Four Common Mistakes in Sweep Frequency Response Analysis Testing
Sweep Frequency Response Analysis (SFRA) testing provides insight into the mechanical and electrical integrity of transformers, reactors, and other equipment with windings. An SFRA instrument sends a signal into the transformer winding at a number of discrete frequencies and measures the returning signals. Damage, physical changes, and electrical changes can be detected by inspection of the graphical results or by comparing them with results from previous test sessions or from testing of similar apparatus.
When set up and used properly, SFRA testing is a powerful tool – for baseline testing of a new transformer in the factory, as part of routine diagnostic testing, or after a system fault. SFRA tests are particularly dependent upon consistent test setup for each test session in order to provide accurate and reliable results. Incorrect setup can create variations that may mask changes in a transformer or create a false positive.
Avoid these common mistakes and save time on return trips for additional testing:
1. Poor Grounding
Poor grounding practices can have a significant impact on test results. The grounding of the transformer is important, as is the entire signal ground path of the measurement setup. When the grounding path introduces additional impedances due to poor connections, the signal is measured with a different reference, causing changes to the traces – from subtle to extreme. For the best outcomes, ensure all safety and measurement grounds are solid!
2. Poor Test Lead Connections
Test leads that are poorly connected to bushing terminals introduce extra impedances into the measurement circuit. These impedance changes impact a wide range of frequencies and will yield poor comparison to prior test results made with good lead connections. SFRA measurements are made at a relatively low voltage. A little extra effort to ensure a clean surface for the measurement lead connections can be critical to getting good results!
3. Inconsistent Tap Changer Positioning
When tap changers change position (either on-load tap changers or de-energized tap changers) the way that the tapped winding interacts with the various RLC networks of the transformer will change. This interaction will have an impact on the response of the transformer to frequency sweeps and different tap positions will cause different responses. It is important to note that changing taps on one specific winding will not only impact that winding’s response but may also impact the responses of other windings. As with all other aspects of the test setup, it is critical to record the tap changer settings in the test record so that the exact setup can be reproduced for future test sessions. Have pity on the next test crew!
4. Inconsistent Configuration of the Stabilizing Tertiary Winding
Manipulating the configuration of a stabilizing tertiary winding can cause the transformer’s response to change. Some stabilizing tertiary windings may come out to a single bushing at one corner of the delta, which could be either grounded or ungrounded while the SFRA tests are performed on the primary and secondary windings. Other stabilizing tertiary windings have one corner of the delta brought out to two bushings, which can either be shorted and left floating, shorted and grounded, open and floating, or finally open and grounded. These changes will impact the interaction of the circuit elements that make up the RLC network inside the transformer and will result in variations to the response while performing SFRA tests. The changing response is likely to be seen in traces captured on all windings within the transformer. Once again, recording the setup details for each test is critical!
The key takeaway is that for Sweep Frequency Response Analysis results to be truly useful over the life of a transformer or other asset, SFRA testing should be done using the same test setup every time. If it can’t be the same for some reason, the difference in setup should be clearly noted in the test record.
The lightweight and versatile Doble M5500 Sweep Frequency Analyzer features easy setup to quickly assess the health of transformers and other equipment with windings. With typical sweep speeds between 15 and 30 seconds, the M5500 offers fast and reliable SFRA testing, often reducing testing time by as much as one-half. Whether the instrument is used at the factory for baseline testing of new transformers, as part of diagnostic maintenance or to determine if a transformer can go back into service after a fault, its highly repeatable measurements detect subtle changes.
Use Doble’s new SFRA Software v6 to analyze diagnostic data collected with the M5500. SFRA Software v6 shares a framework with Doble’s powerful Test Assistant (DTA) application to provide data management and analysis tools that streamline testing and interpretation of results. SFRA Software v6 helps users save time by detecting and importing nameplate data from existing DTA records, streamlining test file creation, and reducing errors. A flexible test plan editor allows for standard and custom test plan sequences. Performing SFRA testing and analysis of your apparatus has never been easier or more productive with the powerful combination of M5500 and SFRA Software v6.
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