Functions of a substation with LV metering
Connection to the MV network
Connection to the MV network can be made:
- By a single service cable or overhead line,
- By dual parallel feeders via two mechanically interlocked load-break switches
- Via a ring main unit including two load-break switches.
Since the ban of PCB in most of the countries, the remaining available insulation technologies for the transformers are:
- Oil-immersed for transformer preferably located outside premises
- Dry-type, cast-resin preferred for transformers located inside premises such as buildings receiving the public.
Local regulations define where the use of cast resin transformers is mandatory.
Most of the LV metering and billing principles take into account the MV/ LV transformer losses.
The characteristics and the location of the VT’s and CT’s dedicated to the metering must comply with the utility’s requirements.
The metering current transformers are generally installed in the LV terminal box of the power transformer, alternatively they can be installed in a dedicated compartment in the main LV switchboard.
The compartments housing the metering VT’s and CT’s are generally sealed by the utility.
The meters are mounted on a dedicated panel accessible by the utility at any time.
Local emergency generators
Emergency standby generators are intended to maintain the supply to the essential loads, in the event of failure of the utility power supply.
A substation with LV metering may include one single emergency generator connected at low voltage level on the main LV distribution switchboard.
The generator may be sized either for the supply of the whole installation or for a part only. In this case a load shedding system must be associated to the generator. The loads requiring an emergency supply may also be grouped on a dedicated LV busbar (see Fig. B31).
An Uninterruptible Power Supply (UPS) may be added when required at LV level to avoid the interruption of the supply during the starting of the emergency generator.
Capacitors are intended to maintain the power factor of the installation at the contractual value specified by the utility. The capacitor banks are connected on the main LV switchboard and can be fixed or adjustable by means of steps controlled by a regulator.
See chapter Power Factor Correction
LV main switchboard
The MV/LV transformer is connected to a main LV distribution switchboard equipped with a LV general circuit breaker ensuring:
- The general protection of the LV installation
- The general isolation of the LV circuits, according to the rules of protection of the persons working in an electrical installations
- The protection of the MV/LV transformer against overload
To comply with the interlocking requirements defined in Interlocks and conditioned operations, the circuit breaker must be equipped with padlocking facilities for locking it in open position.
Simplified electrical network diagram
The diagram Fig. B32 shows:
- The different methods to connect a MV/LV substation to the utility supply:
- Single-line service
- Single line service with provision for future connection to a ring or to dual parallel feeders
- Dual parallel feeders
- Loop or ring-main service
- The protection of the MV/LV transformer, either by a load break switch or by a circuit breaker
- The LV metering
- The main LV switchboard.