Pad-mounted Transformer designed for underground electrical distribution systems

Pad-mounted Transformer

Pad-mounted transformer is a ground-mounted, enclosed distribution transformer installed on a concrete pad. It is designed for underground electrical distribution systems and provides a safe, tamper-resistant method of stepping medium-voltage power down to low-voltage levels for residential, commercial, and industrial customers.

Typical Specifications
·Primary Voltage: 5 kV–35 kV (commonly 11 kV, 13.2 kV, 13.8 kV, 22 kV, 24.9 kV, 33 kV)
·Secondary Voltage: 400/230 V, 415/240 V, 480Y/277 V, 208Y/120 V
·Rated Power: 75–5000 kVA
·Frequency: 50 Hz or 60 Hz
·Number of Phases: Single-phase or Three-phase
·Cooling Method: ONAN (Oil Natural Air Natural)
·Insulation: Mineral oil, natural ester, or synthetic ester fluid
·Installation: Outdoor, ground-mounted on a concrete pad
·Enclosure: Tamper-resistant, lockable steel cabinet

Main Components
·Transformer core
·High-voltage winding
·Low-voltage winding
·Sealed steel tank
·Insulating fluid
·Radiators or cooling fins
·High-voltage compartment
·Low-voltage compartment
·Load-break switches (optional)
·Bayonet and current-limiting fuses
·Surge arresters
·Pressure relief device
·Grounding terminals
·Nameplate

Common Ratings
Typical capacities include:
·75 kVA
·100 kVA
·150 kVA
·225 kVA
·300 kVA
·500 kVA
·750 kVA
·1000 kVA
·1500 kVA
·2000 kVA
·2500 kVA
·3000 kVA
·5000 kVA

Common Voltage Ratios
·11 kV / 400 V
·13.8 kV / 480Y/277 V
·22 kV / 415 V
·24.9 kV / 208Y/120 V
·33 kV / 415 V

Applications
·Residential neighborhoods with underground utilities
·Commercial buildings
·Shopping centers
·Office complexes
·Schools and hospitals
·Industrial facilities
·Data centers
·Renewable energy installations

Advantages
·Compact, space-efficient installation
·Safer than exposed pole-mounted equipment due to enclosed, tamper-resistant design
·Suitable for underground cable systems
·Lower visual impact in urban and residential environments
·Reduced exposure to weather compared with open installations
·Available in loop-feed and radial-feed configurations

Protection Features
·Bayonet fuses
·Current-limiting fuses
·Load-break switch (optional)
·Surge arresters
·Pressure relief device
·Oil level indicator (where applicable)
·Temperature monitoring (optional)

Routine Factory Tests
Typical tests include:
·Winding resistance measurement
·Voltage ratio verification
·Insulation resistance test
·No-load loss and excitation current
·Load loss and impedance measurement
·Applied AC voltage withstand test
·Induced overvoltage test
·Leak test
·Insulating fluid dielectric strength test

Applicable Standards
·IEC 60076 – Power Transformers
·IEEE C57.12.34 – Requirements for pad-mounted, compartmental-type distribution transformers
·IEEE C57.12.90 – Test code for liquid-immersed transformers
·IEC 60296 – Mineral insulating oils

Typical Configurations
·Radial-feed: One incoming medium-voltage feeder supplying the transformer.
·Loop-feed: Two incoming feeders allowing the transformer to remain energized from an alternate source if one feeder is isolated.

Pad-mounted transformers are widely used in modern underground distribution networks because they combine safety, reliability, and ease of maintenance. Their enclosed construction and flexible voltage and capacity options make them well suited for residential developments, commercial campuses, and industrial facilities.