Patent Publication Number: US-2023133674-A1

Title: Wind turbine power generation comprising an ac-dc-ac converter and a transformer with a tap changer

Description:
TECHNICAL FIELD 
     The invention relates to wind turbines, particularly to the control of power generating systems of wind turbines. 
     BACKGROUND 
     A wind turbine typically includes a tower, a generator, a gearbox, a nacelle, and a rotor having one or more rotor blades. The rotor blades transform wind energy into a mechanical rotational torque that drives one or more generators via the rotor. The generators are rotationally coupled to the rotor through the gearbox. A power converter with a regulated DC link controlled by a converter controller is further provided to convert a frequency of generated electric power to a frequency substantially similar to a grid frequency. 
     Renewable energy power generator systems, such as the wind turbine described above, are typically operated within predetermined voltage and power factor tolerance ranges. The operational tolerance ranges enable the wind turbine to supply a reliable transmission of electrical power to the grid over a variety of operating conditions and to provide ancillary functions, such as the injection and absorption of reactive power, in order to support grid stability. For example, a grid voltage tolerance range may extend from 90% to 110% of the nominally rated voltage whilst a typical electrical grid power factor tolerance range extends from +0.9 to -0.9 power factor (pf). These operational tolerance ranges define the electrical parameters for all components connected to the grid including the current rating and power draw for voltages in the lower end of the voltage range and the voltages at the upper end of the voltage range. 
     It is known to configure the grid connected wind turbines so that they operate within a voltage and/or power factor range that is complimentary to the operational tolerance ranges associated with the grid. In particular, each wind turbine is arranged to operate within a pre-determined range of parameter values, which includes power generation, current, voltage, and power factor. 
     In order to accommodate the potential large voltage transients in the electrical power grid, each of the ‘complimentary’ wind turbines may be operated for prolonged periods below their upper power and current parameters. Operating the wind turbines in this way reduces their operating effectiveness and efficiency, leading to an underutilisation of latent wind energy resource, whilst also incurring potential economic losses for the owners/operators of the wind farms. 
     Furthermore, for a typical wind turbine its operating parameters are limited by a maximum voltage for one or more of the components of the power generating system. For example, a wind turbine may be required to provide reactive power to the grid, which would impose over-voltage conditions on transformer windings where the converter is connected. Thus, when the converter provides reactive power, the resulting voltage may exceed a maximum specified continuous operating voltage level. In order to mitigate such over-voltage conditions, the converter can be operated to shift the power factor away from the required value; however, this is not always optimal. 
     Accordingly, there is need to improve systems and methods of optimising wind turbine operation while also maintaining voltage levels within specified operating ranges. It is against this background to which the present invention is set. 
     SUMMARY 
     According to a first aspect of the invention there is provided a method of operating a power generating system for a wind turbine connected to an electrical grid, the power generating system comprising a power generator, a converter, a transformer and a tap changer, the method comprising; when operating the power generating system in a grid-forming configuration, monitoring a signal for detecting a voltage of the electrical grid which requires an increase in output voltage from the power generating system in order to maintain the grid voltage within a predetermined voltage range; and operating the tap changer to tap-up the transformer to provide at least part of the voltage increase required to maintain the grid voltage within the predetermined voltage range. 
     The control method ensures that the converter, which may be decoupled from the grid due to an earlier adjustment of the tap changer, is controlled in dependence on an accurate representation of the grid voltage. In this way, the control method enables the wind turbine to match the demands of the grid whilst enhancing the power output capability of the power generating system. 
     The power generating system may comprise a converter; the method may comprise operating the converter to provide a converter voltage reference to maintain the grid voltage in the predetermined voltage range. 
     The method may comprise determining a tap changer position of the tap changer, and upon determining that the tap changer is not configured in a neutral position, updating the converter voltage reference based on the tap changer position. 
     Updating the converter voltage reference may comprise: monitoring a first signal indicative of a tap position of the tap changer; monitoring a second signal indicative of a converter voltage reference corresponding to the tap changer being operated in a neutral position; and determining a new converter voltage reference based on the first and second signals. 
     Determining the new converter voltage reference may comprise: 
     
       
         
           
             
               U 
               
                 r 
                 e 
                 f 
                 − 
                 n 
                 e 
                 w 
               
             
             = 
             
               
                 
                   n 
                   2 
                 
                 + 
                 Δ 
                   
                 n 
                 ∗ 
                 
                   N 
                   
                     T 
                     C 
                   
                 
               
               
                 
                   n 
                   2 
                 
               
             
             
               U 
               
                 r 
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                 d 
               
             
           
         
       
     
      wherein the new converter voltage reference U ref-new  defines a ‘tap-adjusted’ converter voltage reference, n 2  is a number of windings at the high-voltage side of the transformer, N TC  is a tap position of the tap changer, and Δ n is a change in the number of windings at the high-voltage side of the transformer for a given tap position. 
     The new converter voltage reference may comprise voltage amplitude. 
     The method may comprise operating the power generating system in an islanded configuration. 
     The method may comprise operating the power generating system in a grid-connected configuration. 
     Monitoring the signal for detecting an over-voltage condition in the electrical grid may comprise; monitoring a first signal indicative of a tap position of the tap changer; monitoring a second signal indicative of a voltage at a low-voltage side of the transformer; and determining the voltage at a high-voltage side of the transformer based on the first and second signal. 
     Determining the voltage at the high-voltage side of the transformer may comprise: 
     
       
         
           
             
               U 
               
                 L 
                 V 
                 − 
                 v 
                 i 
                 r 
                 t 
                 u 
                 a 
                 l 
               
             
             = 
             
               
                 
                   n 
                   2 
                 
                 + 
                 Δ 
                   
                 n 
                   
                 ∗ 
                 
                   N 
                   
                     T 
                     C 
                   
                 
               
               
                 
                   n 
                   2 
                 
               
             
             
               U 
               
                 L 
                 V 
               
             
           
         
       
     
     
         
         wherein U LV-virtual  defines a virtual voltage at the low-voltage side of the transformer when the tap changer is configured in a neutral position, n 2  is a number of windings at a high-voltage side of the transformer, N TC  is the tap position of the tap changer, Δ n is a change in the number of windings at the high-voltage side for a given tap position, and U LV  defines the actual voltage at the low-voltage side of the transformer; 
         wherein an over-voltage condition is detected if both the virtual voltage U LV-virtual  and the actual voltage U LV  are determined to be within an over-voltage range. 
       
    
     Determining the voltage at the high-voltage side of the transformer may comprise: 
     
       
         
           
             
               U 
               
                 H 
                 V 
                 − 
                 e 
                 s 
                 t 
                 i 
                 m 
                 a 
                 t 
                 e 
               
             
             = 
             
               
                 
                   n 
                   2 
                 
                 + 
                 Δ 
                   
                 n 
                 ∗ 
                 
                   N 
                   
                     T 
                     C 
                   
                 
               
               
                 
                   n 
                   1 
                 
               
             
             
               U 
               
                 L 
                 V 
               
             
             + 
             
               I 
               
                 r 
                 e 
                 a 
                 c 
                 t 
                 i 
                 v 
                 e 
               
             
             ∗ 
             
               X 
               
                 H 
                 V 
               
             
           
         
       
     
     
         
         wherein U HV-estimate  defines an estimated voltage at the high-voltage side of the transformer when the tap changer is configured in a neutral position, n 2  is a number of windings at a high-voltage side of the transformer, n 1  is a number of windings at the low-voltage side of the transformer, N TC  is the tap position of the tap changer, Δ n is a change in the number of windings at the high-voltage side for a given tap position, U LV  defines the actual voltage at the low-voltage side of the transformer, I reactive  is a reactive current of the transformer, and X HV  is an impedance of the transformer; 
         wherein an over-voltage condition is detected if the estimated voltage U HV-estimate  is determined to be within an over-voltage range. 
       
    
     According to a second aspect of the invention there is provided a controller for controlling a power generating system comprising a power generator, a generator side converter, a grid side converter, a transformer, a tap changer for a wind turbine, the controller being arranged to be connected to the power generating system and configured to control the power generating system according to the method of any preceding claim. 
     According to a third aspect of the invention there is provided a power generating system for a wind turbine which is connected to an external electrical grid, the power generating system comprising a converter, a transformer, a tap changer, and a controller, the controller comprising; an input arranged to receive a signal indicative of a voltage of the electrical grid; a determining module arranged to detect a voltage of the electrical grid which requires an increase in output voltage from the power generating system (20, 120) in order to maintain the grid voltage within a predetermined voltage range; a transformer control module arranged to determine a transformer control signal to control the tap changer to adjust the transformer; and an output arranged, upon detection of the demand to increase the grid voltage, to transmit the transformer control signal to operate the tap changer to tap-up the transformer to provide at least part of the voltage increase required to maintain the grid voltage within the predetermined voltage range. 
     The power generating system may comprise a converter control module arranged to operate the converter to provide a converter voltage reference to maintain the grid voltage in the predetermined voltage range. 
     It will be appreciated that the foregoing represents only some of the possibilities with respect to a control method for controlling a power generating system. Accordingly, it will be further appreciated that embodiments of a control method which include other or additional method steps remain within the scope of the present invention. Additional sub-method steps may relate to other method steps relating to the operation of a wind turbine. 
     The set of instructions (or method steps) described above may be embedded in a computer-readable storage medium (e.g. a non-transitory storage medium) that may comprise any mechanism for storing information in a form readable by a machine or electronic processors/computational device, including, without limitation: a magnetic storage medium (e.g. floppy diskette); optical storage medium (e.g. CD-ROM); magneto optical storage medium; read only memory (ROM); random access memory (RAM); erasable programmable memory (e.g. EPROM ad EEPROM); flash memory; or electrical or other types of medium for storing such information/instructions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Examples of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG.  1    illustrates a wind turbine including nacelle and a plurality of rotor blades; 
         FIG.  2    illustrates a power generating system of the wind turbine of  FIG.  1   ; 
         FIG.  3    illustrates an alternative power generating system of the wind turbine of  FIG.  1   ; 
         FIG.  4    shows a set of International Electrotechnical Commission (IEC) wind distribution charts corresponding to four different wind conditions experienced by the wind turbine of  FIG.  1   ; 
         FIG.  5    is a table indicating the distribution of time that the wind turbine of  FIG.  1    is subjected to different wind conditions; 
         FIG.  6    illustrates a controller of the power generating system of  FIG.  2   ; 
         FIG.  7    is a visual representation of the determined configuration parameters of the power generating system of  FIG.  2   , as determined by the controller of  FIG.  6   ; 
         FIG.  8    is an optimised PQ chart corresponding to the power generating system of  FIG.  2   ; 
         FIG.  9    shows a flow chart illustrating a first method of controlling the power generating system of  FIGS.  2  and  3   ; 
         FIG.  10    shows a flow chart illustrating a second method of controlling the power generating system of  FIGS.  2  and  3   ; 
         FIG.  11    is a table describing a number of strategies for mitigating an extreme over-modulation condition of the power generating system of  FIGS.  2  and  3   ; 
         FIG.  12    shows a flow chart illustrating a third method of controlling the power generating system of  FIGS.  2  and  3   ; 
         FIG.  13    is a schematic representation of a droop control strategy for the power generating system of  FIGS.  2  and  3   ; and 
         FIG.  14    is a schematic of a tap changer of the power generating system of  FIGS.  2  and  3   . 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    shows a wind turbine  10  including a tower  12 , a nacelle  14  rotatably coupled to the top of the tower  12 , a rotor  16  including a rotor hub mounted to the nacelle  14 , and a plurality of wind turbine rotor blades  18  - in the described example, three rotor blades -which are coupled to the rotor hub. The nacelle  14  and rotor blades  18  are turned and directed into the wind direction by a yaw system. The wind turbine  10  is shown in its fully-installed form suitable for operation; in particular, the rotor  16  is mounted on the nacelle  14  and each of the blades  18  are mounted on the rotor hub. The rotor  16  is rotatable by action of the wind during operation of the wind turbine  10  in order to convert the kinetic energy of the wind into rotational energy of the rotor  16 . 
     With particular reference to  FIGS.  2  and  3   , the nacelle  14  houses a power generating system  20 ,  120  which is capable of converting the rotational energy of the rotor  16  into electric power which can be supplied to an electrical grid  28  - or grid  28 . Note that  FIGS.  2  and  3    illustrate two different versions of power generating architectures within which embodiments of the invention may apply, so both generating system will be discussed here in broad terms so as to introduce the main functional components. During operation of the wind turbine  10 , the wind induced rotational energy of the rotor  16  is transferred via a shaft  22  to a generator  24 ,  124 . A first portion  22   a  of the shaft  22  is rotatably coupled at one end to the rotor  16  and at another end to a gearbox assembly  26 . The gearbox assembly  26  is arranged to transfer the rotational speed of the first shaft portion  22   a  to the generator  24  via a second shaft portion  22   b  which is rotatably coupled therebetween. The gearbox assembly  26  includes a step-up ratio which increases the rotational speed of the second shaft portion  22   b  compared to the first shaft portion  22   a . In this way, the gearbox assembly  26  steps up the inherently low rotational speed of the rotor  16  for the generator  24  to efficiently convert the rotational mechanical energy to electrical energy, which is fed into the grid  28  via at least one electrical connection. 
     Each of the exemplary power generating systems  20 ,  120  described herein are provided with a gearbox assembly  26  arranged to transfer power between the rotor  16  and the generator  24 . However, the methods and systems according to the present invention would also apply to a power generating system comprising a direct-drive connection between the rotor and the generator, as would be readily understood by a person having ordinary skill in the art. 
     The generator  24  of the power generating system  20  shown in  FIG.  2    is a three-phase, double-fed induction (asynchronous) generator (DFIG), which includes a generator stator which is magnetically coupled to a generator rotor coil (not shown). A portion of the electrical power generated by the generator  24  is transferred from the generator stator to a converter assembly  30  of the power generating system  20 , and the remainder of the generated power is transferred from the generator rotor coil directly to the grid  28 . 
     According to an alternative arrangement of the power generating system  120 , the generator  24  is a permanent magnet generator  124 , as illustrated in  FIG.  3   . The permanent magnet generator  124  includes a permanent magnet rotor (not shown) from which substantially all of the power generated from the generator  124  is directed to grid  28  via a converter assembly  30 . 
     In each of the power generating systems  20 ,  120  shown in  FIGS.  2  and  3   , the respective generators  24 , are electrically connected to a generator side converter  32  - or a machine side converter - of the converter assembly  30 . The generator side converter  32  is connected to a grid side converter  34  via a DC-link  38  which includes a positive rail, a negative rail and one or more DC-link capacitors coupled therebetween. During operation of the wind turbine  10 , the DC-link capacitors are charged by the DC output current from the generator side converter  32 , which thereby supplies DC power to the grid side converter  34 . An output AC current from the grid side converter  34  is supplied via a transformer  36  to the grid  28 . 
     The transformer  36  of each of the power generating systems  20 ,  120  includes a tap changer assembly  40  - or tap changer  40 , which enables switching, or ‘tapping’, between different turn ratios of the transformer  36  in order to control the voltage output to the grid  28 . The power generating system  20 ,  120  includes a controller  42  arranged to monitor and control the operation of the power generating system  20 ,  120 , as will be described in more detail below. 
     With reference to  FIGS.  4  and  5   , the operation of the power generating system  20 ,  120  will now be described with reference to the different operational conditions of the wind turbine  10 . In particular,  FIG.  4    shows a set of International Electrotechnical Commission (IEC) wind distribution charts corresponding to four different wind conditions I, II, III and IV. Each of the wind distribution charts illustrates a typical distribution vs. frequency of the wind speeds associated with each of the four wind condition categories I-IV. 
       FIG.  5    is a table summarising the data shown in  FIG.  4   , and relates to a wind turbine  10  which is configured to deliver a maximum power output of 6.16 MW. The data in  FIG.  5    represents an example of many possible 
     In particular, the converter assembly  30  is rated to deliver a maximum active power P of 5.6 MW when the tap changer  40  is set to a neutral condition. Tapping up the tap changer  40  by +10% increases the maximum output power to 6.16 MW. Conversely, by tapping down the tap changer  40  by -10%, the minimum active power P is reduced to 4.98 MW. Tapping down the tap changer  40  also increases the current by 11%. 
     The columns labelled W, X, Y and Z represent four different operating conditions of the power generating system  20 ,  120 . Each of the four operating conditions W, X, Y and Z is associated with a different range of wind speeds experienced by the wind turbine  10 . For example, operating condition W is associated with a wind speed of 0 to 10 metres per second (m/s), which corresponds to an operating power output from the power generating system  20 ,  120  of between 0 to 4.98 MW. 
     Operating condition W corresponds to an under-modulation of the wind turbine  10  in which the wind speed is below an optimum level for power generation, whereas operating condition X corresponds to an optimum wind condition for power generation. For certain situations, the operating conditions Y and Z correspond to an over-modulation condition and a cut-out condition, respectively. The over-modulation condition corresponds to the situation in which the wind turbine  10  is required to operate above its optimum operating condition but still within manageable range of the power generating system  20 ,  120 . The cut-out condition corresponds to a situation in which the wind speed is so high that the power generating system  20 ,  120  is shut down in order to protect the components of the wind turbine  10 . 
     Each row of the table describes the relative time in which the power generating system  20 ,  120  is operated according to each of the four operating conditions 1-4, as a percentage of the total wind turbine operation. The four rows correspond to the different wind condition categories I-IV. For example, the power generating system  20 ,  120  is operated according to operating condition W for 63.21% of the time during wind conditions corresponding to wind category IV. More generally, operating condition W occurs between 63-93% of the time, scheme X occurs between 3-7% of the time and scheme Y occurs between 3.5-28% of the time. By contrast, scheme Z has a significantly rarer incidence rate, occurring less than 1.8% of time. 
     The power generating system  20 ,  120  is controlled according to a different control method - or operating scheme - depending on which of the operating conditions W, X, Y and Z the wind turbine  10  is operating within. Each control method is implemented by a controller  42  of the power generating system  20 ,  120  according to the present invention. 
     The controller  42  includes a number of processing modules  50 ,  52 ,  54  including a determining module  50 , a converter control module  52  and a transformer control module  54 , as shown in  FIG.  6   . The controller  42  also includes an input  46  and an output  48  which are arranged to facilitate communications between the controller  42  and the power generating system  20 ,  120 , as illustrated by the connections shown in  FIG.  2    between the controller  42 , the transformer  36  and the converter assembly  30 . 
     The primary function of the controller  42  is to operate the power generating system  20 ,  120  according to one or more control schemes, as will described in more detail below. Each of the modules  50 ,  52 ,  54  is configured to perform a variety of functionalities depending on which control scheme is being implemented by the controller  42 . 
     Generally, the determining module  50  is arranged to monitor and interpret different parameters, including environmental and operational parameters of the wind turbine  10 , which may affect the power generation system  20 ,  120 . The converter control module  52  - or converter module  52  - is configured to control the operation of the converter assembly  30 . Similarly, the transformer control module  54  - or transformer module  54  -is arranged to control the transformer  36  via the tap changer  40 . 
     The input  46  and output  48  each include one or more signal converters which are arranged to permit signals transmitted to and from the controller  42 . For example, the input signal converter is configured to convert the signals received from the components of the power generating system  20 ,  120  into a medium that can be interpreted by the controller  42 . Similarly, the output signal converter is arranged to convert outputted signals into a medium that can be understood by the component of the power generating system  20 ,  120 . 
     The input  46  is arranged to receive a variety of inputs - or input signals - relating to both the configuration and operation of the power generating system  20 ,  120 . A configuration input - or configuration input signal - defines how a particular component of the power generating system  20 ,  120  has been configured so as to carry out its function. 
     An exemplary configuration input includes information relating to the range of voltages which the converter assembly  30  is operable to output during normal use. Alternatively, the configuration input includes a number of tap change positions of the tap changer  40 . The different tap changer positions are, in turn, indicative of a corresponding turn ratio of the transformer  36 . 
     The configuration input signals are transmitted to the controller  42  by a user of the power generating system  20 ,  120  via a human machine interface (not shown). Alternatively, or in addition, each of the configuration inputs may be provided in the form of a lookup table which is stored on a suitable storage medium. 
     The input  46  is also arranged to receive operating inputs - or operating input signals -which define an operating condition of a particular component during operation of the power generating system  20 ,  120 . An exemplary operating input includes voltage and/or electric current measurements received from one or more voltage and electric current sensors (not shown) of the power generating system  20 ,  120 . 
     The voltage and current sensors are electrically connected to the components, or to electrical connections between the components, of the power generating systems  20 ,  120 , as would be readily understood by the skilled person. An alternative operating input may be indicative of the current tap change position of the tap changer  40 . Further exemplary operating inputs relate to the operating parameters of the converter assembly  30 , including the detectable operating parameters of the machine side converter  32 , the grid side converter  34  and the DC-link  38 . 
     Under-Modulation Control Scheme  60   
     An exemplary control method  60  for operating the power generating system  20 ,  120  according to a first aspect of the present invention will now be described with reference to  FIGS.  7 ,  8  and  9   . The control method  60  is particularly suited for controlling the power generating system  20 ,  120  in situations when the wind turbine  10  is operating under schemes W and X. 
     The control method  60  corresponds to a wind turbine  10  which is operated to support the voltage and power levels in the grid  28 . Such grid-feeding, or grid-supporting, configurations require the power generating system  20 ,  120  to deliver active and reactive power P, Q to the grid  28 . In this situation, the controller  42  acts as a primary control by setting an operation point for each of the converter assembly  30  and tap changer  40 , as a function of their respective capacities, in order to maximise the power generating capacity of the wind turbine  10 . Operation of the power generating system  20 ,  120  may be controlled by a secondary control (i.e. an electrical grid controller) which is arranged to set active and reactive power references P*, Q*. The main objective of the secondary control is to minimise the voltage and the frequency deviations within the grid  28 . 
     The determining module  50  is arranged to determine a set of control parameters based on the configuration of the components of the power generating system  20 ,  120 . The converter and transformer modules  52 ,  54  are then arranged to calculate, in dependence on receiving a demand signal, one or more control signal(s) for controlling the power generating system  20 ,  120 . 
     The determining module  50  is primarily arranged to determine the set of control parameters when the power generating system  20 ,  120  is in an offline condition, i.e. when the wind turbine  10  is not supplying power to the grid  28 . Further, the determining module  50  may determine the control parameters prior to the power generating system  20 ,  120  being installed within the wind turbine  10 . In addition, the determining module  50  can be operated to update the control parameters in response to a change in the configuration of the system’s components. Such changes may occur following the replacement of one or more of the components of the power generating system  20 ,  120 . In contrast to the determining module  50 , the converter and transformer modules  52 ,  54  are configured to be operated during an online condition of the power generating system  20 ,  120 , i.e. during the operation of the wind turbine  10 . 
     The operation of the determining module  50  will now be described with particular reference to  FIG.  7   , which shows a schematic of the control parameters as determined by the determining module  50 . The determining module  50  receives a configuration input relating to the converter assembly  30  and transformer  36 . The determining module  50  is arranged to determine, from the configuration input corresponding to the converter assembly  30 , that the converter assembly  30  is configured to operate in seven distinct operating modes, labelled C 1 , C 2 , C 3 , C 4 , C 5 , C 6  and C 7  in  FIG.  7   . 
     Each of the converter operating modes C 1 -C 7  correspond to a converter output voltage Uc which range from 0.87 per unit system (pu), for C 1 , to 1.13 pu, for C 7 . In particular, converter operating modes C 1 -C 7  correspond to converter output voltages U c  of 0.87 pu, 0.9 pu, 0.95 pu, 1.0 pu, 1.05 pu, 1.1 pu and 1.13 pu, respectively. It will be appreciated that the seven voltage modes C 1 -C 7  are described herein as exemplary operating modes of the converter assembly  30 . For example, the converter assembly  30  may be configured to output a range of converter operating voltages which are separated by 0.1 pu, or alternatively less than 0.1 pu. In some situations, the operation of the converter may be defined by a greater or smaller number of operating modes, as would be readily understood by the skilled person. 
     The tap changer  40  is configured to apply a tap change adjustment - or tap change value - to each of the operating modes C 1 -C 7  of the converter assembly  30 . The tap change adjustment defines the extent to which the transformer  36  is tapped up or down by the tap changer  40 . For example, the determining module  50  determines that the tap changer  40  should be configured to apply a tap change adjustment of plus or minus ten percent (+/- 10%) to the converter output voltage U c  which it receives from the converter assembly  30 . It will be appreciated by the skilled person that the tap change adjustment values described above (i.e. +/- 10% represent an exemplary aspect of the determining module  50 . The tap changer may be configured to apply a range of adjustments to the transformer, depending on the requirements of the power generating system. 
     For the exemplary case of operating mode C 1 , the tap changer  40  is capable of tapping-up and tapping-down the converter output voltage Uc (i.e. 0.87 pu) such that the ‘tap change adjusted’ output voltage range is between 0.77 pu and 0.97 pu. Similarly, the nominal range of output voltages corresponding to operating mode C 2  is between 0.7 pu and 0.9 pu. 
     The determining module  50  is arranged to determine the nominal range of output voltages obtained by tapping up and tapping down the converter output voltages U c  which correspond to the converter operating modes C 1 -C 7 . Put another way, the determining module  50  is configured to determine the tap change adjusted output voltage range for each of the converter operating modes C 1 -C 7 . The determined operating parameter data (i.e. the range of ‘tap change adjusted’ converter output voltages) is stored on a memory device  55  of the controller  42 . 
     Owing to the available tap change adjustment, it will be appreciated that there is significant overlap between the different ranges of tap change adjusted output voltage, as represented by the horizontal bars shown in  FIG.  7   . For example, the non-adjusted converter output voltage U c  corresponding to operating mode C 1  is 0.87 pu. A substantially similar output voltage can also be achieved by switching the converter assembly  30  to converter operating mode C 2  (i.e. with a converter output voltage of 0.90 pu) and by tapping down the converter output voltage by -5%. Alternatively, the same output voltage can also be achieved by tapping down the output voltage of converter operating mode C 3  (i.e. 0.95 pu) by 10 %. 
     Each of these three configurations result in a different level of reactive power Q being available to be transmitted into, or absorbed from, the grid  28 , despite producing substantially the same output voltage. By operating the converter assembly  30  according to the first configuration (i.e. applying converter operating mode C 1  with no adjustment to the tap changer 40) it results in a reactive power value Q of 1.018 pu, for example. By comparison, the reactive power value Q corresponding to the second configuration (i.e. converter operating mode C 2  combined with a -5% tap change adjustment) is 1.341 pu and for the third configuration (i.e. converter operating mode C 3  combined with a -10% tap change adjustment) the reactive power value Q is 1.396 pu. 
     Accordingly, the determining module  50  is arranged to determine the reactive power value Q corresponding to each possible configuration of the converter assembly  30  and tap changer  40 . The determining module  50  is also configured to determine the converter assembly  30  and tap changer  40  operating parameters corresponding to each of the respective reactive power values. The operating parameters are then stored, along with the tap change adjusted voltage data, in the memory device  55  of the controller  42 . 
     Having calculated the associated reactive power value for each of the possible operating parameter combinations, the determining module  50  is further configured to determine a set of optimised operating parameters S 1 , S 2 , S 3 , S 4 , S 5 , S 6  and S 7 , by which the system can be controlled in order to deliver a desired output to the grid  28 . Each of the operating parameters S 1 -S 7  is determined to control the system to transmit a required voltage output to the grid. To achieve this, each operating parameter S 1 -S 7  comprises a converter output voltage (i.e. corresponding to one of the converter operating modes C 1 -C 7 ) and a tap changer adjustment (i.e. applying an adjustment to the transformer  36  of between +10% and -10%, for example). 
     The optimised operating parameters S 1 -S 7  are further configured to maximise the power generating capability of the wind turbine generator  10 . According to an exemplary arrangement of the controller  42 , the determining module  50  is configured to determine the converter assembly  30  and the tap changer  40  operating parameters which maximise the reactive power Q output of the power generating system  20 ,  120 . For example, the first optimised operating parameter S 1  comprises operating the converter assembly  30  in operating mode C 3  and with the tap changer  40  being configured to tap down the transformer  36  by -10%. As described above, this combination of converter assembly  30  and tap changer  40  operating parameters provides the maximum reactive power output of 1.396 pu for the corresponding output voltage of 0.87 pu. 
     By tapping down the tap changer  40  when in a partial load condition, the optimised operating parameter S 1  advantageously increases the over voltage ride through (OVRT) capabilities of the system  20 ,  120  compared to a system which does not include a tap changer. This is because the wind turbine generator  10  only operates at full load for a portion of its operational life, and the rest of the time it will operate in a partial load condition. Accordingly, by tapping down the converter output voltage during partial load conditions, and thereby increasing the converter operating current, the associated converter losses are reduced for the majority of the converter’s operational life. 
     As a further example of the operation of the determining module  50  according to the present invention, we now refer to converter operating mode C 7  in  FIG.  7    for which the nominal converter output voltage is 1.13 pu. The optimised operating parameter S 7  corresponding to this output voltage includes operating the converter assembly  30  according to operating mode C 6  and then tapping up the tap changer  40  by 5%. Tapping-up the voltage +5% increases the reactive power output Q by 5% with the same current passing through the converter assembly  36 . Thus, the power generating system  20 ,  120  is able to output more reactive power Q than an equivalent power generating system which does not include a tap changer. 
     Furthermore, by applying the optimised control protocol as described above, the power generating system  20 ,  120  is able to increase the upper limit of its ‘normal’ operating range from 1.13 pu to 1.23 pu. Furthermore, the over-modulation range of the power generating system  20 ,  120  is also increased so that it extends between 1.23 pu and 1.44 pu, owing to the influence of the tap changer control as described herein. By contrast, an equivalent power generating system which does not include a tap changer  40  would have an upper limit of its ‘normal’ operating range of 1.13 pu. 
     The optimised operating parameters S 1 -S 7  are stored on the memory device  55  as a control protocol, which can be read by the converter and transformer modules  52 ,  54  during operation of the power generating system  20 ,  120 , as will be described later. The optimised operating parameter data can then be updated by the determining module  50  in response to receiving updated configuration inputs. 
     The control protocol can be presented as a series of active power vs. reactive power PQ charts, as shown in  FIG.  8   . The PQ charts represent the power generating capability of the power generating system  20 ,  120  when it is controlled with respect to the optimised operating parameters S 1 -S 7 . Each PQ chart indicates the reactive power Q transmitted to the grid  28  with respect to the active power P. The different shaded lines correspond to the power generating capability of the power generating system  20 ,  120 , when it is operated according to the respective operating parameters S 1 -S 7 . 
     The highlighted regions A, B, C and D each represent a point on the PQ chart which is of particular interest to the operation of the power generating system  20 ,  120 . For example, region A corresponds to a situation in which the power generating system  20 ,  120  may be controlled to transmit reactive power +Q into the grid  28 , whereas region C corresponds to a situation in which the system is controlled to absorb reactive power -Q from the grid. 
     The above described determination of the optimised operating parameters S 1 -S 7  (as shown in  FIG.  7   ) corresponds to point A on the PQ charts of  FIG.  8   . The determining module  50  is also configured to determine the optimised operating parameters for each of the points B, C and D. The resulting sets of optimised operating parameters are used to provide maximised PQ charts for the power generating system  20 ,  120 , which thereby define the maximum power generating capability of the wind turbine  10 . 
     As described above, the control protocol comprises a set of operating parameters which result in an optimised power output from the system  20 ,  120 . In this situation, the desired change in reactive power output Q may be achieved without changing the operating mode C 1 -C 7  of the converter assembly  30 . With particular reference to  FIG.  7   , each of the optimised operating parameters S 4 -S 7  involves operating the converter in operating mode C 6 . Accordingly, the desired power output voltage is obtained by switching the tap changer  40  between -10% and + 5%. Thus, the tap changer  40  is used to provide at least a portion of the required increase in reactive power output Q. Advantageously, this control strategy reduces the number of times that the converter assembly  30  must be adjusted in order to change the reactive power output of the system, which thereby increases the operational life of the converter assembly  30 . 
     With particular reference to  FIG.  6   , the controller  42  is configured to control the power generating system  12 ,  120  based on the optimised operating parameters S 1 -S 7 . In particular, the input  46  is arranged to receive a demand input - or demand input signal -which is indicative of a requirement to change the output voltage U o  of the power generating system  20 ,  120 . Such a demand signal may include, for example, a request for the power generating system  20 ,  120  to support the voltage in the grid  28 . 
     The converter module  52  is configured to determine a converter control signal for controlling the converter assembly  30  and transformer module  54  is configured to determine a transformer control signal to control the tap changer  40  in order to meet the required change in output voltage and/or reactive power Q. According to an exemplary arrangement of the controller  42 , the control signals are determined so as to configure the converter assembly  30  and the tap changer  40  based on the optimised operating parameters S 1 -S 7 , as defined in the control protocol. 
     For example, upon receiving an input signal indicative of a demand for the power management system  20 ,  120  to deliver an output voltage corresponding to 0.87 pu, the determining module  50  determines that the optimised operating parameter S 1  is capable of achieving the required output. The converter module  52  then determines a converter control signal to control the converter assembly  30  to output a voltage of 0.9 pu (i.e. to operate the converter assembly  30  according to operating mode C 3 ). The transformer module  54  also determines a tap changer control signal to control the tap changer  40  to tap down the converter voltage by a determined tap change adjustment value, such as -10%. 
     According to this arrangement, the controller  42  is configured to operate the power generating system  20 ,  120  according to the pre-determined control protocol, as previously determined by the determining module  50 . The output  48  is arranged to transmit the control signals to the relevant components of the power generating system  20 ,  120 , in order to control the operation of the converter assembly  30  and the tap changer  40 , as necessary. 
     According to an alternative arrangement of the controller  42 , the processing modules  50 ,  52 ,  54  are again configured to control the power generating system  20 ,  120  in dependence on receiving a demand for an adjustment in the output voltage U o  to the grid  28 . However, in this case the converter and transformer modules  52 ,  54  are arranged to operate the converter assembly  30  and the tap changer  40 , respectively, without the need to consult the pre-determined operating protocol. In this way, the controller  42  is arranged to determine one or more control signals to control the power generating system  20 ,  120  according to a dynamic control scheme  60  - or control method  60 . 
     An exemplary method of operating the power generating system  20 ,  120  according to the dynamic control scheme  60  will now be described with reference to  FIG.  9   . The control method  60  commences with a first step  62  in which the controller  42  receives, via the input  46 , an input signal indicative of a demand for an adjustment in the output voltage U o  of the power generating system  20 ,  120 . The demand for an increase in output voltage U o  may correspond to a demand for increased power output PQ from the power generating system  20 ,  120 . Accordingly, the controller  42  is configured to monitor the input signal to detect a grid voltage U G , which requires an adjustment in the output voltage U o . 
     In a second method step  64 , the determining module  50  determines a partial-load condition of the converter assembly  30 . The partial-load condition corresponds to the converter assembly  30  being configured to output a converter output voltage U c  which is substantially below the maximum converter output voltage. The partial-load condition also refers to situations wherein the power generating system  20 ,  120  is operated below a maximum power output for which the system is rated. In this situation, the partial-load condition refers to when the wind turbine  10  is operated during operating conditions W and/or X, i.e. below the ‘over-modulation’ operating condition Y. 
     It is noted that in each of the exemplary control methods described herein, the turbine load (i.e. the maximum mechanical load that the turbine can accommodate) is not tied to a specific range of modulation index values. Accordingly, the power generating system may not be limited to operating according to prescribed modulation range during certain turbine load conditions. 
     According to an exemplary operation of the wind turbine  10 , the power generating system  20 ,  120  is initially controlled to operate at full active power P with an output voltage U o  of 0.95 pu, To achieve this, the converter assembly  30  is operated in converter operating mode C 3 , and the tap changer  40  is configured in a neutral position,. This combination of operating parameters generates a reactive power output Q of 1.396 pu, which corresponds to region A of the PQ chart shown in  FIG.  8   . 
     The controller  42  then receives an input signal indicative of a demand to increase the reactive power output Q to 1.451 pu. Upon receiving the input signal, the determining module  50  determines a number of possible configurations of the converter assembly  30  and tap changer  40  which would be able to achieve the required reactive power output Q. A first configuration comprises switching the converter assembly  30  to operate in converter mode C 4  (corresponding to a converter output voltage U c  of 1.05 pu) and retaining the tap changer  40  in its neutral position. Alternatively, the same reactive power output Q could also be achieved by switching the converter assembly  30  to operate in converter mode C 4  and then tapping down the transformer  36  by 5% or 10%. 
     In a third method step  66 , the converter module  52  determines a control signal to control the converter assembly  30  to operate in converter mode C 4  and the transformer module  54  determines a control signal to tap down the transformer  36  by either 5% or 10%. Thus, each control signal is configured to operate the tap changer  40  and the converter assembly  30  in order to provide the required adjustment in reactive power output from the power generating system  20 ,  120 . 
     By tapping down the tap changer  40  during the partial-load configuration of the converter assembly  30 , the power generating system  20 ,  120  is able to achieve the requested reactive power output Q whilst minimising its output voltage U o . The advantageous method also increases the potential over-voltage ride-through response during higher load conditions, thereby increasing the power generating system’s  20 ,  120  ability to respond to demands from the grid  28 . Put another way, the controller  42  retains its ability to adjust the tap changer  40  in response to future fluctuations in the grid voltage. 
     According to an aspect of the present invention, the control method  60  comprises controlling the tap changer  40  to adjust the transformer  36  according to a reduced tap changer adjustment, In this way, the tap changer  40  is controlled to adjust the transformer  36  by a tap change adjustment which is less than the maximum available tap change adjustment. For example, the control method  60  may comprise only tapping down the transformer by 5% instead of tapping down by the maximum 10%. Advantageously, the resulting tap change will still achieve the required increase in reactive power output Q whilst also limiting the number of tap changer adjustments - or tappings, which will thereby prolong the life of the tap changer  40 . According to an alternative exemplary control method, the tap changer  40  may be controlled to adjust the transformer  36  by one or more suitable tap change adjustments being less than the maximum tap change adjustment, as would be appreciated by the skilled person. 
     In a final method step  68 , the output  48  transmits the control signal(s) to the converter assembly  30  and to the tap changer  40  in order to control the operation of the power generating system  20 ,  120  to meet the demand from the grid  28 . 
     As described above, the control method  60  is particularly suited for controlling the power generating system  20 ,  120  in situations when the wind turbine  10  is operating under conditions associated with operating conditions W and X. Referring again to  FIGS.  4  and  5   , the operating conditions W and X correspond to an under-modulation condition of the wind turbine  10 . Accordingly, the controller  42  is arranged to receive a wind speed input - or wind speed signal - indicative of a wind speed in the area of the wind turbine  10 . The wind speed signal comprises information relating to the wind speed in the local vicinity of the wind turbine  10 . As such, the input  46  of the controller  42  is configured to receive a wind speed signal from a wind speed sensor which is arranged either on or near to the wind turbine  10 . Alternatively, the wind speed signal may be derived from a measure of the torque which is applied by the wind to the rotor blades  18 , as would be readily understood by the person having ordinary skill in the art. 
     The determining module  52  is configured to determine the speed of the wind that is incident upon the wind turbine  10 , based on the wind speed signal. In turn, the determining module  50  is able to determine whether the power generating system  20 ,  120  is operating in one of the operating conditions W, X, Y and Z. Upon determining that the wind speed falls within at least one of the wind speed ranges associated with W and X (i.e. within an exemplary wind speed range of 0-10 m/s or 10-11 m/s, respectively), the controller  42  then proceeds to control the power generating system  20 ,  120  according tothe control method  60 , as described above. 
     Advantageously, the control method  60  increases the capacity of the power generating system  20 ,  120  to provide an overvoltage to the electrical grid  28 . However, if an overvoltage capability is not desired, then the control method  60  may be disabled. In such a situation, the operation of the power generating system  20 ,  120  may revert back to being determined by the pre-determined operating protocol, as described above. 
     If the determining module  50  detects that the wind speed is outside the wind speed range of operating conditions W and X, then the controller  42  inhibits control of the power generating system  20 ,  120  according to control method  60 . For example, if the wind speed is determined to be in the over-modulation range (i.e. operating condition Y), then the controller  42  will operate the power generating system  20 ,  120  according to an over-modulation control strategy  160 , which is described in more detail below. Alternatively, if the wind speed is determined to be greater than a determined wind speed threshold value, such as 20 m/s for example (i.e. falling within the operating condition Z), then the controller  42  is configured to initiate a shut-down control strategy in order to protect the components of the power generating system  20 ,  120 . 
     The controller  42  is also configured to receive signals indicative of the current wind speed so that the determining module  50  can determine where the current wind speed sits within wind speed range of the relevant operating condition W, X, Y, and Z. 
     If the wind speed is determined to be within the range of operating condition W (i.e. when the wind speed is between 0-10 m/s), then the controller  42  is configured to moderate the extent to which the transformer  36  is tapped down by the tap changer  40 . To achieve this, the controller  42  is configured to only allow tapping down of the transformer  36  when an operating current I c  of the converter assembly  30  is below an operating current threshold value, such as 1.0 pu, i.e. when the output power is between 0 to 5.04 MW, for example. Accordingly, the tapping down of the transformer  36  is inhibited when the operating current I c  of the converter assembly  30  is at or above a threshold current I T . The threshold current I T  defines a converter operating current which, if exceeded, may cause damage to the converter assembly  30  and/or other components of the power generating system  20 ,  120 . 
     By moderating the tapping down of the transformer  36  in this way, the controller  42  is configured to safely increase the overvoltage capabilities of the power generating system  20 ,  120  without causing damage system  20 ,  120  or the grid  28 . This control strategy also reduces harmonic distortion during overvoltage operating conditions. According to an alternative arrangement of the power generating system, the converter assembly  30  is configured such that it operates below 1.0 pu when the output power of the converter assembly  30  is determined to be within a range of power values, for example, between 0 and 4.96 MW. 
     If the wind speed is determined to be within the range of operating condition X (i.e. when the wind speed is between 10-11 m/s), the controller  42  is further determined to moderate the tapping down of the transformer  36 . To achieve this, the determining module  50  determines a tap-down threshold value below which the tap changer  40  cannot be set. The tap-down threshold is determined such that it reduces linearly as the power output of the wind turbine  10  increases from 5.04 MW to 5.6 MW. Accordingly, a maximum tap-down threshold of -10% is determined when the wind speed is at 10 m/s, and a minimum tap-down threshold of 0% is determined when the wind speed is at  10  m/s. This prevents the operating current I c  of the converter assembly  30  from exceeding a threshold current I T . According to an alternative arrangement of the controller  42 , the tap down threshold may be determined based on the operating current I c  of the converter assembly  30 . 
     With reference to the control method  60 , as described above, the determining module  50  is configured to determine a tap change adjustment value which defines the extent to which the transformer  36  is tapped up or tapped down by the tap changer  40 . For example, the determining module  50  is configured to determine whether to tap down the transformer  36  by either 5% or 10%. In this situation, the determining module  50  receives an indication that the wind speed is at 11 m/s. Alternatively, the determining module  50  may detect that the power generating system  20 ,  120  is outputting power at 5.6 MW (i.e. the upper limit of operating condition X). Accordingly, the tap-down threshold is set to 0%, thereby preventing any tapping down of the transformer  36 . The transformer module  54  is then configured to generate a control signal which retains the tap changer  40  in a neutral position. 
     It is noted that the controller  42  is configured to allow tapping up of the transformer  36  at any time whist the power generating system  20 ,  120  is operated according to operating conditions W or X, since this would not cause any adverse effects to either the grid  28  or the power generating system  20 ,  120 . 
     Over-Modulation Control Scheme  160   
     According to a second aspect of the invention, the controller  42  is configured to control the tap changer  40  and the converter assembly  30  according to an over-modulation control scheme  160 , as shown in  FIG.  10   . The control scheme  160  is particularly suited to situations when the wind turbine  10  is operating in wind speeds of between 11-20 m/s (i.e. corresponding to operating condition Y). 
     The primary function of the controller  42 , when operating according to the control scheme  160 , is to increase the operating capabilities of the power generating system  20 ,  120  during over-modulation operating conditions. 
     In a first method step  162  of the over-modulation control scheme  160  - or control method  160  - the input  46  is configured to receive a signal indicative of the grid voltage U G  of the electrical grid  28 . In a second step  164 , the determining module  50  is configured to detect an over-voltage condition requiring a reduction in the output voltage U o  of the power generating system  20 ,  120 . 
     In a third method step  166 , the converter module  52  is arranged to determine a converter control signal to operate the converter assembly  30  in order to provide at least part of the required voltage reduction. The transformer module  54  is also arranged to determine a transformer control signal to adjust the tap changer  40  to tap down the transformer  36  in order provide at least part of the required voltage reduction. 
     The output  48  is arranged, upon detection of the over-voltage condition, to transmit the converter and transformer control signals in a fourth method step  168  to provide the required reduction in the output voltage of the power generating system  20 ,  120 . The converter response mode and the transformer response mode are initiated so that they are implemented at least partially during the same period of time. 
     The converter module  52  is configured to operate in a converter response mode such that it is arranged to control the converter assembly  36  to respond to the over-modulation condition. Similarly, the transformer module  54  is configured to operate in a transformer response mode such that it is able to control the tap changer  40  to adjust the transformer  36  to respond to the detected over-modulation condition. 
     Advantageously, the control method  160  configures the controller  42  to react to extreme over-modulation voltages in the electrical grid  28  by using both the converter and transformer response modes to counteract the over-voltage. The converter assembly  36  can be adjusted quickly to accommodate the voltage demand. By contrast, the tap changer  40  is slower to react but it provides a more efficient and stable means of reducing the output voltage U o  which can therefore be sustained indefinitely. 
     After a period of time following the initiation of the converter and transformer response modes, the converter module  52  is arranged to cancel the converter response mode. The controller  42  therefore reverts to operating the power generating system  20 ,  120  according to just the transformer response mode. The transformer response mode is arranged to retain the tap changer  40  in the required tap changer position so as to provide the required output voltage U o . 
     According to this exemplary control method, the predetermined period of time after which the converter response mode is cancelled is determined by the determining module  50  to be no more than 2 seconds. As such, the predetermined period provides the controller  42  with sufficient time to initiate the tap changer  40  to adjust the transformer  36  in accordance with the transformer response mode control before the converter response mode is shut down. Alternatively, the predetermined time period may be determined in dependence on the severity of the over-modulation, as will be described in more detail below. 
     The transformer response mode represents a tap changer adjustment mode  86  in which the transformer module  54  is arranged to tap down the voltage of the transformer in response to the demand from the grid  28 . The converter response mode includes a number of different converter control strategies, as shown in the table of  FIG.  11   . Each of the converter control strategies can be implemented by the converter module  52  in response to the demand for the change in voltage from the grid  28 . 
     A first converter control strategy comprises a DC voltage U dc  adjustment mode  82  in which the grid side converter  34  is configured to increase a voltage across the DC link  38  above a rated value. 
     A second converter control strategy comprises a reactive power absorption mode  84  in which a reduction in voltage is generated across a grid choke of the utility grid  28 . Reducing the voltage drop across the grid choke leads to a reduction in the voltage output requirement for the converter  34  and therefore a reduced DC voltage output from the power generating system  20 ,  120 . 
     A third converter control strategy comprises an over-modulation mode  88  in which a modulation index of the grid side converter  90  is increased to a value in an over-modulation range. The over-modulation range is determined by the determining module  50  based on the monitored operating parameters of the power generating system  20 ,  120 . Alternatively, a pre-determined over-modulation range may be stored on in the form of a lookup table on the storage device  55 . A fourth converter control strategy comprises a pulse wave modulation blocking mode  90  in which the grid side converter  90  is operated to inhibit pulse wave modulation and allow negative power flow through a DC link chopper of the DC link  38 . 
     Upon detecting the over-modulation condition, the determining module  50  is arranged to determine the severity of the over-modulation condition. To achieve this, the determining module  50  assigns the detected over-modulation with a severity value based on the magnitude of the required change in output voltage U o . The converter and transformer modules  52 ,  54  are then configured to initiate at least one of the converter and transformer control strategies based on the determined severity value. A first severity value corresponds to the grid voltage U G  being within a range that requires an output voltage U o  which can normally be output by the converter assembly  30  (i.e. a modulation index of less than 1 pu). The first severity value corresponds to a grid voltage U G  which corresponds to a modulation index in a linear range of control for the power generating system. A second severity value corresponds to the grid voltage U G  being within a range that requires the converter assembly  30  to generate a voltage that is within an over-modulation voltage range (i.e. a modulation index of between 1 pu and 1.1 pu). A third severity value corresponds to the grid voltage U G  being such that it requires a voltage that exceeds the over-modulation voltage range (i.e. a modulation index greater than 1.1 pu). The third severity value corresponds to a grid voltage U G  value being greater than a maximum synthesizable range of the power generating system. 
     Upon determining that the detected modulation corresponds to the first severity value, the transformer module  54  controls the tap changer  40  to tap down the transformer  36  according to the tap changer adjustment mode  86 , and the converter module  52  controls the converter assembly  30  to reduce the voltage generated across the grid choke of the DC link  38  of the converter in accordance with the reactive power absorption mode  84 . 
     In the situation where the determining module  50  determines that the detected modulation corresponds to the second severity value, then the transformer module  54  is configured to initiate the tap changer adjustment mode  86  and the converter module  52  initiates at least one of the reactive power absorption mode  84 , the over-modulation mode  88  and the U dc  adjustment mode  82 . 
     Upon determining that the detected over-modulation is within the third severity value range, the transformer module  54  is configured to initiate the transformer response mode and the converter module  52  is arranged to initiate at least one of the reactive power absorption mode  84 , the over-modulation mode  88 , the U dc  adjustment mode  82  and the pulse wave modulation blocking mode  90 . Advantageously, the determining module  50  is configured to engage more of the converter control strategies when a severe over-modulation condition is detected, in order to provide a sufficiently large voltage change as required by the grid  28 . 
     For each of the scenarios relating to the three different severity values, the converter module  52  may be configured to initiate one or more of the different converter control strategies  82 ,  84 ,  88  and  90 . Once the determining module  50  has determined which of the severity levels corresponds to the detected over-modulation condition, the determining module  50  is then also configured to determine which of the relevant converter and transformer control strategies should be initiated. The determining module  50  is also arranged to determine when the chosen control strategies should be initiated, and for how long. For example, in some instances the converter module  52  may initiate all of the relevant control strategies at the same time, and in conjunction with the transformer control strategy. 
     The determining module  50  is also arranged to determine the duration of each of the converter and transformer control strategies in order to provide an optimal response to the required voltage change. To achieve this, the determining module  50  is configured to assign a rank to each of the transformer and converter control strategies  82 ,  84 ,  86 ,  88  and  90 . The rankings, as shown in the table of  FIG.  11   , are determined based on the speed at which the control strategy can be implemented in order to counteract the over-modulation condition. The rankings also take into account the stability of the control strategy and their respective capacity to affect the over-modulation condition. 
     The reactive power absorption mode  84  can be applied for relatively long periods (i.e. a number of minutes) it does involve some degree of converter control, which can affect the overall operation of the converter assembly  30 . The U dc  adjustment mode  82  should be limited to a duration of just a few seconds because high DC voltages in the converter assembly  30  can impact the stability of an insulated-gate bipolar transistor of the power generating system  20 ,  120 . For example, the reliable operation of the transistor can be affected by cosmic rays. 
     The over-modulation mode  88  involves allowing the converter assembly  30  to operate with up to 10% additional voltage range and it can be applied quickly (i.e. within 1 to 2 seconds) but this approach can cause high-harmonic output from the converter assembly  30 . The pulse wave modulation blocking mode  90  can be applied quickly but it is most effective when only applied for a very short duration (i.e. less than 2 seconds) because it can generate a negative power flow which must be dissipated in the DC link chopper in order to prevent the voltage in the DC link  38  from increasing too much. The tap changer adjustment  86  can be applied indefinitely, and it is highly efficient but it has a slow response time because it takes several seconds from the initial operation of the tap changer  40  to adjust the transformer  36  before the actual reduction in the voltage can occur. 
     The rankings may be assigned differently to each the severity values, depending on the preferred means of counteracting the voltage demand. The rankings are pre-determined by the determining module  50 , according to the control method  160  and then stored in the storage device  55  in the form of a lookup table. Alternatively the rankings may be pre-loaded into the storage device  55  before the controller  42  is installed within the wind turbine  10 . 
     According to the exemplary arrangement of the control method  160 , as shown in  FIG.  11   , the determining module  50  is configured to assign a ranking of 1 to the tap changer adjustment mode  86 , and a ranking of 2 to the reactive power absorption mode  84 . Due to its greater ranking the reactive power absorption mode  84  will be initiated in preference to the tap changer adjustment mode  86  because the required voltage level is such that it can be addressed by the operation of the converter assembly  36  within its normal operation. If the over-modulation condition continues at the current level (i.e. severity level 1), despite the application of the reactive power absorption mode  84 , then the tap changer adjustment mode  86  is also initiated. The reactive power absorption mode  84  may then be cancelled after a pre-determined period, leaving the tap changer adjustment mode  86  to provide the required change in voltage, for an indefinite period, or until the over-modulation severity level changes. 
     If an over-modulation condition falling within the voltage range of the second severity level is detected, then the tap changer adjustment mode  86  is required to provide at least some of the required change in output voltage U o . Each of lower ranked converter control strategies (i.e. the reactive power absorption mode  84 , the over-modulation mode  88  and the U dc  adjustment mode 82) are initiated at the same time as the tap changer adjustment mode  86  in order so that they can start to address the over modulation in the period before the tap changer adjustment mode  86  can respond. 
     The converter control strategies are then cancelled after a pre-determined period of time according to their assigned ranking. The over-modulation mode  88  and the U dc  adjustment mode  82  are cancelled after a first determined period of time, for example, less than two seconds following the initiation of the converter response mode. The reactive power absorption mode  84  is then cancelled after a second determined period of time, for example one minute. Advantageously, the converter response mode provides a fast response which helps to mitigate the initial risk posed by the over-modulation condition. Then, once the transformer response mode has been implemented, the converter response mode can then be cancelled in a step-wise fashion, in order to increase the operational stability of the converter assembly  30 . 
     Droop Control Scheme  260   
     Each of the first and second control methods  60 ,  160  correspond to a wind turbine  10  which is operated to support the voltage and power levels in a grid  28 . According to a third aspect of the present invention the power generating system  20 ,  120  may be controlled to provide enhanced power generating capacity when operating in a grid-forming mode. 
     Grid-forming power generators are typically arranged to set the voltage that will be supplied to the loads on the grid. It is known to control power generating systems according to a droop control scheme when the wind turbine  10  is configured to operate in a grid-forming mode. Such control schemes are used, typically, in order to control power sharing within electrical power grids, thereby removing the need to provide separate communication networks to coordinate the operation of the power generating systems connected to the electrical grid. However, such droop control strategies can lead to voltage amplitude errors, which compromise the output power capability of the power generating system. 
     According to a third aspect of the present invention the power generating system  20 ,  120  is controlled according to a droop control scheme  260  - or control method  260  - in order to extend the power output capacity of the power generating system  20 ,  120  by controlling the tap changer  40  to overcome the deficiencies of known droop control strategies. 
     The control method  260  will now be described with reference to  FIGS.  12  and  13   . The control method  260  commences with a first step  262  in which the determining module  50  is arranged to determine that the power generating system  20 ,  120  is operating in a grid-forming configuration by outputting a reference voltage U ref  from the converter assembly  30  to the electrical grid  28 . The power generating system  20 ,  120  is also configured to update the reference voltage U ref  in order to maintain the grid voltage U G  within a pre-determined voltage range. 
     In a second method step  264 , the input  46  receives a signal for detecting that the grid voltage U G  requires an increase in output voltage U o  from the power generating system  20 ,  120 , i.e. in order to maintain the grid voltage U G  within the desired voltage range. The determining module  50  is arranged to monitor the input signal and determine whether the grid voltage U G  has dropped to a level which compromises the power output capability of the wind turbine  10  according to a predefined PQ chart. The characteristic drop in grid voltage is represented by the ΔU as illustrated by the schematic graph shown in  FIG.  13   . 
     The transformer module  54  is then configured, in a third method step  266 , to operate the tap changer  40  to tap up the transformer  36  in order to provide at least part of the voltage increase required to maintain the grid voltage U G  within the predetermined range. Hence, the transformer module  54  controls the tap changer  40  to tap back the output voltage U o  to a level which allows the power generating system  20 ,  120  to output the required power P, Q according to the predefined PQ chart without changing the converter voltage reference U ref . The control method  260  thereby provides increased flexibility when configuring the wind turbine  10  in a grid-forming configuration to provide PQ reference values to the electrical grid  28 . 
     Once the tap changer  40  has been adjusted then the reference voltage U ref  which was used previously to control the converter assembly  30  will no longer correspond to the conventional droop control, as determined by the determining module  50  in method step  262 . The change in the tap position of the tap changer  40  effectively causes a decoupling of the converter voltage reference U ref  from the grid voltage U G  such that the previously used converter voltage reference U ref  can no longer be used to control the converter assembly  30  in order to match the demands of the grid  28 . In this way, the previous converter voltage reference U ref  defines an old converter voltage reference U ref-   old  or an un-tapped reference voltage. 
     Consequently, if the old converter voltage reference U ref-old  were used to determine the future droop control strategy of converter assembly  30 , it would likely lead to errors in the grid-forming operation of the wind turbine  10 . For example, the controller  42  may incorrectly configure the power generating system  20 ,  120 , based on the old reference voltage U ref-old  to provide voltage to the grid  28  based on an inaccurate understanding of the voltage which is being outputted from the high-voltage side of the transformer  36 . 
     To address this problem, the controller  42  is configured in a further method step to update the converter voltage reference U ref-old  based on the tap position of the tap changer  40 . To achieve this, the controller  42  is configured to monitor a first input signal indicative of a tap position - or tap changer position - of the tap changer  40 , and a second input signal indicative of the old converter voltage reference U ref-old . The controller  42  is then configured to determine a new reference voltage U ref-new  based on the first and second input signals. 
     The first input signal comprises a tap changer control signal configured to control the tap position of the tap changer  40 . The second input signal is received from a droop control module (not shown) of the controller  42 , which is configured to control the converter assembly  30  when the power generating system  20 ,  120  is arranged in a grid-forming configuration and when the tap changer  40  is configured in a neutral position (i.e. not tapped up or tapped down). Accordingly, the droop control module is configured to operate the power generating system as if it were not fitted with a tap changer  40 , as would readily be understood by a person having ordinary skill in the art. 
     According to an exemplary aspect of the control method  260 , the determining module  52  is configured to calculate a new voltage reference U ref-new  using the following equation: 
     
       
         
           
             
               U 
               
                 r 
                 e 
                 f 
                 − 
                 n 
                 e 
                 w 
               
             
             = 
             
               
                 
                   n 
                   2 
                 
                 + 
                 Δ 
                   
                 n 
                 ∗ 
                 
                   N 
                   
                     T 
                     C 
                   
                 
               
               
                 
                   n 
                   2 
                 
               
             
             
               U 
               
                 r 
                 e 
                 f 
                 − 
                 o 
                 l 
                 d 
               
             
           
         
       
     
      The new converter voltage reference U ref-new  defines the voltage reference which is recalculated following the tap changer adjustment. In this way, the new converter voltage reference U ref-new  represents a ‘tap-adjusted’ converter voltage reference. Furthermore, n 2  represents the number of windings at the high-voltage side of the transformer; N TC  is the tap position of the tap changer; and Δ n is the change in the number of windings at the high-voltage side of the transformer  36  for a given tap position. 
     The control method  260  is particularly applicable for the determination of a voltage amplitude component of the converter voltage reference U ref . By recalculating the voltage amplitude component, based on the tap position of the tap changer  40 , the controller  42  is able to enhance the power output capability of the power generating system  20 ,  120 . 
     The droop control scheme  260  may be implemented to regulate the exchange of active P and reactive Q power within the electrical grid, in order to control the grid voltage frequency and amplitude. In this way, the droop control scheme  260  is arranged to decrease the delivered active power P when the grid voltage frequency increases and decrease the delivered reactive power Q when the grid voltage amplitude increases. The droop control scheme  260  can be implemented when the power generating system  20 ,  120  is operated in an islanded-mode and also in a grid-connected mode, as would be readily understood by the skilled person. 
     Fault Ride Through (FRT) Detection Scheme 
     The above described control scheme  260  is designed to operate the power generating system  20 ,  120  to control the voltage which is provided to the electrical grid  28  by the wind turbine  10 . With reference to the first method step  262  of control method  260 , the demand to adjust the output voltage U o  of the power generating system  20 ,  120  requires that the controller  42  is able to determine the voltage at the high-voltage side of the transformer  36 . This is needed in order to determine the extent to which the output voltage U o  of the power generating system  20 ,  120  must be adjusted in order to meet the demand from the grid  28 . 
     The presence of the tap changer  40  in the system means that the controller  42  is prevented from accurately determining the demand for a change in output voltage U o . In particular, the operation of the tap changer  40  (i.e. the tapping up or tapping down of the tap changer 40) disguises any voltage fault ride through (FRT) from the high-voltage side to the low voltage side of the transformer  36 . In this situation, the controller  42  may incorrectly configure the power generating system  20 ,  120  to provide reactive power to the grid  28  based on an inaccurate understanding of the voltage being outputted from the high-voltage side of the transformer  36 . 
     It is known to monitor the voltage at the high-voltage side of the transformer U HV  in order to detect FRT operating conditions. However, such techniques require additional voltage sensors to be accommodated within the power generating system, which therefore increases the cost and complexity of the wind turbine. 
     The controller  42  is arranged, according to an aspect of the present invention, to determine the voltage at the high-voltage side of the transformer  36 , without the need for additional monitoring equipment. Accordingly, the controller  42  is configured to execute a fault ride through detection scheme. To achieve this, the controller  42  is configured to monitor a first input signal indicative of a tap position - or tap changer position - of the tap changer  40 , and a second input signal indicative of the voltage at the low-voltage side of the transformer  36 . The controller  42  is then configured to control the output voltage U o  of the power generating system  20 ,  120  based on the first and second input signals. 
     The first input signal comprises a tap changer control signal configured to control the tap position of the tap changer  40 . Alternatively, the first input signal may include a tap changer sensor signal from a sensor which is configured to monitor the current tap position of the tap changer  40 . The second input signal comprises sensor data which is received from a voltage sensor that is arranged to monitor the voltage at the low voltage side of the transformer  36 . The voltage data may be measured at any suitable point between the converter assembly  36  and the low voltage side of the transformer  36 , as would be readily understood by the person having ordinary skill in the art. 
     According to a first FRT detection scheme, the determining module  52  is configured to calculate a virtual ‘low-voltage’ U LV-virtual  using the following equation: 
     
       
         
           
             
               U 
               
                 L 
                 V 
                 − 
                 v 
                 i 
                 r 
                 t 
                 u 
                 a 
                 l 
               
             
             = 
             
               
                 
                   n 
                   2 
                 
                 + 
                 Δ 
                   
                 n 
                 ∗ 
                 
                   N 
                   
                     T 
                     C 
                   
                 
               
               
                 
                   n 
                   2 
                 
               
             
             
               U 
               
                 L 
                 V 
               
             
           
         
       
     
      U LV  represents the measured voltage at the low voltage side of the transformer  36  whereas the virtual low voltage U LV-virtual  defines the voltage at the low voltage side of the transformer  36  if the tap changer  40  was not installed. Put another way, the virtual low voltage U LV-virtual  represents the voltage at the low voltage side of the transformer  36  when the tap changer  40  is configured in a neutral tap position.  FIG.  14    illustrates a schematic tap changer  40  of the power generating system  20 ,  120  according to the present invention. The parameter n 2  represents the number of windings at the high-voltage side of the transformer; N TC  is the tap position of the tap changer; and Δ n is a change in the number of windings at the high-voltage side for a given tap position. 
     Upon calculating the virtual low-voltage value U LV-virtual,  the determining module  50  determines whether either the ‘virtual’ or ‘measured’ low-voltage values (i.e. U LV-virtual  or U LV ) are within an FRT voltage range or a continuous voltage range. The FRT voltage range is a voltage range corresponding to a low or high voltage fault ride through condition. The continuous voltage range corresponds to a normal operating condition of the power generating system  20 ,  120 . 
     If the virtual low-voltage value U LV-virtual  is determined to be within the FRT voltage range and the measured low-voltage value U LV  is determined to be within the continuous voltage range, then the controller  42  is configured to generate a reactive current reference based on the virtual low-voltage value U LV-virtual  and an associated k-factor. All other control aspects of the power controller  42  are determined based on the normal operating condition of the power generating system  20 ,  120 . 
     Alternatively, if both the ‘virtual’ and ‘measured’ low-voltage values (U LV-virtual  and U LV ) are determined to be within the FRT voltage range, then the controller  42  is again configured to generate a reactive current reference based on the virtual low-voltage value U LV-virtual  and an associated k-factor. However, in this case the controller  42  is configured to operate the power generating system  20 ,  120  according to a conventional FRT control scheme, i.e. in the same way that would be expected for a power generating system which does not include a tap changer device (i.e. as if it were a directly measured high-voltage transformer voltage U HV ). 
     According to a second FRT detection scheme, the determining module  52  is configured to estimate a ‘high-voltage’ value U HV-estimate  according to the following equation: 
     
       
         
           
             
               U 
               
                 H 
                 V 
                 − 
                 e 
                 s 
                 t 
                 i 
                 m 
                 a 
                 t 
                 e 
               
             
             = 
             
               
                 
                   n 
                   2 
                 
                 + 
                 Δ 
                 n 
                 ∗ 
                 
                   N 
                   
                     T 
                     C 
                   
                 
               
               
                 
                   n 
                   1 
                 
               
             
             
               U 
               
                 L 
                 V 
                 + 
                 I 
                 ∗ 
                 
                   Z 
                   
                     H 
                     V 
                   
                 
               
             
           
         
       
     
      The estimated high-voltage transformer voltage U HV-estimate  represents the voltage at the high-voltage side of the transformer  36  if the tap changer  40  was not installed in the power generating system  20 ,  120 . The parameter n 1  represents the number of windings at the low-voltage side of the transformer  36 . The parameter / represents the current of the transformer  36  and Z HV  represents the impedance at the high-voltage side of the transformer  36 . The remaining parameters of the equation are the same as those described for the first FRT detection scheme. 
     The parameters / and Z HV  are each composed of real and imaginary parts as defined by the following equations: 
     
       
         
           
             I 
               
             = 
               
             
               I 
               
                 a 
                 c 
                 t 
                 i 
                 v 
                 e 
               
             
               
             + 
               
             j 
             
               I 
               
                 r 
                 e 
                 a 
                 c 
                 t 
                 i 
                 v 
                 e 
               
             
           
         
       
     
     
       
         
           
             
               Z 
               
                 H 
                 V 
               
             
             = 
             
               R 
               
                 H 
                 V 
               
             
             + 
             j 
             
               X 
               
                 H 
                 V 
               
             
           
         
       
     
      I active  and I reactive  represent the active and reactive currents corresponding to the transformer  36 , respectively. R HV  and X HV  represent the resistance and reactance of the transformer  36 , respectively. 
     According to an exemplary operation of the controller, the determining module  52  is configured to estimate the ‘high-voltage’ value U HV-estimate  according to the following simplified equation: 
     
       
         
           
             
               U 
               
                 H 
                 V 
                 − 
                 e 
                 s 
                 t 
                 i 
                 m 
                 a 
                 t 
                 e 
               
             
             = 
             
               
                 
                   n 
                   2 
                 
                 + 
                 Δ 
                   
                 n 
                 ∗ 
                 
                   N 
                   
                     T 
                     C 
                   
                 
               
               
                 
                   n 
                   1 
                 
               
             
             
               U 
               
                 L 
                 V 
               
             
             + 
             
               I 
               
                 r 
                 e 
                 a 
                 c 
                 t 
                 i 
                 v 
                 e 
               
             
             ∗ 
             
               X 
               
                 H 
                 V 
               
             
           
         
       
     
      The estimated high-voltage side transformer voltage U HV-estimate  is used by the controller  42  to detect whether there is an FRT condition associated with the power generating system  20 ,  120 . Furthermore, the estimated high-voltage transformer voltage U HV-estimate  can also be used in a conventional FRT control scheme in the same way as a directly measured high-voltage transformer voltage U HV . 
     According to the above described method of determining the voltage at the high voltage side of the transformer  36 , the controller  42  is able to adjust the power generating system  20 ,  120  based on either virtualised or estimated voltage data, and can thereby avoid FRT issues whilst avoiding the need to accommodate additional monitoring equipment within the wind turbine  10 . 
     Although the first and second FRT detection schemes have been described herein with reference to the controller scheme  260 , it will be appreciated that the same FRT detection methods may also be applied in a similar fashion to the control methods  60  and  160 . 
     Efficiency Control Scheme 
     Given the deployment of wind turbines in remote off shore locations, it is considered that a local wind power plant controller of a wind turbine may have access to more accurate weather estimation modelling and data mining capabilities than the traditional distribution system operators (DSOs) and transmission system operators. 
     Accordingly, the local power plant controller (PPC), such as the controller  42  described herein, is configured with additional functionality such as estimation of wind power and distribution. The controller  42  is arranged to receive information relating to how long the wind turbine  10  will be required to run in a high-power, mid-power and low-power configuration. 
     The determining module  50  is arranged to monitor a signal indicative of an operating time corresponding to local wind speed and distribution. The determining module  50  is configured to determine time value in which wind turbine is predicted to operate in each of the high-power, mid-power or low-power configurations. Each of the determined time values is matched to an active and reactive power reference P ref , Q ref  corresponding to the predicted output power of the converter assembly  36  for that power configuration. 
     The determined time values comprise time of day data as well as duration data corresponding to the predicted wind conditions for the location of the wind turbine  10 . The converter module  52  is configured to control the converter assembly  30  based on the determined time values in order to achieve the optimum active and reactive power outputs P, Q for any given operating period of the wind turbine  10 . The transformer module  54  is configured to control the tap changer  40  to adjust the transformer  36  in order to reach a desired voltage level based on the determined time values. The desired voltage levels increase the operational efficiency of the power generating system  20 ,  120  for a given active and reactive power output P, Q. 
     The determining module  50  is configured to produce an efficiency protocol based on determined time values and associated operating parameter data. The efficiency protocol is stored on the storage device  55  and can be updated by the determining module  50  based on new wind condition inputs. The controller  42  is arranged to then control the power generating system  20 ,  120  according to the efficiency protocol. The primary objective of the control method is to determine the most cost efficient operation of the power generating system  20 ,  120  based on the prevailing wind conditions. In particular, the controller  42  is configured to determine whether it is cost effective to adjust the tap changer  40  based on the predicted wind conditions, or whether it would be preferable to maintain the tap changer  40  in its current configuration. 
     If a high power wind condition is predicted to continue for a time period which exceeds a threshold value, then the tap changer  40  is configured to tap up the transformer in order to achieve a more efficient output from the power generating system  20 ,  120 . However, if the high power wind condition is predicted to terminate before exceeding the threshold time period, then the benefit of adjusting the tap changer  40  is reduced such that it will be retained in its current tap position. The required voltage change is then obtained by controlling the converter assembly  30 . 
     The control method  360  can thereby avoid the unnecessary movement of tap changer  40  to extend its lifetime and reduce long term maintenance costs associated with the power generating system  20 ,  120 . The control method  360  can also apply to the operation of the converter assembly  30 , which thereby enables greater control the output voltage U O . 
     The power generating system  20 ,  120  is described herein as comprising a single transformer  36 . The transformer  36  may comprise a medium or a high voltage transformer as would be understood by the skilled person. Furthermore, the power generating system  20 ,  120  may comprise one or more transformer devices without departing form the scope of the present invention. 
     Each of the processing modules  50 ,  52  and  54  are contained within the memory device  55  of the controller. The control protocol is also stored on the memory device  55  can be adapted by a user of the power generating system  20 ,  120  in order to suit the preferred operating parameters of a power plant controller (PPC) of the grid  28 , as would be readily understood by a person having ordinary skill in the art. 
     The processor  44  is configured to perform the computer-implemented functions of the processing modules  50 ,  52  (e.g. performing a control method as will be described in more detail below). The instructions when executed by the processor  44  cause the processor  44  to perform determining and controlling operations, including providing control commands to the various components of the power generating systems  20 ,  120 . 
     The controller  42  forms part of a central control system of the power generating systems  20 ,  120 . As such, the controller  42  may be incorporated into any number of computer based control systems of the power generating systems  20 ,  120 . It should be appreciated by the person having ordinary skill in the art that the controller  42  is described herein as being arranged in electronic data communication with the components of the power generating systems  20 ,  120  using a wired connection, as illustrated by the dotted lines in  FIGS.  2  and  3   . However, in other exemplary arrangements, the power generating systems  20 ,  120  may be coupled to the controller  42  via a wireless connection, such as by using any suitable wireless communications protocol known in the art. Thus, the controller  42  may be configured to receive one or more signals from the power generating system  20 ,  120 , wirelessly.