The main components of a wind turbine system consist of turbine rotor (or blades), gearbox, and generator. The common wind turbine has three blades. The turbine blades capture kinetic energy of flowing air, known as wind. The blades start to rotate and spin a shaft connected from a hub of the rotor to generator. The rotational speed decreases as the as the radius of blades increases. The gearbox is used to adapt the low speed of turbine blades to high speed of the generator. However, the gearbox may not be necessary in multi-pole of generator system. The generator converts the mechanical energy to electrical energy which is then supplied to end users through a power grid system. Sometimes, the electrical energy is stored for future usage by applying newly-developed energy storage methods such as compressed air, pumped hydro, flow batteries, and flywheel. This method is used by people who are living entirely off the grid system.
There are many types of wind turbine, such as variable speed wind turbine system use the power electronic system as interfaces. As name suggested, this type of wind turbine operates at variable speed, the frequency and voltage of electric of the generator varies over the time. Thus, the generated active power must be converted into suitable voltage and frequency to the output. This conversion can be solved by introducing a power electronic converter system to modify the form of electrical energy by changing its voltage, current and frequency. These systems contain switching elements in form of semiconductors. The semiconductor elements are either not controllable (diodes), or controllable by switching on (thyristors) or by switching on and off (bipolar transistors, MOS-FETs, IGBTs, GTO- thyristors). This converter is connected between the generator and grid by an isolating transformer. This device permits different frequency and voltage level in its input and output. The distortion of voltage and frequency may cause a phenomenon called Harmonic to happen. A harmonic of a wave is a component frequency of signal that is an integer multiple of fundamental frequency. All harmonics cause increased currents and possible destructive overheating in capacitors as the impedance of a capacitor is decreasing with the increase of frequency. Certain order of harmonic will cause damage of various kind of electrical equipment. The higher harmonics may further give rise to noise problems in analogue telephone circuits. There are two types of converter, grid commutated and self commutated converter system which are commonly used in modern wind turbine systems.
The grid commutated converters contains mainly thyristor which is a solid state semiconductor device with four layers of alternating N and P-type material. They act as bitable switches, able to conduct when their gate receives a current pulse and continue to conduct as long as they are forward biased. It is cheap and reliable. This type of converter produces integer harmonics that are difficult to filter out. It is not able to control the reactive power and consume inductive reactive power.
Self commutated converter systems utilize pulse width modulation (PWM) method to produce intermediate amounts of electrical power between fully on and off. This converter mainly consists of Insulate Gate Bipolar Transistor (IGBT), which is a three terminal power semiconductor device with highly efficiency and fast switching. Similar to thyristor, IGBT has four layer of alternating N and P-type material. The typical switching frequency of an IGBT lies in range of 2 to 20 KHz. The high frequency in switching enable PWM converter to produce harmonics and inter-harmonics. Usually, the high frequencies of harmonics produced are easily removed by using small size filters. Thus, the disturbances to the power network can be reduced to low levels. This type of converter can control both active and reactive power.
The natural variations in wind speed cause fluctuation of electrical power give rise to a wide interest in energy storage technology. A method called Grid Energy Storage is used for large scale store electricity within an electrical power grid. This storage technique is important especially in area with unpredictable wind speed to provide consistent electricity supply to end users. Electrical energy is stored when the production from wind turbine system exceeds the consumption. Then the stored energy is utilized when the consumption exceeds production. Nowadays, various kinds of energy storage devices are invented. They allow reversible energy conversion in both directions, by charging and discharging either in short time or long time storage of energy. They appear in the forms of pumped water, compressed air, thermal, flywheel, and superconducting magnetic energy, hydrogen and batteries. The working principles of the devices are using the concept of electrochemical, mechanical and electrical.
In this context, the electrical energy storage method is discussed. Traditionally, a conventional capacitor is used as a storage element for electrostatic energy. However, they are not suitable to store large amount of electrical energy. This limitation has been overcome by introducing the electric double layer capacitors which are also known as supercapacitor, pseudocapacitor, ultracapacitor or electrochemical double layer capacitor (EDLC). They have extremely high energy density, typically on the order of thousands of times greater than the common electrolytic capacitors. For example, a typical capacitor has a measured capacitance in nano- or micro-farads. The similar size of electric double layer capacitor would have a capacitance of several farads, which are about two to three orders of magnitude in capacitance larger than the common capacitors.
A capacitor consists of two plates with an insulator called dielectric in between them. When a potential difference is applied across the capacitor, the electrons will be produced and deposited on one plate. This will induce a positive charge on another plate. As a result, an electric field exists between the two conductor surfaces and creates potential difference between them. This process is known as charging the capacitor. The energy stored in the capacitor will then be discharged in external circuit to perform some useful task. This process is called discharging the capacitor.
