Simulation and control of wind turbine equipped with two-way induction generator in unbalanced networks

Dissertation for receiving a master's degree in the field of electricity-power

Electric machines trend

Abstract

In this thesis, the behavior of the wind power plant is investigated using the detailed model of the induction generator with two-way feeding. Electronic converters of the power plant and its control systems and the behavior of the power plant include electrical and aerodynamic simulation parts. Variations of wind speed and operating conditions are studied and the performance of control systems designed using induction machine control is evaluated. In this research, the maximum power strategy, which is integrated with the active power of the DFIG and is responsible for producing the reference active power, has been implemented and the stator flux vector orientation method is used to control the active and reactive power produced by the DFIG based on the wind turbine. The correctness and performance of the method is confirmed by simulating the sample power system in the MATLAB/Simulink software environment. The results can be well used to investigate the mutual performance of power systems with distributed generation that use renewable energy sources.

Introduction

Today, many types of wind turbine systems compete in the market, which can be divided into two main groups. The first group is constant speed wind turbines whose generator is directly connected to the grid. In fact, there is no electrical control for this system. In addition, rapid changes in wind speed are quickly induced on the load (due to power changes). These changes are not pleasant for the wind turbine that is connected to the power system and cause mechanical pressures on the turbine and reduce the life of the turbine and reduce the power quality. In a constant speed wind turbine, there is only one wind speed at which the turbine works at its optimal speed, so the constant speed wind turbine often works outside of its optimal performance and usually the maximum power is not taken from the wind. The variable speed type of wind turbine provides the ability to control the rotor speed, this allows the wind turbine to operate near its optimum point. Most of the wind turbines with power efficiency greater than 1.5 MW are variable speed type. One of the types of variable speed wind turbines is the wind turbines equipped with induction generators from both sides. Today, most of the wind turbines are equipped with induction generators from both sides. In this type, the winding rotor induction generator is connected to the power grid through the stator, and the rotor is connected to the power grid through an ac/dc/ac variable frequency power electronic converter with a rated power of about 25-30% of the rated power of the generator. The power electronic converter consists of a rotor-side converter and a grid-side converter, which are connected back-to-back through a dc coupling capacitor. The main drawback of variable speed wind turbines, especially those equipped with DFIG, is their performance during a short circuit in the grid. A short circuit on the power system, even if it is far from the location of the wind turbine, causes a voltage drop at the connection point of the wind turbine with the power grid. This increases the current in the stator winding. Due to the magnetic coupling between the stator and the rotor, this current is seen in the circuit of the rotor and power electronic converter, because the capacity of the converter is 25-30% of the generator capacity, this current leads to the damage of the converter. In this project, a wind power plant equipped with an induction generator is simulated and controlled from both sides of the power supply, and the effect of wind and grid voltage parameters on the power plant has been investigated. rtl;"> 

1-1- Wind generation mechanism

The radiation received by the sun by the earth causes the air to heat up. Atmospheric and therefore the air moves upwards.The intensity of this warming in the equator; where the sun shines vertically; more than the air around the poles; where the angle of the sun is steep; will be and the air around the poles is less heated than the air at the equator. The air density decreases with the increase in temperature and therefore the lighter air at the equator moves up and spreads around. This action has caused a drop in pressure in this area and causes cold air to be attracted from the poles to the equator [1].

Also, when the sun shines during the day, the air on dry lands heats up faster than the air on seas and waters. Warm air over land rises and cooler, heavier air over water takes its place, and this process creates local winds. At night, since air over land cools faster than air over water, the wind direction reverses. Therefore, the wind is created due to the pressure gradient created by the non-uniform radiation of the sun to the surface of the earth[1].

1-2- The history of wind energy

1-2-1- The beginning of using wind energy

The first windmills for grinding Grains and water pumping were used, and the oldest designed model was of the vertical axis type, which was developed in Iran during 500-900 AD. Apparently, the first use of these windmills was to pump water, but the exact way of working is not known. The first documents related to the design of these windmills are related to Iranians. whose blades or so-called sails were made of wood or straw that were connected to a vertical axis with horizontal beams. Grinding grains is the first documented and very simple use of windmills. So that the millstone is connected to the same vertical axis. All the parts of the windmill are usually enclosed inside a building and the entrance of the building has a play space in the direction of the wind so that the wind can be directed inside rtl;"> 

Using a detailed model of wind power station, including wind turbine, wind turbine governor, wind resources and the DFIG, the performance of wind power station is investigated. It consists of wound rotor induction machine, grid side converter, rotor side converter and associated controllers. The control scheme uses stator flux oriented control for the rotor side converter and grid voltage vector control for the grid side converter. In this research, a maximum power control strategy is incorporated with the DFIG whereby the produced power serves as the dynamic active power reference for the DFIG. Stator flux oriented vector control is applied to decouple the control of active and reactive power generated by the DFIG based WT. MATLAB/Simulink is used for simulation. The main power electronic circuits and controllers are also developed in details and in the same simulation tool. The simulation model is also used to investigate the performance of controllers under different network conditions. The responses of rotor speed, active power output and blade pitch angle are obtained for various conditions of power grid. Capability and modeling accuracy of the proposed method confirmed with simulating a sample power system in MATLAB/Simulink software.