Inside a Box-Type Transformer

Box-type transformers—also known as pad-mounted or compact substations—encapsulate all major transformer and switchgear components within a single, weather-proof enclosure. These units integrate high-voltage (HV) switchgear, the transformer core and windings, low-voltage (LV) distribution gear, protection devices, cooling systems, and monitoring instruments into one compact assembly. Their design maximizes safety, space efficiency, and ease of installation for urban and suburban power distribution networks.

Transformer Core and Windings

At the heart of every box-type transformer lies the magnetic core, typically constructed from laminated silicon-steel sheets to minimize eddy-current losses and provide a low-reluctance path for magnetic flux. Wrapped around the core are two sets of windings—primary (HV) and secondary (LV)—made of enamelled copper or aluminium conductors. These windings are insulated from each other and the core using high-grade insulating paper and pressboard to withstand operating voltages and thermal stresses.

Insulation and Transformer Oil

The core and windings are immersed in mineral oil, which serves as both an electrical insulator and a coolant. The oil fills the sealed tank, eliminating air pockets that could lead to partial discharges, while conducting heat away from the windings to external radiators or cooling fins. Solid insulation—such as Kraft paper, pressboard, and epoxy resin—provides additional electrical separation and mechanical support inside the tank.

Tap Changer and Bushings

Box-type transformers often include an on-load tap changer (OLTC) or off-circuit tap changer to adjust the turns ratio and regulate output voltage under varying load conditions. High-voltage bushings provide insulated passageways for the primary conductors through the tank wall, while LV terminals allow safe cable connections for downstream circuits.

Protection, Control, and Monitoring

Integrated protection devices include fuses, HV/LV circuit breakers, and surge arresters housed within the HV switchgear compartment, which isolate faults and safeguard the transformer. A Buchholz relay detects gas accumulation from internal faults, triggering alarms or tripping to prevent catastrophic failures. Control panels incorporate metering instruments (voltage, current, temperature) and interfaces for remote monitoring and SCADA integration.

Cooling and Accessory Systems

Heat dissipation is facilitated by external radiators, cooling fins, or forced-air fans attached to the oil tank to maintain optimal operating temperatures. A silica-gel breather filters and dries incoming air as the oil expands and contracts, preserving oil quality and insulation integrity. Pressure-relief devices safeguard against overpressure events by venting excess oil or gas safely.

HV/LV Switchgear Integration

The front section of the box-type enclosure contains the HV switchgear—ring-main units, load-break switches, or vacuum circuit breakers—and LV distribution panel, complete with busbars, fuses, and outgoing feeders. This modular arrangement simplifies installation and maintenance by providing clear segregation of HV and LV compartments.

 

By housing all functional elements—magnetic core, windings, insulation, oil, tap changers, protective relays, switchgear, and cooling accessories—in a single enclosure, box-type transformers offer a turn-key solution for reliable, safe, and space-efficient power distribution. Their compact design, along with integrated monitoring and protection, reduces installation time and operational risks in urban utility networks.

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