This guide sets out what the substation and transmission scope usually covers, where its boundaries lie and how to manage the interfaces with the network operator so that energisation is not the step that delays the whole project.
Collector system to project substation
Inside the plant, medium voltage cables collect power from inverters or wind turbines and bring it to the project substation. There, one or more main transformers step the voltage up to transmission level. The substation also houses switchgear, protection, metering, auxiliary supplies and the control building.
Define clearly where the collector system ends and the substation begins, and who designs and builds each part.
The line to the grid
A transmission or sub-transmission line connects the project substation to the network operator’s substation. Depending on distance and terrain, it may be an overhead line or an underground cable. The line needs route surveys, land rights, permits and its own design and construction team.
Long lines through difficult terrain or many land parcels can take longer than the plant itself. Start route work early.
Contestable and network operator works
In many markets, some connection works can be built by the project, while others must be built by the network operator or its approved contractors. The project may build its substation and line, then hand part of them to the operator to own. The operator usually builds or modifies its own substation bay.
Map who builds, who owns and who operates each element. These boundaries drive the contract scope, schedule and cost.
Studies and approvals
Connection studies cover load flow, short circuit, stability, power quality and protection coordination. The network operator often performs some studies and reviews others. Results can change equipment ratings, reactive power requirements and protection settings.
Our article on the interconnection queue for renewable projects explains how these studies fit into the approval process.
Main transformer and switchgear
The main transformer is usually the longest-lead item in the substation, and supply chains for large transformers can be tight. Order it early, with specification and factory tests agreed. High-voltage switchgear, instrument transformers and surge arresters must also meet the operator’s standards.
Plan transport for the transformer, which may need a special route and heavy-haul arrangements.
Protection, SCADA and metering
Protection relays must coordinate with the network operator’s protection. SCADA links let both the plant operator and the network operator monitor and control the plant, and revenue metering must meet market rules. The operator often specifies the equipment and communication protocols.
Test communications end to end well before energisation, because this is a common source of late delays.
Outages on the network side
Connecting a new bay or line to a live network substation usually needs planned outages on the operator’s equipment. Operators schedule outages months ahead and may restrict them to particular seasons. A missed outage window can delay energisation significantly.
Book outages as soon as the schedule allows, and keep the operator informed of progress.
Testing and energisation
Substation testing includes equipment tests, protection injection tests, SCADA point checks and pre-energisation inspections. The network operator often witnesses key tests. Energisation follows a formal procedure with switching instructions agreed by both parties.
The wider commissioning steps are described in our article on the renewable EPC commissioning sequence.
Handover, spares and operation
At handover, the owner needs as-built drawings, test records, protection settings, operation and maintenance manuals and a spare parts set, including critical items such as transformer bushings and protection relays. Agree who operates the substation and under which switching authority.
A clear handover pack makes it easier for the operations team to keep the substation safe and available.
Earthing, civil works and the control building
A substation needs an earthing grid designed for the fault levels found in the connection studies, so that touch and step voltages stay safe during a fault. Soil resistivity tests early in the project feed that design. Civil works include foundations for the transformer, gantries and switchgear, oil containment under the transformer, cable trenches, drainage, roads and a fence with controlled access.
The control building houses protection panels, batteries and chargers, SCADA equipment, communications and often a workshop. Size it for future bays if the plant may expand, and cool it properly, because protection and communications equipment is sensitive to heat.
Reactive power and power quality equipment
Grid codes often require the plant to supply or absorb reactive power over a defined range and to keep harmonics and flicker within limits. Inverters provide much of this capability, but some projects need additional equipment such as capacitor banks, reactors, static var compensators or harmonic filters at the substation.
The need for this equipment usually emerges from the connection studies. Leave space in the substation layout for it until the studies are complete, because adding a filter bank late can force a redesign.
Where EPC risk sits
The substation and line carry risks that are partly outside the EPC contractor’s control: operator approvals, outage windows and land access. Contracts should state how those risks are shared and how delays caused by third parties are treated.
Our overview of EPC for renewable energy projects covers how wider project risks are allocated.
About this guide
Our high-voltage and EPC engineers wrote this guide from experience delivering connections for power projects. USP&E takes on that EPC work directly and does not act as a broker, with 150+ power projects in 45+ countries, 25,000 MW contracted since 2002 and 24+ years of project delivery. Our project work is described on the EPC projects page.
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Substation and line scope questions
What does a renewable project substation include?
Main transformers, high-voltage switchgear, protection, metering, auxiliary supplies, SCADA and a control building, connecting the collector system to the transmission line.
Who builds the connection to the grid?
It depends on the market. The project often builds its substation and line, while the network operator builds or modifies its own substation bay.
Why is the main transformer a schedule risk?
It is often the longest-lead item in the substation, and large transformer supply chains can be tight. Order it early.
What causes delays at energisation?
Common causes are missed network outage windows, late communications testing, approval delays and incomplete test records.
Plan your grid connection scope
Send the connection offer, single-line diagram and site layout, and our engineers will help define the substation and line scope and interfaces.
If a document set is easier than a conversation, send it to info@uspeglobal.com or upload a summary through the EPC enquiry form; to talk it over, dial +27 10 822 2324. Power equipment for project sites is listed in our equipment inventory.