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 winding gear. read more....
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.
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