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A transformer tap is a connection point along a transformer winding that allows a certain number of turns to be selected. By this means, a transformer with a variable turns ratio is produced, enabling voltage regulation of the output. The tap selection is made via a tap changer mechanism.

Contents

Voltage considerations

If only one tap changer is required, tap points are usually made on the high voltage, or low current, side of the winding in order to minimize the current handling requirements of the contacts. However, a transformer may include a tap changer on each winding if there are advantages to doing so. For example, in power distribution networks, a large step-down transformer may have an off-load tap changer on the primary winding and an on-load tap changer on the secondary winding. The high voltage tap is set to match long term system profile on the high voltage network and is rarely changed. The low voltage tap may be requested to change positions once or more each day, without interrupting the power delivery, to follow loading conditions on the low-voltage network.

To minimize the number of windings and thus reduce the physical size of a transformer, a 'reversing' winding may be used, which is a portion of the main winding able to be connected in its opposite direction and thus oppose the voltage. Insulation requirements place the tap points at the low voltage end of the winding. This is near the star point in a star connected winding. In delta connected windings, the tappings are usually at the center of the winding. In an autotransformer, the taps are usually made between the series and common windings, or as a series 'buck-boost' section of the common winding.

Tap changing

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Off-circuit designs (DETC)

In low power, low voltage transformers, the tap point can take the form of a connection terminal, requiring a power lead to be disconnected by hand and connected to the new terminal. Alternatively, the process may be assisted by means of a rotary or slider switch.

Since the different tap points are at different voltages, the two connections can not be made simultaneously, as this would short-circuit a number of turns in the winding and produce excessive circulating current. Consequently, the power to the device must be interrupted during the switchover event. Off-circuit or de-energized tap changing (DETC) is sometimes employed in high voltage transformer designs, although for regular use, it is only applicable to installations in which the loss of supply can be tolerated. In power distribution networks, transformers commonly include an off-circuit tap changer on the primary winding to accommodate system variations within a narrow band around the nominal rating. The tap changer will often be set just once, at the time of installation, although it may be changed later during a scheduled outage in order to accommodate a long-term change in the system voltage profile.

On-load designs

A mechanical on-load tap changer (OLTC), also known as under- load tap changer (ULTC) design, changing back and forth between tap positions 2 and 3

For many power transformer applications, a supply interruption during a tap change is unacceptable, and the transformer is often fitted with a more expensive and complex on-load tap-changing (OLTC, sometimes LTC) mechanism. On-load tap changers may be generally classified as either mechanical, electronically assisted, or fully electronic.

Mechanical tap changers

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. In order 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.

Thyristor-assisted tap changers

Thyristor-assisted tap changers use thyristors to take the on-load current while the main contacts change over from one tap to the next. This prevents arcing on the main contacts and can lead to a longer service life between maintenance activities. The disadvantage is that these tap changers are more complex and require a low voltage power supply for the thyristor circuitry. They also can be more costly.

Solid state (thyristor) tap changers

These are a relatively recent development which uses thyristors both to switch the load current and to pass the load current in the steady state. Their disadvantage is that all of the non-conducting thyristors connected to the unselected taps still dissipate power due to their leakage current and they have smaller short circuit withstand capacity. This power can add up to a few kilowatts which has to be removed as heat and leads to a reduction in the overall efficiency of the transformer, in exchange for a compact design that reduces the size and weight of the tap changer device. Solid state tap changers are typically employed only on smaller power transformers.

References

  • Hindmarsh, J. (1984). Electrical Machines and their Applications, 4th ed.. Pergamon. ISBN 0-08-030572-5.  
  • Central Electricity Generating Board (1982). Modern Power Station Practice: Volume 4. Pergamon. ISBN 0-08-016436-6.  
  • Rensi, Randolph (June 1995), "Why transformer buyers must understand LTCs", Electrical World  .

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