Condition assessment of on load tap changers using dynamic resistance measurements

Resistance measurements on power transformers are a widely accepted method for finding winding defects. Continuity of the tap changer contacts can also be checked when the on-load tap changer (OLTC) is operated during the resistance measurement. Resistance measurements that also record the resistance during OLTC operation are called dynamic resistance measurements. This off-line diagnostic method can be used during regular maintenance to assess the condition of OLTC parts not accessible for inspection. Besides condition assessment of inaccessible parts of the OLTC, this method can also be used to detect maintenance errors that could lead to failure. In particular, these measurements are very helpful in finding contact discontinuity, mechanical defects that influence the operation of the tap changer contacts, problems with the switch time and the transition resistors and can find contact degradation in an early stage.