Tertiary Winding FRA: Detecting Damage in Delta Rings and Buried Winding Structures
Many power transformers include a tertiary winding—typically a delta-connected winding rated at 13.8 kV to 34.5 kV—designed to provide a zero-sequence current path, supply auxiliary loads, or filter harmonics. Tertiary windings are often buried between the HV and LV windings, making direct visual inspection impossible without major disassembly. A Transformer Frequency Response Analyzer, applied with specialized test configurations, provides the only non-invasive method to assess tertiary winding integrity.
Tertiary Winding Functions and Failure Modes
Tertiary windings serve multiple purposes and face unique failure mechanisms:
Zero-sequence path: Delta-connected tertiary provides a return path for ground fault currents, preventing core overheating. Failure of the tertiary can lead to core saturation and tank heating during unbalanced faults.
Harmonic filtering: Tertiary windings are tuned to specific harmonic frequencies (often 3rd or 5th). Damage alters the tuning, increasing harmonic distortion in the power system.
Auxiliary power supply: Many substations use the tertiary to feed station service transformers. Tertiary failure can blackout substation auxiliaries (cooling pumps, control power).
Failure modes unique to tertiary windings include:
Open delta corner: Loss of connection at one corner of the delta ring
Shorted turns in the buried winding (often from through-fault forces)
Lead structure damage to tertiary bushings (often located on the tank side or bottom)
Mechanical deformation from asymmetrical through-fault currents
Specialized Test Configurations for Tertiary Winding FRA
Standard end-to-end and inter-winding tests may not fully characterize the tertiary. Implement these specialized modes:
Tertiary-only loop measurement: For delta-connected tertiary, open the delta at one corner and perform end-to-end FRA across the open corner (e.g., X1 to X2, X2 to X3, X3 to X1). This measures the tertiary winding independently of HV and LV influence.
Transfer function HV to tertiary: Apply source to HV winding, measure response on tertiary (with LV open). Sensitivity to insulation and spacing between HV and tertiary windings.
Transfer function LV to tertiary: Apply source to LV winding, measure response on tertiary. Sensitivity to the inner winding structure.
Tertiary to ground measurement: With all other windings open, measure FRA from each tertiary bushing to tank ground. Detects insulation degradation to ground.
Expected Tertiary FRA Signatures
A healthy tertiary winding produces distinct spectral features:
Delta loop (open corner measurement): A series of evenly spaced resonant peaks corresponding to the delta's electrical length. Typical spacing is 50–200 kHz depending on tertiary size.
Transfer function HV to tertiary: One or two broad resonant peaks in the 10–50 kHz range, with amplitude -40 to -60 dB.
Transfer function LV to tertiary: Higher amplitude (-20 to -40 dB) because of closer proximity between LV and tertiary windings.
Case Example: Detecting Delta Open Corner
A 100 MVA autotransformer with tertiary (230/115/13.8 kV) exhibited elevated zero-sequence current during commissioning tests. Standard DGA and TTR were normal. FRA testing using tertiary-only loop measurement revealed:
X1–X2 measurement: Normal, CC = 0.97 compared to factory baseline
X2–X3 measurement: Normal, CC = 0.96
X3–X1 measurement: Severe deviation, CC = 0.43, with complete loss of resonant structure above 100 kHz
This pattern indicated an open circuit between X3 and X1—a broken connection in the delta ring. Internal inspection found that a brazed joint in the delta ring had cracked during transport. The open delta corner meant zero-sequence currents could not circulate, forcing flux into the tank and causing overheating. The joint was re-brazed, and FRA returned to baseline. Without the tertiary-specific FRA configuration, this hidden open circuit would have remained undetected until tank heating caused a catastrophic failure.
Detecting Shorted Turns in Buried Tertiary Windings
Shorted turns in a buried tertiary winding are difficult to detect by conventional methods:
TTR measures turns ratio between HV and LV only; tertiary is not part of the ratio.
DGA may not show acetylene until the short becomes severe.
Zero-sequence impedance testing can detect gross tertiary faults but not localized shorted turns.
FRA detects tertiary shorted turns as:
Broadband amplitude reduction (5–15 dB) in the tertiary-only measurement
Disappearance of multiple resonant peaks in the transfer function HV-to-tertiary
Phase angle discontinuities (steps > 90 degrees) in the affected frequency range
Establishing Baselines for Tertiary Windings
Given the inaccessibility of tertiary windings, baselines are critical:
Require manufacturer to provide FRA baselines for all tertiary test modes (open corner, transfer HV-to-tertiary, transfer LV-to-tertiary) as part of factory acceptance.
If a transformer is already in service without a baseline, create one using phase-to-phase comparison of the three open-corner measurements (X1-X2, X2-X3, X3-X1). A healthy delta has identical signatures for all three corners; deviation in one corner indicates a fault.
For transformers without tertiary bushings (buried tertiary with no external access), FRA cannot directly measure the tertiary. Use transfer function measurements (HV-to-ground with tertiary effects inferred) as a surrogate.
Correlating Tertiary FRA with Zero-Sequence Impedance
Zero-sequence impedance (Z0) testing is the traditional method for assessing tertiary integrity. FRA provides complementary information:
If Z0 is elevated but FRA is normal: The tertiary winding is intact, but the external grounding or neutral connection may be compromised.
If Z0 is normal but FRA shows deviation: Shorted turns may exist without significantly affecting Z0 (early stage).
If both Z0 and FRA are abnormal: Severe tertiary damage requiring immediate attention.
Practical Testing Challenges
Tertiary windings present unique field testing difficulties:
Accessing open corners: For delta tertiaries with external bushings, physically disconnect the links between bushings to create an open corner. Document original configuration with photographs.
Small bushing size: Tertiary bushings are often smaller (5–15 kV class) with different connection interfaces. Use appropriate adapters.
High current during normal operation: Tertiary windings may carry significant current (hundreds of amps) from auxiliary loads. De-energize and ground both ends before testing.
Buried tertiary without bushings: For transformers where the tertiary is buried (no external bushings), FRA cannot directly access the winding. Use transfer functions and inductive coupling techniques as proxies.
Interpretation Thresholds for Tertiary FRA
Based on industry practice, use these thresholds for tertiary-only measurements:
CC > 0.95: Normal
CC 0.85–0.95: Investigate; possible minor damage or temperature effect
CC < 0.85: Strong evidence of tertiary damage; plan internal inspection
Loss of resonant structure (flat line above certain frequency): Open circuit in the delta ring or severe shorted turns
The Transformer Frequency Response Analyzer, applied with tertiary-specific test configurations, provides the only non-invasive window into buried tertiary winding integrity. For asset managers responsible for three-winding transformers, incorporating tertiary FRA into commissioning, periodic, and post-event testing programs is essential for preventing hidden tertiary failures that can lead to core overheating, harmonic issues, or substation auxiliary loss.
