Procedure for the establishment of a new substation

Large consumers of electricity are invariably supplied at MV.

On LV systems operating at 120/208 V (3-phase 4-wires), a load of 50 kVA might be considered to be “large”, while on a 240/415 V 3-phase system a “large” consumer could have a load in excess of 100 kVA. Both systems of LV distribution are common in many parts of the world.

As a matter of interest, the IEC recommends a “world” standard of 230/400 V for 3-phase 4-wire systems. This is a compromise level and will allow existing systems which operate at 220/380 V and at 240/415 V, or close to these values, to comply with the proposed standard simply by adjusting the off-circuit tapping switches of standard distribution transformers.

The distance over which the energy has to be transmitted is a further factor in considering an MV or LV service. Services to small but isolated rural consumers are obvious examples.

The decision of a MV or LV supply will depend on local circumstances and considerations such as those mentioned above, and will generally be imposed by the utility for the district concerned.
When a decision to supply power at MV has been made, there are two widely-followed methods of proceeding:

1 - The power-supplier constructs a standard substation close to the consumer’s premises, but the MV/LV transformer(s) is (are) located in transformer chamber(s) inside the premises, close to the load centre

2 - The consumer constructs and equips his own substation on his own premises, to which the power supplier makes the MV connection

In method no. 1 the power supplier owns the substation, the cable(s) to the transformer(s), the transformer(s) and the transformer chamber(s), to which he has unrestricted access.

The transformer chamber(s) is (are) constructed by the consumer (to plans and regulations provided by the supplier) and include plinths, oil drains, fire walls and ceilings, ventilation, lighting, and earthing systems, all to be approved by the supply authority.

The tariff structure will cover an agreed part of the expenditure required to provide the service.

Whichever procedure is followed, the same principles apply in the conception and realization of the project. The following notes refer to procedure no. 2.

Preliminary information

The consumer must provide certain data to the utility at the earliest stage of the project.

Before any negotiations or discussions can be initiated with the supply authorities, the following basic elements must be established:

Maximum anticipated power (kVA) demand

Determination of this parameter is described in Chapter A, and must take into account the possibility of future additional load requirements. Factors to evaluate at this stage are:

• The utilization factor (ku)
• The simultaneity factor (ks)

Layout plans and elevations showing location of proposed substation

Plans should indicate clearly the means of access to the proposed substation, with dimensions of possible restrictions, e.g. entrances corridors and ceiling height, together with possible load (weight) bearing limits, and so on, keeping in mind that:

• The power-supply personnel must have free and unrestricted access to the MV equipment in the substation at all times
• Only qualified and authorized consumer’s personnel are allowed access to the substation
• Some supply authorities or regulations require that the part of the installation operated by the authority is located in a separated room from the part operated by the customer.

Degree of supply continuity required

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

• Loss of production
• Safety of personnel and equipment
  

Project studies

The utility must give specific information to the prospective consumer.

From the information provided by the consumer, the power-supplier must indicate:

The type of power supply proposed, and define:

• The kind of power-supply system: overheadline or underground-cable network
• Service connection details: single-line service, ring-main installation, or parallel feeders, etc.
• Power (kVA) limit and fault current level
  

The nominal voltage and rated voltage (Highest voltage for equipment)

Existing or future, depending on the development of the system.

Metering details

which define:

• The cost of connection to the power network
• Tariff details (consumption and standing charges)

Implementation

The utility must give official approval of the equipment to be installed in the substation, and of proposed methods of installation.

Before any installation work is started, the official agreement of the power-supplier must be obtained. The request for approval must include the following information, largely based on the preliminary exchanges noted above:

• Location of the proposed substation
• Single-line diagram of power circuits and connections, together with earthing-circuit proposals
• Full details of electrical equipment to be installed, including performance characteristics
• Layout of equipment and provision for metering components
• Arrangements for power-factor improvement if required
• Arrangements provided for emergency standby power plant (MV or LV) if eventually required
  

Commissioning

After testing and checking of the installation by an independent test authority, a certificate is granted which permits the substation to be put into service

When required by the authority, commissioning tests must be successfully completed before authority is given to energize the installation from the power supply system. Even if no test is required by the authority it is better to do the following verification tests:

• Measurement of earth-electrode resistances
• Continuity of all equipotential earth-and safety bonding conductors
• Inspection and functional testing of all MV components
• Insulation checks of MV equipment
• Dielectric strength test of transformer oil (and switchgear oil if appropriate), if applicable
• Inspection and testing of the LV installation in the substation
• Checks on all interlocks (mechanical key and electrical) and on all automatic sequences
• Checks on correct protective-relay operation and settings

It is also imperative to check that all equipment is provided, such that any properly executed operation can be carried out in complete safety. On receipt of the certificate of conformity (if required):

• Personnel of the power-supply authority will energize the MV equipment and check for correct operation of the metering
• The installation contractor is responsible for testing and connection of the LV installation

When finally the substation is operational:

• The substation and all equipment belongs to the consumer
• The power-supply authority has operational control over all MV switchgear in the substation, e.g. the two incoming load-break switches and the transformer MV switch (or CB) in the case of a Ring-Main Unit, together with all associated MV earthing switches
• The power-supply personnel has unrestricted access to the MV equipment
• The consumer has independent control of the MV switch (or CB) of the transformer(s) only, the consumer is responsible for the maintenance of all substation equipment, and must request the power-supply authority to isolate and earth the switchgear to allow maintenance work to proceed. The power supplier must issue a signed permit-to-work to the consumers maintenance personnel, together with keys of locked-off isolators, etc. at which the isolation has been carried out.