In transformer winding deformation testing, the repeatability of the frequency response curve directly affects the reliability of diagnostic conclusions. If the curves measured on the same transformer under similar operating conditions show obvious drift, engineers often first suspect the winding condition, while the problem sometimes lies in the frequency accuracy of the tester's own signal source or the wiring status. Taking the FRA-9200 transformer winding deformation tester as an example, the frequency accuracy of its signal source output sinusoidal signal is no greater than 0.01%. This indicator provides the basic condition for the comparability of frequency response curves, but in actual use, several maintenance and operation points still need attention.
Question 1: The frequency accuracy meets the standard, so why are the curves still not aligned?
Frequency accuracy is an internal indicator of the instrument, while in field testing, the factors affecting curve consistency come more from the outside. Before testing, confirm whether the transformer winding temperature is stable. Temperature changes will cause slight changes in winding geometric dimensions, thereby affecting frequency response characteristics. At the same time, the grounding method must be reliable. Too long a grounding wire or too large a contact resistance will introduce additional impedance, causing false resonance in the high-frequency band of the curve. The amplitude measurement range of the FRA-9200 is -100dB to +20dB. If the wiring is poor, the attenuation in the high-frequency band may exceed the effective measurement range, making the data incomparable.
Question 2: How should the first inspection be done after transportation and installation?
After transformer transportation, installation, or a short-circuit fault, the winding may undergo axial or radial deformation. At this time, when using the FRA-9200 for inspection, it is recommended to first establish the original frequency response fingerprint of the transformer, and then compare it with subsequent test results. If the original data is missing, data from transformers of the same model and same winding structure can be used as an auxiliary reference, but it should be noted that differences may still exist among different manufacturing batches. Keeping the wiring position and lead routing consistent each time during testing is a basic requirement for improving repeatability.
Question 3: Maintenance precautions during regular overhaul
When the instrument is used at substation sites for a long time, the shielding layer of the test cables should be regularly checked for integrity, and the connectors should be checked for oxidation. The interfaces at the signal source output end and the response measurement end need to be kept clean to avoid abnormal drops in the frequency response curve in a certain frequency band due to poor contact. Before and after each test, a standard impedance network can be used to quickly calibrate the instrument to confirm that the amplitude response of each frequency band is within the allowable deviation. If a decrease in curve repeatability is found, first check the cables and grounding, and then consider the internal calibration of the instrument.
Matching ideas in selection and use
For power transformers of 6kV and above voltage levels, the FRA-9200 can cover winding condition detection during transportation, installation, after short-circuit faults, and in regular overhaul. When selecting, it is necessary to combine the on-site transformer capacity and winding structure to confirm whether the frequency sweep range and dynamic range meet the requirements. At the maintenance level, it is recommended to establish instrument usage records, including wiring photos and ambient temperature for each test, to facilitate subsequent data traceability and horizontal comparison.
