Diagnosing Autotransformers with FRA: Unique Challenges and Optimized Test Strategies
Autotransformers, widely used in transmission networks for voltage step-up and step-down between similar voltage levels (e.g., 230/115 kV), present unique diagnostic challenges for a Transformer Frequency Response Analyzer. Unlike two-winding transformers, autotransformers feature a series winding and a common winding that share both magnetic and galvanic connections. This construction alters FRA interpretation and requires specialized test configurations to isolate potential defects.
Galvanic Coupling: A Double-Edged Sword
In a conventional two-winding transformer, the primary and secondary are electrically isolated. In an autotransformer, the high-voltage and low-voltage windings are conductively connected at the common neutral point. Consequently, when performing an end-to-end measurement on the high-voltage side (e.g., H1 to H0), the signal also propagates through the low-voltage bushings. This can produce additional resonant peaks that are not present in two-winding transformers. Key implications:
Baseline fingerprints for autotransformers are more complex, with 20–30% more resonant notches compared to equivalent two-winding units.
Deviations in the low-voltage winding may appear in measurements taken from the high-voltage side, requiring careful correlation.
Tap changer position changes affect both the series and common windings simultaneously.
Specialized Test Configurations for Autotransformers
To fully assess an autotransformer, perform these specific FRA measurements:
High-voltage end-to-end (H1–H0): Captures the combined response of series and common windings. Sensitive to deformation in either winding.
Low-voltage end-to-end (X1–X0): Only the common winding is directly excited because X1 is connected to the common winding’s midpoint. Deviations here isolate common-winding issues.
Series winding only (H1–X1): By placing the source between H1 and X1 and measuring across the same terminals, the series winding is isolated from the common winding. This is the most sensitive test for series-winding radial buckling.
Capacitive inter-winding (H1–H0 with response on tertiary, if present): For three-winding autotransformers (including a delta tertiary), this mode assesses the insulation geometry between the series/common assembly and the tertiary winding.
Tertiary Winding Influence on FRA Signatures
Most autotransformers above 100 MVA include a tertiary winding (typically delta-connected, 13.8 kV or 34.5 kV) to provide a zero-sequence path and supply auxiliary loads. This tertiary winding is magnetically coupled to both the series and common windings. Its presence introduces additional resonant frequencies in the 1–50 kHz range. When comparing FRA traces, ensure that the tertiary winding is configured identically (open, grounded, or loaded) during baseline and subsequent tests. A change in tertiary grounding (e.g., from solid ground to impedance ground) can produce an apparent deviation that is not a fault.
Case Example: Series Winding Deformation Detection
A 150 MVA, 345/138 kV autotransformer exhibited elevated hydrogen and methane in DGA but no acetylene. The standard high-voltage end-to-end FRA measurement (H1–H0) showed a 3 dB amplitude reduction in the 10–30 kHz band on Phase B, but the low-voltage end-to-end (X1–X0) was normal. The isolated series-winding measurement (H1–X1) confirmed a severe deviation—a resonant peak shift of 12%. Internal inspection revealed that the series winding of Phase B had experienced axial spacer displacement over a 30 cm section. The common winding was intact. The targeted series-winding test mode provided the critical diagnostic evidence.
Practical Recommendations for Autotransformer FRA Programs
Given the complexity of autotransformers, adhere to these best practices:
Establish baseline measurements for all three test modes (HV end-to-end, LV end-to-end, and series-only) during factory acceptance.
Document tertiary winding status (open, grounded, or connected) with every field test.
Use phase-to-phase comparison for autotransformers without baseline: the three phases of a three-phase autotransformer are electrically symmetric, so a single phase deviating from the other two strongly indicates damage.
For units with load tap changers (LTC) on the series winding, always record the LTC position and test at the same position used for baseline. Even small LTC steps shift the FRA response measurably.
When properly configured, a Transformer Frequency Response Analyzer delivers exceptional diagnostic value for autotransformers, detecting incipient mechanical faults that other methods miss. The key lies in understanding the unique series-common winding interaction and deploying the appropriate test modes for each component.
