Frequency support from power plants using power storage units

Abstract

The invention relates to a method for controlling a power plant (100) to provide frequency support to a power grid. The power plant comprises a power storage unit (150) and a power generation system (110), the power generation system comprising one or more power generation units (101), the power generation unit comprising at least one wind turbine generator (102). The method comprises setting a frequency control period, wherein the power storage unit is arranged to be charged or discharged according to a first power set point, and at the beginning of the frequency control period, the power storage unit is charged or discharged according to the power set point. In the event of a frequency deviation, a power change of power from the power storage unit is determined based on the frequency deviation. Frequency support is provided by controlling the power storage unit to be charged or discharged according to a second power set point determined from the power change.

Description

Technical Field

[0001] The invention relates to controlling a power plant, in particular a power plant having one or more wind turbines, and controlling an energy storage system of a power plant, in particular for providing frequency support of a power grid. Background Art

[0002] Electronic storage units, such as battery energy storage systems, can be used to supplement power output from power plants, such as renewable power plants (e.g., wind turbine power plants). Similarly, battery energy storage systems can store excess power output from power plants. The ability of energy storage systems to store and deliver power can be used to support grid frequency deviations by storing and supplying power to the grid when the grid frequency deviation is within the permitted frequency range.

[0003] EP 2 921 698 A1 discloses a system for automatic generation control in a wind farm. The system includes a wind farm controller configured to control a plurality of energy storage elements. The wind farm controller receives an automatic generation control setpoint from an independent system operator, generates a farm-level storage power setpoint for the wind farm based on the automatic generation control setpoint, generates a single storage power setpoint for the plurality of energy storage elements based on the state of charge of the corresponding energy storage elements, and controls the plurality of energy storage elements based on the single storage power setpoint to dispatch the storage power and perform automatic generation control. The wind farm can be controlled to provide additional power to compensate for frequency errors based on a wind farm power reference. Summary of the Invention

[0004] It is an object of the present invention to improve the control of a power plant to alleviate one or more of the above problems and thereby provide a method of improving frequency support of a power grid.

[0005] In a first aspect of the present invention, a method of controlling a power plant for providing frequency support to a power grid is provided, wherein the power plant comprises a power storage unit and a power generation system, the power generation system comprising one or more power generation units, the power generation unit comprising at least one wind turbine generator, wherein the power generation system is connected to a power grid for supplying power to the power grid, and wherein the power storage unit is electrically connected to the power generation system and controllable to charge or discharge depending on a power set point, wherein the method comprises:

[0006] - setting a frequency control period, wherein the power storage unit is arranged to be charged or discharged according to the first power set point,

[0007] - at the beginning of the frequency control period, charging or discharging the power storage unit according to the first power set point,

[0008] - if the grid frequency exceeds a first grid frequency which is above or below a frequency threshold, resulting in a first frequency deviation occurring within the frequency control period, determining a first power change of the power from the power storage unit based on the first frequency deviation,

[0009] - determining a second power set point based on the determined first power variation, and

[0010] -Providing frequency support by controlling the power storage unit to charge or discharge according to a second power set point.

[0011] Advantageously, by scheduling the charging and discharging of the power storage unit within a predetermined frequency control period, with charging or discharging initiated according to a first power set point, the utilization of the power storage system can be improved to provide frequency support. That is, by performing initial charging during the planned frequency support period, the power storage unit's power capacity can be used to support frequency deficits, i.e., the power storage unit provides a change in the power supplied to the grid, for example, by changing from charging power to discharging power. According to one embodiment, when the grid frequency exceeds a second grid frequency that is above or below the first grid frequency, resulting in a second frequency deviation within the frequency control period, a second power change is determined, a third power set point is determined based on the determined second power change, and frequency support is provided by controlling the power storage unit to reverse charging to discharging the power storage unit, or vice versa, according to the third power set point.

[0012] Advantageously, by reversing the direction of charging, maximum variation of the power supplied to the grid is possible when the power storage unit is pre-set to charge or discharge.

[0013] According to one embodiment, the maximum value of the second power variation is given by the sum of the first power setpoint and the nominal charge or discharge power. In other words, by reversing the charging direction and initially charging or discharging at the nominal charge or discharge power, the variation in power supplied to the grid can be maximized to twice the nominal charge or discharge power.

[0014] In one embodiment, providing frequency support is conditional on the level of the state of charge of the power storage unit. Advantageously, if the state of charge is too high or too low, use of the electronic battery to provide frequency support may be inhibited.

[0015] In one embodiment, setting the frequency control period includes:

[0016] - arranging charging of the power storage unit according to the first power set point and arranging discharging of the power storage unit during a frequency control period if the first or second frequency deviation indicates an underfrequency of the grid, or

[0017] - scheduling the power storage unit to discharge according to the first power set point and scheduling the power storage unit to charge during a frequency control period if the first or second frequency deviation indicates an over frequency of the grid.

[0018] Advantageously, by scheduling whether an electronic storage unit should be charged or discharged, for example due to a predicted under- or over-frequency, charging can be reversed to discharging or vice versa, thereby maximizing the variation in power that can be supplied to the grid.

[0019] According to one embodiment, the method comprises:

[0020] - forecasting a time period in which the power plant is able to produce a power plant output power that is higher than the planned required power plant output power, and

[0021] - setting a frequency control period within the forecast time period, wherein the power storage unit is scheduled to be charged according to the first power set point.

[0022] Advantageously, the forecasted excess power can be used to charge the electrical storage unit. The forecasted excess power can be used for scheduled charging with a given power that provides the power capacity of the electrical storage unit to support underfrequency events, for example, changing from charging power to discharging power.

[0023] According to one embodiment, the method comprises:

[0024] - forecasting a time period in which power plants are scheduled to produce power plant output that is lower than the planned required power plant output, and

[0025] - setting a frequency control period within the forecast time period, wherein the power storage unit is arranged to discharge according to the first power set point.

[0026] Advantageously, a predicted low power condition can be used to discharge the electronic storage unit. The discharge can be used to support an overclocking event if such an event occurs simultaneously with or overlaps with a low power period.

[0027] According to one embodiment, the method comprises supplementing the determined power variation with a generator power variation of the power produced by the power generation system. Advantageously, the power variation produced by the power generation system can supplement the electronic storage unit if the power variation available from the electronic storage unit is insufficient.

[0028] According to one embodiment, in the event that the electronic storage unit power variation is insufficient to achieve the determined power variation, the generator power variation is called upon to supplement the determined power variation.

[0029] A second aspect of the invention relates to a central controller for controlling a power plant to provide frequency support to a power grid, the power plant comprising a power storage unit and a power generation system comprising one or more power generation units, the power generation unit comprising at least one wind turbine generator, wherein the power generation system is connected to a power network for supplying power to the power network, and wherein the power storage unit is electrically connected to the power generation system and is controllable to charge or discharge depending on a power set point, and wherein the central controller is arranged to perform the method according to the first aspect.

[0030] A third aspect of the present invention relates to a power plant comprising a plurality of power generation units including at least one wind turbine generator and a central controller according to the second aspect.

[0031] A fourth aspect of the invention relates to a computer program product comprising software code adapted to control a power plant when executed on a data processing system, the computer program product being adapted to perform the method of the first aspect.

[0032] In general, the various aspects and embodiments of the invention may be combined and coupled in any way possible within the scope of the invention.

[0033] These and other aspects, features and / or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0035] Figure 1 shows a power plant comprising an electron storage unit and a plurality of power generation units and wind turbines,

[0036] Figure 2 An example showing the frequency-power function,

[0037] Figure 3 Showing the method steps of an embodiment,

[0038] Figure 4 An example is shown where the frequency control period is scheduled for charging and supporting underfrequency events, and

[0039] Figure 5 An example of frequency support during different frequency control periods is shown. DETAILED DESCRIPTION

[0040] Figure 1A power plant 100 is shown, comprising a power generation system 110 comprising one or more power generation units 101, each comprising at least one wind turbine generator 102. Power plant 100 also comprises a power storage unit 150. Power generation system 110 and power storage unit 150 are connected to a power grid 160 for supplying power to the grid. Power storage unit 150 can be controlled to charge or discharge depending on a power setpoint Pset_esu.

[0041] In general, the power generation units 101 can be composed of different types of power generation units, for example different types of renewable power generation units such as solar power generation units 103 (e.g., photovoltaic solar panels) and wind turbines 102. Different types of power generation units 101 can also include fossil-based power production units, such as diesel engines. According to one embodiment, at least one of the power generation units 101 of the power plant 100 is a wind turbine. The power plant 100 can include at least three power generation units 101 of the same or different types, i.e., a mix of different types of power generation units. For example, the power plant 100 can be composed of only wind turbines 102, in which case it is composed of at least three wind turbines 102. In another example, the power plant 100 includes at least two wind turbines 102 and at least one or two other power generation units 101.

[0042] The power plant 100 is controlled by a central controller 120. The central controller 120 is arranged to control the power generation of the power generation units 101 according to a power plant reference Pplant_ref, which defines the desired power to be supplied from the power plant 100 to the grid. Furthermore, the central controller 120 is arranged to assign power setpoints Pset to the power generation units, i.e., assigning an individual power setpoint to each power generation unit 101 to set the desired power output. The power setpoints Pset can be determined by the central controller 110 based on the power plant reference Pplant_ref, so that the sum of the power setpoints Pset corresponds to the power plant reference Pplant_ref.

[0043] Furthermore, the central controller 120 is arranged to determine a power set point Pset_esu for one or more power storage units 150 so that a given power storage unit 150 can be controlled to charge or discharge with the power set by the power set point Pset_esu.

[0044] Throughout the specification, the term power reference is used for the required power of the power plant 100 , while power set-point is used for the required power of the individual power generation units 101 and power storage units 150 .

[0045] The wind turbine generator 102 may include a tower and a rotor with at least one rotor blade (such as three blades). The rotor is connected to a nacelle mounted on top of the tower and adapted to drive a generator located within the nacelle. The rotor can be rotated by the action of wind. The rotational energy of the rotor blades caused by the wind is transmitted to the generator via a shaft. Thus, the wind turbine is able to convert the kinetic energy of the wind into mechanical energy with the aid of the rotor blades, and then into electrical energy with the aid of the generator. The generator may include a power converter for converting the generator's AC power into DC power and a power inverter for converting the DC power into AC power for injection into the power grid.

[0046] The generators of wind turbines 102 or other power generation units 101 can be controlled to generate power corresponding to a power setpoint Pset provided by central controller 110. For wind turbines, the output power can be adjusted according to the power setpoint by adjusting the pitch of the rotor blades or by controlling the power converter to adjust the power output. Similar adjustment possibilities exist for other power generation units 101.

[0047] Herein, any reference to power, such as plant power Pplant, power setpoint Pset, produced power Pprod, may define active, reactive, or apparent power levels. According to one embodiment, the power levels, such as Pplant, Pset, and Pprod, and other related power levels, are active power levels.

[0048] In one embodiment, the predicted power Pplant_f that a power plant can produce over a period of time in the future can be forecasted. The predicted power Pplant_f can be determined based on the current weather and weather forecast, including the wind speed of the wind turbine and the cloud cover of the solar power unit, as well as other weather parameters related to power output. The power that a wind turbine can produce, i.e., the available power, is the maximum possible output power of the wind turbine under given wind conditions. Therefore, the available power will be close to the power output according to the power curve optimized for the power of the specific turbine. The power curve used here can be understood as a power curve optimized for the power coefficient (Cp) of a specific turbine. In other words, the power curve represents the maximum power output of the turbine under normal operation as a function of wind speed.

[0049] The central controller 120 may include a frequency-power component 121 arranged to determine the requested power Pplant_req to be delivered by the power generation unit 101 and the power storage unit 150 to the grid 160 based on a frequency-power function, for example a function providing the power request as a function of frequency in the form of a curve or a lookup table. Thus, the central controller 120 may be configured to determine the power setpoints Pset, Pset_esu for the power generation unit 101 and the power storage unit 150, for example, with the aid of the frequency-power component 121. Alternatively, the power plant reference Pplant_ref provided as an input to the central controller 120 may be determined by an external frequency-power component (not shown) via the frequency-power function described above.

[0050] For example, for frequencies in the permitted range from a lower frequency threshold fTL, e.g., 50 Hz, below the nominal frequency fn, to a higher frequency threshold fTH, above the nominal frequency, the frequency-power component does not affect the power reference or power setpoint, i.e., the requested power Pplant_req to be supplied to the grid 160. For frequencies above fTH, the requested power Pplant_req is reduced, e.g., by reducing the power reference as a function of frequency. The reduction in requested power supports the grid by assisting in reducing the grid frequency back into the permitted range. Similarly, for frequencies below fTL, the requested power Pplant_req is increased to support the grid in raising the frequency back into the permitted range. For convenience, the lower frequency threshold fTL and the higher frequency threshold fTH are generally referred to as frequency thresholds fT.

[0051] Figure 2 An example of a frequency-power function 200 according to the above examples is shown. Figure 2 The figure shows a first frequency deviation Δf1 relative to the nominal frequency fn due to a first grid frequency f1 exceeding a frequency threshold fT (i.e., a lower or higher frequency threshold fTL, fTH). The frequency-power function results in a first power change ΔP1 due to the frequency deviation Δf1. As shown, for a larger second frequency deviation Δf2, in which the grid frequency exceeds a second grid frequency f2 that is lower or higher than the first grid frequency f1, the requested power Pplant_req can be further increased or decreased (depending on the sign of the frequency deviation). As shown, a second power change ΔP2 is determined based on the second frequency deviation Δf2.

[0052] Figure 3 The method steps of one embodiment are shown.

[0053] In step 301, a frequency control period is set, in which the power storage unit 150 is scheduled to be charged or discharged. The scheduled charging or discharging can be controlled according to the first power set point Pset_esu1. Therefore, a feedback control system can be used to limit the error between the desired first power set point Pset_esu1 and the actual charging or discharging during the frequency control period.

[0054] Frequency control periods may define different scenarios. For example, frequency control periods may define periods during which power plant 100 plans to supply frequency support to grid 160, ie, periods during which power, such as active power, supplied to grid 160 may be increased or decreased.

[0055] Step 302 is initiated at the start of a frequency control period, wherein charging or discharging of the power storage unit is initiated according to the planned first power set point Pset_esu1. The first power set point Pset_esu1 used at the start of a frequency control period may be modified corresponding to the planned first power set point Pset_esu1, for example due to changes in weather conditions.

[0056] In step 303, if a frequency deviation Δf1, Δf2 occurs within the frequency control period, i.e., if the grid frequency f exceeds a first grid frequency f1 that is above or below the respective upper and lower frequency thresholds fTL, fTH, a first change ΔP1 in power from the power storage unit 150 is determined based on the first frequency deviation Δf1.

[0057] In step 304 , if a first change in power ΔP1 is determined and frequency support is required, a second power set point Pset_esu2 is determined based on the first change in power ΔP1 .

[0058] That is, if the required power change ΔP1, i.e., the change in power Pplant to be supplied to the grid 160, is 400 kW, and if the planned first power set point Pset_esu1 is -500 kW (a negative power set point of esu indicates charging), the required power change ΔP1 can be achieved by setting the second power set point Pset_esu2 to -100 kW, thereby reducing the charging power Pesu.

[0059] In another example, if the required power change ΔP1 is 1000 kW and the planned first power set point Pset_esu1 is -500 kW, the required power change ΔP1 can be achieved by setting the second power set point Pset_esu2 to 500 kW, thereby changing the charging power of -500 kW to the discharging power of 500 kW. This enables the power Pplant of the power plant 100 to be increased by 1000 kW.

[0060] Therefore, the second power set point Pset_esu2 may result in a reduction in the magnitude of the charging or discharging power, or change an initial charging into a discharging or vice versa.

[0061] The magnitude and sign of the frequency deviations Δf1, Δf2 for a future period can be predicted, for example based on statistics of historical data. The predicted frequency deviations for the future period can be used to determine the frequency control period. Therefore, in one embodiment, the power storage unit can be arranged to charge or discharge during the frequency control period depending on the predicted frequency deviations Δf1, Δf2, such as the magnitude and sign of the frequency deviations.

[0062] Therefore, if the forecasted or predicted frequency deviation Δf1, Δf2 indicates underfrequency, initial charging of the power storage unit 150 can be planned within the frequency control period so that the maximum discharge power is used to support the grid. That is, if the initial charging power is given by Pset_esu1 and the nominal discharge power is Pnom_esu (i.e., the maximum charging power. The nominal charging and discharge powers here are considered equal and are both referred to as Pnom_esu), the power initially supplied to the grid 160 can be rapidly increased, ΔPgrid=Pnom_esu+Pset_esu1. For example, if the initial charging power is 500kW and the nominal discharge power is 500kW, the power initially supplied to the grid 160 can be rapidly increased to 1000kW, i.e., twice the initial charging power. If the predicted frequency deviation Δf1, Δf2 indicates overfrequency, the opposite is true. Thus, the scheduled charging or discharging enables the power supplied to the grid 160 to be varied, wherein the power variation is given by the sum of the initial charging or discharging power Pset_esu1 and the nominal discharging or charging power Pnom_esu.

[0063] In addition to forecasting the frequency deviation, the future plant output power Pplant_f can also be predicted, for example based on a weather forecast. Thus, a time period can be forecasted during which the power plant 100 can generate a plant output power Pplant_f that is higher than the planned required plant output power Pplant_req_f. During this forecasted time period, the excess power that the power plant 100 can generate in excess of the planned required plant output power Pplant_req_f can be used to charge the power storage unit 150. Thus, a frequency control period can be set depending on (e.g., within) the forecasted time period, during which the power plant can generate power in excess of the planned required power, and the power storage unit can be scheduled to charge according to the first power setpoint Pset_esu1 during the frequency control period. If the forecasted period of excess power occurs before or overlaps with a period of underfrequency in the grid, the excess charging power can be used to satisfy the determined first or second power variation ΔP1, ΔP2, for example, by reversing the charging power into discharging power.

[0064] Similarly, in a period in which the power plant 100 is scheduled to generate a power plant output power Pplant_f that is lower than the planned required power plant output power Pplant_req_f, for example due to a forecasted low available power output of the power generation unit 101, a frequency control period can be set within the forecasted period of low power output, and the power storage unit 150 can be scheduled to discharge according to the first power set point Pset_esu1 during the frequency control period to compensate for the low power output. If this forecasted period of low power occurs before a period of overfrequency of the grid, or overlaps with such a period, the discharged power can be used to meet the determined first or second power variation ΔP1, ΔP2, for example by reversing the discharged power into charging power.

[0065] In step 305 , the power storage unit 150 is controlled to charge or discharge according to the second power set point Pset_esu2 so that the power plant 100 assists in providing frequency support to the grid 160 .

[0066] Figure 4 An example is shown, wherein the frequency control period F_CTRL is arranged for charging, and wherein a first frequency control period F_CTRL1 involves frequency support.

[0067] Curve 402 shows the state of charge SoC of the power storage unit 150 relative to the charge level from 0 to 100%. Curve 401 shows the charging and discharging power of the power storage unit 150, which corresponds to the power set point Pset_esu (such as the first and second power set points Pset_esu1, Pset_esu2) of the power storage unit 150. The curves labeled ΔP1, ΔP2 show the power change determined in response to the frequency deviation, such as the first or second frequency deviation Δf1, Δf2.

[0068] From t=0 to t1, the power storage unit 150 is charged, and the state of charge (SoC) increases. From t1 to t2, there is no charging or discharging. At t2, at the beginning of the first frequency control period F_CTRL1, initial charging occurs according to the first power set point Pset_esu1. At t3, a frequency deviation causes a power change, such as a first or second power change ΔP1, ΔP2. The second or third power set point Pset_esu2, Pset_esu3 is determined based on the determined power changes ΔP1, ΔP2. The determined power changes ΔP1, ΔP2 can be achieved by reversing the charging at Pset_esu1 to discharging at Pset_esu2 / Pset_esu3. From t3 to t4, the state of charge (SoC) decreases. The frequency deviation ends at t4, and the state of charge increases again. The first frequency control period F_CTRL1 ends at t5. During the second frequency control period F_CTRL2, no frequency deviation occurs, and therefore, charging continues during the second frequency control period.

[0069] from Figure 4 It will be appreciated that a certain level of State of Charge (SoC) is required to be able to change the direction or magnitude of the charging current, or equivalently, to change the sign or magnitude of the charging power Pset_esu. Therefore, the ability of the power storage unit 150 to provide frequency support may be conditional on the level of the State of Charge (SoC) of the power storage unit. For example, a State of Charge (SoC) of at least 20% may be required to provide underfrequency support by discharging the power storage unit 150 during frequency deviations. Similarly, a State of Charge (SoC) of up to 80% may be required to provide overfrequency support by charging the power storage unit 150 during frequency deviations.

[0070] Figure 4 An example is also shown, in which the grid frequency f exceeds the first grid frequency f1 (refer to Figure 2). The second frequency deviation Δf2=f2−fn results in a second power variation ΔP2 and a third power set point set_esu3, which can be achieved by reversing the initial charging at the beginning of the frequency control period F_CTRL1 into a discharging according to the third power set point Pset_esu3.

[0071] The first and second grid frequencies f1, f2 are arbitrary frequencies, with f1 serving merely as an example where a frequency deviation may not require a reversal of charging, e.g., from charging to discharging, and f2 serving merely as an example where a frequency deviation may require a reversal of the charging current. This does not exclude the possibility that the first grid frequency f1 does not require a reversal of the charging current, or that the second grid frequency f2 does. The same analogy applies to the first and second frequency deviations Δf1, Δf2, and the second and third power setpoints Pset_esu2, Pset_esu3.

[0072] In some cases, the grid compensation power available to or from power storage unit 150 may not be sufficient to meet the desired power change ΔP, i.e., when the maximum electronic storage unit power change ΔPesu is insufficient to achieve the determined power changes ΔP1, ΔP2. For this reason and other reasons, such as when conserving stored power in storage unit 150 may be prioritized, the desired power change ΔP may be achieved at least in part by varying the generator power Pgen from power generation system 110. Thus, the desired power change ΔP may be achieved by a combination of the generator power change ΔPgen and the electronic storage unit power change ΔPesu. The generator power change ΔPgen may be achieved, for example, by reducing or increasing the power output from wind turbine 102.

[0073] Figure 5An example of frequency support during a first frequency control period F_CTRL1 is shown, in which the power storage unit 150 is pre-set to charge according to a first power setpoint Pset_esu1, here set to the nominal charging power Pnom_esu. During the first frequency control period F_CTRL1, an underfrequency event occurs, resulting in a new power setpoint Pset_esu2. The maximum up-regulation capability of the power storage unit 150 is twice the nominal charging power Pnom_esu, or 2*Pnom_esu. That is, since the first power setpoint Pset_esu1 is set to a maximum charging power of -Pnom_esu, the maximum available power of the power storage unit 150 increases by ΔPesu to 2*Pnom_esu. Therefore, the power of the power plant Pplant can be increased to Pplant_pre + 2*Pnom_esu, where Pplant_pre is the power produced during the frequency control period before the onset of the underfrequency event.

[0074] Similarly, Figure 5 An example of frequency support during the second frequency control period F_CTRL2 is shown, in which the power storage unit 150 is pre-set to discharge according to the first power setpoint Pset_esu1. During the second frequency control period F_CTRL2, an overclocking event occurs, resulting in a new power setpoint Pset_esu2. The maximum downregulation capability of the power storage unit 150 is twice the nominal charging power Pnom_esu, or 2*Pnom_esu. That is, since the first power setpoint Pset_esu1 is set to the maximum discharge power of Pnom_esu, the maximum available power from the power storage unit 150 increases by ΔPesu by 2*Pnom_esu. Therefore, the power of the power plant Pplant can be reduced to Pplant_pre – 2*Pnom_esu.

[0075] Although the invention has been described in conjunction with specific embodiments, it should not be construed as being in any way limited to the embodiments presented. The scope of the invention should be interpreted in light of the set of claims appended hereto. In the context of a claim, the terms "comprise" or "comprising" do not exclude other possible elements or steps. Furthermore, references to reference signs such as "a" or "an" should not be construed as excluding a plurality. The use of reference signs in the claims to elements shown in the figures should also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims may be advantageously combined, and the mention of these features in different claims does not exclude that a combination of features is not possible and advantageous.

Claims

1. A method of controlling a power plant (100) to provide frequency support to a power grid, the power plant comprising a power storage unit (150) and a power generation system (110), the power generation system comprising one or more power generation units (101), the power generation units comprising at least one wind turbine generator (102), wherein: A power generation system is connected to an electricity network (160) for supplying power to the electricity network, and wherein an electricity storage unit is electrically connected to the power generation system and is controllable to charge or discharge depending on a power set point (Pset_esu), wherein the method comprises: - setting a frequency control period, wherein the power storage unit is arranged to be charged or discharged according to a first power set point (Pset_esu1), - at the beginning of the frequency control period, charging or discharging the power storage unit according to the first power set point (Pset_esu1), - determining a first power variation (ΔP1) of the power from the power storage unit based on the first frequency deviation (Δf1) in a case where the grid frequency (f) exceeds a first grid frequency (f1) which is above or below a frequency threshold (fT) resulting in a first frequency deviation (Δf1) occurring within a frequency control period, - determining a second power set point (Pset_esu2) based on the determined first power variation (ΔP1), - providing frequency support by controlling the power storage unit to charge or discharge according to a second power set point (Pset_esu2), The method further comprises: - forecasting a time period in which the power plant is able to produce a plant output power (Pplant_f) that is higher than the planned required plant output power (Pplant_req_f), and - Setting a frequency control period within the forecast time period, wherein the power storage unit is scheduled to charge according to a first power set point (Pset_esu1), thereby increasing the power capacity of the power storage unit to support underfrequency events.

2. A method of controlling a power plant (100) to provide frequency support to a power grid, the power plant comprising a power storage unit (150) and a power generation system (110), the power generation system comprising one or more power generation units (101), the power generation units comprising at least one wind turbine generator (102), wherein: A power generation system is connected to an electricity network (160) for supplying power to the electricity network, and wherein an electricity storage unit is electrically connected to the power generation system and is controllable to charge or discharge depending on a power set point (Pset_esu), wherein the method comprises: - setting a frequency control period, wherein the power storage unit is arranged to be charged or discharged according to a first power set point (Pset_esu1), - at the beginning of the frequency control period, charging or discharging the power storage unit according to the first power set point (Pset_esu1), - determining a first power variation (ΔP1) of the power from the power storage unit based on the first frequency deviation (Δf1) in a case where the grid frequency (f) exceeds a first grid frequency (f1) which is above or below a frequency threshold (fT) resulting in a first frequency deviation (Δf1) occurring within a frequency control period, - determining a second power set point (Pset_esu2) based on the determined first power variation (ΔP1), - providing frequency support by controlling the power storage unit to charge or discharge according to a second power set point (Pset_esu2), The method further comprises: - forecasting a time period in which the power plant is scheduled to produce a power plant output power (Pplant_f) that is lower than the planned required power plant output power (Pplant_req_f), and - Setting a frequency control period within the forecast period, wherein the power storage unit is arranged to discharge according to a first power set point (Pset_esu1) in order to provide support in the event of an overfrequency.

3. The method according to claim 1 or 2, wherein In the event that the grid frequency (f) exceeds a second grid frequency (f2) which is above or below the first grid frequency (f1) resulting in a second frequency deviation (Δf2) occurring within the frequency control period: - determining a second power variation (ΔP2), - determining a third power set point (Pset_esu3) based on the determined second power variation (ΔP2), - providing frequency support by controlling the power storage unit to reverse charging to discharging the power storage unit or vice versa according to a third power set point (Pset_esu3).

4. The method according to claim 3, wherein The maximum value of the second power variation (ΔP2) is given by the sum of the first power set point (Pset_esu1) and the nominal charging or discharging power (Pnom_esu) of the power storage unit (150).

5. The method according to claim 1 or 2, wherein Providing frequency support is conditional on the level of the state of charge (SoC) of the power storage unit.

6. The method according to claim 1 or 2, wherein Frequency control period settings include: - arranging the charging of the power storage unit according to the first power set point (Pset_esu1) and arranging the discharging of the power storage unit during the frequency control period if the first or second frequency deviation (Δf1, Δf2) indicates an underfrequency of the grid, or - Arranging the power storage unit to discharge according to the first power set point (Pset_esu1) and to charge the power storage unit during the frequency control period if the first or second frequency deviation (Δf1, Δf2) indicates an overfrequency of the grid.

7. The method according to claim 1 or 2, comprising supplementing the determined power variation (ΔP1, ΔP2) with a generator power variation (ΔPgen) of the power generated by the power generation system (110).

8. The method according to claim 7, wherein If the electronic storage unit power change (ΔPesu) is insufficient to achieve the determined power change (ΔP1 , ΔP2 ), the generator power change (ΔPgen) is called upon to supplement the determined power change (ΔP1 , ΔP2 ).

9. A central controller (120) for controlling a power plant (100) to provide frequency support to a power grid, the power plant comprising a power storage unit (150) and a power generation system (110), the power generation system comprising one or more power generation units (101), the power generation units comprising at least one wind turbine generator (102), wherein: A power generation system is connected to an electricity network for supplying electricity to the electricity network, and wherein an electricity storage unit is electrically connected to the power generation system and controllable to charge or discharge depending on a power set point (Pset_esu), and wherein the central controller is arranged to perform the method according to any one of claims 1 to 8.

10. A power plant (100) comprising a plurality of power generation units (101) including at least one wind turbine generator and a central controller (120) according to claim 9.

11. A computer program product comprising software code adapted to control a power plant (100) when executed on a data processing system, the computer program product being adapted to perform the method of any one of claims 1 to 8.

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