HUARUI 
Pad Mounted type compartmental transformer
An American-style pad-mounted transformer, also known as a combined transformer, is a compact power distribution device that integrates the transformer body, high-voltage load switch, and protective fuses into a single oil tank. Introduced to China from the United States in the early 1990s, its most significant feature is the direct immersion of the high-voltage switch and fuses in the transformer's insulating oil, utilizing the oil as both insulation and cooling medium, thus significantly reducing the equipment's size.
American-style pad-mounted transformers employ a fully sealed, fully insulated structure, eliminating the traditional transformer's oil conservator (oil tank). The oil tank and radiator are directly exposed to the air, allowing for flexible power supply via either a ring network or terminal connections. This makes it an ideal choice for urban power distribution networks employing a "small capacity, dense distribution" strategy.
Adopting a shared-tank integrated design, the high-voltage components are housed within the transformer tank, resulting in a volume only one-third to one-fifth that of a European-style prefabricated transformer of the same capacity. This minimizes footprint and allows for flexible installation.
All high-voltage live parts are immersed in oil and sealed within the tank. There are no exposed live parts, eliminating the need for insulation distances and reliably ensuring personal safety.
The high-voltage side employs a combination protection scheme of a backup current-limiting fuse and a plug-in dual-sensitive fuse. The plug-in fuse is overload sensitive (temperature + current dual-sensitivity), while the backup fuse provides short-circuit protection. Together, they achieve full-range fault protection, significantly reducing operating costs.
It can be used for both ring network power supply (with ring network cable socket) and terminal power supply, offering flexible and convenient switching. The elbow-type cable connector can be plugged and unplugged under load currents of 200A/600A and can be operated as a load switch in emergencies.
The oil tank and corrugated heat sink are directly exposed to the air, resulting in high heat dissipation efficiency; the oil-immersed structure provides excellent short-term overload withstand capability, suitable for scenarios with fluctuating electrical loads.
The enclosure uses anti-corrosion spraying and special surface treatment, effectively resisting corrosion from harsh environments such as wind and sand, coastal salt spray, and high temperature and humidity.
The core uses high-permeability silicon steel sheets, combined with a Dyn11 connection group design, offering advantages such as neutral point non-drift, high voltage quality, and good lightning protection performance, with a noise level below 52dB.
| Item | Range / Description |
| Rated Capacity | 50 kVA ~ 2500 kVA (standard type); expandable to 7000 kVA (wind/PV type) |
| High Voltage Side | 6 kV ~ 35 kV (common: 12.47 kV, 13.2 kV, 24 kV, 34.5 kV) |
| Low Voltage Side | 0.4 kV / 0.48 kV / 0.69 kV, etc. |
| HV Tapping Range | ±2×2.5% (DETC - De-energized Tap Changing) |
| Connection Group | Dyn11, YNd1, etc. |
| Rated Frequency | 50 Hz / 60 Hz |
| Cooling Method | ONAN (Oil-immersed self-cooled); ONAF (air-cooled) optional |
| 1-min Power Frequency Withstand Voltage | 35 kV (for 10 kV class) 75 kV (for 35 kV class) |
| Lightning Impulse Withstand Voltage (BIL) | 75 kV ~ 95 kV (for 10 kV class) 150 kV ~ 200 kV (for 35 kV class) |
| Ambient Temperature Range | -25℃ ~ +45℃ (standard); expandable to -40℃ ~ +45℃ |
| Applicable Standards | GB/T 17467, JB/T 10217, ANSI C57.12.00/34, IEEE Std C57.12.20, IEC 60076, etc. |
Transformer Body
Includes the core (high-permeability silicon steel sheets laminated) and high and low voltage windings (copper or aluminum wire windings), placed inside the main tank to achieve voltage transformation.
High-Voltage Load Switch
Immersed in transformer oil, compact in size, operable in 360° direction, used for safely switching load current. Ring network type equipped with multi-position switch, supporting ring network power supply mode.
Dual Fuse Protection System
- Backup current-limiting fuse: Used for short-circuit fault protection inside the transformer and on the secondary side, quickly interrupting large fault currents.
- Insert-type fuse: Uses dual-sensitive fuse wire (sensitive to both temperature and current), used for overload protection, can be replaced on-site, reducing operation and maintenance costs.
High-Voltage Cable Accessories (Elbow Plug)
Fully insulated cable connectors that can be plugged and unplugged under load.
Insulating Oil
Mineral oil (IEC 60296) or high flash point environmentally friendly oil (such as FR3 natural ester oil, flash point > 312℃), providing insulation, heat dissipation, and arc extinguishing functions. High flash point oil can be selected for applications with higher fire protection requirements, such as indoor installation.
Oil Tank and Cooling System
A fully sealed oil tank welded from steel plates, eliminating the need for an oil conservator. The outer wall is a corrugated or plate-type radiator, directly exposed to the air for heat dissipation.
Protection and Monitoring Accessories
Includes a pressure relief valve (explosion-proof), oil level gauge (window type or meter type), oil thermometer (with optional temperature controller), drain valve/sampling valve, etc.
High and Low Pressure Chambers
Instrument Box
An independent protective box housing oil thermometers, oil level gauges, vacuum pressure gauges, winding thermometers, etc.
1. Core Product Advantages
① High Cost-Effectiveness: The integrated design significantly reduces costs compared to European-style prefabricated substations (small size, material savings) and traditional distribution room solutions. No dedicated distribution room is required, greatly saving on infrastructure investment.
② High Safety and Reliability: Fully insulated and sealed structure with no exposed high-voltage live parts, eliminating the risk of electric shock. Dual-fuse protection provides full fault coverage, ensuring safe operation of the power grid.
③ Easy and Quick Installation: Shipped as a complete unit, requiring only cable connections on-site for immediate commissioning, resulting in a very short installation period. Suitable for both outdoor ground installation and indoor use (requires fire protection).
④ Flexible Networking Capabilities: Terminal type (Z) and ring network type (H) are available, allowing for rapid connection to existing power distribution networks; cable connectors can be plugged and unplugged under load, facilitating maintenance and temporary power supply operations.
⑤ Excellent overload and short-circuit withstand capabilities: The oil-immersed structure and high insulation strength design give it superior short-term overload and sudden short-circuit withstand capabilities compared to dry-type transformers.
⑥ Excellent protective performance: Anti-corrosion coating and fully sealed design enable it to withstand various harsh outdoor environments such as wind, sand, salt spray, high humidity, and extreme temperatures.
2. Service Life
Under standardized operation and maintenance conditions, the overall design service life of the equipment can reach 20-30 years. The core iron core and windings have no easily damaged parts, resulting in slow lifespan decay. Equipment aging mainly stems from the deterioration of insulating oil and the aging of seals. Regular oil quality testing (such as DGA analysis) and seal replacement can significantly extend the service life.
1. Daily Maintenance
① Daily Inspection: Regularly check the equipment casing for oil leaks, deformation, or corrosion; observe whether the oil level gauge reading is within the normal range; record the oil temperature and operating sound; and troubleshoot any abnormalities.
② Oil Quality Testing: It is recommended to sample and test the dielectric strength, moisture content, and dissolved gas (DGA) of the insulating oil every 1-2 years. Based on the test results, filter, regenerate, or replace the oil.
③ Component Inspection: Regularly calibrate pressure relief valves, oil level gauges, and temperature controllers, etc.; check the high-voltage elbow plug connections for tightness and any signs of discharge.
④ Cleaning and Heat Dissipation: Regularly clean dust and debris from the surface of the oil tank and heat sink to ensure unobstructed heat dissipation and prevent heat buildup.
2. Safety Regulations
① Live-Line Work Safety: Equipment maintenance must be performed after power is disconnected, voltage is tested, and a grounding wire is connected. It is strictly forbidden to plug or unplug elbow cable connectors while the power is on (operations must be performed after power is off). Workers must be certified and wear insulated protective equipment.
② Fire Prevention and Environmental Protection: Open flames and flammable and explosive materials are strictly prohibited around the equipment. Mineral oil is flammable; in situations with strict fire prevention requirements, high-flash-point natural ester oil (FR3) should be used, along with appropriate fire-fighting facilities. When replacing insulating oil, a dedicated container must be used for recycling; indiscriminate dumping that pollutes the environment is strictly prohibited.
③ Compliant Installation Environment: The installation location must meet requirements for ventilation, heat dissipation, maintenance, and safe distances. It must not be placed close to walls or flammable/explosive facilities. Indoor installations must be equipped with fireproof partitions, oil leakage collection tanks, and other compliant fire protection facilities.
④ Fault Handling: If abnormal noises, oil spraying, high-temperature alarms, or pressure relief valve activation occur, immediately stop the machine and disconnect the power. The cause must be investigated before restarting operation; operation with a fault is strictly prohibited.
1. Urban Power Distribution Network Upgrade: As the core equipment for "small capacity, dense distribution points" in load centers, it solves the problems of high line losses and long power supply radius.
2. Residential and Commercial Centers: Terminal power distribution for residential communities, commercial complexes, and office buildings; can be installed along streets and in green belts, aesthetically pleasing and without occupying ground building space.
3. Industrial and Mining Enterprises: Power distribution for factories, mines, ports, petrochemical plants, and other enterprises, suitable for industrial scenarios with large load fluctuations.
4. New Energy Power Generation: Wind power plants (step-up grid connection, suitable for coastal and desert environments) and photovoltaic power stations (step-up grid connection), compatible with wind/photovoltaic dedicated models.
5. Municipal and Infrastructure: Power distribution for public places such as urban road lighting, airports, stations, hospitals, and schools.
6. Temporary Power Supply: Temporary or mobile power distribution needs for construction sites, oil field operations, large-scale events, etc.
1. Environmental Risks
1. Leakage and Pollution Risk: Mineral oil leakage can pollute soil and water sources. High flash point environmentally friendly natural ester oil (FR3) should be used in environmentally sensitive areas (such as water sources and farmland), and the sealing of the enclosure should be strengthened, with corresponding leak-proof collection measures.
2. Fire Risk: Mineral oil is flammable and may ignite in the event of an arc fault.
3. Corrosion and Salt Spray Risks: Coastal areas and chemical plants, where salt spray or corrosive gases can easily corrode the enclosure and metal components. C4-grade or higher anti-corrosion coatings and stainless steel fasteners are required, along with special coating treatment for the enclosure.
4. High Temperature and Heat Dissipation Risks: Prolonged high temperatures accelerate the oxidation of insulating oil and the aging of seals. In high-temperature areas, enhanced heat dissipation configurations or appropriately reduced load rates should be used.
5. Altitude Adaptability Limitations: For altitudes exceeding 1000m, customized high-altitude reinforced insulation products (increasing creepage distance) are necessary to mitigate the risk of insulation breakdown due to low air pressure.
6. Noise Impact: Installation near residential areas requires consideration of noise limits (e.g., ≤55dB at night), necessitating the selection of low-noise models (down to below 45dB).
2. Installation Restrictions
1. Foundation Requirements: A concrete foundation platform is required. The ground must be flat, have sufficient load-bearing capacity, and meet drainage and cable channel requirements.
2. Safety Distances: Equipment installation must meet safety distance requirements for live parts to the ground, buildings, and surrounding facilities, and allow sufficient space for heat dissipation and maintenance operations.
3. Fire Safety Compliance: Indoor or underground installations must be equipped with compliant fire-fighting facilities such as fireproof partitions, oil spill collection tanks, and dry powder fire extinguishers, and meet electrical safety acceptance requirements.
4. Transportation Restrictions: The equipment is shipped as a complete unit; height and width restrictions and hoisting conditions for transportation routes must be considered. During transportation, the equipment must be securely fixed to prevent damage from vibration and bumps.
This product supports full-dimensional personalized customization and can be precisely adapted to project conditions, environmental conditions, industry standards, and customer needs. Key dimensions are as follows:
| Customization Dimension | Optional Range / Description |
| Capacity & Voltage | 50 kVA ~ 7000 kVA; HV ≤ 35 kV, LV customizable (0.36 / 0.4 / 0.48 / 0.69 kV, etc.) |
| Power Supply Mode | Terminal type (Z): standard terminal power supply Ring network type (H): connectable to ring network system for dual or multiple power inputs |
| Insulating Oil Type | Mineral oil (IEC 60296) / High flash point environmentally friendly natural ester oil (e.g., FR3, flash point > 312℃) / Synthetic ester oil |
| Winding Material | All copper / All aluminum / HV aluminum + LV copper, etc. |
| Connection Group | Dyn11, Dyn1, YNd1, Ynd11, etc. |
| Corrosion Resistance Grade | C3 / C4 / C5 grades, suitable for harsh environments such as coastal salt spray and industrial pollution |
| Application Scenarios Customization | Wind power dedicated: adaptable to harsh environments (beaches, deserts, etc.), capacity expandable to > 4,500 kVA Photovoltaic dedicated: suitable for PV grid-connection requirements |
| Low-Voltage Configuration | Optional LV molded case circuit breakers, automatic reactive power compensation switching devices, intelligent undervoltage controllers, metering and remote control devices |
| Intelligent Monitoring | Optional online oil temperature / oil level monitoring, remote data transmission, fault alarm, and automatic voltage regulation functions; suitable for unattended smart grid systems |
| Standards | GB/T 17467, JB/T 10217, ANSI C57.12, IEEE C57.12.20, IEC 60076, etc. |
| Colors & Markings | User-specified color codes (e.g., RAL chart); nameplates and warning labels in Chinese / English |
| Components | European standard (EN) bushings, American standard (IEEE) bushings; optional winding thermometers, sudden pressure relays, etc. |
