Chile: 33 kV Prefabricated Wind-Farm Step-Up Substation

The engineering problem

Before any electrical question gets asked, a wind farm in southern Chile puts three physical constraints on its substation: how equipment gets there, what the wind does to it, and what the cold does to it.

Access comes first. Wind-farm roads are engineered for blades and tower sections — wide radii, controlled gradients, purpose-built crossings — but they are unsealed, budget-built, and they deteriorate through a wet southern winter. A prefabricated substation must fit within the same transport envelope the turbine components already defined, so its width, height, length and mass are design inputs, not consequences worked out afterward. Splitting the unit into two shippable modules is frequently cheaper than upgrading a culvert to take a single larger one.

Wind loading follows. The same resource that justifies siting the farm here also loads the enclosure, its doors and louvres, and any external structure, cable gantry or fence. Under NCh 432, the design wind pressure governs the enclosure's structural framing, its anchoring to the foundation, and the latching on every panel capable of being caught open. Wind-driven rain, and on exposed sites wind-driven grit, reach seals that would never be tested by still-air rain, so ingress protection has to be proved against driven water, not vertical rain.

Cold is the third factor. Low ambient temperature raises oil viscosity and slows convective cooling at start-up, stiffens gaskets and cable insulation, and turns condensation on internal surfaces into the normal state of affairs rather than an occasional problem. Anti-condensation heating, sealed and drained cable entries, and correctly rated steel, seals and cable for low temperature are the practical responses.

Electrically, the collector duty is cyclic and bidirectional, carries converter-derived harmonics, and spends long stretches at light load — all of which makes no-load loss and thermal cycling, rather than steady full-load heating, the dominant design concern.

System configuration

Installation sequence

This contact sheet lays out the four stages a project of this kind moves through: heavy transport along the wind-farm access road, the crane lift onto the prepared foundation, medium-voltage cable and earthing work, and the finished, fenced installation. The order is set by whatever becomes inaccessible afterward. Foundation, earth grid, ducts and drainage are finished and proved before the unit is set down. Collector cables are pulled and left long, terminated only once the enclosure is in place and its entries are positioned. Testing that needs panels open happens before the compound is closed off. Every stage is planned around a weather window, since a southern-Chile storm front can shut down both the road and the lift.

Transport is the stage most likely to throw off the schedule. The route is surveyed for axle loading, gradient, camber, culvert and bridge capacity and turning radii before dispatch — and checked again against the road's actual current condition rather than its as-built drawing, since an unsealed wind-farm road after winter bears little resemblance to the road that was originally handed over. A prefabricated substation is tall for its footprint, so lashing has to restrain lateral movement and roll, not just hold the unit down. On arrival the enclosure is inspected for transport damage, door, louvre and seal alignment, water ingress and shifted internal fixings; where an oil-immersed transformer is fitted, oil level, pressure gauge and gasketed joints are checked before it is accepted.

Placement is the critical lift, and on an exposed site the wind decides when it happens, not the schedule. Crane operations follow a maximum permissible wind speed at hook height, which runs lower than the ground-level reading and lower still for a large-area load — a substation enclosure behaves essentially like a sail. The lift is timed to a forecast weather window, with tag lines used to control rotation. The foundation is confirmed level and its duct positions checked against the enclosure footprint before the unit touches down; landing out of level distorts door seals, defeats the ingress rating and leaves water standing on the roof. Crane capacity is assessed at working radius, with outrigger bearing pressure checked on ground that may well be saturated. Final anchoring follows the wind and seismic detail rather than serving as a nominal fixing.

Collector cable termination is the highest-risk workmanship in the whole installation, and cold makes it harder still. Cable is warmed and handled within its minimum bending radius, because low temperature stiffens the insulation and a radius that would be fine in summer can crack the screen in winter. Terminations follow the usual discipline — cut-back length, semiconducting layer removal, cleanliness, correct stress-cone seating — finished in one controlled session inside a temporary shelter rather than left part-made overnight. Screens bond to the internal earth bar through a conductor rated for the earth-fault duty, and the enclosure, transformer tank and switchgear frames all bond to the site earth grid. Cable entries are sealed and drained, and every busbar and terminal bolt is torqued to value and marked, since contact resistance is what turns a rated joint into a hot spot.

Before energisation the installation is proved as a complete chain. Transformer ratio is measured on every tap and the vector group verified; winding and insulation resistance are recorded, and for oil-filled units dielectric strength is confirmed. Earth continuity is proved from every enclosure part, door and equipment frame back to the main bar, with grid resistance logged. Protection is tested end-to-end by injection so relay, CT circuit, trip coil and breaker are all shown to work together, and grid-code protection functions are verified against the connection requirements. Anti-condensation heating, ventilation, auxiliary supply and battery autonomy are functionally checked. Energisation is staged — transformer charged unloaded, collector feeders brought on in turn, then turbines released progressively.

Specification options

For a comparable wind enquiry, Millenium can tailor the voltage ratio and tapping range, transformer capacity, vector group and impedance to the collector study and the turbine converter's earthing requirement, with copper or aluminium windings. The transformer can be oil-immersed — with a less-flammable ester fluid where fire separation is constrained — or cast-resin dry-type. MV switching can be SF6, solid-insulated or air-insulated with withdrawable breakers, with loss-of-service continuity and internal arc classification set accordingly. Enclosures can be specified for design wind pressure and seismic demand, IP rating, ISO 12944 corrosion category and coating system, low-temperature-rated seals and steel, anti-condensation heating, ventilation or air conditioning, and split into transportable modules to suit the access road. Protection, grid-code functions, auxiliary supply autonomy and reporting over IEC 61850, IEC 60870-5-104 or DNP3 are all configurable.

What we need to quote

• Single-line diagram of the collector system and the intended step-up arrangement
• Collector voltage, transmission connection voltage, tapping range and required vector group
• Transformer capacity, wind farm installed capacity and the expected generation duty profile
• Turbine make and converter type, including harmonic content and earthing requirements
• Fault level at the point of connection, required withstand rating and clearing time
• Grid-code requirements applying at the connection point, including reactive and ride-through obligations
• Protection philosophy and any existing grading or protection coordination study
• Site design wind speed and exposure category, seismic zone, and minimum and maximum ambient temperature
• Snow, icing and wind-driven rain exposure, and the required enclosure IP and corrosion class
• Foundation arrangement, duct positions and available compound area
• Auxiliary supply requirement, battery autonomy and the SCADA protocol and points list
• Access road survey — permissible axle loading, gradients, radii, headroom — available crane capacity and delivery terms