What Tests Should Be Conducted After a Sudden Short Circuit?

—Winding Deformation Testing (FRA Method) Operational Procedure

Transformers are the core equipment of power systems, and their safe and stable operation is critical. When a transformer experiences a sudden short-circuit fault, its windings may undergo mechanical deformation invisible to the naked eye. If these potential damages are not detected promptly, they can lead to degraded insulation performance, increased partial discharge, and ultimately catastrophic failure. According to International Electrotechnical Commission (IEC) standards and State Grid Corporation statistics, winding deformation is one of the primary causes of transformer failures after short circuits. This article details how to scientifically assess winding conditions using the Frequency Response Analysis (FRA) method, an internationally recognized detection technique, to provide reliable data for subsequent maintenance decisions.

 

Inhalt

1. The Impact Mechanism of Sudden Short Circuits on Transformer Windings

● Principles of Short-Circuit Electrodynamic Forces

During a short circuit (especially a three-phase short circuit), the windings carry instantaneous short-circuit currents 10–25 times higher than the rated current. According to Ampere’s force law (F = BIL), these currents generate enormous mechanical stress under the influence of leakage magnetic fields. The forces can be categorized into two types:

(1) Radial forces:Compress the inner winding inward and expand the outer winding outward.
(2)Axial forces: Cause compression or stretching deformation at the winding’s upper and lower ends.

The instantaneous peak force can reach tens of tons, far exceeding the winding’s design strength, leading to the following typical deformation patterns:

Verformungstyp	Verursachen	Mögliche Konsequenzen
Radiale Verformung	Radial electrodynamic force exceeds support structure strength	Altered main insulation distance, increased partial discharge
Axial distortion	Uneven axial force distribution or spacer displacement	Higher risk of inter-turn short circuits
Local depression	Conductor yielding or support bar fracture	Hotspot formation, accelerated insulation aging

● The Development Process of Winding Deformation

Winding deformation after a short circuit typically progresses through three stages:

(1)Instantaneous elastic deformation:Recoverable deformation at the moment of short circuit, which may not be detectable in tests.
(2)Plastic deformation stage: Permanent deformation occurs when the material’s yield point is exceeded.
(3)Cumulative damage effect:Repeated short circuits cause deformation to accumulate, eventually leading to insulation failure.

Research by the International Council on Large Electric Systems (CIGRE) shows that approximately 68% of transformers exhibit measurable winding deformation after severe short circuits, with 30% developing into serious faults during subsequent operation.

2. Technical Principles of FRA for Detecting Winding Deformation

● Basic Concepts of Frequency Response Analysis

The Frequency Response Analysis (FRA) method evaluates the mechanical condition of transformer windings by measuring changes in their impedance characteristics across different frequencies. The theoretical basis is:

A winding can be modeled as a distributed network of inductance (L), capacitance (C), and resistance (R). Its frequency response function H(f) is expressed as:

H(f) = Vout(f)/Vin = Zw/[Zs+Zw]

Where Zw is the winding impedance, and Zs is the signal source impedance.

When physical deformation occurs, the distributed parameters L and C change, causing shifts in the response curve’s characteristic frequency points. By comparing historical data and phase-to-phase data, the location and extent of deformation can be precisely identified.

 

● Advantages of the FRA Method

Compared to traditional methods, FRA offers the following key benefits:

(1)Hohe Empfindlichkeit:Detects minor capacitance changes as small as 0.1%.
(2)Zerstörungsfrei: Test voltage is typically <10V, ensuring no damage to insulation.
(3)Full-frequency analysis: Covers 1kHz–1MHz, reflecting information at different depths.
(4)Quantitative evaluation:Uses metrics like correlation coefficient (CC) and mean squared error (MSE) for objective assessment.

According to IEEE Std C57.156-2016, the FRA method achieves over 95% accuracy in detecting winding deformation and has become a mandatory testing procedure for power companies in Europe and the U.S.

 

3. Standardized FRA Testing Procedure (Compliant with IEC 60076-18)

● Core Preparations

(1)Sicherheitsisolierung:Implement Lockout-Tagout (LOTO) procedures to ensure the transformer is completely de-energized and grounded (residual voltage <50V).

(2)Remove all external connections, especially bushing tap leads, to avoid signal interference. This step eliminates 90% of on-site testing errors.

 

Umweltkontrollen

(1)Temperatur:Record ambient temperature (refer to IEEE C57.152 standard).
(2)Luftfeuchtigkeit:≤85% (high humidity increases surface leakage current).
(3)Elektromagnetische Interferenz:Conduct tests 2 hours after power-off (to avoid system overvoltage).

● Key Wiring Configuration Options

Verdrahtungsart	Anwendungsszenario	International Standard Reference
End-to-end method	Comprehensive diagnosis (recommended post-short circuit)	IEC 60076-18 Annex B
Capacitive coupling method	Rapid on-site screening	CIGRE TB 642
Inductive voltage method	Specialized axial deformation detection	IEEE P1898

Wichtige Schritte:Prioritize the end-to-end method—connect the signal source to the high-voltage bushing and the measurement terminal to the neutral grounding end (as shown in Figure 1). Wiring resistance must be <0.5Ω to avoid distorting low-frequency curves.

● Smart Parameter Settings

fmax = 150 / MVA Rating (MHz)

(1)Frequency range: 1kHz – [calculated value] MHz (e.g., 1.5MHz for a 100MVA transformer).
(2)Scan density: ≥800 points (logarithmic distribution, automatically denser in high-frequency ranges).
(3)Excitation voltage: 10V (balances signal-to-noise ratio and safety).
(4)Signal processing: Apply Hanning window + 32-time averaging (suppresses random noise).

 

4. Data Analysis and Intelligent Diagnosis (Based on IEEE C57.156)

● Core Diagnostic Metrics

Metrisch	Formel	Deformation Sensitivity	Schwelle
CC	∑(xi−x̄)(yi−ȳ)/σxσy	Overall deformation	<0.95 (warning)
RMSD	√[1/N ∑(Htest−Href)²]	Local distortion	>3dB (abnormal)
ASL	∫	logH1(f)−logH2(f)	df

● Typical Fault Signature Patterns

 

Verformungstyp	Low Freq. (1–10kHz)	Mid Freq. (10–500kHz)	High Freq. (>500kHz)
Radial expansion	Resonance peak shifts right ≥5%	Amplitude drops >3dB	Keine wesentliche Änderung
Axiale Verschiebung	Phase lag >10°	Double-peak splitting	New resonance points
Local collapse	No notable change	Narrow-band drop (>6dB)	Nonlinear phase jumps

 

 

Zusammenfassend

Winding deformation testing (FRA method) is the gold standard for post-short-circuit transformer condition assessment. Its scientifically standardized implementation is crucial for ensuring equipment safety. This article provides a comprehensive guide covering test preparation, on-site execution, and data analysis. It is important to note that FRA result accuracy heavily depends on procedural compliance and the completeness of reference data. Companies are advised to establish a full lifecycle FRA database and align with international standards (e.g., IEC 60076-18) to maximize the technology’s value. For critical substations, online FRA monitoring systems can enable real-time winding condition tracking, preventing potential failures at an early stage.

 

 

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