Patent Application
20240356327
This application claims priority of European patent application no. 23169485.2, filed Apr. 24, 2023, the entire content of which is incorporated herein by reference.
The following description relates generally to controlling the operation of a converter, a corresponding apparatus and system, and particularly to converters for doubly-fed induction generators (DFIGs) as commonly used with wind turbines.
DFIG systems include a wound-rotor asynchronous generator and commonly a back-to-back converter including a machine-side inverter, a grid-side inverter, and a DC voltage intermediate circuit connecting both inverters. Commonly the rotor of the generator is connected to the converter and the stator of the asynchronous generator is connected directly to the AC grid. Thus, mechanical drive train components of a wind turbine sustain mechanical stress during grid faults due to oscillations caused by the jump in the grid voltage. This mechanical stress can be limited by applying maximum electrical torque from the generator. The maximum electrical torque, however, is limited by the maximum active power that can be fed into the grid by the DFIG system.
For a reliable operation of a wind turbine, it is desirable to be able to control the wind turbine even at weak grid connection points and in the event of a grid fault.
It is an object to provide a method for controlling a converter, a corresponding apparatus and system that contribute(s) to a reliable operation of a wind turbine in particular at weak grid connection points and in the event of a grid fault.
According to a first aspect, a method for controlling a converter is specified. The converter is configured to be connected to a rotor of a DFIG, for feeding electrical power into an electrical grid. The converter includes a machine-side inverter, a grid-side inverter, and a DC voltage intermediate circuit.
The DC voltage intermediate circuit contains a protection element for dissipating power from the DC voltage intermediate circuit.
The method includes the steps of:
Using the grid-side inverter to “draw” active power from the grid-side AC bus to the DC voltage intermediate circuit and particularly towards the protection element enables a larger capacity of power dissipation for DFIG wind turbines. Compared to conventional control strategies, a limitation of the capacity due to a maximum current carrying capacity of the machine-side inverter is overcome. In particular, in case that both the grid-side inverter and the machine-side inverter are used to divert power to the protection element and have approximately the same current carrying capacity, the capacity of power dissipation for DFIG wind turbines may effectively be doubled.
DFIG wind turbines with a converter employing the above defined method may thus be particularly used on weak grids, that is, grids that are limited as to the amount of power that can be fed to the grid by the DFIG. In other words, such DFIG wind turbines may be used at weak grid connection points.
In an embodiment according to the first aspect, the machine-side inverter is coupled to the grid-side inverter through the DC voltage intermediate circuit. The DC voltage intermediate circuit may also be referred to as “DC link”. The protection element is coupled to a positive DC bus and a negative DC bus of the DC voltage intermediate circuit.
In an embodiment according to the first aspect, the machine-side inverter is configured to be coupled to a machine-side AC bus transmitting three-phase electric power to and from the rotor of the DFIG. The grid-side inverter is configured to be coupled to a grid-side AC bus transmitting three-phase electric power to and from the grid.
In an embodiment according to the first aspect, the protection element includes at least one switching device, in particular a fast switching device, for example, an insulated gate bipolar transistor (IGBT), a gate turn-off thyristor (GTO), or a silicon-controlled rectifier, in series with a resistive device. Once activated, the switching device transmits electric current to the resistive device to be dissipated as heat energy. The protection element may also be referred to as “dynamic braking circuit” or “chopper”. Other circuits that enable a similar operation, or a combination thereof may also be conceivable and expressly fall under the scope of the protection element.
In an embodiment according to the first aspect, the obtained information is indicative of a grid voltage drop or DFIG overpower.
In an embodiment according to the first aspect, the obtained information is indicative of at least one of a measured voltage level at a grid connection point of the DFIG, that is, a point of connection of the DFIG to the grid, a rated voltage level of the grid connection point, a difference of the measured voltage level and the rated voltage level, a measured power supplied to the grid, a steady state active power supplied to the grid, and a difference of the measured power and the steady state active power.
In an embodiment according to the first aspect, the target power value and/or the predetermined threshold is, for example, preset in a controller of the wind turbine, obtained together with the information, or indicated through further parameters obtained, for example, from the grid or grid operator. For example, the target power value and/or the predetermined threshold may represent grid connection regulations.
In an embodiment according to the first aspect, the information represents a grid fault. As used herein, the term “fault” generally refers to any of the following events at the grid including impulses, notches, glitches, interruptions, voltage sag/swells, harmonic distortions, and flickers.
In an embodiment according to the first aspect, the information represents a grid connection interruption.
In the event of grid faults, and in particular a grid connection interruption in weak grids/at weak grid connection points, the power supplied by the generator to the grid may exceed the target power value beyond the predetermined threshold. To compensate for an amount of power that cannot be fed to the grid, a drive train of the DFIG wind turbine may either take up mechanical loads so as to reduce excessive electrical power provided by the generator or by dissipating the excessive electrical power provided by the generator at the protection element. Advantageously, controlling the converter in the event of a grid fault in the above described manner and hence increased capacity of power dissipation may contribute to reduce a mechanical load acting on the drive train of such DFIG wind turbines. Additionally or alternatively, drive train configuration requirements with regard to the ability to withstand such loads may be lowered, thereby contributing to reduce the overall costs of a DFIG wind turbine.
In an embodiment according to the first aspect, the protection element is configured to dissipate a predetermined portion, for example, at least ⅔, of the DFIG wind turbine's rated power. For example, if the DFIG wind turbine has a rated power of 6 MW, both the machine-side inverter and the grid-side inverter have a rated power of at least 2 MW and the protection element is configured to withstand and dissipate at least 4 MW. The skilled person will appreciate that the load energy capability of the protection element depends on a number of configuration criteria, including the rated speed, grid code requirements, cooling capacity, et cetera.
In an embodiment according to the first aspect, the information further indicates a weak grid connection point. In the present disclosure, a weak grid connection point is understood as a grid connection point with a short circuit ratio, SCR, at the point of connection below a specific value, for example with SCR<3, in particular SCR<2, and more particularly SCR<1.5.
In an embodiment according to the first aspect, the grid-side inverter is configured to be controlled based on a set point value representative of a voltage level of the DC voltage intermediate circuit.
Furthermore, the protection element is configured to be controlled based on a threshold value representative of a maximum voltage level of the DC voltage intermediate circuit.
Controlling the converter, in response to obtaining the information, includes adapting the set point value and/or the threshold value such that the set point value is above the threshold value, that is, adapting at least one of the set point value and the threshold value.
By adapting, the converter is enabled to draw a controlled amount of active power from the grid-side AC bus into the DC voltage intermediate circuit and particularly towards the protection element. For example, given a set point value of 1100 V, the threshold value may be set to 1200 V or 1300 V.
In an embodiment according to the first aspect, adapting the set point value and/or the threshold value such that the set point value is above the threshold value specifically includes increasing the set point value.
In an embodiment according to the first aspect, adapting the set point value and/or the threshold value such that the set point value is above the threshold value specifically includes decreasing the threshold value.
In an embodiment according to the first aspect, the protection element is configured to be activated and deactivated based on a duty cycle value.
Furthermore, the controlling the converter, in response to obtaining the information, includes:
The first parameter may be a measured voltage level of the DC voltage intermediate circuit. For example, the first parameter may be obtained in the step of obtaining the information.
The duty cycle value may represent a pulse active time signal for activating and deactivating the protection element. By increasing the pulse active time, the amount of power dissipated at the protection element can be increased.
In an embodiment according to the first aspect, the duty cycle value is determined based on a voltage level difference between the first parameter and the set point value. Advantageously, the duty cycle value is increased at higher voltage level differences and decreased at lower voltage level differences so as to control the amount of power dissipated from the DC voltage intermediate circuit. In particular, changing the duty cycle value may compensate for the adapted set point value and/or threshold value, for example, in the event of a higher set point value, a higher duty cycle value may be useful to faster dissipate the accumulated power in the DC voltage intermediate circuit.
In an embodiment according to the first aspect, the duty cycle value is determined based on the voltage level difference by applying a characteristic. The characteristic can be, for example, a non-linear curve or a droop. In particular, the characteristic is predetermined. For example, the characteristic may be preset in a controller of the wind turbine.
In an embodiment according to the first aspect, the characteristic is predetermined based on a short circuit ratio, SCR, value at the grid connection point of the DFIG, a characteristic value of the protection element and a rated voltage level of the DC voltage intermediate circuit. In particular, based on the characteristic value of the protection element, and in particular its ohmic resistance, the characteristic can account for the amount of power that can be dissipated at the protection element. Furthermore, in particular based on the SCR value of the grid connection point, the characteristic can account for the amount of power that can be supplied to the electric grid. Advantageously, for every operating point of a given DFIG wind turbine connected to the grid, a proper duty cycle value can be determined.
In an embodiment according to the first aspect, adapting the set point value and/or the threshold value such that the set point value is above the threshold value includes:
The second parameter may include a measured voltage level at the grid connection point. For example, the second parameter may be obtained in the step of obtaining the information.
In particular, based on the second parameter, adapting the set point value and/or the threshold value such that the set point value is above the threshold value in an embodiment according to the first aspect specifically includes:
The predetermined amount of power to be supplied to the protection element may particularly compensate an amount of generator overpower and/or an amount of power that cannot or no longer be fed to the grid.
In an embodiment according to the first aspect, the target power value encompasses a target power output for drive train oscillation damping.
Furthermore, determining the set point value such that the grid-side inverter supplies a predetermined amount of power to the protection element includes determining the set point value based on the target power value.
Advantageously, the set point value may be determined such that the protection element implements a function of damping oscillations of the drive train of the DFIG wind turbine. In particular, the grid-side inverter may supply a predetermined amount of power that accounts for mechanic loads acting on the drive train. Additionally or alternatively, drive train configuration requirements with regard to the ability to withstand such loads may be lowered, thereby contributing to reduce the overall costs of a DFIG wind turbine.
According to a second aspect, a method for controlling a converter is specified. Features described in the context of the first aspect apply to the second aspect and vice versa.
Specifically, the converter is configured to be connected to a rotor of a doubly-fed induction generator for feeding electrical power into an electrical grid. The converter includes a machine-side inverter, a grid-side inverter, and a DC voltage intermediate circuit. The DC voltage intermediate circuit contains a protection element for dissipating power from the DC voltage intermediate circuit. The method includes the steps of:
Advantageously, the converter may be controlled such that the protection element implements a function of damping oscillations of the drive train of the DFIG wind turbine. In particular, the grid-side inverter may supply a predetermined amount of power that accounts for mechanic loads acting on the drive train. Additionally or alternatively, drive train configuration requirements with regard to the ability to withstand such loads may be lowered, thereby contributing to reduce the overall costs of a DFIG wind turbine.
According to a third aspect, an apparatus for controlling a converter is specified. Features described in the context of the first or the second aspects apply to the third aspect and vice versa.
Specifically, the converter is configured to be connected to a rotor of a doubly-fed induction generator for feeding electrical power into an electrical grid. The converter includes a machine-side inverter, a grid-side inverter, and a DC voltage intermediate circuit. The DC voltage intermediate circuit contains a protection element for dissipating power from the DC voltage intermediate circuit. The apparatus is configured to perform the steps of the method according to either of the first or second aspect.
According to a fourth aspect, a system is specified. Features described in the context of the first, the second or the third aspects apply to the fourth aspect and vice versa.
Specifically, the system includes a wind turbine, a doubly-fed induction generator, a converter and an apparatus according to the third aspect for controlling the converter. The generator includes a rotor and a stator. The converter includes a machine-side inverter connected to the rotor, a grid-side inverter, and a DC voltage intermediate circuit. The DC voltage intermediate circuit contains a protection element for dissipating power from the DC voltage intermediate circuit. The stator and the grid-side inverter are configured to be connected to an electrical grid for feeding electrical power into the electrical grid.
The invention will now be described with reference to the drawings wherein:
Elements of the same construction or function are marked with the same reference signs across the figures.
During operation, the rotor 12 is set in rotation by an air flow, for example wind. This rotational movement is transmitted to the generator 15 via the rotor shaft and, if necessary, a gearbox. The generator 15 converts the mechanical energy of the rotor 12 into electrical energy.
The converter 26 helps to ensure supply of active and reactive current, for example, in the event of grid faults, and thus compliance with specific grid connection regulations. The converter 26 includes a machine-side inverter (also referred to as rotor-side or generator-side inverter) 30 and a grid-side inverter (also referred to as line-side inverter) 32. Both are connected by a DC voltage intermediate circuit (also referred to as DC link) 34. The DC link 34 includes a protection element (also referred to as chopper) 74 and may further include a capacitive device 75 for maintaining the DC link voltage at a certain level, both coupled between a positive DC bus and a negative DC bus of DC link 34. The chopper 74 may include or substantially consist of a switch coupled in series with a resistor, where the switch may be any device for switching including, without limitation, a GTO or an IGBT. In operation, the switch controllably couples the positive and negative DC bus to the resistor to convert excess energy into heat, for example, in the event of grid faults when it can no longer be fed into the electric grid 22 due to a low grid voltage.
For illustration, a number of elements useful to implement the method of this disclosure are depicted in
For better overview, a controller that controls the operation of the wind turbine is not shown. However, it is conceivable that parts of the method of this disclosure can be implemented in such a controller and that, by way of example, a set point value for an electrical variable in dependence on the grid voltage may be determined by such a controller. It is also conceivable to implement a method of this disclosure in whole or in part by analogue means.
The inverters 30, 32 may be controlled by current controllers 40, 42 and pulse-width modulators 36, 38. More specifically, the current controller 40 may control the pulse-width modulator 36, which in turn may provide the pulse width modulation for the rotor-side inverter 30, and the current controller 42 may control the pulse-width modulator 38, which in turn may provide the pulse-width modulation for the grid-side inverter 32. The chopper 74 may be controlled by pulse-width modulator 72. More specifically, a measured DC link voltage 76 may be present at, for example, set point module 44. The set point module 44 may be particularly configured to determine a set point value 73 for controlling the pulse-width modulator 72, that is, for setting a duty cycle of the chopper 74 and/or setting a threshold for activating the chopper 74.
Set point values 43, 46 and actual values 63, 65 for controlling the rotor-side inverter 30 and the grid-side inverter 32 may be present at the current controllers 40, 42. The set point values 43, 46 may be determined by the set point module 44. Measured values 62, 64 for the currents on the rotor side and on the grid side may also be present at the current controllers 40, 42, as shown in
Measured voltages 58 and measured currents 60 from the three-phase connection 20 to the grid 22 may be present at the controller 27. Furthermore, the measured currents 62 measured in the three-phase connection circuit 24 between the rotor 16 of the generator 15 and the rotor-side inverter 30 and the measured currents 64 measured in the three-phase connection circuit 28 of the grid-side inverter 32 may be present as input quantities at the converter controller 27. The currents 62 measured in the three-phase connection 20 of the rotor-side inverter 30 to the rotor 16 may also be present at the current controller 40 that controls the pulse-width modulator 36 and thus the rotor-side inverter 30. Accordingly, the currents 64 measured in the three-phase connection 28 of the grid-side inverter 32 may also be present at the current controller 42 that controls the pulse-width modulator 38 and thus the grid-side inverter 32. This way it is possible to control the frequency of the pulse-width modulation depending on the currents.
The plural used for the measured quantities 58, 60, 62, 64 is due to the fact that they may be measured and processed in a multi-phase system. For example, in case of the measured voltage 58 there may be voltage measurements for all three phases of the three-phase connection 20 present at the controller 27. Alternatively, for a three-phase system, without a neutral conductor, there may be only two measurements from two phases and the value of the third phase may be calculated.
The input quantities 58, 60, 62, 64 present at the converter controller 27 may be measured by suitable sensors that capture the relevant quantities at the three-phase connections 20, 24, 28 and forward them to the controller 27 as input values. Large dots represent these sensors in
The controller 27 in the embodiment shown in
In comparison to converters that also decouple the generator 15 from the electrical grid 22 on the stator side (“full converter”), which converters usually can convert the full power of the wind turbine 10 into heat for a certain time with the help of a chopper, a current carrying capacity of converters 26 in a DFIG configuration (“partial converter”) is often limited such that only about a third of their rated power is routed through the converter 26 and can thus be diverted to the chopper 74. Specifically, with reference to
where
In addition, the inventor realized that in the above mentioned event the controlling of the chopper 74 as well as the converter 26 may lead to a current limitation of the machine-side inverter 30 where the available capacity of the converter 26 cannot be used for the chopper 74.
In case of a deep voltage sag, the converter 26 is controlled to create an island network up to a location of the grid fault 25. Depending on the impressed reactive current as well as the impedance 23 up to the location of the grid fault 25, the voltage V2 at the wind turbine 10 may remain at up to 50% of the nominal voltage even in the case of a low-impedance “hard” 3-phase fault. Thus, an energy transport from the stator 18 into the chopper 74 could take place via the island network at the stator 18 which the wind turbine 10 generates through reactive current.
According to a first implementation manner of the present disclosure, power can be supplied to the chopper 74 via the grid-side inverter 32 by adapting a set point value 83 of the voltage of the DC link 34 of the grid-side inverter 32. Specifically, the set point value 83 is adapted such that it is greater than a threshold for the activation of the chopper 74, that is, a threshold for the switch to couple the positive and negative DC bus to the resistor.
For example, with reference to
Additionally or alternatively, as indicated in
Additionally or alternatively, a duty cycle for controlling the chopper 74 can be adapted based on the voltage of DC link 34. For example, the pulse-width modulator 72 (cf.
According to a second implementation manner of the present disclosure, power can be supplied to the chopper 74 via the grid-side inverter 32 by adapting a threshold value for activating the chopper 74, for example, as an alternative or in addition to the first implementation manner. Specifically, the threshold is adapted such that it is lower than the set point. Both implementation manners enable, among others, a controlled amount of power being drawn into the chopper 74 as well as the controlled sharing of power being drawn into the chopper 74 between the grid-side inverter 32 and the machine-side inverter 30.
In a step 1, the DFIG 15 and the converter 26 are operated in a power production mode, that is, the converter 26 is controlled to output power P28 to the grid 22. The converter 26 may particularly ensure supply of active and reactive current in compliance with specific grid requirements. When a grid fault is detected (step 2), for example, by a voltage drop in measured voltage 58, the converter 26 is controlled in accordance with the first or second implementation manner described above (step 3) so as to draw power P28 at least partially into the DC link 34 and dissipate power P74 with the aid of the chopper 74. In this respect the threshold and/or duty cycle of the chopper 74 and/or the set point value 83 of the grid-side inverter 32 are adapted. Once the grid fault has been detected to have cleared (step 4), the converter 26 may return to being operated again in power production mode (step 1). In the example shown in
It shall be pointed out that the above described flow diagram is merely exemplary and that particularly the order of steps could vary in practice as can be readily understood by a person skilled in the art. Furthermore, drive train oscillation compensation can be performed independent of a detected grid fault, that is, both during power production mode and during the grid fault.
The use of the grid-side inverter 32 according to the present disclosure for drawing power into the chopper 74 enables a larger power dissipating capacity of DFIG wind turbines and may particularly double the capacity of DFIG wind turbines to dissipate power depending on the fault type. Due to the rapidly growing expansion of wind energy with only moderate growth in grid expansion, wind farms with weak grid connection points are increasingly being created. As a result, only a small part of the rated power of such wind farm can be absorbed by the grid, especially in the event of grid faults. As a result, DFIG wind turbines in particular have to withstand greater loads on the drive train, since the generator stator is rigidly coupled to the faulty grid and the chopper is limited in power consumption by the partial converter.
This may particularly lead to strong torque fluctuations at the generator 15 and thus to an increasing load on the drive train 14, which must be taken into account in the configuration of wind turbines 10 for DFIG configuration. Accordingly, via a method according to the present disclosure, DFIG wind turbines may be used even on weak grids where a power absorption capability of the grid is limited, especially in the event of a grid fault. Furthermore, the load on the mechanical drive train 14 of such DFIG wind turbines particularly in the event of a grid fault can be reduced. Likewise, a reduction of mechanical drive train configuration requirements with a simultaneously larger chopper can reduce the overall costs of the DFIG wind turbine.
It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 23169485.2 | Apr 2023 | EP | regional |
