WO2001097359A1

WO2001097359A1 - A method and a device for stabilizing power generated by groups of generators

Info

Publication number: WO2001097359A1
Application number: PCT/DK2001/000411
Authority: WO
Inventors: Ulrik Krabbe
Original assignee: Ulrik Krabbe
Priority date: 2000-06-14
Filing date: 2001-06-13
Publication date: 2001-12-20
Also published as: DK200000912A; AU7388601A

Abstract

Within groups of wind turbines feeding power to the common utility grid a synchronization may propagate through the grid with the effect that blades of different turbines pass the respective towers simultaneously leading to a short term pulsation of the power fed into the grid. The invention remedies this effect and relates to devices and methods in which the shafts of various turbines are synchronized with a phase-shift in order that the turbines blades do not simultaneously pass the respective towers. The result is that the aggregated power from the group of turbines is stabilized. The invention permits deployment of twin bladed turbines in parks of wind turbines.

Description

A method and a device for stabilizing power generated by groups of generators

The present invention relates to groups of two or more generators feeding into a common electric utility net and driven by independent prime movers inclined to exhibit during operation periodic fluctuations of the driving torque. The invention particularly relates to wind turbines capable of feeding into the utility net while varying the speed of rotor revolution.

Long-term and short-term fluctuations in the power developed by wind turbines are a well known occurrence.

Long-term fluctuations are generally due to variations in the prevailing wind conditions. Generally, the wind turbine is fitted with a speed control serving to keep the operational speed within design limits and to maximize the power developed.

One reason for short-term fluctuations is a shadow effect that reduces the power at every instance when one blade of the wind turbine passes the wind turbine tower. This occurs in situations where the blade is leading and the tower is trailing referring to the wind as well as in cases when the tower is leading and the blade is trailing. The fluctuation may create a brief dip in torque, speed and power generated. Other fluctuations could create a surge or wobble. The fluctuations recur periodically according to the frequency of the blade rotor revolution.

GB-A-2095487 relates to an electric generating apparatus to be driven by a wind turbine which includes a slip ring induction machine and control means to determine the direction and magnitude of current transfer in order to enable the induction generator to feed power into the electricity grid under conditions of variable rotor speed. This publication deals with control strategies adapted for optimizing operation.

EP-A2-0 817 367 relates to a generator system for internal combustion engines which can produce power efficiently while being driven at variable speed.

US-A-4 400 659 relates to a wind turbine combined with a three phase AC generator. The wind turbine comprises a system for pitch control of the turbine blades in order to control the speed of rotor revolution. The rotor windings of the generator are fed with a controlled frequency in order that the stator may produce AC current at a constant frequency even while rotating at different speeds .

US-A-4 246 531 relates to a generator adapted for generating an output signal at a frequency independent of the prime mover shaft rotational rate.

US-A-4 510 433 relates to a variable speed constant frequency alternator compensating for changes in speed of the prime mover by electromagnetically rotating the rotor's magnetic poles around the rotor.

As will appear from the referenced prior art, generators adapted for outputting a constant frequency AC power while being driven by a prime mover exhibiting varying levels of torque and speed belong to the state of the art. The deployment of wind turbines in socalled parks of wind turbines, maybe comprising a substantial number of wind turbines, all feeding into a common utility net, is now widely used.

The short-term fluctuations from different wind tubines within a wind turbine park are generally expected to be randomly scattered over time so as to even out the power fluctuations in the power aggregate. Thus a synchronized short term fluctuation among a number of wind turbines is presumed to be an exceptional occurrence.

However, this has not proven to be true. For reasons yet to be understood, an undesired auto-synchronization effect, presumably propagated through the utility grid, may take place depending on operation conditions such as the prevailing wind conditions. Thus, during service the wind turbines may auto-synchronize in such way that blade-and-tower passings occur simultaneuously among a number of the turbines. The concerted simultaneous blade- and-tower passings give rise to simultaneous fluctuations in the power generated, which is a highly undesirable state of operation.

It is a fact that twin-bladed wind turbines have failed to get wide acceptance, although they have an advantage in manufacturing costs as compared to three-bladed wind turbines. The reason is that the torque generated from the twin bladed wind turbine exhibits substantial short term fluctuations, partly due to the shadowing referred to above and partly due to the level of wind speeds being dependent on the height above ground. The invention in a first aspect provides a method as recited in claim 1. This method provides the concerted operation of a first and a secondary prime mover, each driving a respective electricity generator, in a way that avoids the difficulties enumerated above. The inventive method does not prevent periodic fluctuations within each of the primer movers from occurring but merely prevents the undesirable auto-synchronization of such occurrencies among two independent prime movers and thereby prevents doubling up of the fluctuations as might otherwise occur. Thus, the fluctuations may in fact be present in the electric power aggregated from the pair of generators but only on a small scale as each fluctuation introduced by any one of the generators will be masked by the power developed by the other generator in the pair.

Witin the context of the invention the term singularity phase angle has been used to designate in general any particularity likely to recur periodically and likely to cause a deviation from a steady state of operation. Examples of singularities could be the instance of a wind turbine blade passing closely by a steady obstacle such as the tower of the wind turbine or in a proximity of the ground or other obstacles.

The fluctuation in the power developed in respect of any particular singularity phase angle will depend on particular conditions such as speed of rotor revolution, gearing ratio, extent of obstacle, etc. Thus, there may be varying pictures of fluctuations. By the method according to the invention fluctuations from two generators within a pair will be temporarily offset so as to avoid doubling of the fluctuation fed into the grid. Depending on the duration of the particular occurrences of fluctuations, there may or may not be a temporal overlap between different fluctuations related to different instances of a singularity phase angle, however the phase shift should temporally stagger the peaks of the disturbances sufficiently to avoid adding together the peaks of fluctuation contributions from different generators .

The establishment of speed and phase of rotation of each prime mover should be performed to sufficient accuracy to resolve the phase shift necessary to displace the fluctuations temporarily.

According to a preferred embodiment the singularity phase angles of the secondary prime mover is situated temporarily intermediate the singularity phase angle of the first prime mover. This provides the maximum spacing and minimizes the impact of low frequency disturbances introduced into the grid.

The method according to the invention in the first aspect may be generalized by including more than two prime movers, each associated with a respective independent electricity generator, basically all controlled so as to intentionally offset the disturbances from different prime movers .

The first prime mover is generally regarded as a master unit and thus it will preferably be associated with means for controlling the speed of revolution serving to optimize the performance according to the prevailing conditions as is generally known in the art. The other prime movers preferably operate as slave units, tuning their speed and phase according to the speed and phase of the master unit.

The invention in a second aspect provides a method as recited in claim 8.

By this method a whole group of prime movers, each associated with a respective electricity generator, is synchronized with intentional mutual phase offsets so as to even out fluctuations in the electric power aggregated from the group of electricity generators.

According to a preferred embodiment of this method defined in claim 9, the prime movers are controlled according to a scheme that basically allocates to the set of prime movers a sequence of phase increments together adding up to the cyclic period of the base frequency.

The invention in a third aspect provides a method as defined in claim 10.

In this method, a group of prime movers, each associated with a respective electricity generator, is split up into a selected number of sub-groups. All prime movers within one sub-group are synchronized with coincident phase angle relation. Each of the other sub-groups is also mutually synchronized; however, the rotors of the prime movers of one sub-group are assigned a preselected phase shift relative to the phase of any other sub-group.

Thus, within each sub-group the expected fluctuations in fact add together. However, the expected fluctuations introduced into the grid from the generators within one sub-group is countered by the beat effect of adding power to the grid from each of the other sub-groups in which the periodic fluctuations are temporally offset.

The invention in a fourth aspect provides a device for stabilizing variations in power generated by groups of two or more wind turbines feeding into a common utility net wherein all wind turbines except one are adapted for external speed control, characterized by comprising the synchronization of the wind turbine rotors with off-set phase angles between the blades of different wind turbines in such way as to avoid simultaneous blade-tower passages among the wind turbines.

An embodiment of the invention will be explained in the following with reference to the appended figures, among which

Fig. 1 shows a schematic diagram of a pair of wind turbine generators,

Fig. 2 shows a schematic front view of an angular detector, and

Fig. 3 shows a plot of the detector output signal against time.

A simple embodiment comprises two wind turbines operating as illustrated in figure 1.

The wind turbine 1 comprises generator 13, gear box 11, turbine shaft 9 and angular detector 6. Wind turbine 2 comprises generator 14, rotor 3, gear box 12, turbine shaft 10 and angular detector 7. Both of the generators feed into a common utility grid 5.

Wind turbine 2 comprises multi-phase frequency converter 4 connected to the multi-phase winding of the rotor 3, e.g. as disclosed in GB-A-2095487. This permits controlling the speed of revolution of the shaft 10. There is no requirement for controlling the speed of revolution of generator 1 but generator 1 may be equipped similar as is the case with the wind turbine 2. Generators 13 and 14 may be asynchronous or synchronous.

The two angular detectors 6 and 7, presumed to be mutually similar, transmit signals to the synchronization device 8. This device controls the frequency converter 4 which influences the speed of revolution on wind turbine 2 with the effect to achieve equal speeds of revolution of the two turbine rotor shafts with complete synchronization of the signals transmitted from 6 and 7, and this state of operation is maintained.

Figure 2 provides a schematic illustration of details of the angular detector 6.

The detector comprises a magnet 15 with a north pole and with a south pole connected to the turbine rotor shaft. The south pole is arranged adjacent the blade 16. The static part of the detector 17 comprises a semi-conductor responsive to the magnetic fields with respect to level and polarity. The static part of the detector transmits electric pulses as illustrated in fig 3 which plots voltage on the y-axis against time on the x-axis. Both wind turbines are twin bladed and the south pole is arranged adjacent one of the blades. On turbine 2 the south pole has been offset 90° from one blade. Once the device 8 has synchronized generator 2 to generator 1 with complete coincidence of the positive and negative pulses from the two angular detectors, the blades of generator 1 will be phase-shifted by 90° relative to the generator 2. The shifting polarity of the pulses has the effect that the synchronization device is capable of influencing the frequency of the frequency converter so as to obtain higher or lower values .

Thus, the synchronization has the result that any moment a blade of one turbine passes the respective tower, the tower of the other turbine will be situated between the blades of that other turbine.

This has the effect that the short term drops of torque and power in one generator will be timed to the mid-point between the short term drops of the other generator with the effect that the aggregated power produced from two wind turbines each equipped with twin blades, will appear similar to what would be developed by one turbine with four blades.

The effect of the stabilization will be even more pronounced in case three turbines are used, provided two of the tubines are equipped similarly as tubine 2 although with the detectors phase-shifted 60° and 120°, respectively, relative to one blade. Thus, the power aggregated to the grid from three twin bladed turbines will be similarly to what would be provided from one turbine with six blades. Generally equal spacings of all pulses fed into the grid from n turbines each fitted with A wings can be obtained if the detector for turbine number m is phase-shifted by an angle v relative to one blade presuming the phase shift to be 0 for turbine number 1.

v expressed in degrees should obey the equation:

v = 360* (m-1) /A*n

In case of a wind turbine park with a substantial number of turbines, it is not necessary for the blades of all wind turbines to all have different mutual phase angles. They could e.g. be arranged in groups of four turbines to be synchronized in unison in order that a park of turbines with all three bladed mills will appear similarly as a group of turbines with twelve bladed rotors .

The above description presumes all wind turbines within one group to be synchronized from one turbine 1.

Other considerations may dictate deriving synchronization of e.g. turbine number 3 from turbine number 2. This is also possible within the scope of the invention. The important point is that all groups once they have been synchronized rotate in synchronization without the blades exhibiting coincident phase angles.

Only one type of angular detector has been mentioned above. However, other methods for synchronizing shafts with predetermined mutual phase-shift angles are known in the art. The use of such other methods for synchronization is considered to lie within the scope of the invention.

Figure 1 illustrates two wind turbines with gear boxes.

However, the invention also relates to wind turbines without gears and with other measures for controlling the speed of revolution of the generator e.g. with a frequency converter between the utility grid and the generator.

The invention is particular advantageous for wind turbines with twin-bladed rotors as it enables advantageous use of such wind turbines in wind turbine parks .

Obviously the stabilization of the power is enhanced by increasing the number of synchronized turbines within a given limit.

In the case of wind turbines with facilities for speed control this may serve other purposes than the mutual synchronization. Other purposes could be protection against overloads on wind turbines with pitch control on the blades for precaution in case the pitch control is too slow. Another consideration might be a desire to operate at a speed of revolution which maximizes the power generated at the prevailing level of wind speed. Therefore the control may be adapted to govern the speed of revolution of one turbine, e.g. the turbine 1, so as to meet that requirement while the remaining turbines within the group derive their synchronization from 1. However, since the turbine rotors generally run at low speed revolution bound to lead to slow synchronization, it may in the event of imminent overloading of a turbine be necessary to override the synchronization and thus to allow control to be seized by a master control system.

As appears from Fig. 1 the additional equipment required in order to gain the advantages by the invention is small in the context of the manufacturing of the turbine: a few angular detectors and an electronic synchronization device. In case of a twin-bladed wind turbine rotor this may be balanced by the substantial cost saving gained by using a twin-bladed rotor, i.e. a simpler hub and just two motors for blade pitch control. This is the case even though the blades need to be somewhat wider.

Also in case of turbine parks with three bladed turbine rotors the invention will provide advantages by suppressing fluctuations in the power generated from the aggregated turbines within the park.

Some examples of the procedure followed to establish this set of singularity phase angles shall be explained in the following.

As the singularity phase angles are defined as instances likely to be linked to occurrences of periodic fluctuations of torque, such angles are generally closely related to physical peculiarities of the prime mover. relevant physical peculiarities are generally easily identified by those skilled in the art. If necessary, frequency analysis on the voltage generated from the generator may also be employed to identify any fluctuation recurring at the frequency of rotation of the prime mover.

For instance in the case of a wind turbine, each instance of a turbine blade passing the proximity of the tower is expected to give rise to a fluctuation in the torque developed. The passing of one particular blade recurs by the frequency of rotation of the primary rotor. In the case of a large wind mill, this frequency may typically be in the order of 0.5 Hz, equivalent to 30 rotations per minute. In case of a twin-bladed rotor, the blades are presumed to give rise to exactly similar fluctuations, the aggregate of which then recurring at twice the base frequency, thus e,g, 1.0 Hz. In the case of a three- bladed turbine rotor, the disturbances are likely to recur at three times the base frequency, thus e.g. 1.5

Hz.

Each occurrence of a fluctuation is brief. The establishment of the time picture of any particular periodic fluctuation will be a simple job for those skilled in the art.

In case of a fluctuation caused by a blade of a wind turbine passing in proximity of the ground, the fluctuation caused may take the shape of an extended dip in the torque developed. The establishment of the exact picture of the fluctuation will lie within the capabilities of those skilled in the art.

The phase shifting according to the invention among different prime movers should be selected so as to avoid the fluctuations from adding together. Suitable particular values of the phase shift will be selected according to the picture of the fluctuations, the number of prime movers or sub-groups of prime movers within the group, the accuracy of angular detectors and speed controllers employed etc., as will be evident to those skilled in the art.

The means for speed control of the prime movers generally comprise an angular detector in respect of each prime mover and serving to monitor speed and phase of rotation of the respective prime movers. Initially, the detector signal must be correlated with the singularity phase angles of any particular prime mover by a calibration procedure. The detector signal will generally be transmitted to a control unit associated with means for speed control by any measures known in the art. Suitable detectors, transmission means as well as the procedures necesssary to calibrate the equipment will be evident to those skilled in the art.

Although specific embodiments have been described above it is emphasized that the invention may be exercised in several ways and that the explanation given above exclusively serves to clarify the invention and not to limit the scope of protection conferred, which is exclusively defined by the appended claims.

Claims

P a t e n t c l a i m s

1. A method of operating a first and a second prime mover each driving a respective electricity generator feeding power into a common grid and each being susceptible to exhibit during operation periodic fluctuations of the driving torque, comprising establishing speed and phase of rotation of each prime mover, establishing for each of the prime movers a set of prime mover singularity phase angles likely to be linked to occurrences of periodic fluctuations of torque, providing means for speed control of at least the second prime mover, and using the means for speed control to phase-shift the second prime mover so as to temporally offset the singularity phase angles of the second prime mover from the singularity phase angles of the first prime mover.

2. The method according to claim 1, wherein the two prime movers exhibit a mutually similar set of singularity phase angles, and wherein the means for speed control is used to phase-shift the second prime mover until the singularity phase angles of the second prime mover are situated temporally intermediate the singularity phase angles of the first prime mover.

3. The method according to claim 1, comprising operating a third prime mover similar to said second prime mover and in concert with the first and the second prime mover by way of establishing for the third prime mover a set of singularity phase angles likely to be linked to occurrences of periodic fluctuations of torque, providing means for speed control of the third prime mover, and using the means for speed control to phase-shift the third prime mover so as to temporally offset the singularity phase angles of the third prime mover from the singularity phase angles of the first prime mover as well as of the second prime mover.

4. The method according to claim 1, comprising controlling the speed of revolution of the first prime mover so as to maintain a target value.

5. The method according to claim 1, wherein the prime mover comprises a wind-turbine .

6. The method according to claim 5, wherein the instances of the blades of the wind turbine passing the tower are treated as the singularity phase angles of the wind turbine.

7. The method according to claim 5, wherein the instances of the blades of the wind turbine passing in proximity of the ground are .treated as singularity phase angles of the wind turbine .

8. A method of operating a group of prime movers, each of the prime movers driving a respective electricity generator and each being susceptible to exhibit during operation periodic fluctuations of torque, where all of the generators feed power into a common grid, comprising establishing speed and phase of rotation of each prime mover, establishing for each of the prime movers a set of prime mover singularity phase angles likely to be linked to occurrences of periodic fluctuations of torque, providing means for speed control of all of the prime movers, controlling at least a first one of the prime movers so as to keep the speed of revolution to a target value, and using the means for speed control to phase- shift the remaining part of prime movers within the group according to a predetermined randomized pattern so as to randomize the singularity phase angles from different ones of the prime movers.

9. The method according to claim 8, comprising determining the randomized pattern among a group of N prime movers, each exhibiting a set of A singularity phase angles situated with equal angular spacings by ordering the prime movers in a series in which the phase lag V of the n-th prime mover relative to the first prime mover obeys the equation:

V = 360° * (m - 1) / (A N)

10. A method of operating a group of prime movers, each of the prime movers driving a respective electricity generator and each being susceptible to exhibit during operation periodic fluctuations of torque, where all of the generators feed power into a common grid, comprising establishing speed and phase of rotation of each prime mover, establishing for each of the prime movers a set of prime mover singularity phase angles likely to be linked to occurrences of periodic fluctuations of torque, providing means for speed control of all of the prime movers, splitting the prime movers into subgroups that each comprise a number of prime movers with mutually similar sets of singularity phase angles, using the means for speed control to synchronize all prime movers within at least one subgroup to make the sets of singularity phase angles mutually coincident but phase-shifted relative to the set of singularity phase angles of at least one of the other subgroups according to a predetermined randomized pattern so as to randomize the likely occurrences of fluctuations from prime movers of different subgroups.

11. A device for synchronizing the rotor shafts of two or more wind turbines, of which the wind turbine generators feed into a common utility grid, in which all wind turbine generators except one have facilities for speed control, while the generator is connected to the utility grid, c h a r a c t e r i z e d by the rotor shafts of the wind turbines being synchronized with phase-shift angles among the blades of the turbines in order to prevent a blade of one turbine from passing the tower simultaneously with other passages.

12. The device according to claim 11, c h a r a c t e r i z e d by the angular detectors (6,7) on the rotor shafts (9,10) being adapted for different phase-shift angles of the blades in respect of each turbine and transmitting signals for the synchronization device (8) used in synchronizing the turbine rotor shafts in such way that the signals transmitted from two angular detectors coincide with the result that the turbine blades do not pass the towers simultaneously.

13. The device according to claim 11 or 12, c a r a c t e r i z e d by the turbines comprising twin-bladed rotors .

14. The device according to claim 11 or 12, c h a r a c- t e r i z e d by the phase angle v of the angular detector on a wind turbine with the number m within a group of n mutually synchronized turbine rotor shafts, each fitted with A wings, as referred to one wing and as referred to the turning of the rotor of turbine number 1, v obeying the equation

v = 360* (m-1) /A*n

15. The device according to any of the claims 11 through

14, c h a r a c t e r i z e d by the turbine number 1 within one group being adapted for controlling the speed of rotor revolution according to various functional requirements while the remaining group of turbines being adapted for deriving synchronization from the turbine 1.

16. The device according to any of the claims 11 through

15, c h a r a c t e r i z e d by the control for synchronization of the turbine shafts being suppressed and being replaced by a master speed control whenever required due to turbine safety precautions or other circumstances such as a state of start-up.