A Three-Phase Frequency Adaptive Digital Phase Locked Loop for Measurement, Control, and Protection in Power Systems

A three-phase phase locked loop (3-Phi PLL) for control and protection in power system applications is presented. The 3-Phi PLL is a frequency adaptive digital system that consists of a positive sequence detector and a single-phase predictive phase locked loop (PPLL). The 3-Phi PLL is fully adaptive in extracting frequency, amplitude, and phase angle in time variant systems. The modeling and mathematical properties of the positive sequence detector and the PPLL are presented. The 3-Phi PLL can accommodate a wide range of frequency and amplitude variations in the presence of noise and disturbances. The 3-Phi PLL is implemented on a field programmable gate array (FPGA). A number of analog-to-digital converters (ADCs) have been evaluated for this application. A successive approximation type ADC offers the best tradeoff between resolution and sampling frequency. The synchronization information is extracted within two cycles of the input signal period. The operation of the 3-Phi is verified for balanced and unbalanced loads in the presence of noise, harmonics, and DC-offsets over a frequency range of a fraction of Hz to a few kHz and over an amplitude range of about 3% to 120% of the nominal voltage.