General
Application
Environmental
Standards
An oil-immersed earthing transformer is an auxiliary three-phase transformer that provides a grounded neutral and low-impedance path for zero-sequence currents during earth-fault conditions. Windings are typically arranged in zigzag or Y–Δ configurations and immersed in mineral or ester oil with oil–paper insulation and ONAN/ONAF cooling. The design offers high thermal capacity, low zero-sequence impedance, and defined zero-sequence voltage, supporting stable system earthing, controlled fault current levels, reliable insulation margins, and operation of protection and monitoring devices over the asset lifetime.
Oil-immersed earthing transformers are applied in medium- and high-voltage networks to provide a defined neutral reference and controlled earth-fault current for utility substations, wind and solar farms, industrial plants and transmission or distribution nodes. They are typically connected to MV/HV busbars in solidly grounded, resistance-grounded or Petersen-coil-grounded systems, supporting selective protection, insulation coordination and touch-voltage control. By establishing a stable zero-sequence path, they enhance network reliability, fault localization accuracy and grid-code compliance in complex interconnected power systems.
Application:outdoor installation.
Ambient temperature: -50°C to +40°C
Relative humidity: ≤90% (at 25°C); ambient air RH <93%.
Altitude: ≤1000m above sea level.
Max wind speed: ≤35 m/s.
Seismic withstand: Horizontal ≤0.3 g; Vertical ≤0.15 g.
Special conditions: Custom-built solutions available fo·r project-specific requirements
Ambient temperature: -50°C to +40°C
Relative humidity: ≤90% (at 25°C); ambient air RH <93%.
Altitude: ≤1000m above sea level.
Max wind speed: ≤35 m/s.
Seismic withstand: Horizontal ≤0.3 g; Vertical ≤0.15 g.
Special conditions: Custom-built solutions available fo·r project-specific requirements
IEC 60076-1 – Power Transformers, General
IEC 60076-2 – Temperature Rise for Liquid-Immersed Transformers
IEC 60076-3 – Insulation Levels and Dielectric Tests
IEEE C57.12.00 – General Requirements for Liquid-Immersed Transformers
IEEE C57.12.90 – Test Code for Liquid-Immersed Transformers
IEEE C57.32 – Neutral Grounding Devices
IEC 60076-2 – Temperature Rise for Liquid-Immersed Transformers
IEC 60076-3 – Insulation Levels and Dielectric Tests
IEEE C57.12.00 – General Requirements for Liquid-Immersed Transformers
IEEE C57.12.90 – Test Code for Liquid-Immersed Transformers
IEEE C57.32 – Neutral Grounding Devices
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Why Choose Kerun Intelec
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Fast Lead Times
Our made-to-order production timelines are highly competitive and as low as 8 weeks from order to delivery.
Design Conformance
We maintain rigorous oversight to ensure every transformer adheres precisely to your exact specifications.
Comprehensive Warranty
We stand behind the longevity and reliability of our products with a robust warranty that covers both materials and workmanship.
Competitive Pricing
We deliver exceptional value so you receive top-tier, reliable, high-quality products without the premium price tag
Exceptional Quality
We implement strict quality control measures at every stage of production to ensure our products meet or exceed industry standards.
Dedicated Support
Our support team is always at your service.Offering expert guidance from initial inquiry through post-installation support.
Transformer Features
Engineered cast resin Oil-Immersed Earthing Transformer combining high efficiency, low partial discharge, robust insulation, and indoor operation with minimal maintenance.
Flexible Design - Well-matched to a Range of Applications.
Oil-immersed earthing transformers support zigzag or Y–Δ connections, multiple voltage classes and grounding methods, allowing engineers to match neutral grounding performance to specific network and protection requirements.
Space Saving – Effective, Flexible Layout
Compact tank designs and integrated radiators fit standard substation bays, enabling flexible outdoor placement near MV switchgear or busbars, and reducing additional structures or cable runs.
Laborsaving – Reduction of Manpower
Factory-assembled units with predefined terminals and accessories minimize field wiring and adjustments, simplifying installation, shortening outage windows and reducing the number of technicians required on site.
Energy Saving – Reduced Operating Costs
Optimized core steel, low-loss windings and efficient ONAN/ONAF cooling reduce no-load and load losses, lowering long-term energy consumption and improving total cost of ownership for utilities and plants.
High Reliability – Long-term, Trouble-free Operation
Robust oil–paper insulation, short-circuit-tested windings and conservative thermal design provide stable performance under repetitive ground-fault duty, supporting long service life with predictable operating behavior.
Safe Design – Ensures Operator’s Safety
Grounded tank, clearly segregated HV and LV terminals, provision for protection relays and pressure relief devices help limit touch voltage, arc-energy exposure and oil-related safety risks for personnel.
Minimal Maintenance – Reduced Maintenance Costs
Standardized oil sampling points, sealed bushings and durable gaskets support condition-based maintenance, allowing extended inspection intervals and fewer routine interventions compared with older grounding equipment.
Design Drawings
Detailed technical drawings showcasing optimal transformer layout, connection points, and precise dimensions, ensuring seamless integration and efficient installation in various applications and environments.
20kV-200kVA-Earthing-Transformer-drawing
20kV-1250kVA-Earthing-Transformer-drawing
Oil-Immersed Earthing Transformer Accessories Include
Tank
Core
Coils
Active Part
Conservator
Transformer Oil
Transformer Radiator
HV Bushings
LV Bushings
Earthing Terminals
Neutral Grounding Terminal
Tap Changer
Oil Level Indicator
Oil Level Indicator
Winding Temperature Indicator
Bay-o-net Fuse
Pressure Relief Valve
Gas Relay
Surge Arrester
Surge Arrester
Oil Drain Valve
Filling Valve
Transformer Breather
Wheels
Lifting Lugs
Base Frame
Nameplate
About Construction
Core components combine low-loss step-lap core, robust windings and precision tank systems to deliver stable grounding performance, reliability and long service life.
01
Tanks
02
Core
03
Coils
04
Active Part
05
Conservator
06
Transformer radiator
Tanks
The oil-immersed earthing transformer tank is fabricated from low-carbon steel with customized thickness and dimensions, integrating lifting lugs, base structure and reserved pads for accessories. CNC laser cutting and robotic welding after surface sandblasting ensure dimensional accuracy and weld integrity. Optional hot-dip galvanizing plus electrostatic powder coating with color selection provide durable corrosion protection and consistent outdoor performance.
Dimensions/structure: Customized special configurations supported.
Robotic welding delivers uniform seams and minimizes leakage risk.
Core
The core of the oil-immersed earthing transformer uses either grain-oriented silicon steel laminations or amorphous alloy ribbons, with customizable thickness and geometry to meet loss and noise targets. Fully automatic core cutting and stacking lines control dimensional tolerances and burr height, delivering uniform magnetic paths, reduced no-load losses and repeatable performance across ratings for utilities and industrial distribution networks worldwide.
Material options: Available in grain-oriented silicon steel or amorphous alloy.
Dimension customization: Custom sizes and lamination thicknesses available.
Coils
HV and LV coils use copper or aluminum wire, with LV also available in copper or aluminum foil to optimize current density and short-circuit strength. Insulation systems can be specified with DMD, Nomex® or Weidmann materials, matching thermal class and dielectric requirements. This flexibility supports tailored loss performance, mechanical robustness and reliable earthing duty over demanding medium- and high-voltage lifetimes.
HV/LV Winding materials:Cu/Al wire.LV also available in Cu/Al foil.
Insulation materials: optional DMD, Nomex®, or Weidmann.
Active Part
The active part combines a low-loss step-lap core, precision-wound HV/LV coils, rigid clamping structure and oil-immersed insulation system to control flux density, vibration and acoustic noise. Optimized core steel and winding geometry deliver uniform flux and current distribution, high dielectric strength and lightning impulse withstand, while robust bracing maintains zero-sequence performance and a stable grounding path under repeated ground-fault forces.
Low no-load and load losses improve efficiency and reliability.
Rigid clamping frames reduce vibration, noise and displacement.
Conservator
The conservator is a round, low-carbon steel vessel manufactured with the same blasted, robot-welded and coated process as the main tank. It provides an expansion volume for insulating oil, stabilizing operating pressure and protecting insulation integrity. Configurable oil level indicators and breathers make it easy to monitor condition clearly, limit moisture ingress, and support reliable, long-term operation of the earthing transformer.
Manages oil expansion, reducing tank mechanical stress and deformation.
Separates oil from ambient air, limiting moisture ingress and oxidation.
Transformer radiator
The conservator and radiator system uses cold-rolled steel, robot-welded for consistent seams, mechanical strength and efficient oil cooling. Coatings match the main tank system to maintain corrosion resistance across all surfaces. Radiator height, fin spacing and module quantity are fully customizable, with options for detachable modular panels or tank-integrated sealed radiators to suit site layout and thermal performance requirements.
Automated seam welding ensures oil-tight, uniform radiator chambers.
Coating system matches tank, providing consistent outdoor corrosion resistance.
About Components
Configurable accessories such as bushings, tap changers, monitoring devices and protection hardware enhance safety, observability, maintenance efficiency and long-term grounding reliability performance.
01
Bushings
02
Tap Changer
03
Terminal Box
04
Oil Level indicators
05
Oil Thermometer indicator
06
Winding Temperature Indicator
07
Pressure Relief Valve
08
Gas relay
09
Oil sample Device
10
Oil drain valve
11
Transformer breather
12
Earthing terminals
13
wheel
14
Nameplate
Bushings
Bushings on the oil-immersed earthing transformer provide the insulated transition between internal windings and external connections. Options include porcelain or polymer housings with oil-impregnated paper or RIP/RIS insulation, coordinated to system BIL and creepage requirements. Robust flanges, gaskets and terminals ensure leak-tight sealing, controlled electric field distribution and safe, low-partial-discharge performance under switching surges and earth-fault duties.
Voltage range: HV bushings available from 6–36 kV.
Type options: DIN, EN, or ANSI configurations available.
Tap Changer
The tap changer on an oil-immersed earthing transformer provides discrete HV winding adjustments to fine-tune system voltage and zero-sequence impedance. Off-circuit or on-load designs support accurate setting of earth-fault current, temporary overvoltage and neutral displacement. Robust contacts, clear position indication and interlocking options help ensure repeatable switching, reduced wear and safe, predictable grounding performance over the service life.
Type selection: DETC (off-circuit) or OLTC (on-load) available.
Tap ranges ：(±2.5% to ±10%), 5–33 positions available.
Terminal Box
The terminal box consolidates instrument, protection and auxiliary connections for the oil-immersed earthing transformer. It provides segregated terminals for CTs, PTs, temperature sensors and monitoring devices, with labeled terminal strips and generously sized cable glands. A robust, gasketed enclosure with defined IP rating improves wiring reliability, simplifies testing and maintenance access, and supports safe, orderly interface with substation control systems.
Gasketed door and seals maintain weatherproof, dust-tight internal environment.
IP-rated steel enclosure protects control, protection and monitoring circuits...
Oil Level indicators
Oil level indicators provide continuous visual and, optionally, electrical monitoring of insulating oil volume in the tank or conservator. Clear marking for cold, normal and high levels supports safe loading decisions, accommodates temperature-related expansion, prevents exposure of windings and cores, and helps detect leakage early, enabling planned maintenance, SCADA integration and long-term dielectric strength in oil-immersed earthing transformers..
Clear graduated scales allow quick visual checks during inspection.
Vertical or horizontal mounting options suit different tank designs.
Oil Thermometer indicator
The oil temperature indicator continuously tracks top-oil temperature in the earthing transformer, helping verify safe thermal margins and cooling performance. A clear dial and adjustable alarm contacts support quick local checks and remote warnings. Rugged bulb, capillary and housing designs withstand vibration, outdoor conditions and switching transients, enabling timely load management and condition-based maintenance.
Bimetal sensing element ensuring accurate temperature measurement.
Resettable maximum pointer records peak oil temperatures for diagnostics.
Winding Temperature Indicator
The Winding Temperature Indicator (WTI) is a critical protection device for oil-immersed earthing transformers. It accurately monitors the real-time hot-spot temperature of the windings, providing essential data for proactive maintenance and overload protection. By enabling predictive thermal management, it helps prevent insulation degradation, extends transformer service life, and enhances overall system reliability.
Features high-accuracy sensors for reliable hot-spot temperature measurement.
Integrates configurable alarm and trip contacts for automatic protection.
Pressure Relief Valve
The pressure relief valve provides a controlled venting path for sudden internal pressure rises caused by internal faults or thermal events. A calibrated spring mechanism opens rapidly, limiting tank rupture risk and protecting personnel. Reliable sealing, directional discharge, reset capability and optional alarm contacts support safe fault clearing, post-event inspection and compliance with modern transformer safety practices.
Preset opening pressure protects tank against dangerous overpressure events.
Self-resealing design maintains oil tightness after transient venting operations.
Gas relay
The gas relay on an oil-immersed earthing transformer continuously monitors gas accumulation and oil surge between tank and conservator, providing early warning of internal faults. Float and vane mechanisms detect slow gas generation and rapid oil movement, issuing alarm or trip signals. This improves fault detection sensitivity, limits damage to the active part and enhances overall grounding system reliability.
Two-stage alarm and trip contacts separate minor, major faults.
Robust housing and seals tolerate vibration, temperature cycling and weather.
Oil sample Device
The oil sampling device on an oil-immersed earthing transformer provides a defined point for drawing representative insulating liquid specimens under controlled conditions. A metal-bodied sampling fitting with shut-off mechanism, threaded connection and protective cap supports safe handling, consistent sample quality and contamination control. Its position and clear labeling simplify routine DGA, moisture analysis and condition-based maintenance planning.
Tight sealing prevents moisture ingress and particulate contamination.
Design minimizes trapped air and dead volumes during sampling.
Oil drain valve
The oil drain outlet is located at the lowest point of the tank, providing a controlled path for removing insulating liquid during maintenance, retrofits or decommissioning. A robust metal body, shut-off mechanism and hose connection support safe handling, spill control and contamination management, reducing outage time and simplifying oil replacement procedures in line with utility safety and strict environmental requirements.
Brass construction ensuring excellent corrosion resistance.
Sealing elements maintain tightness, helping prevent leaks during service.
Transformer breather
The transformer breather controls air exchange between the conservator and atmosphere while drying incoming air through desiccant media. This helps limit moisture and contaminants entering the insulating oil, preserving dielectric strength and reducing aging. Durable housings, clear fill-level indication and easy desiccant replacement support reliable operation, straightforward maintenance and long-term insulation performance for oil-immersed earthing transformers.
Color-changing silica gel for easy status monitoring.
Transparent bowl allowing visual inspection of desiccant.
Earthing terminals
Earthing terminals on an oil-immersed earthing transformer provide defined, low-impedance connection points between the tank, core clamping structure and site grounding grid. Robust copper or galvanized steel pads with drilled holes support bolted bars or cables. Clearly marked symbols, multiple connection locations and short, direct paths help control touch voltage, improve fault-current return paths and enhance overall safety and protection performance.
Defined earthing pads provide low-impedance connection to substation grid.
Multiple terminals support separate safety, functional and shield earth conductors.
wheel
Wheels on an oil-immersed earthing transformer support safe movement during installation, alignment and maintenance relocation. Heavy-duty steel hubs with high load capacity are fixed to reinforced base channels and can be configured for rail or floor mounting. Locking and jacking provisions help secure final position, simplify repositioning and reduce crane time, especially in constrained substation layouts.
Wheel materials selectable: cast steel, ductile iron, polyurethane.
Locking devices secure wheelsets, preventing unintended rolling during operation.
Nameplate
Rating plates for Oil-immersed earthing transformer are manufactured from etched stainless steel or anodised aluminium to ensure long-term legibility and corrosion resistance. They present key electrical data such as ratings, tap range, vector group, cooling and insulation levels. Layout, language, units, logo, fixing holes and plate size can be customised to match utility standards and documentation practices.
Layout can be customised for rating data, vector group and tap range.
International standard compliance (IEC/IEEE/ANSI).
About Testing
Comprehensive type and routine tests confirm insulation strength and zero-sequence behavior, ensuring reliable neutral grounding performance and compliance with IEC and IEEE.
Routine Tests
Type Tests
Special Tests
Measurement of winding resistance
Applied AC withstand (hipot) test
Turns ratio & vector group check
Induced overvoltage withstand test
No-load loss & no-load current measurement
Insulation resistance test
Short-circuit impedance & load loss measurement
Temperature-rise test
Lightning impulse withstand test
Short-circuit withstand test
Dielectric type tests (power frequency, induced, impulse)
Sound level measurement (if specified)
Z0 / earthing performance measurement
Dissolved gas analysis (oil condition)
Insulation power factor / tan δ test
Frequency response analysis (FRA)
Harmonic / no-load current distortion test
Mechanical / seismic / vibration tests (if specified)
All transformers undergo strict testing based on the latest IEEE and IEC standards, with detailed reports ensuring traceability. This guarantees each unit meets the highest standards of quality, safety, and performance.
Oil-Immersed Earthing vs Dry-Type Transformer
Compared with dry-type designs, oil-immersed earthing transformers use engineered insulating oil for higher thermal capacity, stronger fault withstand and reliable system grounding.
Technical Comparison
Oil-Immersed Earthing Transformer
Oil-paper insulation, with mineral oil or ester fluid as the main dielectric medium.
ONAN or ONAF cooling, suitable for higher thermal load and longer grounding duty.
Sealed steel tank with radiators, corrugated walls, oil conservator or hermetic design.
Commonly used in MV and HV systems, from distribution networks to large substations.
Higher thermal inertia, suitable for high fault current and longer grounding duration.
Oil quality, moisture control, sealing condition, gasket aging and dielectric strength.
Dry-Type Earthing Transformer
Cast resin or VPI insulation, using air as the main external dielectric medium.
AN or AF cooling, suitable for indoor installation with natural or forced air ventilation.
Open or enclosed dry coils, compact structure, integrated with indoor enclosure if required.
Mainly used in medium-voltage systems, especially 10–35 kV indoor distribution networks.
Lower thermal inertia, suitable for limited fault current and short-duration grounding duty.
Partial discharge, insulation surface cleanliness, air creepage distance and temperature monitoring.
Connection & Grounding
Oil-Immersed Earthing Transformer
Zigzag, YNd, or Yzn connections are widely used in substations, industrial plants, and renewable power systems.
Provides an artificial neutral point for ungrounded or delta-connected MV/HV networks.
Zigzag or Y-Δ winding offers a strong zero-sequence path for larger networks and higher fault levels.
Supports solid grounding, high-capacity neutral grounding resistor, or Petersen coil grounding.
Commonly combined with outdoor NGR cabinets or oil-immersed Petersen coils.
Used on MV/HV busbars, feeders, substations, and multi-bus power systems.
Dry-Type Earthing Transformer
Mainly Zigzag or Zyn connection, suitable for indoor medium-voltage switchgear and compact electrical rooms.
Provides an artificial neutral point for indoor MV busbars where no neutral is available.
Zigzag winding provides a controlled zero-sequence path for moderate fault current applications.
Supports solid grounding or medium-size neutral grounding resistor for indoor systems.
Commonly paired with indoor resistor cabinets or integrated MV switchgear grounding panels.
Used for indoor MV switchgear rooms, plant busbars, data centers, and compact substations.
Application Comparison
Oil-Immersed Earthing Transformer
Preferred for outdoor substations, wind farms, solar yards, mining and utility networks.
Suitable for large networks with high short-circuit capacity and higher energy grounding duty.
Used where solid, resistance or Petersen-coil grounding requires higher energy capability.
Better for greenfield substations, outdoor yards and projects with sufficient civil space.
Used where oil containment and fire protection can be managed by substation design.
Utilities, IPPs, wind and solar developers, oil & gas, mining and large industrial complexes.
Dry-Type Earthing Transformer
Preferred for indoor substations, compact switchgear rooms and fire-sensitive buildings.
Suitable for moderate fault levels and limited network size.
Used where artificial neutral, small-current grounding or resistance grounding is required indoors.
Better for indoor retrofit projects, commercial buildings, OEM panels and infrastructure rooms.
Used where oil-free design, low fire load and safer indoor installation are priorities.
Data centers, commercial complexes, manufacturing plants, infrastructure buildings and indoor MV systems.
Benefits & Advantages Comparison
Oil-Immersed Earthing Transformer
Excellent thermal and mechanical withstand capability for repeated or high-energy faults.
Strong outdoor durability and suitable for higher voltage insulation requirements.
Lower unit cost in many high-capacity applications, but requires oil pit and fire protection.
Reliable for outdoor operation with routine oil test and sealing inspection.
Suitable when oil safety is properly handled by civil design and protection systems.
Better for outdoor, high-fault-current and large-network grounding applications.
Dry-Type Earthing Transformer
Oil-free design reduces fire load, oil leakage risk and environmental protection requirements.
Compact indoor layout, easier enclosure integration and flexible installation near MV switchgear.
Higher unit cost, but may reduce total installed cost in indoor or high-safety projects.
Low routine maintenance, mainly visual inspection, cleaning and temperature checks.
Suitable for occupied buildings, indoor MV rooms and areas with strict fire-safety requirements.
Better for indoor, compact, oil-free and medium-fault-current grounding applications.
Package & Delivery
Engineered packaging and coordinated logistics protect transformers in transit, simplify site handling, and support safe, on-time delivery worldwide
Wooden / Steel Skid with Sealing Film
For nearby deliveries, we recommend skid plus sealing film when you need economical protection, forklift handling and quick onsite positioning.
Wooden / Steel Frame
We suggest wooden or steel framing when you require protection for protruding parts, highway transport, repeated loading and open-deck shipping.
Wooden Crate
Preferred for export and long routes; closed crates protect from weather, dirt and impact, allow stacking and minimise accessory loss.
Container Shipment
Ideal for high-value or overseas projects; containers improve weather protection, security and standardised lashing, simplifying sea–rail–road multimodal logistics.
Transportation & Delivery
We propose skid, frame, crate or container options based on route, handling and budget, ensuring safe, efficient transformer delivery.
Review key FAQ before purchasing an Oil-Immersed Earthing Transformer, covering lead time, grid standard customization,certified components, factory testing, installation support, and long-term reliable operation.
What Is The Lead Time For An Oil-Immersed Earthing Transformer?
The typical lead time for an Oil-Immersed Earthing Transformer is 8 to 12 weeks, depending on rated capacity, system voltage, neutral current,short-time withstand requirement, tank structure, cooling system, accessories, factory testing, drawing approval, and transportation requirements.
Can The Transformer Be Customized For Different Grid Standards?
The transformer can be customized according to IEC, IEEE/ANSI, EN, or local utility standards. Project-required certificates, test reports, CE or related documents, and third-party inspection reports can be provided where applicable.
What Customization Options Are Available For An Oil-Immersed Earthing Transformer?
Customization options include winding material, insulation system, zigzag or star-delta connection, neutral bushing, NGR matching, sealed or conservator tank,bushing type, transformer oil, cooling system, protection devices, monitoring accessories, enclosure coating, and anti-corrosion treatment.
What Tests Are Performed Before Delivery?
Routine tests include winding resistance, voltage ratio, vector group, impedance, zero-sequence impedance, insulation resistance, applied voltage, induced voltage, leak test, and oil tests. Type and special tests can be arranged as required, and third-party witnessed FAT.
Do You Support Installation And Commissioning?
Installation and commissioning support can be provided by documents, remote guidance, video support, or on-site engineers. Support includes
oil level checking, accessory assembly, neutral grounding checks, NGR connection verification, protection testing, and commissioning coordination.
oil level checking, accessory assembly, neutral grounding checks, NGR connection verification, protection testing, and commissioning coordination.
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