Analysis of a tap changer motor current showed to be a powerful diagnostic tool to detect mechanical binding, motor control malfunctions, as well as irregular operation around neutral in reversal switch operations.
This particular case was detected by comparing position of the transition ripple on the DVtest graph depicting the motor current. It was recorded during our workshop at Minel repair shop in Barajevo, Serbia. This tap changer type included a diverter switch and a selector.
DVtest graph below illustrates the motor trace (green) and the tap changer operation diagram (red). One can see the position of transitions is not at the right place. As the motor charges the spring and the spring releases its energy into diverter switch operation – it should be close to the end of the motor operation, not at the beginning, as on this graph.
In order to further check operation of the tap changer and find out a reason for such a strange behavior, we performed the measurement in the reverse direction. The graph below shows the ripple is not at the right position either.
The included video shows the incorrect operation of this tap changer. Fortunately, at this repair shop, the side manhole door on the tank was left opened to verify the selector switch contacts and operation Diverter noise [bang] occurred immediately after the motor and motion of the selector moving contacts started, and the motion ended at the wrong position.
It was decided to correct the mechanical gear set located on the top of the transformer. The mechanic opened the gearbox and moved the gear by one tooth. This seemed to correct the problem. The measurement was repeated after the gear adjustment, and completely correct position of the ripple on the graph was obtained as compared to the motor current trace.
The figure below illustrates the motor current of one operation (green) with three superimposed ripples on the same graph. Two of them are of the OLTC operation before the repair [bad], and the good one after the repair. The gear mechanism was unadjusted and a quick adjustment of the gear box provided a simple corrective action.
The conclusion: due to the incorrect gear driving the tap changer, the spring was not completely charged to release the energy into diverter operation. Only the subsequent motor operation would add-up the required energy and the diverter switch would operate.
It would happen at the very beginning of the subsequent motor operation, and for that reason, the position of the tap changer was always one position off. For example: a transition indicated from 3 to 4, would occur at te beginning of the 4 to 5 operation. The position of the tap changer was always one position off because of that.