Bushing Potential Device (KA-108)
The General Electric Type KA-108 Bushing Potential Device offers advanced design features and superior performance characteristics in a reliable, safe, economical voltage transforming device for the operation of instruments and relays from high-voltage circuits, 115 kV through 765 kV, 60 Hz (may also be applied at other High Voltages and at 50 Hz).
(1) A ground switch for removing high-voltage from the device. The ground switch handle is interlocked with the control panel so that the panel cannot be opened to expose the high-voltage components until the switch has been moved to the “closed” position.
(2) A spark gap that protects the device circuit from abnormally high surge voltages.
(3) A flexible metal covered cable that connects the device to the bushing, so that no live circuits are exposed.
The KA-108 bushing potential device is well suited to operate the usual types of relays, synchroscopes, volt-meters, indicating lamps, wattmeters (not for revenue) and similar instruments requiring a potential source of essentially constant ratio and phase relation with respect to the high-voltage circuit. The device’s major field of application is in protection and control equipment for generating plants, substations, transmission lines, etc. It is not recommended for use where precise accuracy is required, and it is not intended that it should take the place of high-voltage metering equipment.
The device is used in conjunction with a high-voltage capacitance bushing which contains a cylindrical metal plate, thus forming a capacitance voltage divider. Provision is made to connect the bushing potential device to this metal plate through an outlet located in the bushing-mounting flange, and it is connected to the line through the capacitance of the bushing. Being connected between line and ground, the device operates on a single-phase basis. Where three-phase connections are needed, three devices are used, one connected to each bushing of the three-phase circuit. The secondary voltage and its phase angle are adjustable over a wide range of values, making it possible for the device to be used with most bushings having capacitance taps.
The bushing potential device consists of a high reactance transformer, a ground switch, a protective gap, and the required adjustment equipment. An auxiliary capacitor is also provided to augment the capacitance from the high-voltage bushing capacitance tap to ground. These elements are contained in a weatherproof steel housing provided with brackets for mounting the device on the side of a circuit breaker tank or a power transformer tank, either indoors or outdoors. The high-reactance transformer is located in back of the panel. A 38-watt space heater, at either 120 or 240 volts is provided to keep the compartment dry. This heater requires a separate power supply. The ground switch and protective gap assemblies are mounted to the top of the housing. When the ground switch is closed, all voltage is removed from the device. The protective gap is adjusted from inside the housing by means of a threaded stud and lock nuts. Factory setting is for approximately 15kv. The adjustment devices, consisting of an adjustment transformer, a phase-angle adjustment capacitor, and a power-factor-correction-capacitor, are mounted on the back of the adjustment panel. The transformer and both capacitors contain taps to permit the necessary adjustments by means of switches and knobs on the front of the panel. The panel, located just inside the housing door, is hinged to permit easy access to the various devices.
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