Wind farm
Wind farm refers to a group of wind turbines, designed for generating AC power, located in a site for feeding electrical power to the grid[[1]]. This is generally installed by a power utility company. Individual power unit in this case is referred to as a wind generator.
Therefore wind generator refers to a wind driven rotating equipment for power generation converting wind energy to electrical energy. In this article technical details of the wind generator and the connecting details to the grid are brought out.
The wind generators[[2]] are environmentally friendly in contrast to fossil fuelled power generating stations. But the total capacity of each wind form is very low compared to each unit capacity.
Construction
General
The adjacent sketch (to be uploaded) shows the general construction of a wind generator.
The axis of rotation will be generally horizontal. Machines with vertical axis are very rare.
The wind generator consists of a tower of about 24 to 30 M height on which is mounted a housing named as nacelle, enclosing the power generator, allied equipment and controls. The generator capacity is kept low to bring its weight with in the limits of the housing and supports. The generation voltage is also limited to 380 Volts A.C. because of the low generating capacity.
The generator is driven by a propeller outside the housing. Yaw control is provided to enable the propeller to face the wind always to keep the generation at maximum level.
Braking system is provided to keep it stationery or braked whenever necessary such as during maintenance, heavy winds such as typhoons etc.
Connection is taken out of generator by cables for connection to the grid power lines.
Since the generating power of a single unit is low compared to the power generating units of other types of power generating stations, many wind generators are erected in a group to get the total maximum generation. This group is known as wind form and installed in a large windy open area.
Construction details
General
Though the equipment is described here in general referring to some components, the individual construction of the wind generator itself may vary depending on the manufacturer and the technology advances involved. The terminology may also vary from country to country.
Nacelle
This is the housing on the top of the tower. This consists of the generator and allied equipment along with the required controls mounted rigidly on the base of the housing. The shaft protruding in the front has as attached propeller facing the wind to give a rotary motion to the generator.
The allied equipment consists of a brake, a speed control gear and a gear box as main components with necessary lubrication system and hydraulic controls. The propeller is always required to face the wind to get maximum generation. For this purpose a yaw motor is provided at the top. An anemometer is also provided for wind speed monitoring and required control of the propeller.
This nacelle is completely erected on the ground with a cover on and hoisted to the top of the tower and connected to it rigidly.
Tower
The tower is normally to a height of 24 M to 30 M for a wind form. The tower may be of lattice steel or of tubular construction for about 300 KW generator capacity. This tower has the provision for working people to go up the tower for inspection etc.
Propeller
The propeller is mounted on the main shaft and consists of three or two blades. Normally for the size indicated here, three blades are common. They are made of fiber glass reinforced FRP material or similar other material to make it light and long lasting.
Gear box
This is an essential requirement to match the AC power generating unit which normally would be of 1500 or 3000 rpm.
Brake system
This brake unit generally consists of a brake disk with pads operated hydraulically.
Generators
Normally induction type generators are used.
Capacitors
Capacitors are used to improve the power factor of the generation. These are kept in the control cabin down below with necessary cut in and cut out controls.
Control system
The present day trend is to have micro processor based system as compared to older relay system. This micro processor based system offers facility for simultaneous control of several parameters and also facility for remote operation.
Design parameters
Wind speed
Cut in speed
This is the minimum velocity at which the wind generator will start generating power. This wind velocity will normally be 3 to 5 meter per sec. or 10 to 18 KM per hour.
Rated speed
This is the speed at which the generator will generate its maximum out put. At lower wind speed the generation will be lower. The rated wind speed is generally 12 to 15 meter per sec. or 45 to 55 KM per hour.
Survival speed
This is the wind speed for which the machine, structure, blade etc. are designed. This will normally be about 50 to 60 meter per sec. or 180 to 200 KM per hour.
Cut off speed
This is the speed at which the generation will stop. This will be normally being 25 meter per sec or 75 KM per hour.
Power curve
This curve shows the out put of the machine at various wind speeds. This is a normally certified curve for the particular machine.
Pitch control
This is the control of the pitch of the propeller blades. Some manufacturers may supply wind generators with variable pitch or stall regulated ones. This pitch angle is changed by hydraulic controls to obtain maximum out put from the wind and to limit the out put from higher speed winds to the rated one.
Soft starting
Generally the induction type generators of the wind farms are connected to the grid by releasing the brake and allowing the propeller to rotate. When the speed of the same reaches to about 95% of the rated r p m, the supply is cut in through thyristors or resistance to limit the starting current below the full load current. Near about 99% of the speed the thyristor is shorted out and when the RPM crosses the rpm of the power frequency, the machine starts generating power. Soft starting is used to limit the starting current.
Grid connection
Generally the connection to the grid line is done at 11 KV.
The control cabin is provided on ground below. From the generator LT cables of sufficient capacity are brought to the control cabin. This cabin has LT gear, consisting of LT bus bars, and out going feeder controls along with necessary protection and control equipment.
Since the generating voltage is 380 V, a group of 3 to 4 generators are connected to a common step up 1000 KVA transformer with necessary protection. The transformer is connected on the 11 KV side by means of 11 KV cables to the grid. Metering equipment is provided on the out going side of the LT feeders to transformer. This is necessary to measure the out put of generation from the wind generators fed to the Grid.
Siting
This refers to selection of suitable site for location of wind form. The main evaluation criteria for assessment are listed here.
Wind resources
The annual mean wind speed should be preferably above 20 kmph for economic harnessing of wind power. Site specific information on wind speed, direction, frequency, shear, gusts and turbulence are to be obtained. The energy content normally should be more than 1500 kwh/m2/year. If it is a complex terrain then it requires micro level wind resource studies.
Land use
Assuming 200 KW generators with a rotor diameter(D) of 25 m, distance between rows 70 m and between pylons 5 D; assumed also rows perpendicular to wind direction. This works out to total area requirement of 10 ha/MW installed. However the land area required for foundations and roads etc. is only about 5 to 10% of the total. In other areas conjunctive use for grazing, agriculture, salt pans etc. may be provided.
Accessibility
Road accessibility is a must for movement of heavy wind generators and electrical equipment. Movement facility for civil construction materials is also necessary. Only for road construction the contours, topography, terrain and soil characteristics has to be considered.
Grid system
The grid system at the point at which these generators have to be connected must be a stable one and consumers preferably to be near the grid. The consumer distribution system should also be a stable one. Stability of grid implies that it should be free from frequent interruptions. The voltage and frequency variations should also be with in limits. These conditions will not disturb the operation of wind generators.
Normally a stable grid system with wind generators must fulfill the following technical requirements:
- frequency to be with in 48-52 Hs.
- voltage variations at the common coupling point to be +/- 15% even when the wind form is connected or disconnected.
-Voltage increase at each wind generator should be below +15% of the rated voltage.
-asymmetry should be within 15%
-short circuit level at the common coupling point should not be below 5 to 10 times maximum power output of wind form
-each wind generator to be provided with starting current limiting devises to limit it to 1.3 times rated current
-wind generators to be provided with a capacitor bank for full compensation of the idle running reactive power consumption.
Environmental requirement
Though wind forms are environmentally benign, the hazards to bird population and disruption of native bird habitants have been voiced. This is due to possibility of audible low frequency noise. These being minor problems can probably be taken care of by proper siting.
Wind mill
This was generally adopted in the olden days in remote places as isolated units. This mill directly transmits mechanical power with suitable mechanical linkage for operation of equipment such as water pumps. These mills are becoming obsolete now a days since electricity is supplied even to far remote places. The operation and maintenance of electrical pumps and similar equipment are found to be much more convenient and cheaper.
External links
http://www.grist.org/advice/ask/2005/01/03/umbra-wind/index.html
http://www.cus.net/renewableenergy/subcats/wind/wind.html
http://eereweb.ee.doe.gov/windandhydro/
http://en.wikipedia.org/wiki/Wind_farm
"Wind Energy Reference Manual Part 4: Electricity (http://www.windpower.org/en/stat/unitsac.htm)". Danish Wind Industry Association, 2003.
"AC-DC: Inside the AC Generator (http://www.pbs.org/wgbh/amex/edison/sfeature/acdc_insideacgenerator.html)". Edison's Miracle of Light, American Experience. (PBS)