31-08-2012, 10:22 AM
DOUBLY FED INDUCTION GENERATOR IN A WIND TURBINE
DOUBLY FED INDUCTION.doc (Size: 634.5 KB / Downloads: 64)
INTRODUCTION
The conventional energy sources are limited and have pollution to the environment, so the main mission is to short out the global energy shortage and pollution problem by utilizing renewable energy resources. The wind energy is the fastest growing and most promising renewable energy source among them, but the disadvantage is that the wind power is intermittent and depending upon weather conditions. Steady of thesis is relates to tackle the intermittency problem of wind power by means of a short – term energy storage to get a smooth power output from a wind turbine.
Compared with the wind turbine based on synchronous generators, the doubly fed induction generator based wind generation technology has several advantages such as flexible active and reactive power control capabilities, lower converter costs and lower power losses.
In the DFIG concept, the induction generator is directly connected to the grid at the stator terminals, but the rotor terminals are connected to the grid via a variable-frequency AC/DC/AC converter. When the wind speed varies, controlling two back-to-back four-quadrant power converters connected between the rotor side and the grid side can make the rotor flux rotate from a sub-synchronous speed to a super-synchronous speed. The DFIG can produce and inject constant-frequency power to the grid by controlling the rotor flux. The power converter only needs to handle a fraction (typically 20-30%) of the total power to achieve full control of the generator. With this configuration, the power converters could be rated at lower power levels, compared with traditional configuration where the battery is directly tied to the DC link of the rectifier-inverter pair connected to the generator terminal. The energy storage device is controlled so as to smooth out the total output power from the wind turbine as the wind speed varies. Control algorithms are developed for the grid-side converter, rotor-side converter and battery converter, and the control strategies are tested on a simulation model developed in MATLAB/Simulink. The model contains a DFIG wind turbine, three power converters and associated controllers, a DC-link capacitor, a battery, and an equivalent power grid.
Literature review
Renewable energy is increasingly attractive in solving global problems such as the environmental pollution and energy shortage. Among a variety of renewable energy resources, wind power is drawing the most attention from the government, academia, utilities and industry [1]-[9]. However, the disadvantage is that wind power generation is intermittent, depending upon weather conditions. Short-term energy storage is necessary in order to get a smooth power output from a wind turbine [10]-[13].Compared with the wind turbines based on synchronous generators, the doubly-fed induction generator (DFIG) based wind generation technology has several advantages such as flexible active and reactive power control capabilities, lower converter costs and lower power losses [14]-[17].
CONTROL OF POWER CONVERTERS
The objective of the rotor side converter is to manage the active power and reactive power from the stator terminals independently, while the objective of stator side converter is to manage the active power and reactive power from the stator side converter independently. The battery converter is used to keep the DC-link voltage constant regardless of the magnitude and direction of the rotor and stator powers. The goal of these three converters is to maintain a constant total power output from the wind turbine generator.
Control of rotor side converter
The rotor side converter control scheme consists of two cascaded control loops. The inner current control loops regulate independently the d axis and q axis rotor current component, idr and iqr according to a synchronously rotating reference frame where the voltage oriented vector control is used. The outer control loops regulate the active power and reactive power output from the machine stator independently.
In the stator voltage oriented reference frame, the d axis is aligned with stator voltage vector, namely, vs. The stator flux linkage vector λs is then aligned with q axis, i.e., λsq = λs = Lmims and λsd = 0. Considering equation 3-10, we can gt the following relationships
SIMULATION RESULTS
To verify the control strategies designed above, a single machine infinite bus power system is used for simulation studies in MAT LAB Simulik. A 2MW DFIG wind turbine system is connected to the stiff grid through a step-up transformer and transmission line. The proposed integrated power generation and energy storage configuration is used. The parameters of the DFIG wind turbine are given in the Appendix. Three scenarios are studies where the output power to the grid and the wind speed are varied.
4 I Variation in load: Change of load from 500 KW to 1 MW at 5 seconds at constant wind speed of 11m/s
The wind turbine operates at a constant wind speed of 11 m/s with DFIG input mechanical power Pm=2 MW. The reference output power from the stator is set as Ps_ref = 1.8 MW and Qs_ref = 0 Mvar respectively. The reference output power from the grid side converter is initially set as Pg_ref = 0.24 MW and Qg_ref = 0.1 Mvar. This case is simulating the operation condition where there is a change in the load while the power from the stator terminals is maintained constant.
As shown in the figure (4.a) below the DFIG’s input mechanical power and the stator’s output power are kept constant, according to the control method discussed before the DFIG rotor current is constant correspondingly, as shown in the figure 4 c.The battery initially discharges a small amount of power, as shown in figure 4 a, at a low current figure 4 d, through the rotor side converter to meet the difference between the stator output power and input mechanical power. As the stator side converter increases its output power and the input mechanical power, the battery discharges more power to balance the increased power difference. However, when the output power from the stator side converter decreases to 0.2MW, the battery start to be charged figure 4 d and a certain amount of power is delivered to the battery. The power of the stator side converter is controlled by its d axis current, as shown in c, which agrees with equation 47. The change in the output reactive power fig 4 b does not affect the battery current figure 4 d but the q axis current of the stator side converter fig 4 c. Figure 4 d shows that the voltage across the DC link capacitor undergoes very slightly disturbance as the battery current changes, which suggests that the battery can help stabilize the DC link voltage.
Conclusion
An integrated power generation and energy storage system has been presented for doubly fed induction generator based wind turbine system. A battery energy storage system is connected to the DC-link of the back-to-back power converters of the doubly fed induction generator through a bi-directional DC- to-DC power converter. The battery is charged if there is excess power and can supply power to the load if the power demand is higher than the input mechanical power from the wind. The energy storage device is controlled so as to smooth out the output power as the wind speed varies or maintain a desirable power output. Control algorithms are developed for the grid side converter, rotor side converter and battery converter and the control strategies are tested on a simulation model of a 2 MW DFIG wind turbine system developed in MATLAB/ Simulink. The model contains a DFIG wind turbine, three power converters built with individual IGBT switches, a DC-link capacitor and several controllers. Simulation results show that the integrated power generation and energy storage system can supply steady output power as the wind speed changes. As the wind speed is constant, the wind turbine can output varying power due to the existence of the battery. This study suggests that the integrated power generation and energy storage is good for intermittent wind power generation.
DOUBLY FED INDUCTION.doc (Size: 634.5 KB / Downloads: 64)
INTRODUCTION
The conventional energy sources are limited and have pollution to the environment, so the main mission is to short out the global energy shortage and pollution problem by utilizing renewable energy resources. The wind energy is the fastest growing and most promising renewable energy source among them, but the disadvantage is that the wind power is intermittent and depending upon weather conditions. Steady of thesis is relates to tackle the intermittency problem of wind power by means of a short – term energy storage to get a smooth power output from a wind turbine.
Compared with the wind turbine based on synchronous generators, the doubly fed induction generator based wind generation technology has several advantages such as flexible active and reactive power control capabilities, lower converter costs and lower power losses.
In the DFIG concept, the induction generator is directly connected to the grid at the stator terminals, but the rotor terminals are connected to the grid via a variable-frequency AC/DC/AC converter. When the wind speed varies, controlling two back-to-back four-quadrant power converters connected between the rotor side and the grid side can make the rotor flux rotate from a sub-synchronous speed to a super-synchronous speed. The DFIG can produce and inject constant-frequency power to the grid by controlling the rotor flux. The power converter only needs to handle a fraction (typically 20-30%) of the total power to achieve full control of the generator. With this configuration, the power converters could be rated at lower power levels, compared with traditional configuration where the battery is directly tied to the DC link of the rectifier-inverter pair connected to the generator terminal. The energy storage device is controlled so as to smooth out the total output power from the wind turbine as the wind speed varies. Control algorithms are developed for the grid-side converter, rotor-side converter and battery converter, and the control strategies are tested on a simulation model developed in MATLAB/Simulink. The model contains a DFIG wind turbine, three power converters and associated controllers, a DC-link capacitor, a battery, and an equivalent power grid.
Literature review
Renewable energy is increasingly attractive in solving global problems such as the environmental pollution and energy shortage. Among a variety of renewable energy resources, wind power is drawing the most attention from the government, academia, utilities and industry [1]-[9]. However, the disadvantage is that wind power generation is intermittent, depending upon weather conditions. Short-term energy storage is necessary in order to get a smooth power output from a wind turbine [10]-[13].Compared with the wind turbines based on synchronous generators, the doubly-fed induction generator (DFIG) based wind generation technology has several advantages such as flexible active and reactive power control capabilities, lower converter costs and lower power losses [14]-[17].
CONTROL OF POWER CONVERTERS
The objective of the rotor side converter is to manage the active power and reactive power from the stator terminals independently, while the objective of stator side converter is to manage the active power and reactive power from the stator side converter independently. The battery converter is used to keep the DC-link voltage constant regardless of the magnitude and direction of the rotor and stator powers. The goal of these three converters is to maintain a constant total power output from the wind turbine generator.
Control of rotor side converter
The rotor side converter control scheme consists of two cascaded control loops. The inner current control loops regulate independently the d axis and q axis rotor current component, idr and iqr according to a synchronously rotating reference frame where the voltage oriented vector control is used. The outer control loops regulate the active power and reactive power output from the machine stator independently.
In the stator voltage oriented reference frame, the d axis is aligned with stator voltage vector, namely, vs. The stator flux linkage vector λs is then aligned with q axis, i.e., λsq = λs = Lmims and λsd = 0. Considering equation 3-10, we can gt the following relationships
SIMULATION RESULTS
To verify the control strategies designed above, a single machine infinite bus power system is used for simulation studies in MAT LAB Simulik. A 2MW DFIG wind turbine system is connected to the stiff grid through a step-up transformer and transmission line. The proposed integrated power generation and energy storage configuration is used. The parameters of the DFIG wind turbine are given in the Appendix. Three scenarios are studies where the output power to the grid and the wind speed are varied.
4 I Variation in load: Change of load from 500 KW to 1 MW at 5 seconds at constant wind speed of 11m/s
The wind turbine operates at a constant wind speed of 11 m/s with DFIG input mechanical power Pm=2 MW. The reference output power from the stator is set as Ps_ref = 1.8 MW and Qs_ref = 0 Mvar respectively. The reference output power from the grid side converter is initially set as Pg_ref = 0.24 MW and Qg_ref = 0.1 Mvar. This case is simulating the operation condition where there is a change in the load while the power from the stator terminals is maintained constant.
As shown in the figure (4.a) below the DFIG’s input mechanical power and the stator’s output power are kept constant, according to the control method discussed before the DFIG rotor current is constant correspondingly, as shown in the figure 4 c.The battery initially discharges a small amount of power, as shown in figure 4 a, at a low current figure 4 d, through the rotor side converter to meet the difference between the stator output power and input mechanical power. As the stator side converter increases its output power and the input mechanical power, the battery discharges more power to balance the increased power difference. However, when the output power from the stator side converter decreases to 0.2MW, the battery start to be charged figure 4 d and a certain amount of power is delivered to the battery. The power of the stator side converter is controlled by its d axis current, as shown in c, which agrees with equation 47. The change in the output reactive power fig 4 b does not affect the battery current figure 4 d but the q axis current of the stator side converter fig 4 c. Figure 4 d shows that the voltage across the DC link capacitor undergoes very slightly disturbance as the battery current changes, which suggests that the battery can help stabilize the DC link voltage.
Conclusion
An integrated power generation and energy storage system has been presented for doubly fed induction generator based wind turbine system. A battery energy storage system is connected to the DC-link of the back-to-back power converters of the doubly fed induction generator through a bi-directional DC- to-DC power converter. The battery is charged if there is excess power and can supply power to the load if the power demand is higher than the input mechanical power from the wind. The energy storage device is controlled so as to smooth out the output power as the wind speed varies or maintain a desirable power output. Control algorithms are developed for the grid side converter, rotor side converter and battery converter and the control strategies are tested on a simulation model of a 2 MW DFIG wind turbine system developed in MATLAB/ Simulink. The model contains a DFIG wind turbine, three power converters built with individual IGBT switches, a DC-link capacitor and several controllers. Simulation results show that the integrated power generation and energy storage system can supply steady output power as the wind speed changes. As the wind speed is constant, the wind turbine can output varying power due to the existence of the battery. This study suggests that the integrated power generation and energy storage is good for intermittent wind power generation.