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Wind Powered Electrical Power Systems: 'Blowing in the Wind, If Any'
Green Technology Featured Articles
July 14, 2008

Wind Powered Electrical Power Systems: 'Blowing in the Wind, If Any'

By Tom Cross
Technology Columnist

An animated “pictutorial” about winder-powered electricity is available at http://www.techtionary.com.
 
Wind power is great where there is wind and consumers nearby. However, if there is wind power in Wyoming, there is little power in Chicago due to the amount of electrical resistance. Understand resistance is critical to understanding power generation and consumption.

 
The output of the typical electrical power generator is 13 KV-KiloVolts. The voltage is increased to 115 KV and then to 230 KV for metropolitan or long distance transmission. At the destination, the power is reduced to 115K and then to 13 KV and then to 220 Volts for household use. Why increase the voltage? To reduce power loss due to resistance: P = IV = I²R. That is, power loss is due to wire resistance increases as the square of the current as well as decreases as the square of the voltage.  The higher the voltage, the lower the current and the lower the resistance or power lost during transmission. That is, the greater the distance, the higher the resistance and the lower the output at the distant end.
 
Why Resistance Kills Wind Power
 
Electrical power loss due to wire resistance (lost as heat) increases as the square of the current or P = I²R and therefore decreases as the square of the voltage.This relationship is one of Ohm's based on E=IR or voltage equals current multiplied by resistance. Resistance is the measure of electrical current when voltage is transmitted per volt applied. That is, the amount of V-Voltage divided by the current I is the Resistance. R-Resistance is defined by the formula R = V/I. Resistance is measured in units called ohms, represented by the Greek letter omega. Electrical current or amperes (amps) is the flow of electrical charge. However, the charge encounters friction called resistance. Copper, gold, aluminum and other metals have low resistance (without loss of electrical power) to electrical current and are called good conductors. Glass, ceramic, wood and the earth have a high resistance (impede the flow of current) and are called poor conductors. Superconductors like very cold niobium-tin, are special substances that allow current to flow with essentially zero loss. Semiconductors, like silicon, can be either good or poor conductors depending on electrical charges applied to their surface.
 
Tutorial on Wind Power Generators
 
According to GE Energy, "wind is the fastest growing energy source with an AGR-Average Growth Rate of 35.7 percent over the past five years. Globally, installed wind capacity increased to 3,200 megawatts during 2002. In Ireland, wind generates 175 megawatts. In regions of Denmark, Germany and Spain up to 25 percent of the electricity is generated by wind.  Forecasts for electricity wind generation are projected to exceed 83,000 megawatts by 2007."
 
The number of wind blades is generally 2 or 3 with rotor diameters from 84 to 104 meters (341 feet) and rotor speed up to 18 RPM-Revolutions per minute. Propeller pitch (blade angle) control for each blade provides for improved performance.  Each blade and all internal systems have lightning surge protection. Automatic Yaw control (rotation or swivel) of the Nacelle is controlled by the wind direction sensor.
 
Power output is based on wind flow. Beginning with wind speed as little as 3.5 meters per second (~8 MPH-Miles Per Hour) and reaching maximum output of 24-3600 KW-Kilowatts at 14-16 MPS-Meters Per Second (~20 MPH). In technical language, the generator Cut-in (minimum wind speed to generate electricity) wind speed is 3.5 MPS and Cut-Out (the wind speed at which the turbine is shut down to avoid structural overload) wind speed is 25 MPS.  The generator also has a hydraulic breaking system for failsafe protection.
 
The tower and the Nacelle (generator and control housing) are constructed of tubular steel. One of the functions of the propeller pitch control (shown here) is change the angle of interface to the wind to maintain constant output even though wind speeds vary. A three-step planetary gear system is used to reduce excessive torque excursions (differences) to maintain constant speed and electrical output. The input variable for the pitch controller is the rotor speed. The higher the rotor speed, the more the blades are turned out of the wind.
 
In general, fixed speed turbines use stall condition for technical reasons, while variable speed turbines are usually equipped with pitch control to manage stall (reduced wing air flow). In newer systems it is called AS-Active-Stall. This is similar to normal stall power limitation except that the whole blade can be rotated backwards (in the opposite direction as is the case with pitch control) by a few (3-5) degrees at the nominal speed range in order to give better rotor control. The result is known as the 'deep stall' effect, which leads to the power curve bending sharply to a horizontal output line at nominal power and keeping this constant value for all wind speeds between nominal and cut-out.
 
The Nacelle housing contains the AC-Alternating Current three-phase generator with an IGBT-Insolated Gate Bipolar Transistor converter (AC-DC-AC converter).  Most new wind generators use Low Voltage Ride-Through Capability designed to protect interfaces from the electrical from voltage "trips" - faults or drops out resulting from disturbances in the high-voltage electrical grid. The Nacelle also contains control panels, ventilation and a gantry crane for maintenance.
 
Because of the variability of wind, conventional electric generators are not always desirable. Here are some but certainly not all of the key generator types used in wind energy: CT/CS = Constant (fixed) Turbine speed - Classic Stall (fixed blade angle) CT/AS = Constant (fixed) Turbine speed - Active Stall (negative variable blade angle, 3-5 degrees) VTDI = Variable Turbine speed and pitch - Doubly-fed Induction generator VTDD = Variable Turbine speed, Direct Drive synchronous generator VTSGP = Variable Speed/pitch combined with (brushless) Synchronous Generator VT/AGP = Variable Speed /pitch combined with Asynchronous Generator (100 percent current via converter) CT/AGP = combination of fixed speed /pitch with directly connected to asynchronous generator. SVT/OSP = semi-variable speed/pitch combined with (maximum +10 percen tvariation in nominal speed)
 
A CT-Constant (fixed) Turbine speed turbine consists of a rotor and a squirrel cage induction generator connected via a gearbox. The generator stator winding is connected directly to the electrical power grid. The generator slip varies with the generated power so the speed is not always constant. If wind flow drops a squirrel cage generator draws (receives) reactive power from the grid. In order to compensate for the reactive mode, electrical storage capacitors are used to send power to the grid.
 
In a VT-Variable speed Turbine with Doubly-fed (double feeds to the grid) induction generator, the converter feeds the rotor winding, while the stator winding is connected directly to the grid. The electrical rotor frequency can be varied by this converter which can change the mechanical and electrical frequency and making variable speed operation possible.
 
In a VT-Variable speed Turbine with DD-Direct Drive Synchronous generator is managed without a gearbox by the electronic converter to provide for variable speed operation. The advantages of variable speed turbines are greater energy with lower wind speeds with active/reactive power is controlled by the converter.
 
The function of the Converter is to balance electrical output from the wind generator to the demands/needs of the electrical power grid. By changing the pitch of the blades and through AC-DC-AC inverter technology such as an IGBT-Insolated Gate Bipolar Transistor converter (AC-DC-AC converter) power regulation is controlled.
 
AC-Alternating Current generated by the wind generator is stepped up (increased) to 115/230,000 volts for transmission to a neighborhood substation/switching yard. Voltage is then Stepped Down (voltage decreased) by an underground (shown here) or overhead "can" transformer from 13,200 to 7,600 volts to house/business distribution voltage of 277/480 or 110/220 volts AC for use in lights, appliances and other devices.
 
Tom Cross (News - Alert), who has three decades of startup and consulting experience, writes the CrossTalk column for TMCnet. To read more of Tom’s articles, please visit his columnist page.
 

Don’t forget to check out TMCnet’s White Paper Library, which provides a selection of in-depth information on relevant topics affecting the IP Communications industry. The library offers white papers, case studies and other documents which are free to registered users. Today’s featured white paper is Jim Cossetta, President, CEO, 4What Interactive, Creators of The VoIPTrainer, brought to you by 4What Interactive (News - Alert).

 
 
 


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