Zig-Zag Grounding Transformer FRA: Diagnosing Winding Asymmetry and Core Integrity
Zig-zag grounding transformers provide a low-impedance path for ground fault currents in ungrounded or high-resistance grounded systems. Their unique winding configuration—each phase has two halves on different core limbs, connected in a zig-zag pattern—creates a distinctive frequency response. A Transformer Frequency Response Analyzer is ideally suited to detect winding asymmetry, inter-turn faults, and core damage that compromise grounding performance.
Zig-Zag Transformer Construction and FRA Implications
Key design features affecting FRA:
Split windings per phase: Each phase consists of two equal winding halves on different core limbs. This creates a more complex frequency response than conventional transformers, with twice as many resonant peaks.
No secondary winding: Zig-zag grounding transformers have only primary windings; there is no secondary to provide capacitive inter-winding measurements.
Low zero-sequence impedance: The zig-zag connection cancels flux for zero-sequence currents, resulting in low impedance to ground. This affects low-frequency FRA (
<500 hz="">Continuous neutral current: In systems with harmonic distortion, neutrals may carry significant triplen harmonics, thermally stressing windings.
Expected FRA Signatures for Healthy Zig-Zag Transformers
A healthy zig-zag grounding transformer produces:
Low-frequency band (10 Hz – 1 kHz): Higher amplitude than conventional transformers (-10 to -20 dB vs. -30 to -40 dB) due to low zero-sequence impedance.
Mid-frequency band (1–100 kHz): Multiple resonant peaks (10–20 peaks) corresponding to individual winding halves and their interactions. Peaks are closely spaced (every 5–15 kHz).
High-frequency band (100 kHz – 10 MHz): Slower roll-off (15–20 dB per decade) than conventional transformers because the zig-zag connection maintains capacitive coupling across phases.
Phase-to-phase symmetry: In a healthy unit, FRA signatures of phases A, B, and C are highly symmetrical (CC > 0.95 between phases).
Detecting Winding Asymmetry
Winding asymmetry—a common failure mode in zig-zag transformers—appears as:
Asymmetrical phase-to-phase CC: For example, A vs. B CC = 0.97, B vs. C CC = 0.96, but A vs. C CC = 0.72. This indicates that Phase C is the outlier.
Split-winding imbalance: If one of the two winding halves of a phase is damaged, FRA will show a "split peak" where a single resonant peak becomes two closely spaced peaks (e.g., 25 kHz and 28 kHz instead of a single peak at 26 kHz).
Amplitude mismatch between phases: One phase showing 2–5 dB lower amplitude in the mid-band while others are normal.
Case Example: Inter-Turn Fault Detected by Asymmetry
A 2 MVA zig-zag grounding transformer in an industrial plant experienced unexplained neutral current imbalance. DGA showed trace acetylene (1 ppm). FRA testing revealed:
Phase A vs. B CC = 0.98
Phase B vs. C CC = 0.97
Phase A vs. C CC = 0.68
Phase C showed a 4 dB amplitude reduction in the 10–30 kHz band
This asymmetric pattern (one phase deviating) indicated a fault on Phase C. Internal inspection found two shorted turns on one of the Phase C winding halves. The shorted turns were repaired, and FRA returned to symmetrical. Without FRA, the inter-turn fault would have progressed to a phase-to-ground fault, potentially causing a plant shutdown.
Testing Protocol for Zig-Zag Transformers
Follow this specialized procedure:
De-energize the transformer and disconnect from the bus. Open the neutral grounding connection as well.
Perform end-to-end measurements on each phase (H1 to H0, H2 to H0, H3 to H0) just like a conventional transformer.
Additionally, perform phase-to-phase measurements (H1-H2, H2-H3, H3-H1) to assess the zig-zag interaction.
Compare phases using CC thresholds: healthy units have phase-to-phase CC > 0.95 in mid-band.
If a baseline exists, compare each phase individually to its baseline.
Core Damage Detection
Zig-zag transformers have cores similar to conventional transformers but may experience unique core damage from:
Overvoltage during single-phase-to-ground faults (the healthy phases see 1.73× line-to-neutral voltage)
GIC (geomagnetically induced currents) in long transmission lines connected to the same bus
Core damage appears as:
Low-frequency band (10–500 Hz) amplitude reduction > 3 dB on all phases (symmetrical)
Phase shift > 10 degrees at 50/60 Hz (can be observed by comparing phase angle at power frequency)
Hysteresis: FRA differs between increasing-frequency and decreasing-frequency sweeps
Establishing Baselines for Grounding Transformers
Because zig-zag transformers are often installed in harsh environments (outdoor substations, industrial yards), establish baselines:
Factory baseline before shipment
Commissioning baseline after installation
Baseline after any fault event (e.g., cleared ground fault)
Correlating FRA with Ground Fault Performance
Monitor grounding transformer health by correlating FRA with:
Zero-sequence impedance measurements: A healthy zig-zag transformer has low Z0 (typically 1–5 ohms). FRA asymmetry often precedes Z0 increase.
Neutral current harmonics: If FRA shows asymmetry, check neutral current for triplen harmonics—asymmetry reduces cancellation, increasing harmonic current.
Ground fault relay logs: Frequent ground fault trips may correlate with FRA-detected winding asymmetry.
Practical Testing Considerations
Field testing of zig-zag transformers presents:
Neutral access: The neutral bushing (H0) is often grounded directly. Disconnect the ground strap for testing, but re-connect before energizing.
Small size: Many zig-zag transformers are dry-type or small oil-filled units (500 kVA – 5 MVA). Use portable FRA instruments with alligator clips.
Harmonic-rich environment: Industrial plants with VFDs and rectifiers inject harmonics. Perform FRA during plant shutdowns for clean measurements.
The Transformer Frequency Response Analyzer, applied with phase-to-phase comparison and split-winding analysis, provides essential diagnostics for zig-zag grounding transformers. For industrial and utility engineers responsible for system grounding integrity, FRA is the most sensitive tool for detecting inter-turn faults and winding asymmetry before they compromise ground fault protection.
