Sequence filters for determining the symmetrical components of multiphase AC power systems

Apparatus for generating the positive or negative sequence voltage from a three phase AC source voltage, said apparatus comprising:

• means for generating a first line-to-line voltage as the difference between a first two of the source voltages;
• means for generating a second line-to-line voltage as the difference between one of said first two source voltages and the third source voltage;
• means for phase shifting one of said line-to-line voltages 120.degree. to bring it into phase with the other line-to-line voltage, said means including a potentiometer having a tap point, a capacitor connected between the tap point and ground, means for connecting said first line-to-line voltage to one end of the potentiometer, an operational amplifier and means connecting the other end of the potentiometer to the inverting input of the operational amplifier;
• means for summing said phase shifted line-to-line voltage and said other line-to-line voltage to produce said positive or negative sequence voltage;
• and
• wherein said means for generating said first line-to-line voltage includes a second operational amplifier, means for connecting first and second of said source voltages to the inverting input of said second operational amplifier, a non-reactive impedance connected in series with the output of said second operational amplifier and a junction, and means for applying the first and third source voltages to said junction, wherein said junction is connected to the first end of said potentiometer in said phase shifting means to shift said first line-to-line voltage 120.degree. in phase, wherein said means for generating said second line-to-line voltage and for summing said second line-to-line voltage with the phase shifted first line-to-line voltage includes means for applying the output of said second operational amplifier and said second source voltage to the inverting input of the operational amplifier in said phase shifting circuit.

Apparatus for generating the positive or negative sequence voltages from a three phase AC source voltage, said apparatus comprising:

• a first operational amplifier;
• means for applying one of said source voltages to the inverting input of said operational amplifier;
• a resistive impedance connected in series between the output of said first operational amplifier and a junction;
• means for applying a second of said source voltages to said junction;
• means for shifting the voltage generated at said junction 120.degree. in phase;
• a second operational amplifier;
• and
• means for applying the third source voltage, the output of the first operational amplifier and the output of the phase shifting means to the inverting input of the second operational amplifier.

Apparatus for generating the positive or negative sequence voltages from a three phase AC source voltage, said apparatus comprising:

• a first operational amplifier;
• means for applying a first and second of said source voltage to the inverting input of said first operational amplifier;
• a resistive impedance connected in series to the output of said first operational amplifier and a junction;
• means for applying the first and third source voltages to said junction;
• means for shifting the voltage generated at said junction 120.degree. in phase;
• a second operational amplifier;
• means connecting the output of said phase shifting means to the inverting input of said second operational amplifier;
• and
• means for also applying the output of said first operational amplifier and the second source voltage to the inverting input of said second operational amplifier.