Prediction of harmonic distortion and application of power-factor correction in AC power systems

Method of applying power-factor-correction capacitance to an AC power system, including predicting values of harmonic distortion defined by, for each of one or more harmonics, a harmonic impedance and a damping resistance, the method comprising the steps of:

• (1) imposing a first step change of capacitance on said system;
• (2) determining as a result of said first step change, values of said harmonic impedance for respective said harmonics;
• (3) determining from said harmonic-impedance values, values of said damping resistance for respective said harmonics;
• (4) determining from corresponding said harmonic-impedance and damping-resistance values, values of an electrical load parameter for respective said harmonics for a second, as yet unimposed, step change of capacitance, wherein said second step capacitance change is a desired change for the purposes of power-factor correction;
• (5) determining from said electrical load parameter values obtained in step (4) whether the imposition of said second step capacitance change would result in an undesirable resonance condition for the system;
• and, where it is determined that a resonance condition would result,
• (6) imposing a step capacitance change other than said second step change, thereby to avoid said resonance condition.

A power factor controller for correcting lagging power factor in an AC power supply system operating at a variable power factor, the controller comprising:

• measurement means for periodically measuring electrical load parameter values of the power supply system,
• switching means for individually controlling connection of each of a plurality of capacitances to the power supply system,
• digital signal processor means connected to the measurement means and the switching means for calculating the power factor of the power supply system from the electrical load parameter values and deciding whether correction of lagging power factor is required and, if correction is required, deciding which power factor correction capacitances should be applied to the power supply system by the switching means to correct the power factor, said digital signal processor means being operative for predicting values of harmonic distortion defined by, for each of one or more harmonics, a harmonic impedance and a damping resistance, the processor means including
• means for imposing a first step change of capacitance on said system;
• means for determining as a result of said first step change, values of said harmonic impedance for respective said harmonics;
• means for determining from said harmonic-impedance values, values of said damping resistance for respective said harmonics;
• means for determining from corresponding said harmonic-impedance and damping-resistance values, values of an electrical load parameter for respective said harmonics for a second, as yet unimposed, step change of capacitance, wherein said second step capacitance change is a desired change for the purposes of power-factor correction;
• means for determining from said electrical load parameter values whether the imposition of said second step capacitance change would result in an undesirable resonance condition for the system;
• and, where it is determined that a resonance condition would result, means for imposing a step capacitance change other than said second step change, thereby to avoid said resonance condition.