A method for controlling the boosted power output of a power generation unit

Abstract

A method for controlling the power output of a power generation unit (18, 21) is disclosed. The cumulative power output of the power generation unit (18, 21) during a predefined time interval is predicted. The actual power output of the power generation unit (18, 21) is measured during the predefined time interval, and the actual cumulative power output is estimated based on the measured actual power output of the power generation unit (18, 21) during the predefined time interval. The difference between the predicted cumulative power output and the estimated actual cumulative power output is obtained. If the estimated actual cumulative power output is lower than the predicted cumulative power output and the difference between the predicted cumulative power output and the estimated actual cumulative power output is greater than a predefined threshold, the power output of the power generation unit (18, 21) is increased.

Description

Technical Field

[0001] The present invention relates to a method for controlling the power output of a power generation unit, such as a wind turbine. More particularly, the method according to the present invention ensures that a predicted power output is achieved without overloading the power generation unit. Furthermore, the present invention relates to a renewable power plant comprising at least one power generation unit controlled in accordance with this method. Background Art

[0002] Transmission system operators (TSOs) forecast grid load demand and / or power production to ensure supply and demand reliability, serving as the foundation for operating the power system. In order for TSOs to properly manage the grid, independent power producers (IPPs) in the electricity market, such as wind farm owners, must provide the TSO with a forecast of the cumulative power production they expect to be able to provide to the grid over a predefined future time interval. If the forecasted value is not met, the IPPs are often penalized by the TSO.

[0003] Therefore, independent power producers have a significant concern about providing the power they actually produce in accordance with the power forecast. The difference between the power forecast and the power actually produced is sometimes referred to as the forecast error. Summary of the Invention

[0004] An object of the present invention is to provide a method of controlling the power output of a power generation unit in which the prediction error is reduced without causing excessive fatigue loads to the power generation unit.

[0005] According to a first aspect, the present invention provides a method for controlling power output of a power generation unit, the method comprising the steps of:

[0006] forecasting the cumulative power output of a power generation unit during a predefined time interval,

[0007] Measuring the actual power output of a power generation unit during a predefined time interval,

[0008] estimating the actual cumulative power output in a predefined time interval based on the measured actual power output of the power generation unit,

[0009] Determine the difference between the forecasted cumulative power output and the estimated actual cumulative power output, and

[0010] When the estimated actual cumulative power output is lower than the predicted cumulative power output and the difference between the predicted cumulative power output and the estimated actual cumulative power output is greater than a predefined threshold, the power output of the power generation unit is boosted.

[0011] Thus, according to a first aspect, the present invention provides a method for controlling the power output of a power generation unit. As used herein, the term "power generation unit" should be interpreted as a unit capable of generating electricity and supplying all or part of the generated electricity to a power grid. The power generation unit may be a renewable power generation unit such as a wind turbine, photovoltaic cell, or the like.

[0012] In this document, the term "grid" should be interpreted as an interconnected electrical network that delivers electricity from producers to consumers. Furthermore, grids can have different topologies. Examples of such topologies include radial and meshed grids.

[0013] In the method according to the first aspect of the present invention, the cumulative power output of the power generation unit during a predefined time interval is initially forecasted. Thus, an estimate is made of how much energy the power generation unit is expected to supply to the grid during a specific future time interval. This estimate can, for example, be based on a given set of expected conditions, such as environmental conditions, such as weather forecasts, fatigue levels of the power generation unit, requirements of the transmission system operator, etc.

[0014] A weather forecast may be a prediction of atmospheric conditions for a given location and time. In addition, a weather forecast may depend on variables such as temperature, air pressure, humidity, precipitation, wind speed, wind direction, solar radiation, etc.

[0015] Fatigue levels are a measure of the weakness in a material caused by repeated changes in stress. Components subjected to repeated loading, for example in the form of bending, such as wind turbine rotor blades, towers, and drivetrains, develop fatigue levels that can ultimately lead to damage or fracture of the component. Components are usually designed for a specific service life, and the fatigue level of a component ideally corresponds to its current "age." If a component's fatigue level is higher than expected, the fatigue load on the component should be reduced. On the other hand, if a component's fatigue level is lower than expected, the load on the component can be increased without reducing the expected service life below the design life.

[0016] Predefined time intervals can be defined by different time scales ranging from minutes to days. For example, a predefined time interval can have a duration of several hours, such as 1-10 hours. A predefined time interval can start immediately or be a time interval in the future, such as the next week.

[0017] Next, the actual power output of the power generation unit during a predefined time interval is measured. This measurement can be performed directly, for example, using a power meter. Alternatively, the power output can be derived from other parameters that can be measured. For example, the power output can be derived by measuring the voltage and current of the power generation unit.

[0018] Thus, the power output actually produced by the power generation unit and provided to the grid is monitored during predefined time intervals and preferably reviewed against the forecasted cumulative power output to determine whether the produced power output is in accordance with the forecast.

[0019] Next, the actual cumulative power output for the predefined time interval is estimated based on the measured actual power output of the power generation unit. In this context, the term "actual cumulative power output" should be interpreted as an estimate of the expected total capacity of the power generation unit during the predefined time interval, i.e., the total amount of power that the power generation unit is expected to produce during the entire predefined time interval. This estimation is performed based on the actual power produced by the power generation unit during the predefined time interval. Thus, the estimation is performed at a point in time within the predefined time interval, i.e., after the start of the predefined time interval but before the end of the predefined time interval, and is based on the power produced to date and the capacity provided at the time the estimation is performed.

[0020] At any particular time during the predefined time interval, the cumulative power that has been produced since the beginning of the predefined time interval can be derived from the measured actual power output of the power generation unit. The actual cumulative power output for the entire predefined time interval can then be estimated by estimating how much power the power generation unit is expected to produce during the remainder of the predefined time interval and adding this to the power already produced. The estimated portion can, for example, be equal to the portion of the forecasted cumulative power output that corresponds to the remainder of the predefined time interval.

[0021] Alternatively or additionally, the estimate of the actual cumulative power output for the remainder of the predefined time interval may, for example, depend on a weather forecast, i.e. a weather forecast of expected meteorological conditions such as temperature, air pressure, humidity, precipitation, wind speed, wind direction etc. in and around the area of ​​the power generation unit during the remainder of the predefined time interval.

[0022] If the power generation unit is a wind turbine, the power generation capacity depends on wind conditions, particularly wind speed and direction. Therefore, a weather forecast regarding wind speed and direction at the wind turbine's location can be crucial for predicting the actual cumulative power output of the wind turbine. For example, if the expected wind speed is low, the expected power generation capacity is also low, while an expected wind speed above the rated wind speed results in an expected power generation capacity corresponding to the rated power. Furthermore, an expected wind speed above the cut-out wind speed results in an expected power generation capacity of zero, as the wind turbine would have to be shut down in this case.

[0023] Other relevant factors that are expected to affect production capacity during the remainder of the predefined time interval may also be considered.

[0024] Therefore, the estimated actual cumulative power output is a measure of the expected total power output of the power generating unit during the entire predefined time interval, but taking into account the power that has been produced since the beginning of the predefined time interval. The closer to the end of the predefined time interval, the more accurate the estimated actual cumulative power is expected to be.

[0025] Next, the difference between the forecasted cumulative power output and the estimated actual cumulative power output is calculated. The difference between the forecasted cumulative power output and the estimated actual cumulative power output is a measure of how much the expected total capacity differs from the forecasted capacity. Therefore, this difference indicates how far the estimated actual cumulative power output differs from the forecasted cumulative power output, and thus whether the forecast can be expected to be met within the predefined time interval. Furthermore, this difference indicates whether the estimated cumulative power output is higher or lower than the forecasted cumulative power output.

[0026] In the event that the estimated actual cumulative power output is greater than the forecasted cumulative power output, the total capacity is expected to be higher than forecasted, and therefore there is excess capacity. Therefore, unless something changes, it can be expected that the power generation units will provide a higher total power output than forecasted at the end of the predefined time interval. In this case, the power generation units can simply continue to operate in the same manner as they have been operating, thereby achieving a greater cumulative power output than forecasted, and the excess capacity can be sold at the market price.

[0027] Alternatively, the power generation unit can be operated to provide a reduced capacity during the remainder of the predefined time interval, thereby achieving a cumulative capacity closer to the forecasted cumulative capacity. This results in reduced wear and fatigue loads on the power generation unit, potentially extending the expected life of the power generation unit. Furthermore, this can also allow the power generation unit to be operated in a manner that increases fatigue levels without reducing the life of the power generation unit below the design life, for example, to increase capacity if required later.

[0028] Furthermore, operating a generation unit at reduced capacity allows the generation unit to have spinning reserve, which is additional generating capacity made available by increasing the power output of the generation unit. Such spinning reserve is valuable because transmission system operators are willing to pay generators to make spinning reserve available because the spinning reserve can be activated to support the grid, for example, in the event of a fault or emergency in the grid.

[0029] If the estimated actual cumulative power output is lower than the forecasted cumulative power output, the expected total capacity is lower than forecasted, resulting in a gap in capacity and the forecasted power output may not be achieved. As mentioned above, this may result in penalties, especially if the difference is significant. Therefore, in this case, it is beneficial to increase capacity during the remainder of the pre-defined time interval to avoid penalties and meet obligations to the grid.

[0030] The difference is therefore a measure of the expected difference between the forecasted cumulative power output and the estimated actual cumulative power output of the power generation unit at the end of the predefined time interval and provides an indication as to whether the forecasted power output is likely to be achieved for the entire predefined time interval.

[0031] Finally, when the estimated actual cumulative power output is lower than the predicted cumulative power output and the difference between the predicted cumulative power output and the estimated actual cumulative power output is greater than a predefined threshold, the power output of the power generation unit is increased.

[0032] As mentioned above, if there is a significant difference between the forecasted cumulative power output and the actual cumulative power output at the end of the pre-defined time interval, this may result in a penalty. Furthermore, if the estimated actual cumulative power output is significantly lower than the forecasted cumulative power output, this indicates that the actual cumulative power output will not reach the forecasted cumulative power output, and thus may result in a penalty.

[0033] Therefore, to avoid these penalties, when this occurs, the power output of the generating unit can be increased for the remainder of the predefined time interval, which is the case if the estimated actual cumulative power output is lower than the forecasted cumulative power output, and the difference is greater than a predefined threshold. Thus, the predefined threshold defines an upper limit on the size of the difference between the forecasted cumulative power output and the estimated actual cumulative power output, which represents the maximum acceptable difference before triggering / initiating the increase of the generating unit. For example, the threshold can represent a difference that will not result in a penalty, or will only result in an acceptable penalty.

[0034] This ensures that the difference is sufficiently large before increasing the power output of a generating unit. Therefore, the power output of a generating unit is increased only when the difference is too large. This ensures that obligations to the grid are met and penalties are avoided.

[0035] The ramp-up operation is configured with hysteresis around a predefined threshold or a timer applied to the ramp-up operation. The hysteresis can be zero or non-zero, and the timer can also be zero or non-zero. The purpose of using hysteresis or timers is to ensure that the ramp-up operation does not cause a sudden and continuous increase and decrease in power plant output, which may cause interference to the power grid.

[0036] In this document, the term "increasing the power output of a power generation unit" should be interpreted as increasing the power output of the power generation unit above the normal maximum permitted power output. In the case where the power generation unit is a wind turbine generator, the increased power output may correspond to a power output above the maximum power output for the wind turbine generator to extract power from the wind, for example, above the rated power output, wherein the wind turbine generator experiences an increased load that was not permitted in the original setting.

[0037] Therefore, the power output of the power generation unit is increased only when the estimated actual cumulative power output is lower than the forecasted cumulative power output and the difference between the forecasted cumulative power and the estimated actual cumulative power output is higher than a threshold value.

[0038] It is advantageous to increase the power output of a generating unit only when necessary, as this ensures that obligations to the grid are met and penalties are avoided without excessive strain on the generating unit. This discrepancy provides a quick and reliable way to determine whether the forecasted power output is likely to be achieved. Furthermore, since the decision on whether to increase power output is based on the estimated actual cumulative power output obtained before the predefined time interval has elapsed, forecast errors can be avoided, as capacity can be increased during the remainder of the predefined time interval, thereby narrowing the gap between actual capacity and forecast.

[0039] In the event that the expected total capacity is higher than forecasted, the excess capacity can be sold at market prices, or the generation unit can be operated to provide reduced capacity during the remainder of the predefined time interval, thereby extending the expected life of the generation unit. By reducing the power output of the generation unit, the generation unit is allowed to have spinning reserve, which can be used by the transmission system operator in the event of an imbalance between load and generation. In the event that the expected total capacity is lower than forecasted, the power output of the generation unit is increased during the remainder of the predefined time interval. As a result, the generation unit is able to provide a power output during the predefined time interval that is consistent with the forecasted cumulative power output of the generation unit during the same predefined time interval, which in turn allows for the reduction of forecast errors.

[0040] The power generation unit may be a wind turbine generator. A wind turbine generator is a power generation unit that converts the mechanical rotational power produced by the wind into electrical energy. A wind turbine generator typically has a tower carrying a nacelle that carries a rotor and a set of wind turbine blades mounted thereon. The nacelle may include electrical devices such as a generator, power electronics, etc. The nacelle may further be mounted on top of the tower, but may also be mounted on other parts of the tower, for example, in the case where the wind turbine is a multi-rotor wind turbine comprising two or more rotors. The wind turbine generator may be a stand-alone wind turbine generator, or it may form part of a wind farm comprising two or more wind turbine generators. Alternatively, the power generation unit may be in the form of another renewable power generation unit such as one or more photovoltaic cells, a hydroelectric unit, etc.

[0041] The step of forecasting the cumulative power output may be based on a weather forecast for the location of the wind turbine generator. The weather forecast may be a prediction of atmospheric conditions at the location of the wind turbine generator and during a predefined time interval. Furthermore, the weather forecast may depend on variables such as temperature, air pressure, humidity, precipitation, wind speed and direction, solar radiation, and the like.

[0042] For wind turbines, power generation depends on weather conditions, particularly wind conditions such as wind speed and direction. Therefore, a weather forecast related to wind speed and direction at the location of the wind turbine can be important for predicting the cumulative power output of the wind turbine. Therefore, basing such a forecast on a weather forecast that predicts weather conditions at the site and during the relevant time interval is advantageous.

[0043] The step of increasing power output may include changing the wind turbine generator's pitch control strategy. The goal of a pitch control strategy is to control the pitch angle of the wind turbine blades—that is, the angular position of the wind turbine blades about an axis disposed longitudinally relative to the wind turbine blades—in order to achieve a specific power output and adhere to a given power curve. Changing the pitch angle changes the angle of attack between the wind turbine blades and the incident wind, thereby altering the wind turbine's ability to extract energy from the wind.

[0044] By changing the wind turbine generator's pitch control strategy, the blade pitch is controlled according to the modified power curve. For example, a pitch control strategy can be selected that operates the wind turbine at a power rating higher than the design rating at wind speeds above the rated wind speed, thereby increasing power production. To achieve this, a more aggressive pitch strategy must be selected, causing the wind turbine blades to rotate further into the wind. This will result in increased loads on the wind turbine, which may not be acceptable in the initial setup. However, this is acceptable because it only occurs during a limited time interval.

[0045] Alternatively or additionally, the wind turbine generator's pitch control strategy may be modified to extend the wind turbine generator's cut-out wind speed. The cut-out wind speed is the wind speed at which the wind turbine's rotor is stopped by pitching the blades out of the wind to avoid damage from high winds. By extending the wind turbine generator's cut-out wind speed, energy can be extracted from the wind for a longer period of time at high wind speeds before pitching the wind turbine blades out of the wind, thereby increasing the amount of electricity produced.

[0046] Therefore, changing the pitch control strategy of a wind turbine generator is a suitable way to increase the power output of the wind turbine generator.

[0047] Alternatively or additionally, the step of increasing power output may include modifying the generator control strategy. The purpose of a generator control strategy is to control the generator torque and / or generator speed to achieve a specific power output and adhere to a given power curve. Generator torque refers to the rotational and torsional force acting on the generator shaft at a specific speed, while generator speed refers to the angular velocity of the generator shaft. Therefore, power generation can be controlled by appropriately controlling the generator torque and / or speed.

[0048] By changing the wind turbine generator's generator control strategy, the generator torque is controlled according to the modified torque-speed curve. For example, a generator control strategy may be selected in which the wind turbine generator is operated with increased generator torque, thereby increasing power generation. To achieve this, the current fed to the wind turbine generator's rotor may be increased, thereby increasing the magnetic field. This magnetic field generates a torque that rotates the generator shaft, thereby increasing the amount of power produced.

[0049] The power generation unit may form part of a renewable power plant comprising two or more renewable power generation units, which may be coupled to an electrical grid. In this document, the term "renewable power plant" should be interpreted as a collection of two or more renewable power generation units, such as wind turbines, photovoltaic cells, etc., which are arranged within a limited geographic area and may share various forms of infrastructure, such as access roads, communication networks, substations, power electronics, and grid connections.

[0050] According to this embodiment, the forecast of the cumulative power output of the power generation units during the predefined time interval can be the expected production capacity of each power generation unit of the renewable power plant. In this case, the cumulative power output is forecast for each power generation unit, and each power generation unit is controlled to provide a power output that meets the forecasted cumulative power output of the corresponding power generation unit.

[0051] Alternatively or additionally, the forecast of the cumulative power output of the power generation units during the predefined time interval can be the expected production capacity of the entire renewable power plant. In this case, for example, a cumulative power output is forecast for the entire renewable power plant based on the forecasts for the individual power generation units. The power generation units of the renewable power plant can then be controlled in such a way as to achieve a total power output of the renewable power plant that conforms to the forecasted cumulative power output of the entire renewable power plant.

[0052] A renewable power plant may include a power plant controller configured to control the power output of each power generation unit within the renewable power plant. The power plant controller is thus communicatively coupled to each renewable power generation unit. The power plant controller may further be communicatively coupled to a power grid. The power plant controller may derive control signals, including, for example, power setpoints, for each renewable power generation unit based on power grid requirements to ensure that the entire renewable power plant (i.e., the combination of all renewable power generation units) meets power grid requirements. Thus, the power plant controller ensures that the contributions from each power generation unit add up to the total required output of the renewable power plant.

[0053] When the renewable power plant includes such a power plant controller, at least some of the above-described method steps can advantageously be performed by the power plant controller. Thus, in this case, the power plant controller can, for example, estimate the actual cumulative power output based on the measured actual power output, compare it to the forecast, and determine whether one or more power generation units need to be boosted. Furthermore, if it is determined that one or more power generation units need to be boosted, the power plant controller can advantageously select which power generation units to boost. Finally, the control signals generated by the power plant controller for the power generation units can include boost signals when the relevant power generation units have been selected for boosting.

[0054] The method may further include the step of selecting one or more renewable power generation units that meet the power boost conditions, and the step of boosting the power output of the power generation units may be performed only when the power generation units selected meet the power boost conditions. According to this embodiment, only power generation units with the required qualities or meeting the required conditions are selected to provide the boosted power output. Therefore, according to this embodiment, the predicted cumulative power output of the renewable power plant can be achieved by boosting the output of only some of the power generation units.

[0055] The step of selecting one or more renewable power generation units can be based on the fatigue level of the power generation unit. Fatigue level is a measure of material weakness caused by repeated changes in stress. Components that are subjected to repeated bending, such as rotor blades on a wind turbine generator, may eventually develop cracks that may eventually lead to component failure. By first selecting renewable power generation units with the lowest fatigue level, it can be ensured that renewable power generation units with higher fatigue levels do not wear out before their design lifespan.

[0056] For example, in a renewable power plant that includes multiple wind turbines, the wind turbines may be exposed to winds from different directions. However, wind turbines facing the prevailing wind direction may typically be more heavily loaded and experience higher fatigue levels than wind turbines in the rest of the renewable power plant, particularly those located away from the boundaries of the renewable power plant (i.e., the central portion of the renewable power plant), because these wind turbines do not directly face the incident wind, regardless of wind direction. Therefore, wind turbines in the central portion of the renewable power plant may experience lower fatigue levels than wind turbines in the rest of the renewable power plant. Therefore, it may be appropriate to select wind turbines that are not currently facing the incident wind for upgrading.

[0057] Alternatively or additionally, the step of selecting one or more renewable power generation units can be based on the power generation unit's ramp-up history. For example, power generation units that have not been previously selected for ramp-up, or that have been selected for ramp-up for a relatively short period of time, can be selected before power generation units that have been frequently selected for ramp-up in the past. This ensures that the additional load caused by ramp-up is appropriately distributed among the power generation units in the renewable power plant, thereby achieving a more even wear and / or fatigue level across the power generation units. For example, it can ensure that no single power generation unit is selected every time a ramp-up is required.

[0058] Alternatively or additionally, the step of selecting one or more renewable power generation units can be based on the current production capacity of the power generation units. Renewable power generation units that produce high power outputs may already be nearing their limits, while renewable power generation units that produce low power outputs may be further from their limits. Therefore, increasing the power output of renewable power generation units that are already producing high power outputs may cause excessive wear on these power generation units, while increasing the power output of renewable power generation units that are currently producing low power outputs may not cause excessive wear on these power generation units.

[0059] The individual renewable power generation units may be selected based on a priority list indicating that the renewable power generation units meet the conditions for power boosting. The priority list may, for example, be based on the remaining life and / or age of the renewable power generation units and may be drawn up in advance, i.e. before a boost is required. In this case, the priority list is at hand when a boost is required. Thus, when it is determined that a boost is required, the power generation unit(s) to be boosted may be immediately selected from the priority list. The priority list may further be continuously updated during operation of the renewable power station. For example, a renewable power generation unit that has been boosted and / or has a higher fatigue level may have a lower position in the priority list than a renewable power generation unit that has not yet been boosted and / or has a lower fatigue level. The selection of wind power generation units for boosting may start from the highest ranked wind power generation unit to the lowest ranked wind power generation unit.

[0060] Therefore, in situations where a boost is needed, it may be an advantage to select renewable units with lower power output for boosting, as these renewable units can increase their power output more before reaching their limits without incurring excessive wear and tear. Therefore, renewable units with lower power output can take on more load without reducing their life expectancy below their design life.

[0061] The step of estimating the actual cumulative power output of the power generation unit may include estimating the cumulative power output at the end of the predefined time interval, assuming that the power output remains constant for the remainder of the time interval. According to this embodiment, at a certain point in time during the predefined time interval, the cumulative power output is estimated for the remainder of the predefined time interval, assuming that the power output for the remainder of the predefined time interval is constant. Thus, what is estimated is what the expected cumulative power output would be at the end of the predefined time interval if the power generation unit continued with the same power output, in particular the power output currently being produced, for the remainder of the predefined time interval, taking into account the power already produced. This is a simple and easy way to estimate the actual cumulative power output.

[0062] Alternatively or additionally, the step of estimating the actual cumulative power output of the power generation unit may take into account variations in known and expected operating conditions. For example, meteorological conditions such as wind speed, wind direction, precipitation, etc. may make the estimate more accurate.

[0063] The step of determining the difference between the forecasted cumulative power output and the estimated actual power output can include comparing the median value of the forecasted cumulative power output at a specific point in time during the predefined time interval with the actual cumulative power output at that specific point in time. According to this embodiment, the actual cumulative power output produced at a specific point in time during the predefined time interval is compared with the median value of the forecasted cumulative power output corresponding to the same point in time. Thus, it can be determined whether the actual production capacity complies with the forecast. Based on this, it can be determined whether the forecasted cumulative power output can be expected to be achieved by the end of the predefined time interval.

[0064] The method may further include the steps of repeatedly measuring the actual power output of the power generation unit, estimating the actual cumulative power output and deriving the difference, and the method may further include the step of stopping the increase in power output if the difference between the predicted cumulative power output and the estimated actual cumulative power output is lower than a predefined threshold.

[0065] According to this embodiment, after a decision is made to increase the capacity of a power generation unit, the actual capacity is continuously monitored and at a later point in time, but before the end of the predefined time interval, a new estimate of the actual cumulative capacity is derived in the above manner to forecast and assess whether the increase is still meaningful.

[0066] Therefore, if it turns out that the capacity has been increased to the point where it is now possible to reach the forecasted cumulative power output level without the increased capacity provided by the boost, the boost is stopped. This is the case if the difference is below a predefined threshold. This minimizes the fatigue load on the generating units.

[0067] Alternatively or additionally, the boosting of the power output of the power generating unit may simply be stopped after a predefined time interval has elapsed.

[0068] Therefore, by stopping the boosting of the power output of the power generation unit if the difference is below a predefined threshold, it is possible to ensure that the power output of the power generation unit is boosted only when needed.

[0069] On the other hand, if it turns out that the forecasted cumulative power output can only be achieved by continuing the ramped-up operation, then the ramp-up will not be stopped. Furthermore, if the new cumulative power output is lower than the forecasted cumulative power output, even assuming the current ramp-up level is maintained, then a decision may be made to increase the ramped-up capacity. Where the power generation units form part of a renewable power plant, this may include selecting one or more additional power generation units for ramp-up.

[0070] The method may further comprise the following steps:

[0071] receiving a signal indicating a need to increase the supply of power to a grid to which the power generating unit is connected, and

[0072] In response to the received signal, the power output of the power generation unit is increased.

[0073] According to this embodiment, the power output of the power generation unit may be increased in response to a signal indicating a need to increase power supply to the grid. The signal indicating the need to increase power supply may be a price signal indicating the price of electricity. A high price may indicate a shortage of power, while a low price may indicate a surplus of power. Therefore, if the price signal increases above a certain threshold, this may indicate a need for increased power supply, which is sufficient to justify increasing the power generation unit.

[0074] Alternatively or additionally, the grid may provide a signal indicating the need to increase power supply in the event of a fault, a sudden event in the grid, or a disconnection of one or more power generation units. Thus, for example, in the event of a power loss in the grid, the power output of the power generation units may be increased to stabilize the grid.

[0075] According to a second aspect, the invention provides a renewable power plant comprising a plurality of power generation units coupled to a grid, wherein each power generation unit is adapted to provide an electrical output to the grid, wherein the electrical output of at least one power generation unit is controlled according to the method according to the first aspect of the invention.

[0076] Therefore, according to a second aspect, the present invention provides a renewable power plant. As used herein, the term "renewable power plant" should be interpreted as a collection of two or more renewable power generation units, such as wind turbines, photovoltaic cells, etc., adapted to provide power output to a power grid, located within a limited geographic area, and sharing various forms of infrastructure, such as access roads, communication networks, substations, power electronics, and grid connections. According to the method of the first aspect of the present invention, the power output of the power generation units is controlled in the manner described above. Therefore, the comments above regarding the first aspect of the present invention apply equally here.

[0077] At least one of the power generation units may be a wind turbine generator. A wind turbine generator is a power generation unit that converts mechanical rotational power generated by wind into electricity. A wind turbine generator typically has a tower carrying a nacelle that carries a rotor and a set of wind turbine blades mounted thereon. The nacelle may include electrical equipment such as a generator and power electronics. The nacelle may further be mounted on top of the tower, but may also be mounted elsewhere on the tower, for example, if the wind turbine is a multi-rotor wind turbine including two or more rotors. The wind turbine generator may be a standalone wind turbine generator or may form part of a wind farm including two or more wind turbine generators.

[0078] Alternatively, the power generation unit may be in the form of another renewable power generation unit such as one or more photovoltaic cells, a hydroelectric unit or the like. BRIEF DESCRIPTION OF THE DRAWINGS

[0079] The present invention will now be described in further detail with reference to the accompanying drawings, in which

[0080] Figure 1 is a block diagram illustrating a method according to one embodiment of the present invention,

[0081] Figure 2 is a flow chart showing a method according to one embodiment of the present invention,

[0082] Figure 3 shows two power curves of a wind turbine generator operated according to a method according to an embodiment of the invention,

[0083] Figure 4 shows the production capacity of a power generation unit operated according to a method according to an embodiment of the invention as a function of time,

[0084] Figure 5 showing a renewable power plant operated according to a method according to a first embodiment of the invention,

[0085] Figure 6A renewable power plant is shown operating according to a method according to a second embodiment of the invention, and

[0086] Figure 7 is a block diagram illustrating a power plant controller used in a method according to an embodiment of the present invention. DETAILED DESCRIPTION

[0087] Figure 1 is a block diagram illustrating a method according to one embodiment of the present invention. A forecasting unit 1, in communication with a boost controller 2, receives meteorological data 3, such as temperature, air pressure, humidity, precipitation, wind speed, and wind direction, in the vicinity of a power generation unit (not shown) controlled according to the method. Based on this data, the forecasting unit 1 derives a forecast of the cumulative power output of the power generation unit during a predetermined future time interval. The predefined time interval can be defined by different time scales, ranging from a few minutes to a few days. The forecasted cumulative power output is an estimate of the expected production capacity of the power generation unit during the predefined time interval. The forecast generated in this manner is supplied to a summing point 4 of the boost controller 2 and can further be supplied to an estimator 5 of the boost controller 2.

[0088] At the beginning of a predefined time interval, the power generation unit begins producing electricity. The actual power produced is monitored, and information about the produced power is provided to the estimator 5. Information related to meteorological data is also provided to the estimator 5. Based on this information, the estimator 5 estimates the actual cumulative power output of the power generation unit during the predefined time interval. The estimated actual cumulative power output is the expected amount of total power that the power generation unit can produce during the predefined time interval.

[0089] At any particular time during the predefined time interval, the cumulative power that has been produced since the start of the predefined time interval can be derived from the measured actual power output of the power generating unit. The actual cumulative power output for the predefined time interval can then be estimated by estimating how much power the power generating unit is expected to produce during the remainder of the predefined time interval and adding this to the power already produced.

[0090] The estimated actual cumulative power output is supplied from the estimator 5 to a summing point 4 where the forecasted power output supplied by the forecast unit 1 is compared with the estimated actual cumulative power output to investigate whether it is likely that the forecasted cumulative power output will be reached at the end of the predefined time interval.

[0091] The summing point 4 outputs the algebraic sum of the inputs, ie, the difference between the predicted cumulative power output and the estimated actual cumulative power output.

[0092] This difference is provided from summing point 4 to boost decider 6, which forms part of boost controller 2. This difference is a measure of how much the expected total capacity differs from the forecasted capacity. Therefore, the difference indicates how much the estimated actual cumulative power output differs from the forecasted cumulative power output, and thus whether the forecast is expected to be met within a predefined time interval. Furthermore, the difference indicates whether the estimated cumulative power output is higher or lower than the forecasted cumulative power output. Based on this difference, boost decider 6 decides whether the power output of the generating unit should be boosted.

[0093] In the event that the estimated actual cumulative power output is greater than the forecasted cumulative power output, the total capacity is expected to be higher than forecasted, and therefore there is excess capacity. Therefore, unless something changes, it is expected that the total power output provided by the generation units at the end of the predefined time interval will be higher than forecasted, and therefore no boost signals will be sent to the generation units.

[0094] In the event that the estimated actual cumulative power output is lower than the forecasted cumulative power output, the expected total capacity is lower than forecasted, and therefore a gap in capacity exists, potentially preventing the forecasted power output from being achieved. This may result in a penalty, particularly if the difference is significant. Therefore, if the difference signal indicates that the estimated actual cumulative power output is lower than the forecasted cumulative power output, and if the difference is greater than a predefined threshold, the boost determiner 6 generates a boost signal and provides it to the generation unit, causing the generation unit to increase its capacity for the remainder of the predefined time interval. Thus, the generation unit's capacity is increased sufficiently to achieve the forecasted cumulative power output.

[0095] The boost determiner 6 may further receive a signal indicating power demand in the power grid. If this signal indicates power demand in the power grid, the boost determiner 6 may send a boost signal to the power generation unit, causing the power generation unit to increase its output. This may be useful, for example, in the event of a sudden power demand. For example, an imbalance between load and power generation may occur due to a fault, an emergency, or the disconnection of one or more power generation units in the power grid. In such cases, power from the power generation unit may be needed to stabilize the power grid, and therefore the power output of the power generation unit may be increased.

[0096] Alternatively or additionally, the signal indicative of the demand for electricity in the grid may be a price signal from the electricity market indicative of the price of electricity. A high price may indicate a shortage of electricity, whereas a low price may indicate an excess of electricity.

[0097] Figure 2is a flow chart showing a method according to one embodiment of the present invention. The method starts at step 7, where the cumulative power output of the power generation unit during a predefined time interval is forecasted. Figure 1 The forecast of cumulative power output is performed in a manner similar to that of FIG.

[0098] In step 8, the power generation unit is operated under normal conditions, for example following the design power curve, during which the actual power output of the power generation unit is measured.

[0099] In step 9, the actual cumulative power output in the predefined time interval is estimated based on the measured actual power output of the power generation unit. Figure 1 The estimation of the actual cumulative power output is performed in a manner similar to that of FIG.

[0100] In step 10, the difference between the predicted cumulative power output of the power generation unit and the estimated actual cumulative power output is obtained. Figure 1 The difference is obtained in this way.

[0101] In step 11, an investigation is performed to determine whether the estimated actual cumulative power output is lower than the forecasted cumulative power output, and whether the difference between the forecasted cumulative power output and the estimated actual cumulative power output is higher than a threshold value. If the actual cumulative power output is lower than the forecasted cumulative power output and the difference is higher than the threshold value, this indicates that the forecasted cumulative power output will not be reached by the end of the predefined time interval. Therefore, in this case, the process proceeds to step 12, where the power output of the power generation unit is increased for the remainder of the predefined time interval to ensure that the forecasted cumulative power output is reached. This increase can be achieved, for example, by operating the power generation unit according to another power curve that allows the power generation unit to operate above rated power.

[0102] If step 11 reveals that the actual cumulative power output is higher than the forecasted cumulative power output or the difference between the forecasted cumulative power output and the estimated actual cumulative power output is lower than the threshold, this indicates that it is possible to achieve the forecasted cumulative power output by the end of the predefined time interval, and therefore there is no need to increase the capacity of the power generation unit. Therefore, in this case, the process returns to step 8 to continue operation under normal conditions.

[0103] Figure 3 Two power curves 13, 14 of a wind turbine generator operated according to a method according to an embodiment of the present invention are shown. One of the power curves 13 represents the design power curve of the wind turbine generator. Therefore, under normal operating conditions, the wind turbine generator will be operated according to the power curve 13.

[0104] Another power curve 14 represents the boosted power curve of the wind turbine generator. Figure 3 It can be seen that when the wind turbine generator is operated according to the boosted power curve 14, a higher power output is obtained than when the wind turbine generator is operated according to the normal power curve 13. This applies to the part load region below the rated wind speed, as well as the full load region above the rated wind speed.

[0105] The wind turbine generator can switch between a normal power curve 13 and a boosted power curve 14. For example, the wind turbine generator may follow the normal power curve 13 in a partial load region, and then receive a boost signal indicating that the wind turbine generator's power output should be boosted. Upon receiving the boost signal, the wind turbine generator may switch to the boosted power curve 14 and control the wind turbine generator's power output according to the boosted power curve 14. When the boost is no longer required, the wind turbine generator may switch back to the normal power curve 13.

[0106] Figure 4 The power generation capacity of a power generation unit operated according to a method according to an embodiment of the present invention is shown as a function of time. The diagram shows a solid line 16 representing the operation of the power generation unit without an increase in capacity, and a dashed line 15 showing the operation of the power generation unit at an increased capacity. It can be seen that the capacity is increased during the time interval when the capacity was originally low.

[0107] Figure 5 A renewable power plant 17 is shown operating according to a method according to a first embodiment of the present invention. The renewable power plant 17 comprises a plurality of wind turbine generators 18, three of which are shown, and is connected to a power grid 19. The renewable power plant 17 further comprises a power plant controller 20, which is configured to derive control signals, for example including power set points, for the individual wind turbine generators 18 based on a demand from the power grid 19 and based on information provided by each wind turbine generator 18, in order to ensure that the entire renewable power plant 17, i.e., the combination of all wind turbine generators 18, meets the demand of the power grid 19. Thus, the power plant controller 20 ensures that the contributions from the individual wind turbine generators 18 add up to the total required output of the renewable power plant 17.

[0108] The power plant controller 20 performs at least some of the method steps of the claimed invention, namely, the power plant controller estimates the actual cumulative power output, compares it to the forecast, and determines whether one or more wind turbine generators 18 need to be ramped up. The power plant controller 20 further selects which wind turbine generators 18 need to be ramped up and forwards a ramp-up signal to the selected wind turbine generators 18. Thus, as described above, obligations to the grid 19 can be met and penalties can be avoided.

[0109] Figure 6 A renewable power plant 17 is shown operating according to a method according to a second embodiment of the invention. Figure 6 17 renewable power plants and Figure 5 The renewable power plant 17 is very similar and will not be described in detail here.

[0110] Figure 6 The renewable power plant 17 comprises two different types of renewable power generation units, namely as described above with reference to Figure 5 The wind turbine generator 18 and at least one photovoltaic cell 21. Figure 6 In the embodiment shown, the power plant controller 20 is configured as described above with reference to Figure 5 The control signals are provided to the wind turbine generator 18 and the photovoltaic cells 21 in a manner that is consistent with the present invention. Thus, in the event that a boost is determined to be required, the power plant controller 20 may select one or more photovoltaic cells 21 for boosting if deemed appropriate.

[0111] Figure 7 FIG. 2 is a block diagram showing a power plant controller 20 used in a method according to an embodiment of the present invention. Figure 7 The power plant controller 20 is applied, for example, to Figure 5 and Figure 6 renewable power plants.

[0112] The power plant controller 20 includes a power plant ramp-up control function 22 that receives input data in the form of power demand from the grid, actual measured power output of renewable power generation units in the form of wind turbine generators 18, meteorological data such as temperature, air pressure, humidity, precipitation, wind speed, wind direction, etc. in the vicinity of the renewable power plant, and a forecast of the cumulative power output of the renewable power plant during a predefined future time interval.

[0113] The lifting control function 22 is for example as described above. Figure 1 The received data is processed in a manner that generates a control signal for each wind turbine generator 18 within the renewable power plant, indicating whether the power output of each renewable power generation unit should be increased. Depending on the desired increase, the power plant controller 20 may increase the power output of one or more wind turbine generators 18 within the renewable power plant to avoid forecast errors at the renewable power plant level, as described above.

[0114] The power plant controller 20 may select which wind turbine generator(s) 18 to boost, as well as the boost level for the selected wind turbine generator(s) 18. In the event that two or more wind turbine generators 18 are selected for boosting, the boost levels may be the same for the selected wind turbine generators 18, or they may vary from one wind turbine generator 18 to another. The selection of the wind turbine generator(s) 18 may be based on, for example, the age and / or fatigue level of the respective wind turbine generator(s) 18.

Claims

1. A method for controlling the power output of a power generation unit (18, 21), the method comprising the following steps: Forecasting the power that a power generation unit (18, 21) will output during a predefined time interval, measuring the power output by the power generation unit (18, 21) from the beginning of the predefined time interval to a point in time before the end of the predefined time interval, estimating the power that the power generation unit (18, 21) will output from the beginning of the predefined time interval to the end of the predefined time interval based on the measured power, deriving a difference between the forecasted power and the estimated power, and increasing the power output of the power generation unit (18, 21) if (i) the estimated power is lower than the forecasted power and (ii) the difference between the forecasted power and the estimated power is greater than a predefined threshold value.

2. The method according to claim 1, wherein The power generation unit (18, 21) is a wind turbine generator (18).

3. The method according to claim 2, wherein The step of forecasting the power to be output by the power generation unit (18, 21) during the predefined time interval is based on a weather forecast at the location of the wind turbine generator (18).

4. The method according to claim 2 or 3, wherein The step of increasing power output includes changing the pitch control strategy of the wind turbine generator (18).

5. The method according to claim 2 or 3, wherein Steps to boost power output include changing generator control strategies.

6. The method according to any one of claims 1 to 3, wherein The power generation unit (18, 21) forms part of a renewable power plant (17) comprising two or more renewable power generation units (18, 21), the renewable power plant (17) being coupled to a power grid (19).

7. The method according to claim 6, further comprising the step of selecting one or more renewable power generation units (18, 21) that meet the power boost conditions, and wherein, The step of increasing the power output of the power generation unit (18, 21) is performed only when the power generation unit (18, 21) is selected to meet the power increase condition.

8. The method according to any one of claims 1 to 3, wherein The step of estimating the power that the power generation unit (18, 21) will output from the beginning of the predefined time interval to the end of the predefined time interval is based on a constant power output from said point in time to the end of the predefined time interval.

9. The method according to any one of claims 1 to 3, wherein The step of deriving the difference comprises comparing the median value of the forecasted power at a specific point in time during the predefined time interval with the actual cumulative power output at the specific point in time.

10. The method according to any one of claims 1 to 3, further comprising the step of stopping boosting power output when the difference between the forecasted power and the estimated power falls below a predefined threshold.

11. The method according to any one of claims 1 to 3, further comprising the steps of: A signal is received indicating a need to increase power supplied to a grid (19) to which the power generation unit (18, 21) is connected, and in response to the received signal, the power output of the power generation unit (18, 21) is increased.

12. A renewable power plant (17) comprising a plurality of power generation units (18, 21) coupled to a power grid (19), wherein: Each power generation unit (18, 21) is adapted to provide an electrical output to a power grid (19), wherein the electrical output of at least one power generation unit (18, 21) is controlled according to a method according to any of the preceding claims.

13. A renewable power plant (17) according to claim 12, wherein At least one of the power generation units (18, 21) is a wind turbine generator (18).

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