Patent Description:
In order to allow a much higher penetration of renewable energy sources such as wind turbine into the electrical grid, some countries propose some requirements to equip the power converters with grid-forming properties similar to conventional synchronous generators. These requirements can be address by configuring the renewable power generating units as virtual synchronous machines VSM.

With normal implementations of virtual synchronous machines, the power delivered to the grid may be heavily fluctuating after a low voltage condition and the power of the wind turbine generator is unloaded during the fault and fluctuating after the fault.

Accordingly, it is a problem the implementation of the virtual synchronous machine generates power oscillations after a grid fault and generates increased mechanical loads on the drive train and other mechanical components. Accordingly, an implementation of the virtual synchronous machine which addresses these problems is strongly needed.

Rosyadi et. discloses in the paper: "Low voltage ride-through capability improvement of wind farms using variable speed permanent magnet wind generator", low voltage ride-through (LVRT) capability improvement of wind turbine generator system is investigated, in which two wind farms model connected with multi-machine power system is considered. The wind farms considered consist of fixed speed wind turbine based induction generator and variable speed wind turbine with permanent magnet synchronous generator. A new control scheme is developed for back-to-back converters of PMSG with DC link protection controller embedded to support fixed speed wind generators during fault condition. Simulation results by PSCAD/EMTDC show that the proposed control scheme is very effective to improve the LVRT of wind farm during severe network disturbance.

Olve Mo et. discloses in the paper: "Evaluation of Virtual Synchronous Machines With Dynamic or Quasi-Stationary Machine Models" a comparison of the small-signal stability properties for virtual synchronous machines (VSMs) with dynamic and quasi-stationary representation of the internal synchronous machine (SM) model. It is shown that the dynamic electrical equations may introduce poorly damped oscillations when realistic stator impedance values for high-power SMs are used. The quasi-stationary implementation is less sensitive to the impedance of the virtual machine model, but depends on filtering of the measured d- and q-axes components of the ac-side voltage to avoid instability or poorly damped oscillations. It is demonstrated how both implementations can be made stable and robust for a wide range of grid impedances. However, the dynamic electrical model depends on a high virtual resistance for effectively damping internal oscillations associated with dc components in the ac currents during transients. Thus, when using SM parameters with low virtual stator resistance for decoupling the active and reactive power control, the quasi-stationary VSM implementation is preferable.

discloses in the paper: "Placement and Implementation of Grid-Forming and Grid-Following Virtual Inertia and Fast Frequency Response", The electric power system is witnessing a shift in the technology of generation. Conventional thermal generation based on synchronous machines is gradually being replaced by power electronics interfaced renewable generation. This new mode of generation, however, lacks the natural inertia and governor damping, which are quintessential features of synchronous machines. The loss of these features results in increasing frequency excursions and, ultimately, system instability. Among the numerous studies on mitigating these undesirable effects, the main approach involves virtual inertia (VI) emulation to mimic the behavior of synchronous machines. In this paper, explicit models of grid-following and grid-forming VI devices are developed for inertia emulation and fast frequency response in low-inertia systems. An optimization problem is formulated to optimize the parameters and location of these devices in a power system to increase its resilience. Finally, a case study based on a high-fidelity model of the South-East Australian system is used to illustrate the effectiveness of such devices.

LIANG XIAODONG et al. discloses in the paper "Emerging Power Quality Challenges Due to Integration of Renewable Energy Sources", that renewable energy becomes a key contributor to our modern society, but their integration to power grid poses significant technical challenges. Power quality is an important aspect of renewable energy integration. The major power quality concerns are: <NUM>) Voltage and frequency fluctuations, which are caused by noncontrollable variability of renewable energy resources. The intermittent nature of renewable energy resources due to ever-changing weather conditions leads to voltage and frequency fluctuations at the interconnected power grid. <NUM>) Harmonics, which are introduced by power electronic devices utilized in renewable energy generation. When penetration level of renewable energy is high, the influence of harmonics could be significant. In this paper, an extensive literature review is conducted on emerging power quality challenges due to renewable energy integration. This paper consists of two sections: <NUM>) Power quality problem definition. Wind turbines and solar photovoltaic systems and their power quality issues are summarized. <NUM>) Existing approaches to improve power quality. Various methods are reviewed, and the control-technology-based power quality improvement is the major focus of this paper. The future research directions for emerging power quality challenges for renewable energy integration are recommended.

Jingyang Fang et. discloses in the paper "The Role of Power Electronics in Future Low Inertia Power Systems" that Inertia plays an essential role in maintaining the frequency stability of power systems. Nevertheless, the increase of power electronics-based renewable generation can dramatically reduce the inertia of modern power systems. This issue has already challenged the control and stability of small-scale power systems. It will soon be faced by larger power systems as the trend of renewable integration continues. In view of the inertia challenge, this paper presents a comprehensive review of conventional and emerging inertia enhancement techniques. It is revealed that the inertia emulation by wind turbines has successfully demonstrated its effectiveness and will receive widespread adoptions. In addition, the virtual inertia generated by the DC-link capacitors of power converters has a great potential due to its low cost. The same concept can also be applied to ultracapacitors. Moreover, batteries will serve as an alternative inertia supplier. In future power systems where most of the generators and loads are connected via power electronics, virtual synchronous machines (VSMs) will become the main inertia provider. In general, it is concluded that advances in semiconductors and control promise to make power electronics an enabling technology for inertia control in future power systems.

It is an object of the invention to improve control of wind turbines to alleviate one or more of the above mentioned problems, and therefore to provide a method which provides improved control methods of virtual synchronous machines.

The invention is defined in the indepenent Claims.

The chopper power dissipated by the chopper may be added to the grid power and the difference between the power reference and the sum of the chopper power and the grid power is feed into the inertial integration model which determines the integration of the power difference.

Thus, in view of the inertial integration it follows that the derivative of the synchronous machine rotational speed is indicative of a deviation, e.g. difference, between the power reference for the desired power output of the wind turbine and the sum of grid power supplied by the wind turbine to a power grid, the chopper power and the damping power.

The idea of the first aspect of the invention is to dissipate power in the DC link chopper and use this power dissipation in a swing equation of the virtual synchronous machine model. Due to this, a smoother performance during low voltage and over voltage ride through events is achieved with a more stable and controllable current injection during the fault and reductions in power oscillations and mechanical loads in the voltage recovery phase after a grid fault.

Advantageously, by including the chopper power in the determination of the power deviation between the power reference and the produced grid power, the power generated by the generator can be reduced gradually so that drive train oscillations are reduced.

According to an embodiment, the power output from the wind turbine is controlled based on the synchronous machine angle.

According to an embodiment, a chopper power reference for the chopper is determined based on a comparison of a DC-link voltage with a DC-link voltage reference and/or a comparison of the grid power with the power reference.

According to an embodiment, the determination of the damping power based on the virtual synchronous machine rotational speed comprises high-pass filtering the synchronous machine rotational speed and determining the damping power based on high-pass filtered signal.

According to an embodiment, the method comprises.

According to an embodiment the comparison of the DC-link voltage with the DC-link voltage reference comprises determining a contribution to the chopper power reference dependent on a voltage difference of the DC-link voltage and the DC-link voltage reference and a comparison of the voltage difference with a voltage threshold.

According to an embodiment the comparison of the grid power with the power reference comprises determining a contribution to the chopper power reference dependent on a power difference between the grid power and the power reference and a comparison of the power difference with a power threshold.

According to an embodiment the chopper power is determined dependent on a voltage measurement of the DC-link voltage, alternatively dependent on both DC-link voltage and a measured current flowing to/from the DC link capacitor.

Advantageously, the DC link voltage is controlled by feeding forward the grid power reference and by use of a DC link controller which adjusts the generator power reference to keep the DC link voltage at its reference value.

A second aspect of the invention is as defined in claim <NUM>.

A third aspect of the invention relates to a computer program product comprising software code adapted to control a wind turbine when executed on a data processing system, the computer program product being adapted to perform the method of the first aspect.

A fourth aspect of the invention relates to a wind turbine comprising a control system according to the first aspect.

In general, the various aspects and embodiments of the invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

<FIG> shows a wind turbine <NUM> (WTG) comprising a tower <NUM> and a rotor <NUM> with at least one rotor blade <NUM>, such as three blades. The rotor is connected to a nacelle <NUM> which is mounted on top of the tower <NUM> and being adapted to drive a generator situated inside the nacelle via a drive train. The rotor <NUM> is rotatable by action of the wind. The wind induced rotational energy of the rotor blades <NUM> is transferred via a shaft to the generator. Thus, the wind turbine <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 generator is connected with a power converter which comprises a machine side converter and a line side converter. The machine side converter converts the generator AC power into DC power and the line side converter converts the DC power into an AC power for injection into the utility grid.

<FIG> shows an example of a power system <NUM> of a wind turbine <NUM> according to an embodiment. The power system comprises a generator <NUM> and a power converter <NUM>. The power converter <NUM> comprises a machine side converter <NUM>, a line side converter <NUM>, a DC-link <NUM> and a resistor <NUM> connected with a controllable switch <NUM>. The resistor and switch forms a power dissipation device, also known as a chopper <NUM>, for dissipating active power. The DC-link <NUM> comprises on or more DC-link capacitors which are charged by the DC output current from the machine side converter <NUM> or current from the line side converter <NUM>. The output AC current from the line side converter <NUM> is supplied via output inductors <NUM> and possibly via a wind turbine transformer <NUM> to the power line <NUM>.

The power line <NUM> may be a medium voltage power bus which receives power from other wind turbines <NUM>. The power line <NUM> may be connected with a high voltage network, e.g. via further transformers. Thus, the power line <NUM> and one or more power systems <NUM> of corresponding wind turbines constitutes a wind power plant or park arranged to supply power to a utility grid for distribution of electrical power.

The power converter <NUM> may be full-scale converter configured according to different principles including forced-commutated and line-commutated converters.

The power system <NUM> is principally illustrated and therefore does not explicitly reveal that the system may be a three phase system. However, principles of the described embodiments apply both to single and multi-phase systems.

The line side converter <NUM> uses some variant of pulse width modulation (PWM) for converting the DC power into AC power. The control system <NUM> is used for controlling the modulation of the line side converter <NUM> and for controlling the reactive current and the active current generated by the line side converter <NUM>.

<FIG> shows that the grid voltage Ugrid, here the voltage at the low voltage LV side of the transformer <NUM>, can be measured. The grid voltage Ugrid can be used for determining a virtual synchronous machine angle θVSM (as described elsewhere) and for controlling the power output of the converter, based on determining Pgrid from grid voltage Ugrid and grid current Igrid. Alternatively, the grid voltage Ugrid may be measured on the high voltage HV side of the transformer and corrected based on the turns ratio of the transformer, or the internal voltage reference Uqref is used instead of the measured voltage Ugrid.

Thus, in an alternative, internal voltage references such as Uqref may be used for determining Pgrid and consequently synchronous machine angle θVSM. Thus, the grid current Igrid supplied to the grid can also be measured. The angle of the grid voltage θgrid can for example be determined from the grid voltage Ugrid.

<FIG> shows an example of a control components <NUM> arranged for controlling the generation of active current Iq and active power Pgrid and reactive current Id and reactive power Q supplied to the grid at the power output <NUM> from the wind turbine. The control components <NUM> may form part of the control system <NUM>. Alternatively, the control components <NUM> receive control signals from the control system <NUM>.

References for the active and reactive current references may be received from a Power Plant Controller, PPC, or a Transmission System Operator, TSO, or determined from active and reactive power references, e.g. from the grid operator.

The active power, Pgrid, is controlled via the virtual synchronous machine angle θVSM. Examples for determining the synchronous machine angle θVSM is given elsewhere.

The synchronous machine angle θVSM may be used to transform the signals from the rotating DQ frame into a non-rotating frame such as the αβ or abc frame, or vise-versa. Based on the synchronous machine angle θVSM and the voltage magnitude reference Uqref, control signals for the desired active power and reactive power are determined.

Thus, the synchronous machine angle θVSM may be defined in a rotating DQ frame defined by the angular position θVSM. Based on the synchronous machine angle θVSM, control signals, i.e. the angle of the modulation voltage signals for the pulse-width-modulator PWM, <NUM> are determined and transformed into a non-rotating frame such as the αβ or abc frame. The modulation Uqref voltage signal controls the reactive current Id and the active current Iq.

The frame conversion and control unit <NUM> determines the voltage reference signal and transforms the voltage control signal from the DQ frame into the αβ or abc frame. The frame converted output signals from the control unit <NUM> unit are converted by the pulse-width-modulator PWM, <NUM> into a modulation signal for the grid side converter <NUM> in order to generate the voltage based on the θVSM angle that will give the grid power according to the grid power reference.

The reactive power Q is controlled with the amplitude of the grid voltage reference Uqref which is determined based on a reactive power reference. The voltage reference Uqref is converted from the DQ frame to the αβ or abc frame and outputted from the control unit <NUM> as a control signal to the pulse-width-modulator PWM, <NUM> which determines the modulation signal for the grid side converter <NUM>.

<FIG> show examples of control systems <NUM> for determining the synchronous machine angle θVSM. The synchronous machine control systems <NUM> may be comprised by the control system <NUM>.

The synchronous machine angle θVSM is determined based on a virtual synchronous machine control concept which aims at generating a power response which corresponds to the power response from a real synchronous generator, including the inertia of the synchronous generator.

In response to grid voltage fluctuations, e.g. reflected in the measured Ugrid and Pgrid, which causes the virtual machine to either accelerate or decelerate to reach a new equilibrium condition. The new equilibrium is reached when the measured grid power Pgrid is again following Pref.

The virtual synchronous machine control concept is utilized on the line side converter <NUM> using a swing equation to calculate θVSM.

During an Under Voltage Ride Through (UVRT) event, the angular speed ωVSM will increase faster than the grid angular speed ωL and at low grid voltage and long duration faults the turbine is at risk of becoming unstable and trip or shut down. It is possible to change the inertia constant H of the swing equation to a high value during the fault, but then the control will not adapt to phase changes or real frequency changes during the UVRT. Embodiments and examples of the present invention dissipates energy in the DC link chopper <NUM> and use the dissipated energy in the swing equation to have a smoother performance during UVRT/OVRT with a more stable and controllable current injection during the fault and reduce the power swings in the voltage recovery phase. The advantage is wider voltage tolerance curve without losing synchronism and less mechanical loads in the voltage recovery phase.

<FIG> shows an example of an implementation of the virtual synchronous model <NUM>. The virtual synchronous model <NUM> includes a closed loop where the virtual synchronous machine rotational speed ωVSM is determined based on a combination of a feedback of a damping power Pd, a power reference Pref for the desired active power output of the wind turbine, the active grid power Pgrid supplied by the wind turbine to the grid via the power line <NUM> and a chopper power Pchop dissipated by the chopper <NUM> and an inertial integration model <NUM>. The inertial integration model <NUM> is implemented as <NUM>/(2Hs) where H is the inertia time constant and <NUM>/s is the integration in s-domain. Accordingly, the combination of powers Pref - Pd - Pgrid - Pchop is used as input for the inertial integration model <NUM>.

Since the derivative of the synchronous machine rotational speed ωVSM corresponds to the deviation between the power reference Pref and the grid power Pgrid, the integration of the difference Pref - Pd - Pgrid - Pchop gives the synchronous machine rotational speed ωVSM.

The grid power Pgrid can be determined based on the measured grid voltage Ugrid and the measured current Igrid, e.g. measured at the LV or HV side of the transformer.

Variations in the power reference Pref, i.e. variations per time unit, may be slope limited according to the slope limiter <NUM>.

The damping power Pd is determined as the difference between the rotational speed of the grid ωL and the synchronous machine rotational speed ωVSM multiplied with the damping factor Dp. The rotational speed of the grid ωL, i.e. the grid frequency is determined from the measured grid voltage Ugrid.

The synchronous machine angle θVSM is determined based on an integration of the synchronous machine rotational speed ωVSM according to ωr/s, where ωr is the rated synchronous generator speed.

The chopper power Pchop can be determined dependent on a measured DC-link voltage, e.g. by means of a voltage detector arranged to measure the voltage over the DC-link capacitor <NUM> according to the equation Pchop= chop_on*UDC*UDC/Rchop. Rchop is the resistance of the chopper resistor <NUM> and chop_on is a value between zero and one which indicates the duty-cycle of the switch <NUM>, i.e. the fraction of time where the switch <NUM> is closed, where chop_on=<NUM> may indicate that the switch is closed <NUM>% of a switching period.

<FIG> shows an alternative virtual synchronous model <NUM> which is not based on a measured grid voltage Ugrid, but the damping part, e.g. the damping power Pd, is determined based on a high-pass filtering <NUM> of the synchronous machine rotational speed ωVSM.

In general, the virtual synchronous model <NUM> determines the angle θVSM of the virtual machine based on the combination of powers Pref - Pd - Pgrid - Pchop, the inertial integration model <NUM>, e.g. implemented as <NUM>/(2Hs) and a feedback of the damping power Pd determined based on ωVSM and an integration of ωVSM.

The control systems <NUM> are implementable based on power values Pref, Pd, Pgrid, Pchop but may equivalently be implemented based on corresponding torque values Tref, Td, Tgrid, Tchop based on the relationship where power equals torque times rotation frequency, e.g. the rotational speed of the grid ωL.

<FIG> illustrates a circuit <NUM> for determining a chopper power reference Pchop_ref for the chopper <NUM>. As illustrated, Pchop_ref can be determined based on the difference of the DC-link voltage, UDC, and the DC-link voltage reference UDC_ref. The DC-link voltage is the voltage across the DC-link capacitor <NUM>. An increase of the DC-link voltage above the reference can be compensated by dissipating DC-link capacitor energy in the chopper according to the chopper reference Pchop_ref. Alternatively, Pchop_ref can be determined based on the difference between the measured grid power Pgrid and the power reference Pref.

If the power reference Pref is greater than the grid power Pgrid, e.g. due to a low voltage grid event, the excess energy will cause an increase of the DC-link voltage UDC. The excess energy can be compensated by activating the chopper according to the chopper reference Pchop_ref.

The contribution to the chopper power reference Pchop_ref based on the DC-link voltage UDC and/or the grid power Pgrid may be dependent on a comparison of the voltage and/or power difference with a respective voltage and power thresholds as defined by the voltage and power limit functions <NUM>, <NUM>. Thus, the contribution to the chopper reference from any of the comparisons or difference calculations may be zero if the difference is below the threshold, and if the difference is above the threshold, the limit functions <NUM>, <NUM> provides a monotonically increasing output as a function of the difference, i.e. voltage or power difference.

As illustrated, the chopper reference Pchop_ref can also be determined based on a combination, e.g. a sum, of the contributions from the DC-link voltage deviation (UDC-ref - UDC) and the contribution of the active power deviation (Pref - Pgrid).

<FIG> illustrates a circuit <NUM> for determining a power reference P_MSC_ref for the machine side converter <NUM> based on the difference between the DC-link voltage reference UDC_ref and the measured DC-link voltage UDC and the power reference Pref. The DC-link controller <NUM> determines a power adjustment P_corr to be combined with the power reference Pref. Thus, if the DC-link voltage UDC is too high, e.g. above a threshold, with respect to the DC-link reference UDC_ref, P_corr becomes negative so that the machine side power reference P_MSC_ref is reduced relative the power reference Pref. In this way the voltage across the DC link capacitor <NUM> is controlled.

<FIG> shows response curves in the event of a Low Voltage Ride Through event for a wind turbine configured with a Virtual Synchronous Machine which is not adapted to include the chopper power Pchop, i.e. the power deviation ΔP equals Pref-Pgrid-Pd.

Curve <NUM> shows the voltage drop at the measured Ugrid. Due to the voltage drop, the electrical power to the grid Pgrid (curve <NUM>) is instantly reduced. The grid power is increased during the fault, because the virtual synchronous machine (VSM) control will increase the angle between the grid and the synchronous machine angle θVSM. After the grid fault the grid power Pgrid oscillates since the virtual synchronous machine has accelerated up during the fault and will oscillate back to its prefault power level.

The power dissipated in the DC link chopper Pchop, e.g. determined according to the diagram in <FIG> is shown in curve <NUM>. The chopper is activated due to a high DC link voltage UDC or a mismatch between the generator power, i.e. the machine side power P_MSC, and the grid power Pgrid. Pchop may be reduced during the fault, as illustrated, to reduce energy capacity needs in the chopper, e.g. by adjusting the chop_on duty cycle.

Curve <NUM> shows that the machine side power P_MSC, in this example, is kept steady because the chopper is dissipating the power not delivered to the grid. After the fault the P_MSC power is oscillating until the grid side VSM control has returned to its steady state condition.

The active current Iq (curve <NUM>) increases due to the VSM response.

The reactive current Id (curve <NUM>) towards the grid is increased during the fault due to the VSM response with a reactive current to support the grid voltage.

The synchronous machine angle difference (θgrid -θVSM) between the grid angle and the synchronous machine angle (curve <NUM>) increases during the fault due to the deviation between the power reference Pref and the grid power Pgrid.

<FIG> shows response curves in the event of a Low Voltage Ride Through event for a wind turbine configured with a Virtual Synchronous Machine which is adapted to include the chopper power Pchop, i.e. so that the power deviation ΔP equals Pref-Pgrid-Pd-Pchop.

Curve <NUM> shows the voltage drop at the measured Ugrid. Due to the voltage drop, the electrical power to the grid Pgrid (curve <NUM>) is instantly reduced. After the grid fault the grid power Pgrid recovers slowly without oscillations. This is due to the control method including the chopper power Pchop which has the effect that the virtual synchronous machine is not accelerated up during the fault. That is, the angle difference between the grid θgrid and the synchronous machine angle θVSM is not significantly changed.

Curve <NUM> shows that the machine side power P_MSC is slightly unloaded during the fault and recovers after the grid fault. During normal non-fault conditions, the machine side power P_MSC is typical equal to grid power, but it can be useful to reduce generator power slower than grid power, during the fault, to avoid drive train loads and tower oscillations.

The active current Iq (curve <NUM>) is constant but could also be increased or reduced during the fault to match grid requirements.

The reactive current Id (curve <NUM>) towards the grid is increased during the fault to match either a VSM response or a more converter controllable value and to provide grid voltage support.

Claim 1:
A method for controlling a wind turbine (<NUM>), the wind turbine comprises a power generator (<NUM>), a machine side converter (<NUM>), a line side converter (<NUM>), a DC link (<NUM>), with a DC link voltage (UDC) and a chopper (<NUM>) electrically connected to an output of the machine side converter and an input of the grid side converter, characterized in that the method comprises
- determining a virtual synchronous machine rotational speed (ωVSM) and/or a synchronous machine angle (θVSM), where the virtual synchronous machine rotational speed (ωVSM) is determined based on a combination of a feedback of a damping power (Pd), a power reference (Pref) for a desired power output of the wind turbine, a grid power (Pgrid) supplied by the wind turbine to a power grid and controlling a chopper power (Pchop) dissipation by the chopper (<NUM>) and an inertial integration model (<NUM>), where the synchronous machine angle (θVSM) is determined based on an integration of the synchronous machine rotational speed (ωVSM),
- determining a chopper power reference (Pchop_ref) for the chopper, based on a comparison of the DC link voltage (UDC) with a DC-link voltage reference (UDC_ref) and/or a comparison of the grid power (Pgrid) with the power reference (Pref), and
- determining the damping power (Pd) based on the virtual synchronous machine rotational speed (ωVSM).