GB2206930A - Wind turbine operating system - Google Patents

Abstract

A variable speed wind turbine operating system comprises a speed limiting system comprising turbine 11, driving an alternator 16 via a gearbox 14, a controller 19 and a shaft mounted brake means 13. The turbine has a rated condition and a speed/torque characteristic such that when wind speed increase tends to run the turbine above its rated condition, the brake is applied to slow the turbine to a speed at which the blades are operating so inefficiently that further wind speed increase cannot increase shaft speed or torque unacceptably. The brake may be a disc brake the hydraulic calipers of which are supplied with fluid pressure by a pump driven by the turbine. In alternative arrangements the brake is replaced by means for retarding the shaft such as an electrical generator (Fig 5) driving a switchable resistive control load. Alternatively generators having variable pole arrangements may be used (Fig 6). Alternatively two similar generators may be used with variable connections to a supply grid (Fig 7). <IMAGE>

Description

WIND TURBINE OPERATING SYSTEM This invention relates to wind turbines and operating systems therefor.

Some wind turbines are designed to operate at constant speed for the purpose of generating constant frequency electricity by directly driving an a.c.

generator. Other turbines are designed to operate at variable speed for optimum power capture over a range of wind speeds. The constant speed turbines are regulated by alteration of the turbine blades, as by pitch change.

The variable speed turbines may have fixed pitch blades and be designed to operate at constant tip speed ratio over the operating wind speed range.

In either case some control of the power capture of the blades is usually provided in the form of pitch control or as air brakes or spoilers so as to avoid overspeeding and/or overstressing the blades and/or drive train in high wind conditions outside the operating range.

The operation of air brakes or spoilers and the control of pitch angle are, however, fairly complicated matters, involving moving components that rotate with the turbine and which, if not always prone to failure, require regular maintenance which necessitates shutting down the turbine periodically. The mechanisms involved are subject to stress and need to be solidly engineered, which adds capital cost and also a weight penalty to the turbine which decreases its operational flexibility and ultimately its efficiency. So far as concerns fixed pitch wind wheels, the need for having moving parts rotating with the blades tends to negate one of the major advantages of such turbines, namely simplicity of manufacture and hence low capital cost.

On the other hand, relatively inexpensive turbine shaft mounted braking systems - such for example as disc brakes - are unsuited to prolonged or repeated application on account of such problems as wear, heat and fade.

The present invention, however, provides a wind turbine operating system which enables shaft retarding means to be used instead of the aerodynamic means hitherto considered essential.

The invention comprises a variable speed wind turbine operating system comprising a speed limiting system comprising retarding means acting on the turbine shaft and control means adapted to operate said retarding means, the turbine having a rated condition (as herein defined) and a speed/torque characteristic such that at some critical shaft speed, for a given maximum permitted torque loading on the turbine shaft, lower than a shaft speed appropriate to said rated condition, wind speed at or above said rated condition does not increase turbine torque on the shaft beyond the maximum permitted, and said control means operating said retarding means to reduce shaft speed to or below or maintain shaft speed at or below said critical speed, whereupon said critical speed will not thereafter be exceeded though said retarding means need not be continuously operated to retard shaft speed, despite further wind speed increase.

By "rated condition" is meant a nominal shaft speed and wind speed not normally to be exceeded.

Said retarding means may comprise friction brake means such as disc brake means. Or a generator driven (directly or indirectly e.g. through gearing) may be controlled so as to retard the turbine shaft.

The turbine may be such as to have kneed (i.e.

having the appearance of the outline of a bended knee) iso-torque lines on a shaft speed/wind speed diagram.

An iso-torque line is a line joining points of constant torque on a graph of shaft speed against wind speed. A fixed pitch wind wheel has iso-torque lines which are kneed, that is to say are smooth, bowed curves having a single value of shaft speed at a minimum wind speed with a vertical tangent at that point and upper and lower values at higher wind speeds, so that for each wind speed, two values of shaft speed lie on the curve, aside from the single shaft speed value at the minimum wind speed for the iso-torque line. The upper part of the curve indicates sharply increasing shaft speed for increasing wind speed. The lower part of the curve, however, indicates decreasing shaft speed with increasing wind speed to a minimum shaft speed (the "knee" of the curve) and thereafter only gently increasing shaft speed for increasing wind speed.Thus if a wind wheel, accelerating against a constant torque load in a high wind, is braked to just below the shaft speed corresponding to the minimum wind speed on the curve, increasing wind speed does not increase torque at a given speed of the wheel. This prevents further shaft speed increase or at least maintains shaft speed below the desired level.

This reduced shaft speed level corresponds to lower values of tip speed ratio, which, in turn, correspond to lower power capture levels, and so the wind wheel is operating at reduced efficiency (because it is partially stalled) but nevertheless safely. Of course, turbines with aerodynamic controls operate also at reduced efficiency whenever aerodynamic braking is applied.

Provided the rated condition of the turbine is chosen sensibly having regard to the prevailing wind conditions it can be arranged that the shaft retarding means are called upon only infrequently, and then only applied for such time as is necessary to slow the turbine down to below the critical speed. Problems of fade, heat generation and undue wear of friction brake material are thus avoided, as are unacceptably prolonged high loading of the generator or drive train in the case of electrical braking.

The control of the operation of the retarding means may be effected by means which are not prone to failure or inactivation in the event of, say, a failure of a generator driven by the turbine or of a mains network into which the generator is supplying power.

Such means are much more readily available for a shaft braking arrangement than for a blade mounted arrangement, and a shaft braking arrangement can respond much more quickly, in general, than where long mechanical linkages, e.g. to spoilers, are involved.

A self-contained hydraulically operated friction braking system taking its operating power directly from the wind turbine shaft can be used, for example, the shaft driving a pump providing hydraulic braking pressure and having an associated control applying said braking pressure to a disc brake when required. As braking for the purpose of the invention is required only when the shaft is at speed there will always be guaranteed a supply of hydraulic power.

The braking may be effected continuously for as long as required to bring down the turbine speed to the new, safe level. On the other hand, the speed may be reduced in steps.

Electrical braking may be effected by, for example, switching a resistive load across the output terminals of the generator. This could increase the torque felt by the turbine to say 2.5 times, which would cause the turbine to slow down. This retarding torque, must, of course, pass through the drive train, so that if there is a gearbox it must be capable of accepting the transient high torque occurrences needed for braking the turbine.

Of course, with variable turbine speed, constant frequency alternating current can be produced only with some arrangement such as a static inverter. If, within the context of the present invention, however, it is accepted that there be only two turbine speeds, the lower speed corresponding to reduced efficiency operation in high winds as well as high efficiency operation in low winds, then the generator configuration can be changed by changing the number of poles to give the same output frequency at the two speeds.

It is known already to arrange for such pole switching in wind turbine operation, but so as to run the turbines only at low speed in low winds in six pole configuration and at higher speed in high winds in four pole configuration.

Embodirents of variable speed wind turbine operating systems according to the invention will now be described with reference to the accompanying drawings, in which : Figure 1 is a diagrammatic representation of one system as applied to a wind turbine electric generator; Figure 2 is a shaft speed/wind speed diagram showing an iso-torque line; Figure 3 is a diagram like Figure 2, showing a family of iso-torque lines; Figure 4 is a diagrammatic illustration of a shaft brake system; Figure 5 is a diagrammatic illustration of another system; Figure 6 is a diagrammatic illustration of yet another system; and Figure 7 is a diagrammatic illustration of a further system.

The system illustrated in Figure 1 is applied to a three bladed aerogenerator 11 of 31 metres diameter having stiff, twistless blades having NACA 0025 aerofoil section.

Mounted on the shaft 12 of the aerogenerator 11 is a disc brake 13 and a gearbox 14. The output shaft 15 of the gearbox 14 drives an alternator 16. The arrangement is intended to operate at variable shaft speed dependent upon the wind conditions. For network connection, therefore, a static convertor 17 is connected to the alternator 16 output.

A tachometer 18 senses shaft 12 speed and this together with alternator power output information is supplied to a controller 19 which controls the alternator field coils so as to apply a torque loading on the shaft 11 proportional to the square of shaft speed. In this way, the wind wheel operates - over its normal operating range - at constant tip speed ratio, that is to say the ratio of the speed of the blade tip to the speed of the wind is a constant over the entire range (actually, this is an ideal situation in practice the alternator field coil control may be stepwise so that there is only an approximately constant tip speed ratio) In addition, the controller 19 controls the application of the brake 13.

Figure 2 illustrates the way in which the control is effected. Figure 2 is a plot of turbine shaft speed against wind speed. The wind speed is shown in metres/second and the shaft speed in revolutions per minute. The kneed curve is an iso-torque line - it is, in fact, in the diagram, the 80,000 Nm iso-torque line. Any point on the line represents a wind speed and shaft speed that together generate a torque of 80,000 Nm on the shaft.

If there is a 80,000 Nm torque load on the shaft, any shift in wind speed or shaft speed that specifies a point lying inside the bow of the curve will result in a higher torque and thus the shaft will accelerate. Conversely, any point lying on the convex side of the curve implies a lower torque and hence deceleration of the shaft against a constant 80,000 Nm torque loading.

With a wind velocity of 12.5 metres/second and a tip speed ratio of 5.1:1, a torque of 80,000 Nm is generated at 42 rpm. The line YZ in Figure 2, which is the 5.5:1 tip speed ratio line for the 31 m wheel referred to is divided in 10,000 Nm torque steps. Other tip speed ratio lines are shown for ratios in 0.5:1 steps down to 2:1.

Figure 3 shows a family of iso-torque lines at 10,000 Nm intervals together with the effect of an increase in wind speed and the controlled braking.

The point 'A' on Figure 3 represents the rated output of the wind turbine generator. The turbine in this condition develops 80,000 Nm torque (equivalent to 357 kW or 470 shaft horse power) at 42 rpm in a wind speed of 12.5 metres/second. Up to this point, the turbine has been operating along the line YZ, which is the line corresponding to square-law operation at a constant tip speed ratio of 5.1:1 and it has been constrained so to do by the controller 19 controlling the alternator field coils to apply an appropriate torque loading on the shaft proportional to the square of shaft speed.

Wind speeds above the rated condition will increase the torque available from the turbine and will give rise, therefore, to a torque imbalance which will tend to speed up the system at a rate proportional to the imbalance. A gust to only 15 metres/second would increase torque by some 40,000 Nm, and would cause a rapid increase in rpm. For the drive train to restrain the turbine to its rated speed of 42 rpm, regardless of wind speed increase, it would increase the torque loading until, round abput a wind speed of 23 metres/second the torque has increased to 200,000 Nm.

In other words, the drive train has to be capable of sustaining loads of two and a half times the rated capacity and will on that account need to be much more robustly (and expensively) constructed than is necessary for operation under normal conditions. Since the abnormal conditions would prevail for only a minute fraction of the operational time, this would be uneconomic.

However, if the brake 13 is applied to reduce turbine speed in a controlled way when high wind conditions are experienced, these high torque loadings are avoided and the drive train can be made much lighter and less expensive.

The brake 13 is, in these circumstances, applied, according to the invention, until shaft speed is reduced to 33 rpm, and then released. The turbine is then operating at point 'B' on Figure 3, namely on the 88,000 Nm iso-torque line. Turbine speed has now been reduced by a factor 33/42 = 0.79, while torque has increased by a factor 88,000/80,000 = 1.1. The power is therefore reduced to 0.86% or by about 13%. Of course, the torque is now higher than the rated torque of 80,000 Nm, but the drive train will normally have a service factor of at least 1.5, which enables it to operate at a torque of 80,000 Nm x 1.5 or 120,000 Nm for short periods. At 33 rpm, that torque can never be reached regardless of windspeed. For a site with average wind speed of 6.7 metres/second, the proportion of time such high winds are experienced is acceptably small.

In these particular circumstances, the rated condition is the point 'A' corresponding to the generation of 80,000 Nm of torque at 12.5 metres/second wind speed and 42 rpm shaft speed. The maximum permitted torque loading is 120,000 Nm and the critical shaft speed is 33 rpm. Other systems will, of course, have different values for these quantities.

So, the controller 19 monitors the operation of the turbine and generator to decide whether the turbine is operating above its rated condition. If so, the controller 19 causes an immediate application of the brake means 13 to reduce turbine speed. The braking is continued until the point 'D' on Figure 3 is reached, that is to say, speed has been reduced to 33 rpm. The brake is then taken off.

The wind turbine is now in a mode of operation appropriate to being "below rated condition" which it can easily maintain indefinitely.

As explained above, in this condition, no matter what the wind speed, there will never be generated a torque above the maximum permitted torque. The turbine will remain in this condition until falling wind speed restores the turbine to its normal operating mode.

Here there are several possibilities, but one is that the controller 19 maintains a shaft speed of 33 rpm until the wind speed falls to about 10 metres/second, when the wind wheel again reverts to its normal mode on the 5.5:1 tip speed ratio line YZ, generating torque of 50,000 Nm at 10 metres/second and moving up and down this line with changing wind speed.

Essentially, the brake means 13 are adapted to slow the turbine down, in high wind conditions, to such a speed that the blades are at least partially stalled, or at least not generating maximum lift, whereupon no increase in wind speed can cause higher than permitted shaft speeds. The condition is vacated by falling wind speed to the point where normal operation becomes possible.

Figure 4 illustrates a braking arrangement in which the brake means 13 comprise a disk 41 on the turbine shaft 12 with hydraulic calipers 42 operated by hydraulic pressure in a line 43 branched from a circuit 44 including a reservoir 45 and a pump 46 driven by gearing 47 from the shaft 12. Control means 48 themselves controlled by the controller 19 of Figure 1 (or otherwise) regulate the pressure in the circuit 44 to apply the calipers 42 when necessary.

The control means 48 may include an arrangement limiting shaft speed in any event, even if the controller 19 fails for any reason; such an arrangement may include such throttling as becomes effective above a certain predetermined shaft speed to cause pressure to increase in the circuit 44 to an extent sufficient to apply the calipers 42.

The invention is, of course, not restricted to fixed pitch wind wheels of the kind described - it is also applicable, for example, to vertical axis turbines, so long as there is a low speed condition of such turbines such that the vanes are not operating at such efficiency as causes the turbine to overspeed in high wind, the control system being adapted to attain such low speed condition in high wind conditions and vacate it when wind conditions return to normal.

Figure 5 illustrates diagrammatically an arrangement in which a wind turbine 51 drives an induction generator 53 through gearing 52. Retarding of the generator shaft and hence, through the drive train including the gearing 52, of the turbine itself, is effected by the control means 54 switching a resistive load 55 across the output of the generator 53.

Figure 6 illustrates an arrangement in which the turbine 61 drives, either directly or through gearing (not shown) one of two induction generators 62, 63, there being a mechanical selector 64 for switching the drive from one generator to the other under the control of the control system 65.

Generator 61 is a four pole generator, while generator 62 is a six pole generator.

At low wind speeds, the drive is to the six pole generator 62, whereas at higher wind speeds the drive is to the four pole generator 61 so that, because of the pole configuration, the same frequency is produced. The generators 61,62 can of course be connected to mains distribution network and run synchronously therewith.

According to the invention, however, at higher wind speeds still, the drive can be switched back to the six pole generator 62 which will still operate synchronously with the distribution network at the lower shaft speed, the turbine now operating less efficiently, but at least operating and producing power when otherwise it could not, at or below the critical speed so that no undue strain is placed on the drive train components.

Of course, the two generators 61 and 62 could, and would normally in practice, be a single generator with a variable pole configuration, and the selector 64 would be replaced by a pole configuration changing arrangement.

Figure 7 illustrates another arrangement involving two induction generators 72, 73 driven from a wind turbine 71 via a gearbox 74 with two output shafts 74(2), 74(3) driving the generators 72, 73 respectively at the same time but at different speeds. In this instance, both generators are four pole generators.

Only one of them at a time is connected to the grid (or a grid-like load that supplies energising current and forms a stiff load forcing the generators to synchronism) by a switch arrangement 75 operated by selector means 76.

The output shafts 74(2) and 74(3) have step-up ratios relative to the input shaft 71a of the turbine 71 such that a speed of 1500 rpm (plus 40 rpm to allow for slip) is imparted to generator 72 at a turbine speed of 33 rpm, and to generator 73 at a turbine speed of 42 rpm.

Clearly, only one of these generators is running at synchronous speed, namely generator 72 when the turbine speed is 33 rpm and generator 73 when the turbine speed is 42 rpm. The turbine not operating at synchronous speed is disconnected from the grid, and so rotates harmlessly at non-synchronous speed.

Assuming operation commences at windspeed equal to zero, but increasing steadily, the arrangement would accelerate to just over 33 rpm, when the shaft brake 13 would be applied momentarily and the control system would select generator 72 to be connected to the grid.

Further wind speed rise would first disconnect generator 72 and permit shaft speed to rise to 42 rpm, when generator 73 would be connected to the grid.

Further windspeed rise still would apply the brake 13 to slow the turbine down again to 33 rpm and connect generator 72 to the grid, after first disconnecting generator 73. The arrangement is now operating inefficiently at a speed below the critical speed, and further increase in windspeed cannot increase the turbine speed.

The speeds chosen for the above illustration are, of course, arbitrary, and more than two generators can be provided, each coupled to the wind turbine through different output shafts of a gearbox, all to suit site conditions.

Claims (20)

1. A variable speed wind turbine operating system comprising a speed limiting system comprising retarding means acting on the turbine shaft and control means adapted to operate said retarding means, the turbine having a rated condition (as herein defined) and a speed/torque characteristic such that at some critical shaft speed, for a given maximum permitted torque loading on the turbine shaft, lower than a shaft speed appropriate to said rated condition, wind speed at or above said rated condition does not increase turbine torque on the shaft beyond the maximum permitted, and said control means operating said retarding means to reduce shaft speed to or below or maintain shaft speed at or below said critical speed, whereupon said critical speed will not thereafter be exceeded though said retarding means need not be continuously operated to retard shaft speed, despite further wind speed increase.

2. A system according to claim 1, in which said retarding means comprise friction brake means.

3. A system according to claim 2, in which said brake means comprise disc brake means.

4. A system according to any one of claims 1 to 3, in which a generator is controlled so as to retard the turbine shaft.

5. A system according to any one of claims 1 to 4, in which the turbine is such as to have kneed iso-torque lines (as herein defined) on a shaft speed/wind speed diagram.

6. A system according to any one of claims 1 to 5, in which the retarding means are applied to reduce shaft speed when torgue exceeds a predetermined value.

7. A system according to any one of claims 1 to 6, in which the turbine is a fixed pitch wind wheel.

8. A system according to claim 7, comprising a constant tip speed ratio operating system.

9. A system according to claim 8, comprising a control computer supplied with torque and shaft speed information.

10. A system according to claim 9, applied to electric power generation from the turbine in which torque information is derived from power output information.

11. A wind turbine comprising an operating system according to any one of claims 1 to 10.

12. A wind turbine according to claim 11, having turbine blade having an aerofoil section having a gentle stall characteristic.

13. A wind turbine according to claim 11 or claim 12, having turbine blades of high stiffness.

14. A wind turbine according to any one of claims 11 to 13, having twistless turbine blades.

15. A wind turbine according to any one of claims 11 to 14, comprising retarding means comprising a selfcontained hydraulic braking system taking its operating power directly from the turbine shaft.

16. A wind turbine according to claim 15, in which the turbine shaft drives a pump providing hydraulic braking pressure and having an associated control applying braking pressure when required.

17. A wind turbine according to any one of claims 11 to 16, driving a generator having a switchable resistive load for retarding the shaft.

18. A wind turbine according to any one of claims 11 to 17, driving a generator having a changeable pole configuration and arranged to operate at different speeds according to the pole configuration so as to generate constant frequency alternating current, and arranged to operate, in high winds, at a low speed so as to be at or below the said critical speed.

19. A wind turbine according to any one of claims 11 to 17, driving one of at least two generators having different pole configurations and arranged to operate at different speeds so as to generate constant frequency alternating current, and selector means selecting which one of said generators is driven according to the wind speed so that the turbine is operated, in high winds, at a low speed so as to be at or below the said critical speed.

20. A wind turbine according to any one of claims 11 to 17, driving at least two indution generators having a similar pole configuration at different speeds from different output shafts of a common gearbox on which the input shaft is the turbine shaft, and selector means selecting which generator is connected to supply current to a grid, the other generator or generators being disconnected from the grid, according to the wind speed so that the turbine is operated, in high winds, at or below said critical speed.