DESCRIPTION Wind energy plant Background An electricity grid distributes power provided by various power generation plants to the consumers of electricity. Base load power generation plants that burn fossil fuels are being phased out and replaced by power generation plants that convert renewable energy sources to electricity. An example of such a "renewable" power generation plant is a wind farm. Any power generation plant is generally required to fulfil certain regulations in order to contribute to grid stability. It is important to maintain the grid frequency at an essentially constant value, for example 50 Hz in Europe. However, a sudden increase in load can result in a noticeable drop in grid frequency, generally referred to as a grid under-frequency event. The grid operator must ensure that the grid frequency quickly returns to normal. For this reason, a mode of frequency response can be agreed between the grid operator and the operator of a power generation plant. For example, in exchange for remuneration, the operator of a wind energy plant may commit to delivery of a certain quantity of active power for a certain duration following a grid under- frequency event. This commitment to provide the agreed quantity of active power may be a permanent arrangement or may remain valid for a contractually agreed time. However, because of the fluctuating nature of wind and the unpredictability of grid under-frequency events, it is essentially impossible for the operator of a prior art wind energy plant to predict with any certainty the quantity of active power that can be delivered in response to a grid under-frequency event that might occur at any time in the future. In one approach, as proposed in EP2733812A2, a separate or stand-alone power storage facility can be controlled to store power which can be released as required to assist in responding to an under-frequency event. However, since a power storage facility deteriorates over time, the he operator of the wind energy plant may be unable to fulfil the contractual obligation. It is known to curtail the output power of variable-speed wind turbines of a wind park so that, if necessary, the wind turbines can increase their output power in order to respond to a grid under-frequency event. An example of this approach is given in WO2021249603A1. Typically, the output power of such a wind park might be curtailed to 80% of rated power output, i.e. the wind turbines are operated to generate 20% less output power than is possible. However, curtailing the output power means that the wind park is never exporting as much electricity as it could, and the revenue is less than what it could be. The loss of potential revenue may outweigh the remuneration in exchange for commitment to provide an agreed amount of active power. Furthermore, in order to fulfil normal grid demand, a "curtailed" wind farm may require a larger number of wind turbines, which further increases the cost of energy (CoE). It is therefore an object of the invention to provide a wind energy plant that overcomes the problems outlined above. This object is achieved by the claimed wind energy plant and by the claimed method of operating a wind energy plant. Summary of the invention According to the invention, the wind energy plant comprises a plurality of wind turbines and is committed to provide a specified quantity of active power over a specified duration in response to a grid under-frequency event. The wind energy plant is characterized by a plurality of power storage devices, and each power storage device is arranged in a wind turbine of the wind energy plant. The rated storage capacity of the plurality of power storage devices is at least as large as the specified quantity of active power. The wind energy plant is configured to deliver an active power deficit when the collective stored power available in the power storage devices is less than the rated storage capacity at the time of occurrence of a grid under-frequency event. The specified quantity of active power can have been stipulated or agreed at a previous stage. It shall be understood that the agreed quantity of primary reserve power is a contractual obligation on the part of the plant operator. The wind energy plant must then deliver the agreed amount of active power (the "primary reserve") for the specified duration in order to honour the commitment. The grid operator can remunerate the park operator for undertaking to commit to the provision of primary reserve. The timespan during which the primary reserve power is to be provided following a grid under-frequency event may also be referred to as the "event response duration" in the following. An advantage of such wind energy plant is that it is set up to provide an agreed amount of active power following a grid under-frequency event, but does not need to always operate the wind turbines at curtailed output power in order to be able to deliver the primary reserve. Instead, a configuration of power storage devices is provided with a total rated capacity that is at least as large as the agreed primary reserve. Following a grid under-frequency event, the power storage devices feed their stored power into the grid. If, at the time of a grid under-frequency event, the power storage devices collectively store less than the rated capacity, the wind energy plant reacts by controlling the wind turbines in one of several ways so that the deficit can be made up. According to the invention, the method of operating such a wind energy plant comprises the steps of determining the actual stored power available in the power storage devices; computing the active power deficit between the actual stored power and the rated storage capacity of the power storage devices; feeding the stored power from the power storage devices to the grid; and operating the wind turbines to deliver the active power deficit. Particularly advantageous embodiments and features of the invention are given by the dependent claims, as revealed in the following description. Features of different claim categories may be combined as appropriate to give further embodiments not described herein. In the following, it may be assumed that the wind energy plant comprises many wind turbines and can contribute significantly to the stability of the grid into which it feeds. For example, a wind energy plant may comprise several hundred wind turbines. Of course, the invention applies equally to smaller wind energy plants. It may be assumed that the wind energy plant is controlled by a park controller that performs various functions, for example a park controller issues control commands in response to the fluctuating grid requirements. In the wind energy plant, the park controller comprises a frequency response module configured to identify an optimum way of delivering the agreed primary reserve, as will be explained in the following. In the following, the terms "frequency response module" and "frequency response controller" are regarded as synonyms and may be used interchangeably. It may also be assumed that each wind turbine has a local wind turbine controller which receives power setpoints from the park controller, and which generates local control commands accordingly. The number of power storage devices deployed throughout the wind energy plant may be determined on the basis of the agreed primary reserve. Equally, the agreed quantity of primary reserve may be determined based on the number of power storage devices deployed throughout the wind energy plant. The power storage device of a wind turbine is preferably configured to discharge into the grid in response to a grid under-frequency event, for example in response to a command from the wind turbine controller or from a controller of the wind energy plant. The power storage device of a wind turbine is preferably configured to be charged by its wind turbine during normal grid conditions. In other words, when the grid is stable, electricity generated by the wind turbine is used to charge the power storage device and to maintain the fully-charged state of the power storage device. In the following, without restricting the invention in any way, the power storage device of a wind turbine may be assumed to be a battery arrangement comprising one or more batteries connected in any suitable manner. The terms "power storage device", "battery arrangement" and "battery" may be used interchangeably. Since the batteries are distributed between the wind turbines of the wind energy plant, they may be referred to collectively as a distributed storage arrangement. Of course, the distributed storage arrangement can comprise various types of power storage devices. As mentioned above, as long as all batteries are in good health and the total stored battery power is the same as the rated battery power, the batteries can collectively deliver the agreed primary response following a grid under-frequency event. However, power storage devices such as batteries do not have an unlimited lifetime and their performance can deteriorate over time. Therefore, sooner or later, the total stored battery power will no longer reach the rated level. The wind energy plant must then make up this deficit in another way. In a particularly preferred embodiment of the invention, during normal operation of the wind energy plant, the frequency response module observes the health of the distributed storage arrangement and identifies a deficit, for example a diminishing capacity owing to wear and tear, or owing to a fault in a battery. The frequency response module may then issue power setpoints that effectively curtail the output power of one or more wind turbines based on the discrepancy between the actual storage capacity of the batteries and the rated storage capacity. Then, should a grid under-frequency event occur, the curtailed wind turbines will be able to release their spinning reserve to augment the power released form the batteries. In combination, the battery power and the spinning reserve deliver the agreed amount of active power. In a preferred embodiment of the invention, the wind energy plant comprises a monitoring means that can monitor the health of the power storage devices so that a deficit can be identified ahead of a grid under-frequency event. For example, a monitoring means may observe the charging cycles of the batteries and can identify any deterioration in performance. The monitoring means can be realised by providing each battery with appropriate voltage and/or current measuring devices, and a means of communicating the measurements to the local wind turbine controller or to the park controller. In a preferred embodiment of the invention, the primary reserve can be re-negotiated as the storage capacity of the power storage devices deteriorates over time. For example, the revenue that may be earned by not curtailing the wind turbines further may outweigh the remuneration for commitment to provide primary reserve above the basic minimum. Depending on various unpredictable factors, a situation may arise in which the batteries are unable to collectively deliver the required power in response to an under-frequency event. Therefore, as explained above, the wind energy plant controller can curtail the output power of the wind turbines to some extent during normal grid conditions. In the event of a grid under-frequency event, the wind energy plant will then be able to avail of a quantity of spinning reserve to "top up" the battery output power. The power stored as spinning reserve can be released, i.e. fed into the grid, when the batteries are depleted. The following describes an exemplary embodiment of the invention. A wind energy plant comprises ten wind turbines, each with a rated power of 10 MW, so that the wind energy plant has a rated power of 100 MW. Each wind turbine is equipped with a storage element, and each of the ten storage elements has a rated capacity of 1 MW, i.e. each storage element can store 1 MW of power. The wind energy plant is committed to providing up to 8 MW of additional active power during frequency response, for a duration of up to 30 minutes. At some point in time t ₀ , a grid under-frequency event occurs and the wind farm must deliver the agreed active power P _FR . The park controller determines the available power being generated by the wind turbines, the power stored in the batteries, and the wind conditions in the near future. In this simple example, each of the ten wind turbines is generating 9 MW at time t ₀ , and each of the ten batteries can provide 0.5 MW. Therefore, at time t ₀ , the batteries can collectively provide 5 MW, leaving a deficit of 3 MW for the wind energy plant to provide. The park controller forecasts that during the specified response time t _FR , the wind turbines can be operated at their rated power. In other words, the wind turbines can be controlled to increase their output power as the batteries are being depleted. Once the batteries are depleted, the "missing" 3 MW will then be provided by the wind energy plant spinning reserve. To this end, appropriate setpoints are issued to the ten wind turbines depending on their individual forecasts in order to distribute the 3 MW according to their capabilities. In this simple example, the ten wind turbines are collectively generating 85 MW at time t ₀ , and the ten batteries collectively store 5 MW. Therefore, at time t ₀ , there is a deficit of 3 MW for the wind energy plant to provide. The park controller forecasts that during the specified response time t _FR , the wind turbines can be operated at their rated power. In other words, the wind turbines can be controlled to increase their output power as the batteries are being depleted. Once the batteries are depleted, the "missing" 3 MW will then be provided by the wind energy plant spinning reserve. To this end, appropriate setpoints are issued to the ten wind turbines depending on their individual forecasts in order to distribute the 3 MW according to their capabilities. During normal operation of the wind energy plant, the power storage devices are in standby, ready to deliver active power if required in response to a grid under-frequency event. However, the power available in the batteries may also be needed for other purposes. For example, the power available in the power storage device of a wind turbine may be used to power up that wind turbine during low-wind conditions following a shut-down duration. As soon as the wind speed increases, the wind turbine resumes generating power, some of which is used for its auxiliary devices, and some of which is used to recharge the power storage device. During a grid under-frequency event, as much active power as possible should be fed into the grid. Therefore, in another approach to ensuring delivery of the agreed primary reserve, the method preferably comprises a step of reducing the power consumption of the auxiliaries of the wind turbines. In this way, during a grid under-frequency event, the frequency response controller can reduce the power consumption of one or more such devices so that as much active power as possible can be fed into the grid. The power consumption of these devices returns to normal after the specified event response duration has completed. It shall be assumed that the frequency response controller assesses the feasibility of such load reduction, for example the motors, fans, pumps etc. of a cooling arrangement will only be operated at lower power if critical temperatures (e.g. the generator temperature) are within safe levels. Power transported through a cable generally undergoes some loss, depending on parameters such as cable length and conductor cross-sectional area. The cables connecting the various wind turbines of a large wind park to a point of common connection (PCC) can have significantly different lengths, depending on how the wind turbines are distributed throughout the wind park. For example, a wind turbine that is far away from the PCC in a wind park may be associated with 5% cable losses, compared to only 1 % cable losses for a wind turbine that is closer to the PCC. Therefore, in a further preferred embodiment of the invention, the frequency response controller is provided with information regarding the cable layout of the wind park and distributes the power setpoints to the wind turbines according to their cable connection lengths to the PCC. In this way, the uneven distribution of setpoints can minimize cable losses in order to ensure that as much active power can be fed into the grid following an under-frequency event, while at the same time ensuring that the wind turbines are curtailed by a favourable minimum during normal operation. For example, in a large offshore wind park, the frequency response controller may choose to curtail any "near" wind turbines to a greater extent than the more "remote" wind turbines, i.e. a wind turbine with a shorter cable connection to the PCC can be curtailed to a greater extent than a wind turbine with a longer cable to the PCC. Following a grid under-frequency event, the more severely curtailed "closer" wind turbines can immediately increase their output to the rated level, and only a small fraction of the resulting active power is lost through the shorter cable connections. The more remote wind turbines are curtailed less during normal operation to counteract the larger cable losses. Similarly, if a power storage device of a "remote" wind turbine becomes faulty, the frequency response controller may respond to counteract the ensuing deficit by increasing the curtailment of a "near" wind turbine in order to be able to compensate for the overall reduction in storage capacity. Brief description of the drawings Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention. Figure 1 shows an exemplary embodiment of a wind energy plant; Figure 2 is a block diagram to illustrate control of the wind energy plant of Figure 1; Figures 3 and 4 illustrate further embodiments of the invention; Figure 5 illustrates ancillary services related to frequency response; Figure 6 illustrates the effect of deteriorating battery capacity; Figure 7 illustrates a further embodiment of the invention; and Figure 8 illustrates a flow chart of a method of operating a wind energy plant according to an example. In the diagrams, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale. Detailed description Figure 1 shows an exemplary embodiment of a wind energy plant 1 or wind park 1. The wind energy plant 1 may comprise a plurality of wind turbines 10_1, …, 10_n that export power to a grid, represented herein by a point of common connection (PCC). The diagram shows a wind park controller 1C or wind park operation centre, which may be local to the wind park 1 or at a remote location. Each wind turbine 10_1, …, 10_n may be equipped with a power storage device 10S_1, …, 10S_n. Since the power storage devices 10S_1, …, 10S_n may be distributed over the wind park 1, they may be referred to collectively as a distributed storage arrangement. The total rated capacity of the power storage devices 10S_1, …, 10S_n, i.e. the capacity of the distributed storage arrangement, may be at least as large as a power reserve P _FR agreed between the grid operator and the wind park operator: P10S _rated ≥ P _FR (1) Figure 2 is a block diagram to illustrate control of the wind park 1 of the example of Figure 1. The operation centre 1C may inform a frequency response controller 15 of the agreed active power P _FR to be supplied during frequency response, and the agreed duration t _FR . In the event of a grid under-frequency event at time t ₀ , the frequency response controller 15 may be provided with at least the following inputs: the present power output P10_t ₀ _i of each wind turbine, the forecast or projected power output P10_t _FR _i that could be achieved by each wind turbine 10_1, …, 10_n during the response duration t _FR , and the power P10S _avail available in the batteries 10S_1, …, 10S_n of the distributed storage arrangement at time t ₀ . The present power output P10_t ₀ _i of each wind turbine 10_1, …, 10_n is known by the wind park controller 1C. The power output P10_t _FR _i that could be achieved by each wind turbine 10_1, …, 10_n may be determined by a forecasting module 17. The total power P10S _avail available in the batteries 10S_1, …, 10S_n of the distributed storage arrangement may be determined from a battery monitoring means 19 that monitors the charging cycles of a battery 10S_1, …, 10S_n. Each wind turbine 10_1, …, 10_n of the wind park 1 may have its own battery monitoring means 19, or a single battery monitoring means 19 may monitor all batteries 10S_1, …, 10S_n of the wind park 1. If the batteries of the distributed storage arrangement are all healthy and fully charged at time t ₀ , satisfying equation (1), the frequency response controller 15 may issue commands to commence discharging the batteries 10S_1, …, 10S_n, and the wind energy plant 1 will have honoured its commitment by the end of the response time t _FR . However, if the batteries 10S_1, …, 10S_n are not fully charged at time t ₀ , i.e. the available power P10S _avail stored in the batteries is less than the rated storage capacity P10S _rated , the frequency response controller 15 may respond in an appropriate manner in order to ensure that the wind energy plant 1 can honour its commitment to deliver the agreed quantity of active power P _FR . To this end, the frequency response controller 15 may determine the power deficit: ∆P10S= P10S _rated - P10S _avail (2) The frequency response controller 15 then may determine how to control the wind turbines 10_1, …, 10_n so that the spinning reserve of one or more wind turbines 10_1, …, 10_n makes up the deficit ΔP10S. The frequency response controller 15 may then issue appropriate setpoints SP_1, …, SP_n to the wind turbine controllers 10 _WTC of the wind turbines 10_1, …, 10_n. The setpoints SP_1, …, SP_n may be scheduled so that the spinning reserve provided by the wind turbines 10_1, …, 10_n is timed to optimally augment the active power being released by the discharging batteries 10S_1, …, 10S_n. In addition to the functionality described in Figure 2 above, the frequency response controller 15 may assess the impact of activating inertial response of each wind turbine 10_i, and may activate or deactivate this feature as a result of the assessment. As a result, the setpoints SP_1, …, SP_n issued by the wind turbine controllers 10 _WTC will enable the release of spinning reserve of some wind turbines and disable the release of spinning reserve of the remaining wind turbines. This is illustrated in Figure 3, in which setpoints SP_1, SP_j that result in the release of spinning reserve are indicated by solid lines, and setpoints SP_i, SP_n that do not result in the release of spinning reserve are indicated by dotted lines. In addition to the functionality described in Figure 3 above, the frequency response controller 15 may determine whether it is expedient to reduce the power consumption of auxiliary loads in the wind turbines. For example, if it seems that the available stored power P10S _avail and the projected wind power P10_t _FR _i may be unable to provide the entire primary reserve P _FR over the agreed duration t _FR , the frequency response controller 15 may command a temporary reduction in power consumed by motors, pumps etc. of a certain auxiliary devices such as the cooling systems of one or more wind turbines. For example, as indicated in Figure 4, the frequency response controller 15 may assess the impact of reducing the consumption of specific auxiliary systems of one or more wind turbines and then may communicate to the relevant wind turbine(s) the auxiliary device(s) 10 _aux to be regulated, the amount 10 _{aux_Δ} by which power is to be reduced, and the duration 10 _{aux_t} . The power that is "borrowed" from the auxiliaries for a short time can help fulfil the commitment to deliver the agreed amount of active power P _FR . Figure 5 illustrates various types of ancillary services related to frequency response. The diagram indicates the total power from various power generation plants being fed into a grid following a grid under-frequency event at time t ₀ . Depending on the capabilities of the power generation plants, these provide inertial response 50, primary response 51, secondary response 52 or tertiary response 54. These services 50, 51, 52, 53 are delivered in chronological order, i.e. inertial response 50 and primary response 51 can be delivered essentially immediately. Secondary response 52 is delivered if primary response 51 failed to stabilize the grid. Similarly, tertiary response 53 is delivered if secondary response 52 was insufficient to stabilize the grid. A wind park is generally only able to provide some inertial response 50 (by releasing kinetic energy of the aerodynamic rotors) and some primary response 51 (from spinning reserve). The method is concerned with a way of ensuring that a wind energy plant 1 can honour a commitment to provide a set amount of primary reserve while avoiding the revenue sacrifice attendant with a prior art approach of curtailed operation. As long as the batteries 10S_1, …, 10S_n of the wind energy plant 1 are fully charged, the wind turbines can operate at rated power output and need not be curtailed. In the event of a grid under-frequency event, the batteries can provide the entire quantity of agreed primary reserve P _FR . As the batteries deteriorate over time and lose capacity, the wind turbines can be curtailed by an amount to make up for the deficit in stored battery power. Figure 6 illustrates the effect of deteriorating battery capacity during the lifetime of the wind energy plant. Initially, the distributed batteries have a rated capacity P10S _rated that is at least as large as the agreed primary reserve P _FR . The health of the batteries declines over time. During an initial phase 60, the discrepancy between rated capacity P10S _rated and primary reserve P _FR may be small, and it may be possible to make up the difference by reducing the load on the batteries of the wind turbines during the response time t _FR following a drop in grid frequency. Sooner or later, during a subsequent phase 61, the collective storage capacity may deteriorate to a point at which a more effective remedy is required to make up the difference between the rated capacity P10S _rated and the primary reserve P _FR . The frequency response module will then react by curtailing the power output of a suitable number of wind turbines so that, in the event of a grid under- frequency, it can then issue setpoints to release the spinning reserve of those wind turbines. Figure 7 shows an exemplary embodiment of a wind park 1 with many wind turbines. Some wind turbines 10 _far may be at a significant distance 10 _far from the PCC, while other wind turbines 10 _near may be closer to the PCC. It shall be understood that the wind park 1 may comprise many more wind turbines and only a small selection is shown here for the sake of clarity, and that any number of wind turbines may be connected to the PCC by cables of intermediate length. It shall be understood that any of the wind turbines 10 _far , 10 _near shown here can be any of the wind turbines 10_1, …, 10_n mentioned in the description of the previous diagrams, and each of these may be assumed to comprise a power storage device. Of course, a wind park need only be equipped with a sufficient number of batteries 10S (and a corresponding number of wind turbines) which together can deliver the agreed active power P _FR in primary response. The "remote" wind turbines 10 _far are connected to the PCC by cables with long length L _long , while the "closer" wind turbines 10 _near are connected to the PCC by cables with comparably short length L _short . As explained above, power transported through a cable generally undergoes some loss, depending on parameters such as cable length and conductor cross-sectional area. The frequency response module 15 of Figure 2 is provided with information regarding the cable layout of the wind park, i.e. the cable lengths connecting each wind turbines to the PCC, and distributes the power setpoints SP_1, …, SP_n to the wind turbines 10 _far , 10 _near according to their cable connection lengths L _long , L _short to the PCC. The "closer" wind turbines 10 _near can be curtailed more during normal grid conditions, while the wind turbines 10 _far further away are curtailed less. This "weighted" distribution of power setpoints SP_1, …, SP_n effectively minimizes cable losses and also ensures that as much active power as possible can be fed into the grid following an under-frequency event. Figure 8 depicts an example of a flow chart of a method 800 of operating a wind energy plant according to any of the examples disclosed. The actual stored power P10Savail available in the power storage devices 10S_1, …, 10S_n may, in block 801, be determined. Then, the active power deficit ΔP10S between the actual stored power P10Savail and the rated storage capacity (P10Srated) of the power storage devices 10S_1, …, 10S_n may, in block 802, be computed. The stored power P10Savail from the power storage devices 10S_1, …, 10S_n may, in block 803, be fed to the grid. The wind turbines 10_1, …, 10_n may, in block 804, be controlled to deliver the active power deficit ΔP10S. Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. For example, the aerodynamic rotor of a variable-speed wind turbine can also be used as an energy storage device, and the kinetic energy stored in the rotor can be released in a brief contribution to frequency response. In a further preferred embodiment of the invention, the active power provided by the wind energy plant in response to a grid under-frequency event can be augmented by inertial response if necessary. Again, the release of energy stored in the aerodynamic rotors of one or more wind turbines can be released by issuing appropriate setpoints and commands to those wind turbines. Furthermore, the batteries of the distributed storage system may be used to respond to an over-frequency event, which occurs when supply outweighs grid demand. Following a grid over-frequency event any partially-charged batteries of the distributed storage system can be fully charged using surplus grid power. The step of charging the batteries can be combined with reducing the active power output by the wind turbines, so that the wind energy plant can quickly reduce its power output faster to respond to the over-frequency event. If the over-frequency event coincides with a low-wind conditions and/or the wind turbines are not producing output power for other reasons, absorption of the surplus power is done by the batteries alone. For the sake of clarity, it is to be understood that the use of "a" or "an" throughout this application does not exclude a plurality, and "comprising" does not exclude other steps or elements. The mention of a "unit" or a "module" does not preclude the use of more than one unit or module.