Frequency measurement for electric power delivery system

A method for determining a variable period of an input waveform, the method comprising:

• receiving a representation of the input waveform;
• and
• performing an iterative process to determine the variable period of the input waveform, the iterative process comprising:
• selecting an estimated window length;
• determining an autocorrelation value based on the estimated window length;
• determining an adjustment value to the window length to identify a maximum of the autocorrelation value;
• and
• determining the variable period based on the window length based on the window length associated with the maximum of the autocorrelation value;
• a control action subsystem configured to implement a control action based on the period of the input waveform;
• wherein the period determination subsystem is further configured to perform the iterative process to track changes in the variable period of the input waveform.

A system configured to monitor a periodic waveform in an electrical power system and to determine a variable period of the periodic waveform, the system comprising:

• an input configured to receive a representation of the periodic waveform;
• and
• a period determination subsystem configured to perform an iterative process to determine the variable period of the periodic waveform, the iterative process comprising:
• selection of an estimated window length;
• determination of an autocorrelation value based on the estimated window length;
• determination of an adjustment value to the window length to identify a maximum of the autocorrelation value;
• and
• determination of the variable period based on the window length associated with the maximum of the autocorrelation value;
• a periodicity determination subsystem configured to evaluate a periodicity of the periodic waveform and to provide a measurement quality indicator based on the periodicity of the input waveform for each of a corresponding plurality of values of the variable period;
• a control action subsystem configured to selectively disconnect a load connected to the electric power distribution system when the variable period exceeds a threshold;
• wherein the period determination subsystem is further configured to selectively discard values of the variable period when the measurement quality indicator is below a threshold.