Functions of the substation with MV metering

MV/LV Transformers and internal MV distribution

As mentioned for substation with LV metering, only oil-immersed and dry type cast-resin transformers are allowed with the same rules of installation.

When the installation includes several MV/LV transformers and/or secondary MV/ LV substations an internal MV distribution network is required.

According to the required level of availability, the MV supplies to the transformers and the secondary substations may be made,

• By simple radial feeders connected directly to the transformers or to the secondary substations
• By one or several rings including the secondary MV/LV substations (Fig. B10)
• By duplicate feeders supplying the secondary MV/LV substations.

For the two latter solutions the MV switchboard located in each secondary substation includes two load break switch functional units for the connection of the substation to the internal MV distribution and one transformer protection unit, for each transformer installed in the substation.

The level of availability can be increased by using two transformers operating in parallel or arranged in dual configuration with an automatic change over system.

It is not recommended to use MV/LV transformers above 2500 kVA due to:

• The high level of the short circuit current generated on the main LV switchboard.
• The number of LV cable required for the connection of the transformer to the LV switchboard.

Metering

The characteristics and the location of the VT’s and CT’s dedicated to the metering shall comply with the utility requirements.

The VT’s and CT’s are generally installed in the MV switchboard. A dedicated functional unit is in most of the cases required for the voltage transformers while the current transformers may be contained in the functional unit housing the circuit breaker ensuring the general protection of the substation.

The panel that contains the meters shall be accessible by the utility at any time.

Local emergency generators

Emergency standby generators are intended to maintain the power supply to the essential loads in the event of failure of the utility power supply.

According to the energy needs an installation may contains one or several emergency generators.

The generators can be connected:

• At MV level to the MV main substation (see Fig. B34).The generator(s) 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(s).

• At LV level on one or several LV switchboards requiring an emergency supply. At each location, the loads requiring an emergency supply may be grouped on a dedicated LV busbar supplied by a local generator (see Fig. B31).

Fig. B34: Connection of emergency generators at MV level

Capacitors

Capacitors are intended to maintain the power factor of the installation at the contractual value specified by the utility. The capacitor banks can be fixed or adjustable by means of steps. They can be connected:

• At MV level to the main MV substation
• At LV level on LV switchboards.

LV main switchboard

Every MV/LV transformer is connected to a main LV switchboard complying with the requirements listed for substation with LV metering (see 4.2.6).

Simplified electrical network diagram

The diagram (Fig. B35) shows:

• The methods of connection of a MV/LV substation to the utility supply:

- Spur network or 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

• General protection at MV level
• MV metering functions
• Protection of MV circuits
• LV distribution switchboard

Compared with a substation with LV metering, a substation with MV metering includes in addition:

• A MV Circuit breaker functional unit for the general protection of the substation
• A MV metering functional unit
• MV Functional units dedicated to the connection and the protection of:

- MV/LV transformers

- MV feeders supplying secondary substations

- MV capacitor banks

- Emergency generators

The general protection usually includes protection against phase to phase and phase to earth faults. The settings must be coordinated with the protections installed on the feeder of the primary substation supplying the installation.