A novel control system design to improve LVRT capability of fixed speed wind turbines using STATCOM in presence of voltage fault

The design and implementation of a new control system for reactive power compensation and mechanical torque, voltage regulation and transient stability enhancement for wind turbines equipped with fixed-speed induction generators (IGs) in power systems is presented in this study. The designed optimal linear quadratic regulator (LQR) controller provides an acceptable post fault performance for both small and large perturbations. Large disturbance simulations demonstrate that the designed controller enhances voltage stability as well as transient stability of the system during low-voltage ride-through transients and thus enhances the LVRT capability of fixed-speed wind generators. Further verifications based on detailed time-domain simulations are also provided. Calculations, simulations and measurements confirm how the increased STATCOM rating can provide an increased transient stability margin and consequently enhanced LVRT capability. A concept of critical clearing time has been introduced and its utility has been highlighted. 2015 Elsevier Ltd. All rights reserved. Introduction WIND turbines are one of the renewable energy technologies that, today, face a growing progress. This developments cause rapid progress of economic and environmental issues [1]; therefore, the study about connecting the turbines to the grid is very important [2]. Many countries have their own grid codes which monitor the behavior of the wind turbines connected to the grid [3]. All network grid codes for wind turbines include requirements such as low-voltage ride-through capacity (LVRT), voltage control, power-quality and protection requirements. In 2005, LVRT requirement was introduced the wind turbine rotor speed try to achieve stability on the index which requires a certain voltage, an example of the voltage profile is shown in Fig. 1. If a fault or voltage drop occurs at stator terminals of wind turbine generator, according to (1), electrical torque will decrease while mechanical torque still exists because wind keeps blowing. According to Eq. (2), these conditions will cause rotor speed to increase. If this voltage drop continues, it may cause rotor of turbine to accelerate and make rotor speed unstable. Te / vs ð1Þ d dt xr 1⁄4 1J Tm Te ð Þ ð2Þ The maximum voltage drop (either in terms of magnitude or in terms of time) which wind turbine is able to withstand without suffering from rotor speed instability is called wind turbine low voltage ride through capability. Wind turbine technologies include Fixed and variable speed wind turbines [5]. Since fixed-speed turbines are easy to install, durable and costeffective, nowadays, most of the installed turbines are chosen from this category [6]. In Iran, about 91 MW of installed wind turbines are fixed-speed squirrel cage induction generator turbines [7]. Squirrel cage induction generators show slight stability margin against voltage drop; consequently, it is necessary to use compensator devices to improve rotor speed stability margin [8]. In order to determine the type and the way of compensation, it is necessary to examine characteristic curves of induction generator. The absorbed reactive power, slip and slip-torque characteristics of induction machine are shown in Fig. 2(a) and (b) respectively. As it can be observed in these figures, during the normal operation, the generator has a very low slip close to zero, and, in this case, little reactive power is absorbed by generator. But if http://dx.doi.org/10.1016/j.ijepes.2015.11.011 0142-0615/ 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: [email protected] (M.H. Khooban). Electrical Power and Energy Systems 77 (2016) 280–286