A Novel Frequency Domain Based Technique to Detect Transformer Inter-turn Faults

Magnetic balance tests and Buchholz relays can usually provide indication of winding inter-turn faults in transformers. However their usefulness in determining such faults at an incipient stage remains questionable. Ratiometer test, which is the standard method used for determining voltage ratio of the transformer, can also be used in an indirect way to determine if an inter-turn short circuit exists in the winding of a transformer. However, this test is essentially a bridge method and hence is very sensitive to the accuracy and calibration of the bridge resistors. Additionally, phase angle adjustments to nullify the phase shift between the primary and the secondary are also required for accurate measurements. Increased no-load losses have also been shown to give very good indication of inter-turn faults in case of shorted turns. However the effect of core degradation and looseness can influence no-load losses. Other detection techniques include magnetizing current and magnetic balance tests. A technique proposed here detects the inter-turn faults even to the extent of a single turn by utilizing the saturation effects of the core of the transformer. The test does not require any delicate adjustments and the sensitivity even for one turn fault is very high. It would however require the use of two other similar and healthy transformers. In case it is a three phase transformer, the remaining two of the three phases can be used provided there is no fault in them and the magnetic circuit of the three phases are reasonably balanced. The method has been verified using both simulation and experiments on a bank of three single-phase transformers.