Dr. Ahmed G. Abo-Khalil

Electrical Engineering Department

Mechanical tap cha

A mechanical tap changer physically makes the new connection before releasing the old using multiple tap selector switches, but avoids creating high circulating currents by using a diverter switch to temporarily place a large diverter impedance in series with the short-circuited turns. This technique overcomes the problems with open or short circuit taps. In a resistance type tap changer, the changeover must be made rapidly to avoid overheating of the diverter. A reactance type tap changer uses a dedicated preventive autotransformer winding to function as the diverter impedance, and a reactance type tap changer is usually designed to sustain off-tap loading indefinitely.

In a typical diverter switch powerful springs are tensioned by a low power motor (motor drive unit (MDU)), and then rapidly released to effect the tap changing operation. To reduce arcing at the contacts, the tap changer operates in a chamber filled with insulating transformer oil, or inside an SF6 vessel. Reactance-type tap changers, when operating in oil, must allow for with the additional inductive flyback generated by the autotransformer and commonly include a vacuum bottle in parallel with the diverter switch. During a tap-change operation, the flyback raises the potential between the two electrodes in the bottle, and some of the energy is dissipated in an arc discharge through the bottle instead of flashing across the diverter switch.

Some arcing is unavoidable, and both the tap changer oil and the switch contacts will slowly deteriorate with use. To prevent contamination of the tank oil and facilitate maintenance operations, the diverter switch usually operates in a separate compartment from the main transformer tank, and often the tap selector switches will be located in the compartment as well. All of the winding taps will then be routed into the tap changer compartment through a terminal array.

One possible design (flag type) of on-load mechanical tap changer is shown to the right. It commences operation at tap position 2, with load supplied directly via the right hand connection. Diverter resistor A is short-circuited; diverter B is unused.

In moving to tap 3, the following sequence occurs:

  1. Switch 3 closes, an off-load operation.
  2. Rotary switch turns, breaking one connection and supplying load current through diverter resistor A.
  3. Rotary switch continues to turn, connecting between contacts A and B. Load now supplied via diverter resistors A and B, winding turns bridged via A and B.
  4. Rotary switch continues to turn, breaking contact with diverter A. Load now supplied via diverter B alone, winding turns no longer bridged.
  5. Rotary switch continues to turn, shorting diverter B. Load now supplied directly via left hand connection. Diverter A is unused.
  6. Switch 2 opens, an off-load operation.

The sequence is then carried out in reverse to return to tap position 2.

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