Patent Description:
Sub-synchronous resonances (SSR) is a well-known phenomenon for conventional power plants, but it is a relatively newly discovered phenomenon for wind power plants. This type of interaction phenomenon is generally referred to as a sub-synchronous control interaction (SSCI). This is a purely electrical phenomenon without shaft torsional dynamics being involved. Another characteristic of SSCI is that the resonance frequency varies depending on the network impedances. Other oscillating modes may include shaft torsional dynamics and can also be dealt with in the present invention.

The conventional sub-synchronous torsional interaction (SSTI) can occur in wind power plants. Wind turbines generally include several sub-synchronous torsional modes. It would be likely to excite one of these low-frequency torsional modes if the frequency of grid current coincides with one of them.

Wind power plants generally make extensive use of power electronic controllers such as turbine converters, and sometimes SVC and STATCOM for reactive power compensation and voltage control, and HVDC links for connection of offshore wind farms. These components can be located in the vicinity of rotating equipment such as those of the wind turbine and thermal power plants, or adjacent to variable frequency drive generators.

When created, these Sub-synchronous resonances (SSR) oscillations may cause damage to turbine-generator shafts and components attached to the shaft. The causes and consequences of sub-synchronous resonance are exacerbated by the continued growth of power transmission system interconnections.

It is known that e.g. frequency and voltage of the electric power transmitted in the grid may start oscillating due to disturbances or bad control coordination between different generation units. It is also known that such oscillations in the grid may be counteracted or damped by injecting electric power with the right phase relative to the grid oscillations. However, injection of such electric power may excite mechanical resonances in the power generator device which produces the damping electric power. <CIT> discloses a power generator, e.g. a wind power generator, comprising a compensating circuit having a dump load resistor and a dump load capacitor for dissipating and temporarily storing excess power, when grid frequency is outside of a predetermined range, thereby balancing an instantaneous difference is power demand and generation.

Accordingly, it is a problem that when a power generator is controlled to damp grid oscillations vibrations in the power generator may inadvertently be excited.

In an aspect, the present invention relates to a wind turbine generator according to claim <NUM>.

An advantage of first aspect is that the mechanical production variation of the wind turbine generator can be none oscillating, whereas the power supplied to the grid has an oscillating shape, provided by the oscillating second reference signal and the dump load unit, in order helps and support the reduction of the grid resonances.

According to one embodiment of the invention the first reference signal is an envelope curve of the damping reference signal.

An advantage of this embodiment is that the first reference signal can provide an envelope around the curve needed to reduce an oscillation in the grid.

According to one embodiment of the invention the first reference signal is a monotonic function.

An advantage of this embodiment is that monotonic functions tend to move in only one direction as x increases. A monotonic increasing function always increases as x increases, i.e. f(a)>f(b) for all a>b. A monotonic decreasing function always decreases as x increases, i.e. f(a)<f(b) for all a>b. A monotonic decreasing function's derivative is always negative. A monotonic increasing function's derivative is always positive. Thus the mechanical stress on the wind turbine generator structure is reduced.

According to one embodiment of the invention the first reference signal is decreasing and starting from a positive value, and wherein the wind turbine power controller further is arranged for receiving a positive power reference.

An advantage of this embodiment is that the first reference signal can be seen as a temporary increasing of the power from the wind turbine generator, and thus it utilizes overload capabilities in the wind turbine generator.

According to one embodiment of the invention the wind turbine power controller is arranged for receiving the damping reference signal and to derive the first reference signal and the oscillating second reference signal from the damping reference signal, said wind turbine power controller communicates the oscillating second reference signal to the damping controller.

According to one embodiment of the invention the power plant controller is arranged to calculate the first reference signal and the oscillating second reference signal, and dispatching the first reference signal and the oscillating second reference signal, to the wind turbine power controller and to the damping controller, of the at least one wind turbine generator respectively.

In a second aspect, the present invention relates to a method according to claim <NUM>.

The advantages of the second aspect and its embodiments are equivalent to the advantages for the first aspect of the present invention.

Any of the attendant features will be more readily appreciated as the same become better understood by reference to the following detailed description considered in connection with the accompanying drawings. The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the invention.

The present invention will now be explained in further details. While the invention is susceptible to various modifications and alternative forms, specific embodiments have been disclosed by way of examples. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications and alternatives falling within the scope of the invention as defined by the appended claims.

For large Wind Power Plants (WPPs) a Power System Stabilizer (PSS) may be required. It is a well-known that a controller prevents one power plant from electro-mechanically oscillating against another or even one area of a continent from oscillating against another. Helping with these oscillations is something that Transmission System Operators look very favourably on.

Sub-synchronous resonances (SSR) are well-known for conventional power plants, but it is a relatively newly discovered phenomenon for wind power plants. This type of interaction phenomenon is generally referred to as a sub-synchronous control interaction (SSCI). This is a purely electrical phenomenon without shaft torsional dynamics being involved. Another characteristic of SSCI is that the resonance frequency varies depending on the network impedances. Other oscillating modes may include shaft torsional dynamics and can also be dealt with in the present invention.

To damp oscillations in the power system by using a wind power plant, some challenges are present, such as the possibility of mechanical resonances in the Wind Turbine Generators (WTGs). This could be tower oscillations or oscillations in the blade, but oscillations in the drive train, i.e. between the two mass systems, the rotor of the generator <NUM> and the aero dynamical rotor <NUM>.

The present invention proposes a solution to avoid this risk of mechanical resonances.

It is known that e.g. frequency and voltage of the electric power transmitted in the grid may start oscillating due to disturbances or bad control coordination between different generation units. It is also known that such oscillations in the grid may be counteracted or damped by injecting electric power with the right phase relative to the grid oscillations. However, injection of such electric power may excite mechanical resonances in the power generator device which produces the damping electric power.

In a WPP a PSS would work by injecting a signal to the plant active power reference, Pref or reactive power reference Qref. This signal could be repetitive with frequency typically between <NUM> and <NUM>. Instead of sending this oscillation demand directly to the generator of the wind turbine generator or the generator side of the variable frequency drive, the generator demand could be without the oscillating part, if a dump load unit is taken into account. For example, the overall WPP could produce a <NUM> oscillation by having a dump load unit operate with a <NUM> oscillation, but no WTG generators are excited at <NUM>.

<FIG> shows a wind turbine generator <NUM> comprising a tower <NUM> and a nacelle <NUM>. The rotor assembly of rotor blades <NUM> is rotatable by action of the wind. The wind induced rotational energy of the rotor blades <NUM> can be transferred via a shaft to a generator in the nacelle. Thus, the wind turbine generator <NUM> is capable of converting kinetic energy of the wind into mechanical energy by means of the rotor blades and, subsequently, into electric power by means of the generator.

The wind turbine generator can be connected to the grid for supplying the generated electric power to the grid.

It is known that e.g. frequency, voltage and current of the electric power transmitted in the grid may start oscillating due to disturbances. It is also known that such oscillations in the grid may be counteracted or damped by injecting a power signal into the grid with the right phase for counteracting the oscillations. In principle the damping power signal may be in anti-phase with the oscillations, although normally the damping signal is phase shifted relative to the principal anti-phase damping signal in order to generate the optimum damping.

The present invention is about a wind turbine generator <NUM> with an electrical generator <NUM>, a dump load unit <NUM> (see <FIG>) for dissipating power, a wind turbine power controller <NUM> (see <FIG>) and a damping controller <NUM> (see <FIG>) both arranged to control wind turbine components based on a damping reference signal <NUM> (see <FIG>).

The damping reference signal <NUM> is a combined signal, and comprises a first reference signal <NUM> (see <FIG>), <NUM> (see <FIG>) and a second reference signal <NUM> (see <FIG>), <NUM> (see <FIG>), wherein the second reference signal <NUM>, <NUM> is an oscillating part, the wind turbine power controller <NUM> (see <FIG>) is controlling the power from the electrical generator <NUM> (see <FIG>) according to the first reference signal <NUM>, <NUM> and the damping controller is controlling the dump load unit to dissipate power according to the second reference signal <NUM>, <NUM>.

In one embodiment of the present invention the wind turbine power controller and the damping controller is located in one unit <NUM>, with separate outputs, a power output (not shown) and a damping reference signal <NUM>.

The main purpose of the dividing the damping reference into a first and second reference signal is that the dump load unit <NUM> can operate very fast and is by no means bandwidth limited to operate within the frequency range of <NUM> to <NUM>. The dump load may although be restricted in the amount of energy that can be dissipated into the resistor, this can be handled in the dimensioning of the dump load components or alternatively by monitoring the state of the dump load components and thus only operate whenever the dump load resistor is healthy.

In an embodiment of the invention the first reference signal is an envelope curve of the damping reference signal, so that the first reference signal provides an envelope around the damping reference curve <NUM>, <NUM> needed to reduce the oscillation in the grid. While the second reference signal provides the oscillating part.

In another embodiment of the invention the first reference signal is a monotonic function. A monotonic function tends to move in only one direction as x increases. A monotonic increasing function always increases as x increases, i.e. f(a)>f(b) for all a>b. A monotonic decreasing function always decreases as x increases, i.e. f(a)<f(b) for all a>b. A monotonic decreasing function's derivative is always negative. A monotonic increasing function's derivative is always positive. Thus the mechanical stress on the wind turbine generator structure is reduced, as no oscillations happen in the generator <NUM>.

<FIG> illustrates an oscillation in the grid, e.g. in the form of an oscillating voltage amplitude <NUM> (i.e. the sinusoidal peak amplitude or RMS amplitude). From a measurement of the grid oscillation <NUM> a power reference <NUM> for damping the oscillation can be generated. By supplying the power reference <NUM> e.g. to a wind turbine generator <NUM> or a plurality of wind turbines generators <NUM> electric power is generated with the right phase with the grid oscillation and injected into the grid. As examples, oscillations in grid voltage or grid frequency can be damped by injecting electric power to the grid in anti-phase or with the undesired grid oscillations or having a particular phase relative to the undesired grid oscillations.

An explanation of why grid oscillations can be damped by injecting power to the grid is given here. If grid oscillations are present such as oscillations in grid frequency, then the speed of a main generator <NUM>, <NUM> (e.g. the generator of a nuclear power station, not shown in the figures) is oscillating. By accelerating and breaking the generator at the right moments the grid oscillations can be damped. The breakings and accelerations of the main generator <NUM>, <NUM> are done by changing the electrical torque experienced by the generator by injecting a damping power oscillation to the grid.

The power damping reference <NUM> may be determined on basis of a model of the power generator unit and the grid so that the power reference optimizes the damping of the grid oscillations. For example, the power damping reference <NUM> may be determined by determining a reference signal which when applied to the model optimizes the damping of the grid oscillations where the grid oscillations may be derived from the measured or estimated electrical parameters. The power damping reference <NUM> can be determined by using the feedback of one or more signals <NUM> from the grid, and processing them by a filter which will extract the oscillatory part of the grid feedback signals. Then the phase and magnitude of the damping reference signal <NUM> is corrected according to the feedback signals <NUM> and the control structure.

When an oscillating power reference <NUM> is used for controlling the power production of e.g. a wind turbine generator, the reference signal may cause e.g. the blade pitch to adjust in order to adapt the power production to the power reference <NUM>. The adjustments of the pitch may inadvertently excite structural vibrations of different wind turbine components, e.g. vibrations of the blades <NUM>, the shaft or the tower <NUM>. Such excitation of vibrations is undesired since the vibrations may reduce the lifetime or damage components.

Other reference signals that the main power damping reference <NUM> may be used for controlling the amount of active and/or reactive power injected to the grid. For example, a current reference may be defined which when applied to the controller of e.g. a wind turbine generator affects the amount of power injected into the grid. Since different types of damping references <NUM> may be used for controlling the damping power injected to the grid, reference is generally made to a main damping reference <NUM> which could be main power damping reference <NUM>, a current reference or equivalent reference signals.

Other power generator units than wind turbine generators may be power controlled for damping grid oscillations <NUM>. Such power generator units may also have structures which inadvertently can be mechanically excited, such as the turbine shaft of thermal solar power plants or other de-central gas turbine plants. Therefore, such other power generator units may cause the same challenges as wind turbine generators with respect to minimizing excitation of structural vibrations. Since, wind turbine generators and other power generator units with turbine driven generators give the same challenges with respect to inadvertently excited mechanical resonances only wind turbine generators <NUM> or wind turbine plants are used as examples for general power generator units.

<FIG> illustrates schematics of wind turbine generator according to an embodiment of the present invention. The embodiment shows a wind turbine generator with full conversion of the electrical power through a power converter <NUM>, <NUM>. Starting from left to right, the aero dynamical rotor <NUM> is mechanically connected to an optional gear box <NUM>, if no gear box is present the aero dynamical rotor <NUM> is mechanically connected directly to an electrical generator <NUM>. In the present embodiment the gear box is mechanically connected to an electrical generator <NUM>, which is electrically connected to a generator side power converter <NUM> via an induction link <NUM>. The generator side power converter <NUM> converts variable AC electrical power to DC electrical power, and is connected to a grid side power converter <NUM> via a DC capacitor link <NUM>. Parallel with the DC capacitor link <NUM> is a DC dump load unit <NUM>. The dump load unit, with a dump load switch can dissipate electrical power by short circuit the DC link voltage through a resistor, the actual short circuit occurs by using an electrical switch, e.g. a semiconductor type such as IGBT, thyristors etc. or a mechanical switch.

The grid side power converter <NUM> converts the DC electrical power in "fixed" frequency electrical AC power, ("fixed" because the converter <NUM> follows the frequency of the grid, so if the frequency of the grid varies a little so will the frequency of the grid side power converter). The grid side power converter is electrically connected to a transformer <NUM> via a grid impedance <NUM>. The transformer <NUM> is yet again connected to the electrical grid see more in <FIG>.

In one embodiment the dump load can store the energy, instead of dissipating the energy, and can thus provide energy during the sections of second reference signal that are positive. In this embodiment the dump load resistor is replaced by an energy storage device, , such a storage device can be, but not limited to, a super capacitor bank, a battery bank or other storage devices known to the person skilled in the art.

According to one embodiment of the invention the wind turbine power controller <NUM> is arranged for receiving the damping reference signal and to derive the first reference signal and the second reference signal from the damping reference signal, said wind turbine power controller communicates the second reference signal to the damping controller.

According to one embodiment of the invention the power plant controller is arranged to calculate the first reference signal and the second reference signal, and dispatching the first reference signal and the second reference signal, to the wind turbine power controller and to the damping controller, of the at least one wind turbine generator respectively.

<FIG> shows a generator <NUM> - e.g. a generator of a nuclear power station - which supplies electric energy to a main grid <NUM>. Power generator units <NUM>, e.g. in the form of wind turbine generators, also supplies electric power to the grid via transformer stations <NUM>. A plurality of power generator units <NUM> may be grouped in a power generator plant <NUM>.

A power generator unit <NUM> may be a single wind turbine generator <NUM>, a wind power plant <NUM>, other individual power generator such as a thermal solar power generator, or a generator plant comprising a plurality of power generators.

A damping controller <NUM> is connected to the point of common connection, i.e. a point located at the grid side of the transformer stations <NUM>, for determining a main power damping reference <NUM> from measured or estimated electrical parameters relating to the utility grid. from measured or estimated values of grid voltage, active or reactive grid current, active or reactive grid power, grid frequency or generator speed of the generator <NUM>.

The determined main power damping reference is supplied to a damping dispatcher <NUM> which determines individual damping references to each of the individual wind turbine generators in the wind power plant <NUM> and communicates an individual damping reference signals <NUM>.

<FIG> illustrates one embodiment of a wind turbine power controller <NUM> of an embodiment of the invention where the wind turbine power controller <NUM> comprises both the damping controller <NUM> and a damping dispatcher <NUM>. According, to this embodiment, the wind turbine power controller <NUM> comprises an input for receiving measured or estimated electrical parameters relating to the utility grid, and an output for transmitting the determined reference signals <NUM>, <NUM> to power generator units <NUM> or a distributor for distributing the damping reference signals <NUM>, <NUM>.

According to an embodiment, the wind turbine power controller <NUM> may further comprise an input <NUM> for receiving vibration values indicative of a structural vibration state of each of a plurality of power generator units, and to apply or assign the reference signals to the power generator units in dependence of the structural vibration states of each power generator unit. A structural vibration state may comprise vibration amplitude of an element of a power generator unit. The vibration state of a power generator unit may be measured or estimated by a sensor system of the power generator unit and transmitted to the wind turbine power controller <NUM>.

According to another embodiment the wind turbine power controller <NUM> may further comprise another input <NUM> which provides information about the dump load unit back to the wind turbine power controller <NUM>, the information can be, but limited to, temperature and operating time of the various components in the dump load unit, operating time.

According to another embodiment the individual damping reference signals <NUM> are determined without use of measured or estimated electrical parameters relating to the utility grid, e.g. measured grid power values, but from a main power damping reference. In this embodiment of the invention the wind turbine power controller <NUM> comprises a processor for determining the reference signals <NUM> from the main power damping reference (i.e. the processor is equivalent to the damping dispatcher <NUM>), an input for receiving the main power damping reference and an output for the determined damping reference signals <NUM>.

According to yet another embodiment the individual damping reference signals <NUM> may be determined directly from the electrical parameters of the grid without initial determination of the main power damping reference <NUM>. In this embodiment of the invention the wind turbine power controller <NUM> comprises a processor for determining the damping reference signals <NUM> directly from the electrical parameters of the grid as described above, an input for receiving the electrical parameters and an output for the determined damping reference signals <NUM>.

<FIG> illustrates three embodiments of the processor <NUM>-<NUM> of the control device <NUM>. In <FIG> the processor <NUM> is equivalent to the damping controller <NUM> and the damping dispatcher <NUM> so that the individual damping reference signals <NUM>, <NUM> are determined from the electrical parameters <NUM>.

In <FIG> the processor <NUM> determines the individual reference signals <NUM> directly from a main power damping reference <NUM>. In <FIG> the processor <NUM> determines the individual reference signals <NUM> directly from the electrical parameters <NUM>.

The processor, e.g. the damping dispatcher <NUM>, may further be configured to distribute the individual power damping reference signals <NUM> among the power generator units <NUM>. Alternatively or additionally, the capability of distributing the individual reference signals <NUM> may be located in a separate dispatcher unit, e.g. at the location of a wind turbine plant. It is understood that the individual reference signals <NUM> may be applied to generator units <NUM> located in a single location, such as a single plant <NUM>, or in different locations.

Accordingly, the function of the processor depends on how the individual control references are determined. Accordingly, the processor is understood as a processor or control system capable of determining the individual damping reference signals <NUM> according to anyone of the three embodiments explained above, i.e. the processor may also be understood as being configured to carry out the function of the damping controller <NUM> and the damping dispatcher <NUM>.

In other embodiments the damping controller <NUM> is located in each of the wind turbine generators <NUM>, <NUM> wherein the need for a dispatcher is eliminated.

<FIG> shows the schematics of how the damping signals are handled in an embodiment. The input signal <NUM> is communicated to the damping controller <NUM> and the damping controller <NUM> communicates a damping reference signal <NUM>.

In some embodiments of the invention the damping controller <NUM> is located centrally in a power plant controller where it through the damping dispatcher <NUM> dispatches the individual damping reference signals <NUM>.

In <FIG> there is no dispatcher and the reference signal <NUM> is handled in a damping signal splitter <NUM> that computes the first reference signal <NUM> and the second reference signal <NUM>.

The damping controller <NUM> calculates the active power reference, also called the damping reference <NUM>, <NUM>, to be injected into the electrical grid in order to provide some damping support, the injection might not eliminate the oscillation. The damping reference <NUM>, <NUM>, is split into two signals a first reference signal and a second reference signal <NUM>, <NUM>. The first reference signal <NUM>, <NUM> is considered to be an envelope curve around the actual damping reference.

In another embodiment there is a Dispatcher <NUM>. In some embodiments the dispatcher and the damping signal splitter <NUM> is one unit and communicates to all WTGs <NUM> in the wind power plant. Here the dispatcher dispatches a vector damping reference signal <NUM> wherein the vector comprising a first <NUM> and second reference signal <NUM> is contained.

In an embodiment several wind power plants work in a coordinated way for damping an oscillation, depending on the location the amplitude and the phase of the damping reference signal may differ.

<FIG> shows an example of an electrical grid setup wherein a wind power turbine <NUM> is connected to an AC system operating at <NUM> kV. In reality the single wind turbine generator <NUM> is a plurality of turbines.

<FIG> shows a schematic diagram of an electrical power system under consideration. A wind turbine generator <NUM> is connected to a wind turbine transformer <NUM>, in some embodiments the transformer <NUM> of <FIG> may be the same transformer as transformer <NUM> also called WTT <NUM>. The WTT <NUM> is connected to a substation transformer (SST) <NUM> that again is connected to a high voltage transformer 704a and 704b, for large wind power plants a dual source for the high voltage transformer 704ab is often used. The electrical grid is also equipped with filter components such as inductors 705a, capacitors 705b and tuned filers 705c, it is all connected to a transmission line <NUM>. At the other end of the transmission line <NUM> is another power generating source <NUM> attached together with filter components 707a and 707b.

<FIG> is divided into four parts A, B, C and D. The figure shows an example of how the reference signals can look like in a situation where turbine is operating in an overrating mode to handle the oscillation. <FIG> shows an example of a damping reference signal <NUM>, an embodiment of the invention where the first reference signal is decreasing after a short increase, the increasing part is not a real ramp but more an effect from using a discrete controller that dispatches reference signals with fixed intervals, so a first sample is at zero and a second sample is at the peak. After the increase the reference signal <NUM> is then going from a positive value, and wherein the wind turbine power controller is receiving a positive power reference, down to zero. Where <FIG> shows the first reference signal <NUM>, the first reference shows an increase in the power production that declines to normal level. In reality the signal <NUM> is added together with the actual power reference to the WTG, which is known to person skilled in the art. <FIG> shows how the first reference signal <NUM> fits together with the damping reference signal <NUM>. The second reference signal is in <FIG> derived by the area between the two signals <NUM>, <NUM>, giving the second reference signal <NUM>.

<FIG> is divided into four parts A, B, C and D. The figure shows an example of how the reference signals can look like in a situation where turbine is operating in a derating mode to handle the oscillation. <FIG> shows an example of a damping reference signal 901a, an embodiment of the invention where the first reference signal is increasing after an short decrease, also due to a discrete controller and a sampled system, and then going from a negative value, and wherein the wind turbine power controller is also receiving a positive power reference. Where <FIG> shows the first reference signal <NUM>, the first reference shows an decrease in the power production, in reality the signal <NUM> is added together with the actual power reference to the WTG, which is known to person skilled in the art. <FIG> shows how the first reference signal <NUM> fits together with the damping reference signal 901b, where the damping reference signal 901a is adjusted with first order function equal or similar to the trend function of damping reference signal 901a. The second reference signal is in <FIG> derived by the area between the two signals <NUM>, <NUM>, giving the second reference signal <NUM>.

In an embodiment related to the situation in <FIG>, the envelope curve <NUM> can have a shape that actually allow the whole damping reference signal to be enveloped, i.e. so there will be no negative parts of the second reference signal, to compare it to <FIG>, will all the darkened area be above the zero line.

<FIG> is divided into three parts A, B and C. The figure shows an example of how the reference signals can look like in a situation where turbine is operating no power mode to handle the oscillation. <FIG> shows an example of a damping reference signal <NUM>, an embodiment of the invention where the first reference signal is zero, and wherein the wind turbine power controller is receiving a power reference that could be zero. <FIG> shows the first reference signal <NUM>. <FIG> shows how the second references signal <NUM> which is the negative part of the damping reference signal <NUM>.

According to one embodiment of the invention the first reference signal is substantially zero and wherein the dump load unit dissipates the negative sections of the second reference signal. Even if the first reference signal is substantially zero the dump load can still provide some contribution to the reduction of the oscillation.

In another embodiment the dump load can store the energy in storing devices such as batteries, capacitors or others, the shape of the first reference signal is changed, as an envelope curve <NUM>, <NUM> no longer needs to cover both the positive and negative part of the oscillations, as the dump load unit <NUM> either absorb power or inject power depending on the sign damping signal <NUM>, <NUM>.

In an embodiment of the present invention the generator <NUM> of the WTG is producing the positive part of the damping reference signal, whereas the negative part is absorbed by the dump load unit.

<FIG> shows a flow chart of a method according to the present invention, for damping electrical grid oscillations in a wind turbine generator with an electrical generator, a dump load unit, for dissipating power, a wind turbine power controller and a damping controller, the method comprises step <NUM>, <NUM> and <NUM>. Where step <NUM> is receiving a damping reference signal for damping the grid oscillations, and step <NUM> is determining a first reference signal of the damping reference signal and dispatching said first reference signal to the wind turbine power controller for controlling power generation of the electrical generator and step <NUM> is determining a second reference signal with an oscillating part of the damping reference signal and dispatching said second reference signal to the damping controller for controlling the dump load unit.

It will further be understood that reference to 'an' item refer to one or more of those items.

Claim 1:
A wind turbine generator (<NUM>) comprising an electrical generator (<NUM>), a dump load unit (<NUM>) for dissipating power from the electrical generator (<NUM>) and for damping electrical grid oscillations, a wind turbine power controller (<NUM>) and a damping controller (<NUM>) both arranged to control wind turbine components based on a damping reference signal,
- the damping reference signal being a combined signal, and comprising a first reference signal and an oscillating second reference signal,
- said wind turbine power controller (<NUM>) is arranged to control the power from the electrical generator (<NUM>) according to the first reference signal in order to avoid excitation of mechanical vibrations in the electrical generator (<NUM>), and
- said damping controller (<NUM>) is arranged to control the dump load unit (<NUM>) to dissipate power from the electrical generator (<NUM>) according to the oscillating second reference signal in order to dampen electrical grid oscillations.