Pad Mounted Transformer: Powering Progress, Safely and Quietly.
01 General
1.1 Project Description
250 kVA three phase pad mounted transformer was delivered to Guyana in 2024. The rated power of the transformer is 250 kVA with ONAN cooling. The high voltage is 13.8 kV with ±2*2.5% tapping range (NLTC), the low voltage is 0.22y/0.127 kV, they formed a vector group of Dyn11.
The Pad-Mounted Transformer is the cornerstone of modern, safe, and reliable underground electrical distribution. Engineered for performance and public safety, this compact, ground-level unit is designed to step down high-voltage electricity to the usable levels required by residential neighborhoods, commercial centers, and industrial facilities. Unlike traditional pole-mounted transformers, its fully enclosed, tamper-resistant steel housing securely contains all live components, making it an ideal choice for areas where public accessibility is a concern. The robust design protects against environmental elements, vandalism, and accidental contact, ensuring uninterrupted service and enhanced safety for the community.
A key advantage of the Pad-Mounted Transformer is its minimal footprint and aesthetic flexibility. It is discreetly installed on a concrete pad at ground level, easily blending into its surroundings or being screened with landscaping. This design not only preserves the visual appeal of an area but also allows utility crews safe and easy access for maintenance and load management.
From powering smart cities to integrating renewable energy sources, the Pad-Mounted Transformer stands as a silent, reliable, and efficient guardian of the power grid, delivering electricity where it is needed-safely and seamlessly.
1.2 Technical Specification
250 kVA pad mounted transformer specifications type and data sheet
| Delivered to | Guyana |
| Year | 2024 |
| Type | Pad mounted transformer |
| Standard | IEEE Std C57.12.34 |
| Rated Power | 250 kVA |
| Frequency | 60HZ |
| Phase | 3 |
| Feed | Loop |
| Front | Dead |
| Cooling Type | ONAN |
| Primary Voltage | 13.8 kV |
| Secondary Voltage | 0.22y/0.127 kV |
| Winding Material | Copper |
| Vector Group | Dyn11 |
| Impedance | 4% |
| Efficiency | 98.94% |
| Tap Changer | NLTC |
| Tapping Range | ±2*2.5% |
| No Load Loss | 0.535 kW |
| On Load Loss | 3.56 kW |
1.3 Drawings
250 kVA pad mounted transformer diagram drawing and size.
02 Manufacturing
2.1 Core
| This pad-mounted transformer features a compact three-phase, three-limb core made of laminated silicon steel. The design provides a efficient magnetic path for the main flux while allowing zero-sequence flux to return through the air, resulting in a robust and space-saving unit. |  |
2.2 Winding
 | The windings utilize copper for both the low-voltage and high-voltage sections. The low-voltage winding is of foil construction, providing excellent axial short-circuit strength and efficient heat dissipation. Conversely, the high-voltage winding employs a layer-wound design with round or rectangular magnet wire, offering the necessary voltage endurance and flexibility in turns arrangement. This combination of foil and layer-wound techniques results in a robust, compact, and highly reliable coil assembly. |
2.3 Tank
| The transformer tank is constructed from mild steel, providing a robust and cost-effective enclosure. It is hermetically sealed to ensure the internal insulating oil remains isolated from moisture and contaminants. A durable exterior coating is applied to withstand harsh environmental conditions, making it suitable for long-term outdoor service. |  |
2.4 Final Assembly
 | The final assembly integrates the core-and-coil unit into the mild steel tank, which is then filled with purified dielectric oil. All internal connections are secured, and external components such as the compartmentalized secondary compartment, primary terminations (like elbows or bushings), pressure relief device, and grounding lugs are installed. The tank is hermetically sealed, and the entire unit undergoes rigorous testing to ensure operational integrity, safety, and reliability before being approved for service. |
03 Testing
The tests included routine items: winding resistance measurement (max unbalance rate 1.57%), ratio test (deviation -0.07% to -0.05%), phase-relation test (confirming Dyn11), no-load losses and current measurement (0.450kW, 0.76%), load losses, impedance voltage and efficiency test (3.354kW load loss, 3.93% impedance, 99.04% efficiency), applied voltage test (34kV for HV, 10kV for LV, 60s each with no voltage collapse), induced voltage withstand test (27.6kV, 150Hz, 48s, no voltage collapse), leak test (20kPa for 12h, no leakage), insulation resistance measurement, and oil dielectric test (55.13kV). All measured values met acceptance requirements, leading to a “Pass” conclusion. The report is valid only with signatures of the tester, verifier, and approver; objections must be submitted within 7 days of receipt, and copying is not allowed without Scotech’s written permission.
| No. | Test Item | Unit | Acceptance Values | Measured Values | Conclusion |
| 1 | Resistance Measurements | % | Maximum resistance unbalance rate≤5% | 1.57 | Pass |
| 2 | Ratio Tests | % | The deviation of voltage ratio on the principal tapping: ≤0.5% Connection symbol: Dyn11 | -0.07%-0.05% | Pass |
| 3 | phase-relation tests | / | Dyn1 | Dyn1 | Pass |
| 4 | No-load losses and excitation current | / | I0 :: provide measured value | 0.76% | Pass |
| P0: provide measured value(t:20℃) | 0.450kW | ||||
| the tolerance for no load loss is +10% | / | ||||
| 5 | Load losses impedance voltage and efficiency | / | t:85℃ the tolerance for impedance is ±7.5% the tolerance for total load loss is +6% | / | Pass |
| Z%: measured value | 3.93% | ||||
| Pk: measured value | 3.354kW | ||||
| Pt: measured value | 3.804 kW | ||||
| Efficiency not less than 98.94% | 99.04% | ||||
| 6 | Applied Voltage Test | kV | HV: 34kV 60s LV: 10kV 60s | No collapse of the test voltage occurs | Pass |
| 7 | Induced Voltage Withstand Test | kV | Applied voltage (KV):2Ur | No collapse of the test voltage occurs | Pass |
| Induced voltage(KV): 27.6 | |||||
| Duration(s):48 | |||||
| Frequency (HZ): 150 | |||||
| 8 | Leakage Test | kPa | Applied pressure: 20kPA | No leakage and no Damage | Pass |
| Duration: 12h | |||||
| 9 | Insulation Resistance Measurement | GΩ | HV-LV to Ground : | 46.0 | / |
| LV-HV to Ground: | 109.6 | ||||
| HV&LV to Ground: | 42.6 | ||||
| 10 | Oil Dielectric Test | kV | ≥45 | 55.13 | Pass |
04 Packing and Shipping
4.1 Packing
| Place a tin foil bag on its tray to cover the transformer, add desiccant, leave an opening for vacuuming, then seal the bag. Next, fit corner protectors (foam, plastic or pressed cardboard) around it and wrap with protective film. Finally, package it in a wooden box, which is sprayed with forklift and center-of-gravity marks. |  |
4.2 Shipping
 | The fully assembled and tested transformer is prepared for shipment to the port of destination, Georgetown, Guyana. All external protrusions are protected, and the unit is accompanied by the necessary shipping documents, including detailed packing lists and technical manuals, to facilitate customs clearance and final delivery. |
05 Site And Summary
In summary, this pad-mounted transformer delivers a robust and reliable power distribution solution, engineered for longevity and safety. Its optimized design, combining an efficient magnetic core, durable copper windings, and a sealed steel tank, ensures consistent performance in demanding outdoor environments. We are confident this unit will provide years of trouble-free service, making it a trusted and valuable asset for your electrical infrastructure.
