Compact Substations

Each compartment has its own doors opening from outside. The MV compartment takes a ring main unit, or radial or dual-supply switchgear, rated 200–⁠630 A, with MV metering if required and full mechanical interlocking. The transformer compartment holds a low-loss oil-immersed unit (noise below 55 dB) or a dry-type (below 65 dB), with automatic forced-air cooling and lighting, plus rails on request to roll the transformer out through the side doors. The low voltage compartment has the LV distribution board, panel or cabinet type, rated 100–⁠3000 A, with circuits for power and lighting, reactive compensation, energy metering and measurement.

In line, the three compartments run end to end. At 200–⁠630 kVA that makes 4140 × 2590 × 2320 mm, the size usually seen at mines, oilfields and construction sites; at 800–⁠1250 kVA it makes 5184 × 2500 × 2626 mm, more common in residential areas. Clustered, either the transformer or the MV compartment takes one side and the remaining two share the rest, all in 2500 × 2400 × 2320 mm for 50–⁠400 kVA, also common in residential areas.

One to three movable sealed boxes hold the primary and secondary equipment, typically a 35 kV switch box, a main transformer box and a 10 kV switch box with cable between them. Ratios are 35/10, 35/6.3 and 35/0.4 kV at 400–⁠20000 kVA, with 35 kV breakers at 1250, 1600 or 2000 A and 10 kV breakers up to 3150 A. The 35 kV lines can come in and go out overhead or by cable. Oil transformers above 4000 kVA usually stand outdoors behind a safety guard. Shipment is in several transport units, with withdrawable breakers packed separately.

Transformer and MV switch sit in two separate oil tanks, so arcing from operating the load-break switch stays in the switch tank’s oil and does not affect the transformer oil. The MV side has a four-position 630 A and a two-position 100 A load-break switch with backup and plug-in fuses, and phase-loss protection can trip the incoming load-break switch and the LV breaker. Ratings are 12/0.4 kV, 100–⁠1000 kVA in 11 steps, Dyn11, ONAN, ±2 × 2.5% off-circuit tapping, 50 dB noise and IP34. Footprint is always 2000 × 1620 mm; height and weight go with capacity, from 1600 mm and 1560 kg at 100 kVA to 2060 mm and 4650 kg at 1000 kVA. For a solar or wind plant, see the Pad-Mounted Transformer for Solar and Wind.

Choose from composite steel panels, stainless steel, aluminium alloy or cast fibre cement, all with corrosion treatment. Colour and appearance can follow the site; the photo shows a louvred front in dark blue and orange. Ventilation is both natural and forced: the transformer and LV compartments have air ducts, and exhaust fans start and stop at a set temperature. Coastal and chemical sites can have the coating upgraded, and where sand, dust or salt mist is a problem the protection can go up to IP54 per project.

Radial means a single incoming cable. A ring main has one cable in and one out, with a tee-off through a load-break switch and fuse to the transformer; tenders usually specify this as a ring main unit (RMU). Dual supply brings in two cables, again feeding the transformer through a load-break switch and fuse. Switch type, number of ways and metering position come from the project single-line; on the LV side you get incomer, feeders, reactive compensation and energy metering.

AC output from the inverters lands on the LV incoming panels, a double-split transformer with two LV windings steps it up, and an MV outgoing panel connects to the collector circuit. Each stage has its own compartment, in line or in an L, and the LV compartment can also hold a remote control device and an auxiliary transformer. Capacity, LV voltage and MV panels depend on the inverter and the collector scheme. Where you only need step-up and MV switching, a pad-mounted transformer takes less space.

For projects shipped overseas, the station can be built as a 20 ft high-cube container with a CSC plate, and the MV, transformer and LV rooms each open from outside. The photo shows one from ETENZ. If you only want the housing, go to the BESS Step-Up Substation Enclosure (20 ft).