Prime-mover dynamo plants – Electric control – Fluid-current motors
Reexamination Certificate
2002-01-23
2004-05-18
Le, Dang (Department: 2834)
Prime-mover dynamo plants
Electric control
Fluid-current motors
C290S043000, C290S005000
Reexamination Certificate
active
06737757
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns a method for operating windmills where a primary generator is driven by the windmill rotor, possibly with a gear mechanism, with constant or approximately constant rotational speed. The invention also concerns a windmill where a primary generator is driven by the rotor of the windmill, possibly with a gear mechanism, with constant or approximately constant rotational speed.
It is known that certain optional benefits may be achieved in windmills if one may establish operation with variable rpm.
Many modern types of windmills are provided with a directly network connected asynchronous generator. This kind of generator has significant advantages. Even if certain adjustments in the winding have been made, the directly network connected asynchronous generator is in principle just a directly network connected asynchronous motor driven at an supersynchronous rpm by an external energy source. An asynchronous motor with short-circuited rotor is the most simple and robust form of electric motor, and the asynchronous generator has the same advantages. The only wear parts are constituted by the bearings. Large production numbers on the motor side implies that the price per kW is the lowest possible.
The directly network connected asynchronous generator with short-circuited rotor has, however, also significant drawbacks in connection with windmill operation. The drawbacks are connected to the largely constant rpm for this kind of generator. By larger power outputs the generator may only be made with a slip exceeding 1% with difficulty since the power loss deposited in the rotor in principle is proportional to the slip. If the slip exceed the normal limit of the 1%, the rotor losses become so great that thermal problems may arise. With a slip of 1%, or less, the rpm of the windmill remains largely constant.
A largely constant rpm is a prerequisite for one of the two normal forms of power control, stall regulation. While it simultaneously is a prerequisite for the control, too small slip may, on the other hand, give rise to problems with power variations as a result of torsion oscillations in the transmission system. A small slip means small dampening in the generator, and therefore continuous oscillations of a certain, not insignificant, magnitude may occur.
By stall regulation the advantages by the largely constant rpm will normally exceed the disadvantages. Otherwise with the other of the two normal kinds of power control, pitch regulation, where it gives rise to considerable problems. Pitch regulation is based on mechanically setting the wings to another pitch angle on the rotor hub when the power deviates from the desired power. If the rotor absorbs other power from the wind than absorbed by the generator, the generator will accelerate until there is balance again between absorbed and yielded power. If the generator slip is small, only a slight acceleration is required for the generator to yield a significantly different power. The time for the control system to adjust the wings therefore becomes very short, and in practice pitch regulated mills with directly network connected asynchronous generator have great power variations due to variations in the wind speed.
The directly network connected asynchronous generator also has certain considerable deficiencies in connection with network quality. First, consideration to voltage variations in the net requires the coupling in of the generator to occur with power electronics since the coupling in with traditional contactors will imply large voltage variations. Second, the asynchronous generator has a not insignificant consumption of reactive power for magnetising. Usually it is necessary to provide a windmill having a directly network connected asynchronous generator with phase compensation, typically in the form of a capacitor battery.
The problem with the reactive power consumption may be solved in principle by using a directly network connected synchronous generator. This type of generator has its own technical drawbacks, including a winded rotor. On the other hand the net conditions are good. If the requirements to the net conditions were great, it could be argued that the drawbacks of the synchronous generator were acceptable. The reason that this type of generator cannot be used at all in a directly network connected version without special measures is that the slip of the synchronous generator is 0. The above mentioned drawbacks by the asynchronous generator with small slip assume their most extreme form in the directly network connected synchronous generator, and operation with 0 slip is practically impossible because of power variations. The synchronous generator may only be used in direct network connection if a slip between gear and generator is established in other ways. Such a slip may e.g. be provided with a hydraulic coupling. However, it is difficult to achieve more than a few percent slip in this way, and normally it will not be sufficient to ensure a completely satisfactory regulation.
Greater slip may be achieved by means of an electric eddy current coupling. If such a coupling is provided with adjustable magnetisation, the slip may be regulated and the coupling may be adjusted so that the torque from a certain slip becomes e.g. a hyperbolic function of the rpm, whereby the output power may be kept at nominal power. Though an eddy current coupling thus gives the necessary regulating possibility, it has, however, some very significant drawbacks. The most important drawback is probably that the power from the slip is deposited as heat in the coupling. If the windmill has e.g. a nominal power of 1 MW and if a slip of 10% is desired, up to 100 kW will be deposited as heat in the coupling. In practice, this implies such requirements to size and cooling of the coupling that this solution is not economically feasible. A secondary drawback is that a certain slip is necessary also by part load since otherwise a synchronous generator will cause power fluctuations. Also, in this range of operation the slip power will be deposited as heat. While the loss by operation at nominal power may be said to be unimportant from the view of efficiency since ample input power is available and the loss thus only has influence on the dimensioning and cooling of the coupling, by part load the loss is clearly unfavourable from an efficiency view. At wind speeds by which the windmill does not yield maximum power it is important that the efficiency is as good as possible, and a slip occurring as waste heat is only a disadvantage here.
The deficiencies inextricably associated with the directly network connected asynchronous generator with short-circuited rotor have been known in general for a long time. For stall regulated windmills where the power regulation presupposes a roughly constant rpm, the asynchronous generator is normally considered to be a solution close to optimum, and the effort has therefore been concentrated on relieving the problems connected thereto. Methods for adjusting the slip in the making of generator itself have been developed so that the specifications of the generator may be optimised for the dynamic properties of the actual type of windmill. Electronic coupling systems have been developed, and both fixed and adjustable phase compensating systems may be supplied as standard.
The situation is different for pitch-regulated windmills. The drawbacks associated with operation by pitch-regulation and small slip have appeared to be significant, and largely all commercial windmills with pitch regulation by now have some form of variable rpm.
The variable rpm may be established in different ways.
In a simple embodiment, the directly network connected asynchronous generator with short-circuit rotor may be substituted by a likewise directly network asynchronous generator with winded rotor, slip rings and external resistors. In this configuration the greater part of the rotor loss is deposited in the external resistors, and the slip is proportional with the rotor power. An arbitrarily lar
Bonus Energy A/S
Creighton Wray James
Le Dang
Mohandesi Iraj A.
Narasimhan Meera P.
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