Controlling the recovery of a renewable energy power plant after a fault

Abstract

There is provided a method (500) for operating a renewable energy power plant (12) comprising a plurality of renewable energy generators (14), the method (500) comprising: calculating (502) a rate of change of a reactive power reference level for transitioning between a current reactive power reference level output by a reactive power controller (206) and a target reactive power reference level, the target reference level being suitable for normal operation of the renewable energy power plant (12); and, after a fault in an electrical power network to which the renewable energy power plant (12) is connected, dispatching (506) a control signal for controlling the renewable energy power plant (12) to generate or consume reactive power at the calculated rate of change, thereby causing a reactive power level of the renewable energy power plant (12) to transition to the target reactive power reference level.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method of controlling a renewable energy power plant, to a renewable energy power plant controller, and more generally to a wind turbine power plant. BACKGROUND

[0002] Newly commissioned renewable power plants, in particular wind power plants, are expected to be able to operate and adapt to a variety of different situations when connected to an electricity network. A wind power plant (WPP) typically comprises a plurality of wind turbine generators and is also referred to as a wind park or wind farm. The regulation and general operation of the power plant is controlled by a power plant control system or controller (PPC) which enforces operational limits and requirements set by the transmission system operator (TSO) or country specific grid interconnection requirements or "grid codes". The TSO also communicates power delivery demands to the PPC.

[0003] The grid codes typically include specific requirements for the supply of reactive and / or active current by each power plant when the network is subject to a fault, such as a voltage deviation. These requirements ensure that the network is adequately supported throughout the fault and that the voltage level is maintained through the supply of active and / or reactive current by one or more wind power plants.

[0004] After the fault, the TSO expects the power plant controller to quickly restore the operation of the power plant to the normal state prior to the fault. In some situations, the network conditions are not satisfactory and conventional restoration control can result in an even more unstable power network. This is particularly the case in "weak" connections between the power network and the power plant, where changes in the active or reactive power supplied by the plant to the network can result in undesirable voltage fluctuations.

[0005] It is an object of the present invention to improve upon conventional solutions. SUMMARY

[0006] According to an aspect of the present invention, there is provided a method of operating a renewable energy power plant comprising a plurality of renewable energy generators. The method comprises: calculating a rate of change of a reactive power reference level for transitioning between a current reactive power reference level output by a reactive power controller and a target reactive power reference level, the target reference level being suitable for normal operation of the power plant; and, after a fault in an electricity network to which the renewable energy power plant is connected, dispatching control signals for controlling the power plant to generate or consume reactive power at the calculated rate of change, thereby causing a reactive power response of the power plant to transition to the target reactive power reference level.

[0007] By calculating the rate of change of the reactive power, or the "ramp rate" of the reactive power, conditions of the power plant, connected network and wider power network are taken into account when resuming normal operation. By tailoring the ramp rate to dynamically react to current conditions, the likelihood of power network instability caused by power plant operation is reduced.

[0008] The fault can be a voltage deviation. The fault can be caused by a voltage deviation. The voltage deviation can be an under-voltage event.

[0009] The method can comprise generating a difference between the current reactive power reference level and the target reactive power reference level. The reactive power reference level change rate can be calculated based on the generated difference.

[0010] The reactive power reference level change rate can be indirectly proportional to the generated difference. In other words, a larger generated difference results in a smaller reactive power reference level change rate, while a smaller generated difference results in a larger reactive power reference level change rate. Thus, larger transitions are implemented slowly, while smaller transitions are implemented quickly, ensuring stability of the power grid by not transitioning the reactive power level too quickly.

[0011] The method can comprise determining the target reactive power reference level. The target reactive power reference level can be a reactive power reference level generated based on normal operating conditions of the power plant. The target reactive power reference level can be a reactive power reference level generated prior to the fault. The target reactive power reference level can be a reactive power reference level generated most recently prior to the fault. Implementing the target reactive power reference level based on a previous level allows for resuming operation as if the fault had not occurred.

[0012] The target reactive power reference level can be a reactive power reference level generated during the fault as if the fault had not occurred. In other words, the reference level can be calculated during the time the fault is occurring as if the power plant had been operating in normal operation mode the whole time. By basing the target reactive power reference level on a level generated during the fault as if the fault had not occurred, normal operation of the power plant can be resumed as quickly as possible.

[0013] Determining the target reactive power reference level can comprise operating two reactive power controllers to output the reactive power reference level. A first of the two reactive power controllers can be configured to operate in a fault ride-through mode during a grid fault. A second of the two reactive power controllers can be configured to operate in a normal operation mode during a grid fault. The target reactive power reference level can be an output of the second reactive power controller. Advantageously, this arrangement makes use of existing components within a wind turbine, and therefore no additional modifications to the power plant are required to implement the method. Using the second reactive power controller does not require adjustment of the first reactive power controller.

[0014] The rate of change of the reactive power reference level for the transition can be calculated based at least in part on a short circuit ratio of the power network. The use of the short circuit ratio is beneficial as it can be used to assess the state of the power network and its stability, and therefore how quickly the transition can be made. The rate of change of the reactive power reference level for the transition can be calculated based at least in part on a weakness of the power network. The short circuit ratio can be used to calculate the weakness of the power network. A longer transition time can be used for a weak power network than a strong power network.

[0015] The rate of change of the reactive power reference level for the transition can be calculated based at least in part on a voltage level.

[0016] According to another aspect of the application, there is provided a power plant controller for controlling operation of a renewable energy power plant comprising a plurality of renewable energy generators and connected to a power network, the power plant controller comprising: a main reactive power controller configured to generate a reactive power reference level; and a dynamic ramp rate controller configured to calculate a rate of change of the reactive power reference level for a transition between the reactive power reference level output by the main reactive power controller and a target reactive power reference level, the target reference level being suitable for normal operation of the power plant.

[0017] The dynamic ramp rate controller can comprise an additional reactive power controller configured to generate the target reactive power reference level. During a fault, the main reactive power controller and the additional reactive power controller can be configured to operate in different modes. During a fault, the main reactive power controller operates in a fault ride-through mode and the additional reactive power controller operates in a normal operation mode.

[0018] The dynamic ramp rate controller can comprise a ramp rate logic configured to calculate the rate of change of the reactive power reference level for the transition based at least in part on a difference between the main reactive power reference level and the target reactive power reference level.

[0019] According to another aspect of the application, there is provided a controller configured to control a renewable energy power plant in accordance with the above-described method.

[0020] The renewable energy power plant can be a wind power plant. The renewable energy power generator can be a wind turbine generator.

[0021] The renewable energy power plant can be a solar power plant. The renewable energy power generator can be a solar power generator.

[0022] According to another aspect of the application, there is provided a computer program downloadable from a communications network and / or stored on a machine-readable medium, comprising program code instructions for implementing the above-described method.

[0023] According to another aspect of the application, there is provided a renewable energy power plant comprising a controller as described above.

[0024] It is expressly intended that aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims, and / or in the following description and drawings, and their combinations, can be implemented independently and / or in any combination thereof. That is, not all of the described aspects, embodiments, examples and alternatives may be required, as some can be mutually exclusive in certain contexts. It is intended that the application not be limited by any of the features described herein, but instead that the application can be implemented and / or practiced with only features described, and / or with other features not specifically described. Applicants reserve the right to change, modify, amend, add, replace, or delete features described herein, including in the claims, and / or to substitute therefor, in any currently or subsequently filed application, any additional or alternative features, whether or not such features are or were a part of the application. Applicant also reserves the right to amend or update the claims in any currently or subsequently filed application, including presenting new claims to replace former claims, to add new claims, or to present new claims that are not a substitute for former claims. BRIEF DESCRIPTION OF DRAWINGS

[0025] One or more embodiments of the application will now be described, by way of example only, with reference to the accompanying drawings in which:

[0026] Figure 1 is a schematic architecture of an electricity network comprising a wind power plant and a main electricity grid;

[0027] Figure 2 is a schematic representation of a control architecture for a power plant controller;

[0028] Figure 3 is a representation showing the voltage levels and Figure 2 the response of the power plant controller under an example fault scenario;

[0029] Figure 4 is a schematic representation of an alternative control architecture for a power plant controller;

[0030] Figure 5 is a method of operating a wind power plant;

[0031] Figure 6 is a schematic representation of an example dynamic ramp rate controller for a power plant controller of Figure 4 ;

[0032] Figure 7 is a schematic representation of an example ramp rate logic for a dynamic ramp rate controller for a power plant controller of Figure 6 ;

[0033] Figure 8 is a flowchart illustrating a method of operation of a power plant controller of Figure 4 ;

[0034] Figure 9 is a representation showing the response of a power plant controller of Figure 4 to voltage levels and fault scenarios under an example fault scenario; and

[0035] Figure 10 is a representation showing the response of a power plant controller of Figure 4 to voltage levels and fault scenarios under another example fault scenario. DETAILED DESCRIPTION

[0036] Figure 1 shows a typical architecture in which a wind power plant (WPP) is connected to a main transmission grid as part of a wider power network. Figure 1 The schematic of Figure 1 should only be representative of a power network. Alternative configurations of power networks and power plants are known and it is expected that other known components can be incorporated in addition to or as an alternative to those shown and described. Such variations are within the capabilities of the skilled person. For example, depending on the number of WTGs included in the WPP, it is expected that a substation or additional transformers will be incorporated in the WPP.

[0037] As will be appreciated by the skilled reader, a WPP includes at least one wind turbine generator (WTG), more typically simply referred to as a "wind turbine". A WPP is also known as a wind park or wind farm. The example shown is representative only, and the skilled reader will appreciate that other specific architectures relating to wind power plants, other renewable energy power plants, wind turbine generators and other renewable energy generators are possible. Accordingly, the present disclosure relates generally to renewable energy power plants and renewable energy generators, and not specifically to wind power plants and generators as in the figures. Furthermore, the skilled reader will appreciate that the methods, systems and techniques described below are also applicable to many different configurations of power networks. Furthermore, the components of wind power plants and power networks are conventional, and therefore familiar to the skilled reader. It is contemplated that other known components can be incorporated in addition to or as an alternative to those shown and described in the figures. Such variations are within the capabilities of the skilled person. In particular, it will be appreciated that a WPP can also be considered to include STATCOM devices, substation devices, cables, plant controllers and other components required to connect a WPP to the main grid.

[0038] Furthermore, the skilled reader will appreciate that, Figure 1 is a schematic diagram, so the manner of transmission of control commands is not explicitly depicted. The interconnection can be a direct or "point-to-point" connection, or can be part of a local area network (LAN) operating under a suitable protocol such as CAN bus or Ethernet. In addition, it will be appreciated that, rather than using cables, control commands can be transmitted wirelessly over a suitable wireless network such as a wireless network operating under the WiFi TM or ZigBee TM standards (IEEE 802.11 and 802.15.4 respectively).

[0039] Figure 1 A power network 10 incorporating a WPP 12 is shown. The WPP 12 includes a plurality of WTGs 14. Each of the plurality of WTGs 14 converts wind energy into electrical energy, which is transmitted from the WTGs 14 to a main transmission network or main grid 16 for distribution. A connection network 17 extends between the WPP 12 and the main grid 16.

[0040] The WTGs 14 generate both active and reactive power. The main grid 16 typically has specific active and reactive power level requirements, which the WPP 12 is required to adhere to, and the output of the WTGs 14 can be varied in real time to match these requirements. Other grid requirements can be specified in relation to the reactive and active current levels, as well as voltage levels, at specific points within the power network.

[0041] The output of each WTG 14 is controlled using the associated WTG controller 15. During normal operation of the WPP 12, the WTG controllers 15 operate to achieve the active and reactive power reference levels set by the power plant controller (PPC) 32. Under special conditions, the WTG controllers 15 operate to meet predetermined network requirements and also act to protect the WTGs 14 from any potentially harmful conditions. In these special cases, the WTG controllers can autonomously operate the WTGs 14 separately from the PPC 32. The PPC 32 and the WPP 12 together operate in accordance with the methods and processes described herein to facilitate a safe and stable return to normal operating conditions and normal operating reactive power level output to the main grid 16 following operation in a fault mode.

[0042] Each WTG 14 of the WPP 12 is connected to a local grid (not shown) that links the WTGs 14. The WPP 12 is connected to the main grid 16 by a connection network. The WPP 12 and the main grid 16 are connected at a point of interconnection (PoI) 68, which is the interface between the WPP 12 and the main grid 16.

[0043] The connection network is known in the art and includes a combination of transmission lines, buses and / or transformers to couple the WPP to the main grid. Other components such as circuit breakers, reclosers and other systems known in the art can also be incorporated into the connection network.

[0044] Returning to the control of the WTGs, the PPC 32 is connected to the power network 10 at a point of measurement (PoM) 34 and is also directly connected to the WPP 12. The PPC 32 is a suitable computer system for performing the controls and commands described above and is equipped to measure various parameters. The parameters measured are representative only since the line losses between the PoM 34 and the PoI 26 and between the PoM 34 and the PPC 32 can have an effect on the measurements, as the PoM 34 is not at the PoI 26. Appropriate compensation can be made to account for these losses to ensure that the measurements are accurate.

[0045] The role of the PPC 32 is to act as a command and control interface between the WPP 12 and the grid operator or Transmission System Operator (TSO) 36. The TSO 36 is responsible for instructing the PPC 32 of the requirements and demands of the main grid 16. The PPC 32 monitors parameters of the power output, such as frequency and voltage, as well as the reactive current or power exchange between the WPP 12 and the main grid 16 and other parameters of the main grid 16, such as voltage levels. In its role as a command and control interface, the PPC 32 interprets the power delivery demands of the TSO 36 on it and manages the WTGs 14 in the WPP 12 to meet these demands, while taking into account other operational factors, such as the monitored parameters, and sudden changes in the output or measured grid voltage, and / or, as set out in the following description, grid faults. The WTGs 14 are able to change their current or power output in accordance with commands received from the PPC 32.

[0046] As will be understood by the skilled person, a grid fault or grid fault event is generally defined as a period in which the voltage level of the grid 16 or wider power network falls outside of an acceptable normal operating voltage range.

[0047] During a grid fault in which the grid voltage level falls below a pre-defined threshold (a so-called "under-voltage" event), the WPP 12 and PPC 32 operate in an under-voltage mode, such as a Low Voltage Ride Through (LVRT) mode. The WPP 12 operates in the under-voltage mode to avoid the WPP 12 disconnecting from the grid 16 and supporting the grid 16 through the fault. In the LVRT mode, the WPP 12 supports the grid 16 during the fault by supplying reactive current to energise the voltage level to rise. The WPP 12 is required to supply the reactive current until the voltage level recovers to a level within the normal operating voltage range. In many cases, this operating range is between a voltage level of 0.9 per unit (p.u.) voltage and 1.1 p.u. voltage.

[0048] As will be understood by the skilled person, per unit voltage is an expression of voltage relative to a reference value, which is used as a reference. Similarly, per unit active current or per unit reactive current is an expression of power / current relative to a reference reference value. The use of per unit systems can normalise numerical values for transformers and other components, which can vary by orders of magnitude.

[0049] The WPP 12 operates to comply with a set of grid requirements specific to the main grid 16. When operating in the under-voltage mode, the WTGs 14 of the WPP 12 are initially acting autonomously and are controlled by the WTG controllers 15. In other words, during at least part of the under-voltage mode, the WTGs 14 are not controlled according to the signals output by the PPC 32. In the under-voltage mode, the WTGs 14 operate to control the active and reactive power currents according to pre-defined criteria. This is in contrast to the normal operating mode, in which the WTGs are essentially continuously controlled according to the control signals dispatched by the PPC 32.

[0050] During the fault, the PPC 32 enters a fault ride-through mode when the WTGs 14 are acting autonomously. The first phase of the fault ride-through mode is the fault state, in which the control loop of the PPC 32 is effectively frozen. In other words, the output of the PPC 32 is fixed at one pre-set level when the WTGs 14 are not receiving commands based on the output of the PPC 32, in preparation for exiting the fault state. Typically, this pre-set level is unity power factor, in which the reactive power reference output is set to 0 p.u.

[0051] The fault state is maintained until a trigger condition is met. Once the trigger condition is met, the WPP 12 and the PPC 32 enter the post-fault state. The trigger condition is typically that the voltage level has reached a pre-determined threshold, referred to as V LVRT_OUT ), although other trigger conditions can be specified.

[0052] In the post-fault state, the PPC 32 outputs a reactive power reference level to control the WTGs 14 to support the recovery of the voltage level. The WTGs 14 are controlled in voltage control mode according to the reactive power reference level.

[0053] Once the voltage level has recovered to a normal level (i.e. between 0.9 and 1.1 p.u.), the post-fault state ends as there is no longer a need to supply a high reactive power level. After the post-fault state, the WTGs 14 and the PPC 32 are configured to return to the operating mode in which they were operating prior to the fault. The reactive power reference level output by the PPC 32 is generated according to the normal operating mode. As the post-fault state requires a high reactive power supply, the difference between the reactive power reference level of the post-fault state and the normal operating mode can be very large or very small, depending on which mode is the normal operating mode.

[0054] However, an instantaneous or near-instantaneous change from the reactive power reference level of the post-fault state to the reactive power reference level of the normal operating mode is undesirable as it can lead to instability in the voltage level of the grid. The change in reference level required to return to the normal operating mode can be large when the normal operating mode of the PPC 32 is a reactive power or power factor control mode.

[0055] This is a particular problem in so-called "weak grid interconnections". In some cases, the interconnection between a remote power plant and the power network can be described as a "weak grid interconnection" because small changes in the reactive / reactive power exchange between the plant and the network can result in large voltage oscillations. Weak grid interconnections occur when the grid 16 is operating at or near its design limits, and so the level of fault is low. Weak grid interconnections are most common in facilities in remote locations where the infrastructure is insufficient, resulting in a high likelihood of reaching the design limits.

[0056] One way in which the transition between the post-fault state and the normal operating mode can be improved is to implement a transition state between the post-fault and normal states. In the transition state, there is a transition between the different reactive power reference levels. This is a form of "bumpless transfer". Using this approach, in the transition state, the reactive power reference level specified by the PPC 32 is changed according to a rate of change or 'ramp rate' determined prior to the first use of the WPP 12 or PPC 32. The pre-determined ramp rate is designed to comply with the grid code requirements of the host grid. For example, the grid code can specify a particular time period within which the transition must be completed, thereby providing a minimum ramp rate for the transition period. The PPC 32 implements this ramp rate by using a series of step changes to the reactive power reference level that it provides to the WTGs 14. Using a pre-determined ramp rate advantageously allows a controlled transition between the post-fault and normal operating states, and reduces the likelihood of grid instability. The skilled person will appreciate that the ramp rate can be implemented in any conventional manner.

[0057] The PPC 32 implements the pre-determined ramp rate using a control architecture 200 as shown in Figure 2 . Figure 3 A fault scenario and the reaction of the PPC 32 incorporating the architecture 200 of Figure 2 to the voltage changes experienced during the fault is shown.

[0058] Figure 2The architecture 200 includes a voltage controller 202, a power factor controller 204, and a reactive power controller 206 (also referred to as a Q controller). The voltage controller 202 is sometimes referred to as a voltage slope controller. The voltage controller 202 and the power factor controller 204 each receive one or more inputs 208, 210 and each generate an output 212, 214 based on the received inputs 208, 210. Each output 212, 214 is a reactive power setpoint. The inputs 208, 210 can include, for example, an operating mode, power factor or voltage setpoints, and / or one or more indications of measured and / or calculated parameters associated with the connected network or main grid. These parameters can include measured voltage at the PoI or PoM, and / or measured active power at the PoI or PoM.

[0059] The reactive power setpoint outputs 212, 214 generated by the voltage and power factor controllers 202, 204 are used by the Q controller 206 in determining the reactive power reference level that should be communicated to the WPP 12. When the normal mode of operation of the PPC 32 and the WPP 12 is voltage control mode, the output 212 of the voltage controller 202 is used by the Q controller 206. When the normal mode of operation of the PPC 32 and the WPP 12 is power factor control mode, the power factor controller output 214 is used by the Q controller 206. In order to select which controller output 212, 214 to use, the Q controller 206 receives an input 216 indicating the mode of operation. In some cases, the Q controller 206 incorporates a switching mechanism and receives input from only one of the voltage or power factor controllers 202, 204.

[0060] As will be appreciated by the skilled person, a WPP operating in voltage control mode is controlled to maintain a voltage level, a WPP operating in power factor control mode is controlled to maintain a power factor, and a WPP operating in reactive power control mode is controlled to consume or supply a reactive power level.

[0061] In addition to the input 212, 214 received from one of the voltage controller 202 or the power factor controller 204, the Q controller 206 receives further inputs and generates a reactive power reference level output Q ref , 220 based on these inputs. Signal conditioning is performed on the reactive power reference level output 220 before dispatch to the WPP 12, where the WTGs 14 are controlled so as to meet the reactive power reference level.

[0062] In the architecture 200 of Figure 2 the Q controller 206 receives additional inputs including a measured reactive power level Q, 222 at the PoM, PoI, and / or output by the WPP / WTGs, and a fault ride through (FRT) input 224.

[0063] It will be appreciated that in other configurations, other inputs, such as a reactive power (Q) setpoint according to grid code or storage relationship, can be received at the Q controller 206. In other configurations, certain inputs can be combined. For example, the operating mode input 216 and the fault ride-through input 224 can be combined.

[0064] The fault ride-through input 224 causes the Q controller 206 to enter a fault state in the event of a voltage deviation or other fault on the main grid 16. As described above, in the fault state, the reactive power reference level of the Q controller 206 is frozen at a predetermined setpoint, and the WTG 14 acts as an autonomous current source. Control of the WTG 14 by the PPC 32 will resume in the post-fault state once the trigger condition is met.

[0065] Figure 3 The response of the output 220 of the Q controller 206 to a voltage deviation is shown when the normal operating mode of the PPC 32 is the power factor mode. Figure 3 Two graphs are provided: the lower graph is a voltage profile of the voltage level measured at the PoM; the upper graph is a simultaneous operating of the reactive power reference level (Q ref ) in response to the voltage.

[0066] Initially, the voltage measured at the PoM 34 is within the normal operating range, which is indicated by the parallel dashed lines either side of the 1 p.u. voltage level in Figure 3 Since the PPC 32 is operating in the power factor mode, the Q controller 206 receives an input from the power factor controller 204. The output 220 of the Q controller 206 is based on the signal received from the power factor controller 204. The input from the voltage controller 202 is not used in this mode. Thus, when the voltage level is within the normal operating range, the reactive power reference level is the minimum reactive power level, Q min . The reactive power level of Q min indicates that the WPP 12 is instructed to consume reactive power.

[0067] In this case, a fault occurs at time ti. The voltage level deviates from its normal operating level. Thus, the Q controller 206 receives a signal from the fault ride-through input indicating that a fault has occurred, and the Q controller 206 enters its fault state. The reactive power reference level moves from Q min to the unity power factor of 0 p.u. When in the fault state, the Q controller 206 maintains its reference output at 0 p.u.

[0068] At time t2, the triggering condition to move from the fault condition to the post-fault condition is met. In this case, the triggering condition that has been met is that the voltage level has recovered to a predetermined level. In the depicted case, the predetermined level is the voltage level at which the fault event is considered to have been completed, here labelled V LVRT_OUT . When V LVRT_OUT is reached, the output and operation that were effectively "frozen" are "unfrozen" to allow the operation to continue. The PPC 32 enters the post-fault condition and resumes its communication with the WPP 12 to control according to the reference level output by the Q controller 206. In the post-fault condition, the voltage recovery is further supported by the Q controller 206 by specifying a reactive power reference level that is or is close to supplying maximum reactive power to the grid, Q max .

[0069] During the post-fault condition, the voltage level is expected to recover within its normal operating level. Once it is determined that the recovery is complete, that the voltage level is within its normal operating range, and that the fault has been cleared, the Q controller 206 switches to a transition condition. In Figure 3 , the transition condition starts at time t3.

[0070] In the transition condition, the reactive power reference level output by the Q controller 206 for controlling the WPP 12 is changed from the post-fault condition reference level to a normal operating mode reference level. The normal reference level can be the same as the reference level immediately prior to the fault, or it can be different and based on new conditions. This change is implemented using a predetermined ramp rate. The ramp rate can be implemented by stepping the reactive power reference level at predetermined time intervals.

[0071] Once the reactive power reference level of the normal operating mode is recovered, the normal operation of the PPC 32, and thus of the WPP 12, is recovered. This occurs at time t4 in Figure 3 .

[0072] In an alternative approach, an alternative control architecture can be incorporated into the PPC 32 of the system of Figure 1 to facilitate the dynamic calculation of the ramp rate for the transition between the post-fault condition and the normal operation. In other words, this alternative architecture allows the ramp rate to be calculated in real time according to the instantaneous conditions of the grid, the WPP, or the connected network, or according to other criteria.

[0073] Figure 4 An alternative architecture is depicted, Figure 6 and Figure 7 provide example features of the alternative architecture. In particular, Figure 4 shows the overall alternative architecture 400, which includes the components of the architecture 200 of Figure 2 .Figure 2 and Figure 4 The shared components in the architecture perform the same functions described above. Therefore, the shared components are assigned the same... Figure 2 The same reference figures are used and will not be described further.

[0074] Apart from Figure 2 In addition to the components, Figure 4 The alternative architecture 400 also includes a dynamic rate-increase controller 402. The dynamic rate-increase controller 402 is arranged to receive input signals 412, 414, and 420 from the voltage controller 202, the power factor controller 204, and the Q controller 206. The dynamic rate-increase controller 402 is also configured to receive an external input 428.

[0075] The dynamic rate-of-rise controller 402 allows for dynamic control of the rate of change of the reactive power reference level during transition states. This is achieved by dispatching the output rate-of-rise (Q... ramp_out This is achieved using a 430. Dynamic control of the rate of change of reactive power reference level is a particularly effective mechanism for reducing the likelihood of voltage instability. Minimizing the rate of increase and decrease is useful in weak grid interconnections, where rapid transitions can lead to undesirable fluctuations. Conversely, adjusting the rate of increase and decrease is also useful in stronger grid interconnections, allowing for increases in the rate of increase and decrease where possible. During operation, the strength of grid interconnections typically changes due to variations in conditions and parameters; therefore, the system's ability to adapt to new situations is also beneficial. Another advantage is that when changes to the requirements for WPP are needed, these changes do not require reprogramming the entire system but can be implemented using the rate of increase and decrease controller.

[0076] exist Figure 4 In the diagram, the dynamic rate of increase / decrease controller 402 is shown arranged in parallel with the Q controller 206, although it is understood that these systems can be arranged in series or as completely independent systems. The inputs provided to the dynamic rate of increase / decrease controller can also be changed according to the operator's requirements.

[0077] Figure 4 The architecture 400 is based on Figure 5 The described method 500 is used for operation. In method 500, the rate of change (i.e., the rate of increase / decrease) of the reactive power reference level is calculated. The rate of increase / decrease is calculated to apply to the reactive power reference level (Q) output by the Q controller. ref ) and the target reactive power reference level (Q) applicable to the normal operation of power plants. targetThe transition between the reactive power reference level and the target reactive power reference level is achieved after a fault. The calculated rate of increase / decrease is then used to assign a 506 control signal to control the power plant and its generators to generate or consume reactive power at that rate, thus facilitating the transition between the reactive power reference level and the target reactive power reference level. It is conceivable that the rate of increase / decrease is calculated based at least on the magnitude of the difference between the target reactive power reference level and the primary reactive power reference level. In some examples, the rate of increase / decrease may be indirectly proportional to this difference. The rate of increase / decrease may also be based in part on other inputs, which will be discussed later.

[0078] Figure 6 The details show what can be used to implement Figure 5 An example of the method is a dynamic rate-increase controller 402. The dynamic rate-increase controller 402 includes an additional Q controller 406 and a rate-increase calculation logic unit 440. For distinction, by... Figure 2 and Figure 4 The Q controller 206 shared by the architecture 200 and 400 will be referred to as the main Q controller below.

[0079] The supplementary Q controller 406 receives inputs 412 from the voltage controller and power factor controller 414, and may also receive other inputs 442 (such as measured reactive power levels and / or mode inputs). Importantly, however, the supplementary Q controller 406 and the main Q controller 206 are distinguished by a fault ride-through input 224; the main Q controller 206 receives the fault ride-through related input 224, while the supplementary Q controller 406 does not. Because the supplementary Q controller 406 does not receive the fault ride-through input, it does not enter fault ride-through mode. Instead, while the main Q controller is in fault and post-fault states, the supplementary Q controller 406 continues to output a reactive power reference level according to the normal operating mode. In this system, the output 444 of the supplementary Q controller 406 becomes the target reactive power reference level used for transition during the transition state, Q target .

[0080] Outputs 420 and 444 from the main Q controller 206 and the auxiliary Q controller 406 are input to the rate of increase / decrease calculation logic 440, which calculates the dynamic rate of increase / decrease accordingly. The rate of increase / decrease logic 440 may receive further inputs 446 indicating the state of the power network, as well as any limiting characteristics or conditions that may affect the maximum or minimum rate of increase / decrease or may determine the method of calculating the rate of increase / decrease.

[0081] Figure 7 An example acceleration / deceleration rate calculation logic unit 440 is shown. This logic unit 440 includes a difference calculator 448 configured to calculate the reactive power reference level Q provided by the main Q controller 206.ref the absolute difference between the target reactive power reference level Q target provided by the additional Q controller 406. The output 450, Q ramp_in of the difference calculator 448 is communicated to a ramp rate calculator 452, which is configured to calculate a ramp rate, Q ramp_out using any limiting input 446 and communicate it as an output 430. The ramp rate can be calculated, for example, according to a look-up table, a pre-determined formula or according to modelling.

[0082] The calculated ramp rate is utilized by another controller to control the actual reactive power control of the WTG.

[0083] An example input 446 to the ramp rate calculator 452 is the short circuit ratio (SCR) and the voltage measured at the PoM. Changes in voltage level and / or phase angle can also be used. The SCR is used to assess whether there is a weak grid interconnection. The SCR can be calculated in real-time by measuring the change in voltage level for a given change in reactive power on the bus and given as the ratio of the change in reactive power to the change in voltage level. These values are typically sampled in a short sampling window. The SCR is typically compared to a pre-set threshold value. If the SCR is below the threshold value, a weak grid interconnection is identified and a limit is applied to the ramp rate accordingly.

[0084] In some embodiments, the threshold SCR value for identifying a weak grid interconnection is 3.0. In other embodiments, the threshold is less than 3.0. For example, the threshold can be a value between 3.0 and 2.5. In special cases, the threshold can be lower than 2.5.

[0085] Figure 8 One mode of operation of the dynamic ramp rate controller comprising the modules Figure 6 and Figure 7 is illustrated. It is envisaged that Figure 8 The method 800 illustrated in is continuously performed, and when triggered by a triggering condition, the output ramp rate is used as part of the control signal. It is understood that in the present example, the triggering condition is the end of the post-fault condition, but it can also depend on the voltage level recovering to normal operating levels and staying within normal operating levels for a period of time, and / or can be based on a measure of the stability of the voltage level.

[0086] Figure 8 In the main Q controller 206 and the additional Q controller 406 are run in parallel 802, 804 to output the reactive power reference level and the target reactive power level.

[0087] The output of each of the controllers 206, 406 is received 806 at a ramp rate logic. A difference calculator 448 of the ramp rate logic calculates 808 a difference between the reference level and the target level. A ramp rate calculator 452 calculates 810 a ramp rate based on the difference.

[0088] The ramp rate calculator 452 also applies 812 any limiting conditions to update the ramp rate. The updated ramp rate is output 814 from the ramp rate logic and the dynamic ramp rate controller 402, ready for signal conditioning and dispatch.

[0089] Figure 9 and Figure 10 The response of the parameters used by the Q controller 206 and the dynamic ramp rate controller 402 incorporating the additional Q controller 406 is illustrated in response to different operating scenarios. Figure 9 The response is shown when the normal operating mode is a reactive power or power factor control mode, while Figure 10 The response is shown when the normal operating mode is a voltage control mode.

[0090] In each of Figure 9 and Figure 10 five graphs are shown, each showing the variation of a different parameter over time. These graphs show, respectively: the voltage measured at the PoM, V PoM ; the reactive power reference level, Q ref ; the target reactive power level, Q target ; the output of the difference calculator, Q ramp_in ; and the output of the ramp rate calculator, Qramp_out .

[0091] In Figure 9 , the system is first shown operating in its normal power factor / reactive power control mode. V PoM is measured to be within its normal operating range. Because V PoM is within its normal range, and the WPP 12 and PPC 32 are operating in reactive power / power factor control mode, Q ref and Q target are both at the minimum reactive power value, Q min , which is the maximum reactive power consumption of the WPP 12. In normal operation, Q ref and Q target are equal. This is because both the primary and additional Q controllers 206, 406 are operating in the same mode - no fault ride-through is initiated. Q ramp_in , which is the difference between Q ref and Q target , is zero because Q ref and Q targetIt is the same value. Therefore, the rate of increase / decrease calculator 452 will use Q. ramp_out The calculation is set to the default level.

[0092] At point t1, a fault occurs, and the voltage deviates from its normal operating range. The main Q controller 206 receives an input from its fault crossover input 224 to enter a fault state. The auxiliary Q controller 406 does not receive this input. Therefore, Q... ref From Q min It becomes a unity power factor of 0 p.u., while Q target Then remain at Q min Therefore, there now exists an equal to Q. max The difference in the rate of ascent and descent results in a reduction in the rate of ascent and descent to account for the difference in the rate of ascent and descent.

[0093] At t2, the voltage has recovered, allowing the start of the post-fault state and enabling PPC 32 to resume control of WTG 14. The post-fault state requires WPP 12 to inject reactive power into the main grid 16, therefore Q ref Rising to the maximum level of reactive power supply, Q max Since it is still operating in its normal operating mode, the additional Q controller 406 will Q target Maintain at Q min The current difference is equal to 2Q. max This is the largest difference, and the acceleration / deceleration rate calculator 452 reacts accordingly to reduce the acceleration / deceleration rate to the lowest possible value. min .

[0094] At point t3, the voltage has returned to normal operating levels, and the system can enter a transition state. Ramp-up rate Q ramp_out Used from Q ref Value transition to Q target The value. Since the acceleration and deceleration rates are at their minimum levels, the length of the transition state increases from its default level.

[0095] Once the transition is complete, normal operation will resume.

[0096] exist Figure 10 In the middle, V PoM Approaching the lower limit of its normal operating range. WPP 12 and PPC 32 operate in voltage control mode, where the reactive power supplied to the grid by WPP 12 is at its maximum level, Q max Therefore, Q ref and Q target All in Q max Therefore, the difference is zero. In this case, the default rise / fall rate is the maximum rise / fall rate to ensure a fast transition.

[0097] At t1, a fault occurs, and V PoM drops to a lower level. The main Q controller 206 enters its fault state, so Q ref is 0 p.u. Q target is maintained at Q max , so the difference is Q max . The larger difference results in a lower ramp rate.

[0098] When entering the post-fault state after the fault state, at t2, Q ref changes to a post-fault level, which is slightly lower than Q Figure 10 in max . The post-fault level is slightly lower than Q max because it depends on the voltage level. It will be appreciated that the post-fault reactive power level is set according to the control protocol of the turbine. Since the post-fault state is a voltage control state, the reactive power set point for the voltage between V LVRT_OUT and the normal operating voltage level in the additional Q controller and the main Q controller is similar, since both operate in the form of a voltage control state. Therefore, Q ref and Q target are aligned with the same or similar values, and the difference becomes zero or close to zero. As a result, the ramp rate returns to close to its maximum value, since Q ref and Q target are very small. Therefore, the transition state time is short.

[0099] It will be appreciated that the above description of the dynamic ramp rate controller is one way in which the dynamic ramp rate is achieved according to the method described herein. In alternative examples, a memory unit is incorporated in the PPC which stores the pre-fault Q ref value to be used as Q target after the post-fault state. In other examples, a memory unit is used which stores values of Q target and / or ramp rate.

[0100] In some embodiments, alternative renewable energy generators or hybrid power plants can be operated according to the method described herein.

[0101] Many modifications can be made to the above described examples without departing from the scope of the present application as defined in the appended claims.

Claims

1. A method (500) for operating a renewable energy power plant (12) comprising a plurality of renewable energy generators (14), the method (500) comprising: operating a first reactive power controller (206) in a fault ride-through mode during a grid fault to output a current reactive power reference level; operating a second reactive power controller (406) in a normal operation mode during the grid fault to output a target reactive power reference level; calculating (502) a reactive power reference level change rate for transitioning between the current reactive power reference level and the target reactive power reference level, the target reactive power reference level being suitable for normal operation of the renewable energy power plant (12); and, after a fault in a power network to which the renewable energy power plant (12) is connected, dispatching (506) a control signal for controlling the renewable energy power plant (12) to generate or consume reactive power at the calculated change rate, thereby transitioning a reactive power level of the renewable energy power plant (12) to the target reactive power reference level.

2. The method (500) of claim 1, comprising generating (808) a difference between the current reactive power reference level and the target reactive power reference level, and wherein the reactive power reference level change rate is calculated based on the generated difference.

3. The method (500) of claim 2, wherein The reactive power reference level change rate is indirectly proportional to the generated difference.

4. The method (500) of any preceding claim, the target reactive power reference level being a reactive power reference level generated based on normal operating conditions of the renewable energy power plant (12).

5. The method (500) of claim 1, wherein, The target reactive power reference level is a reactive power reference level generated during the fault as if the fault had not occurred.

6. The method (500) of claim 1, wherein The reactive power reference level change rate for transitioning is calculated based at least in part on a short circuit rate of the power network.

7. The method (500) of claim 1, wherein The reactive power reference level change rate for transitioning is calculated based at least in part on a voltage level.

8. A power plant controller (400) for controlling operation of a renewable energy power plant (12) comprising a plurality of renewable energy generators (14) and connected to a power network, the power plant controller (400) comprising: a primary reactive power controller (206) configured to operate in a fault ride- through mode during a fault to generate a reactive power reference level; and a dynamic ramp rate controller (402) comprising an additional reactive power controller (406) configured to operate in a normal operation mode during the fault to generate a target reactive power reference level, wherein the dynamic ramp rate controller (402) is configured to calculate a reactive power reference level change rate for transitioning between the reactive power reference level generated by the primary reactive power controller (206) and the target reactive power reference level, the target reactive power reference level being suitable for normal operation of the renewable energy power plant (12).

9. The power plant controller (400) according to claim 8, wherein The dynamic ramp rate controller (402) includes a ramp rate logic (440) configured to calculate a rate of change of the reactive power reference level for the transition based at least in part on a difference between the reactive power reference level generated by the main reactive power controller (206) and the target reactive power reference level.

10. A computer program product downloadable from a communication network and / or stored on a machine readable medium, the computer program product comprising program code instructions for implementing a method (500) according to any one of claims 1 to 7.

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