Characteristics and Diagnosis of Inter-Turn Short Circuit Faults in Power Transformer Windings

1. Overview of Inter-Turn Short Circuit Faults

• Insulation Aging: Degradation of paper/oil or epoxy insulation due to thermal stress, moisture, or chemical reactions.

• Electrical Overstress: Lightning surges, switching transients, or harmonic voltages that breach insulation withstand limits.

• Mechanical Damage: Winding deformation from short-circuit forces or improper installation.

• Contamination: Particles or moisture in the insulation system leading to partial discharges (PD) and breakdown.

2. Key Fault Characteristics

2.1 Electrical Characteristics

a. Increased Winding Current

• A shorted turn acts as a low-impedance loop, drawing excessive current ($I_{\text{sc}}$) proportional to the number of shorted turns. For a winding with $N$ turns and $n$ shorted turns:$I_{\text{sc}}\approx \frac{V}{Z_{\text{turn}}}\propto \frac{n}{N-n}$

• $Z_{\text{turn}}$: Impedance of a single turn.

• This causes unbalanced currents in multi-phase transformers, triggering differential protection relays.

b. Reduced Impedance

• The fault lowers the winding’s effective inductance, reducing the transformer’s short-circuit impedance ($Z_{\%}$). A decrease of 3–5% in $Z_{\%}$ may indicate winding damage.

c. Voltage Regulation Distortion

• Shorted turns alter the turns ratio, leading to voltage mismatches between primary and secondary sides. For example, a 1% turn loss in a 10 kV winding may cause a 100 V output deviation.

2.2 Thermal Characteristics

• Localized Overheating: Fault currents generate joule heating ($I^{2}R$), raising temperatures in the shorted region. Infrared (IR) imaging may detect hotspots exceeding 100°C, compared to the normal winding temperature (≤95°C for oil-immersed transformers).

• Oil Degradation: Overheating decomposes transformer oil, producing gases like methane (CH₄), ethylene (C₂H₄), and hydrogen (H₂). DGA (Dissolved Gas Analysis) ratios exceeding IEC 60599 codes (e.g., C₂H₄/C₂H₆ > 3) indicate thermal faults.

2.3 Mechanical Characteristics

• Vibration Amplification: Asymmetrical magnetic forces from shorted turns induce mechanical vibrations at frequencies of $2f$ (twice the power frequency). Vibration sensors may detect acceleration levels >5 g (normal: <2 g).

• Winding Deformation: Prolonged faults can cause axial or radial displacement of windings, detected via frequency response analysis (FRA) or short-circuit impedance tests.

2.4 Acoustic and PD Characteristics

• Arcing Noise: High-energy faults produce audible hissing or buzzing sounds due to partial discharges.

• PD Activity: Insulation breakdown creates PD pulses, measurable via HFCT (High-Frequency Current Transformer) sensors or ultrasonic detectors. PD levels >100 pC are indicative of serious defects.

3. Diagnostic Techniques

3.1 Electrical Tests

a. DC Resistance Measurement

• Compares resistance values between phases or windings. A deviation >2% from the average indicates possible shorted turns:$\Delta R=\frac{R_{\text{test}}-R_{\text{avg}}}{R_{\text{avg}}}\times 100\%$

• Example: A 10 MVA transformer’s winding resistance shifts from 0.12 Ω to 0.11 Ω, suggesting a 5–8% turn loss.

b. Turns Ratio Test

• Measures the voltage ratio between primary and secondary windings. A sudden change in the ratio (e.g., >0.5%) signals winding deformation or shorted turns.

c. Frequency Response Analysis (FRA)

• Applies a swept-frequency signal (10 Hz–1 MHz) to the winding and compares the impedance frequency response to a baseline. Deviations in the resonance peaks (e.g., shift >2 kHz) indicate mechanical deformation from short circuits.

3.2 On-Line Monitoring

a. DGA (Dissolved Gas Analysis)

• Monitors gas concentrations and ratios to classify fault severity:

• Low-Temperature Pyrolysis (<300°C): CH₄, CO dominate.

• High-Temperature Thermal Fault (>700°C): C₂H₄, H₂ dominate.

• IEC 60599’s Duval triangle method correlates gas ratios to fault types.

b. Partial Discharge (PD) Monitoring

• Uses inductive sensors (HFCT) or ultrasonic probes to detect PD pulses. A PD trend showing increasing pulse magnitude (>500 pC) or repetition rate (>100 pulses/s) indicates evolving insulation damage.

c. Dynamic Load Testing

• Applies step loads while monitoring winding temperature rise and voltage regulation. A faster-than-normal temperature increase (e.g., >5°C/h under 75% load) may indicate hidden short circuits.

3.3 Mechanical and Thermal Diagnostics

a. Vibration Analysis

• Uses accelerometers to capture vibration signals at the tank surface. Fourier transform analysis of the signals reveals fault-related frequencies (e.g., 2f, 3f) with amplitude increases >30% compared to baseline.

b. Infrared Thermography

• Detects temperature gradients across windings and bushings. A hotspot with a temperature delta ($\Delta T$) >20°C relative to adjacent areas suggests a localized fault (Figure 1).

c. Winding Deformation Test (WDT)

• Uses low-voltage impulse (LVI) or sweep-frequency impedance (SFI) methods to assess winding geometry. Changes in the impulse response waveform’s timing or amplitude indicate turn-to-turn short circuits.

4. Fault Severity Assessment and Mitigation

4.1 Severity Classification

4.2 Mitigation Strategies

• Immediate Shutdown: For critical faults (e.g., smoking, explosive noises), isolate the transformer to prevent catastrophic failure.

• Offline Testing: Perform FRA, DGA, and DC resistance tests to confirm fault location.

• Repair or Rewinding: Replace damaged winding sections or rewind the entire winding if >10% of turns are shorted.

• Insulation Refurbishment: Replace aged oil and paper insulation to prevent recurrence.

5. Industry Standards and Case Studies

5.1 Standards

• IEEE Std C57.12.90: Guidelines for transformer diagnostic testing.

• IEC 60076-5: Specification for withstand voltage tests and partial discharge measurements.

• ASTM D3612: Method for gas analysis in transformer oils.

5.2 Case Study: 220 kV Transformer Inter-Turn Fault

• Diagnostics:

• DGA showed C₂H₄ = 280 ppm (alert limit: 150 ppm), H₂ = 500 ppm (alert limit: 150 ppm).

• FRA revealed a 3 kHz shift in the resonance peak of the high-voltage winding.

• DC resistance test showed a 2.5% decrease in the faulty phase’s resistance.

• Outcome: A 5-turn short in the HV winding was found during disassembly. The winding was rewound, and the transformer returned to service with normal parameters.

6. Future Trends in Fault Diagnosis

• AI-Driven Diagnostics: Machine learning models (e.g., convolutional neural networks) to analyze FRA and PD data for early fault detection.

• IoT-Based Predictive Systems: Real-time integration of sensor data with cloud platforms for automated fault triaging.

• Non-Destructive Testing (NDT) Advances: Use of terahertz imaging to visualize insulation defects without disassembly.

7. Conclusion