Abstract: Apparatus controls a power converter 34 to interchange power between first and second electric sources, 30 and 32. Typically the first source 30 is a standard utility bus and the second source 32 is a storage battery. A controller generates control signals in response to the utility bus voltage, to a command specifying the required power interchange and to a feedback signal indicative of the actual power being interchanged. These signals control the power converter 34 to produce the required power interchange.

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.