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With the increasing scarcity of land resources in urban core areas, traditional ground-mounted distribution substations and pad-mounted transformers are facing growing challenges in terms of space utilization, landscape integration, and safety protection. In addition, complex operating conditions such as high temperature, high humidity, salt mist corrosion, frost heave and thaw settlement in frozen soil regions, as well as power grid fluctuations, pose severe challenges to the long-term reliability of underground power distribution equipment.
Leveraging its extensive technical expertise in oil-immersed transformers, dry-type transformers, and amorphous alloy transformers, Zisheng Electric has developed underground transformers specifically designed for extreme environmental conditions. The products can be customized in accordance with international standards such as IEC 60076 and GOST, providing highly reliable underground power distribution solutions for urban infrastructure projects and overseas regions with special climatic conditions.
An underground transformer is a type of underground power distribution equipment developed for urban environments with limited space resources. By installing the main transformer body within an underground foundation structure, it achieves the integration of reliable power supply and efficient urban space utilization.
Installed below the outdoor ground level, the equipment occupies no surface space and is widely applicable to lighting and power supply systems in locations such as urban streets, highways, bridges, tunnels, parking lots, airports, ports, tourist attractions, and other public facilities.
During the development of urban core areas, commercial centers, transportation facilities, and public spaces impose increasingly higher requirements on ground space utilization. Traditional above-ground power distribution equipment is gradually facing challenges related to space occupation and landscape integration. The pressure placed on urban space by conventional pad-mounted transformers and distribution rooms is becoming increasingly evident.
As an environmentally friendly compact transformer, the underground transformer features energy efficiency, reliability, high performance, safety, and low power loss. Its application in underground distribution network design has become one of the important directions for modern urban power grid development. In the construction of urban roads, commercial areas, and public facilities, underground transformers can reduce the footprint of above-ground equipment and improve the integration between power distribution facilities and the urban environment. Infrastructure upgrade projects in regions such as the Middle East, South America, Africa, and Russia are also increasingly focusing on this technology approach.
Land resource saving. The main body of the underground transformer is installed in a concrete underground pit, occupying no surface space. Taking a typical configuration as an example, a 1250kVA liquid-immersed non-flammable underground transformer occupies approximately 6.25 m² underground, while the above-ground section occupies only about 2.3 m². Compared with a conventional pad-mounted transformer, which typically occupies approximately 7.21 m², the underground transformer can significantly improve land utilization efficiency. The surface area can be restored for use as green belts, parking lots, roads, or other public spaces.
Urban landscape enhancement. The above-ground section of the underground transformer can be designed as an integrated structure combining advertising panels and low-voltage switchgear cabinets. The interior of the enclosure contains a low-voltage distribution box, while illuminated advertising panels can be installed on both sides for public service advertisements, traffic guidance, or road information displays. This design aligns with modern urban ecological concepts and enables better integration with surrounding landscapes.
Noise reduction and electromagnetic shielding. By installing the main transformer body underground, the underground transformer effectively reduces operating noise, especially the impact of no-load noise at night on nearby residents. The underground pit structure also provides electromagnetic shielding capability.
High operational reliability. The underground transformer adopts a fully sealed structure with a protection rating of up to IP68, allowing temporary operation while submerged in water. The fully sealed, fully insulated, and fully shielded connection method improves operational safety and enables application in harsh environments such as mudslides and flooding conditions.
Environmental protection and energy efficiency. The underground transformer can be equipped with amorphous alloy or cold-rolled silicon steel sheet cores. The windings use oxygen-free copper conductors, combined with S11/S13/S15 energy-saving transformer technologies, achieving lower losses compared with conventional products.
Comparison Item | Traditional Pad-Mounted Transformer | Underground Transformer |
|---|---|---|
Above-ground footprint (1250kVA) | Approx. 7.21 m² | Approx. 2.3 m² |
Underground footprint | None | Approx. 6.25 m² |
Surface space restoration | Not available | Green belts / parking lots / roads |
Noise impact | More noticeable | Low (shielded underground) |
Protection rating | IP54 | IP68 (temporary water immersion capability) |
Urban landscape integration | General | High |
The underground transformer generally adopts a split-type modular structure, consisting of two main parts: above-ground facilities and an underground installation unit.
The underground section is centered around the underground transformer body, which is installed inside a prefabricated concrete pit or a prefabricated underground transformer foundation chamber. The transformer tank is manufactured using stainless steel or corrosion-resistant materials treated with processes such as hot-dip zinc spraying or powder coating, ensuring full sealing and corrosion resistance. The tank integrates components including high-voltage bushings, low-voltage terminals, high-voltage load switches, backup fuses, plug-in surge arresters, pressure relief valves, temperature control devices, and oil level indicators.
The above-ground section adopts a combined structure integrating advertising panels and low-voltage switchgear cabinets. The enclosure contains a low-voltage distribution box, while illuminated advertising panels can be installed on both sides. The high-voltage and low-voltage electrical cabinets are located above ground and adopt a compact structural design.
The auxiliary systems include a prefabricated pit foundation, automatic drainage system, temperature control system, and ventilation system. A water collection sump and automatic drainage system are installed inside the underground pit, enabling automatic drainage through water level sensors.
Capacity and voltage level. The capacity range of underground transformers is typically from 30kVA to 2000kVA. During selection, the transformer capacity and type should be determined according to the power load size, load characteristics (lighting or power applications), and distribution of electrical equipment. For inductive loads (such as motors), the starting current and operating current must be considered to ensure sufficient impact resistance.
The rated voltage is generally 6kV/10kV on the high-voltage side and 0.4kV on the low-voltage side, with a frequency of 50Hz or 60Hz. The typical vector groups are Dyn11 or Yyn0.
Selection Parameter | Typical Range | Description |
|---|---|---|
Rated capacity | 30kVA - 2000kVA | Determined according to load calculation |
High-voltage side voltage | 6kV / 10kV / 11kV / 33kV | Matched with the local power grid |
Low-voltage side voltage | 0.4kV / 0.415kV | Matched with terminal equipment requirements |
Frequency | 50Hz / 60Hz | Based on the power grid standard of the project location |
Vector group | Dyn11 / Yyn0 | According to system requirements |
Protection rating | IP68 | Suitable for submerged operation |
Operating sound level | ≤55dB | Meets urban environmental requirements |
Protection and environmental performance. The enclosure protection rating should reach IP68, meaning that the equipment can operate normally while being submerged in water for a certain period. Energy-efficient transformers should be prioritized to reduce energy consumption, and the operating sound level should be controlled below 55dB.
Intelligent monitoring functions. With the development of smart grid technologies, some underground transformers integrate remote monitoring and fault diagnosis functions. The intelligent monitoring system can perform temperature monitoring, remote meter reading, and anti-theft alarms through wireless communication modules (GPRS/GSM). During equipment selection, the installation of an intelligent safety monitoring terminal and an electrical fire monitoring system can also be considered.
Pre-installation preparation. Before installation, the transformer shall undergo a comprehensive inspection to ensure that all components are intact and the oil filling level meets the requirements. The installation site should be relatively flat and located away from areas prone to flooding.
Foundation construction. After excavation of the foundation pit, all debris shall be removed, and the foundation shall be designed according to the transformer’s dimensions and weight. The ventilation openings on the top shall be designed with rainproof and waterproof protection. The side walls shall be treated with waterproof concrete materials to prevent water infiltration from the sides of the pit. A waterproof concrete load-bearing base shall be installed at the bottom, elevated above the pit floor by a certain height, allowing any seepage water to flow into the annular drainage channel beneath the base and preventing the transformer from being immersed in water.
Equipment installation. The transformer shall be lifted into the prefabricated pit using lifting equipment, secured with foundation bolts, and properly grounded. During installation, the transformer’s external condition shall be carefully inspected for any damage and properly documented.
Cable connection. The high-voltage and low-voltage incoming and outgoing cables shall adopt a waterproof fully sealed, fully insulated, and fully shielded connection method. After the low-voltage terminal box is pre-installed with cables, the sealing modules shall be adjusted according to the cable diameter, and the bolts shall be tightened until a fully sealed condition is achieved.
Grounding system. The grounding conductor and grounding connections shall be capable of withstanding the rated short-time and peak withstand currents of the grounding circuit. In areas with high soil resistivity (such as Middle Eastern deserts and certain regions of Africa), the number of grounding electrodes and their installation depth shall be adjusted according to the measured soil resistivity.
Soil Conditions | Soil Resistivity (Ω·m) | Recommended Grounding Electrode Configuration |
|---|---|---|
Moist clay, marshland | ≤30 | Standard configuration |
Clay, moist sandy soil | 30 - 100 | Standard configuration |
Sandy clay, moist sand | 100 - 500 | Add 1-2 grounding electrodes |
Dry sand, gravel | 500 - 1000 | Increase the number of grounding electrodes and installation depth |
Desert, rocky soil | ≥1000 | Use resistance-reducing agents or extended grounding systems |
Design standards reference. The design of the underground substation can refer to relevant international standards and the regulations of the country where the project is located.
Routine inspection. Inspection items include checking whether the nameplate and other markings are intact, whether there are abnormal noises, whether the high-voltage cable joints have oil leakage, and whether the oil temperature, oil color, and oil level are normal.
Maintenance includes insulation resistance testing, infrared temperature measurement, oil level and oil quality management, cleaning, and inspection. Key components such as the tap changer, lead connections, and core grounding system shall be inspected and tested regularly. Periodic insulation performance testing shall also be carried out, including DC high-voltage testing and AC withstand voltage testing.
After floods, prolonged heavy rainfall, or the rainy season, the water accumulation condition inside the underground pit shall be checked. If excessive water is found, pumping and cleaning measures shall be implemented promptly.
A maintenance mode combining “monitoring center + inspection operation” can be adopted. Through the intelligent transformer management system, remote control functions can be achieved, including main circuit switch operation, drainage, ventilation, cooling, and other functions.
Maintenance Item | Period | Main Content |
|---|---|---|
Routine inspection | Weekly | External inspection, oil temperature, oil level, abnormal noise |
Insulation testing | Annually | Insulation resistance, DC withstand voltage |
Oil quality testing | Every 1-2 years | Breakdown voltage, moisture content, dielectric loss |
Infrared temperature measurement | Every 6 months | Joint temperature, hot spot detection |
Grounding resistance testing | Annually | Integrity of grounding system |
Drainage system inspection | Quarterly | Water level sensors, drainage pump operation |
The underground transformer is suitable for the following typical applications:
Municipal projects: Urban main roads, street lighting systems
Transportation facilities: Lighting and power systems for highways, bridges, tunnels, airports, and ports
Landscape areas: New lighting enhancement projects in parks, green spaces, tourist attractions, and urban plazas
Commercial centers: Power distribution upgrades in busy commercial districts and pedestrian streets
Residential communities: High-end residential developments and urban public distribution systems
Special locations: Parking lots and power capacity expansion projects in urban villages
The coastal commercial complex project in Doha, the capital of Qatar, has a total construction area of approximately 180,000 square meters, including a high-end shopping mall, office buildings, and hotels. Located in the core coastal area of Doha, the project site has extremely limited above-ground space. The owner required that power distribution equipment must not occupy above-ground developable space and must comply with urban landscape design regulations.
The region features a typical Gulf coastal climate, where summer ambient temperatures can exceed 45°C, while relative humidity remains between 80% and 90% throughout the year. Severe salt mist corrosion is a major environmental challenge. In addition, the project site has a relatively high groundwater level, placing higher requirements on the equipment’s waterproof sealing performance.
Based on the project conditions, we provided a complete underground transformer solution. The equipment adopts a fully sealed stainless steel enclosure with a protection rating of IP68, allowing reliable operation under submerged conditions. The enclosure is treated with hot zinc spraying combined with multi-layer anti-corrosion coatings, making it suitable for harsh salt mist coastal environments. The underground pit foundation is equipped with an automatic drainage system and a temperature control and ventilation system. Water level sensors enable automatic drainage control, ensuring safe operation of the transformer under rainy-season conditions and groundwater level fluctuations.
The main equipment parameters are as follows: rated capacity 1250kVA, high-voltage side 11kV, low-voltage side 0.415kV, frequency 50Hz, vector group Dyn11, cooling method ONAN, and design standard IEC 60076. The transformer core adopts high magnetic flux density grain-oriented silicon steel, with both no-load losses and load losses meeting first-class energy efficiency requirements. The equipment is equipped with an intelligent monitoring terminal, which enables oil temperature monitoring, remote operating status data reading, and fault warnings through a wireless communication module.
Project Information | Detailed Content |
|---|---|
Project Name | Underground power distribution project for a coastal commercial complex in Doha |
Project Location | Doha, Qatar |
Equipment Type | Underground oil-immersed distribution transformer |
Rated Capacity | 1250kVA |
Voltage Level | 11kV / 0.415kV |
Protection Rating | IP68 |
Special Design Features | Fully sealed anti-corrosion enclosure, automatic drainage system, intelligent monitoring terminal |
Design Standard | IEC 60076 |
Operating Period | 24 months |
The construction period for the underground pit was approximately 6 weeks. After completion of the transformer installation and cable connections, only the low-voltage switchgear cabinet and ventilation openings remained visible above ground, while the surrounding area was restored into a landscaped green zone.
As of today, the equipment has been operating continuously for more than 24 months, successfully undergoing three summer high-temperature periods and multiple heavy rainfall events. No shutdown failures caused by high temperature, humidity, or salt mist corrosion have occurred. Compared with traditional pad-mounted transformers, the underground transformer solution reduced above-ground space occupation by approximately 5 square meters, and the landscape integration effect met the project design requirements.
Relatively difficult maintenance and inspection. Since the equipment is installed underground, maintenance and troubleshooting require professional personnel and technical support, resulting in relatively higher maintenance costs.
Higher initial investment cost. The underground transformer has a more complex structure and higher construction requirements, so the overall investment cost may be higher than traditional solutions.
Heat dissipation requires special attention. Underground environments generally have poorer heat dissipation conditions. Comprehensive solutions such as low-temperature-rise design, optimization of surface treatment processes, and installation of foundation ventilation openings are required to effectively address thermal management issues.
Waterproofing and drainage systems must be fully considered. The waterproof design and drainage system of the underground pit foundation require comprehensive planning and engineering design.
With the increasing requirements for refined urban management and the continuous development of smart grid technologies, underground transformers are evolving toward the following directions:
Oil-free design — adopting new technologies such as non-flammable liquid-immersed transformers to eliminate potential fire hazards;
Invisible integration — further reducing above-ground facilities and achieving deeper integration with the urban environment;
Landscape-oriented design — improving the appearance of above-ground structures while adding information display and communication functions;
Miniaturization — reducing equipment size through technological innovation to adapt to more limited installation spaces;
Intelligent operation — integrating functions such as remote monitoring, fault diagnosis, automatic drainage, and automatic cooling control.
Zisheng Electric provides customized underground transformer solutions covering different capacity ranges and voltage combinations, designed and manufactured according to international standards such as IEC 60076. For target markets including the Middle East, South America, Africa, and Russia, where special operating conditions such as high temperature, high humidity, salt mist, frozen soil, and sandstorms are common, we have developed technical capabilities in equipment corrosion protection, sealing performance, heat dissipation, and low-temperature start-up design. These capabilities enable us to provide reliable underground power distribution solutions adapted to local environmental conditions for urban power distribution and infrastructure projects.
Zisheng Electric — with years of experience in power equipment research, development, and manufacturing. Our product portfolio covers dry-type transformers, oil-immersed transformers, amorphous alloy transformers, and switchgear equipment. We provide customized manufacturing according to international standards including IEC 60076 and GOST, serving markets across the Middle East, South America, Africa, and Russia.
For underground transformer project selection, technical specifications, and procurement support, please send your project requirements to Zisheng Electric’s official channels for professional technical assistance.
