Configuration of LV circuits

Main possible configurations:

• Single feeder configuration (fig.D20): This is the reference configuration and the most simple. A load is connected to one single source. This configuration provides a minimum level of availability, since there is no redundancy in case of power source failure.

Fig. D20: Single feeder configuration

• Parallel transformers configuration (fig.D21): The power supply is provided by more than 1 transformer generally connected in parallel to the same main LV switchboard.

Fig. D21: Parallel transformers configuration

• Variant: Normally open coupled transformers (fig.D22): In order to increase the availability it is possible to split the main LV switchboard into 2 parts, with a normally open bus-coupler (NO). This configuration may require an Automatic Transfer Switch between the coupler and transformer incomers.

These 2 configurations are more often used when power demand is greater than 1 MVA.

Fig. D22: Normally open coupled transformers

• Main LV switchboard interconnected by a busway (fig D23): Transformers are physically distant, and operated in parallel. They are connected by a busway, the load can always be supplied in the case of failure of one of the sources. The redundancy can be:
  • Total: each transformer being able to supply 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 transformer failure.

Fig. D23: Main LV switchboard interconnected by a busway

• LV ring configuration (fig. D24): This configuration can be considered as an extension of the previous configuration with interconnection between switchboards. Typically, 4 transformers connected in parallel to the same MV line, supply a ring using busway. A given load is then supplied by several transformers. This configuration is well suited to large sites, with 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 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.

As the previous configuration this type of installation is commonly used in automotive industry or large site manufacturing industry.

Fig. D24: Ring configuration

• Double-ended power supply (fig. D25): 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.

Fig. D25: Double-ended configuration with automatic transfer switch

• Configuration combinations (fig D26): 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 busways).

Fig. D26: Example of a configuration combination

1: Single feeder, 2: Main LV switchboard interconnected by a busway, 3: Double-ended

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

Characteristic to be considered	Configuration
	Single feeder (fig. D20)	Parallel transformer or transformers connected via a coupler (fig. D21-D22)	Main LV switchboard interconnected by a busway (fig D24)	LV ring	Double-ended
Site topology	Any	Any	1 level 5000 to 25000 m2	1 level 5000 to 25000 m2	Any
Power demand	< 2500kVA	Any	≥ 2500kVA	> 2500kVA	Any
Location latitude	Any	Any	Medium or high	Medium or high	Any
Load distribution	Localized loads	Localized loads	Intermediate or uniform load distribution	Intermediate or uniform load distribution	Localized loads
Maintainability	Minimal	Standard	Standard	Standard	Enhanced
Disturbances sensitivity	Low sensitivity	High sensitivity	High sensitivity	High sensitivity	High sensitivity

Fig. D27: Recommendations for the configuration of LV circuits