OLTC Integrity Assessment with FRA: Detecting Contact Wear and Diverter Switch Faults
On-load tap changers (OLTCs) are among the most failure-prone components in power transformers, yet they are often excluded from routine FRA testing. Standard Transformer Frequency Response Analyzer measurements on the main windings may not reveal OLTC deterioration. However, specialized FRA configurations—focusing on the regulating winding and the tap changer's internal impedance—can detect contact wear, transition resistor degradation, and diverter switch timing anomalies. This article presents practical FRA techniques for OLTC assessment.
How OLTC Condition Affects FRA Signatures
The OLTC is connected to the regulating winding, which is part of the transformer's overall R-L-C network. OLTC degradation alters this network in measurable ways:
Contact wear: Increased contact resistance (milliohms to ohms) adds series resistance that damps high-frequency resonances (>100 kHz), reducing amplitude by 2–8 dB.
Transition resistor degradation: Carbonized or cracked transition resistors change the parallel impedance across the tapped section, creating localized notches or amplitude dips at specific frequencies.
Diverter switch timing asymmetry: Uneven contact transition times produce non-linear effects that appear as amplitude instability (variation between successive FRA sweeps).
Contact bounce or arcing residues: Carbon deposits create capacitive paths that add extra resonant peaks in the 200–500 kHz range.
Specialized FRA Test Configurations for OLTC
Standard end-to-end measurements on the main winding may not sensitively detect OLTC issues. Implement these dedicated modes:
Regulating winding only measurement: Apply FRA source and response across the regulating winding terminals (typically separate bushings labeled R, S, T or via the tap changer access panel). This isolates the OLTC from the series and common windings.
Tap-to-tap measurement: With the OLTC in a specific position (e.g., position 5), measure FRA between successive taps within the diverter switch (requires access to tap leads). Detects transition resistor degradation localized to specific tap pairs.
Position sweep measurement: Perform repeated FRA measurements as the OLTC moves through all positions (e.g., position 1 to position 17). Plot CC versus tap position. A healthy OLTC shows consistent CC > 0.95 across all positions; a drop at specific positions indicates localized wear.
Dynamic FRA (during tap change): Advanced instruments can trigger a sweep while the OLTC operates, capturing transient impedance changes. Anomalous transients indicate diverter switch timing errors.
Expected Signatures: Healthy vs. Degraded OLTC
A healthy OLTC produces:
Stable amplitude (±0.5 dB) across all tap positions in the regulating winding measurement
Correlation coefficient > 0.95 when comparing any two tap positions (except for design-based differences at extremes)
Smooth phase response without discontinuities
A degraded OLTC shows:
Progressive amplitude reduction in high-frequency band as contact wear accumulates
Sudden CC drop (
<0.85) at="" specific="" tap="" positions="">Narrowband notches (1–5 kHz wide) at frequencies corresponding to the electrical length of the tapped section
Case Example: Detecting Transition Resistor Cracking
A 40 MVA transformer with 17-position OLTC experienced occasional DGA spikes (acetylene at 2–5 ppm) but no pattern. Routine FRA on the main windings was normal. Dedicated regulating winding FRA with tap position sweep revealed:
Positions 1–8: CC > 0.96 compared to baseline
Position 9: CC = 0.81, with a 4 dB notch at 320 kHz
Positions 10–17: CC > 0.94
This localized deviation (only position 9) indicated an issue specific to that tap's transition resistor. Internal OLTC inspection found a cracked transition resistor on the tap 9–10 transition. The resistor was replaced, and post-repair FRA returned to CC > 0.96 across all positions. Without tap-position-resolved FRA, the cracked resistor would have continued to erode, eventually causing a diverter switch failure and transformer outage.
Establishing OLTC FRA Baselines
For effective OLTC trending, establish baselines at:
Factory acceptance: Perform regulating winding FRA at all tap positions before the OLTC is filled with oil.
Commissioning: Repeat after installation and oil filling (oil changes permittivity and affects high-frequency response).
After any OLTC maintenance (contact replacement, oil change): Establish new baseline for future comparisons.
Testing Frequency and Range for OLTC FRA
OLTC-related effects dominate at higher frequencies. Optimize instrument settings:
Frequency range: 10 kHz to 10 MHz (focus on 100 kHz – 5 MHz band where contact resistance and transition resistor effects are most visible)
Sweep points: 500–1,000 (logarithmic)
Averaging: 10–20 sweeps to overcome noise from diverter switch springs and mechanical vibration
Correlating OLTC FRA with Other Diagnostics
Combine FRA with OLTC-specific tests:
Dynamic resistance measurement (DRM): Measures contact resistance during tap change. FRA complements DRM by detecting high-frequency impedance changes that DRM misses.
Tap changer DGA: If OLTC has a separate oil compartment, DGA showing acetylene > 10 ppm correlates with FRA high-frequency amplitude loss.
Motor current signature: OLTC motor current peaks that are asymmetric may correlate with FRA-detected contact wear on specific positions.
Practical Testing Challenges
Field testing of OLTC with FRA presents unique difficulties:
Access to regulating winding terminals: Some transformers have separate bushings; others require opening the tap changer compartment. Follow safety procedures—OLTC compartments may have separate oil or gas preservation.
Multiple tap positions: Testing all 17–35 positions takes 2–4 hours. Prioritize by testing neutral, minimum, maximum, and every 5th position for trending; test all positions only when anomalies appear.
Mechanical noise: Diverter switch springs create vibration that can couple into measurements. Perform FRA with the OLTC stationary (not during tap change). Use additional averaging.
The Transformer Frequency Response Analyzer, when configured for OLTC-specific measurements, provides a non-invasive window into tap changer condition. By establishing position-resolved baselines and trending high-frequency amplitude, utilities can predict OLTC contact wear and schedule maintenance before diverter switch failures occur.
