Patent Description:
Modern wind turbines are typically variable-speed turbines. In such turbines, the power output is controlled, at least in certain operational modes, by controlling the speed at which the blades can rotate. For example, a torque can be applied to the rotor to limit rotational speed during high winds. The rotor speed may be controlled using a power controller or a torque control system. The power or torque control system is in turn controlled using a power reference signal, typically generated by the turbine's main controller. As used herein, a power or torque control system may refer to a converter of the wind turbine, or to a power controller or torque controller.

The power reference signal may be generated based on the requirements of various control systems of the turbine. For example, the power reference signal may set an average rotor speed based on semi-static wind conditions, as well as providing for small variations to that average rotor speed for temporary condition changes. For example, small variations in the power reference signal may be used to provide damping to counteract vibrations in the turbine.

However, the extent to which such control systems can change the power generated by the turbine is limited by the needs of the electricity grid to which the turbine is connected. Power fluctuations caused by the control systems risk being passed to the grid. As a result, there may be a need impose limits on the amount of power variation the turbine control systems can apply, limiting the effectiveness of these systems.

Some grid operators are introducing a requirement that new turbines should emulate a synchronous generator. In a synchronous generator, there is a direct link between the grid and the rotor of the generator, enabling the generator to smooth out power disturbances in the grid. Such a virtual synchronous machine concept, when implemented on a wind turbine, would mean that disturbances on the grid are fed directly back into the mechanical power of the turbine, disrupting the control system of the turbine.

<CIT> discloses a power generation system in the form of a renewable power source. The system comprises a source side converter, a DC link and a grid side converter to output power to a grid. An electrical energy storage device is coupled to the DC link, and an energy storage controller is arranged for achieving a desired power balance on the DC link.

<CIT> discloses a wind turbine converter having a generator side converter portion and a grid side converter portion. A damping power signal is determined based on an actual rotor frequency, and the generator side converter portion and the grid side converter portion are controlled based on the damping power signal.

According to the invention a method according to claim <NUM> of controlling a wind turbine connected to an electrical grid is provided, the method comprising:.

The turbine control reference signal thus comprises a primary signal and one or more secondary control system signals. The primary static power level signal may be a semi-static signal, semi static in the sense that any variations are noise related.

The time-dependent difference between the turbine control reference signal and the grid control reference signal averages to zero within a predetermined time window. The duration of the predetermined time window may for example be between <NUM> seconds and <NUM> minutes, or between <NUM> seconds and <NUM> minute.

In some embodiments, controlling the amount of energy generated by the turbine may comprise controlling the amount of energy provided by the machine side unit to, or extracted by the machine side unit from, a battery connected between the machine side unit and the line side unit. Alternatively or additionally, controlling the amount of energy transferred to the electrical grid may comprise controlling the amount of energy extracted by the grid from, or provided by the grid to, the battery.

In some embodiments the method may further comprise receiving a grid performance indicator. Controlling the amount of energy provided to, or extracted from, the battery may be based on the grid performance indicator.

In some embodiments the method may further comprise adjusting the turbine control reference signal and/or the grid control reference signal based on a remaining storage capacity of the battery.

In some embodiments controlling the amount of energy generated by the turbine may comprise controlling the amount of energy provided, by the turbine, to a resistor connected between the machine side unit and the line side unit.

In some embodiments the power or torque control system may be or comprise a converter. And here it is understood that the term converter is used in a broad understanding including both power electronics and a logic controller, so that the converter as a whole may control the actual counter torque imposed on the rotor and/or control the actual power injected into the grid.

In some embodiments, controlling the energy transferred to the electric grid may comprise emulating a synchronous generator.

The control system may be further configured to perform the method of any embodiment of the method according to the present invention.

A preferred embodiment of the invention provides a wind turbine comprising a control system according any embodiment of the wind turbine control system according to the invention.

<FIG> illustrates, in a schematic perspective view, an example of a wind turbine <NUM>. The wind turbine <NUM> includes a tower <NUM>, a nacelle <NUM> at the apex of the tower, and a rotor <NUM> operatively coupled to a generator housed inside the nacelle <NUM>. In addition to the generator, the nacelle houses miscellaneous components required for converting wind energy into electrical energy and various components needed to operate, control, and optimize the performance of the wind turbine <NUM>. The rotor <NUM> of the wind turbine includes a central hub <NUM> and a plurality of blades <NUM> that project outwardly from the central hub <NUM>. In the illustrated embodiment, the rotor <NUM> includes three blades <NUM>, but the number may vary. Moreover, the wind turbine comprises a control system. The control system may be placed inside the nacelle or distributed at a number of locations inside the turbine and communicatively connected. The wind turbine <NUM> may be included among a collection of other wind turbines belonging to a wind power plant, also referred to as a wind farm or wind park, that serve as a power generating plant connected by transmission lines with a power grid. The power grid generally consists of a network of power stations, transmission circuits, and substations coupled by a network of transmission lines that transmit the power to loads in the form of end users and other customers of electrical utilities.

<FIG> schematically illustrates an embodiment of a turbine control system <NUM> together with elements of a wind turbine <NUM>. The wind turbine comprises rotor blades <NUM> which are mechanically connected to an electrical generator <NUM> via gearbox <NUM>. In direct drive systems, and other systems, the gearbox <NUM> may not be present. The electrical power generated by the generator <NUM> is injected into a power grid <NUM> via aelectrical converter <NUM>. The converter <NUM> comprises a machine side unit, a DC-link, and a line side unit. Power generated by the turbine <NUM> is passed from the machine side unit to the DC-link, and then onto the line side unit to be passed to the grid <NUM>. The electrical generator <NUM> and the converter <NUM> may be based on a full scale converter (FSC) architecture or a doubly fed induction generator (DFIG) architecture, but other types may be used.

The turbine control system <NUM> comprises a number of elements, including at least one main controller <NUM> with a processor and a memory, so that the processor is capable of executing computing tasks based on instructions stored in the memory. In general, the wind turbine controller ensures that in operation the wind turbine generates a requested power output level. This is obtained by adjusting the pitch angle of the blades <NUM> and/or the power extraction of the converter <NUM>. To this end, the control system comprises a pitch system including a pitch controller <NUM> controlled using a pitch reference signal <NUM>, and a power system including a power controller <NUM> controlled using a power reference signal <NUM>. The wind turbine rotor comprises rotor blades that can be pitched by a pitch mechanism. The rotor comprises an individual pitch system which is capable of individual pitching of the rotor blades, and may comprise a common pitch system which adjusts all pitch angles on all rotor blades at the same time. The turbine control system, or elements of the turbine control system, may be placed in a power plant controller (not shown) so that the turbine may be operated based on externally provided instructions.

The power controller <NUM> and the converter <NUM> (comprising the machine side converter unit, DC link, and line side converter unit) may together be considered to be an example of a power or torque control system. The power controller <NUM> may comprise separate control blocks for controlling each of the machine side unit and line side unit of the converter. As used herein, 'machine side unit' and 'line side unit' are to be understood as referring generally to both the respective power electronics and the respective logic controller, so that line side unit/machine side unit as a whole may control the actual counter torque imposed on the rotor/ the actual power injected into the grid. Thus, in embodiments described below in which the machine side unit receives a turbine control reference signal, and controls the power extracted from the wind based on the reference signal, it is to be understood that a machine side logic controller receives the turbine control reference signal and controls machine side power electronics to control the power extracted from the wind. Similarly, where a line side unit bases the amount of power transferred to the grid on a grid control reference signal, it is to be understood that a line side logic control receives the grid control reference signal and controls line side power electronics to control the power transferred to the grid. The machine side logic controller and/or line side logic controller may be incorporated into the power controller <NUM>.

In conventional systems, a single power reference signal <NUM> is generated to control the amount of power extracted from the wind by the turbine <NUM> and passed to the grid. In embodiments such single power reference signal <NUM> may comprise a primary component and a secondary component. The primary component sets the static power output of the turbine <NUM>. The secondary component comprises signals from one or more secondary control systems, such as damping systems of the turbine <NUM>, which act as small variations on the primary signal. These small variations in the power reference signal <NUM> provide time limited fluctuations in the mechanical power or torque of the turbine <NUM>, and so can be used, for example, to damp vibrations in the turbine. The secondary control systems feeding into the secondary component may include a side-side tower damping (SSTD) system, a drive train damping (DTD) system, and/or an extended power control (EPC) system.

The secondary components of the power reference signal <NUM> cause variations in the amount of power generated by the turbine <NUM>. These variations may then be passed to the electrical grid <NUM> via the line side unit of converter <NUM>, causing power fluctuations such as flicker. As a result, the amount of power variation that can be applied by the secondary control systems of a turbine must be limited.

Grid operators are also implementing new requirements that wind turbines act as a virtual synchronous generator, emulating the response of a traditional synchronous generator. In a synchronous generator, there is a direct link between the rotor used to generate electricity, and the electricity grid. Disturbances on the grid are fed back into the generator, smoothing out the disturbances. Whilst this is beneficial for grid operation, it can have a detrimental impact on the performance of a wind turbine acting as a virtual synchronous machine (VSM).

A variety of grid disturbances and expected VSM generator responses are shown in <FIG> illustrate disturbances on a grid, and <FIG> illustrate the corresponding responses expected of a virtual synchronous machine.

<FIG> illustrates small time dependent oscillations in the AC frequency of electricity in the grid from its nominal value of <NUM>. <FIG> illustrates the response expected of a VSM, in which the power generated by the VSM correspondingly oscillates to dampen the oscillations in the grid. Such power oscillations in a wind turbine will counteract the turbine's own damping systems, such as SSTD, and so limit the mechanical damping control available to the turbine.

<FIG> illustrates large deviations in the AC frequency, and <FIG> shows the corresponding VSM response. In this case the VSM is expected to increase or decrease the generated power by around <NUM>% to smooth the transitions in the grid frequency. Such large scale power changes conflict with the aims of the power (or partial load) controller, reducing the efficiency of the turbine, and potentially damaging the mechanical components of the turbine.

<FIG> illustrates a sharp change in the grid angle of the grid (i.e. the phase difference between current and voltage in the grid). As shown in <FIG>, the VSM is expected to compensate with large impulses in the generated power. Such large impulses would likely cause damage to the components of the wind turbine, notably the drive train.

<FIG> illustrates elements of a turbine control system <NUM> for a wind turbine <NUM>, which can be used to decouple the power generated by the turbine <NUM> from the power provided to the grid <NUM>. This decoupling protects the grid <NUM> from the power variations used to control the turbine <NUM>, and protects the turbine <NUM> from the requirements of the grid <NUM>, such as VSM requirements.

Decoupling the turbine <NUM> and grid <NUM> in this way provides a number of advantages. Damping systems of the turbine <NUM> may be able to apply larger magnitude variations to the power generated than would conventionally be possible, as the variations will no longer be passed on to the grid <NUM>. This may allow for reduced pitch activity, as more power control can be used in preference to pitch control. Pitch activity may also be reduced as constant power destabilisation of speed control is limited. Similarly, drivetrain loads may be reduced by more aggressive use of DTD damping than would conventionally be possible, and limiting of constant power strategy destabilisation. Reducing pitch activity and drivetrain loads reduces the rate of fatigue of the components of the turbine <NUM>, and so increases turbine lifetime. Furthermore, the decoupling reduces flicker in the grid <NUM>, as turbine damping system activity is no longer seen in the grid, and the annual energy production (AEP) of the turbine may be increased due to more effective turbine control.

The control system <NUM> generates two distinct power control reference signals, a turbine control reference signal <NUM>; and a grid control reference signal <NUM>. Turbine control reference signal <NUM> is provided to a machine side unit <NUM> of a power or torque control system of the turbine <NUM>, such as the machine side unit of converter <NUM>. The machine side unit <NUM> adjusts the mechanical operation of the turbine <NUM> based on the turbine control reference signal <NUM> to control the amount of energy extracted by the turbine <NUM> from the wind.

The grid control reference signal <NUM> is provided to a line side unit <NUM> of the power or torque control system (e.g. the line side unit of converter <NUM>) configured to control the amount of energy transferred from the turbine <NUM> to the electrical grid <NUM> based on the grid control reference signal. The line side unit <NUM> may also base the power supplied to the grid <NUM> on a current grid condition, for example to emulate a synchronous machine response to a disturbance on the grid <NUM>. In some embodiments, a feedback mechanism from the grid <NUM> may be used to provide the turbine controller/s with information about the current state of the grid, such as grid performance indicator. The grid performance indicator may provide an estimate of the health of the grid <NUM>, and/or may indicate any disturbances in the grid <NUM> such a frequency or grid angle changes. This feedback may be used to inform generation of the control reference signals <NUM>, <NUM>, so that the reference signals are based in part on the current condition of the grid <NUM>.

Each turbine <NUM> in a wind farm may be associated with an individual line side unit <NUM>, or a plurality of turbines may supply energy to a common line side unit.

In conventional systems with only a single power control signal, the power generated by the turbine <NUM> equals the power transferred to the grid <NUM> at all times. In the present invention, the turbine control reference signal <NUM> can be different from the grid control reference signal <NUM> - so the energy generated by the turbine at a particular time can be different from the energy transferred to the grid <NUM> at that time, providing the decoupling of the turbine <NUM> and grid <NUM> described above.

The turbine control reference signal <NUM> comprises a primary component, setting a static power level for the turbine operation based on the current conditions experienced by the turbine. The primary component may be generated by a main controller <NUM> (or partial load controller) of the turbine <NUM>. As shown in <FIG>, this primary component may be the grid control reference signal <NUM>.

As shown in <FIG>, the turbine control reference signal <NUM> also comprises a number of secondary control system signals <NUM>-<NUM>, each generated by a corresponding secondary control system <NUM>-<NUM>. The secondary control system signals <NUM>-<NUM> are summed with the primary component of the turbine reference signal (i.e. the grid control reference signal <NUM> in the illustrated embodiment) to form the turbine control reference signal <NUM>. The secondary control system signals <NUM>-<NUM> include damping system signals intended to dampen oscillations in the turbine by applying small variations to the static power generated by the turbine <NUM>. The secondary control system signals are thus time varying signals. In an embodiment the signals may be oscillating signals with an amplitude and a phase set or determined by the respective secondary control system in accordance with the desired damping result. In the particular embodiment illustrated in <FIG>, the damping system signals include an SSTD control signal <NUM>, generated by SSTD controller <NUM>; and DTD control signal <NUM> generated by DTD controller <NUM>. The turbine control reference signal <NUM> is also formed from an EPC control signal <NUM> generated by EPC controller <NUM>. Although illustrated as distinct components, any of the secondary control systems <NUM>-<NUM> may actually be implemented in the main controller <NUM> of the turbine <NUM>.

The inclusion of the secondary control system signals <NUM>-<NUM> in the turbine control reference signal <NUM> means that at a given instant in time the amount of energy extracted from the wind by the turbine may be greater than the energy passed to the grid <NUM>. This excess energy may be dumped in a resistor connected between the machine side unit and the line side unit, for example a resistor connected on the DC link between the machine side unit and line side unit of converter <NUM>. In such cases, the differences between control reference signals <NUM>, <NUM> may be controlled to ensure that the machine side-line side power difference can be safely dumped onto the resistor, without the resistor overheating. For example, there may be a feedback mechanism from the resistor to the turbine controller/s so that the control reference signals <NUM>, <NUM> can be generated based on the current state of the resistor, as well as based on the current conditions experienced by the turbine <NUM>.

Alternatively, excess energy may be stored in a battery connected between the machine side unit and the line side unit. <FIG> illustrates elements of a converter <NUM> incorporating such a battery <NUM>.

Converter <NUM> comprises a machine side unit <NUM> connected to a line side unit <NUM> by a DC link <NUM>. The line side unit <NUM> transfers power to an electrical grid <NUM>.

The machine side unit <NUM> supplies generated energy to the DC link <NUM> based on a turbine control reference signal <NUM>, generated as described above. The line side unit <NUM> extracts energy from the DC link <NUM> and supplies it to the grid <NUM> based on a grid control reference signal <NUM>, also generated as described above. In some embodiments, the line side unit <NUM> may additionally extract energy from the DC link <NUM> based on a current grid condition, for example to provide a virtual synchronous machine response.

As noted above, converter <NUM>, and its machine side unit <NUM> and line side unit <NUM>, should be considered to refer generally to both the logic controllers which receive the control reference signals <NUM>, <NUM>; and to the power electronics controlled by those logic controllers. As such, the converter <NUM> may be considered to be an example of a power or torque controller. The logic controllers may be implemented in a power controller, such as power controller <NUM>. In such cases, the converter power electronics together with the power controller may be considered to form a power or torque controller.

Due to the different control reference signals <NUM> and <NUM>, at any instant of time the energy transferred to the DC link <NUM> by the machine side unit <NUM> may not equal the energy extracted from the DC link by the line side unit <NUM>. A battery <NUM> connected to the DC link <NUM> provides a buffer against such energy differences. When excess energy is provided by the turbine <NUM>, it can be stored on the battery <NUM>. When more energy is required by the grid <NUM> than is currently being generated by the turbine <NUM>, for example to smooth a disturbance in the grid <NUM> by providing a VSM response, the excess energy can be extracted from the battery <NUM>.

As the battery <NUM> has a limited storage capacity, a feedback mechanism may be used to provide the turbine controller/s with information about the current state of the battery <NUM>. In this way, the turbine control reference signal <NUM> and grid control reference signal <NUM> may be generated based on the current state of the battery <NUM>, ensuring that excess energy can be stored safely in the battery <NUM>. In particular, the difference between generated power and extracted power may be limited based on the current capacity of the battery <NUM>.

The capacity of the battery <NUM> available for buffering differences between generated and extracted power may also be dynamically determined based on a current grid condition. A storage controller may receive a grid performance indicator, representing the current state or health of the grid <NUM>. Based on this grid performance indicator, the storage controller may determine a capacity of the battery <NUM> that should be reserved for fulling requirements of the grid <NUM>, such providing a VSM response. Thus only the non-reserved capacity of the battery <NUM> may be used as a buffer. The dynamically determined non-reserved capacity of the battery <NUM> may be fed back to the turbine controller/s, so that the turbine control reference signal <NUM> and grid control reference signal <NUM> are generated based on the current dynamic capacity of the battery <NUM>.

It will be appreciated that when averaged over time, the power generated by the turbine <NUM> should substantially equal the energy supplied to the grid <NUM>, <NUM> (less any inefficiency losses). The turbine control reference signal <NUM> and grid control reference signal <NUM> may be generated to ensure that the difference between energy generated and transferred to the grid <NUM>, <NUM> averages to zero over a certain time window. For example, the time window may be between <NUM> seconds and <NUM> minutes, or between <NUM> seconds and <NUM> minute.

<FIG> illustrates a method <NUM> of controlling a wind turbine <NUM> according to the present invention.

At step <NUM>, a turbine control reference signal is generated, and at step <NUM>, a grid control reference signal is generated. The turbine control reference signal and grid control reference signal may be generated by controllers of the turbine <NUM>, as described above in relation to <FIG>. The turbine control reference signal may be a power reference signal or a torque reference signal.

At step <NUM>, the turbine control reference signal is provided to a machine side unit of a power or torque control system of the turbine, such as a machine side unit of a converter.

At step <NUM>, the amount of energy generated by the turbine <NUM> is controlled by the machine side unit based on the turbine control reference signal.

At step <NUM>, the grid control reference signal is provided to a line side unit of the power or torque control system, such as a line side unit of the converter. The line side unit may be connected to the machine side unit by a DC link.

At step <NUM>, the amount of energy transferred to the electrical grid may be controlled using the line side unit based on the grid control reference signal.

Claim 1:
A method of controlling a wind turbine (<NUM>) connected to an electrical grid (<NUM>), the method comprising:
generating a turbine control reference signal (<NUM>) comprising a primary static power level signal (<NUM>) and one or more secondary control system signals (<NUM>-<NUM>), the secondary control system signals include damping signals to dampen oscillations in the turbine, and wherein the turbine control reference signal is the sum of the primary static power level signal and the one or more secondary control system signals, and wherein a time-dependent difference between the turbine control reference signal (<NUM>) and the grid control reference signal (<NUM>) averages to zero within a predetermined time window;
generating a grid control reference signal (<NUM>) as the primary static power level signal;
providing the turbine control reference signal (<NUM>) to a machine side unit (<NUM>) of a power or torque control system of the turbine;
controlling, using the machine side unit (<NUM>), the amount of energy generated by the turbine based on the turbine control reference signal (<NUM>);
providing the grid control reference signal (<NUM>) to a line side unit (<NUM>) of the power or torque control system; and
controlling, using the line side unit (<NUM>), the amount of energy transferred to the electrical grid (<NUM>) based on the grid control reference signal.