PERFORMANCE ANALYSIS OF GRID-CONNECTED WIND ENERGY SYSTEMS

Abstract

Wind energy has recently emerged as one of the most promising and significant sources of renewable energy that is replenishable. Unlike conventional power plants, wind power plants emit no air pollutants or greenhouse gases and therefore, wind energy provides clean and non-polluting form of electricity. Today, more than 341,320 wind turbines are spinning all over the world. However, the growing penetration of wind energy system (WES) into electric network poses significant challenges on a wide range of issues, the major ones are as follows: a) Variable Wind speed - Wind speed is intermittent and stochastic in nature and the output of wind turbine (WT) is proportional to the cube of the wind speed. This causes the wind generator output power to fluctuate largely for even small changes in wind velocity. The power in the grid determines the frequency at which the grid will operate. When the penetration of WES in the grid is large, the wide power variations may result in significant fluctuations in grid frequency. b) Transient faults- The induction generator is extremely sensitive to the grid faults as its stator winding is directly connected to the grid. If short-circuit faults occur in the power network, induction type WES tend to drain relevant amount of reactive power potentially causes rotor instability due to voltage collapse. In recent years this has become a major concern as the wind power penetration is increasing in the grid power mix. (c) Grid code requirement- The grid codes are originally defined keeping in mind with conventional generators. But increasing penetration level of wind generators into power system has pushed wind farm operators to set new grid code requirements for reliable grid operation. During the disturbances, wind generating stations connecting to the grid must satisfy the grid code requirements in order to ensure power system stability and reliable operation. However, designing a robust controller for effective power smoothing, fault ride through and smooth grid interaction operation subjected to above issues is a challenging task to the control engineers as wind energy system become highly uncertain. In response to these challenges, this thesis mainly proposes a novel control strategy with interval type-2 fuzzy sets, for handling the uncertainties in the network operating conditions. The third dimension membership functions (MFs) and foot-print-of-uncertainty (FOU) offer an additional degree-of-freedom in the controller design to take uncertainties into account. The feasibility of the controller is also investigated in some test cases by developing the real time simulations using OPAL-RT digital simulator. iv Therefore, the main focus of the thesis is to investigate the applicability of the interval type-2 fuzzy logic controller (FLC) for fixed speed and variable speed wind energy systems to improve the operational performance under varying wind speed and network faults. The core objectives of the thesis are as follows:  Transient stability enhancement and power smoothing of WES using type-2 fuzzy logic based pitch-angle controller.  Stability enhancement of fixed speed wind farm using STATCOM equipped with type-2 fuzzy logic based damping controller.  Performance analysis of a fixed speed wind farm using unified voltage and pitch-angle control (UVPC) strategy.  Design and analysis of adaptive type-2 FLC-PI for a variable speed (DFIG) wind energy system. In general, pitch-angle controller regulates the generator output power when the wind speed exceeds the rated wind turbine speed. Besides, this it can also be employed to stabilize the WES rotor speed during the transient disturbances. In this part of the thesis, therefore, a logical pitch angle controller strategy (in power and speed control modes) has been developed and an interval type-2 fuzzy logic technique is proposed to design the controller. To evaluate the effectiveness of the type-2 fuzzy logic based pitch-angle controller, the simulations have been carried out for severe network faults and fluctuating wind conditions and results are compared with conventional PI and fuzzy logic controller (called as type-1 fuzzy logic controller) that has been reported in the literature. Moreover, some key factors that affect the transient stability of wind generator have also been investigated. The electrical torque as well as mechanical torque versus rotor speed results are obtained under different pitch-angle conditions, and concept of stable and unstable electrical-mechanical equilibrium points are established. This type of investigation is very important to expand the operating limitations of the wind turbine driven induction generator under the severe faults. The WES has an undesirable characteristic in which its power output varies with wind speed, resulting in fluctuations in the grid frequency and voltage. This part of the work initially employed exponential moving average (EMA) concept to generate reference power. Later on, an interval type-2 fuzzy logic based pitch-angle controller is implemented and designed for good reference tracking and therefore, it can smoothen out the WES output power more effectively. Different types of wind speed patterns are employed to validate the effectiveness of the proposed controller. Real time simulations are also developed to show the applicability of the proposed controller using the OPAL-RT digital simulator. The results show that the proposed type-2 fuzzy logic based pitch-angle controller offers better v performance in tracking reference power and hence, it offers good smoothing of output power fluctuations than conventional proportional-integral (PI) and traditional fuzzy logic (type-1) controllers. The performance of the proposed controller is also estimated using power smoothing and energy loss functions in terms of performance indices. Fixed speed wind farms employing squirrel-cage induction generator still exist in the world with a considerable number due to their advantages. In the event of grid faults, they are extremely sensitive as their stator winding is directly connected to the grid. As for cases in which if the penetration of wind farm is large in the generator mix and therefore, supplying power to the grid have adverse impact on the power system to which they are connected. Therefore, stability of wind farm becomes an important issue and has recently attracted considerable attention. As a preliminary study, this part of the work, therefore, investigates the impact of fault ride-through on the stability of fixed speed wind farm. The effect of fault locations and fault time durations on the stability of fixed speed wind farm are studied for different types of fault such as line-to-ground (LG), double-line-to-ground (LLG) and three-line-to-ground (LLLG) faults. The simulations are then repeated incorporating a static synchronous compensator (STATCOM) to study its contribution to support the wind farm during different fault conditions. The outputs include the affected voltage profile, active and reactive power magnitudes. Later on, an interval type-2 fuzzy logic based damping controller for STATCOM is proposed to contribute an adequate damping characteristic which improves the transient stability of wind integrated power system. In this regard, three different scenarios (STATCOM-without damping controller, STATCOM-with type-1 FLC based damping controller and STATCOM-with type-2 FLC based damping controller) are considered to evaluate the effectiveness of the proposed method by comparative analysis. The simulation results are obtained for different fault cases such as LLLG, LLG and LG. All the simulations are carried out on a test system using MATLAB/Simulink® software. The increasing penetration of wind energy system into power system has pushed grid operators to set new grid code requirements in order to maintain acceptable and reliable operation of the system. One of the most relevant grid code requirements is low voltage ride-through (LVRT) capability of wind generators. In case, whenever large voltage dip occurs due to severe network disturbances, the wind generators must remain connected, instead of tripping, in order to avoid other sequence of disturbances triggering in the power system. This part of the thesis proposes Unified Voltage and Pitch-angle Control (UVPC) strategy for fixed speed wind farm. The focus is put on guaranteeing the grid code compliance (i.e. LVRT) when the wind farm subjected to severe network fault. The UVPC consists of STATCOM voltage control and pitch-angle control loops which are coordinated in order to enhance the vi low-voltage ride-through capability of the wind generators thereby, fulfilling the prescribed LVRT grid code requirement. To evaluate the effectiveness of UVPC strategy, different test cases have been considered which are as follows: (a) System without STATCOM and pitch-angle controller (b) System with STATCOM only (c) System with STATCOM as well as pitch-angle control (i.e. UVPC). The simulation results show that the adoption of STATCOM and pitch-angle controller helps in complying with LVRT requirement so as to ensure the continuous operation of wind turbines. Moreover, UVPC strategy is also employed and investigated for its applicability in smoothing out the output power and voltage regulation of a fixed-speed wind generator in the partial load region subjected to varying wind speed. In both conditions (transient fault and varying wind speed) the pitch angle control loop of UVPC has been designed using type-2 fuzzy logic controller, as it offers effective performance than other control technique such as conventional PI and type-1 FLC. Doubly fed induction generator (DFIG) is very sensitive to voltage variations in the grid, which pose limitations for wind plants during the grid interaction. Handling the disturbances, which cause voltage variations in the grid thereby, affecting DFIG is a major challenge to make it complaint with the modern grid code requirement. This work proposes an advanced interval type-2 fuzzy logic-proportional integral (PI) controller for torque and voltage control loops of rotor-side converter (RSC) of DFIG. The gains of PI controller are determined and tuned by interval type-2 fuzzy logic method according to system operating condition. Thus, the adaptive nature of type-2 fuzzy logic and the robust nature of PI controller combined together, eventually exhibits good steady-state and dynamic responses. The performance of the proposed controller has been evaluated for different operating conditions of DFIG such as severe fault and voltage sag with reference to varying wind speed. A 1.5MW DFIG connected to the grid is modeled using MATLAB/Simulink® and then it exported to OPAL-RT digital simulator for real time simulation. The performance of the proposed controller scheme is verified through a comparative analysis with its traditional fuzzy logic-PI counterpart. The transient analysis of tuned PI gains with interval type-2 fuzzy logic shows the improved performance subjected to disturbances (i.e. three phase short circuit fault and voltage sag) as desired by the grid codes.