Winding Deformation Tester
Winding Deformation Tester (also known as Transformer Winding Deformation Tester, Frequency Response Analyzer/FRA) is a specialized diagnostic device used to detect mechanical deformation of power transformer windings. It scans the amplitude-frequency characteristic curve of the winding through Frequency Response Analysis (FRA), enabling the detection of issues such as winding distortion, bulging, displacement, and inter-turn short circuits without the need for lifting the transformer cover. It serves as the "structural doctor" ensuring the safe operation of transformers.
Core Principle - Frequency Response Analysis (FRA)
Under high-frequency voltage, each transformer winding can be equivalent to a passive linear two-port network composed of distributed parameters such as linear resistance, inductance (mutual inductance), and capacitance. Its internal characteristics can be described by the transfer function H(jω).
When mechanical deformation occurs in the winding, the internal distributed inductance, capacitance, and other parameters inevitably change, causing shifts in the zeros and poles of the transfer function, resulting in identifiable deviations in the frequency response characteristic curve. The instrument performs diagnosis through the following steps:
Signal Injection: The DDS signal source injects a sinusoidal sweep excitation signal ranging from 1kHz to 2MHz (some models extend to 10Hz-10MHz) into the winding.
Signal Acquisition: Dual-channel synchronous acquisition of voltage signals at the excitation end and response end.
Transfer Function Calculation: Calculate the amplitude ratio of the two channel signals to generate the amplitude-frequency response curve.
Comparative Analysis: Perform correlation coefficient analysis between the current curve and the factory "fingerprint" data (longitudinal comparison) or three-phase data of the same equipment (horizontal comparison) to determine the degree of deformation.
Frequency Band Diagnostic Significance
Anomalies in different frequency bands reflect different types of winding defects:
Low Frequency Band (1kHz-100kHz): Reflects overall inductance characteristics; anomalies indicate overall deformation, core displacement, or inter-turn/inter-disc short circuits.
Mid Frequency Band (100kHz-600kHz): Sensitive to local deformation; indicates bulging, collapse, or distortion.
High Frequency Band (600kHz-1MHz and above): Reflects inter-turn distributed parameter characteristics; anomalies indicate overall winding displacement or lead displacement.
System Composition
The equipment typically consists of the following components:
Measurement Main Unit (Black Box): Built-in DDS signal generator, dual-channel high-speed acquisition module, and automatic range adjustment circuit.
Analysis Software: Installed on a laptop or the instrument's built-in touchscreen, it performs curve display, correlation coefficient calculation, deformation level determination, and report generation.
Connection Cables: Excitation cable, response cable, grounding cable, etc.
Accessories: USB/Ethernet communication cables, micro printer (built-in on some models).
Key Technical Parameters
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Parameter Typical Range
Sweep Frequency Range 1kHz-2MHz (High-end models: 10Hz-10MHz)
Number of Sweep Points 1000-4000 points
Frequency Accuracy ≤0.01% (some up to 0.001%)
Amplitude Measurement Range -100dB to +20dB
Amplitude Measurement Accuracy 0.1dB
Sampling Rate 20Msps to 100Msps
Quantization Accuracy 12-16 bits
Signal Output Impedance 50Ω
Signal Input Impedance 1MΩ
Signal Output Amplitude Maximum ±5V to ±10V
Single-Phase Test Time <60 seconds to several minutes
Communication Interface USB / Ethernet / WiFi / Bluetooth
Detection Methods
In addition to the mainstream Frequency Response Analysis (FRA), there are the following auxiliary methods:
Short-Circuit Impedance Method (Reactance Method)
Measure the winding impedance or leakage reactance at 50Hz and judge deformation based on the rate of change.
Judgment thresholds: Impedance change rate ≤2% normal; 2%-3% slight deformation; >3% obvious deformation, recommended to shut down for internal inspection.
Execution standard: DL/T 1093-2008
Low Voltage Impulse Method
Inject a low-voltage pulse signal into the winding and analyze the reflected waveform to determine the deformation location.
High sensitivity, but strict requirements on the test environment.
Result Judgment Standards
According to DL/T 911-2016, the degree of deformation is quantitatively determined by correlation coefficients:
Judgment Based on Correlation Coefficient R
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Deformation Degree Correlation Coefficient Range
Normal >0.99 (Longitudinal) / >0.98 (Horizontal)
Mild Deformation 0.97 to 0.99
Moderate Deformation 0.95 to 0.97
Severe Deformation <0.95
Judgment Based on Frequency Band Correlation Coefficients
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Deformation Degree Criteria
Normal RLF≥2.0 and RMF≥1.0 and RHF≥0.6
Mild Deformation 2.0>RLF>1.0 or 1.0>RMF>0.6
Obvious Deformation 1.0>RLF>0.6 or RMF<0.6
Severe Deformation RLF<0.6
RLF, RMF, and RHF are the correlation coefficients for the low, mid, and high frequency bands, respectively.
Main Application Scenarios
Before commissioning of new transformers: Collect factory "fingerprint" data to establish baseline curves.
After short-circuit impact: Mandatory testing after a nearby short circuit to determine if the winding is damaged.
After transportation collision: Detect whether vibration during transport caused winding displacement.
Routine preventive tests: Once every 1-3 years in normal maintenance cycles.
After winding replacement or major repair: Verify the quality of the repair.
Renewable energy stations: Health assessment of wind/solar box transformer windings.
Applicable equipment covers oil-immersed power transformers (110kV-1000kV), dry-type transformers, rectifier transformers, converter transformers, autotransformers, traction transformers, etc. Test Conditions and Precautions
Test Conditions
The transformer must be de-energized and reliably grounded.
Ambient temperature -15°C to +50°C, relative humidity ≤90%.
All external leads must be disconnected; testing should be performed at the maximum tap position or a fixed tap position.
Sufficient demagnetization is required before testing; testing should not be conducted during thunderstorms.
Precautions
Test connections must be firm and reliable, and the grounding wire cross-section must meet requirements.
For multiple tests on the same transformer, keep the connection method consistent to ensure data comparability.
Test data should be properly stored as the historical "fingerprint" file for the equipment.
When curve anomalies occur, comprehensive judgment should be combined with auxiliary methods such as dissolved gas analysis (acetylene content >5μL/L as a warning) and DC resistance testing.
