What Should I Do If The Transformer Is Short-circuited Suddenly?

Transformer short-circuit accidents require systematic inspection and testing to identify root causes and ensure safe repairs—this is a core technical capability of CHH Power’s after-sales and maintenance team. When a transformer experiences a sudden short circuit, both high- and low-voltage sides endure massive short-circuit currents. Before circuit breakers can trip, these currents generate electromotive forces (proportional to the square of the current) that act on windings as radial forces and axial forces. Below is CHH Power’s standardized process for diagnosing, inspecting, and resolving transformer short-circuit faults.

Contents hide

1 1. Mechanism of Transformer Damage During Short Circuits

2 2. Core Inspection & Testing Procedures for Short-Circuit Faults

3 3. Key Precautions for Short-Circuit Fault Handling

4 OUR CERTIFICATE

5 OUR PROJECTS

1. Mechanism of Transformer Damage During Short Circuits

Understanding how forces affect components is critical to targeted inspections. CHH Power’s engineering team has summarized the key damage patterns:

(1) Radial Force Impact

Radial forces stretch high-voltage windings and compress low-voltage windings.
Since circular structures are more vulnerable to compression than tension, low-voltage windings are more prone to deformation (e.g., bulging, ovalization).

(2) Axial Force Impact

Axial forces compress windings, causing axial displacement of high- and low-voltage windings.
These forces also transfer to the iron core and clamps, potentially loosening or damaging these components.

(3) Component Vulnerability Ranking

From CHH Power’s fault statistics, the order of damage risk is:

Low-voltage windings and balance windings (highest risk of deformation/broken strands)
High- and medium-voltage windings
Iron cores and clamps

2. Core Inspection & Testing Procedures for Short-Circuit Faults

CHH Power follows a multi-step inspection protocol to fully assess damage, combining visual checks, electrical testing, and oil/gas analysis.

(1) Winding Inspection & Condition Assessment

Short-circuit forces subject windings to compression, tension, and bending—faults here are often concealed. CHH Power uses two key methods:

Visual Inspection: After lifting the winding, check for obvious deformation (e.g., bulging, misalignment), broken strands, or insulation damage.
Specialized Testing: Deploy winding deformation testers (e.g., low-voltage impulse test equipment) to detect subtle changes in winding inductance or impedance, which indicate hidden deformation.

(2) Transformer DC Resistance Measurement

DC resistance testing is a quick way to identify winding damage. CHH Power’s process:

Measure the DC resistance of each winding phase and calculate the unbalance rate (must comply with IEC standards: ≤2% for medium-voltage windings, ≤4% for low-voltage windings).
Compare results with historical data (from factory tests or previous maintenance). A significant increase often signals broken strands.
Case Example: After a short-circuit accident in a 31.5MVA transformer, CHH Power measured a 10% increase in DC resistance on the low-voltage C-phase. Subsequent inspection revealed one broken strand in the C-phase winding.

(3) Transformer Winding Capacitance Measurement

Winding capacitance reflects gaps between turns, layers, and the iron core—deformation alters these gaps. CHH Power’s testing logic:

Capacitance components include inter-turn, inter-layer, inter-cake, and winding-to-ground capacitance.
Deformation (e.g., bending) reduces the winding-to-iron core gap, increasing winding-to-ground capacitance. Larger capacitance changes indicate more severe deformation.
Use a precision capacitance bridge to measure capacitance values, comparing them with baseline data to assess deformation severity.

(4) Post-Decompression Internal Inspection (Hood Removal)

CHH Power performs this critical inspection after draining oil and removing the transformer tank cover:

Check for molten copper/aluminum slag or high-density cable paper fragments—these confirm severe winding deformation or broken strands.
Inspect winding spacers (displacement/falling), pressure plates (uneven compression), and pressure nails (displacement)—these indicate axial force damage.

(5) Iron Core & Clamp Inspection

Iron cores and clamps must withstand axial forces; CHH Power’s inspection focuses on:

Iron Core Yoke Laminations: Check for vertical movement (a sign of clamp failure).
Insulating Grommet Screws: Measure insulation resistance between screws and the core; check for damaged insulation sleeves (prevents core grounding faults).
Pull Plates & Connections: Inspect for cracks or loosening (weakness here leads to core displacement).
Winding Pressure Plates:
- Check pressure nails for damage or displacement.
- Verify reliable grounding between pressure plates, pressure nails, and the upper iron yoke (prevents overcurrent burning of grounding straps).

(6) Transformer Oil & Gas Analysis

Short circuits generate gas (e.g., hydrogen, methane) that accumulates in the gas relay. CHH Power’s analysis process:

Gas Relay Sampling: Test gas composition to determine fault nature (e.g., high hydrogen indicates winding overheating or arcing).
Oil Testing: Analyze oil for dissolved gas content, dielectric strength, and moisture—degraded oil (low dielectric strength) requires filtration or replacement.

3. Key Precautions for Short-Circuit Fault Handling

CHH Power’s maintenance team adheres to strict guidelines to avoid secondary damage and ensure long-term reliability:

(1) Insulating Part Replacement Standards

Test all new insulating parts (e.g., lead bracket wood blocks) to meet IEC 60641 standards before installation.
For wooden insulating blocks: Soak in 80°C transformer oil for 24 hours to eliminate moisture and ensure insulation performance.

(2) Timing of Transformer Insulation Testing

Insulation resistance and dielectric loss tests must be conducted 24 hours after oil filling.
Reason: Damp insulating parts release moisture into hot oil over time; testing immediately after filling may miss hidden insulation defects.
Case Example: A 31.5MVA 110kV transformer showed normal insulation after oil filling, but 24 hours later, low-voltage side insulation resistance dropped to 1MΩ—root cause: unqualified insulation in the 10kV copper bar support wood block.

(3) Iron Core Reassembly: Avoid Sharp Corner Risks

When reinstalling iron core yoke laminations, file sharp corners to prevent piercing oil passage insulation.
Measure oil passage insulation resistance in real time during reassembly (target: ≥100MΩ).
Lesson Learned: A 120MVA 220kV transformer had 0Ω oil passage insulation after reassembly—sharp core laminations had shorted the oil passage, requiring days of rework.

(4) Use Short-Circuit-Resistant Winding Materials

CHH Power prioritizes materials that enhance winding durability for repairs:

Conductors: Use semi-hard copper wire or self-adhesive transposed wire (improves tensile/compression resistance).
Radial Support: Adopt high-strength cardboard tubes or increase strut quantity (resists radial deformation).
Axial Protection: Use spring pressure nails or reinforced pull plates (absorbs axial forces).
Pre-Sales & Repair Focus: CHH Power’s technical team evaluates winding short-circuit resistance during sales contract demonstrations and repair planning to ensure long-term reliability.

(5) Transformer Drying Measures

Short-circuit repairs often take weeks—CHH Power prevents moisture ingress with two methods:

Vacuum-Nitrogen Preservation:
- Seal the transformer tank and evacuate to -0.095MPa to remove surface moisture.
- Fill with dry nitrogen (dew point ≤-40°C) when stopping work overnight.
- After repairs, circulate hot oil (60–70°C) for 24 hours before commissioning.
Hot Oil Spray Drying:
- Implement rainproof measures for the transformer during repairs.
- After work, use hot oil (80–90°C) to spray the core and windings for 7–10 days to eliminate moisture.

(6) Comprehensive Fault Judgment & Repair Planning

Combine test results (oil/gas, DC resistance, capacitance, deformation) to determine fault nature and severity.
For severe winding deformation (e.g., broken strands, large-scale bulging), replace windings and recheck core/clamp alignment.
Document all repairs (materials, test data, photos) for future maintenance reference.

By following this systematic process, CHH Power ensures that transformer short-circuit faults are resolved thoroughly, minimizing downtime and restoring reliable operation. For power utilities and industrial clients, this approach also extends the transformer’s service life and reduces long-term operational risks.