GB2534396A

GB2534396A - Method of controlling a turbine rotor

Info

Publication number: GB2534396A
Application number: GB1501059.8A
Authority: GB
Inventors: Harrison Matthew; Reed Matthew
Original assignee: Marine Current Turbines Ltd
Current assignee: Marine Current Turbines Ltd
Priority date: 2015-01-22
Filing date: 2015-01-22
Publication date: 2016-07-27
Also published as: GB201501059D0

Abstract

A method of controlling a turbine rotor, such as a wind or water turbine rotor, to mitigate wake flow disturbance effects 5 in operation comprises modifying a parameter of the turbine rotor at one or more specific rotor azimuth angles for each rotation of the rotor. Preferably the parameter of the turbine rotor comprises turbine speed, turbine torque or blade pitch angle. The method may further comprise obtaining load fluctuation data in the rotor during each rotation of the rotor; deriving recurring load fluctuations present over the rotations, together with a rotor azimuth angle at which the fluctuations occur; and modifying the parameter at the one or more rotor azimuth angles. The parameters may be modified and then the above process may be repeated. The invention is intended to mitigate the effects of the change in a fluid stream in the zone of the wake of the turbines supporting tower 1.

Description

METHOD OF CONTROLLING A TURBINE ROTOR

This invention relates to a method of controlling a turbine rotor to mitigate wake flow disturbance effects in operation, for example in tidal or wind turbines.

When a rotor is operating downstream of its support structure, at some point in a full rotation, each blade of the rotor passes through the wake of the support structure, which has a reduced velocity relative to that of the general flow. Reducing the velocity of the flow reduces the dynamic pressure on the blade as it passes through the wake and consequently, the lift coefficient of the hydrofoil or aerofoil sections of the blade. This combination introduces cyclic fluctuations which may cause fatigue damage to the blades, powertrain and support structure and additionally may cause fluctuations in rotor torque and therefore power.

In accordance with the present invention, a method of controlling a turbine rotor to mitigate wake flow disturbance effects in operation comprises modifying a parameter of the turbine rotor at one or more specific rotor azimuth angles for each rotation of the rotor.

Preferably, the parameter of the turbine rotor comprises one of turbine speed, turbine torque, or blade pitch angle.

Preferably, the method further comprises obtaining data relating to load fluctuations in the rotor during each rotation of a plurality of rotations of the rotor; deriving recurring load fluctuations present over the plurality of rotations, together with a rotor azimuth angle, or arc of rotor azimuth angles, at which the fluctuations occur; and modifying the parameter at the one or more specific rotor azimuth angles, or over an arc of rotor azimuth angles.

Preferably, the method further comprises modifying the parameter, then repeating the step of obtaining data relating to load fluctuations in the rotor.

Preferably, a minimum number of rotations is set for obtaining data relating to load fluctuations in the rotor during each rotation of a plurality of rotations of the rotor. Preferably, the minimum number of rotations required for obtaining data is set to a higher value after a parameter has been modified.

Preferably, the load fluctuations comprise one of rotor speed fluctuations, rotor power fluctuations; blade strain measurement fluctuations; or turbine rotor structural strain measurement fluctuations.

Preferably, the steps of obtaining data relating to load fluctuations and deriving recurring load fluctuations are carried out in a simulation of turbine rotor operation.

Typically, the flow disturbance effects result from one of velocity shear, crossbeam wake, or pile wake.

Preferably, the method further comprises initiating a change to a blade pitch angle at a rotor azimuth angle in advance of the rotor azimuth angle at which wake flow disturbance effects occur, or a load fluctuation commences.

Preferably, the method further comprises initiating a change to a blade pitch angle at a rotor azimuth angle in advance of the rotor azimuth angle at which wake flow disturbance effects cease, or a load fluctuation terminates.

Preferably, the step of modifying a parameter of the turbine rotor is carried out by a turbine controller An example of a method of controlling a turbine rotor according to the present invention will now be described with reference to the accompanying drawings in which: Figure la shows a side view of a typical rotor configuration; Figure lb shows a plan view of the configuration of Fig. la; Figure 2 illustrates resultant velocity vector and blade pitch angle when using the configuration of Fig.l a; Figure 3 shows variation in lift coefficient with angle of attack; Figure 4 illustrates blade root bending moment fluctuation with rotor azimuth angle; Figure 5 is a flow diagram of a first embodiment of an example of a method according to the present invention; and, Figure 6 is a flow diagram of a second embodiment of an example of a method according to the present invention.

The present invention controls a turbine parameter, such as rotor speed, rotor torque or rotor blade pitch angle in order to compensate for the effects of flow disturbance in the wake of a support structure. The specific examples given below are described with respect to the blade pitch angle option, but the invention is not limited to this parameter. In Fig. la, a side view of a typical rotor configuration shows a support structure 1 on which a rotor 2 is mounted. Arrow 3 illustrates the direction of flow past blades 4 of the rotor. Fig. lb shows the same arrangement in plan view. As the flow comes into contact with the support structure 1, the flow is diverted around the structure, forming a wake 5 of disturbed flow. The rotating blade then has to pass through this area of disturbed flow. The same problem occurs with rotors mounted on crossbeams, rather than directly to the support structure (not shown), although, in general, the crossbeam provides a smaller cross-section to the flow, so the change from normal flow to disturbed flow and back again may occur more quickly and the overall impact may be less.

The problem is one that occurs in tidal turbines more than in wind turbines because wind turbines can avoid this type of flow disturbance by rotor yawing -i.e. the rotor is maintained upstream of the tower at all times. Yawing is needed in wind turbines as the wind can come from any direction, so yawing does not add cost to a wind turbine, as it would already be present. However, for tidal turbines, the tidal flow is generally bidirectional, so yawing systems add complication and expense that would not otherwise be required, particularly in terms of maintenance of subsea turbines.

Thus, it is desirable to find a solution which avoids incorporating yawing in tidal turbines. However, that does means that the effects of tower or crossbeam shadow need to be mitigated.

When a rotor is operating downstream of its support structure, each blade will pass through the wake of at least one support structure 1, for example a tower or crossbeam, which gives rise to a shadow region in which there is disturbed flow. In the wake of the structure, the inflow velocity is reduced, which reduces the dynamic pressure on the blade. Fig.2 shows the support structure 1 and blade 4 along with the vectors, UA, UB and tic which represent the freestream flow, UA and wake flow, UB and Uc, illustrating the relationship of the pitch angle of the blade to the resultant flows.

An initial pitch angle for the blade is set, shown by outline 4a, and in freestream flow 7, vector UA gives a resultant velocity vector 8 at an angle OA. For the reduced wake flow 9, vector UB, the resultant velocity vector 10 on to the blade is at a reduced angle OB to the blade, i.e. at a reduced angle of attack. A reduction in angle of attack reduces the lift coefficient of the hydrofoil, or aerofoil, section of the blade. The combination of the reduced dynamic pressure due to the reduced velocity of the wake and the reduced lift coefficient due to the reduced angle of attack at the reduced velocity causes a transient reduction in the forces on the blade (and therefore the torque on the rotor) as the blade passes through the wake of the support structure. This transient reduction is cyclic and may cause fatigue damage to the blades, powertrain and support structure, or give rise to fluctuations in rotor torque and therefore power. To overcome these problems, an operating parameter of the rotor may be adjusted. Although, more than one parameter may be adjusted at the same time, this adds complexity to the process, so typically only one parameter is adjusted at a time.

As illustrated in Fig.2, by modifying the blade pitch angle in the wake, as illustrated by blade 4b in its new position, the angle Oc at which the resultant velocity vector 10, from reduced wake flow vector Uc, is incident on the blade, is increased, i.e. the angle of attack is increased, increasing the lift coefficient to more closely match that achieved in freestream flow. The graph of Fig.3 illustrates the relationship of lift coefficient and angle of attack for the blade in freestream flow 12; for the blade in the same orientation, but in the tower wake 13; and for the blade in the tower wake 14, but a different orientation, the blade pitch angle having been modified to compensate for the reduced velocity. As can be seen in the graph of Fig.3, adapting the pitch of the blade to the nature of the flow may even mean that the lift coefficient obtained is higher than with freestream flow. Fig.4 illustrates how the blade root bending moment is affected in the part of a rotation in which the blade is in the tower shadow over arc 17 and by comparison, the substantial improvement achieved when the pitch angle of the blades rotating through the tower shadow is modified to compensate for the flow velocity reduction in this region. Rotation through the tower shadow with no blade pitch angle correction is illustrated by line 15 and rotation through the tower shadow with blade pitch angle correction is illustrated by line 16.

As a specific example, if the blade is designed so that in freestream flow the angle of attack on to the hydrofoil, or aerofoil is 0A=5 degrees, which gives a lift coefficient of 0.9 from the graph of Fig.3, then when the blade enters the support structure wake, the free stream velocity reduces with the blade pitch angle remaining the same, so the angle of attack reduces to 0,3=3 degrees, giving a lift coefficient of 0.7. The reduction in dynamic pressure (resulting from reduced inflow velocity) and reduced lift coefficient combine to cause a fluctuation in blade root bending moment, measured in kNm, for a given rotor azimuth angle, or arc of angles, measured in degrees, as illustrated in Fig. 4 and may also cause rotor torque fluctuation. By reducing the blade pitch angle, as described above, the angle of attack is maintained, or increased slightly. In this specific example, the angle of attack is increased to compensate for reduced dynamic pressure, resulting in Oc = 6 degrees and a lift coefficient of 1. This gives a much smaller fluctuation in blade root bending moment over the range of rotor azimuth angles 17 where there is tower shadow as shown in Fig.4, as well as there being less torque fluctuation. This means that fatigue on the blade, powertrain and structure is reduced.

In its simplest form, the invention controls the blade pitch angle, so that as an individual blade passes through the wake of the support structure, the blade pitch angle is reduced resulting in the angle of attack onto the blade foil sections being increased slightly. Thus, the lift coefficient increases and fluctuations in blade root bending moment and rotor torque are minimised. Similarly, the invention may control the rotor speed or rotor torque to compensate for the effects of wake flow disturbances.

However, tidal turbine tower shadow is particularly abrupt in nature and limits the effectiveness of real time load fluctuation measures, so a further improvement to the invention is to control more closely, exactly when the blade pitch angle is adjusted.

A significant proportion of rotor blade load fluctuations occur at the same position, or rotor azimuth angle, within each cycle of rotation. This is due the spatially fixed positions of features within the water column that cause load fluctuations, i.e. the flow disturbance or velocity variations, such as velocity shear, crossbeam wake and pile wake. Instead of just detecting disturbance and modifying the blade pitch angle in response, then detecting a return to normal flow and resetting the blade pitch angle, the point at which the disturbance begins and ends in terms of the rotor azimuth angle over each 360° rotation is detected for a plurality of cycles.

For example, during commissioning, or after installation, over a number of rotation cycles, information about where flow disturbance begins and ends is recorded and averaged to give an average start point rotor azimuth angle 17a and an average end point rotor azimuth angle 17b. In the example of Fig.4, the disturbance extends over an arc 17. From this a controller determines the correct rotor azimuth angle 18 at which a change to the blade pitch angle needs to begin, in order that the blade is already at the new blade pitch angle when it enters the disturbed flow of the wake of the support structure. Similarly, the rotor azimuth angle 19 at which to change back to the blade pitch angle for freestream flow is controlled, so that the change has been completed by the time the blade re-enters the freestream flow.

If the parameter to be modified were turbine speed, or turbine torque, then a change of speed or torque may be initiated at a rotor azimuth angle in advance of the rotor azimuth angle at which wake flow disturbance effect first occurs, or a load fluctuation commences and a further change, or change back, of the turbine speed or turbine torque may be initiated at a rotor azimuth angle in advance of the rotor azimuth angle at which wake flow disturbance effects cease, or a load fluctuation terminates. By implementing a cyclic feedback technique, blade pitching, or other load fluctuation limiting actions may be initiated in advance of the flow disturbance being encountered, and therefore the extent and effectiveness of the limiting action is increased. The cyclic feedback technique of the present invention monitors the angular position, rotor azimuth angle, of repeating load fluctuations in order to initiate load fluctuation reducing measures in advance of encountering the disturbance. Similarly, the technique may be used to begin to return to a standard angle in advance of exiting the disturbance.

This reduces the load fluctuations experienced by an axial flow tidal stream turbine as the rotor blades turn within the water column during generation. By reducing these load fluctuations, there is less fatigue suffered in components and structural or load carrying sections of the turbine, so the turbine cost can be reduced as the rotor does not have to be constructed to cope with these loads as a matter of course.

The controller may make use of one or more of a number of parameters to detect load fluctuations, including rotor speed fluctuations, blade strain gauge measurements or structural strain gauge measurements. The controller detects, records and processes the arc of rotor angles across which each fluctuation occurs and from the combination of rotor angle and fluctuation data, the controller is able to determine the changes required to a turbine parameter to reduce blade load fluctuations, resulting from the blade passing through a flow disturbance during power generation. The controller then controls the required turbine parameter, such as speed, torque or blade pitch angle.

The invention is able to use a control strategy that initiates the measures to reduce load fluctuations in advance of the flow disturbance by mapping magnitude and angular positions (arc lengths) over which the disturbance is encountered for a number of cycles, so that the mitigating action can be initiated in advance of encountering the disturbance.

The controller is not limited to using real world measurements to determine the correct point in a rotation cycle to start and stop a change to the blade pitch angle, but this information can be obtained from a simulation of the expected flow behaviour for the support structure and turbine combination and the start and stop points set accordingly before installation. Clearly, this could be combined with optimisation from actually recorded measurements and analysis of the effects of any changes applied in response to these.

Figs.5 and 6 illustrate examples of a method of operating a turbine system using flow disturbance mitigation in accordance with the present invention. In one embodiment, the system is provided with sensors to detect one or more measurements from the turbine in operation. In another embodiment, a simulation of the turbine and support provide the data on which to base control of turbine parameters when the turbine is installed and operating. The operator selects which option will apply for the turbine in question. If obtaining data from the turbine, the operator may set 20 which measurements are being detected, based on the sensors available on that turbine. A minimum number of cycles over which data is to be collected is also set 21. The turbine may already have been commission and be in operation, or the measurements may be taken during commissioning, but from the perspective of the cyclic feedback, operation of the turbine starts 23 with the first of the minimum number of cycles over which measurements are being made 24.

The parameter which is to be modified in response to the outcome of the measurements is chosen, either during set up by the operator, or when the minimum number of cycles has been completed and the data is ready for processing. Before processing the data to determine what correction to apply, the controller determines 25 which parameter is being modified, so that it can calculate what adjustments to that parameter are required as part of the processing 26. Having made the modification to whichever parameter has been chosen, then the monitoring may be continued 27 throughout the period of operation, or may be carried out at intervals, or only on request from the operator. When further measurements are made, they are monitored to see whether they fall outside and expected range over the minimum number of cycles and if so, the data is processed and appropriate further adjustment to the parameters can be made. The minimum number of cycles may be different for the long term monitoring to that set for commissioning or initial set-up to avoid occasional inconsistencies giving rise to constant adjustment.

If the data for setting the cyclic adjustment of a parameter of the turbine is being determined by simulation as illustrated in Fig.6, the necessary design details about the turbine and its support are entered 30 in the simulation. A minimum number of cycles over which the simulation is to run is set 31 and the simulation is run 32 for at least that number of cycles. The results are then processed 33 to determine where cyclic disturbances are expected in terms of the rotor azimuth angle over a full rotation. The parameter to be modified is determined 24 and the modification applied to that parameter. The simulation is then run again for the minimum number of cycles and the data processed and checked to see whether any further adjustment is required. The run and check stages may be repeated for a number of times, which the operator can set. When the operator has completed all the test runs, then he will take the results and use this as the basis for setting up the turbine control systems, so that the installed turbine modifies the chosen parameter at the appointed angles around each full rotation of the rotor. A combination of simulation for the initial set-up and recorded actual measurements in operation for fine tuning could also be used.

Claims

CLAIMS1. A method of controlling a turbine rotor to mitigate wake flow disturbance effects in operation, the method comprising modifying a parameter of the turbine rotor at one or more specific rotor azimuth angles for each rotation of the rotor.
2. A method according to claim 1, wherein the parameter of the turbine rotor comprises one of turbine speed, turbine torque, or blade pitch angle. 10
3. A method according to claim 1 or claim 2, wherein the method further comprises obtaining data relating to load fluctuations in the rotor during each rotation of a plurality of rotations of the rotor; deriving recurring load fluctuations present over the plurality of rotations, together with a rotor azimuth angle, or arc of rotor azimuth angles, at which the fluctuations occur; and modifying the parameter at the one or more specific rotor azimuth angles, or over an arc of rotor azimuth angles.
4. A method according to claim 3, wherein the method further comprises modifying the parameter, then repeating the step of obtaining data relating to load fluctuations in the rotor.
5. A method according to claim 3 or claim 4, wherein a minimum number of rotations is set for obtaining data relating to load fluctuations in the rotor during each rotation of a plurality of rotations of the rotor.
6. A method according to claim 5, wherein the minimum number of rotations required for obtaining data is set to a higher value after a parameter has been modified.
7. A method according to any of claims 3 to 6, wherein the load fluctuations comprise one of rotor speed fluctuations, rotor power fluctuations; blade strain measurement fluctuations; or turbine rotor structural strain measurement fluctuations.
8. A method according to any of claims 3 to 7, wherein the steps of obtaining data relating to load fluctuations and deriving recurring load fluctuations are carried out in a simulation of turbine rotor operation.
9. A method according to any preceding claim, wherein the flow disturbance effects result from one of velocity shear, crossbeam wake, or pile wake.
10. A method according to any preceding claim, wherein the method further comprises initiating a change to a blade pitch angle at a rotor azimuth angle in advance of the rotor azimuth angle at which wake flow disturbance effects occur, or a load fluctuation commences.
11. A method according to any preceding claim, wherein the method further comprises initiating a change to a blade pitch angle at a rotor azimuth angle in advance of the rotor azimuth angle at which wake flow disturbance effects cease, or a load fluctuation terminates.
12. A method according to any preceding claim, wherein the step of modifying a parameter of the turbine rotor is carried out by a turbine controller