The invention is related to a control system with the features of claim 1 and a control method with the features of claim 15.

A utility-tied wind turbine generator (WTG) transforms the mechanical power of the wind into electrical power which is injected into a utility grid via a generator and a power electronic converter system utilized for maximum power production. This is generally independent of the utility grid transient conditions.

The wind turbine systems can use a full size converter (FSC) rectifying the power produced by the generator and inverts/injects it into a fixed frequency utility grid or it can use a partial scale converter (PSC) used for Double Fed Induction Generator (DFIG) operation.

In either case, wind turbine systems can be grouped into wind power parks and used for bulk power production, facing challenging grid code requirements.

The increased use of wind generated power fed into utility grids creates issues compared with the known approach and layout of the power system. Systems addressing the feeding of wind power into the grid are known e.g. from EP 2 384 540 B1 , WO 2012 / 163355 A1 , WO 2013 / 010332 A1 or EP 2 306 001 A2.

Therefore, control systems for wind turbine systems are required which allow an active role in the stability of the utility grid.

The control system comprises an inertia management system for obtaining at least one electrical property data of the utility grid as input, the inertia management system being further configured for a comparison of the obtained electrical property data with respective reference values. Based on this comparison, a control signal is generated which can be used to control a plurality of energy storage devices. At least two of the energy storage devices are configured to be charged or discharged in particular during electrical transient conditions of the utility grid, the control signal comprising energy sharing coefficients for dynamically controlling the individual charging or discharging of the respective energy storage devices. This allows the flexible and dynamic (i.e. time dependent) use of several energy storage devices depending on the requirements.

In one embodiment of the control system, the energy sharing coefficients are being chosen, in particular optimized to achieve a nominal or predetermined power output of the wind turbine system. The frequency is one example of the electrical property data.

In the context of a wind turbine the at least two energy storage devices comprises at least one supercapacitor, at least one battery device and / or at least one a flywheel device.

It is also possible, that control system comprises at least one device for controlling the energy in input and / or output, in particular energy stored in rotating masses of blades of the wind turbine system and / or at least one means for curtailing the power of the wind turbine system. Curtailment in this context means that the power production is reduced from the maximum available power.

In particular in one embodiment, one of the at least two energy storage devices is charged or discharged through the wind turbine system, even though in principle it is also possible that the at least two energy storage device can be charged at least over some time from an external source.

Furthermore, in one embodiment the control signal comprises at least one set point of the at least two energy storage devices.

In one embodiment, it is also possible that the inertia management system monitors the energy level of the at least two energy storage devices during nominal operation and / or transient operation of the utility grid. The control signal can e.g. be generated by the inertia management system in dependence of the energy level of the at least one energy storage device. In a further embodiment, at least two energy storage devices are each utilized from 0% to 50% of the respective capacity required by the inertia management system.

The input data for the inertia management system can be obtained in different ways. The at least one electrical property data can be measured through direct measurements of the at least one electric property from the grid. It can use the data transmission of the at least one electric property from some other data center and / or a computation based on measured or transmitted data.

Embodiments can comprise inertia management systems which are configured to obtain, in particular measure and / or compute voltage data, current data, frequency data and / or power data from utility grid. This can e.g. include time dependent data and / or measured and / or computed first derivatives of the data. With the derivative data the transient behavior can be assessed in more detail.

In a further embodiment, the measurements of the inertia management system are taken between a power electronics converter device and the utility grid or within the power electronics converter device. The power electronics converter device can be a full power converter or a partial power converter.

The issues are also addressed by a control method with the features of claim 15.

In this method, inertia management system obtains at least one electrical property data of the utility grid as input.

The inertia management system is subsequently comparing the obtained electrical property data with respective reference values and is then generating a control signal in dependence of the comparison.

At least two energy storage devices are charged or discharged in particular during electrical transient conditions of the utility grid and the control signal comprises energy sharing coefficients for dynamically controlling the individual charging or discharging of the respective energy storage devices. Embodiments of the invention are described in an exemplary way in the following figures, wherein

Fig. 1 schematically shows a transient behavior of a frequency signal in a utility grid;

Fig. 2 schematically shows an embodiment of a control system for a wind turbine system and its connection to the utility grid;

Fig. 3 schematically shows the response of an embodiment of a control system during a transient behavior of the utility grid;

Fig. 4 schematically shows the effect of an energy storage device for kinetic energy stored in the rotating blades.

One aspect in the utility grid 200 stability is the frequency stability which represents balancing the response of the power generation (e.g. large synchronous-based hydro or fossil fueled power plants) to the demanding characteristics of the load on the utility grid 200. The balance between power generation and load results in a constant frequency reflecting the normal operating conditions of the utility grid.

In Fig. 1 the dependency of a frequency signal over time is shown. The time scale of the x-axis is not to scale to show the transient events in more detail.

At t=0 the frequency is at the nominal value of 50 Flz with a tolerance of +/- 0,2 Hz. In other embodiments, the nominal frequency values (e.g. 60 Hz) and / or the tolerances can be different.

This frequency situation changes under a disturbance, if e.g. a major utility grid fault occurs or a generation unit with substantially installed capacity (e.g. in the hundred MW range) is lost, causing the frequency to experience excursions with an increased rate of change in the under-frequency region. In Fig. 1 , the disturbance sets in during the time range 0 < t < 10 s indicated by the hatched area, the Inertial Response Region (IR).

In this IR the transient frequency deviates from its rated value. The energy stored in the rotating masses of convention power plants is synchronously released by all generators in a time range of several seconds (see Figure 1 - Inertial Response (IR) region).

The rate of change in the frequency signal along with the entire frequency deviation is moderated down to its nadir by the cumulative inertia of all generating units in the wind power system. This damping capability of synchronous generators is composed of all rotating masses which comprehend the architecture of a conventional power plant starting from the turbine and ending with the synchronous generator own momentum of inertia.

This is followed by a recovery period in the frequency signal representing the action of the conventional power plant governors trying to bring back the frequency (speed of the generators w = 2*pi*f) to its rated point (see Fig. 1 - Primary Frequency Control (PFC) region). This can usually be completed within 30 s.

In the extraordinary case the amount of reserves is not enough, the restoration reserves are deployed which represents the action of the Secondary Frequency Control (SFC). This brings up the frequency up to the nominal frequency value in about 30 min.

The transient behavior in Fig. 1 is just exemplary. In other embodiments different time frames for the deterioration of the frequency and the recovery of the frequency are used.

The volatility of the frequency in the utility grid is expected to increase due to the upcoming decommissioning of conventional power plants in favor of renewable power plants, including wind power plants. Therefore, the power frequency output of the wind turbine systems needs to be controlled in case of such events.

In Fig. 2 an embodiment of a control system for a wind turbine system 100 is shown which controls the energy output of the wind turbine system 100 to address some of the issues discussed in connection with Fig. 1.

The wind turbine system 100 as such is known so that here for the sake of clarity only the blades 102 of the wind turbine system 100 and the generator 101 are depicted. Under nominal operations the electrical energy output of the generator 101 is fed into the utility grid 200.

The control system comprises an inertia management system 1 which obtains electrical property data E from the utility grid 200. The inertia management system shown in Fig. 2 can also be implemented in the turbine controller.

The electrical property data E in particular characterizes the performance of the utility grid 200. It allows e.g. to distinguish nominal from non-nominal behavior. The electrical property data E can e.g. be voltage data, current data, frequency data (like the frequency signals discussed in connection with Fig. 1 ) and / or power data.

The electrical property data can comprise stationary data (e.g. data points measured in longer time intervals) or time dependent data (e.g. data taken online). The data obtained can be used to generate first derivative information (e.g. by numerical analysis) to assess changes e.g. in the frequency or the power.

The inertia management system 1 can obtain the electrical property data E in a variety of ways. The electrical property data (e.g. the frequency) can be measured directly from the utility grid 200. It is also possible to measure some data directly from the utility grid 200 and to compute the electrical property data E (e.g. the frequency) from that data. Furthermore, it is possible that the electrical property data E is transmitted to the inertia management system 1 from some other measurement unit, e.g. in a centralized utility control facility. It is also possible to combine these methods for obtaining data.

The obtaining of data is particularly relevant during electrically transient conditions of the utility grid 200 as described in connection with Fig. 1.

The inertia management system 1 compares the obtained electrical property data E with reference electrical property data Eref by e.g. forming a difference |E - Eref|. Based on this comparison a control signal S is generated. The control signal S can e.g. comprise a dataset with instructions to different units in the wind turbine system 100.

To mitigate possible transient effects in the utility grid 200, the wind turbine system 100 comprises a plurality of energy storage devices 11 , 12, 13 which can be utilized or not according the status of the utility grid 200, the status being defined by the obtained electrical property data E. It also comprises means for controlling the energy input and / or output within the system, e.g. controlling the energy stored in rotating masses of the blades 102 and / or means for curtailing the power. The means 14, 15 for controlling the energy input and / or output operate with sharing coefficients as the energy storage devices 11 , 12, 13 under the control of the inertia management system 1. This will be explained below.

If, e.g., the frequency in the utility grid 200 shows a significant excursion from the nominal behavior it might be necessary that the wind turbine system 100 counteracts this transient behavior by providing more power to the utility grid 200.

The inertia management system 1 is sensitive to grid measurements and supervises all the energy sources used and reacts in accordance to stabilize the faulty condition of the utility grid 200. This inertia management system 1 is coupled with a central controller of the wind turbine generator 101 and is also activated in accordance with the specifications provided by the existing grid code.

The inertial response provided by the conventional generation and adopted by the grid codes can be specified by an inertia constant H (in seconds) or it can be specified as a percentage portion of generation which has to be released during frequency faults.

In the first case, the inertia constant H represents the energy stored in the rotating masses of a generator and with this damping characteristic, the transmission system operators calculate the total damping needed in the system. This feature is imposed to the wind power plants and has to be provided starting from the wind turbine level up. Inertia constants demanded by operators can be in the range of 1 to 15 seconds which means that the wind power plant backed-up by wind turbine generators with multiple energy sources has to provide this nominal power for the above mentioned seconds. The cumulative answer is based on the sum of each individual wind turbine generator and/or auxiliary storage unit used in the plant. The second requirement adopted is specified in terms of percentages of power over time which has to be released during the frequency faults. Typical values used for frequency support during the inertial phase are in between 1 to 20 percent. The wind turbine generator has to provide this type of response during under-frequencies.

In both cases, the wind turbine generator has to artificially inherit the inertial response of conventional generation, has to deploy the reserves and contribute to the decrease of rate of change of frequency until its nadir.

In Fig. 3, the power output in reaction of the transient behavior as indicted in Fig. 1 is shown.

As soon as the inertia management system 1 detects a deviation from the nominal range (e.g. excursion out of the tolerated frequency band) the power output of the wind turbine system 100 is increased. In Fig. 3 the ramping over 10 s is a response to the Inertial Response (IR) shown in Fig. 1. In the example shown, the maximum of the cumulative response is reached after 10 s, i.e. the time in which the nadir of the frequency deviation is reached (see Fig. 1 ).

After this maximum the power output is reduced till the transient behavior (here the frequency transient) has returned to its nominal value. The power output of the wind turbine system 100 makes use of the plurality of energy storage device 11 , 12, 13 which are under the control of the inertia management system 1.

In the embodiment described here, the first energy storage device 11 is a supercapacitor (also called ultracapacitor). Typically, supercapacitors use electrostatic double-layer capacitors (EDLC) (e.g. using carbon electrodes) and electrochemical pseudocapacitors (e.g. using metal oxide or conducting polymer electrodes), both of which contribute to the total capacitance of the capacitor, however, with different amounts.

The second energy storage device 12 comprises a rechargeable battery device, such as e.g. a lithium ion battery.

The third energy storage device 13 comprises a flywheel device which can store rotational energy.

The system comprise also some control features as e.g. means 14 for controlling the energy stored in rotating masses of blades 102 of the wind turbine system 100. The blades 102 of wind turbine system 100 have considerable kinetic energy due to their mass and their rotational momentum. By, e.g., boosting or curtailing the rotation, the power output can be controlled (see Fig. 4).

The system also comprise means 15 for curtailing the power of the wind turbine systems 100.

In other embodiments, a different combination of the energy storage devices 11 , 12, 13 and the means 14, 15 for controlling the energy can be used. In particular, more than one device of one particular type can be used.

The total power output of the combined energy storage devices 11 , 12, 13, and the input and / or output of the means 14, 15 for controlling can be controlled through the control signal S - using an appropriate communication protocol - which comprises energy sharing coefficients a-i, a ₂ , a _3, a ₄ , a ₅ for dynamically controlling the individual charging or discharging of the respective energy storage devices 11 , 12, 13 and / or the input and the output under the control of the means 14, 15 for controlling the energy input and / or output.

For instance - depending on the transient behavior of the utility grid 200 - at one point the control signal S indicates that the sharing coefficients a-i, a ₂ , a _3, a ₄ , a ₅ have the following values: a-i=0,5, a ₂ =0, a ₃ = 0,2 _, a ₄ =0,1 , a ₅ =0,2.

This means, e.g., that the first energy storage device 11 provides 50% of the output power, the second energy storage device 12 is not used at all. The total output of all energy storage devices 1 1 , 12, 13 and the energy input and / output under the control of the means 14, 15 is 100%.

The sharing coefficients a-i, a ₂ , a _3, a ₄ , a ₅ are dynamically adapted by the inertia management system 1 as demanded by the transient behavior of the utility grid 200. In addition, the adapting can also take into account the actual power level of the energy storage devices 11 , 12, 13 and / or the energy input and / or output of the means 14, 15 for controlling the energy. If one energy storage device 11 , 12, 13 is becoming depleted, the inertia management system 1 can switch to another one.

The control signal S can also include set points for the operation of the energy storage devices 11 , 12, 13 and / or the means 14, 15 for controlling the energy input and / or output..

The inertia management system 1 can also use some objective function to optimize the power output. One embodiment uses an objective function which trades off a rapid return to nominal conditions against power conservation.

In Fig. 4, the controlled power output of stored kinetic energy (inertial energy) is exemplarily shown. The actual operation point (curtailment mode) is A at the rotation speed of w _2. In case of a frequency event, the turbine starts from point A and starts to increase the production and moves to Point B which is the maximum power production. Usually all turbines are operated in point B if curtailment is not applied.

After the B point is reached there is still some energy left in the rotating blades of the turbine and the controller will increase the reference of power until reach point C is reached. Increasing the power reference to point C means more power is extracted from the turbine which consequently will slow it down and the speed w is decreased. After point C follows a period of recovery which coincides with point A.

If the rotational speed has to increase to wi due to an external demand from the utility grid 200, the power output raises to P _avaiiabie at operating point B. If at constant rotational speed additional rotational energy is supplied from the third energy storage device 13, the power output is raised to P _mcrease in operating point C.

Reference numbers

1 inertia management system

1 1 first energy storage device

12 second energy storage device

13 third energy storage device

14 device for controlling the energy stored in rotating masses

15 device for curtailing the power

100 wind turbine system

101 generator

102 blade

200 utility grid a, energy sharing coefficient for one energy storage device E electrical property data (e.g. measured)

Eref reference electrical property data

S control signal