Performing reliable and reproducible frequency response measurements on power transformers
The Sweep Frequency Response Analysis (SFRA) has become a standard method to assess the mechanical and electrical integrity of the power transformer’s active part. It provides a very high sensitivity to evaluate possible damages after transportation or for troubleshooting after a specific event such as a near failure with high short-circuit forces. However, users often struggle to reach a high reproducibility which is essential for a reliable condition assessment. Deviations, caused by reproducibility issues, can lead to misinterpretation, unnecessary inspections or cost-intensive maintenance activities. This paper focuses on best practices in order to perform highly repeatable and reproducible SFRA measurements.
Keywords: IEC 60076-18, power transformer, reproducibility, SFRA, Sweep Frequency Response Analysis
The Sweep Frequency Response Analysis (SFRA) method was introduced to verify the integrity of the active part of a power transformer. After manufacturing, power transformers are transported on site, often over long distances using different types of transportation, such as ship, train or truck. Both during transportation and loading from one vehicle to another, the transformer might be exposed to mechanical shocks. Such shocks can also be caused by earthquakes or mechanical impacts due to short-circuit forces after a failure. All these impacts can lead to a deformation or partial movement in the active part. Common diagnostic measurements such as transformer turns ratio, including exciting currents, short-circuit impedance at nominal frequency as well as frequency response of stray losses (FRSL), may have disadvantages regarding to their sensitivity to detect and prove mechanical deformations. For example, a buckling of a winding does not typically influence ratio or insulation resistance measurements and is hard to detect in a change of capacitance. Compared to them, the SFRA is the most sensitive method for reliable core and winding assessment. This paper illustrates several best practices to perform SFRA measurements in order to ensure highly repeatable and reproducible test results.