GB2534578A

GB2534578A - Turbine control method

Info

Publication number: GB2534578A
Application number: GB1501374.1A
Authority: GB
Inventors: Harrison Matthew
Original assignee: Marine Current Turbines Ltd
Current assignee: Marine Current Turbines Ltd
Priority date: 2015-01-28
Filing date: 2015-01-28
Publication date: 2016-08-03
Also published as: GB201501374D0

Abstract

A method of controlling operation of a turbine comprises setting a value of optimal mode gain for turbine operation, monitoring (36) one or more of a plurality of sensors, or other data sources and determining a change in data received from the one or more sensors, or the other data sources. Values are obtained (38) for one or more control parameters from stored data in response to the change in data received from the one or more sensors, or other data sources and the value of optimal mode gain set is modified (39) according to the obtained values. In operation the recalculated optimal mode gain is used by the controller to change the generator torque to ensure maximum power coefficient is achieved in the operating conditions identified. The controller may monitor data input to determine further changes in operating conditions and to allow torque to be further adjusted.

Description

TURBINE CONTROL METHOD

This invention relates to a method of controlling operation of a turbine, such as a wind or water current turbine, in particular for optimising power generation performance of a turbine operating in varying site conditions.

In power generation from wind turbines, or water current turbines, such as tidal turbines, the generator torque demand varies as a function of the square of the rotor generator speed and the optimal mode gain. Conventionally, the optimal mode gain has been set at the time of commissioning the turbine and the rotor generator speed depends upon the wind speed or current speed. However, optimal mode gain is affected by a number of parameters and applied torque is not always balanced with the rotor generator speed correctly to achieve maximum power coefficient for the turbine. In accordance with a first aspect of the present invention, a method of controlling operation of a turbine comprises setting a value of optimal mode gain for turbine operation; monitoring one or more of a plurality of sensors, or other data sources; determining a change in data received from the one or more sensors, or the other data sources; obtaining values for one or more control parameters from stored data in response to the change in data received from the one or more sensors, or other data sources; and modifying the value of optimal mode gain set according to the obtained values.

The present invention addresses the problem of accurately balancing the torque demand and the rotor generator speed by adjusting the optimal mode gain according to changes in the conditions which affect the value of the optimal mode gain Preferably, the sensors are chosen from bathymetric sensors; water density sensors; air density sensors, barometric sensors; water temperature sensors; and turbine condition sensors.

Preferably, the other data sources include simulated turbine wear data, or operational status of other turbines in an array of turbines in which the turbine is operating.

Preferably, outputs from sensors mounted on other turbines in a common body of fluid are monitored.

Preferably, operational status of other turbines in a common body of fluid is monitored.

Preferably, the control parameters comprise one of rotor radius, water density, air density, specified tip speed ratio; gearbox ratio; and power coefficient at the specified tip speed ratio.

Preferably, the method further comprises adjusting a value of generator torque in response to a change of set optimal mode gain.

Preferably, the method further comprises deriving and storing values of optimal mode gain and associated control parameters during commissioning of the turbine. Preferably, the turbine comprises a tidal turbine, a river current turbine, or a wind turbine.

An example of a method of controlling a turbine according to the present invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a typical example of a subsea tidal turbine, installed on the seabed, for which the method of the present invention may be applied; Figure 2 shows an array of the subsea tidal turbines of Fig.1, Figure 3 is a graph of power coefficient against tip speed ratio of a turbine, for different blockage scenarios for use in the method of the present invention; Figure 4 compares power coefficient for constant optimal mode gain to power coefficient with varying optimal mode gain, on the graph of Fig.3; Figure 5 is a flow diagram of commissioning steps for a method of controlling operation of a turbine according to the present invention; and, Figure 6 is a flow diagram of a method of controlling a turbine according to the present invention.

An example will now be described for a water current turbine, in this case a tidal turbine, although it could be a turbine installed in a river. In a water current turbine operating below the sea surface 13, either mounted to a pile 16 on the seabed 18 or to a surface piercing support (not shown), a turbine rotor 10 comprising hub 17 and blades 11 rotates as a tidal flow 21 passes through the blades. Rotation of the hub 17 turns a shaft 19 which is connected to a gearbox 14 within a turbine powertrain 20. The gearbox converts the relatively slow speed of rotation, of the order of 15 revolutions per minute (rpm) to a higher rotational speed, for example of the order of 1000 rpm suitable for generating electricity in a generator 15, also within the powertrain. Alternatively, if a direct drive generator is used, also working at 15 rpm, then the gearbox is not required. In Fig.2, an array of turbines 10A, 10B, 10c, 10D mounted on piles 16 on the seabed is shown. Conditions in the array may vary, due to changes in height of tide 13a, 13b, or due to which turbines are operating and which are not.

Optimal mode gain is a relationship between generator speed and generator torque demand. The ratio is chosen so that the rotor 10 is always operating at a specified tip speed ratio, i.e. the ratio between the tangential speed of the tip 12 of a blade 11 and the actual velocity of the flow 21. The optimal mode gain is a function of fluid density, i.e. water density or air density, as well as rotor radius, power coefficient at the specified tip speed ratio, the specified tip speed ratio and the gearbox ratio. Power coefficient Cp is the efficiency of the turbine relative to the free stream kinetic energy (free stream flow). Conventionally, tidal turbine control systems have been designed to operate the rotor at a constant optimal mode gain in any scenario.

However, it is desirable that the tidal turbine is at its optimal power coefficient, even when site conditions change, such as depth of water due to change in tidal height, or proximity, bathymetry and number of adjacent turbines. The values of desired tip speed ratio and the corresponding power coefficients are derived from mathematical modelling of the turbine and from commissioning tests. Conventionally, once these values have been derived, the rotor is always operated at the same optimal mode gain, no matter what the tidal height, site conditions, meteorological conditions, bathymetry, proximity of other tidal turbines and support structures and the operating conditions of other turbines (i.e. operating, not operating, or operating at reduced power).

Kopt = 5Cp 2X3G3 where Kopt = Optimal mode gain; p = water density; R = rotor radius; Cp = power coefficient at X; X = desired tip speed ratio and G = gearbox ratio The torque demand is then given by Qd = Koptn2 where Qd = generator torque demand; Q = measured generator speed In the paper "A free-surface and blockage correction for tidal turbines", J. Fluid Mech. (2009), vol 624, pp 281-291, it is demonstrated that the power coefficient of a turbine is dependent on blockage ratio, where blockage ratio is the area of turbines as a proportion of the area of the channel they are installed in. The greater the blockage ratio the higher the power coefficient will be, up to a limit where the flow chooses to avoid the turbine rather than pass through it.

Blockage ratio changes as the height of the tide changes, or when surrounding turbines are switched on or off, and therefore the optimal mode gain which ensures that the turbine is generating maximum power will vary depending on these conditions. This is illustrated for the array of Fig.2. A change in tidal height from low tide level 13b to high tide level 13a influences the optimal mode gain, as does the effect of switching turbines into or out of operation. If turbines 10A, 10c, were not operating, the optimal mode gain setting for turbines 10a, 1 OD would need to be different to that used when all turbines were operating, in order to generate maximum power efficiently.

In a working example of a tidal turbine, installed at Strangford Lough, the tidal height varies between 24.1m (lowest astronomical tide -LAT) and 28.4m (highest astronomical tide -FIAT). The geometry of the channel can be simplified as a simple rectangle of width 300m, as at the point of installation Strangford narrows is around 600m wide, and the channel is assumed to have a triangular cross-section, with varying depth as shown in Table 1 below.

Site LAT HAT Approx. width of equivalent rectangular channel (assuming triangular section) Strangford 24.1m 28.4 300m

Table 1

In order to improve efficiency of operation of the turbine, the present invention provides a method in which the optimal mode gain is varied to reflect the variation in the optimal tip speed ratio and power coefficient with varying operating conditions, as illustrated in the examples of Fig.3 and Table 2. This example is for a 16m SeaGen S system. Four different conditions are shown in Table 2, and Figure 3.

Scenario Optimal TSR Cp at optimal TSR Optimal mode gain No blockage correction 5.64 0.419 0.361 Depth 24.1m 6.06 0.469 0.326 Depth 28.4m 6.01 0.439 0.313 Depth 24.1m, 1 rotor operating 5.72 0.441 0.365

Table 2:

Fig.3 shows the relationship of power coefficient to tip speed ratio. Graph 1 shows the variation of power coefficient with tip speed ratio for a depth of 24.1m, graph 2 shows the variation of power coefficient with tip speed ratio for a depth of 28.4m; graph 3 shows the variation of power coefficient with tip speed ratio for a depth of 24.1m, with only 1 rotor operating, out of 2 and graph 4 shows the variation of power coefficient with tip speed ratio, with no blockage correction. With no blockage correction the graph 4 of Fig.3 shows that a maximum Cp of 0.419 will be reached at TSR of 5.64, giving an optimal mode gain of 0.361. With blockage correction applied graph 3 shows the optimal model gain would 0.365 at LAT with one rotor running with a Cp of 0.441 at optimal TSR of 5.72. Graph 2 shows that with a depth of 28.4m, i.e.at HAT the optimal mode gain is 0.313 with Cp of 0.439 at optimal TSR of 6.01. Graph 1 shows that the optimal mode gain is 0.361 with Cp of 0.419 at optimal TSR of 5.64. Table 3 below illustrates the effect on power generation efficiency for the different blockage conditions, if a constant optimal mode gain from the unblocked model is applied and compares this with the effect of varying the optimal mode gain according to the blockage conditions. Although not all situations give rise to losses, the losses in efficiency may be up to 0.6% at LAT in this example. Over the long timescales for which the turbines are expected to remain in operation, this loss of efficiency may add up to a significant amount, with corresponding cost implications.

For a larger array, the potential for loss of efficiency, or increased costs, is greater.

Scenario Optimal TSR from model without blockage correction Cp (Power coefficient) at fixed TSR Optimal TSR from model including blockage Cp at optimal TSR efficiency lost by operating at constant optimal mode gain Depth 24.1m 5.64 0.466 6.06 0.469 0.6% Depth 28.4m 5.64 0.459 6.01 0.439 0.5% Depth 24.1m, 1 rotor operating 5.64 0.441 5.72 0.441 0%

Table 3

Figure 4 shows the same graphs of power coefficient to tip speed ratio as in Fig.3, but with the power coefficient at fixed optimal mode gain marked and also the power coefficient when the optimal mode gain has been varied to take account of the operating conditions. It can be seen that the conventional mode of operation, which operates the turbine at constant optimal mode gain, and hence constant tip speed ratio (TSR), means that the turbine does not always operate at its maximum power coefficient. On graphs 1 to 4, this is indicated by mark 5. By contrast, when the turbine is operated in accordance with the method of the present invention, adapting the optimal mode gain better to the conditions in which the turbine is actually operating, then the turbine may be operated at the maximum power coefficient for the scenario, in this example depth. As can be seen, in several cases the tip speed ratio shown by mark 6 is greater than the tip speed ratio for fixed optimal mode gain The examples given above have been described with reference to water current turbines, but the invention is also applicable to wind turbines. For a wind turbine, water depth, or tidal height is not a concern, but operation of other turbines nearby does have an impact, so the data inputs and sensors are chosen accordingly.

The present invention varies the optimal mode gain during turbine operation to maximise the energy generated, rather than keeping optimal mode gain constant as has been the practice in the past. Optimal mode gain may be varied during operation to ensure that a predefined operating scenario is catered for. The optimal mode gain may be changed to a predefined value, chosen from a number of values which have been gathered by modelling and during commissioning tests, then stored along with the criteria on which they were based. The criteria include which parameters are measured or used to generate the predefined values. The measured parameters may be derived from data supplied by sensors which gather information on factors such as water depth, tidal height, meteorological conditions and operating conditions of other turbines. In operation, the controller determines the current conditions for the parameters which were used to determine the optimal mode gain values and uses the value which has been set for the most closely comparable set of conditions. The parameters chosen depend upon whether a wind or water current turbine is being considered.

Fig.5 illustrates some of the commissioning steps for a turbine to be operated according to a method of operating a turbine according to the present invention. The first step is to determine 30 which parameters are to be used to generate each power coefficient to tip speed ratio graph for the different blockage conditions envisaged for the turbine in operation. This may be done as part of the commissioning, or defined beforehand. For water current turbines, these parameters include height of tide, or for a river system, depth of the stream, which may vary at different times of year. For water or wind turbines, the parameters may include operational state of other turbines in an array of turbines in which the turbine is installed, e.g. generating or not generating; or physical state of the turbine, e.g. turbine roughness, which typically increases with age.

Measurements are then made 31 for those parameters which can be measured and the measurements are used 32, in combination with other data, for example turbine operational data which has been reported separately to a controller, rather than being determined from sensor data, or turbine roughness, which may have been simulated for aging effects and stored. A graph is then generated 33 for the specified blockage conditions. The graph of power coefficient versus tip speed ratio is stored 34, along with the criteria on which it was based. This process is repeated 35 until all the graphs that were specified in step 30 have been generated. The process then stops.

In operation, the data that has been derived during commissioning is used to adapt the optimal mode gain to the prevailing conditions to improve the efficiency of power generation from the turbine. This is illustrated in Fig.5. The controller may set up the turbine with default state conditions for its initial operation, until measurements from the sensors have been received and processed to determine 36 the actual blockage ratio for the turbine. For the determined blockage ratio, a stored power coefficient to tip speed ratio graph is chosen 37. The number of graphs produced can be adapted according to the effect on power generation, for example, in a water current turbine, the graphs may be generated for high tide, mid tide and low tide, or alternatively, they may be generated based on a fixed change in height from one to the next, e.g. every 0.1m, every 0.5m. If the primary concern is the impact of other turbines in the vicinity, for example for a wind turbine, graphs may be generated for a change in operational state of any one immediate neighbour, or for a change in state of a fraction of the number of immediate neighbours, e.g. 0.1, 0.25, 0.33. The required tip speed ratio for the maximum power coefficient is derived 38 from the chosen graph and the optimal mode gain for the chosen tip speed ratio and power coefficient is recalculated 39. The controller then uses this information to adjust generator torque 40, so that the maximum power coefficient is achieved for the measured set of operating conditions. The controller continues to monitor 41 inputs from the sensors and other data signal inputs, e.g. stored data, to see if the blockage ratio has changed. If a change is detected, steps 37 to 40 are repeated. Meanwhile, the monitoring step 41 continues.

The present invention allows for continuous control to keep the rotor and generator in balance, so that the generator runs neither under speed nor over speed. The present invention improves efficiency of the turbine over a range of operating conditions.

Claims

CLAIMS1. A method of controlling operation of a turbine, the method comprising setting a value of optimal mode gain for turbine operation; monitoring one or more of a plurality of sensors, or other data sources; determining a change in data received from the one or more sensors, or the other data sources; obtaining values for one or more control parameters from stored data in response to the change in data received from the one or more sensors, or other data sources; and modifying the value of optimal mode gain set according to the obtained values.
2. A method according to claim 1, wherein the sensors are chosen from bathymetric sensors; water density sensors; barometric sensors; water temperature sensors; and turbine condition sensors.
3. A method according to claim 1 or claim 2, wherein the other data sources include simulated turbine wear data, or operational status of other turbines in an array of turbines in which the turbine is operating.
4. A method according to any preceding claim, wherein outputs from sensors mounted on other turbines in a common body of fluid are monitored.
5. A method according to any preceding claim, wherein operational status of other turbines in a common body of fluid is monitored.
6. A method according to any preceding claim, wherein the control parameters comprise one of rotor radius, water density, air density, specified tip speed ratio; gearbox ratio; and power coefficient at the specified tip speed ratio.
7. A method according to any preceding claim, wherein the method further comprises adjusting a value of generator torque in response to a change of set optimal mode gain.
8. A method according to any preceding claim, wherein the method further comprises deriving and storing values of optimal mode gain and associated control parameters during commissioning of the turbine.
9. A method according to any preceding claim, wherein the turbine comprises a tidal turbine, a river current turbine, or a wind turbine.