On Load Tap Changer Monitoring (OLTC)

The type of OLTC should influence the plan for inspections and online/out-of-service testing. There are two distinct types of OLTCs. First, bolt-on or welded-on OLTCs fit onto the sidewall of a transformer. Second, in-tank type OLTCs install inside the tank and suspend from the cover. Both designs can include arcing in oil-type diverter switches using resistors, series reactors or vacuum interrupters. These various components are designed for installation inside the tank and to be suspended from the cover.

The OLTC is the major mechanical moving device on a transformer. As with any mechanical device, movement equates to the wear of components over time and frequency of operation. The OLTC failure modes can be the same or segregated, depending on the type of testing or monitoring performed. Some failure modes can be detected with sensors and systems. Others can only be determined with a combination of sensors and laboratory testing (oil, for example). To learn more about various diagnostic methods, read this article from Transformer Technology Magazine.

Implementing online monitoring for an OLTC is relatively easy. There is a mechanical operations counter on every OLTC. It records an event (cumulative) every time the OLTC operates. The tap position indicator on every OLTC is a mechanical device. It provides the current tap position and the ‘range of movement’ the mechanism has operated. The movement is recorded using resettable drag hands on the indicator.

By adding a remote tap position sensor, a monitor will track and trend the total operations, number of operations at each position, and the I2T to provide an idea of contact wear. The monitoring system can provide control if desired. It can also monitor the frequency of reversing switch operations. In other words, the time since it last moved through that position to “wipe the contacts”.