Positive-sequence flux estimator based on Second-Order Generalized Integrators for grid synchronization and motor control under imbalanced conditions

Abstract

The estimation of flux as the integral of voltage presents challenges due to known integrator issues such as drift and saturation, while requiring high-pass filters to suppress DC offsets and low-frequency harmonics. High-pass filters in applications such as motor control need to be adaptive, in order to allow operation in an extended frequency range. The estimated fluxes are commonly fed to a Phase-Locked Loop (PLL), to calculate the voltage/flux angle and frequency. Common PLLs for three-phase systems operate on the basis of balanced input signals. Their estimates deteriorate in the existence of imbalance, which can affect the response of the controllers acting based on them. This paper presents an adaptive positive-sequence flux estimator based on Second-Order Generalized Integrators (SOGIs). The balanced fluxes generated by the estimator can be used for synchronization with an imbalanced grid or for sensorless control of a motor over an imbalanced cable. Simulation results are included to illustrate the method's application to an imbalanced grid and its response to transient conditions. Experimental results using a commercial motor drive running a permanent magnet motor (PMM) connected to the drive via a long cable are presented to compare the method with the conventional alternative. A representative example of instability and loss of control occurring when using conventional integrators for sensorless control of a PMM over a flat armoured cable is included to illustrate the advantage offered by the proposed approach.