During the regular maintenance, resistance measurement can be performed in order to observe the condition of the OLTC. One of the measurement methods is Dynamic Contact Resistance Measurement (DCRM) also and can be implemented as OLTC Diagnostic System.
Due to its dynamic and continuous operation under load condition, OLTC is prone to many different defects, caused by the load current or high operation temperature.
- The current leads to direct impact on contacts deterioration by eroding the contact material.
- Another source of contacts deterioration is a resistive layer. It is developed on contacts due to the heating of insulation oil by the load current.
As a major cause of OLTC failure, these defects might take place after some years of operation and their frequency increases with the aging time.
Dynamic resistance measurement is used as a method to diagnose the contact condition of on-load tap changers offline. This method is able to detect many types of defects or deterioration of change-over selector contacts and arcing switch contacts without accessing these contacts.
A DC current is injected into the transformer and flows through the tap changer. The measurement reads the flow of current through the tap changer during its operation. DCRM measures the OLTC while it is moving through its entire taps and the resistance measurement is dynamic.
The defects can be seen as deviations of the DCRM graph . The location, timing and the length of deviations can be used to determine the type of the defects as shown in Figure below.