Effect of Erroneous Position Measurements in Vector-Controlled Doubly Fed Induction Generator

Vector control of the doubly fed induction generators (DFIGs) depends on accurate information from a stator voltage phase lock loop (PLL) and a rotor position encoder for the reference frame transformations required by theory. The effect of erroneous information due to noise picked up by commonly used hardware PLLs and incremental position encoders was analyzed in this paper. The analysis shows that erroneous information can give rise to destructively large currents due to: 1) the DFIG system wrongfully perceiving very large P and Q references; and 2) system instability. It is shown that instability does not arise from inside the inner current feedback loop, but from the outer feedback loop that regulates the complex power. Simulation and experimental test results validate the analysis. The experimental results are taken from a 1.5-MW DFIG intended for a wind turbine. A simple scheme to protect against the erroneous information has been tested successfully in a brief trial period.