A short circuit in a 35kV power system is not simply a temporary overload. Within a very short time, the transformer windings may carry current far above their normal operating level. The conductors heat rapidly, while electromagnetic forces act on the winding assembly, leads, spacers, clamping components, and other parts of the active structure.
A transformer may continue operating normally for years without experiencing a severe external fault. However, when a fault does occur, insufficient short-circuit withstand capability can lead to winding displacement, conductor deformation, insulation damage, loose clamping, changed impedance, or internal electrical failure.
For this reason, the short-circuit performance of a 35kV power transformer must be considered during system study, electrical design, mechanical design, manufacturing, testing, transportation, and final technical review.
HONLE’s 35kV oil-immersed power transformer design combines winding construction, oil-duct arrangement, structural reinforcement, core clamping, and tank configuration to support mechanical stability and short-circuit withstand performance in substations, industrial plants, mining facilities, agricultural power systems, and regional distribution networks.
Quick Answer
The short-circuit withstand capability of a 35kV power transformer is evaluated through four connected areas:
- System fault conditions, including the available short-circuit current, fault duration, network impedance, transformer impedance, and protection clearing time.
- Thermal withstand, which determines whether the winding conductors can tolerate the heating caused by the fault current.
- Dynamic mechanical withstand, which evaluates whether the windings, leads, supports, clamping structure, and active part can resist the electromagnetic forces created during the fault.
- Design and manufacturing verification, including calculations, design review, controlled production, agreed factory tests, and special short-circuit testing when required.
IEC 60076-5 specifically addresses the ability of power transformers to withstand external short circuits. It includes calculation procedures for thermal withstand and permits dynamic capability to be demonstrated through theoretical evaluation or a special short-circuit test.
Table 1: Main Elements Used to Evaluate 35kV Transformer Short-Circuit Withstand
| Evaluation Area | Main Question | Typical Evidence |
| System fault duty | What fault current can flow through the transformer? | Short-circuit study and single-line diagram |
| Fault duration | How long will the current continue before protection clears it? | Relay and breaker coordination data |
| Thermal withstand | Can the conductors tolerate the resulting temperature rise? | Thermal calculation |
| Dynamic withstand | Can the winding assembly resist radial and axial forces? | Mechanical calculation and design review |
| Impedance | How does transformer impedance limit fault current? | Approved data sheet and impedance calculation |
| Winding support | Are the conductors adequately supported and compressed? | Construction drawings and manufacturing records |
| Design verification | Has the design been theoretically or physically verified? | Calculation report or special test report |
| Manufacturing control | Was the approved design reproduced accurately? | Inspection records and quality-control documents |
| Post-production condition | Is there evidence of movement or deformation? | Dimensional checks and comparative FRA where specified |
Short-Circuit Withstand Includes Both Thermal and Mechanical Performance
A 35kV transformer must withstand two different effects during an external short circuit. These effects occur at the same time but must be evaluated separately.
Thermal withstand protects the winding conductors
Short-circuit current produces intense resistive heating in the winding conductors. The temperature can rise quickly because the energy is concentrated within a short fault period.
Thermal withstand depends on factors such as:
- The symmetrical short-circuit current;
- Fault duration;
- Conductor material;
- Conductor cross-sectional area;
- Initial winding temperature;
- Current distribution within the winding;
- Protection and breaker clearing time;
- Applicable standard requirements.
The thermal assessment determines whether the winding conductor temperature remains within the permitted short-time limit.
This calculation should use the actual project fault duty. A generic statement such as “high short-circuit capacity” is not sufficient when the available fault current and clearing time have not been confirmed.
Dynamic withstand protects the winding geometry
The mechanical effect is created by the interaction between short-circuit current and the transformer’s leakage magnetic field. The resulting electromagnetic forces act on the conductors and winding assembly.
These forces can:
- Compress an inner winding inward;
- Expand an outer winding outward;
- Apply axial pressure at the winding ends;
- Cause conductors to tilt or shift;
- Move winding sections relative to each other;
- Stress winding leads and terminal connections;
- Reduce the original clamping pressure;
- Change the transformer’s leakage field and measured impedance.
A power transformer can remain electrically connected after a fault while still suffering internal mechanical deformation. The damage may not immediately produce a complete failure, but it can weaken insulation, reduce clearances, or increase the risk of a later fault.
The design must therefore protect both conductor temperature and winding geometry.
Evaluation Begins with the Actual 35kV Network
Short-circuit withstand capability cannot be evaluated from transformer capacity alone.
Two transformers with the same rated power and voltage ratio may be exposed to very different fault duties because they are connected to different networks. A transformer supplied by a strong utility grid may experience a higher available fault current than a similar transformer connected to a weaker industrial network.
The technical review should begin with the system single-line diagram and short-circuit study.
Table 2: Network Information Required for Short-Circuit Evaluation
| Project Parameter | Why It Matters |
| Rated transformer capacity | Establishes rated winding current |
| Primary and secondary voltage | Defines the system connection and winding rating |
| Available short-circuit level | Determines the fault current that may pass through the transformer |
| Transformer percentage impedance | Influences the current limited by the transformer |
| Upstream source impedance | Affects the total available fault current |
| Network X/R ratio | Influences the asymmetrical peak current |
| Fault duration | Determines thermal energy and mechanical exposure |
| Protection clearing time | Shows how quickly relays and breakers isolate the fault |
| Earthing arrangement | Influences ground-fault current and fault behavior |
| Parallel operation | Can increase the fault contribution through each transformer |
| Generator contribution | May increase or change the short-circuit duty |
| Future system expansion | Helps prevent the selected design from becoming inadequate later |
For industrial projects, information about large motors, generators, parallel transformers, and future network expansion should also be included. Motor and generator contributions may affect the initial fault level, while parallel transformers can significantly change the current flowing into a downstream fault.
The approved fault duty should be confirmed before the winding and mechanical structure are finalized. Late changes to the short-circuit level can require a new review of conductor dimensions, winding geometry, impedance, insulation, support, and clamping.
Transformer Impedance Influences Fault Current but Is Not the Only Measure of Strength
Percentage impedance is one of the most important parameters in a short-circuit study. It limits the current flowing through the transformer during a downstream fault.
In general, a lower transformer impedance allows a higher short-circuit current, while a higher impedance reduces the fault current contribution. However, selecting a higher impedance is not automatically the best solution.
Transformer impedance also affects:
- Voltage regulation under load;
- Motor-starting voltage drop;
- Parallel load sharing;
- Downstream switchgear duty;
- Protection coordination;
- Winding geometry;
- Leakage flux distribution;
- Transformer dimensions and cost.
Impedance is partly determined by the physical relationship between the high-voltage and low-voltage windings. Changing the impedance may therefore require changes to winding dimensions, radial clearances, axial arrangement, conductor configuration, or magnetic leakage paths.
This is why impedance should not be changed as an isolated commercial parameter near the end of a project. It must be coordinated with the electrical system and the transformer’s mechanical design.
A power transformer with higher impedance can still have inadequate mechanical strength if the windings are not properly supported. Likewise, a transformer with a lower specified impedance can be designed for the required fault duty when the conductor arrangement, winding support, clamping, and active-part structure are correctly engineered.
Winding Construction Is Central to Dynamic Short-Circuit Strength
The winding assembly carries the short-circuit current and receives most of the resulting electromagnetic force. Its mechanical stability depends on far more than conductor material alone.
Important design factors include:
- Winding type;
- Conductor dimensions;
- Number and arrangement of parallel conductors;
- Radial winding build;
- Axial winding height;
- Insulation thickness;
- Oil-duct arrangement;
- Spacer location;
- Pressboard density and strength;
- End-block configuration;
- Radial support;
- Axial compression;
- Lead routing and support;
- Manufacturing tolerances.
Radial support controls inward and outward deformation
In a typical concentric winding arrangement, short-circuit forces can compress the inner winding and stretch the outer winding.
An inner winding without adequate radial support may buckle inward. An outer winding may experience tensile stress and outward expansion. The design must therefore consider the conductor’s mechanical properties, winding diameter, support arrangement, current distribution, and expected peak force.
Radial ducts and insulation spaces must provide the required cooling and dielectric clearances without leaving the winding mechanically unstable.
Axial clamping keeps winding sections in position
Axial forces act along the height of the winding, particularly near the winding ends and where the magnetic balance between windings is imperfect.
Adequate axial compression helps prevent winding movement during transportation, normal operation, and external faults. The clamping system may include pressure rings, end blocks, tie structures, and other mechanical components according to the transformer design.
The original compression must also remain stable after:
- Insulation drying;
- Active-part assembly;
- Oil filling;
- Transportation;
- Thermal cycling in service.
Cellulose insulation can settle during drying and operation. Manufacturing procedures must account for this behavior so that the winding does not lose critical clamping pressure.
Lead support must not be overlooked
The winding body may be well supported while the internal leads remain vulnerable.
Short-circuit current also passes through:
- Winding exits;
- Tap leads;
- Neutral leads;
- Bushing connections;
- Internal terminal joints;
- Tap-changer connections.
These conductors require adequate clearances, insulation, fixing points, and mechanical support. An unsupported lead can move during a fault, placing stress on insulation or terminal connections even when the main winding remains stable.
Core Clamping and Active-Part Assembly Support Overall Stability
The transformer core does not carry the load current in the same way as the windings, but the core frame and active-part structure contribute to overall mechanical stability.
The assembly should maintain:
- Correct winding alignment;
- Stable axial compression;
- Controlled clearances;
- Secure lead routing;
- Reliable core clamping;
- Consistent positioning inside the tank;
- Adequate lifting and transport strength.
HONLE’s 35kV oil-immersed power transformer uses structural measures around the core, pressing components, high- and low-voltage windings, oil ducts, and connection areas to improve mechanical strength and short-circuit withstand capability. The available tank configurations include bell-type and core-lifting structures according to the applicable design.
Accurate assembly is important because even a technically correct design can lose mechanical performance if production dimensions, winding alignment, spacer positions, or pressing conditions deviate from the approved drawings.
Tank Strength and Transport Restraint Must Be Evaluated Separately
A strong transformer tank does not automatically prove that the windings can withstand a short circuit.
The tank mainly provides:
- Mechanical enclosure;
- Oil containment;
- Environmental protection;
- Mounting for bushings, radiators, valves, and accessories;
- Lifting and jacking interfaces;
- Support during transportation and installation.
Short-circuit withstand is primarily related to the active part, especially the windings and their supporting structure. Tank construction remains important, but it should not be used as a substitute for winding mechanical calculations.
Transportation also requires separate consideration. A transformer may be designed for the required electrical fault duty but still be damaged by uncontrolled lifting, road impact, vibration, or improper securing.
For large 35kV power transformers, the transport review may include:
- Overall transport dimensions;
- Shipping weight;
- Center of gravity;
- Lifting points;
- Jacking locations;
- Internal transport restraints;
- Removable radiators and accessories;
- Nitrogen-filled or oil-filled transport conditions;
- Shock or impact recorders where specified;
- Receiving inspection at the project site.
Transport restraints should protect the active part without introducing excessive force into windings, leads, bushings, or the tank structure.
Short-Circuit Capability Must Be Verified with the Correct Evidence
No single document or factory test provides a complete evaluation of short-circuit withstand capability.
The evidence should be matched to the purpose of the review.
Table 3: Methods Used to Verify Transformer Short-Circuit Performance
| Verification Method | What It Demonstrates | Important Limitation |
| Thermal calculation | Ability of conductors to tolerate fault heating | Depends on correct fault current and duration |
| Dynamic mechanical calculation | Expected winding forces, stresses, and structural margins | Accuracy depends on the design model and input data |
| Design review | Coordination of winding, conductor, support, impedance, and clamping | Must reflect the final approved design |
| Special short-circuit test | Physical performance of a transformer under specified fault conditions | Not a routine test for every manufactured unit |
| Routine electrical tests | Compliance of the manufactured transformer with agreed electrical values | Do not independently prove full dynamic short-circuit strength |
| Impedance measurement | Confirms the manufactured impedance value | Does not alone confirm winding mechanical capability |
| Dimensional and assembly inspection | Confirms production follows approved drawings | Requires documented acceptance criteria |
| Frequency response analysis | Provides comparative information about active-part mechanical condition | Requires a suitable reference and does not replace design verification |
A special short-circuit test is not normally a routine test
Routine tests are performed on each transformer according to the applicable standard and contract. They commonly verify winding resistance, voltage ratio, vector group, impedance, losses, dielectric performance, tap-changer operation, auxiliary circuits, and general mechanical condition.
These tests are important, but they do not reproduce the full thermal and dynamic forces of an external short circuit.
A short-circuit withstand test is a special test performed at a suitable high-power laboratory when required by the project or agreed verification plan. IEC 60076-5 also permits theoretical evaluation of dynamic withstand according to its defined procedures.
The project specification should state whether acceptance will be based on:
- Design calculations;
- A short-circuit test on the offered transformer;
- A valid test report for an equivalent design;
- A combination of calculations, design similarity, and previous test evidence.
When a report from a similar transformer is submitted, the technical review should compare the relevant design characteristics rather than relying only on the same voltage class or rated capacity.
FRA is a condition-assessment tool, not a replacement for short-circuit verification
Frequency Response Analysis, often referred to as FRA or SFRA, compares the winding response across a range of frequencies. Changes in the measured response can indicate possible movement or deformation of the active part.
The comparison may use:
- An earlier baseline measurement from the same transformer;
- A corresponding phase of the same transformer;
- A sister transformer with an equivalent design.
CIGRE describes FRA as a method commonly used to assess the mechanical integrity of a transformer’s active part through comparison with suitable reference data.
FRA can be useful:
- Before shipment as baseline data;
- After long-distance transportation;
- After a severe external fault;
- When winding movement is suspected;
- During condition assessment.
However, a satisfactory FRA result does not by itself demonstrate that the transformer was designed for the specified short-circuit duty.
Manufacturing Control Must Preserve the Approved Mechanical Design
Short-circuit calculations are based on specific dimensions, materials, supports, and clamping conditions. Manufacturing control must ensure that these assumptions remain valid in the completed transformer.
Important production controls include:
- Incoming inspection of conductors and insulation materials;
- Verification of conductor dimensions;
- Controlled winding tension;
- Winding diameter and height measurement;
- Spacer and oil-duct positioning;
- Insulation build control;
- Axial pressing and compression control;
- Lead forming and fixing;
- Core stacking and clamping;
- Active-part dimensional inspection;
- Drying-process control;
- Final tightening and assembly inspection;
- Tank welding and leak inspection;
- Correct installation of transport restraints;
- Final electrical and mechanical inspection.
A drawing can show the correct number of spacers, but production must confirm that the spacers are positioned correctly. A calculation can specify axial compression, but assembly procedures must ensure that the required clamping condition is achieved and retained.
For HONLE’s 35kV oil-immersed power transformers, controlled winding construction, reasonable oil-duct and connection-area layout, core reinforcement, tank structure, insulation processing, and factory inspection form part of the manufacturing process supporting mechanical stability.
Project Specifications Should Define the Required Fault Duty Clearly
A power transformer specification should not contain only a sentence such as:
The transformer shall have high short-circuit withstand capability.
That statement does not provide enough information for calculation, design, quotation, or technical acceptance.
Table 4: Information Required for a 35kV Short-Circuit Withstand Review
| Required Information | Project Detail to Provide |
| Rated capacity | kVA or MVA |
| Voltage ratio | Primary, secondary, and tertiary voltage where applicable |
| Highest voltage for equipment | Applicable 35kV-class insulation basis |
| Frequency | 50Hz or 60Hz |
| Vector group | Required winding connection |
| Percentage impedance | Required value and permitted tolerance |
| System short-circuit level | Fault level at the relevant bus |
| Fault duration | Maximum clearing time |
| Earthing system | Solid, resistance, reactance, or other arrangement |
| Parallel transformers | Number, rating, and impedance of connected units |
| Motor or generator contribution | Relevant rotating-machine fault contribution |
| Applicable standard | IEC, national, utility, or project standard |
| Verification method | Calculation, equivalent report, or special test |
| Required documents | Calculation report, drawings, test reports, and inspection records |
| Transport conditions | Road, sea, site access, and impact-monitoring requirements |
Providing this information before detailed design reduces later revisions and allows the transformer impedance, winding arrangement, mechanical supports, insulation system, and protection scheme to be evaluated together.
Common Mistakes When Evaluating Short-Circuit Strength
Evaluating only the transformer impedance
Impedance affects fault current, but it does not describe conductor strength, winding support, axial clamping, lead fixing, or manufacturing quality.
Treating routine tests as complete short-circuit proof
Routine tests confirm important electrical characteristics of the finished transformer. They do not create the same forces as a high-current short-circuit test.
Accepting a similar test report without reviewing design equivalence
Two transformers can have the same capacity and voltage but different winding geometry, conductor type, impedance, tap range, clamping structure, or active-part arrangement.
Ignoring parallel operation and future network changes
A transformer selected for the present fault level may become inadequate after another transformer, generator, motor, or stronger grid connection is added.
Changing impedance late in the project
Impedance is linked to winding geometry and leakage flux. A late change can affect dimensions, losses, voltage regulation, fault current, and mechanical calculations.
Focusing on the tank while overlooking the active part
Tank strength is important for enclosure, transport, oil containment, and accessory support. Short-circuit strength depends primarily on the windings and their supporting structure.
How HONLE Supports 35kV Power Transformer Projects
HONLE manufactures 35kV oil-immersed power transformers for urban and rural power grids, industrial and mining enterprises, transmission and distribution systems, agricultural projects, and other medium-voltage applications.
The 35kV product design focuses on:
- Low-loss core construction;
- Controlled core stacking and clamping;
- Reinforced active-part structure;
- High- and low-voltage winding arrangement;
- Oil-duct and connection-area layout;
- Mechanical winding strength;
- Short-circuit withstand capability;
- Bell-type or core-lifting tank configuration;
- Factory inspection and testing;
- Technical documentation for project review.
Before production, HONLE can review the project’s:
- Single-line diagram;
- Rated capacity;
- Voltage ratio;
- Frequency;
- Vector group;
- Percentage impedance;
- System fault level;
- Fault duration;
- Tap-changing requirement;
- Cooling method;
- Installation environment;
- Accessories;
- Applicable standards;
- Required tests and documents.
This technical information provides the basis for coordinating the winding design, conductor selection, insulation structure, oil ducts, supports, clamping components, tank layout, and protection interfaces.
Frequently Asked Questions
Is a short-circuit withstand test required for every 35kV power transformer?
No. A short-circuit withstand test is generally treated as a special test rather than a routine test performed on every unit.
The agreed verification method may include thermal calculations, dynamic mechanical evaluation, applicable test reports from an equivalent design, or a special short-circuit test when required by the contract.
Does higher transformer impedance always provide better short-circuit performance?
Higher impedance generally reduces the fault current passing through the transformer, but it can also increase voltage drop and affect motor starting, regulation, parallel operation, and system protection.
Mechanical strength must still be evaluated through winding design, conductor stress, radial support, axial clamping, and manufacturing control.
Can routine impedance measurement detect winding movement?
A significant change in impedance may indicate a change in winding geometry, but impedance measurement alone may not identify every type of deformation.
Where mechanical movement is suspected, the investigation may also include FRA, winding resistance, ratio measurements, insulation tests, oil analysis, internal inspection, and comparison with previous baseline data.
What documents should be reviewed before approving the transformer design?
Typical documents include:
- Approved technical data sheet;
- General arrangement drawing;
- Nameplate drawing;
- Winding and connection information where contractually available;
- Impedance and short-circuit calculation documents;
- Thermal withstand calculation;
- Dynamic withstand evaluation;
- Applicable short-circuit test report;
- Inspection and test plan;
- Routine test schedule;
- Transport drawing and lifting arrangement.
The exact document scope should be agreed during the technical review.
Does winding material determine short-circuit strength?
Winding material is one factor, but copper or aluminum alone does not determine whether a transformer has adequate short-circuit strength.
The result also depends on conductor dimensions, current density, winding geometry, insulation system, support arrangement, axial compression, lead fixing, thermal limits, and manufacturing quality.
Conclusion
Short-circuit withstand capability is a coordinated electrical, thermal, mechanical, and manufacturing requirement.
For a 35kV power transformer, the evaluation should begin with the actual network fault level and protection clearing time. Transformer impedance, winding current, conductor temperature, radial forces, axial forces, support structure, clamping pressure, lead fixing, active-part assembly, and transport conditions must then be reviewed as one complete design.
Routine factory tests remain essential for confirming the manufactured transformer’s electrical performance, but they should not be confused with direct verification of dynamic short-circuit strength. Depending on the project, the final acceptance basis may include calculations, equivalent design evidence, comparative diagnostic data, and a special short-circuit test.
HONLE supports 35kV power transformer projects from technical parameter review through design, production, factory testing, documentation, packaging, and delivery. To prepare a technical proposal, the project team can provide the single-line diagram, rated capacity, voltage ratio, system short-circuit level, transformer impedance, fault duration, vector group, tapping method, cooling requirement, installation conditions, applicable standards, required tests, quantity, and delivery destination.
For project-specific short-circuit withstand requirements, technical drawings, test documents, or 35kV power transformer selection, contact HONLE with the available electrical system information.