The invention relates to a wind turbine according to the preamble of claim 1 and to a method for controlling a wind turbine according to the preamble of claim 13.

Such a wind turbine comprises a rotor being rotatable by wind, and a generator operatively coupled to the rotor for generating electrical power from the motive power of the rotor. The generator has at least one electrical output for providing at least part of the generated electrical power by providing electrical current. The wind turbine further comprises a converter unit. The converter has an electrical input and an electrical output. The input of the converter unit is coupled to the output of the generator, e.g. via a rotor circuit. The converter unit is adapted for converting the electrical current provided by the generator, e.g. by modifying the frequency, strength and/or voltage of the current, and providing the converted electrical current directly or indirectly to an external electrical grid. The wind turbine further comprises an additional energy source.

EP 2 306 001 A2 describes a wind turbine having an additional energy source in the form of an energy storage. The energy storage is coupled either to a DC link of a converter, or to the electrical grid.

In an electrical grid, power generation and load have to be balanced for maintaining stability of the grid. One aspect in the grid stability is frequency stability. Frequency stability represents the balancing response of power generation to the demanding characteristics of the load. A balance between generation and load can result in a substantially constant value of frequency, reflecting the normal operating conditions of the grid.

Referring to Fig. 4, under normal conditions, the frequency (shown per unit, p.u. for simplicity) is within a standard frequency range (SFR) around a nominal value, e.g. 50 Hz. A balance between power generation and power consumption in the electrical grid results in a value of frequency within the standard frequency range. This situation may change in case of a grid fault or disturbance, e.g. in the case when one or more power generation units (e.g. power plants) stop providing power, in particular when said generation units have an installed capacity in the hundred-MW range. Such a fault may cause the frequency of the electrical grid to experience high excursions with an increased rate of change in an under-frequency region. The rate of change of frequency along with the entire frequency deviation is moderated down to a nadir by the cumulative inertia of all generating units in the power system. Damping may be provided by the inertia of rotating masses of synchronous generators in conventional power plants. During a transient condition of a grid disturbance, which causes the frequency of the electrical grid to deviate from its standard frequency range, the energy stored in the rotating masses is synchronously released by all generators in a time range of several seconds within an inertial response (IR). This is followed up by a recovery period, which normally includes an action of conventional power plants to restore the balance between power generation and load, and bring back the frequency, e.g. by adjusting a speed of one or more generators, within its standard frequency range. The frequency restoration or recovery may be divided into a primary frequency control (PFC) region or phase which is active, e.g., within the first seconds after the frequency drop. In the case the reserves activated by PFC are not enough to restore the frequency, so called secondary frequency control (SFC) mechanisms are deployed, e.g. by increasing power generation by means of power plants that do not participate in the primary control. Secondary frequency control may be active within the first minutes or half hour after the disturbance.

With the increasing impact of renewable energy sources, in particular wind turbines, grid code requirements become more and more demanding. The grid code serves to maintain grid stability, e.g. by specifying a required behavior of a connected wind turbine when facing disturbances in the grid.

It is an object of the present invention to provide an improved wind turbine and an improved method for controlling a wind turbine which may fulfil demanding grid code requirements.

This object is solved by a wind turbine according to claim 1.

Accordingly, the additional energy source is electrically coupled to the input of the converter unit for transmitting electrical power between the additional energy source and the input of the converter unit.

By connecting the additional energy source to the rotor circuit, the inertial response behavior of the wind turbine when facing a grid disturbance is improved. This is achievable by releasing energy stored in the additional energy source into the electrical grid, e.g. in response to a detection of the grid disturbance. Transmitting the energy via the rotor circuit allows an efficient use of the converter unit of the wind turbine, in particular without substantial modifications of the converter unit. Therefore, the wind turbine may fulfil demanding grid code requirements in a particularly effective manner.

The generator may be a double fed induction generator (DFIG). Double fed induction generators conventionally comprise a stator circuit and a rotor circuit. The stator circuit may be connected to the electrical grid, e.g. without an intermediate converter. The rotor circuit may be connected (e.g. via slip-rings) to the converter unit of the wind turbine. A double fed induction generator allows to control the rotor voltage and/or current by controlling the converter unit. Therefore, the double fed induction generator may be kept synchronized with the electrical grid while the wind turbine speed varies, thus increasing the wind turbine efficiency. Using a double fed induction generator may allow to use partial scaled converters. Alternatively, the generator is another type of induction generator. The converter unit may alternatively also be connected with a stator circuit of (any type of) the generator.

The converter unit may comprise a rotor-side converter and/or a grid-side converter. The rotor-side converter and the grid-side converter may be connected with each other via a DC link. The rotor-side converter and/or grid-side converter may comprise an AC input or AC output. For an increased efficiency, the converter unit, in particular the rotor-side converter and/or the grid-side converter may be controllable, e.g. for adjusting a voltage, current and/or frequency at the output of the converter unit.

The rotor-side converter may be an AC-to-DC converter. An AC input of the rotor-side converter may serve as the input of the converter unit. The grid-side converter may be a DC- to-AC converter. An AC output of the grid-side converter may be connected to the electrical grid, e.g. via a transformer. A DC output of the rotor-side converter may be electrically coupled to a DC input of the grid-side converter, forming the DC link. The additional energy source may comprise a converter (an additional converter). An AC output and/or input of the converter of the additional energy source may be electrically connected to the AC input of the converter unit and/or to an AC output of the generator. The converter of the additional energy source may be a bidirectional converter. The converter of the additional energy source may serve as an extra rotor-side converter. In this way, the additional energy source may efficiently provide and/or receive electrical power from the generator. The converter of the additional energy source may be a DC-to-AC converter. By this a DC current provided by the additional energy source may be converted to an AC current and provided to the converter unit (and, optionally, vice versa). The additional energy source may comprise a power storage unit for storing power for an autonomous operation.

The power storage unit can comprise or consist of a battery, a supercapacitor and/or a flywheel. Supercapacitors (also referred to as ultra-caps) have the advantage of a comparably quick response. Flywheels may operate at wide temperature ranges, have a long life span and need little maintenance.

The output of the generator is electrically coupled to the input of the converter unit via a circuit, e.g. the rotor circuit. The circuit may be a three-phase circuit.

The input of the converter unit is an AC input for receiving AC current from the generator, in particular from a rotor of the generator.

The wind turbine may further comprise a wind turbine generator controller (WTG controller). The wind turbine generator controller may control the operation of the wind turbine, in particular of a converter controller and the additional energy source. The wind turbine generator controller may be adapted for monitoring a condition parameter of the electrical grid. Based on the monitored condition parameter, the wind turbine generator controller may control the additional energy source to provide electrical current to the input of the converter unit and/or to receive electrical current from the generator.

Said condition parameter may particularly be a grid frequency or a grid voltage. A deviation from a predefined nominal frequency and/or voltage may be detected by the wind turbine generator controller. In response to this detection, the wind turbine generator controller may command the additional energy source to provide energy to the converter unit and/or to adjust a frequency, voltage and/or current provided by the converter of the additional energy source.

The object is also solved by a method for controlling a wind turbine, in particular a wind turbine according to any aspect or embodiment described herein, the method comprising the steps of:

Generating electrical power by means of a generator mechanically coupled to a rotor of the wind turbine; and providing the electrical power from the generator to an input of a converter unit. The converter unit converts the electrical power (e.g. by modifying a voltage, frequency and/or current) from the generator and provides the converted electrical power at an output to an electrical grid. Therein, the further step of transmitting electrical power between an additional energy source and the input of the converter unit is provided, the additional energy source being electrically coupled to the input of the converter unit.

The method may have the same advantages as the wind turbine described above, so reference is made thereto. The method may further comprise the step of monitoring a condition parameter of the electrical grid, and commanding the additional energy source to provide electrical power to the input of the converter unit based on the condition parameter.

The condition parameter may be a grid frequency or a grid voltage.

The steps of the method may be performed in the order as described, or they may be performed in another order.

Embodiments of the invention are shown in the figures, where

Fig. 1 shows a schematic view of a wind turbine with a tower, a nacelle and a rotor;

Fig. 2 shows a block diagram of various components of the wind turbine according to Fig. 1 ;

Fig. 3 shows a method for controlling a wind turbine; and

Fig. 4 shows a frequency disturbance in an electrical grid and measures for recovering the nominal frequency against time.

Fig. 1 shows a wind turbine 1 for generating electrical energy by rotation of a rotor 12 by means of wind. The rotor 12 is mounted at a nacelle 11 arranged at an upper end 100 of a tower 10 of the wind turbine 1. The tower 10 extends between its upper end 100 and a foundation 101 at the ground. The tower 10 is elongate and has a longitudinal axis. The rotor 12 comprises several, in the present case three blades 121 mounted on a hub 120. The rotor 12 is rotatable around a rotor axis X with respect to the nacelle 11. The rotor blades 121 can be revolved within a rotor plane. In use, the rotor axis X is oriented substantially horizontally. For an efficient extraction of wind energy, the rotor 12 is oriented towards the wind. In particular, the rotor plane is oriented perpendicular to the direction of the incoming wind. For this purpose, the nacelle 1 1 together with the rotor 12 is rotatable around a yaw axis Z with respect to the tower 10. The yaw axis Z corresponds to the longitudinal axis of the tower 10. The yaw axis Z is substantially perpendicular to the rotor axis X.

The wind turbine 1 produces electrical power and provides the electrical power to an electrical grid. A plurality of wind turbines 1 can be grouped into wind power parks and used for bulk power production.

Since more and more wind turbines are deployed in the electrical grid and contribute to an increasing share in the total generated energy, there is a need for wind turbine generators to provide a contribution to the inertial response in accordance with specifications provided by the grid code.

For this purpose, the wind turbine 1 according to Fig. 1 comprises an additional energy source, as will be described in detail with reference to Fig. 2 below.

According to Fig. 2, the wind turbine 1 comprises a wind turbine generator 14 that transforms mechanical power of the wind into electrical power and provides it to the electrical grid 2. For this purpose, the generator 14 is mechanically coupled to the rotor 12 of the wind turbine 1 (via an optional gearbox 13). When the rotor 12 is rotated by the wind, a wind turbine torque Twt is applied to the generator 14. The wind turbine 1 may generally comprise a full size converter (FSC), which rectifies the entire power produced by the generator 14 and inverts and/or injects it into the electrical grid 2, or a partial scale converter (PSC), in particular for a use with a double fed induction generator. In the present example, the generator 14 of the wind turbine 1 is a double fed induction generator. The generator 14 has a rotor output 140 providing electrical power from a rotor of the generator 14. The generator 14 further has a stator output 141 providing electrical power from a stator of the generator 14. Both outputs 140, 141 provide three-phase currents. The stator output 141 of the generator 14 is connected to the electrical grid 2 by means of a stator circuit 171. The stator circuit 171 may be interrupted by an optional switch. Other wind turbine generators may be connected to the stator circuit 171 via corresponding connectors L1 , L2, L3. The stator circuit 171 is connected to the electrical grid 2 via a transformer 19 for adjusting a voltage of the supplied electrical energy. Alternatively, no transformer 19 is provided and the stator circuit 171 is directly coupled to the electrical grid 2.

The rotor output 140 of the generator 14 is connected to the electrical grid 2 by means of a rotor circuit 170 and via a converter unit 15. The rotor circuit 170 electrically connects the rotor output 140 with an input 154 of the converter unit 15. The rotor circuit 170 is a three- phase circuit. The converter unit 15 may be utilized for maximum power production, impartial to utility grid transient conditions. In the present example, the converter unit 15 is a partial scale converter what may reduce the cost of the converter unit 15 when compared to a full size converter.

The converter unit 15 converts electrical power provided at its input 154 by the generator 14 and provides it at an output 155. The output 155 of the converter unit 15 is electrically connected with the stator circuit 171 via an optional filter 153, e.g. for reducing high- frequency components in the current provided by the generator unit 15, and an optional switch.

The converter unit 15 comprises a rotor-side converter 150 and a grid-side converter 151. The rotor-side converter 150 is an AC-to-DC converter. The rotor-side converter 150 has an alternating-current (AC) input that is adapted to receive alternating current from the rotor output 140 of the generator 14. The AC input of the rotor-side converter 150 forms the (AC) input 154 of the converter unit 15. The rotor-side converter 150 is adapted to convert the alternating current provided at the input 154 to direct current (DC) and provide the direct current at a DC output.

The DC output of the rotor-side converter 150 is electrically connected to a DC input of the grid-side converter 151. The electrical connection of the rotor-side converter 150 with the grid-side converter 151 forms a DC link. Optional capacitors 152 in the DC link, in particular between two poles of the DC link, may filter possible AC components in the direct current provided by the rotor-side converter 150.

The grid-side converter 151 is a DC-to-AC converter. It is adapted to convert direct current provided at its DC input (via the DC link) into alternating current and provide the alternating current at an AC output. The AC output of the grid-side converter 151 forms the (AC) output 155 of the converter unit 15.

A converter controller 161 controls the converter unit 15. For example, the converter controller 161 may receive instructions by an external control of the electrical grid 2 or by a wind turbine generator controller 160 of the wind turbine 1. Depending on a condition parameter, e.g. a frequency, of the electrical grid 2, the converter controller 161 adjusts the converter unit 15. The adjusting may comprise an adjustment of an amplitude of the current provided at the output 155. For the case of a grid frequency disturbance, the converter controller 161 may be adapted to adjust the amplitude of the current provided at the output 155 for bringing the frequency back to its nominal value. As power production of the wind turbine 1 is coupled to the wind speed, power provided by the generator cannot easily be increased for contributing to the balance between power generation and load in the electrical grid 2.

Therefore, the wind turbine 1 comprises an additional energy source 18. The additional energy source 18 may provide additional power when needed for balancing the electrical grid 2. According to Fig. 2, the additional energy source 18 is electrically connected to the rotor circuit 170. Therefore, the additional energy source 18 may provide electrical power to the input 154 of the converter unit 15 via the rotor circuit 170.

In case of a grid disturbance, the additional energy source 18 may provide additional electrical power to the converter unit 15 as if the wind speed would have been increased for providing additional power to the grid. Therefore, by using the converter 180 of the additional energy source 18 as a supplementary rotor-side converter, the inertial response of the generator 14 may be improved. On the other hand, the additional energy source 18 may also receive electrical power from the rotor output 140 of the generator 14, e.g. when there is too much wind energy for the electrical grid 2 to accept. In this way, excess energy may be buffered by means of the additional energy source 18 and provided at a later stage when the load in the electrical grid 2 exceeds the power generation. For this purpose, the converter 180 of the additional energy source 18 may be a bidirectional converter.

This arrangement of the additional energy source has several advantages. The control of the converter unit 15 may be kept simple. No major modifications of the setup of the wind turbine 1 , in particular of the converter unit 15 (e.g. its controls and its architecture), have to be done with respect to a wind turbine without an additional energy source. Existing wind turbines may be equipped with the additional energy source 18.

The additional energy source 18 comprises a power storage unit 181. The power storage unit 181 is adapted to store power. For example, the power storage unit 181 comprises a battery, a flywheel, a supercapacitor, or a combination thereof. The power storage unit 181 is electrically connected to a converter 180 of the additional energy source 18 via a DC link. The converter 180 of the additional energy source 18 is provided in addition to and may be arranged separated from the converter unit 15. The converter 180 of the additional energy source 18 has a three-phase AC output that is connected with the rotor circuit 170.

The wind turbine generator controller 160 controls the additional energy source 18, e.g. whether it provides or receives power, and how much. The wind turbine generator controller 160 also controls the converter controller 161. In particular, the wind turbine generator controller 160 may measure and/or monitor a condition parameter, in particular a grid frequency and/or a grid voltage, of the electrical grid 2. Alternatively or additionally, the wind turbine generator controller 160 may receive a control message from a controller of the electrical grid 2, in particular a required rate of change of frequency (ROCOF) and/or a steady-state deviation, and control the additional energy source in dependence of the control message (in particular a voltage, a current and/or an amount of transferred power).

Depending on the condition parameter, the wind turbine generator controller 160 may control the wind turbine 1 to provide an additional power reference. In particular, the wind turbine generator controller 160 may control the additional energy source 18 to provide electrical power to the AC input 154 of the converter unit 15 or to receive and store energy from the generator 14. The wind turbine generator controller 160 may alternatively or in addition command the converter controller 161 to control the converter unit 15 to adjust the electrical power provided at the AC output 155 of the converter unit 15, e.g. a voltage, frequency and/or current.

The converter unit 15 and/or the additional energy source 18 may be located within the nacelle 11 or tower 10 of the wind turbine 1 , or outside thereof, e.g. in a separate building.

Fig. 3 shows a method for controlling a wind turbine, in particular the wind turbine 1 according to Fig. 1 and 2. The method comprises the following steps:

At step S100, electrical power is generated by means of a generator 14 (directly or indirectly, e.g. via a gearbox) mechanically coupled to a rotor 12 of the wind turbine 1 , the rotor having rotor blades 121.

At step S101 , the electrical power from the generator 14 is provided to an AC input 154 of a converter unit 15. The converter unit 15 converts the electrical power from the generator 14 and provides the converted electrical power at an AC output 155 of the converter unit 15 to an electrical grid 2 (directly or indirectly, e.g. via a transformer). At step S103, additional electrical power is transmitted between an additional energy source 18 and said AC input 154 of the converter unit 15. The additional energy source 18 is electrically coupled to the input 154 of the converter unit 15.

In this way, energy from the additional energy source 18 may be used for stabilizing the electrical grid 2 in a particularly easy way and to fulfil demanding grid code requirements in a simple manner. At an optional step S102, a condition parameter of the electrical grid 2 is monitored. The additional energy source 18 may be instructed to provide electrical power to the input 154 of the converter unit 15 based on the condition parameter. Alternatively, the additional energy source 18 may be instructed to receive electrical power from the generator 14 based on the condition parameter. The condition parameter is, for example, a grid frequency or a grid voltage. For example, the additional energy source 18 may be instructed to provide electrical power to the input 154 of the converter unit 15 when the grid frequency is below a predefined standard frequency range of the electrical grid. When the grid frequency is above the standard frequency range of the electrical grid, the additional energy source 18 may be instructed to receive electrical power from the generator 14 and store it in the power storage unit. When the condition parameter is within a predefined range, e.g. when the grid frequency is within the standard frequency range, it may be provided that in step S102 the additional energy source is not instructed to receive or provide electrical power, or it may be instructed to not receive and provide any electrical power. Step S102 may be performed before performing step S103.

Step S102 may be repeated until the condition parameter is outside of a predefined range, e.g. when the grid frequency is above or below the standard frequency range. Optionally, the wind turbine generator controller 160 and the additional energy source 18 are adapted to damp power oscillations of the wind turbine generator 14 and/or of other generators in the electrical grid 2.

As another option, the wind turbine generator controller 160 and the additional energy source 18 may be adapted to provide power during a black start sequence.

The power oscillations and/or a black start may be detected by the wind turbine generator controller 160 and/or may be communicated to the wind turbine generator controller 160 by a controller of the electrical grid 2. List of Reference Numbers

1 wind turbine

10 tower

100 upper end

101 foundation

11 nacelle

12 rotor

120 hub

121 blade

13 gearbox

14 generator

140 rotor output

141 stator output

15 converter unit

150 rotor-side converter

151 grid-side converter

152 capacitor

153 filter

154 input

155 output

160 wind turbine generator controller

161 converter controller

170 rotor circuit

171 stator circuit

18 additional energy source

180 converter

181 power storage unit

19 transformer

2 electrical grid

X rotor axis

Z yaw axis