 4 transformers connected to the same MV line, supply a ring using busbar trunking. A given load is then supplied power by several clustered transformers. This configuration is well suited to extended installations, with a high load density (in kVA/m²). If all of the loads can be supplied by 3 transformers, there is total redundancy in the case of failure of one of the transformers. In fact, each busbar can be fed power by one or other of its ends. Otherwise, downgraded operation must be considered (with partial load shedding). This configuration requires special design of the protection plan in order to ensure discrimination in all of the fault circumstances.

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Fig. D23: Ring configuration

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• Double-ended power supply: This configuration is implemented in cases where maximum availability is required. The principle involves having 2 independent power sources, e.g.:

   - 2 transformers supplied by different MV lines,
   - 1 transformer and 1 generator,
   - 1 transformer and 1 UPS.
An automatic transfer switch (ATS) is used to avoid the sources being parallel connected. This configuration allows preventive and curative maintenance to be carried out on all of the electrical distribution system upstream without interrupting the power supply.

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Fig. D24: Double-ended configuration with automatic transfer switch

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• Configuration combinations: An installation can be made up of several sub-asssemblies with different configurations, according to requirements for the availability of the different types of load. E.g.: generator unit and UPS, choice by sectors (some sectors supplied by cables and others by busbar trunking).
  

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Fig. D25: Example of a configuration combination
1: Single feeder, 2: Switchboard interconnection, 3: Double-ended

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For the different possible configurations, the most probable and usual set of characteristics is given in the following table:

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Characteristic to  be considered	Configuration
	Radial	Two-pole	Sheddable load	Interconnected   switchboards	Ring	Double-ended
Site topology	Any	Any	Any	1 level 5 to 25000m²	1 level 5 to 25000m²	Any
Location latitude	Any	Any	Any	Medium or high	Medium or high	Any
Maintainability	Minimal	Standard	Minimal	Standard	Standard	Enhanced
Power demand	< 2500kVA	Any	Any	≥ 1250kVA	> 2500kVA	Any
Load distribution	Localized loads	Localized loads	Localized load	Intermediate or uniform distribution	Uniform distribution	Localized loads
Interruptions sensitivity	Long interruption  acceptable	Long interruption  acceptable	Sheddable	Long  interruption acceptable	Long interruption acceptable	Short or no  interruption
Disturbances sensitivity	Low sensitivity	High sensitivity	Low sensitivity	High sensitivity	High sensitivity	High sensitivity
Other constraints	/	/	/	/	/	Double-ended  loads