Asynchronous Generators

            The type of generator used in a Gulf Stream turbine will almost certainly be the asynchronous (cage wound) generator, also called induction generator.  This type of generator is not widely used outside the wind turbine industry.  The curious thing about the asynchronous generator is that it was originally designed as an electric motor.  In fact, one third of the world’s electricity consumption is used for running these motors.  One reason for choosing this type of generator is that it is very reliable and tends to be comparatively inexpensive.  These generators also have some mechanical properties that are useful, such as generator slip and certain overload capability.  In order for the asynchronous generator to generate power, its stator must be magnetized by the AC from the grid and it is the cycles of the stator’s field current that determines the generator’s rotational speed. 

If we mechanically spin the shaft of an asynchronous generator so that its rotor spins at exactly the synchronous speed of the grid, nothing will happen.  This is because the magnetic field would be rotating at exactly the same speed as the rotor, thereby producing no electricity by induction in the rotor to interact with the stator.  However, if we apply more torque to increase the shaft’s rotational speed, the rotor moves a little bit faster than the rotating magnetic field being produced by the stator, which will induce a strong current in the rotor.  The harder the shaft is cranked, the more electricity produced.  The difference between the rotational speed at peak power and at idle is very small, only about one percent.  If the shaft of an asynchronous generator is cranked at slower rotational speeds than the synchronous speed of the grid, the generator will operate as a motor, pulling power from the grid to increase the shaft’s spin to the synchronous speed.  Because these generators can turn into motors if insufficient spin is applied to the shafts, they must be disconnected from the grid when their rotational speeds are not fast enough to produce usable power.

Although the generators and rotors can be any size as long as they are matched to each other and to the water velocity, we should perhaps make a few assumptions to provide examples of what is possible.  If we assume that each turbine has a rated capacity of 600 kW, the generators used in the Gulf Stream Turbine could be virtually identical to those used in the Nordex N43/600 kW wind-driven system.  These machines use generators of two types.  They use a single wound 4-poler asynchronous 600 kW generator, or a double wound 4/6-pole asynchronous generator.  Although the Nordex literature states that the synchronous speeds are 1,500 rpm for their 4-pole generators and 1,000 rpm for their 6-pole generators, these speed are based on the 50 Hz current that is used in Europe.  If these same generators were hooked into US grids where the AC cycle is at 60 Hz, the more rapid excitation rates in the stators would increase the synchronous speed of the armatures to 1,800 rpm for the 4-pole and to 1,200 rpm for the 6-pole generators.

The speed of the rotors and the number of magnetic lines per pole fix the magnitude of the voltage generated.  The more poles there are, the more lines of magnetic force.  This also means that the more poles there are, the lower the rpm required to produce the same amount of power at the same frequency.  The following formula can be used to calculate the rpm required to generate electricity of with differing numbers of poles: 

rpm  =  (C x 60) / (  P / 2)                    C = frequency in cycles per second 

                                                            P = the number of poles (an even number)

Conventional wind-powered machines have compact generators that have 4 or 6 poles and use a rotor-gearbox-generator drive train.  The Lagerway wind machines, made in Australia, use large diameter ring generators with more than 80 poles and no gearbox.  Rather than using a generator similar to the Nordex 600 kW generators, an adaptation of the Lagerway ring generator could be used to reduce – or, with enough poles, even eliminate the requirements for a gearbox.  A disadvantage of the Lagerway-type ring generators is that the nacelles containing them must have much larger diameters and they would be more expensive.  Because these larger streamlined nacelles would otherwise add displacement below the center of gravity where it would decrease the unit’s stability, that displacement should be reduced by flooding ballast compartments inside the nacelles.  Drawing of the how the Lagerway-type generators can be mounted are in the patent.  


Power Control for Asynchronous Generators

            A problem can be caused by the water having too much velocity for what a turbine power plant, equipped with asynchronous generators, can safely handle.  If the generators should be overpowered by an extremely strong current so that it is forced to rotate too fast to remain in sync with the grid’s 60-cycle current, serious damage can occur. 

As the water current velocity increases, the generator’s rotational speed can be controlled to some degree by increasing the strength of the electrical current in the stator to increase the strength of the magnetic field.  Beyond that, there could be a need for limiting the power input to the generator.  Because wind turbines have this problem, we should look at the systems that they use.  They use both pitch controlled and stall controlled systems.  

            On a pitch controlled wind turbine the turbine’s electronic controller checks the power output of the turbine several times per second.  When the power output becomes too high, it sends an order to the blade pitch mechanism, which immediately turns the rotor blades slightly out of the wind.  Conversely, the blades are turned back into the wind whenever the wind drops.  On a pitch controlled wind turbine, a computer changes the pitch of the blades a few degrees every time the wind changes to keep the rotor blades at the optimum angle to maximize output for all wind speeds.  Because of its complexity, the pitch-controlled system might not be the best choice for the Gulf Stream turbines.

            The stall controlled wind turbines have rotor blades that are bolted onto the hub at a fixed angle.  The geometry of the rotor blade profile has been aerodynamically designed to ensure that the moment the wind speed becomes excessive, it creates turbulence on the side of the rotor blade that is away from the wind.  This produces a stall that prevents the lifting force of the rotor blade from acting on the rotor.  As the wind speed increases, the angle of attack of the rotor blade increases until at some point the blade starts to stall.  On the stall corrected wind turbines, the blade twists slightly along its longitudinal axis.  This is partly to ensure that the rotor blade stalls gradually rather than abruptly when the wind speed reaches its critical value.  The stall control systems present a very complex aerodynamic design problem.  Around two thirds of the wind turbines currently being installed in the world are of this type.  Because the stall-controlled system has no moving parts or complicated control system to break or malfunction, it should be considered for those submersible turbines that will be placed where the current’s velocity may occasionally become excessive for the generators.  A reason not to use the stall corrected system is that, as the blades stall, there would be a substantial increase in drag, which would pull the machines down to greater depths and could put an excessive strain on the anchoring system.

            Another method for controlling the power input to the asynchronous generator would be to have a fluid coupling or a magneto-rheological fluid clutch on the high-speed shaft, between the gearbox and the generator, that would allow some slippage, with no mechanical wear, should the power input become excessive.  This slippage can be controlled by the power output of the generator.  Because the slippage would generate heat, a system to cool the fluid would be required.

            A rather drastic approach to preventing damage to generating units in unusually strong currents would be to simply shut off the current from the grid to the generator’s stator.  Cutting off this current would eliminate the magnetic forces that induce the electricity in the spinning armature.  The turbine and generator could then spin harmlessly at high speeds while producing no power.

            Because of the relative uniformity of a current’s velocities, it is possible that no power-limiting device will be required. 

The Variable-Speed Generators

            Another approach to solving the problems caused by variations in current strength is to use variable-speed generators that have indirect hookups to the grid.  Instead of using an asynchronous generator that has its rotational speed controlled by the 60 Hz cycles of the grid, the variable-speed generator runs on its own, on a separate mini AC-grid.  This grid is controlled electronically (using an inverter) so that the frequency of the alternating current in the stator of the generator may be varied.  In this way it is possible to run the turbine and generator at variable rotational speeds.  The turbine generates alternating current at the variable frequency applied to the stator; then the current is converted electronically to direct current; then back again to AC with the frequency in sync with the grid.  Theoretically, the variable speed generator offers an advantage in terms of annual production for the wind-powered turbines because it is possible for them to run at an optimal rotational speed, depending on the wind velocity.  Using the variable-speed generator for the water-powered turbines would avoid those problems that could otherwise be caused by an excessively strong current overpowering an asynchronous generator as well as permit the production of usable power from rotation speeds that are too slow for the asynchronous generator.  A negative for the variable-speed generator is that they have more complicated electronics that could cause service problems.