Efficient energy conversion-Three-phase pad-mounted transformers make your power usage worry-free!
01 General
1.1 Project Background
In 2024, a 1,000 kVA pad-mounted transformer was delivered to the United States for the Belmont Municipal Lighting Department. This three-phase distribution transformer is designed to operate on a 13.8 kV grounded wye (GrdY)/7.97 kV system and features a ±2 × 2.5% no-load tap changer (NLTC) for flexible voltage adjustment. The secondary voltage is 0.208/0.12 kV, forming a YNyn0 vector group, and it is configured as a loop-feed transformer for reliable network operation.
This transformer doesn’t use PCB and is filled with FR3, an environmentally friendly oil. It has a dead-front, loop-feed design, which makes it really safe and reliable in everyday operation. Being pad-mounted, it can be installed outdoors on a concrete base, so it works well in cities, residential neighborhoods, parks, industrial areas, airports-you name it.
A key feature of this transformer is its separate box structure: the transformer itself and the high-voltage components-like load switches, plug-in fuses, and backup current-limiting fuses-are all in their own sealed tanks, divided by a partition. The partition even has an insulated through-wall sleeve, which makes connecting the transformer to the load switches much simpler. It also means you can maintain or replace parts without messing with the other components, which is really convenient. This design makes maintenance and component replacement easier, without affecting the normal operation of other parts.
Overall, this transformer is compact, easy to install and maintain, low-noise, low-loss, anti-theft, and capable of handling strong overloads. It provides full protection and is suitable for loop power supply, dual power supply, or terminal power supply systems, serving as a device for power transformation, metering, compensation, control, and protection.
1.2 Core & Coil Highlights
Five-legged, high-quality silicon steel core; low-loss, high permeability, anti-aging.
Copper windings for high efficiency and durability.
Firm fixation reduces vibration, noise, and transport distortion.
Filled with FR3 less-flammable, eco-friendly fluid, ensuring safe and efficient long-term outdoor operation.
1.3 Technical Specification
1000 kVA transformer specifications type and data sheet
| Delivered to | South America |
| Year | 2024 |
| Type | Pad mounted transformer |
| Standard | IEEE C57.12.34-2022 |
| Rated Power | 1000KVA |
| Frequency | 60HZ |
| Phase | 3 |
| Feed | Loop |
| Front | Dead |
| Cooling Type | KNAN |
| Primary Voltage | 13.8GrdY/7.97 KV |
| Secondary Voltage | 0.48Y/0.277 KV |
| Winding Material | Copper |
| Liquid Insulant | FR3 Oil |
| Angular displacement | YNyn0 |
| Impedance | 5.75% |
| Tap Changer | NLTC |
| Tapping Range | ±2*2.5% |
| No Load Loss | 1.15KW |
| On Load Loss | 7.56KW |
| Accessories | Standard Configuration |
1.4 Drawings
1000 kVA pad mounted transformer diagram drawing and size.
02 Manufacturing
2.1 Core
| The core shall be five-legged and constructed of the highest quality, non-aging, cold-rolled, grain oriented, stress-free, thin silicon steel laminations. The core shall have high permeability and low hysteresis losses. The steel core laminations shall be properly annealed and shall have smooth surfaces at the edges. Each sheet shall have an insulated surface, which is impervious to hot transformer oil. The core shall be rigidly clamped and blocked to prevent deteriorating vibrations, interference with oil circulation, objectionable noise conditions, and short circuit and shipment distortions. The core shall be securely grounded to the tank. |  |
2.2 Winding
 | The transformer windings are made of copper. The foil-wound low-voltage design increases surface area for better heat dissipation, supporting continuous high-load operation. It ensures more uniform current distribution, enhancing overall stability, resists electromagnetic forces from short circuits, reduces leakage losses, and lowers temperature rise within the windings. This combination improves efficiency and extends the transformer’s service life. |
2.3 Tank
| The transformer tank is a weatherproof, welded steel, liquid-tight structure mounted on a steel skid base for rolling or skidding, with lifting hooks and jacking facilities. It includes two copper-faced ground pads, dead-front tamperproof terminal compartments with hinged doors, steel barriers, and brass hardware. Tubular radiators are permanently welded at the rear. Doors can be propped open 150° for safe maintenance. Surfaces are sandblasted, interior treated, and exterior coated with primer and two finish layers (min. 3 mils) in Munsell Green 7GY 3.29/1.5, with extra protection at the bottom and sill. |  |
2.4 Final Assembly
 | Component Inspection: Confirm the integrity of the transformer core, enclosure, and electrical components. Transformer Assembly: Combine the core with the windings and perform insulation treatment. Enclosure Installation: Assemble the stainless steel housing and ensure that it is sealed to prevent corrosion. Electrical Connections: Connect high and low voltage terminals as well as the grounding system. Safety Devices: Install protective devices to prevent overloads and short circuits. Cooling System: Install a cooling system to ensure effective heat dissipation. |
03 Testing
| No. | Test Item | Unit | Acceptance Values | Measured Values | Conclusion |
| 1 | Resistance Measurements | % | Maximum resistance unbalance rate
| 0.29 | Pass |
| 2 | Ratio Tests | % | The deviation of voltage ratio on the principal tapping: ≤0.5% | 0.11-0.12 | Pass |
| 3 | Phase-relation tests | / | YNyn0 | YNyn0 | Pass |
| 4 | No-load losses and excitation current | % | I0 :: provide measured value | 0.30 | Pass |
| kW | P0: provide measured value | 1.115 | |||
| / | the tolerance for no load loss is ±10% | / | |||
| 5 | Load losses , impedance voltage, total losses and efficiency | / | t:85℃ the tolerance for impedance is ±7.5% the tolerance for total load loss is ±6% | / | Pass |
| % | Z%: measured value | 5.88 | |||
| kW | Pk: measured value | 7.264 | |||
| kW | Pt: measured value | 8.379 | |||
| % | Efficiency not less than 99.43% | 99.45 | |||
| 6 | Applied Voltage Test | / | LV: 10kV 60s | No collapse of the test voltage occurs | Pass |
| 7 | Induced Voltage Withstand Test | / | Applied voltage (KV):0.96 | No collapse of the test voltage occurs | Pass |
| Duration(s):30 | |||||
| Frequency (HZ): 240 | |||||
| 8 | Insulation Resistance Measurement | GΩ | HV-LV to Ground: | 2.54 | / |
| LV-HV to Ground: | 2.76 | ||||
| HV&LV to Ground: | 2.89 | ||||
| 9 | Leakage Test | / | Applied pressure：50kPA | No leakage and no Damage | Pass |
| Duration：12h | |||||
| 10 | Oil Test | kV | Dielectric Strength | 51.7 | Pass |
| mg/kg | Moisture Content | 106.4 | |||
| % | Dissipation Factor | 0.02106 | |||
| mg/kg | Furan Analysis | / | |||
| / | Gas Chromatography Analysis | / |
04 Packing and Shipping
05 Environmental & Operational Benefits
FR3 Oil: Biodegradable, less flammable than mineral oil, eco-friendly.
Robust Outdoor Installation: Weatherproof, corrosion-resistant, secure core and winding fixation.
Energy Efficiency: Low iron loss, optimized copper windings, controlled impedance.
Balances reliability, safety, and environmental responsibility.
