In order to test for a transformer’s internal condition, it is necessary to remove it from its tank. However, this method can be tedious and takes too much time. A more practical method is to use a sweep frequency analyzer.
A sweep frequency response analyzer lets you diagnose the internal condition of a transformer by using injecting an excitation signal and analyzing the spectrum bounced back by by the windings. This is a more efficient way of detecting core and winding problems. One of the most popular test sets used is the Doble M5400 SFRA. It has an output voltage of 20 Volts peak-to-peak at 50 Ohms output impedance. Its input circuitry can analyze difference, cross-relation and sub-band at frequencies from 10 Hertz to 25 MegaHertz. This bandwidth is more than sufficient in analyzing common problems in transformers. It also features a sampling rate of 100 MS/s with a dynamic range of >90 dB. If you plan investing in one, consider renting from a test equipment rentalcompany. This lets you evaluate its features to ensure it meets your specific needs.
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