Patent Application
20130187384
The present invention relates generally to a method of operating a wind turbine, a wind turbine, a wind turbine controlling system, and a processing system usable for operating a wind turbine.
With the rapid increase of large offshore wind farms, a new problem associated with the response of wind turbines to temporary overvoltages has arisen: The majority of wind turbines use voltage source converters with a DC-link. When the grid voltage exceeds a certain voltage limit, the current flow through the line-side converter of the wind turbine may reverse, resulting in a rapidly increasing DC-link voltage.
One possibility to handle such situations is to interrupt the connection between the wind turbine and the grid. However, it is desirable to keep the wind turbines connected to the grid even under such circumstances. Thus, different approaches are currently under investigation which enable to keep the wind turbines connected to the grid during temporary overvoltages, while at the same time avoiding rapidly increasing DC-link voltages.
According to an embodiment, a method of operating a wind turbine comprising a DC-to-AC voltage converter is provided, the wind turbine being connectable to a grid via the DC-to-AC voltage converter, the method comprising: determining a line voltage of a power line connected between the DC-to-AC voltage converter and the grid; if the determined line voltage exceeds a particular grid voltage threshold value, directly injecting reactive current into the power line, wherein the amount of reactive current injected is chosen such that an output voltage of the DC-to-AC voltage converter is kept within a predetermined voltage range.
According to an embodiment, the line voltage is determined using a phase lock loop algorithm.
According to an embodiment, a reactive current reference is calculated based on a maximum output voltage of the DC-to-AC voltage converter, the line voltage, and an active current reference.
According to an embodiment, the active current reference is derived from a DC link controller output.
According to an embodiment, the reactive current reference is calculated according to the following formula:
I
R≦(√{square root over (m2·Umax2−(UGD−ωL·IA)2)}−UGQ)/ωL
wherein UGD, UGQ are the line voltages of the power line along the d/q axis in dq frame, and IA, IR are the active and reactive currents, m is the maximum allowed modulation index and Umax is the maximum allowed converter output voltage {right arrow over (U)}v. Further, L is the inductance of an inductor (commonly known as “grid inductor” or “grid choke”), and ω is the voltage frequency. IR calculated in the above equation has a negative value. Thus, the minimum reactive current may be chosen (in terms of amplitude) if IR is set equal to the right term of the equation. Since a maximum current Imax is fixed, this will leave more space for injection of active current IA. Thus, IR should preferably be equal to the right term of the equation. According to an embodiment, the reactive current reference is converted into an optimized reactive current reference if a converter output current reference amplitude calculated from the active current reference and the reactive current reference exceeds a predetermined converter output current reference amplitude threshold value.
According to an embodiment, the active current reference is converted into an optimized active current reference if a converter output current reference amplitude calculated from the active current reference and the reactive current reference exceeds a predetermined converter output current reference amplitude threshold value.
According to an embodiment, the converter output current reference amplitude is determined according to the following formula:
I
ref=√{square root over (Ir ref cal2+Ia ref cal2)}
According to an embodiment, the optimized active current reference/reactive current reference are calculated based on the following formula:
According to an embodiment, the converter output current is controlled based on the optimized active current reference and the optimized reactive current reference. That is, the amount of reactive current injected is controlled based on the optimized reactive current reference, and the amount of active current injected is controlled based on the optimized active current reference. One effect of this embodiment is that the converter output voltage is kept within Umax, and the converter output current is kept within Imax. A further effect of this embodiment is that, at the same time, the maximum possible active power can be injected into the grid.
According to an embodiment, a processing system usable for operating a wind turbine comprising a DC-to-AC voltage converter is provided, the wind turbine being connectable to a grid via the DC-to-AC voltage converter, the processing system comprising: an input unit being configured to receive a line voltage signal indicating the line voltage of a power line connected between the DC-to-AC voltage converter and the grid; a processing unit coupled to the input unit, the processing unit being configured to determine whether the line voltage exceeds a particular line voltage threshold value, and to determine an amount of reactive current which, if directly injected into the power line, keeps an output voltage of the DC-to-AC voltage converter within a predetermined voltage range; and an output unit coupled to the processing unit, the output unit being configured to output a signal indicative of the reactive current to be directly injected into the power line.
According to an embodiment, the input unit is further configured to receive an active current reference signal, wherein the processing unit is configured to calculate a reactive current reference indicative of the reactive current to be injected based on a maximum output voltage of the DC-to-AC voltage converter, the line voltage signal, and the active current reference signal, wherein the output signal outputted by the output unit is derived by the processing unit from the reactive current reference.
According to an embodiment, the processing unit is further configured to convert the reactive current reference into an optimized reactive current reference if a converter output current reference amplitude calculated from the active current reference and the reactive current reference exceeds a predetermined converter output current reference amplitude threshold value, wherein the output signal outputted by the output unit is derived by the processing unit from the optimized reactive current reference.
According to an embodiment, the processing unit is configured to convert the active current reference into an optimized active current reference if a converter output current reference amplitude calculated from the active current reference and the reactive current reference exceeds a predetermined converter output current reference amplitude threshold value, wherein the output signal outputted by the output unit is derived by the processing unit from the optimized active current reference.
According to an embodiment, a wind turbine controlling system for controlling a wind turbine comprising a DC-to-AC voltage converter is provided, the wind turbine being connected to a grid via the DC-to-AC voltage converter, the controlling system comprising a processing system according to any one of the embodiments as described above.
According to an embodiment, the wind turbine controlling system further comprises a phase lock loop unit being coupled to the input unit of the processing system, the phase lock loop unit being configured to determine the line voltage signal using a phase lock loop algorithm, and to supply it to the input unit.
According to an embodiment, the wind turbine controlling system further comprises a DC link controller unit coupled to the input unit of the processing system, wherein the DC link controller unit is configured to control a DC link voltage according to a DC link voltage reference level (the DC link controller unit controls the DC link voltage so that it is always kept at a reference voltage; the DC link controller does this by injecting an appropriate active current), and to output the active current reference signal which is supplied to the input unit.
According to an embodiment, the wind turbine controlling system further comprises a converter output current controlling unit coupled to the output unit of the processing system, wherein the converter output current controlling unit is configured to control the converter output current based on the active current reference and the reactive current reference, or based on the optimized active current reference and the optimized reactive current reference.
According to an embodiment, a wind turbine comprising a wind turbine controlling system according to any one of the embodiments as described above is provided.
In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the invention are described with reference to the following drawings, in which:
a shows a schematic drawing of an electrical system having a full scale converter configuration;
b shows a schematic drawing of an electrical system having a full scale converter configuration according to an embodiment of the present invention;
The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.
a shows an electrical system 100 of a wind turbine having a converter configuration which may be used in a wind turbine according to embodiments of the present invention. The electrical system 100 is connected to a generator 102 of a wind turbine. The electrical system 100 comprises an AC-to-DC voltage converter 104 (generator-side converter) connected to the generator 102, a DC-to-AC converter 108 (line-side converter) and a DC link 106 connected between the AC-to-DC converter 104 and the DC-to-AC converter 108. The DC-to-AC converter 108 is connected via a power line 116 to a transformer 112 which in turn is connected to a power grid 110. An inductor 114 is located along the power line 116. The power line 116 is further connected to a capacitor 118. The electrical system 100, the generator 102 and the transformer 112 may be part of a wind turbine as shown in
In other words: In case the grid voltage of the power grid 110 exceeds for example beyond 1 p.u. (p.u.=“per unit”, i.e. a ratio of voltage/normal voltage), the DC-to-AC converter 108 (line-side converter) of the wind turbine is required to adjust a line current vector such that the required converter output voltage of the DC-to-AC converter 108 can be kept below the maximum converter output voltage Umax and the converter output current does not exceed the converter output current limit of the DC-to-AC converter 108. In grid swell condition, since the maximum output voltage Umax of the DC-to-AC converter 108 is limited by the DC-link voltage UDC, the converter output current IG may become uncontrollable. Also, in a grid swell condition, a significant overshoot of the DC-link voltage UDC may occur which may lead to an activation of a chopper (not shown) for the dissipation of unknown/uncontrolled power transfer to the grid 110. In order to avoid such uncontrollable situations, countries like Australia have stipulated High Voltage Ride Through (HVRT) requirements for wind turbines to withstand an over-voltage of 1.3 p.u. for 60 ms without being disconnected.
The grid voltage on the grid 110 (high voltage side of the transformer 112) is correlated to the line voltage of the power line 116 (low voltage side of the transformer 112) by the transformer ratio. Thus, in the present invention, the terms “line voltage” and “grid voltage” may be used interchangeably.
However, the situation may be kept under control if reactive power is injected into the power line 116 connected between the DC-to-AC converter 108 and the transformer 112 (and thus into the power grid 110) by adjusting the required output voltage of the DC-to-AC converter 108, as will become apparent in the description below.
In the context of
In the following description, a theoretical background for embodiments of the present invention will be given.
The output voltage of the DC-to-AC converter 108 in
{right arrow over (U)}
v
={right arrow over (U)}
G
+{right arrow over (U)}
L
={right arrow over (U)}
G
+jωL{right arrow over (I)}
G
wherein {right arrow over (U)}v is the converter output voltage of the DC-to-AC converter 108, {right arrow over (U)}G is a line voltage at a part of the power line 116 close to the low voltage end of the transformer 112, {right arrow over (U)}L is the voltage drop across the inductor 114, and {right arrow over (I)}G is the output current of the DC-to-AC converter 108. Further, L is the inductance of the inductor 114, and ω is the voltage frequency.
In case of a “healthy” grid (power line) (in a case where there is no grid swell and where the line voltage {right arrow over (U)}G is within its normal line voltage range), the converter output current {right arrow over (I)}G is aligned with the line voltage {right arrow over (U)}G as illustrated in the phasor diagram of
Under the grid voltage swell condition, according to an embodiment of the present invention, the converter output current {right arrow over (I)}G is controlled in such a way that the amplitude of converter output voltage {right arrow over (U)}v is limited at or below a maximum converter output voltage amplitude Umax. The phase of converter output current {right arrow over (I)}G leads the phase of the line voltage {right arrow over (U)}G and it can be projected into reactive current Ireactive and active current Iactive as shown in the phasor diagram of
It should be noted that in the case of grid voltage dip (LV (low voltage) events), reactive (capacitive) current is also normally required by grid operators to be injected into the grid 110 to help in stabilizing the grid 110. In the case of LV event, the converter output voltage amplitude is always below the limit Umax. The phasor diagram as shown in
The following equations can be derived from a DQ frame model of line current control.
wherein UVD, UVQ are the converter output voltages along the d/q axis in the dq frame, UGD, UGQ are the line voltages along the d/q axis in dq frame, and IA, IR are the active and reactive currents. Further, L is the inductance of the inductor 114, and ω is the voltage frequency.
In the steady state condition, the converter output voltage {right arrow over (U)}v can be calculated as below
The following expression can be used based on the converter voltage limit,
U
VD
2
+U
VQ
2=(UGD−ωLIA)2+(UGQ+ωLIR)2≦(m·Umax)2 (1)
wherein “m” is the maximum allowed modulation index and Umax is the maximum allowed converter output voltage {right arrow over (U)}v.
Using the equations given above, under the steady state condition, the required reactive current for the HVRT condition can be derived as follows
I
R≦(√{square root over (m2·Umax2−(UGD−ωL·IA)2)}−UGQ)/ωL (2)
The voltage ride through functionality system 402 comprises an input unit 404, an output unit 406 and a processing unit 408.
The input unit 404 is coupled to a phase lock loop unit 410 being configured to generate a line voltage signal S1 indicative of the d,q components of the line voltage of the power line 116 using a phase lock loop algorithm, and to supply the line voltage signal S1 to the input unit 404.
The input unit 404 is further coupled to a DC link controller unit 412 being configured to generate an active current reference signal S2, and to supply the active current reference signal S2 to the input unit 404.
The output unit 406 is coupled to a converter output current controlling unit 414 being configured to control the converter output current based on an active current reference signal S3 and an reactive current reference signal S4. The active current reference signal S3 may be an optimized active current reference signal, and the reactive current reference signal S4 may be an optimized reactive current reference signal.
The processing unit 408 comprises a first reactive current generator unit 416, a second reactive current generator unit 418, a line voltage classifying unit 420, a switching unit 422, and a current optimizing unit 424.
Based on a nominal line voltage signal S5, the line voltage classifying unit 420 classifies the line voltage into three categories—normal, dip or swell.
Depending on the line voltage category determined, a reactive current reference signal S6 is generated by either the first reactive current generator unit 416 (in case of a dip condition) based on information as shown in
Signal S9 is a measured DC link voltage signal (measured at the DC link 106, signal S10 is an inductance input parameter (a parameter indicating the inductance of the inductor 114), signal S11 is a DC link voltage target value, and signal S12 is a measured converter output current signal of the converter 108.
In the case that a voltage dip condition is detected by the line voltage classifying unit 420, the reactive current reference signal S6 is calculated by the first reactive current generator unit 416 based on the percentage of the dip according to a grid code as shown in
The current optimizing unit 424 (optimal current trajectory control block) is optional and works as follows. A converter output current reference amplitude is computed by the current optimizing unit 424 from the reactive current reference signal S6 and the active current reference signal S2, and is compared with a converter output current reference amplitude limit Imax. If the converter output current reference amplitude is less than Imax, an active current reference signal S3 and a reactive current reference signal S4 are set as following:
That is, in this case, the active current reference signal S3 corresponds to the active current reference signal S2, and the reactive current reference signal S4 corresponds to the reactive current reference signal S6.
However, if the current reference amplitude exceeds the maximum limit Imax, an optimization process is applied. The optimization process is carried out as follows: Optimized current references (signals S3, S4) are determined using equation (1) as follows:
Since UGD is equal to zero in the steady state (this is because the rotation frame, the voltage is aligned with the Q axis in stable condition, and in vertical position to the D axis; UGD is the voltage that the line voltage is projected to D axis, so it is zero) in the current vector plane, the equations above can be written as following:
By solving the above equations (3), the following optimal current references (signals S3, S4) are obtained:
That is, if the converter output current reference amplitude exceeds the maximum limit Imax, the signals S3, S4 are obtained from signals S2, S6 using equations (4).
If the equations (4) are plotted in the current plane, the optimized solutions for active/reactive current references in line voltage swell condition can be visualized. This is shown in
This optimal current trajectory control ensures maximum active current and power output during HVRT while maintaining both the converter output current and converter output voltage within its limits.
In the condition of line voltage dip, the situation of
Based on the above description, it has become apparent that, according to an embodiment of the present invention, the following method is applied: 1) First, the active current (Ia_ref) and reactive current (Ir_ref) that need to be injected into the power line are determined. Ir_ref is determined using equation (2) and optimized, if necessary. 2) To cause the DC-to-AC converter 108 to output a converter output current IG which includes the respective active and reactive current components, the DC-to-AC converter 108 is controlled to output the necessary converter output voltage Uv. The control of the DC-to-AC converter 108 is done using PWM (Pulse Width Modulation) signals generated based on the signal S8 (i.e. the output signal from the converter output current control unit). 3) The PWM signals are used to control the operation of the DC-to-AC converter 108 such that it outputs the Uv (which is below Umax) together with a corresponding converter output current IG (comprising the corresponding Ia and Ir components determined) which is below the converter output current limit. Thus, first the reactive current to be injected into the power line 116 is determined such that Uv needed to generate the reactive current is below Umax. Then, the DC-to-AC converter 108 is controlled to have a voltage output Uv which will imply the converter output current IG with corresponding Ir component as determined earlier.
As has become apparent, embodiments of the present invention solve current and power control problems in case of a grid voltage swell.
According to embodiments of the present invention, by selecting a proper trajectory of current, the optimal active and reactive current can be injected such that the maximum active power can be transferred to the grid during HVRT.
According to embodiments of the present invention, the possibility of turbine tripping and chopper activation during a HVRT condition is minimized.
According to embodiments of the present invention, real and reactive power is controlled during voltage ride through condition.
According to embodiments of the present invention, by using an optimal trajectory current control, it is possible to provide the optimal active and reactive current injection so that the maximum active power can be transferred to the grid during HVRT.
According to embodiments of the present invention, a reactive current reference is computed, thereby minimizing the possibility of chopper activation. In HVRT condition with no activation of a chopper, the power is fed to the grid rather than being dissipated as heat into a chopper resistor.
While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.
| Number | Date | Country | Kind |
|---|---|---|---|
| PA 2010 00272 | Mar 2010 | DK | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/DK2011/050095 | 3/23/2011 | WO | 00 | 4/12/2013 |
| Number | Date | Country | |
|---|---|---|---|
| 61319305 | Mar 2010 | US |
