Test procedure and test circuit considerations for on load tap changer dynamic resistance measurement

Resistance measurements on power transformers are offline diagnostic methods that are performed without opening the transformer main tank. In case the power transformer is equipped with an on-load tap changer (OLTC), this resistance measurement can be used to find degradation and defects inside the OLTC. Many OLTC defects as well as contact degradation can be measured by means of dynamic resistance measurements, which is a resistance test during OLTC operation. A low (DC) test current through the on-load tap changer is used during dynamic resistance measurements. This method is in particular sensitive to maintenance errors, contact degradation and contact timing problems. In general, distinction can be made between defects that interrupt the current through the OLTC, defects that affect the timing of the OLTC contacts and contact degradation resulting in abnormal contact resistance. Firstly, this paper evaluates two ways to perform dynamic resistance measurements on OLTCs: 1) a direct measurement of the OLTC resistance by measuring the current through the OLTC and the voltage of the corresponding transformer terminals and 2) an indirect measurement of the induced voltage on the (secondary) windings without OLTC when the OLTC switches a DC current. The measurement procedure for the direct method is discussed for condition diagnosis of in-tank high-speed resistor tap changers. Secondly, the effect of the test circuit and test parameters on the measurement results is discussed. This influences the readings directly, in particular at service-aged contacts. Current amplitude, the secondary short-circuit and circuit resistance are considered. Simulation results and experimental data are presented to demonstrate those effects and to select an optimal test setup.