The Ferranti Tapchanger (tap changer) Limited DC3 On-Load Tap Changing Gear is a three phase, high speed resistor transition tap changer for 33kV insulation class transformers, which can be used in either 'Star' or 'Delta' operation at a maximum full line current of 300 amps. on normal tappings
Principal of Operation
Three single phase diverter switches of the high speed transition variety using resistors for bridging between running positions during the tap change, are mounted in a separate compartment on top of the selector switches, and a hinged cover to the diverter switch compartment replaces the usual drop tan to give easier accessibility. read more....
The Ferranti Tapchanger Type DC3 On Load Tap Changing Gear , comprises in effect, three separate component parts in a common housing, viz tapping selector switches, diverter switches and driving gear, and associated control mechanisms. All these component parts are mounted in oil filled and isolated compartments of the common housing on the side of the main transformer tank, and the selector switch compartment is separated from the main transformer tank by three oil-tight insulated 'bath tub' style barrier board panels which carry the terminals for the tapping connections from each phase of the transformer windings. The diverter switch compartment is provided with a hinged bolted cover which allows easy access to the divertor switches for inspection after draining off the oil. A barrier panel with through type oil-tight insulators separates the diverter switch compartment from the selector switch pocket.
The motor drive and timing gear are housed together in a compartment situated at the end of the tapping selector switch compartment. The motor is of single-three phase capacitor start and run type and is coupled to the main tapping selector switch and diverter switch operating shaft through reduction spur gearing. The limit switches are single pole, directly connected and no contactors are involved in the limit switch circuit. Reversal of the supply phases which can remove the limit switch protection on a three phase motor has no effect on this circuit design. A mechanical stop is provided and is accurately set during assembly of the gear. If this stop operates, the driving motor will stall from full speed into the mechanical stop without damage. An auxiliary solenoid-operated brake is not required to bring the driving gear to rest since the oil damping is sufficient for the purpose, thus eliminating the need for brake adjustment. At one end of the main driving shaft is a spur gear which operates a Geneva wheel mechanism and in turn drives the selecter switch lead screws. The Geneva wheel mechanism incorporates an arrangement giving 180deg lost motion on reversal, which provides the correct selector switch sequence. The use of a Geneva wheel mechanism ensures that the tapping selector switches are only moved one step at a time and that they are locked in position except during the actual period of rotation. A crankshaft is fitted to the other end of the main driving shaft which, through a connecting rod, gives a reciprocating motion to the diverter switch operating lever. This lever loads up the operating springs and drives a toggle mechanism which operates the assembly carrying the moving diverter switch contacts. The control or timing gear is mounted in a compartment adjacent to the motor and reduction gear train, and since all the equipment required for control of the tap change gear, with the exception of remote indicators and control switches, which reduces the number of leads between the transformer and the control point to a minimum. A cam-shaft driven from one of the gears in the reduction gear train, times the whole sequence of each tap change operation, drives a local mechanical tap position indicator, a transmitting device for remote indication of tap position, and operates the electrical limit switches. A cam-shaft operates a normally open contact which closes during a tap change, a changeover or 'sense' contact which ensures that the gear continues to operate in the appropriate direction, and a normally closed contact which opens during a tap change. Limit switches are fitted in the supply leads to the starting circuit to prevent another tap change from being initiated after the end stop has been reached. All the control switches have open contacts which are positively opened and are immersed in transformer oil.
Motor Gearbox and Timing Gear
The driving motor, gearing and timing gear are all contained in the lower half of the gearbox. All these items are immersed in oil to ensure constant lubrication of the motor, gearing etc., to give long trouble free operation. It also reduces the number of oil seals, only one oil seal being required between the gearbox and the diverter switches, and one oil seal between the diverter and selector switch compartments.
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