Frequency control for paralleled AC systems

An apparatus for producing a controlled frequency output, comprising:

• (a) a reference oscillator having an output;
• (b) phase shift means for introducing a desired phase shift to said reference oscillator output;
• (c) first discriminator means for providing an output signal proportional to the phase difference between said output of said phase shift means and a signal present at an input;
• (d) second discriminator means for producing an output signal proportional to the phase difference between said reference oscillator output and a signal present at an input;
• (e) first filter means for furnishing a filtered version of said output of said first discriminator means;
• (f) second filter means for supplying a filtered version of said output of said second discriminator means;
• (g) first logic means having a clock input effectively connected to said output of said second filter means and an input effectively connected to said output of said first filter means;
• (h) second logic means having a clock input effectively connected to said output of said first filter means and an input effectively connected to said output of said second filter means;
• (i) third logic means having a first input effectively connected to said output of said first logic means, a second input effectively connected to said output of said second logic means, and an output;
• (j) counter means having a reset input effectively connected to said output of said third logic means, a clock input effectively connected to said reference oscillator output, and an output effectively connected to set or reset inputs of said first and second logic means;
• (k) amplifier means having a first input effectively connected to said output of said first logic means, a second input effectively connected to said output of said second means, and an output,
• (l) control signal input means effectively connected to one of said inputs of said amplifier means;
• and
• (m) a controlled oscillator having an input effectively connected to said output of said amplifier means and having an output effectively connected to said inputs of said first discriminator means and second discriminator means, said output providing a controlled frequency signal.

A method of connecting and controlling alternating current systems having individual frequency controls with independent reference oscillators, comprising the steps of:

• (a) connecting said alternating current systems for paralleled operation;
• (b) measuring the circulating current between said parallel-connected alternating current systems;
• (c) producing a first control signal proportional to a component of said circulating current;
• (d) Adjusting the individual frequency controls of each said alternating current system in accordance with said first control signal, including
• applying said control signal to a signal controlled oscillator;
• and
• simultaneously applying a second signal to said signal controlled oscillator having a magnitude proportional to the frequency difference between the output of said signal controlled oscillator and the output of one of said reference oscillators, and having a polarity indicative of the frequency relationship between said output of said signal controlled oscillator and said output of said one of said reference oscillators;
• and
• (e) approximately synchronizing the frequency and phase of said alternating current systems prior to connecting said systems together in accordance with step (a).