picture shows the Ferranti Tapchanger Rotary Range of On Load
The Ferranti Rotary Type Tapchanger (tap changer) employs
the continuous rated bridging reactor method of operation, in
which duplicate tapping selector switches and series switches
are used in conjunction with continuous rated reactors. The tapping
selecter and series switches are each mounted in their own separate
oil filled and isolated compartment on the side or on the end
of the main transformer tank. The selector switch compartment
is separated from the main tank by an oil-tight insulated panel
which carries terminals for the tapping connections. The series
switches are also housed in a separate compartment with a drop
down tank which can be lowered for inspection by means of hand-operated
The principle of operation - on the initiation of a tap change
one of the series switches is opened and the corresponding tapping
selector switch is moved round to the next contact. The series
switch is then re-closed. The same process is repeated by the
other series switch and its corresponding tapping selecter switch.
The continuous rated reactor connected between the series switches
limits the circulating current in the bridging position. Often
the bridging position is used as a running position to give a
voltage half way between that of any two main tappings. In this
case the reactor is serving as an auto transformer.
The bridging reactors are oil immersed and continuously rated.
They are installed in the main transformer tank. Ample air gaps
are provided in the cores to prevent magnetic saturation either
under normal load or transient tap changing conditions. Surges
and objectionable harmonics are thereby eliminated.
The Series Switches which are mounted in a separate oil filled
compartment are of the quick break contactor type. Every switch
is insulated for a pressure equal to the test pressure of the
transformer and is designed to carry full load current continuously.
Their duty is actually much lighter than this as they carry only
half full load current and break only the voltage difference
between adjacent tappings. Thus the switches deal with only a
small percentage of the transformer kVA and are of ample proportions
electrically. As with all the Ferranti Tap Change equipment they
are very robustly designed from a mechanical point of view. The
contacts are self aligning and the need for flexible braids for
conduction has been eliminated.
The Tapping Selector Switches are mounted in their own separate
compartment built on to one of the walls of the main transformer
tank. They are completely isolated from the switches and as they
break no current cannot be contaminated by carbonised oil. Drain
and filling valves for the compartment are provided and upon
inspection it is only necessary to drain the oil from the tapping
selector switch compartment. The operation of the series switches
and tapping selector switches in their correct sequence is ensured.
The layout of the gear box is
reduced to an exceptionally simple and robust arrangement by
the use of a single actuating cam.
The control or timing gear is housed in a compartment partly
filled with oil, mounted above the gear box. All the equipment
required for the tap change gear, with the exception of the remote
indicators and control switches, is mounted in this compartment,
which reduces the number of pilot wires between the transformer
and the control point to a minimum.
parts and major assemblies are available from Ferranti Tapchangers.
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