A method for controlling a wind farm during shallow grid faults

Abstract

A method for controlling a wind farm (1) is disclosed. A grid voltage phase angle, θ, of an electric grid (3) is monitored, and in the case that a change in angle, Δθ, of the grid voltage phase angle, θ, exceeds a predefined threshold value, it is investigated whether or not the power plant controller (5) has entered into a fault mode. In the case that the power plant controller (5) has not entered into a fault mode, the power setpoints to the wind turbines (2) are freezed for a predefined period of time. A fast recovery period, following a grid fault, is obtained.

Description

[0001] A METHOD FOR. CONTROLLING A WIND FARM DURING SHALLOW GRID FAULTS

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a method for controlling a wind farm during grid faults, in particular during shallow grid faults. The method according to the invention ensures a fast active power recovery of the wind farm following the grid fault.

[0004] BACKGROUND OF THE INVENTION

[0005] Generally, inverter- based power producers, such as wind turbines or photovoltaic panels, when being connected to an electric grid, provide active power to the electric grid. Moreover, in the case of grid faults, the inverterbased power producers are required to provide a level of active and / or reactive power which is sufficient to comply with grid code requirements, during the grid fault as well as during a recovery phase following the grid fault. In order to do this, inverter- based power producers may be able to detect when a grid fault is occurring. For instance, inverter- based power producers may detect a grid fault in the electric grid by monitoring the grid voltage phase angle and / or the voltage of the electric grid.

[0006] In traditional electric grids being dominated by synchronous machines, a change in grid voltage phase angle and a change in voltage magnitude, caused by a grid fault, occur almost simultaneously, and this allows a grid fault to be detected easily and reliably. However, in electric grids being dominated by inverter- based power producers, and in particular in electric grids with a high presence of grid forming power producers, it is not straight forward to reliably detect a grid fault based on grid voltage phase angle and / or voltage measurements, due to variations in the design and performance of controllers of grid forming power producers. For instance, behaviour of grid forming power producers may impact grid fault detection approaches of grid following power producers. Thus, a wind farm operating in grid following mode might not detect a grid fault accuratelyand reliably, and this may result in unintended performance of the wind farm during a grid fault and the following recovery phase.

[0007] DESCRIPTION OF THE INVENTION

[0008] It is an object of embodiments of the invention to provide a method for controlling a wind power plant during shallow grid faults in which accurate, reliable and fast reaction to grid faults is obtained.

[0009] It is a further object of embodiments of the invention to provide a method for controlling a wind power plant in which fast recovery following a grid fault is ensured.

[0010] The invention provides a method for controlling a wind farm during shallow grid faults, the wind farm comprising a plurality of wind turbines connected to an electric grid at a point of common coupling and a power plant controller being configured to dispatch power setpoints for the wind turbines to generate power, the method comprising the steps of:

[0011] - monitoring a grid voltage phase angle, 0, of the electric grid,

[0012] - in the case that a change in angle, AO, of the grid voltage phase angle, 0, exceeds a predefined threshold value, investigating whether or not the power plant controller has entered into a fault mode, and

[0013] - in the case that the power plant controller has not entered into a fault mode, freezing the power setpoints to the wind turbines for a predefined period of time.

[0014] Thus, the method according to the invention is a method for controlling a wind farm. In the present context the term 'wind farm' should be interpreted to mean a plurality of wind turbines arranged within a specified geographical area, and which share some infrastructure, such as internal power grid, connection to an external electric grid, substations, access roads, etc. Thus, the wind farm comprises a plurality of wind turbines connected to an electric grid at a point ofcommon coupling, the wind turbines of the wind farm thus supplying power to the electric grid. The wind farm further comprises a power plant controller being responsible for the overall control of the wind turbines within the wind farm. For instance, the power plant controller is configured to generate and dispatch power setpoint to the wind turbines within the wind farm, so as to cause the wind turbines to generate power in accordance with the power setpoints, in order to ensure that the total power production of the wind farm meets requirements of the electric grid.

[0015] In the method according to the invention, a grid voltage phase angle, 0, of the electric grid is monitored. In the present context the term 'grid voltage phase angle' should be interpreted to mean the phase angle of the voltage of the electric grid at the point of common coupling of the wind farm.

[0016] The grid voltage phase angle, 0, can be expected to change fast in the case that a grid fault is occurring in the electric grid, and therefore monitoring the grid voltage phase angle, 0, of the electric grid can be expected to provide a fast and reliable indication of the occurrence of a grid fault, especially for shallow grid faults.

[0017] Accordingly, it is investigated whether or not and to which extent the grid voltage phase angle, 0, changes. To this end, a change in angle, A0, of the grid voltage phase angle, 0, is derived or obtained, in the form of the difference between grid voltage phase angles, 0, measured at two specified points in time. In the case that the change in angle, A0, exceeds a predefined threshold value, this is an indication that a grid fault is likely occurring, e.g. in the form of a voltage ride through event, such as a low voltage ride through (LVRT) event.

[0018] When this occurs, it is investigated whether or not the power plant controller has entered into a fault mode. As described above, a change in the grid voltage phase angle, 0, provides a fast and reliable indication of a grid fault may be occurring. However, the power plant controller may base its decision of entering into a fault mode on other parameters, such as voltage level at the point of common coupling. This may be slower than the change in grid voltage phase angle, 0, in particular if the grid fault occurs far away from the point of commoncoupling. Moreover, this may be slower than the sampling rate at which the power plant controller dispatches power setpoints to the individual wind turbines of the wind farm. Accordingly, if a change in grid voltage phase angle, 0, above a predefined threshold level is detected, and the power plant controller has not entered into a fault mode, there is a risk that a grid fault is in fact occurring, but the power plant controller continues to dispatch power setpoints to the individual wind turbines that does not take this into account. This may even worsen the effects of the occurring grid fault, and may, e.g., result in a delay in the recovery of active power of the wind farm, following the grid fault.

[0019] Therefore, following a detection of a change in the grid voltage phase angle, 0, as described above, it is investigated whether or not the power plant controller has entered into a fault mode. If this is not the case, the power setpoints to the wind turbines are freezed for a predefined period of time, in order to prevent the power plant controller from providing new setpoints to the wind turbines. Thus, it is prevented that the wind turbines are provided with power setpoints that will result in a delay of the recovery of active power of the wind farm, and a fast recovery is therefore ensured. Moreover, the true grid fault is detected and handled fast and reliably.

[0020] The method may further comprise the step of, in the case that the power plant controller has entered into a fault mode when the predefined period of time has elapsed, continue to freeze the power setpoints to the wind turbines until the power plant controller is no longer in the fault mode.

[0021] According to this embodiment, when the predefined period of time has elapsed, it is once again investigated whether or not the power plant controller has entered into a fault mode. If this is the case, the power plant controller has, during the predefined period of time, detected that a grid fault is occurring, and has accordingly entered into the fault mode, e.g. by setting a relevant flag.

[0022] However, due to the freeze of the power setpoints to the wind turbines, this has not resulted in the power plant controller dispatching new power setpoints to the wind turbines, or if the power plant controller has dispatched new power setpoints to the wind turbines, this has not caused the wind turbines to operate according to the new power setpoints. However, the individual wind turbines arealready reacting to the grid fault, and therefore the freeze of the power setpoints to the wind turbines is continued in order to ensure that changed, e.g. reduced, power setpoints from the power plant controller do not interfere with the recovery phase following the grid fault. Moreover, the freeze of the power setpoints is maintained until the power plant controller is no longer in the fault mode.

[0023] Alternatively or additionally, the method may further comprise the step of, in the case that the power plant controller has not entered into the fault mode when the predefined period of time has elapsed, discontinuing the freeze of the power setpoints to the wind turbines.

[0024] According to this embodiment, the investigation performed when the predetermined period has elapsed reveals that the power plant controller has not entered into the fault mode. This may be an indication that the change in grid voltage phase angle, 6, was not due to a grid fault, or that the grid fault has already been handled, and that a recovery phase may already have been entered. Therefore, when this is the case, the freeze of the power setpoints to the wind turbines is discontinued, and the power plant controller is allowed to operate in a normal manner.

[0025] The predefined period of time may be within the range of 50 ms to 300 ms, such as within the range of 75 ms to 250 ms, such as within the range of 100 ms to 200 ms.

[0026] The operating cycle of the power plant controller will typically be of the order of 40 ms. Thus, according to this embodiment, it is ensured that the predetermined period of time exceeds the operating cycle of the power plant controller. Thereby the power plant controller is prevented from reacting fast, i.e. within an operating cycle, to changes in the electric grid that may be perceived by the power plant controller as a frequency event.

[0027] The step of monitoring a grid voltage phase angle, 0, of the electric grid may be performed by the power plant controller. According to this embodiment, the information regarding the change in angle, A0, applied as a trigger forinvestigating whether or not the power plant controller has entered into the fault mode, is readily available to the power plant controller. This allows a required freeze of the power setpoints to the wind turbines to be performed promptly and without delay, thus efficiently preventing the power plant controller from dispatching power setpoints to the wind turbines that may cause a delay in the recovery following a grid fault.

[0028] As an alternative, the step of monitoring a grid voltage phase angle, 0, may be performed by the wind turbines or by a separate device or entity.

[0029] The fault mode of the power plant controller may be a voltage ride through mode, such as a low voltage ride through (LVRT) mode. Voltage ride through events, such as LVRT events, are often detected based on measurements of magnitude of the grid voltage, and this is often somewhat slower than detection of frequency events. Accordingly, in the case of shallow grid faults, where the change in magnitude of the grid voltage is slow, this may very well lead to a situation where the detection of the grid fault by the power plant controller, causing the power plant controller to enter into a fault mode, occurs significantly later than the detection of a change in grid voltage angle, 0, and possibly several sampling cycles of the power plant controller later. This may result in the power plant controller dispatching undesired new setpoints to the individual wind turbines for a significant time period. This is exactly the situation which is avoided by the method according to the invention. A shallow grid fault is to be understood as a faults where the grid voltage drops, with retained voltage of 0.6 pu. - 0.85 pu.

[0030] The electric grid may be dominated by inverter based power sources. Examples of inverter based power sources are renewable power sources, such as wind turbines or photovoltaic panels. In electric grids dominated by inverter based power sources, the fraction of traditional power sources, such as large power plants, in the electric grid is smaller than in traditional electric grids. It is therefore necessary that the inverter based power sources contribute to maintaining the stability of the electric grid. To this end, at least some of the inverter based power sources may be in the form of grid forming power sources. The behaviour of such grid forming power sources makes it more difficult toaccurately and fast detect grid faults, and in particular to operate grid following power sources accordingly. It is therefore particularly relevant to apply the method according to the invention in wind farms being connected to electric grids that are dominated by inverter based power sources.

[0031] The step of monitoring a grid voltage phase angle, 0, may comprise monitoring the grid voltage phase angle, 0, at the point of common coupling. According to this embodiment, the grid voltage phase angle, 0, forming the basis of the decision of freezing the power setpoints to the wind turbines, is measured at the point that forms an interface between the wind farm and the external electric grid. It can therefore be assumed that the monitored grid voltage phase angle, 0, appropriately represents the condition of the external electric grid.

[0032] BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The invention will now be described in further detail with reference to the accompanying drawings in which

[0034] Fig. 1 is a schematic view of a wind farm being controlled in accordance with a method according to an embodiment of the invention,

[0035] Fig. 2 shows graphs illustrating active power output, voltage, active power setpoints and power plant controller fault flag setting, as a function of time, related to a wind farm being controlled in accordance with a prior art method,

[0036] Fig. 3 shows graphs illustrating active power output and power setpoints, as a function of time, in a wind farm being controlled in accordance with a prior art method and in accordance with a method according to an embodiment of the invention, respectively,

[0037] Fig. 4 illustrates voltage, phase angle and frequency experiences by a power plant controller, as a function of time, and a comparison between a prior art method and a method according to an embodiment of the invention,Fig. 5 is a block diagram of a power plant controller of a wind farm being controlled in accordance with a method according to an embodiment of the invention, and

[0038] Fig. 6 is a flow chart illustrating a method according to an embodiment of the invention.

[0039] DETAILED DESCRIPTION OF THE DRAWINGS

[0040] Fig. 1 is a schematic view of a wind farm 1 being controlled in accordance with a method according to an embodiment of the invention. The wind farm 1 comprises a plurality of wind turbines 2, three of which are shown, and it is connected to an electric grid 3 at a point of common coupling 4. The wind farm 1 further comprises a power plant controller (PPC) 5 being communicatively connected to the wind turbines 2, via a communication connection 6. Thus, the power plant controller 5 is able to dispatch power setpoints to the wind turbines 2, so as to make the wind turbines 2 generate power in such a manner that the total power output from the wind farm 1 to the electric grid 3 is in accordance with the requirements of the electric grid 3.

[0041] The wind farm 1 is controlled in accordance with a method according to an embodiment of the invention, e.g. as described below with reference to any of Figs. 2-6.

[0042] Fig. 2 shows three graphs illustrating active power output at the point of common coupling, voltage, active power setpoints to a wind turbine and power plant controller fault flag setting, as a function of time, related to a wind farm being controlled according to a prior art method.

[0043] The upper graph 7 shows power output of the wind farm as a function of time. It can be seen that until a point in time of approximately t=15 s, the power output of the wind farm is substantially constant, but after that the power output starts to decrease. This indicates that a grid fault has occurred.Approximately at t=15.1 s, the power output of the wind farm starts increasing again, and at approximately t=16.3 s the previous power output level has essentially been reestablished. The time period from approximately t=15 and approximately t=16.3 may be regarded as a fault duration.

[0044] In the middle graph 8 the dashed line 9 illustrates the voltage at the point of common coupling between the wind farm and the electric grid, and the solid line 10 illustrates a power setpoint to one of the wind turbines of the wind farm, dispatched by the power plant controller. It can be seen that the voltage 9 varies in a similar manner to the power output illustrated in graph 8.

[0045] It can be seen that shortly after the occurrence of the grid fault, at approximately t=15.05 s, the power setpoint 10 to the wind turbine is decreased significantly. This is because the power plant controller reacts to the decrease in voltage 9, and perceives this as a frequency event in the electric grid. The power plant controller then seeks to handle the frequency event, and therefore it dispatches the decreased power setpoint 10 to the wind turbines. However, since a frequency event is in fact not occurring, the decreased power setpoint 10 is in fact inappropriate, and instead of handling a grid fault, the decreased power setpoint 10 results in a prolonged recovery period following the actual grid fault.

[0046] The lower graph 11 illustrates a fault flag setting of the power plant controller. It can be seen that the fault flag setting is initially 'Low', indicating that the power plant controller has not entered into a fault mode. At approximately t=15.08 s, the fault flag setting switches to 'High', indicating that the power plant controller has detected that a grid fault is occurring, e.g. a voltage ride through event, and has therefore entered into the fault mode. However, this occurs after the point in time where the decreased power setpoint 10 was dispatched to the wind turbines, as illustrated in the middle graph 8. Accordingly, even though the correct grid fault has now been detected, the wind turbines have already received the decreased power setpoint 10 that is supposed to handle a (nonoccurring) frequency event, and the prolonged recovery period can therefore not be avoided.In Fig. 3, the upper graph 12 illustrates active power output of a wind farm, and the lower graph 13 illustrates power setpoints in the wind farm. The solid lines 14, 15 illustrate a prior art control method, and correspond essentially to the middle graph 8 of Fig. 2. It can be seen that the power setpoint 15 is decreased shortly after the decrease in power output 14, and that the recovery period is relatively long, in the sense that the original power output is not reached until approximately t=29 s.

[0047] The dashed lines 16, 17 illustrate a control method according to an embodiment of the invention. According to this method, a grid voltage phase angle, 0, of the electric grid is monitored. In the case that a change in angle, AO, of the grid voltage phase angle, 0, exceeds a predefined threshold value, this is regarded as an indication that a grid fault may be occurring. In the example illustrated in Fig.

[0048] 3, this occurs at approximately t=15 s.

[0049] In response to detecting that the change in angle, AO, is above the predefined threshold value, it is investigated whether or not the power plant controller has entered into a fault mode. As described above with reference to Fig. 2, this is not yet the case. Therefore, in order to prevent the power plant controller from dispatching the decreased power setpoints 15, the power setpoints 17 to the wind turbine are freezed for a predefined period of time. It can be seen from the lower graph 13 that this has the consequence that the power setpoint 17 remains at the level applied before the detection of the change in angle, AO. Moreover, it can be seen from the upper graph 12 that this results in a significantly faster recovery of the power output 16 following the grid fault.

[0050] Fig. 4 illustrates a comparison between a prior art method and a method according to an embodiment of the invention. The upper graph 18 illustrates voltage magnitude, the middle graph 19 illustrates phase angle of the electric grid, and the lower graph 20 illustrates frequency of the electric grid, all measured by the power plant controller at the point of common coupling. In each graph 18, 19, 20, the solid lines 21, 23 25 represent the prior art method, and the dashed lines 22, 24, 26 represent the method according to an embodiment of the invention. It can be seen, also when comparing Fig. 4 to Figs. 2 and 3, that the method according to an embodiment of the invention isvery effective in active power recovery of the wind farm under the same grid fault conditions. In particular, it can be seen that the grid conditions are the same before and after the disturbance.

[0051] Fig. 5 is a block diagram of a power plant controller 5 of a wind farm being controlled in accordance with a method according to an embodiment of the invention. Voltage, Vmeas, frequency, Fmeas, power output, Pmeas, and grid voltage phase angle are measured at the point of common coupling of the wind farm. The measured voltage, Vmeas, is supplied to an LVRT detection unit 27, and in the case that an LVRT event is detected, an LVRT state machine 28 sets a flag and provides this to a Q-V controller 29, handling voltage or reactive power control of the wind farm, and to a P controller 30, handling active power control of the wind farm. The setting or activation of a flag will normally cause the power plant controller 5 to enter into a fault mode.

[0052] A frequency controller 31 of the P controller 30 further receives the measured frequency, Fmeas, power, Pmeas, and grid voltage phase angle. The frequency controller 31 then monitors the grid voltage phase angle, and in the case that a change in angle is detected which exceeds a predefined threshold value, it is investigated whether or not the LVRT state machine 28 has set a flag. If this is not the case, the frequency controller 31 instructs a power setpoint dispatcher 32 to freeze the power setpoints to the wind turbines of the wind farm for a predefined period of time. Thereby it is ensured that the power setpoints to the wind turbines are not lowered erroneously, and a fast recovery following a grid fault is obtained.

[0053] Fig. 6 is a flow chart illustrating a method according to an embodiment of the invention. The process is started at step 33. At step 34 a grid voltage phase angle, 0, is monitored, and at step 35 it is investigated whether or not a change in angle, AO, of the grid voltage phase angle, 0, exceeds a predefined threshold value. If this is not the case, the process is returned to step 34 for continued monitoring of the grid voltage phase angle, 0.

[0054] In the case that step 35 reveals that the change in angle, A0, exceeds the predefined threshold value, this is an indication that a grid fault or disturbance isoccurring. The process is then forwarded to step 36, where it is investigated whether or not an LVRT flag has been set, indicating that the power plant controller has entered into a fault mode. If this is the case, the process is returned to step 34 for continued monitoring of the grid voltage phase angle, 0.

[0055] In the case that step 36 reveals that an LVRT flag has not been set, the process is forwarded to step 37, where the power setpoints to the wind turbines are freezed. This will prevent the power setpoints to the wind turbines from being lowered.

[0056] At step 38 it is investigated whether or not a predefined time period has lapsed, following the freeze of the power setpoints to the wind turbines. If this is not the case, the process is returned to step 37 for a continued freeze of the power setpoints to the wind turbines.

[0057] In the case that step 38 reveals that the predefined period of time has lapsed, the process is forwarded to step 39, where it is investigated whether or not an LVRT flag has been set in the meantime. If this is the case, then the process is returned to step 37 in order to maintain the freeze of the power setpoints to the wind turbines until the flag is no longer set, thus continuing to prevent that the power setpoints to the wind turbines are erroneously lowered. Thereby it is ensured that a fast recovery is obtained.

[0058] In the case that step 39 reveals that a flag is not set, then the process is forwarded to step 40, where the freeze of the power setpoints to the wind turbines is discontinued, thus allowing the power plant controller 5 to operate in a normal manner. Finally, the process is returned to step 34 in order to monitor the grid voltage phase angle, 6.

Claims

CLAIMS1. A method for controlling a wind farm (1) during shallow grid faults, the wind farm (1) comprising a plurality of wind turbines (2) connected to an electric grid (3) at a point of common coupling (4) and a power plant controller (5) being configured to dispatch power setpoints for the wind turbines (2) to generate power, the method comprising the steps of:- monitoring a grid voltage phase angle, 0, of the electric grid (3),- in the case that a change in angle, AO, of the grid voltage phase angle, 0, exceeds a predefined threshold value, investigating whether or not the power plant controller (5) has entered into a fault mode, and- in the case that the power plant controller (5) has not entered into a fault mode, freezing the power setpoints to the wind turbines (2) for a predefined period of time.

2. A method according to claim 1, further comprising the step of, in the case that the power plant controller (5) has entered into a fault mode when the predefined period of time has elapsed, continue to freeze the power setpoints to the wind turbines (2) until the power plant controller (5) is no longer in the fault mode.

3. A method according to claim 1 or 2, further comprising the step of, in the case that the power plant controller (5) has not entered into the fault mode when the predefined period of time has elapsed, discontinuing the freeze of the power setpoints to the wind turbines (2).

4. A method according to any of the preceding claims, wherein the predefined period of time is within the range of 50 ms to 300 ms.

5. A method according to any of the preceding claims, wherein the step of monitoring a grid voltage phase angle, 0, of the electric grid (3) is performed by the power plant controller (5).

6. A method according to any of the preceding claims, wherein the fault mode of the power plant controller (5) is a voltage ride through mode.

7. A method according to any of the preceding claims, wherein the electric grid (3) is dominated by inverter based power sources.

8. A method according to any of the preceding claims, wherein the step of monitoring a grid voltage phase angle, 0, comprises monitoring the grid voltage phase angle, 0, at the point of common coupling (4).

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