Abstract: A negative sequence occurring in a polyphase power system control including a continuous wave frequency transducer causes a second harmonic to be generated which for a 60 Hz system comprises a 120 Hz signal. In a preferred embodiment of the invention, the continuous wave frequency transducer includes an active 120 Hz notch filter having one stage which provides a component of the 120 Hz second harmonic. This signal is coupled to a second harmonic detector implemented by a semiconductor diode connected to the output of an operational amplifier. The detector diode is coupled to one input of a comparator circuit whose other input is coupled to a selectable DC reference signal. The comparator operates to generate a fault indicating signal indicative of a negative sequence when the output from the second harmonic detector exceeds the reference.

Abstract: An improved surge current limiting method and apparatus is provided for an inverter circuit. The inverter includes electronic switching devices serially connected between relatively positive and negative voltage buses and alternately conductive for providing alternating current paths through a load connected to a terminal intermediate the two switching devices. An inductor connected in series with the switching devices is provided with a permanent magnet core which is magnetically polarized in a direction such that current through the inductor attempts to reverse the magnetic polarity of the permanent magnet. The inductor is sized such that normal current through the inductor is not appreciably attenuated by the inductance. Under fault current conditions, the magnetic field generated by the higher currents through the inductor is opposed by the magnetic field of the permanent magnet so as to provide a substantial current limiting effect.