Atlantic Orient Corporation’s 15/50 Wind Turbine
Following are brief descriptions of the principal parts of the AOC 15/50 wind turbine. Jump or scroll down to view a description of how the wind turbine and control system act together.
The rotor consists of the three blades with their tip ‘air’ brakes and the hub to which they are bolted. The tip brakes are held with an electromagnetic coil which is de-energized to release the tip brake plates which will slow the rotor down to a safe speed in any wind condition should the wind turbine become disconnected from the KEA grid. The tip brakes are held in running position loosely to the ends of the blades by a small spring when the magnets are de-energized and the rotor is stopped or turning slowly. Power to the tip brakes is provided through a rotary transformer located between the rotor hub and the gearbox. The tip brakes will release on their own to slow the wind turbine should there be a controller failure.
The rotor blades are shaped like the wing of an aircraft and will generate power if the wind causes them to rotate at about 64 rpm. If the wind speed becomes very high, the blades become inefficient and stall which causes the power to decrease. The control system senses high wind conditions and stops the wind turbine to prevent excessive mechanical and structural loads. A pair of anemometers on the tower measures the wind speed for the controller. The controller automatically uses the highest wind speed from the two sensors.
The gearbox supports the rotor on its low speed shaft and the generator on its high-speed shaft. The gearbox causes the generator to turn at 1800 rpm when the rotor turns at 63.7 rpm. The wind turbine generates power if the generator turns above 1800 rpm and consumes power like a motor if the wind is not able to drive it above this speed. The control system senses rotor speed very accurately and stops the wind turbine if the wind speed drops much below 1800 rpm.
The generator in essentially a 1750 rpm induction motor which when driven at a speed greater than 1800 rpm becomes a generator capable of producing as much power at 1850 rpm as it would consume at 1750 rpm as a motor.
A parking brake is located on the back of the generator which is used to hold the wind turbine rotor stationary when the turbine is turned off or when the wind sped is not high enough to operate. A manual release for the brake for use only during servicing is located under the small cover on the brake enclosure. There is a short time delay of about 3 seconds for the parking brake to operate after it is de-energized. A generator speed sensor is located under the parking brake cover, which is used to measure turbine speed for the controller. Turbine speed is a direct indicator of power produced by the turbine and is used instead of a power (kilowatt)meter.
The gearbox is supported on the main tower top casting. A large diameter bearing allows the gearbox to turn on the vertical yaw axis. The wind pushing on the rotor blades causes a force, which will position the blades on the down-wind side of the tower. A locking pin is located under the casting that enables locking of the yaw motion for safety reasons during servicing.
A pair of long flexible cables droop from the wind turbine to a junction box near the tower bottom. These cables carry the generator power and also are used to communicate signals to and from the parking brake, tip brakes, generator speed sensor and other features such as a generator heater, generator over-temperature switch and vibration safety switch. The last is not yet used on this wind turbine.
The controller contains the control switches, test switches, control relays, a main contactor and a Programmable Logic Controller (PLC). The PLC replaces the many relays and timers that once were used to control of machinery like wind turbines. The PLC measures wind speed and rotor speed and also sense the position of the control switches. A computer program in the PLC then makes decisions about which control relays to operate in order to place the machine in operation or to stop it. The controller consists of three cabinets. The smaller tow cabinets that have no external control switches contain the dynamic brake.
The dynamic brake is an electrical circuit containing capacitors and resistor which when switched across the generator immediately following disconnection from the grid, will cause a large electrical load which helps to stop the machine. In normal stopping operation all three brakes are applied at the same time.
The following is a simplified description of how the wind turbine and control system act together. The Turbine Switch should be on ON, all faults are cleared and the brake cools down light out. We assume that the wind has just commenced to increase and will increase to about 35 mph for a few hours and then decrease to below 5 mph. should the wind speed continue to increase above 50 mph the controller action is also described.
When the controller senses the wind speed above 11mph the controls cause the parking brake to be released, the tip brakes to be held and the dynamic brake to be disconnected. The wind should then cause the rotor to turn and to increase in speed until the generator turns at 1800 rpm. At this instant the controller connects the generator to the grid.
As the wind speed increases from 11 mph to 25 mph the power generated will continue to increase to about 50 kW. As the wind speed continues to increase to 30 mph the power will rise to about 66kW. Above 30 mph the power will begin to fall slightly as the rotor blades stall. When the wind speed reaches 50 mph the controller will shut the wind turbine down. This is a “High Wind Shutdown”.
If the rotor speed falls below 1790 rpm for 10 seconds or the wind speed falls below 8 mph the controller will shut down the wind turbine. These are “Under Speed” and “Low Wind” shutdowns respectfully.
If the controller signals the wind turbine to shut down it will not attempt to start again until 15 minutes have passed. This is to allow the dynamic brake resistors and parking brake to cool down.
If when operating in high winds the rotor speed exceeds 1860 rpm for 60 seconds the wind turbine will be stopped. This is an “Over Power” shutdown.
Following Low Wind Speed, High Wind Speed and Over Power shutdown the controller will enable the wind turbine to start again after the 15 minute brake cooling delay if wind speed again is between 11 and 50 mph.
Should there be a cause for the wind turbine to exceed 1875 rpm the controller will initiate an “Over Speed Stop.” Following an Over Speed Stop the controller will not allow re start until it is reset. An investigation of the reason for the Over Speed Stop should be made before operating the wind turbine again.
Atlantic Orient Corporation P.O. Box 1097 Farrell Farm Rd. Rt. 5N Norwich, VT, 05055