Quick Answer
A 35kV oil-immersed power transformer transfers electrical power from a 35kV-class network to the voltage required by substations, industrial plants, mining facilities, agricultural systems, and regional distribution networks.
Selecting the right power transformer involves more than confirming its rated capacity and voltage ratio. Buyers should also evaluate the load profile, vector group, impedance, tapping method, winding material, cooling arrangement, insulation level, installation environment, testing requirements, and future expansion plans.
Where IEC standards are specified, transformer design and testing should follow the applicable requirements of IEC 60076, together with the project specification and relevant national or utility requirements.
The final transformer configuration should always be based on approved project data rather than a general online specification.
What Is a 35kV Oil-Immersed Power Transformer?
A 35kV oil-immersed power transformer is a medium-voltage transformer in which the magnetic core and windings are immersed in insulating oil.
The oil performs two essential functions. It provides electrical insulation between energized components and transfers heat from the windings and core to the tank and cooling surfaces.
A typical 35kV oil-immersed transformer includes:
- A laminated magnetic core;
- High-voltage and low-voltage windings;
- Solid and liquid insulation;
- A steel transformer tank;
- High-voltage and low-voltage bushings;
- A tap-changing device;
- Radiators or other cooling surfaces;
- Oil-level and temperature indicators;
- Pressure-protection devices;
- A control or marshalling box when required.
In a substation or industrial power system, the transformer receives electricity from the incoming 35kV network and converts it to the voltage required by downstream switchgear, motors, production equipment, distribution transformers, or other electrical loads.
Two transformers with the same capacity and voltage ratio may still have different operating performance. Core material, winding construction, insulation design, guaranteed losses, mechanical strength, cooling arrangement, accessories, and manufacturing quality all influence reliability and service life.
For this reason, buyers should not evaluate a 35kV transformer only by its nameplate rating.
Where Are 35kV Oil-Immersed Transformers Used?
35kV oil-immersed power transformers are widely used in utility substations, industrial plants, mining operations, agricultural power systems, infrastructure projects, and other medium-voltage distribution networks.
Utility and Regional Substations
In a utility or regional substation, the transformer converts the incoming 35kV voltage to a lower voltage for local distribution.
These projects normally focus on:
- No-load and load losses;
- Voltage regulation;
- Insulation coordination;
- Lightning impulse performance;
- Short-circuit withstand capability;
- Protection coordination;
- Monitoring and alarm functions;
- Long-term maintenance requirements.
Because utility transformers may remain energized continuously, energy losses can have a significant effect on lifecycle cost.
The lowest initial quotation is therefore not always the most economical option. Buyers should compare the guaranteed losses, expected loading, service conditions, and total operating cost.
Industrial Plants
Industrial facilities may use 35kV transformers to supply:
- Large motors;
- Pumps and compressors;
- Production lines;
- Furnaces;
- Crushers;
- Ventilation systems;
- Processing equipment;
- Plant-wide medium-voltage distribution systems.
Industrial loads are often more demanding than conventional distribution loads. Large motors may create high starting currents, while furnaces, converters, variable-frequency drives, and welding equipment may introduce harmonics or rapidly changing demand.
The engineering review should consider:
- Maximum simultaneous demand;
- Motor-starting current;
- Operating cycles;
- Harmonic content;
- Permissible voltage drop;
- Temporary overload requirements;
- Standby or redundancy arrangements;
- Future production expansion.
Transformer capacity should be selected according to the actual operating profile rather than the total nameplate power of all connected equipment.
Mining and Heavy Industry
Mining, cement, metal processing, and other heavy industrial projects may expose transformers to demanding site conditions.
Common concerns include:
- Dust and contamination;
- High or low ambient temperatures;
- Elevated installation altitude;
- Vibration;
- Frequent load variation;
- Restricted transportation access;
- Difficult maintenance conditions.
A transformer installed in a remote mining area may therefore require additional monitoring, stronger environmental protection, suitable spare parts, and carefully planned transportation.
Agricultural and Rural Systems
Agricultural power systems may use 35kV transformers for irrigation, pumping stations, cold storage, food processing, livestock facilities, and rural distribution networks.
These applications often have seasonal load variation and long feeder distances. The transformer design should therefore account for:
- Seasonal peak demand;
- Motor starting;
- Voltage drop;
- Outdoor operating conditions;
- Future agricultural expansion;
- Local maintenance capability.
Renewable Energy and Infrastructure Projects
A 35kV transformer may also be used in renewable-energy collection systems and infrastructure projects.
Depending on the application, engineers may need to consider:
- Bidirectional power flow;
- Harmonic currents;
- Repeated loading cycles;
- Grid-code requirements;
- Remote monitoring;
- Ambient temperature;
- Installation altitude.
The transformer should be selected as part of the complete electrical system rather than as an isolated item.
What Specifications Should Be Confirmed Before Ordering?
A complete request for quotation allows the manufacturer to prepare a more accurate technical proposal and reduces revisions later in the project.
Rated Capacity
Transformer capacity is normally expressed in kVA or MVA.
The required rating should be based on:
- Maximum demand;
- Load diversity;
- Power factor;
- Motor-starting requirements;
- Harmonic loads;
- Permissible overload;
- Ambient conditions;
- Future expansion.
Adding together the rated power of every connected device may result in an oversized transformer because not all equipment operates at the same time.
However, selecting a transformer only according to the current average load may leave insufficient capacity for peak demand, motor starting, or future expansion.
The objective is to balance initial cost, operating efficiency, voltage performance, thermal margin, and long-term capacity requirements.
Primary and Secondary Voltage
The inquiry should clearly state:
- Primary voltage;
- Secondary voltage;
- Frequency;
- Normal voltage variation;
- Required tap range;
- Number of tap positions;
- Voltage step between positions.
The secondary voltage must be coordinated with downstream switchgear, motors, protection systems, distribution transformers, and other connected equipment.
Vector Group
The vector group defines the winding connection and phase displacement between the high-voltage and low-voltage sides.
It affects:
- System grounding;
- Protection settings;
- Zero-sequence current paths;
- Harmonic behavior;
- Parallel operation.
When a new transformer will operate in parallel with an existing unit, the voltage ratio, vector group, impedance, polarity, tapping arrangement, and operating characteristics must be compatible.
Impedance
Transformer impedance affects both fault current and voltage regulation.
Lower impedance can reduce voltage drop during normal loading but may allow higher short-circuit current.
Higher impedance can help limit fault current but may increase voltage drop during heavy loading or motor starting.
The required impedance should therefore be coordinated with:
- The system short-circuit study;
- Switchgear breaking capacity;
- Protection settings;
- Motor-starting requirements;
- Permissible voltage drop;
- Parallel transformer operation.
Winding Material
Copper and aluminum can both be used when the transformer is properly engineered and manufactured.
Copper provides high conductivity and may support a more compact winding design. Aluminum may reduce material cost and weight but requires suitable conductor dimensions, joints, insulation, and mechanical support.
Buyers should not compare transformers only by winding material.
A complete comparison should include:
- Guaranteed no-load loss;
- Guaranteed load loss;
- Temperature rise;
- Dimensions and weight;
- Short-circuit strength;
- Connection quality;
- Testing scope.
Tapping Method
An off-circuit tap changer is adjusted while the transformer is de-energized. It is normally suitable where the incoming voltage is relatively stable and tap changes are infrequent.
An on-load tap changer allows the voltage ratio to be adjusted while the transformer remains energized and carries load.
It may be required where:
- Network voltage varies significantly;
- Load conditions change frequently;
- Automatic voltage regulation is necessary;
- The utility specification requires continuous voltage control.
An on-load tap changer increases equipment cost and complexity and may also require additional control, protection, and maintenance.
It should therefore be selected according to the actual voltage-regulation requirement.
Installation Environment
Accurate site information should be provided before the transformer is designed.
Important conditions include:
- Maximum and minimum ambient temperature;
- Installation altitude;
- Indoor or outdoor installation;
- Humidity;
- Dust and sand;
- Coastal salt pollution;
- Industrial contamination;
- Seismic requirements;
- Noise limits;
- Available installation space;
- Transportation restrictions.
High altitude can affect external insulation and cooling performance. Coastal or chemically aggressive environments may require stronger corrosion protection. Restricted road access may limit transformer dimensions and transport weight.
How Do Core, Winding, Insulation, and Tank Design Affect Reliability?
Transformer performance depends on the coordination of electromagnetic, thermal, and mechanical design.
Core Design
The magnetic core carries the alternating magnetic flux required for voltage transformation.
Its performance is influenced by:
- Electrical-steel grade;
- Lamination thickness;
- Cutting accuracy;
- Joint design;
- Stacking method;
- Clamping pressure;
- Assembly quality.
For 35kV oil-immersed transformers, HONLE applies controlled core cutting and stepped-lap construction to improve magnetic flux distribution and help reduce no-load loss, excitation current, vibration, and noise.
Good core material alone is not enough. Cutting, stacking, clamping, and handling during production also affect the final measured performance.
Winding Design
Transformer windings must withstand electrical, thermal, and mechanical stress.
During an external short circuit, strong electromagnetic forces act on the conductors. The winding structure therefore requires adequate radial and axial support.
Important design factors include:
- Conductor dimensions;
- Insulation thickness;
- Spacer arrangement;
- Mechanical clamping;
- Lead routing;
- Oil-duct design.
HONLE designs the high-voltage and low-voltage windings together with the oil passages and mechanical supports to achieve the required cooling performance and short-circuit strength.
Insulation System
The insulation system includes transformer oil, paper insulation, pressboard barriers, spacers, winding insulation, bushings, and electrical clearances.
It must withstand normal operating voltage as well as temporary overvoltage, lightning impulse, thermal ageing, and environmental stress.
Moisture control is particularly important. Excessive moisture reduces dielectric strength and accelerates insulation ageing.
Material storage, active-part drying, oil treatment, assembly cleanliness, and final testing all contribute to long-term insulation reliability.
Tank and Sealing
The transformer tank protects the active part and contains the insulating oil.
It must withstand:
- Lifting;
- Transportation;
- Vacuum treatment;
- Oil filling;
- Thermal expansion;
- Normal operating pressure.
HONLE selects the tank, cooling surfaces, sealing system, and structural design according to transformer capacity, installation conditions, transport limits, and maintenance requirements.
Reliable welding, sealing, pressure testing, and surface treatment help reduce the risk of oil leakage, moisture ingress, and corrosion during long-term operation.
What Is the Difference Between ONAN and ONAF Cooling?
Transformer losses generate heat in the core and windings. This heat must be transferred to the surrounding environment to keep the insulation system within its design temperature limits.
ONAN Cooling
ONAN means Oil Natural Air Natural.
The oil circulates naturally as it is heated and cooled. Heat is transferred through the tank and radiators to the surrounding air without cooling fans.
Its advantages include:
- Simple construction;
- Low auxiliary power consumption;
- Fewer moving components;
- Lower fan-related noise;
- Straightforward maintenance.
ONAN is suitable where the required capacity and operating conditions can be handled through natural cooling.
ONAF Cooling
ONAF means Oil Natural Air Forced.
The oil still circulates naturally, but fans force air across the radiators or cooling surfaces.
ONAF may be used where:
- Peak load is higher than normal load;
- Additional thermal capacity is required;
- Available cooling space is limited;
- The project requires multiple loading stages.
The buyer should confirm:
- ONAN and ONAF ratings;
- Number of fans;
- Fan redundancy;
- Starting and stopping temperatures;
- Manual and automatic control;
- Auxiliary power supply;
- Alarm contacts;
- Noise requirements.
Not every transformer requires an ONAF stage. The cooling arrangement should be selected according to capacity, loss level, ambient temperature, loading cycle, and available installation space.
What Tests Should Be Completed Before Delivery?
IEC 60076 is an important international standard series for power transformers.
Where IEC standards are specified, design and testing should follow the applicable IEC 60076 requirements, together with the project specification and relevant national or utility requirements.
The purchase contract should clearly define the applicable standard edition and testing scope.
Routine Tests
Routine tests are generally performed on each completed transformer and may include:
- Winding resistance measurement;
- Voltage-ratio measurement;
- Vector-group verification;
- No-load loss and no-load current measurement;
- Load loss and impedance measurement;
- Applied-voltage withstand test;
- Induced-voltage withstand test;
- Accessory and control-circuit checks;
- Tank leakage or pressure checks.
The measured values should be compared with the guaranteed values in the approved technical data sheet.
Type Tests
Type tests verify the performance of a transformer design and may include:
- Temperature-rise testing;
- Lightning-impulse testing;
- Sound-level measurement.
A type test is not necessarily repeated for every transformer.
During the technical review, Transformer Manufacturer and the buyer should confirm whether an available test report is applicable to the proposed power transformer or whether a new test is required.
Special Tests
Depending on the project, additional tests may include:
- Partial-discharge measurement;
- Zero-sequence impedance measurement;
- Frequency-response analysis;
- Special sound testing;
- Harmonic-related measurements;
- Third-party witnessed inspection.
Special tests can affect production time, test scheduling, and price. They should therefore be confirmed before the order is placed.
What Determines the Price of a 35kV Transformer?
There is no universal price for every 35kV oil-immersed transformer.
The final quotation depends on:
- Rated capacity;
- Voltage ratio;
- Guaranteed losses;
- Copper or aluminum windings;
- Off-circuit or on-load tap changer;
- ONAN or ONAF cooling;
- Accessories and monitoring;
- Environmental conditions;
- Testing scope;
- Documentation;
- Transportation and site services.
Lower guaranteed losses may require higher-grade core material, more conductor material, or a larger transformer structure.
Accessories such as temperature indicators, pressure-relief devices, oil-level indicators, cooling controls, remote contacts, and monitoring interfaces also affect the price.
The quotation should clearly state whether it includes:
- Export packaging;
- Inland transportation;
- Ocean freight;
- Insurance;
- Unloading;
- Installation supervision;
- Spare parts;
- Commissioning support.
A low quotation may not represent a lower total project cost when important accessories, tests, or services are excluded.
Why Choose HONLE as Your 35kV Transformer Manufacturer?
HONLE manufactures transformers and related electrical equipment for substations, industrial plants, mining operations, agricultural systems, and infrastructure projects.
For 35kV oil-immersed transformers, HONLE focuses on:
- Low-loss core design;
- Controlled winding construction;
- Oil-duct and cooling design;
- Mechanical strength;
- Short-circuit withstand capability;
- Reliable insulation processing;
- Tank sealing and surface protection;
- Factory testing and technical documentation.
HONLE supports the project through the following stages:
- Reviewing the electrical system and load requirements;
- Confirming capacity, voltage ratio, impedance, vector group, and tapping method;
- Designing the core, windings, insulation, cooling system, and accessories;
- Completing production and quality-control procedures;
- Performing the agreed factory tests;
- Preparing drawings, test reports, manuals, and shipping documents;
- Providing delivery and technical support according to the agreed project scope.
To prepare an accurate proposal, HONLE should receive:
- The single-line diagram;
- Rated capacity;
- Primary and secondary voltage;
- Frequency;
- Vector group;
- Required impedance;
- Tap-changing method;
- Cooling requirement;
- Installation conditions;
- Applicable standards;
- Accessory list;
- Testing scope;
- Delivery destination.
Complete project information helps improve quotation accuracy and reduces technical revisions after the order is placed.
Frequently Asked Questions
Can a 35kV transformer use different secondary voltages?
Yes. The voltage ratio can be designed according to the project requirements.
The primary voltage, secondary voltage, frequency, insulation level, vector group, and tapping arrangement should be provided for technical evaluation.
Is an on-load tap changer always necessary?
No.
An on-load tap changer is appropriate where voltage must be adjusted while the transformer remains energized. If the supply voltage is stable and tap changes are infrequent, an off-circuit tap changer may be sufficient.
Should buyers choose copper or aluminum windings?
Both materials can be used in a properly designed transformer.
The decision should consider losses, temperature rise, dimensions, weight, mechanical strength, cost, and project requirements.
How should transformer capacity be selected?
Capacity should be based on maximum demand, load diversity, motor starting, power factor, harmonics, ambient conditions, overload requirements, and future expansion.
A load list and single-line diagram provide a better basis for selection than simply adding all equipment nameplate ratings.
What documents should be checked before shipment?
Typical documents include:
- Approved technical data sheet;
- General arrangement drawing;
- Nameplate drawing;
- Wiring diagrams;
- Routine test report;
- Applicable type test reports;
- Packing list;
- Installation and maintenance manual.
Conclusion
A 35kV oil-immersed power transformer is a critical part of a substation or industrial power system.
Its reliability depends on the coordination of the core, windings, insulation, cooling system, tank structure, accessories, factory testing, and site conditions.
Buyers should avoid selecting a transformer only by capacity, winding material, or initial price.
A complete inquiry should describe the electrical system, load characteristics, environmental conditions, applicable standards, required tests, accessories, and delivery scope.
As a transformer manufacturer, HONLE can evaluate these requirements and prepare a 35kV oil-immersed transformer solution covering technical design, production, testing, documentation, delivery, and agreed technical support.
