Measurements in milliohmeter mode and in in-place check mode complement each other and provide a comprehensive picture of the state of transformer.
OLTC "in-place" check mode allows carrying out of in-place check and diagnostics of OLTC with current-limiting resistors without removing the contactor tank covers. This mode involves measuring of instantaneous current passing at first through the transformer cover and then through OLTC contracts at switching from one tap to another. A graph of current variance at contacts switching is drawn up and the time of switching as well as the general technical condition of the instrument is checked on its basis.
Analysis of the acquired graphs provides not only for sorting out of elements by fault-free/faulty criteria but also for defining the nature of the defect allowing eliminating of fault-free OLTCs opening.
- express diagnostics of OLTC transformer technical condition at any weather environment;
- construction of assessment diagram of contactors’ operation without OLTC tank opening;
- review of the graphs of the measured item on the instrument itself;
- defining of OLTC fault location: for instance, identifying of interruption of current-limiting resistors, poor selector contacts, etc.
For example, for type RS, RNOA, etc., on-load tap changers the correctness of operation of the main and arcing contactor contacts, their condition (burned or not burned contact), current-limiting resistor integrity may be determined. For type RNTA – 35/320 tap changers and in the same on-load tap changers, in which the contactor and selector are changed for a fine control circuit breaker, main and arcing contacts, current-limiting resistor condition, as well as condition of surface of all fixed contacts of the circuit breaker, through which the moving contacts move, can be checked.