Low Voltage Ride Through Enhancement of Doubly Fed Induction Generator Wind Turbines with Fuzzy Logic Controlled Crowbar and Series Dynamic Braking Resistor

Abstract

Doubly fed induction generator (DFIG) type wind turbines are widely used nowadays as they offer benefits such as low-cost power electro-converters, different speed operations, separate control of reactive and active powers, and reduced filter requirements. However, the direct coupling of their stator winding to the power network and utilisation ofsmall back to-back converter power ratings make them highly susceptible to power network faults, particularly voltage drops. When these generators sense voltage dips at the point of interconnection (POI), their stator voltage diminishes immediately, which further causes oscillation of the stator as well as rotor currents, DC bus over-voltage, and torque fluctuations. This may produce damage to the generator stator and rotor circuits, including the fault-prone converters. In the past years, these wind turbine-type generators were rapidly isolated in the event of severe voltage sags. As their installation is significantly increasing, transmission system operators (TSO) need them to stay coupled with the grid and provide support in maintaining voltage stability, currently. The ability of wind turbine generators to keep connected to power system networks under severe voltage sags is named to as low voltage ride-through capability. To fulfil such capacity while withstanding the aforementioned challenges, an appropriate enhancing scheme or protection should be applied. Hence, in this thesis, a hybrid fuzzy logic controlled (FLC) crowbar and series dynamic braking resistor is employed. The DFIGs’ performance under steady as well as transient states with this improving technique was examined. To observe its effectiveness, this scheme was compared with the conventional and fuzzy logic-controlled (FLC) crowbars under 90% symmetrical voltage dips for 150 milliseconds. The torque control loss was preserved at 3 seconds, 3.44 seconds, and 4.1 seconds with FLC combined crowbar and SDBR, FLC crowbar, and conventional crowbar following the dip at 2.5 seconds, respectively. The stator voltage was stabilised at 4 seconds, 3.48 seconds, and 3.2 seconds using the traditional crowbar, FLC crowbar and FLC hybrid crowbar and SDBR correspondingly. The rotor current, however, regains its nominal value at 3 seconds, 3.44 seconds, and 4.1 seconds employing the hybrid FLC crowbar and SDBR, FLC crowbar, and conventional crowbar, respectively. As a result, the hybrid FLC crowbar and SDBR perform better LVRT capability-enhancing schemes. The primary data were taken from Adama II wind power, and MATLAB/Simulink 2021a software was used for modelling and simulation.