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Abstract

The disclosure is directed to a transformer circuit apparatus which receives an input signal and generates an output signal representative of the input signal. For example, the input signal may be an AC current, and the output signal to be generated may be a voltage that is proportional to the current. A transformer is provided, and has a core, a primary winding for receiving the input signal, a sense winding, and a feedback winding. A first circuit is coupled to the sense winding, and is responsive to the magnetic flux sensed by the sensed winding, for applying a signal to the feedback winding that tends to reduce the flux sensed by the sense winding. A feedback circuit is provided, and includes a low-pass filter, for feeding back a signal that depends on the output signal to the first circuit means. A circuit is provided for determining the presence of input signal offsets, and for generating offset indication signals in response thereto. A control means, responsive to the offset indication signals, is provided for controlling the low-pass filter. In the illustrated embodiment, there is disclosed current transformer circuit suitable for utilization in a protective relaying system. The low-pass filter includes a capacitor, and the circuit for controlling the low-pass filter includes a switch for discharging the capacitor, the switch being controlled by the offset indication signals. In this embodiment, the offset indication signals depend upon the time between zero-crossings of the input signal and the amplitude of the input signal. By discharging the capacitor in response to offsets at the input, the problem of saturating the input amplifier of the sense winding circuit is eliminated, and this permits use of a transformer core that need only be large enough to handle the small flux that is uncancelled by the feedback winding.

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First Claim