How does FDS testing work?

The test set up for an FDS test is virtually the same as that used for conventional tan δ testing. The big difference is that conventional tan δ tests are carried out only at power frequency, whereas as FDS tests involve injecting a test signal that sweeps over a wide range of frequencies, typically from a millihertz or so up to around a kilohertz. By recording the response of the CT to this test signal during the frequency sweep and analysing the results, it is possible to reliably estimate the percentage moisture in the solid insulation. No oil sampling is of course required with this method, so not only is more convenient, it also means that testing can be carried out regularly with no risk of depleting the volume of oil in the transformer.