Procedure for the establishment of a new substation

Large consumers of electricity are always fed by a medium voltage supply.

On LV systems operating at 120/208 V (3-phases 4-wires), a load of 50 kVA may be considered as "large", while on a 240/415 V (3-phases 4-wires) system this limit is generally above 100 kVA. These two systems of LV distribution are widely used in most of the parts of the world. See also Definition of voltage ranges

The IEC recommends a "world" standard low voltage system of 230/400 V for 3-phases 4-wires. This is a compromise which will allow the existing systems operated at either 220/380 V or 240/415 V, or close to these values, to comply simply with the proposed standard by just adjusting the off load tap changers of the MV/LV distribution transformers.

The choice of the most appropriate level of supply of a new electrical installation is under the responsibility of the utility operating the network to which the installation is supposed to be connected.

The decision is mainly based on two criteria:

• The distance to the primary substation that will feed the installation
• The total power required by the installation.

Other criteria such as the rate of availability of the electricity supply are also taken into account.

There are two possibilities for the construction of a substation dedicated to the supply of a new electrical installation:

1 - The utility builds the substation according to its own specifications, close to the consumer’s premises. The MV/LV transformer(s) however, remain located inside the installation, close to the loads.

Depending on local rules the MV/LV transformer(s) may belong or not to the utility, they may be installed or not by the owner of the installation, the utility has or not unrestricted access to their locations

2 - The owner of the electrical installation builds and equips the substation inside his premises.

In most of the cases the utility must at least have unrestricted access to the metering and to the part of the substation ensuring the connection of the installation to the MV utility network. The utility may require a separate room for the equipment dedicated to these two functions.

The following chapters only refer to the construction of the substation by the owner of the electrical installation.

Preliminary information

In most of the cases the project of the construction of a new substation must be submitted before any further detailed studies to the approval of the utility operating the MV network that will feed the substation. The list of information to provide for this approval may be the result of preliminary discussions with the utility. Among all information, the following are generally required:

Maximum anticipated power (kVA) demand

The method of evaluation of this parameter is detailed in section Power loading of an installation, it must take into account the future additional loads. As described there, two factors associated to the loads are used in the proposed method:

• The factor of maximum utilization (ku)
• The diversity factor (ks).

Layout and arrangement drawings of the proposed substation

The following information may be required:

• Situation of the substation with regard to traffic lane
• Location of the substation inside the installation
• Dispositions foreseen for the unrestricted access of the utility operating staff
• Arrangement drawings showing clearly the location of the electrical equipment (MV Switchboard, transformers, Metering panel,…)
• Routing of MV cables
• Single line diagram and type of protections against phase to phase and phase to earth faults
• Main characteristics of electrical equipment
• Solution foreseen for the compensation of the reactive energy
• Principle of the earthing system
• Presence in the installation of a power generator operated in parallel with the MV network
• Etc.

Degree of supply continuity required

The consumer must estimate the consequences of a failure of supply in terms of:

• Safety of the persons
• Impact on the environment
• Safety of the installation
• Loss of production.

He shall indicate to the utility the acceptable frequency of the interruptions of the electricity and their durations.

Information and requirements provided by the utility

Based on the information provided by the consumer, the utility must provide his proposition, his commitment and his own requirements concerning the connection of the substation:

• Level of voltage
• Supply by overhead line
• Supply by underground cables
• Single-line service, ring type supply, parallel feeders, etc.
• Rated values concerning the voltage
• Rated value concerning the current
• Details concerning the applicable tariff and the billing of the electrical energy
• Comments on drawings and information provided by the consumer
• Specific requirements applicable to the substation.

The detailed studies of the substation must take into account all these parameters and requirements.

Commissioning, testing, energizing

When required by the local authority, commissioning tests and checking must be successfully completed to get the authorization to energize a new installation.

The following tests and checking are generally mandatory and applicable to the whole installation:

• Verification that the main substation complies with all the requirements expressed by the utility
• Measurement of earth-electrodes resistances
• Electrical continuity of all equipotential and bonding conductors
• Inspection and functional testing of all MV components
• Dielectric test of switchgears and transformers
• Inspection and testing of the LV parts of the installation
• Mechanical and electrical interlocks checking
• Protective-relays checking
• Other additional tests and checking mat be required.

As soon as the conformity official document is issued, the utility proceeds with the energizing of the installation.