The following is a summary of the core knowledge points about this instrument:
1. Core Working Principle
Currently, mainstream winding deformation testers are mainly based on two methods, among which Frequency Response Analysis (FRA) is the most commonly used and has the highest sensitivity:
Frequency Response Analysis (FRA):
Principle: The transformer winding is considered as a complex passive linear network composed of resistance (R), inductance (L), and capacitance (C). The instrument injects a sinusoidal sweep signal within a certain frequency range (usually 1 kHz to 2 MHz) into the winding.
Detection: It measures the response characteristics (amplitude and phase) of the winding to signals at different frequencies, generating a "frequency response curve."
Judgment: If the winding deforms, its internal distributed inductance and capacitance parameters change, causing the frequency response curve to shift. By comparing lateral data among the three phases or longitudinal comparison with historical data, the degree and location of deformation can be determined.
Standard: Complies with Chinese power industry standard DL/T 911-2004 "Frequency Response Analysis Method for Power Transformer Winding Deformation."
Low Voltage Short-Circuit Impedance Method (Impedance Method):
Principle: Measures the short-circuit impedance of the transformer under low voltage.
Judgment: If the short-circuit impedance value changes by more than a certain threshold (usually 3%) compared to the factory value or historical data, it suggests possible winding deformation.
Standard: Complies with DL/T 1093-2008 "Guide for Detection and Judgment of Power Transformer Winding Deformation by Reactance Method."
Features: Often used as an auxiliary verification method for the FRA method, especially for large transformers.
2. Main Application Scenarios
Post-fault diagnosis after short circuit: After a transformer experiences an outlet short circuit or near-area short circuit, it must be tested immediately to confirm whether the winding is damaged, determining whether a cover lift inspection or scrapping is needed.
Inspection after transportation and installation: Large transformers may be subjected to severe vibration during long-distance transportation or installation. Testing before commissioning is required to confirm the internal structure is intact.
Preventive testing: As part of regular maintenance, establish a winding fingerprint database to monitor long-term trends in winding condition.
Factory acceptance: Manufacturers provide baseline frequency response curves when the transformer leaves the factory, serving as a standard for subsequent comparisons.
3. Types of Faults Detectable
This instrument can effectively identify the following mechanical defects:
Winding inter-turn short circuit or open circuit
Overall or partial winding distortion, bulging
Winding axial or radial displacement
Poor contact of tap changer or lead displacement
Poor core grounding or displacement (indirect reflection)
4. Typical Test Steps
Preparation: Ensure the transformer is de-energized and fully discharged, and remove all external connections (such as surge arresters, neutral grounding wires, etc.).
Wiring: Usually adopt "end-to-end" or "end-to-ground" methods. For three-phase transformers, test the high-voltage and low-voltage windings of phases A, B, and C separately.
Parameter setting: Input the transformer nameplate parameters and select the scanning frequency range (e.g., 1k-1000kHz).
Data acquisition: Start the instrument for automatic sweep and record the amplitude-frequency characteristic curve.
Data analysis:
Three-phase comparison: Observe the coincidence of the three-phase curves in low, medium, and high frequency bands. If a certain phase curve deviates significantly from the other two phases, it suggests possible deformation in that phase.
Historical comparison: Overlay the current curve with the factory or previous test curve, and calculate the correlation coefficient (R). If the R value is below the standard (e.g., <0.95 or lower, depending on the frequency band), it is judged as abnormal.
5. Common Equipment Models and Manufacturers
Common brands on the market include Wuhan Moen, Dingsheng Electric, Huatian Electric, Yangzhou Dary, etc. Model names vary, such as DCBX-H, MERB-I, BWD series, etc., but the core functions all follow the above standards.
Summary: The winding deformation tester is the "CT machine" for transformers, capable of "seeing through" the mechanical health of the internal winding without disassembly. It is an indispensable diagnostic tool for ensuring the safe operation of the power grid.
