Tap (transformer)
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A transformer tap is a connection point along a transformer winding that allows the number of turns to be selected. By this means, a transformer with a variable turns ratio is produced, enabling voltage regulation of the secondary side. Selection of the tap in use is made via a tap changer mechanism.
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[edit] Voltage considerations
Tap points are usually made on the high voltage, or low current, side of the winding in order to minimise the current handling requirements of the contacts. To minimise the number of windings and thus reduce the physical size of a transformer, use may be made of a 'reversing' winding, 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 centre 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.
[edit] Tap changing
[edit] Off-load designs
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.
Because the different tap points are at different voltages, the two connections should not be made simultaneously, as this short-circuits a number of turns in the winding and would result in an excessive circulating current. This therefore demands that the power to the load be physically interrupted during the switchover time. Off-load tap changing is also employed in high voltage transformer designs, though it is only applicable to installations in which loss of supply can be tolerated.
[edit] On-load designs
Because interrupting the supply is usually unacceptable for a power transformer, these are often fitted with a more expensive and complex on-load tap-changing mechanism. On-load tap changers may be generally classified as mechanical; or as electronic, which in turn may be either assisted or solid state.
[edit] Mechanical tap changers
A mechanical tap changer physically makes the new connection before releasing the old, but avoids the high current from the short-circuited turns by temporarily placing a large diverter resistor (sometimes an inductor) in series with the short-circuited turns before breaking the original connection. This technique overcomes the problems with open or short circuit taps. The changeover nevertheless must be made rapidly to avoid overheating of the diverter. Powerful springs are wound up, usually by a low power motor, and then rapidly released to effect the tap changing operation. To avoid arcing at the contacts, the tapchangers is filled with insulating transformer oil. Tapping normally takes place in a separate compartment to the main transformer tank to prevent contamination of its oil.
One possible design 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:
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- Switch 3 closes, an off-load operation.
- Rotary switch turns, breaking one connection and supplying load current through diverter resistor A.
- 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.
- Rotary switch continues to turn, breaking contact with diverter A. Load now supplied via diverter B alone, winding turns no longer bridged.
- Rotary switch continues to turn, shorting diverter B. Load now supplied directly via left hand connection. Diverter A is unused.
- Switch 2 opens, an off-load operation.
The sequence is then carried out in reverse to return to tap position 2.
[edit] Thyristor-assisted tapchangers
Thyristor-assisted tap changers use thyristors to take the on-load current whilst 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.
[edit] Solid state (thyristor) tap changers
These are a relatively recent development which use 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. 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. They are therefore only employed on smaller power transformers.
[edit] 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. Pergamon. ISBN 0-08-016436-6.
