Advances in the Determination of Control Parameters for Static Compensators

The purpose of the paper is to discuss the advantages of synchronizing modern programmable controllers to the phase locked loop synchronization circuit for power electronic control applications. Accurate phasor values of system quantities are required for fast feedforward and feedback control of the firing angle of dc converters and static compensators. The combination of a high-speed programmable controller and an accurate high frequency phase-locked loop (PLL) provides the ability to measure specific phasor quantities of any harmonic in the presence of system unbalances and frequency excursions. A generalized method of transforming the instantaneously measured quantities to suitable phasor quantities is described. The problem with generating phasor quantities is that a reference phasor, accurate in frequency, phase and magnitude is required. The phase-locked loop is used to provide the reference phasor. ## Modern synchronization circuits essentially lock on to the fundamental positive sequence component of the system voltage, and can follow (with acceptable error transiently) frequency and phase excursions of the system. The reference phasor for the control system is therefore locked onto the positive sequence component of the system voltage and remains constant in magnitude and phase, even in the presence of unbalances and harmonics. Use of PLL to generate the reference phase offers advantages over more conventional techniques especially under unbalanced and harmonically distorted waveform conditions.