Sweep Frequency Response Analyzer: Practical Guide for New & Experienced Power Engineers

Winding Deformation Tester

What specific inspection does this instrument perform on the transformer?

Many newcomers to the field often ask when first encountering this device: "Is this used for voltage measurement or resistance measurement?" In reality, neither is the case. The winding deformation tester essentially performs a CT scan on the transformer's internal windings—it allows detection of deformation, loosening, or displacement without requiring transformer disassembly or cover removal.

The simplified principle: Treat the winding as a "special filter".

You can envision each winding of a transformer as a complex network composed of resistors, inductors, and capacitors—a customized "filter." When fed with signals of varying frequencies, it produces specific responses, generating a unique "fingerprint curve."

If the winding remains intact, the characteristics of this "filter" remain unchanged, and so does the fingerprint curve. However, if the winding is deformed due to short-circuit impacts or transportation shocks—such as wire warping or altered turn-to-turn spacing—the inductive and capacitive parameters within the network change accordingly, resulting in a modified fingerprint curve. Our tester determines whether the winding is faulty by analyzing these curve variations.

Figure: Typical appearance of a winding deformation tester, compact and portable, suitable for field operations

What issues can it detect?

1. Axial/radial deformation of windings, twisting, bulging, inter-turn short circuits, and inter-patch misalignment
2. Winding loosening, lead wire displacement
3. Interphase short circuit, core loosening

Moreover, the entire testing process is non-destructive and causes no damage to the transformer, which is why it has become the standard for preventive testing.

Practical Demonstration: Testing of Transformers

Step 1: Pre-test preparations – do not overlook these points

1. Transformer condition verification: First, disconnect the transformer from power supply and remove all external leads (including busbars and cables), then relocate these leads at least 20 cm away from the bushing to avoid interference with test signals.
2. Complete Discharge: After the transformer is de-energized, ensure the windings are fully discharged to prevent residual charges from causing injury or interfering with testing.
3. Record key information: Note the tap position of the transformer! This is crucial, as different tap positions yield entirely distinct curves; data comparisons must be conducted at the same tap position.
4. Environmental inspection: Check the site for strong interference sources such as welding machines or large motors. If present, either turn them off or schedule the measurement at a different time; otherwise, the measured curves will exhibit erratic fluctuations.

Step 2: Wiring

Figure: Schematic wiring diagram of Phase A winding in the Yn configuration, clearly indicating the connection positions of each clamp.

• Grounding is crucial: Connect the tester's grounding terminal, the transformer's casing, and the iron core together to a single grounding point! Many cases where the measured curve appears abnormal are due to improper grounding or connecting multiple grounding points, which introduces interference.
• The clamp must be securely tightened: Before testing, thoroughly clean the sleeve terminal and clamp it firmly to avoid loose connections, as this would alter the contact resistance and cause signal distortion.
• Avoid excessively long shielded cables: Use shielded test cables, preferably not exceeding 3 meters in length, as longer lengths may introduce interference.

Step 3: Parameter Setup and Testing

After completing the wiring, open the testing software on the computer and connect the instrument; then you can configure the parameters.

• Generally, a frequency sweep range of 1 kHz to 2 MHz is sufficient, as most transformers can be covered by this range.
• For scanning, start with linear frequency scanning to quickly obtain the overall result. If any abnormalities are detected, then use segmented frequency scanning for detailed analysis.
• Then click "Start Test," and the instrument will automatically inject a frequency sweep signal into the winding to collect responses. Each phase test takes less than one minute, and completing measurements for all three phases (high, medium, and low) of a transformer requires approximately 10 minutes in total.

Figure: On-site testing scenario, where engineers are conducting winding deformation detection

Step 4: Result Analysis

• Horizontal comparison: Compare the curves of phases A, B, and C at the same voltage level. Under normal conditions, all three curves should appear similar; if one curve deviates significantly, that phase is likely faulty.
• Vertical comparison: Compare the current curve with the transformer's original factory curve and the historical curves from previous tests. If the deviation exceeds 3.5 dB, it indicates deformation of the windings.

Common misuse examples

1. After completing the DC resistance test, do not immediately proceed with the winding deformation test!

This is the most common pitfall! Many practitioners conduct tests on-site and, to save time, directly connect the deformed winding wire for measurement after measuring the DC resistance. However, did you know? During the DC resistance test, the winding is magnetized, leaving residual magnetism in the iron core. This residual magnetism directly affects the winding's inductance parameters, resulting in completely inaccurate frequency response curves that can easily be misinterpreted as evidence of winding deformation.

Correct procedure: After completing the DC resistance test, demagnetize the transformer or wait at least 30 minutes for the residual magnetism to dissipate before performing the winding deformation test.

2. Dry-type transformers cannot be tested.

Many people, upon receiving the testing equipment, measure all transformers indiscriminately. In fact, the national standard explicitly states that winding deformation testing is primarily intended for oil-immersed transformers! The windings of dry-type transformers are open, exhibiting high stray capacitance, resulting in highly inconsistent frequency response curves. Moreover, the national standard does not specify requirements for winding deformation testing of dry-type transformers; thus, the obtained results lack practical reference value—there's no point in conducting such tests.

3. First eliminate these interference factors for abnormal curves.

Many people, upon noticing an abnormal curve, immediately conclude that the winding is deformed; in reality, this is often caused by interference.

1. Low-frequency band misalignment? First, check the wiring: If the low-frequency band curve at several tens of kHz does not align, there is a 90% chance that the issue lies not with the windings but with improper wiring or poor contact. Re-test after re-clamping the clamp.
2. High-frequency band interference jumping? Check for interference: If the curve in the high-frequency band exhibits numerous spikes, it indicates the presence of electromagnetic interference at the site. Either shut down the nearby welding machine or ensure proper shielding of the test lines.
3. Does the curve exhibit a double peak? This may indicate axial distortion: If a distinct double peak appears in the low-frequency band, this is not interference but rather a typical characteristic of winding axial distortion! Previously, a 220 kV transformer exhibited this feature following a short-circuit shock; disassembly revealed an axial distortion of 8 mm in the high-voltage winding, demonstrating remarkable diagnostic accuracy.

4. Perform the measurement twice on the same phase to ensure repeatability.

Many people complete the measurement after just one attempt. However, standard specifications require at least two measurements for the same phase, with the similarity between the two curves exceeding 0.95. If significant discrepancies are observed between the measurements, it indicates either unstable wiring or excessive environmental interference; in such cases, re-measurement is mandatory to ensure reliable data.

5. For small transformers of 35 kV and below, do not simply compare the three phases.

Many small 35 kV transformers exhibit inherent differences in their three-phase frequency response curves due to their winding configurations. In such cases, avoid relying solely on horizontal comparisons; instead, conduct vertical comparisons against their original measurement data to prevent misinterpretations.

application scenarios

1. After a short-circuit impact: Following a near-field short circuit in the transformer, the immense electrodynamic force can easily deform the windings. Immediate measurement is essential to confirm whether the windings are damaged; otherwise, operating with the fault could lead to serious accidents.
2. Installation and Acceptance of New Transformer: Upon delivery to the site, the newly installed transformer must undergo testing to establish an original "fingerprint profile," which will serve as the benchmark for all subsequent tests.
3. After transportation/histing: During long-distance transport of large transformers or after maintenance under a hoist cover, vibrations can easily cause winding displacement. In such cases, measure the displacement to confirm that the windings remain undamaged during transport and maintenance.
4. Preventive Testing: For operating transformers, winding deformation testing is a mandatory item in preventive tests conducted every 3 to 5 years to identify potential hazards in advance.

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