GB2429342A

GB2429342A - Turbine powered electricity generation apparatus

Info

Publication number: GB2429342A
Application number: GB0516892A
Authority: GB
Inventors: Frank Moeller
Original assignee: Nexxtdrive Ltd; Drivetec UK Ltd
Current assignee: Nexxtdrive Ltd; Drivetec UK Ltd
Priority date: 2005-08-17
Filing date: 2005-08-17
Publication date: 2007-02-21
Also published as: GB0516892D0

Abstract

Turbine powered electricity generation apparatus includes a turbine (2) and a synchronous AC generator (26) connected thereto via a transmission system (10) comprising a three branch epicyclic gearset. A first shaft (8) of the gearset is connected to the turbine (2), a second shaft (38) is connected to the generator (26) and a third shaft (16) is connected to the rotor of an electric motor/generator (24). The electrical connections of the stator of the motor/generator (24) are connected to the electrical output connections of the generator (26) via a controller (28), which is arranged to control the flow of electrical power to and from the motor/generator (24) such that the rate of rise of torque applied to the electric generator (26) does not exceed a predetermined value. The apparatus may be used in a wind turbine, a water-powered turbine, or may be applied to an electrically powered fluid-impelling apparatus with the synchronous operator (26) replaced by a synchronous motor.

Description

I

TURBINE POWERED ELECTRICITY GENERATION APPARATUS

The present invention relates to turbine powered electricity generation apparatus and is particularly, though not exclusively, concerned with wind turbine generators. The invention is concerned in particular with the transmission system connecting the turbine to the generator.

Wind turbine generators are rapidly becoming increasingly popular as the search intensifies for practicable and cost-effective sources of electrical power that do not involve the burning of the dwindling stocks of fossil fuels.

However, existing wind turbine generators suffer from a number of problems.

Earlier, relatively large wind turbine generators included a synchronous AC generator coupled to a turbine via a gearbox with a fixed and high transmission ratio. The generator was driven at a fixed and relatively high speed, e.g. 1,000, 1,500 or 3,000 RPM, and its output was locked as regards frequency and phase to those of the mains supply. If the wind speed increased gradually, then the torque applied to the generator also increased progressively and the power output of the generator thus increased progressively also. However, if the wind speed increased suddenly, e.g. as a result of the wind being gusty, the torque applied to the generator and its power output also increased abruptly and this resulted in "torque wind-up" of the transmission, that is to say all of the components of the transmission being subjected to a shock torsional loading and this can result in vibration and possible failure.

It was appreciated that the average overall efficiency of the installation could be improved by permitting the turbine to run at a variable speed depending on the wind strength. However, with a transmission system of fixed ratio this meant that the generator also ran at variable speeds and this meant that the locking in of the generator to the frequency of the mains supply was no longer possible.

The generators used were therefore of variable speed type producing AC or DC and these could only be coupled to the mains supply or grid via expensive power electronics, i.e. inverters and/or converters. However, due to the fact that the inertia of the relatively slowly rotating turbine is substantially less than that of the relatively rapidly rotating generator, a sudden increase in the speed of the wind again results in "torsional wind up" of the transmission, that is to say a nearly instantaneous increase in the output torque of the turbine results in only a gradual increase in the speed of the generator due to its high inertia. All the components of the transmission thus again become very highly loaded, resulting in the problems referred to above. It is believed that up to 50% of wind turbine generators fail within two years of installation, many of them precisely for this reason.

Wind turbine generators are rapidly becoming larger and generators with a capacity of up to 2.5 MW are currently being installed and it is anticipated that capacities of up to 5MW or more will be installed in the relatively near future.

Such large generators frequently include an automatic pitch control mechanism.

Such a mechanism operates in response to the wind speed to alter the pitch of the turbine blades to optimise the efficiency of the turbine that is to say to maximise its output torque. Thus at low wind speeds, the pitch of the blades is relatively shallow and the torque output is relatively low and at high wind speeds, the pitch of the blades is relatively steep and the torque output is relatively high. It might be thought that such a pitch control mechanism could be operated to overcome the problem of torque wind-up, referred to above, by immediately adjusting the pitch of the blades, when a wind gust occurs, to an angle which is significantly less than optimum, whereby the torque output of the turbine does not abruptly increase but instead increases only slightly. If the pitch control mechanism were then operated to make the operation of the turbine increasingly close to the optimum, the torque applied to the generator could be controlled to rise only gradually, that is to say at a rate less than the threshold rate at which the problems of torque wind-up and vibration occur.

Unfortunately, the pitch control mechanism used with large turbines is itself so large and thus slow that it takes an appreciable time for it to react to a change in wind speed and it is in practice incapable of responding sufficiently rapidly to gusty conditions. The pitch control mechanism is therefore not capable of being controlled to overcome the problem discussed above.

It is, therefore, the object of the invention to overcome the problems described above and to provide a turbine powered electricity generator which is both relatively cheap and is not subject to premature failure and does not require the usual power electronics, which are expensive and inherently associated with a reduction in efficiency, to couple the output of the generator to the mains grid.

According to the present invention turbine powered electricity generation apparatus includes a turbine and a synchronous electric generator connected thereto by a transmission system comprising a three branch epicyclic gearset, a first shaft of which is connected to the turbine, a second shaft of which is connected to the generator and a third shaft of which is connected to the rotor of an electric motor/generator, the electrical connections of the stator of which are connected to the electrical output connections of the generator via control means arranged to control the flow of electrical power to and from the motor/generator such that the torque applied to the electric generator does not increase at a rate above a predetermined value.

Thus in the generation apparatus in accordance with the present invention the transmission system includes a three branch epicyclic gearset, of which one shaft is connected to a motor/generator. The control means adjusts the flow of electrical power between the output of the generator and the stator connections of the motor/generator to vary the amount of torque applied to the electric generator and ensure that the rate at which the applied torque changes does not exceed a predetermined value, that is to say a value at which it is determined the transmission system is likely to be subject to the problems discussed above.

This means in practice that there will be a torque sensor, that is to say a sensor which directly or indirectly measures the torque applied to the generator. If there should be a sudden increase in the wind speed, the output torque from the turbine will increase rapidly and this increase is sensed by the sensor. The signal from the sensor is supplied to the control means which operates to direct an increased amount of electrical power from the motor/generator, if it is operating as a generator, to the output of the generator or alternatively a reduced amount of electrical power from the output of the generator to the motor/generator, if it is operating as a motor. The majority of the increased torque produced by the turbine is thus initially absorbed by the motor/generator and does not result in a rapid rate of rise of torque at the input of the generator.

The electrical outputs of the generator and the motor/generator are therefore additive and the motor/generator may provide up to say 20% of the total electrical output of the apparatus, though it will be appreciated that if the motor/generator is operating as a motor, its electrical output is negative, that is to say it is an absorber of electrical power but a producer of mechanical power, which is applied to the input of the synchronous generator. If, on the other hand, the wind speed should suddenly drop, the torque output from the turbine will decrease. This decrease is sensed by the torque sensor which operates to direct power or an increased amount of power from the output of the generator to the motor/generator, if it is operating as a motor, or alternatively to direct a reduced amount of electrical power to the output of the generator from the motor/generator, if it is operating as a generator. The torque produced by the motor/generator, which will be negative if it is operating as a generator, and the torque produced by the turbine are additive and the control means is arranged such that the sum of these two torques and thus the input torque to the generator, does not fall at a rate below a predetermined rate. The generator can therefore run at constant speed and can remain locked to the frequency of the mains network despite variation in the wind speed. The fact that variations in torque are effectively smoothed out by the motor generator and are not applied to the generator means that the transmission system is not subject to the "torsional wind up" referred to above and the service life of the apparatus is thereby extended.

The torque sensor may take various forms but it is preferred that it comprises means connected to or forming part of the control means for detecting the magnitude of the current flowing in the stator of windings of the generator or the motor/generator. Thus a slight increase in the torque applied to the generator will result in a slight shift of its phase relative to that of the mains network and this slight shift results in a substantial increase in current flowing in the stator windings and this increase can readily be detected and measured.

Similarly, an increase in the torque output from the turbine will result in an increase of torque applied to the motor/generator and this again results in an increase in the current flow in the stator of windings of the motor/generator which may be readily detected and measured. An increase in current in the stator of the motor/generator is therefore indicative of an increase in the torque applied to the generator. Accordingly, although the torque applied to the generator will vary with variations in wind speed, the rate at which it varies is not permitted to exceed a predetermined, safe level and it is possible to maintain the generator frequency and phase locked to those of the mains network.

Precisely the same problems can also arise in certain types of electrically powered fluid impelling apparatus. Thus, for instance, electrically driven marine propellers can be subjected to substantially varying resistance forces, e.g. due to water currents or waves, and this rapidly changing resistance can result in a torsional wind-up of the transmission system in precisely the manner described above. The invention is equally applicable to solving this problem with driven impellers, such as propellers, fan wheels, centrifugal impellers and the like intended for use with gaseous or liquid fluids, particularly air or water and thus according to a further aspect of the present invention there is provided electrically powered fluid impelling apparatus including a fluid impeller and synchronous electric motor connected thereto by a transmission system comprising a three branch epicyclic gearset, a first shaft of which is connected to the impeller, a second shaft of which is connected to the motor and a third shaft of which is connected to the rotor of an electric motor/generator, the electrical connections of the stator of which are connected to the electrical input connections of the motor via control means arranged to control the flow of electrical power to and from the motor/generator such that the rate of rise of the torque applied to the impeller does not exceed a predetermined value.

Operation of this second aspect of the present invention is precisely the same as that of the first aspect. Thus if a marine propeller driven by a synchronous electric motor should encounter a large wave or current, the suddenly increased resistance to its movement could result in a substantial increase in torque on the components of the transmission system and thus in vibration and ultimately failure. However, the controller operates to direct an increased amount of electrical power from the motor/generator to the input connections of the motor and thus effectively absorbs most of the additional torque and this limits its rate of rise.

The epicyclic gearset may take various forms but it is of course desired that it has a relatively high transmission ratio because the speed of the turbine or impeller will in practice be relatively low whilst it is desired that the speed of the generator or motor is relatively high, that is to say 1,000 RPM or above, so that its overall size can be maintained relatively small. It is therefore preferred that the gearset comprises a relatively large first sun gear, which is in mesh with one or more relatively small first planet gears, each of which is connected to rotate with a respective relatively large second planet gear about a respective planet shaft, the second planet gears being in mesh with a relatively small second sun gear, which is connected to rotate with the second shaft. The second sun gear will thus be connected to the generator or motor and the gearset will include one or more pairs of first and second planet gears of different sizes connected to rotate together. In practice, each pair of associated planet gears may constitute a single "stepped planet", that is to say a single gear wheel with two portions of different diameter.

The sets of associated planet wheels will include a common carrier and the turbine or impeller may be connected to either the common carrier or to the first sun gear but it is preferred that the first shaft is connected to rotate the first sun gear and the third shaft is connected to the planet carrier common to the first and second planet gears.

Wind turbines of different sizes rotate at different speeds and whilst a relatively small turbine, with a power output of, say, 3 to 10 kW, may rotate at around 200 RPM, a large turbine with an output in excess of 1 MW, may rotate at a speed of only between 10 and 20 RPM. This latter speed is slower than desired if the generator is to be rotated at an appropriately high speed and in this event it is convenient for the first shaft to be connected to the turbine via step up gearing, whereby it will rotate faster than the turbine, e.g. 10 times faster.

Similarly, it is desirable for the motor/generator to be connected to the third shaft via step up gearing so that it will rotate faster than the third shaft. This will enable the motor/generator to be relatively small thus to be able to react extremely rapidly to any changes in the output torque from the turbine.

The maximum benefit to efficiency from the presence of the motor/generator will be obtained if it is so constructed that the maximum electrical power which it can generate, when operating as a generator, is substantially equal to the maximum electrical power which it can consume, when operating as a motor.

The transmission system of the turbine in accordance with the invention can respond very rapidly to changes in wind speed in order to ensure that the torque applied to the generator does not change at a rate greater than a predetermined "safe rate". The motor/generator will thus smooth out the effects caused by wind gusts, that is to say relatively short-lived changes in wind speed.

However, the wind may of course substantially increase in speed for a prolonged period of time and it is therefore desirable, particularly with a wind gcnerator of relatively large size, for the turbine to have blades of variable pitch type in order to maximise the output of the generation system.

It would be possible for the control means to be set such that the motor generator always operates as a motor or always operates as a generator but this would not maximise the benefits obtainable from it and it is preferred that the turbine or motor has a range of output torques, outside which it will in practice not be used, and that the controller is so arranged that, when the turbine or motor is producing a torque at the lower end of this range, it transmits electrical power from the generator to the motor/generator, which thus operates as a motor and serves to increase the torque applied to the second shaft and, as the torque output of the turbine increases, the electrical power transmitted to the motor/generator progressively decreases until it reaches zero, whereafter the motor/generator operates as a generator and contributes to the electrical power output of the generation apparatus or returns electrical power to the input of the motor.

Although many wind turbines will start to rotate from rest once the wind speed reachcs a certain threshold value, others will not and some means have to be provided to start rotation of such turbines. There is, however, no such problem with the turbine generator in accordance with the present invention because the motor/generator can simply be operated as a motor, powered by the mains power supply, to start rotation of the turbine. Once the rotary speed of the turbine has reached a minimum value, it will be powered by the wind, provided that the wind speed is sufficient, and operation of the motor/generator as a motor can then be terminated.

Further features and details of the invention will be apparent from the following description of one specific embodiment of wind turbine generator which is given by way of example only with reference to the single accompanying highly diagrammatic drawing.

The wind turbine generator illustrated in the drawing is of large capacity with a maximum output of, say, 2.5 MW. It includes a wind turbine 2 carried by a shaft 4 and mounted for rotation about a horizontal axis. It also includes a synchronous AC generator 26, which is connected via an isolator 30 to an AC bus 34, which is typically connected to the mains power supply. The turbine 2 is connected to the generator 26 by means of a transmission system comprising a three branch epicyclic gearset accommodated within an outer casing 10. The gearset includes a first relatively large sun gear 12, which is carried by a shaft 8 rotatably mounted in bearings. The sun gear 12 is in mesh with a number of relatively small first planet gears 14, which are fixedly carried on respective planet shafts 20 which are mounted to rotate in bearings carried by a planet carrier 16. Also fixedly carried by the shafts 20 is a number of relatively large second planet gears 18. The first and second planet gears 14,18 are therefore associated in pairs fixedly connected to a respective shaft 20, which means that each pair rotates as a solid body. In this case, the shafts 20 rotate in bearings but it would be equally possible for the planet gears 14 and 18 to be connected together in pairs and mounted to rotate on stationary planet shafts 20. The second planet gears 18 are in mesh with a relatively small second sun gear 36 carried by a shaft 38, which constitutes the input shaft to the generator 26. The carrier 16 is connected to rotate with a motor/generator 24 and in this case the connection is by way of a carrier gear 40, which is connected to the carrier 16 and is in mesh with a pinion gear 42. The shaft carrying the pinion gear 42 constitutes the input shaft to step up gearing 22, the output shaft of which constitutes the input shaft to the motor/generator 24. The purpose of the step up gearing 22 is to permit the motor/generator to rotate substantially faster than the carrier 16, so as to permit its size to be relatively small, whereby it has a low inertia. The stator windings of the motor/generator 24 are connected to a controller 28, which is connected in turn via a further isolator 32 to the AC bus 34. The controller 28 includes means for sensing the magnitude of the current flowing in the stator windings of the motor/generator and means for controlling the operation of the controller in dependence on the change in the measured current, as will be discussed below.

The synchronous generator 26 will need to be driven at a relatively high speed, e.g. 1,000 RPM or more but the turbines of such large wind turbine generators rotate at only a very low speed of e.g. between 14RPM and 20 RPM. It is not readily practicable for a three branch epicyclic gearset to provide a step up ratio of the magnitude that would be required by itself and the shaft 4 carrying the turbine 2 is therefore connected to the input shaft 8 of the epicyclic gearset via further step up gearing 6 with a step up ratio of e.g. 10 so that the speed of the input shaft 8 is typically between 140 RPM and 200 RPM. With relatively large wind turbines such as that of the present embodiment, it is desirable for the turbine blades to be of variable pitch type but with smaller wind turbines this may not be necessary.

In use, the turbine 2 is rotated by the wind and this rotation is transmitted through the epicyclic gearset to the generator 26 which produces a three-phase AC output in phase with the mains supply to which the AC bus 34 is connected.

If the wind should suddenly increase in strength, the relatively low inertia of the turbine 2 means that it very rapidly produces an increased output torque. This increasing torque acts on both the generator 26 and the motor/generator 24. The increasing torque on the generator will tend to produce a very slight shift in the phase of the generator rotor with respect to the phase of the mains supply and this change in phase results in the flow of substantial currents which counteract the change. The increasing torque on the motor/generator results in a change in the current in the stator windings and this is immediately sensed by the controller 28 which operates to direct an increased amount of electrical power to the AC bus 34. The motor/generator has a low inertia and thus rapidly changes speed and this increase in speed prevents the torque applied to the generator 26 increasing at a significant rate. Thereafter, the controller operates progressively to reduce the amount of power directed from the motor/generator to the AC bus. The torque applied to the input of the generator thus increases progressively and the controller is programmed to ensure that the rate of rise of the applied torque does not exceed a predetermined level. This explanation assumes that the motor/generator is operating as a generator, which is the case when the wind speed is above a predetermined threshold value. If, however, the wind speed is below this threshold value, the controller is programmed to ensure that the motor/generator is operating as a motor and is producing a torque, which is applied to the input of the generator in addition to the torque produced by the turbine. If the wind speed suddenly increases when the motor/generator is operating as a motor, the controller is programmed to reduce the amount of electric power supplied to the motor/generator, whereby its torque output decreases. The motor/generator therefore again acts to limit the rate at which the torque applied to the generator can increase. Put in other words, the increased torque and power produced by the turbine when the wind strength increases is initially largely absorbed by the motor/generator and the controller ensures that the torque applied to the generator 26 increases but only at a "safe" rate. Thus if the motor/generator is operating as a generator, the electrical output of the generation apparatus is produced not only by the generator 26 but also, to a lesser extent, by the motor generator 24. The output of the motor generator may be up to 20% and is more preferably between 10% and 15% of that of the generator 26. The turbine also includes a conventional pitch control mechanism and a change in the wind speed is sensed and the mechanism operated to adjust the pitch to optimise the efficiency of the turbine, i.e. maximise its torque output. This may take several minutes to be fully effective. However, if the increasing wind speed should be a short lived gust, the duration of the gust may well be less than the response time of the pitch control mechanism and in this event all the compensation for the changing wind speed is effected by the motor/generator and its controller. If, on the other hand, the wind speed should drop, then the torque applied to the generator and to the motor/generator begins to drop also. This is again sensed by the controller 28 which operates to transfer power from the AC bus 34 to the motor/generator 24, which then acts as a motor. The torque provided by the motor/generator is then added to that provided by the turbine. In this event, the power consumed by the motor/generator is taken from the output of the generator and thus although the output of the generator 26 may remain substantially constant, the output of the whole apparatus does of course decrease.

Although the embodiment described above relates to a wind-powered turbine, it will be appreciated that it may also be a water-powered turbine. Waterpowered turbines may be powered by the flow of a river, by tidal flow or by waves and similar problems relating to the change in speed of the flow and surges in flow arise with water-powered turbines also and the present invention will operate in an analogous manner with such turbines and deal with the problem that would otherwise arise.

As referred to above, the invention may also be applied to an electrically powered fluid impelling apparatus. One specific embodiment of such an apparatus is substantially the same as that illustrated in the Figure, the only difference being that numeral 26 designates a synchronous motor and numeral 2 designates a fluid impeller, such as a water propeller. The operation of this embodiment and particularly of the motor/generator and the controller are substantially the same as those described above.

Claims

1. Turbine powered electricity generation apparatus including a turbine and a synchronous electric generator connected thereto by a transmission system comprising a three branch epicyclic gearset, a first shaft of which is connected to the turbine, a second shaft of which is connected to the generator and a third shaft of which is connected to the rotor of an electric motor/generator, the electrical connections of the stator of which are connected to the electrical output connections of the generator via control means arranged to control the flow of electrical power to and from the motor/generator such that the rate of rise of torque applied to the electric generator does not exceed a predetermined value.

2. Apparatus as claimed in Claim I including means arranged to produce a signal indicative of the torque applied to the generator comprising means connected to the control means for detecting the magnitude of the current flowing in the stator windings of the generator or the motor/generator.

3. Apparatus as claimed in Claim 1 or Claim 2 in which the turbine has blades of variable pitch type.

4. Apparatus as claimed in any one of the preceding claims in which the turbine has a range of practicable output torques and the controller is arranged so that, when the turbine is producing a torque at the lower end of this range, it transmits electrical power from the generator to the motor/generator, which thus operates as a motor and serves to increase the torque applied to the second shaft and, as the torque output of the turbine increases, the electrical power transmitted to the motor/generator progressively decreases until it reaches zero, whereafter the motor/generator operates as a generator.

5. Electrically powered fluid impelling apparatus including a fluid impeller and a synchronous electric motor connected thereto by a transmission system comprising a three branch epicyclic gearset, a first shaft of which is connected to the impeller, a second shaft of which is connected to the motor and a third shaft of which is connected to the rotor of an electric motor/generator, the electrical connections of the stator of which are connected to the electrical input connections of the motor via control means arranged to control the flow of electrical power to and from the motor/generator such that the rate of rise of the torque applied to the impeller does not exceed a predetermined value.

6. Apparatus as claimed in Claim 5 including means arranged to produce a signal indicative of the torque applied to the impeller comprising means connected to the control means for detecting the magnitude of the current flowing in the stator windings of the motor or the motor/generator.

7. Apparatus as claimed in Claim 5 or 6 in which the motor has a range of practicable output torques and the controller is arranged so that, when the motor is producing torque at the lower end of this range, it transmits electrical power to the electrical input connections of the motor from the motor/generator, which thus operates as a generator and serves to decrease the torque applied to the first shaft and, as the torque output of the motor increases, the electrical power transmitted from the motor/generator progressively decreases, whereafter the motor/generator operates as a motor.

8. Apparatus as claimed in any one of the preceding claims in which the epicyclic gearset comprises a relatively large first sun gear, which is mesh with one or more relatively small first planet gears, each of which is connected to rotate with a respective relatively large second planet gear on a respective planet shaft, the second planet gears being in mesh with a relatively small second sun gear, which is connected to rotate with the second shaft.

9. Apparatus as claimed in Claim 8 in which the first shaft is connected to rotate with the first sun gear and the third shaft is connected to a planet carrier common to the first and second planet gears.

10. Apparatus as claimed in any one of the preceding claims in which the first shaft is connected to the turbine or impeller via step up gearing and thus rotates faster than the turbine or impeller.

11. Apparatus as claimed in any one of the preceding claims in which the motor/generator is connected to the third shaft via step up gearing and thus rotates faster than the third shaft.

12. Apparatus as claimed in any one of the preceding claims in which the motor/generator is so connected that the maximum electrical power which it can generate, when operating a generator, is substantially equal to the maximum electrical power which it can consume, when operating as a motor.