From substation to energisation: the stages of a high-voltage project

What a high-voltage substation does

A substation is a node in the grid. It receives energy from one or more lines, changes its voltage through transformers, distributes it between feeders and automatically isolates the faulty section when an incident occurs. In France, HTB refers to voltages above 50 kV AC: notably 63, 90, 225 and 400 kV on the French grid.

With industrial electrification and the connection of renewables and data centers, these projects are multiplying. RTE plans to triple its annual investment rate by 2030.

1. Studies

Everything starts with grid studies: power to be transferred, short-circuit currents, stability, power quality. They set the substation architecture (number of bays, busbar arrangement, protection scheme) and the sizing of equipment.

Basic and then detailed design follow: layout, civil works, earthing, cabling, protection and control. This is when a mistake is cheapest to fix.

2. Permits

Depending on the size and location of the project, planning and environmental permits are needed, and sometimes local consultation. These steps run in parallel with the studies and often weigh on the schedule.

3. Long-lead procurement

Power transformers, circuit breakers, disconnectors, gas-insulated switchgear: some equipment takes several years to manufacture. It is ordered as soon as the specification is stable, sometimes before permits are final. Its delivery date often sets the project's critical path.

Since 2026, switchgear choices must also take into account the European SF6 phase-out timetable, which applies to the date of putting into operation.

4. Civil works and electrical erection

Platforms, foundations, transformer oil containment pits, cable trenches, control building: civil works prepare for the equipment's arrival. Electrical erection follows: steel structures, switchgear, busbars, connections, earthing grid.

This phase requires tight coordination between contractors and strict safety follow-up. Work near live equipment is governed in France by the NF C 18-510 standard and the electrical authorisation rules.

5. Protection and control

Protection, automation, remote control, metering: protection and control is the brain of the substation. It is going digital. RTE has begun deploying fully digital systems based on the IEC 61850 standard, which lets devices from different manufacturers communicate on the same network. Designing, configuring and securing these systems are becoming skills in their own right.

6. Testing and energisation

Testing takes place in two stages. First, individual tests of each piece of equipment: insulation measurements, transformer tests, circuit breaker checks, wiring verification. Then integrated functional tests: secondary injection into protection relays, automation tests, exchanges with the grid operator's control centre.

Energisation is prepared as an operation of its own, with an approved programme, isolation and de-isolation procedures, and minute-by-minute coordination with the grid operator.

Where lead time is won or lost

On a substation project, delays rarely come from a single task. They come from interfaces: a civil works drawing that does not match the equipment delivered, a protection setting approved too late, a missed outage window. Hence the value of technical management that follows the project end to end, from studies to commissioning, with a single point of contact facing the grid operator.