With the expansion of urban construction and environmental considerations, traditional centralized voltage reduction, long-distance power distribution, and overhead power grids no longer meet the needs of modern urban power supply development. Urban grid upgrades require high-voltage direct access to urban areas, substation equipment close to load centers, underground cable transmission, and coordination between distribution equipment and the surrounding environment. Padmount Transformers, due to their compact structure, easy installation, operational flexibility, safety, reliability, and minimal maintenance, have gained rapid acceptance.

Key Features:​

1.High Performance: Utilizes 10/11-type or amorphous alloy cores for low loss, low noise, and high short-circuit resistance.

2.Functionality: Capable of load current interruption, full-range current protection, flexible high-voltage input (ring/terminal networks), phase-loss protection, and basic substation functionality.

3.Space-Saving & Cost-Effective: Compact size (≈1/3 of traditional substations), minimal installation effort (four bolts and cable connections), and rapid deployment.

4.Safety: Fully enclosed design with insulated elbow cable plugs ensures personnel safety and eliminates exposed conductive parts.

Definition:​

A Padmount Transformer integrates a transformer core, switchgear, fuses, tap changers, and auxiliary equipment into a sealed oil tank, using transformer oil as insulation. It combines high-voltage circuits and the transformer core within the tank, while low-voltage components (circuit breakers, current transformers, meters) are housed in a separate low-voltage chamber.

Key Components & Design:

1.Core Design: 

Uses high-permeability silicon steel or amorphous alloy cores, with D,yn winding configuration to minimize voltage distortion and neutral drift.

2.High Voltage System and Outgoing Line Configurations
The high voltage system connection methods include Ring Network Type, Terminal Type, and Terminal Dual-Circuit Type.

Ring Network Type​ ("one incoming, two outgoing lines"): One power supply incoming line, one outgoing line from the transformer’s low-voltage side, and another line connected to another transformer or electrical device to provide power for its low-voltage side or operation.

Terminal Type​ ("one incoming, one outgoing line"): One power supply incoming line and one outgoing line from the transformer’s low-voltage side.

Terminal Dual-Circuit Type​ ("one incoming, one outgoing, one backup"): One power supply incoming line, one outgoing line from the transformer’s low-voltage side, and a backup power supply that takes over when the main incoming power fails.

3.High Voltage Incoming Line
The high-voltage incoming line for pad-mounted transformers is connected via underground cables using high-voltage cable accessories. These accessories consist of three components: Elbow Connector, Load Break Bushing Insert, and Bushing Well.

Load Break Bushing Insert​ comes in two types: Single Bushing Insert and Loadbreak Feed-Thru Insert. The Load Break Bushing Insert connects directly to the cable for high-voltage grid access, while the Loadbreak Feed-Thru Insert connects one end to a surge arrester and the other to the grid.

ANSI-standard cable accessories include brands such as Cooper Power System, Elastimold, Hubbell, and Chardon. Most models are interchangeable. Cooper Power System has the highest brand recognition and is widely chosen by pad-mounted transformer manufacturers for transformer components.

4.Surge Arresters​
Surge arresters primarily protect against lightning strikes and are categorized into internal and external types.

Internal surge arresters​ are placed inside the oil tank near the high-voltage outgoing lines for protection.

External surge arresters​ (Elbow Arresters) are externally connected to the transformer’s high-voltage bushings. The Chardon brand is recommended for external arresters due to its excellent delivery performance, making it a preferred choice among South American transformer manufacturers.

5.Other Components​
Components include tap changer switches, oil level gauges, pressure gauges, temperature gauges, pressure relief valves, and optional additions like low-voltage air circuit breakers, secondary-side metering, and compensation devices. Fuses, multi-position load break switches, and no-load tap changers are integrated into the transformer assembly inside the oil tank, allowing easy fuse replacement or load switch operation from outside the tank to adjust operating conditions. High-voltage power is supplied via load-break elbow cable connectors, while a low-voltage outgoing line compartment is externally mounted to accommodate protective devices and meters. All live components are enclosed within the oil tank, eliminating exposed live parts.

6.Protection System​
The pad-mounted transformer protection system uses a series connection of high-voltage current-limiting fuses and overload protection fuses on the high-voltage side, and air circuit breakers on the low-voltage side for overload and short-circuit protection.

1.Bay-O-Net Fuses​ (insert-type overload protection fuses): Cooper offers variants such as Current Sensing Bayonet Fuse, Dual Sensing Bayonet Fuse, Dual Element Bayonet Fuse, and High Amp Overload Bayonet Fuse. These fuses can handle up to 38kV and provide robust protection against transformer surges, primarily safeguarding against overloads and low-voltage side short circuits.

2.Current Limiting ELSP Backup Fuses​ : These safely interrupt all fault currents that could cause melting, protecting the transformer and distribution system during internal faults. Cooper provides fuses rated for 8.3kV, 15.5kV, 23kV, and 38kV, with the CBUC38140D100 model being widely used by pad-mounted transformer manufacturers.

3.Fuse Coordination: The selection between Bay-O-Net Fuses and Current Limiting Fuses should follow fuse coordination guidelines. Refer to the recommended combinations for 34.5kV three-phase transformers in the table below. For further assistance, utilize Eaton’s Cooper Power series TransFusion Coordination tool.

Advantages:​

Compact "triad" structure (transformer, high/low-voltage chambers).

Fully insulated, oil-sealed, and maintenance-free.

Ideal for urban grids due to modularity, underground compatibility, and dual-power supply support.