At first glance, a transformer substation is simply a transformer substation: a transformer, medium-voltage switchgear and distribution equipment.
However, a substation that supplies a factory and one that delivers electricity from a solar power plant to the grid perform two different functions. The differences in their design are significant enough for an incorrect assumption to become expensive.
In a conventional consumer substation, electricity flows from the grid towards the loads, and all equipment, protection systems and metering are configured for that direction of power flow.
In a solar power plant, the transformer operates as a step-up transformer and electricity flows from the plant towards the grid. At facilities that also have their own electricity consumption, the direction of power flow can change throughout the day: electricity is imported in the morning and exported around midday.
The equipment must therefore be selected and configured for power flow in both directions, while the metering system must distinguish between electricity imported from and exported to the grid.
A consumer substation is sized according to the consumer’s peak load.
A solar power plant substation is sized according to the plant’s rated output, which may remain at full power for several hours on a clear day, day after day, throughout the production season.
This is a more demanding thermal operating profile than typical electricity consumption with short peaks, so the transformer must be selected without optimistic assumptions regarding spare capacity.
Another important consideration is low-load operation at night. When the solar power plant is not producing electricity, the transformer remains energised and continues to generate no-load losses, which must be included in the project’s financial model.
A grid-connected power plant is not a passive network user.
Through the grid connection conditions, the distribution system operator defines how the plant must behave, including protection settings at the point of connection, operating requirements during grid disturbances and, where required, participation in reactive power control.
Compliance with these settings must be demonstrated during commissioning, and the relevant documentation is required before the plant can be connected to the grid.
This additional layer of requirements is largely absent in a conventional consumer substation. It is also where the difference is most evident between a contractor experienced in power generation projects and one that has worked only on traditional transformer substations.
The most common industrial application today is a combined system: an existing facility installs a solar power plant, so the same substation both supplies the facility and exports surplus electricity to the grid.
In this case, it is necessary to verify whether the existing transformer and medium-voltage switchgear can withstand the new operating regime, whether the protection and metering systems support power flow in both directions and whether sufficient grid connection capacity is available for electricity export.
Sometimes the solution requires only a minor modification. In other cases, an additional switchgear cell or a higher-capacity transformer may be required.
The important point is that this assessment must be completed during the solar power plant design stage, rather than becoming an unexpected issue during the technical inspection.
That is the difference between a planned budget item and a delay affecting the entire project.
