Abstract:
A wind turbine converter system with a rectifier and an inverter and a converter controller has at least first and second converter strings. The converter system is controlled by a master-converter controller and a slave-converter controller. The master-converter controller controls the first converter string and the slave-converter controller controls the second converter string. The master-converter controller receives commands from a superordinate wind turbine controller, provides the slave-converter controller with string-control commands on the basis of the superordinate control commands, and controls the conversion operation of the first converter string on the basis of the superordinate control command. The slave-converter controller receives the string-control commands from the master-converter controller and controls the conversion operation of the second converter string on the basis of the string-control commands received. The first and the second converter strings can be arranged in a bipolar configuration giving access to a neutral point. Fault detection can be performed based on current through the neutral. The system is capable of fault ride-through. Also, in case of failure of the master-converter controller, a redundant unit takes its place.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to wind-turbine converter control and, for example, to a wind-turbine converter system, a wind turbine with such a converter system, and method for converting variable-frequency electrical power produced by a variable-speed wind turbine into fixed-frequency electrical power to be fed into an electricity grid. 
       BACKGROUND 
       [0002]    EP 2 475 061 A1 describes a controller arrangement of a power transfer system of a wind turbine. The system has two converters and two controllers. Normally, the first and second controllers control the first and second converters, respectively. In the event of a fault of a converter controller control of the associated converter is handed over to the other controller, which then controls both converters. 
       SUMMARY OF THE INVENTION 
       [0003]    A wind-turbine converter system arranged to convert variable-frequency electrical power produced by a variable-speed wind turbine into fixed-frequency electrical power to be fed into an electricity grid is provided, wherein operation of the wind turbine is controlled by a wind-turbine controller. The wind-turbine converter system comprises a converter and a converter controller. The converter comprises a plurality of converter strings that are arranged to perform conversion operation, wherein at least a first converter string and a second converter string are provided. The converter controller comprises a plurality of converter-string controllers associated with the converter strings, wherein at least a first converter-string controller and a second converter-string controller are provided to control the conversion operation of the first and second converter strings, respectively. The first and second converter-string controllers are arranged to operate in a master-slave relation relative to each other. The first converter-string controller is arranged to operate as a master-converter controller, and the second converter-string controller is arranged to operate as a slave-converter controller. The master-converter controller is arranged to receive superordinate control commands from the wind-turbine converter system, to provide the slave-converter controller with string-control commands on the basis of the superordinate control commands, and to control the conversion operation of the first converter string on the basis of the superordinate control commands. The slave-converter controller is arranged to receive the string-control commands from the master-converter controller and to control the conversion operation of the second converter string on the basis of the string-control commands received. 
         [0004]    According to another aspect a wind turbine comprising such a converter system, a nacelle housing a generator and mounted on a tower is provided. 
         [0005]    According to another aspect a method is provided of controlling a converter for converting variable-frequency electrical power produced by a variable-speed wind turbine into fixed-frequency electrical power to be fed into an electricity grid. The operation of the wind turbine is controlled by a wind-turbine controller. The method comprises: performing conversion operation by a converter with a plurality of converter strings, including at least a first converter string and a second converter string; and controlling the converter by a converter controller having a plurality of converter-string controllers associated with the converter strings, wherein at least a first converter-string controller and a second converter-string controller are provided to control the conversion operation of the first and second converter strings, respectively. The first and second converter-string controllers operate in a master-slave relation relative to each other, the first converter-string controller operates as a master-converter controller, and the second converter-string controller operates as a slave-converter controller. The master-converter controller receives superordinate control commands from the wind-turbine controller, provides the slave-converter controller with string-control commands on the basis of the superordinate control commands and controls the conversion operation of the first converter string on the basis of the superordinate control commands. The slave-converter controller receives the string-control commands from the master-converter controller and controls the conversion operation of the second converter string on the basis of the string-control commands received. 
       GENERAL DESCRIPTION, ALSO OF OPTIONAL EMBODIMENTS OF THE INVENTION 
       [0006]    The wind-turbine converter system is arranged to convert variable-frequency electrical power produced by a variable-speed wind turbine into fixed-frequency electrical power to be fed into an electricity grid. 
         [0007]    Such converter systems of a variable speed wind-turbine convert three-phase AC power with a variable frequency, produced by a wind turbine generator in depending on wind speed to AC power of a fixed-frequency, e.g. 50 Hz or 60 Hz, to be fed into the grid. To achieve this, variable frequency AC current is first converted to DC current by the converter system and this DC current is converted to the AC current corresponding to the fixed-frequency electrical power. 
         [0008]    The operation of the wind turbine is controlled by a wind-turbine controller. The wind turbine controller provides the converter system with superordinate control commands, as for example, an active power target or reactive power target to be produced by the wind turbine with a corresponding power factor in response to grid side commands, for example, provided by a wind park controller. Furthermore, the wind turbine controller initiates e.g. certain operation modes, such as a shut-down operation mode, a synchronization mode (when starting up the wind turbine), or a fault ride through operation mode (when a grid fault, such as a voltage dip is detected). 
         [0009]    The wind-turbine converter system comprises a converter and a converter controller. The terms “converter” and “converter controller” are to be understood as functional entities. A converter is for example an entity that converts the aforementioned AC current of variable frequency to a DC current of a DC link and converters this DC current to an AC current of variable frequency, i.e. it performs a conversion operation. The converter controller is an arrangement that is capable of receiving superordinate control commands from the wind turbine controller, interpreting these commands and finally, of turning these commands into concrete control commands for the converter or to functional parts of the converter. The converter controller is further arranged to collect and analyze current status data from the converter and to send status reports on the basis of this collected data to the superordinate wind-turbine controller. 
         [0010]    The converter, controlled by the converter controller, includes a plurality of converter strings that are arranged to perform the conversion operation, wherein at least a first converter string and a second converter string are provided. The converter strings are a functional part of the converter system, as a converter string is capable of rectifying at least a part of the variable frequency AC voltage produced by the generator and/or is capable of inverting at least a part of the DC link voltage resulting from this rectification. The first and second converter strings are, for example, electrically coupled in parallel or, alternatively, the converter strings are fed by separate armature windings of the generator that are electrically isolated from each other and are coupled to separated transformer windings that are isolated from each other, i.e. the converter strings are separated and functionally parallel to each other. These converter strings may have a common DC link or may have separated DC links. 
         [0011]    The converter controller that controls the converter, is equipped with a plurality of converter-string controllers associated with the converter strings. Each of these converter-string controllers is, for example, arranged to control its associated converter string. Alternatively or additionally, the converter-string controller is, for example, arranged to control its associated string, as well as at least one other converter-string. In any case, at least a first converter-string controller and a second converter-string controller are provided to control the conversion operation of the first and second converter string, respectively. The conversion operation of the first and second converter-string is, for example, controlled by these first and second converter-string controllers by selecting switching states and switching times of the switching elements of the inverters of the respective strings. These switching elements are, for example, part of two level inverters with a three-phase current output and may be realized as insulated-gate bipolar transistor switches (IGBT switches). 
         [0012]    The term “inverter” is used in this document to refer to an element of a converter string that converts AC power to DC power as well as to an element of the converter string that converts DC power to AC power. 
         [0013]    The first and second converter-string controllers are arranged to operate in a master-slave relation relative to each other. The first converter-string controller is arranged to operate as a master-converter controller, and the second converter-string controller is arranged to operate as a slave-converter controller. This master-slave relationship between those two converter string controllers is further described below: 
         [0014]    The master-converter controller is arranged to receive superordinate control commands from the wind-turbine controller, such as an active power or reactive power target value to be fed to the electricity grid by the wind turbine. The master-converter controller is further arranged to provide the slave-converter controller with string-control commands on the basis of the superordinate control commands received from the superordinate wind turbine controller. This means that the string-control commands for the slave-converter controller are derived by the master-controller such that the superordinate control commands of the wind turbine controller are respected, e.g. the overall active power production target value, demanded by the wind turbine controller, is reached. 
         [0015]    These string-control commands for the slave-converter controller are, for example, adapted to a current energy conversion status of the second converter string, i.e. the converter string controlled by the slave-converter controller. Examples for the current energy conversion status are an amount of active power produced by the at least one inverter of this converter string and the voltage level at which this amount of active power is created or, to provide another example, a momentary phase angle between current and voltage produced by this at least one inverter of this converter string. 
         [0016]    The slave-converter controller is arranged to receive the string-control commands from the master-converter controller that were provided to the slave-converter controller as described above. The slave converter is arranged to control the conversion operation of the second converter string on the basis of the string-control commands received. Also this conversion operation is, for example, controlled by providing switching states and switching times for the switching elements of the inverters of the second string. 
         [0017]    The master-controller is further arranged to control the conversion operation of the first converter string on the basis of these superordinate control commands. The first converter string is the converter string associated with the master-converter controller. The master-converter controller derives string-control commands for the first string, i.e. the string associated with the master-converter controller, on the basis of the superordinate control commands received from the wind-turbine controller. These string commands are adapted to the current energy conversion status of the associated first converter string. These string commands for the first string, derived from the superordinate control commands, may then be used by the master-converter controller to control the conversion operation of the first string. 
         [0018]    Alternatively, the master-controller controls the conversion operation of the first string directly on the basis of the superordinate control commands, e.g. the master controller directly derives switching states and switching times for its associated string, i.e. the first string, and derives string control commands for the slave controller, i.e. the controller that is arranged to control the second string, on the basis of the superordinate control commands, but also with respect to the conversion operation of the first string. 
         [0019]    An example of a control sequence in the converter arrangement described above may be as follows: the wind turbine controller sets an active power target value to be provided to the grid by the wind turbine. Optionally, the master-converter controller may add an internal power consumption of the wind turbine to this active target value (the internal power consumption is the power consumed by auxiliary circuits in the wind turbine, such as circuits for safety and control functions. This power consumption is typically extracted from the active power supplied to the grid). The resulting active power target, optionally corrected by the internal power consumption, is then split to result in active power targets for the first and second string. These active power targets are provided as separate string-control command to the first and the second string. The conversion operations of these strings is controlled on the basis of these string commands, respecting the current active power production of the first and second strings. 
         [0020]    By introducing such a master-slave relationship between the converter-controllers for the respective converter-strings, it is ensured that only one of the converter-controllers, namely the master converter controller has to be reprogrammed if, for example, there is a change in the way string commands are derived from superordinate wind turbine controller commands changes or there is a change in the way power-conversion-control commands are derived. To stick with the above example, if the power required to cover the internal power consumption is only going to be extracted from the first converter string, and not from the power of all converter strings together, the string-control active power command for the first string would be an active power command from the wind turbine controller divided by the number of converter strings plus the internal power consumption. The active power targets for the other converter strings would still be the overall active power target divided by the number of converter strings. 
         [0021]    As the slave converter controller is arranged to receive only the corresponding string-control command from the master-converter controller and to follow this command, the slave-converter controller does not need to be reprogrammed if such changes in the processing of superordinate control commands occur. If both controllers were arranged to derive the string-control commands for their respective strings independently from each other, both controllers would have to be reprogrammed. 
         [0022]    Furthermore, only the master-converter controller has to be equipped with software and hardware that is needed to derive string-operation control commands on the basis of said superordinate control commands received by the wind-turbine controller. 
         [0023]    In some embodiments, the master-converter controller comprises a master-control module, and at least one string-operation-control module. These modules are, for example, (alt1) separate electronic circuits/processors physically integrated in the master-converter controller, meaning that these modules are separate physical entities. Alternatively (alt2) they are, for example, different processes performed concurrently by the same processors integrated into the master-converter controller, meaning that these modules are, e.g. programs performed on the same physical entity(-ies). The master-control module is arranged to derive string control commands on the basis of the commands received from the superordinate wind turbine controller. The string-operation control module is, for example, arranged to carry out these string-control commands. The master-converter controller is further equipped with an interface to the wind-turbine controller, and an interface to the slave-converter controller. The interface to the wind turbine controller enables the master-converter controller to receive commands and/or send status reports to the wind turbine controller. 
         [0024]    If alternative 1 mentioned above (alt 1) is implemented, the master-controller module may be equipped with interfaces to transmit and receive commands and status reports, which may, for example, be realized as serial interfaces, such as interfaces of a high speed Ethernet connection between these controllers, using, for example a transport control protocol, such as TCP, combined with an internet protocol, such as IP, for receiving commands and sending status reports. Other examples for such interfaces are high speed serial link (HSSL) interfaces, or control area network (CAN) interfaces. 
         [0025]    If alternative 2 mentioned above (alt 2) is implemented, the master-controller module and the at least one string-operation control module may, for example, be connected to common interfaces for both modules, as they are different processes, performed on the same electronic circuits/processors. 
         [0026]    In these embodiments, the slave-converter controller comprises at least one string-operation-control module and an interface to the master-converter controller. This interface is, together with the interface of the master-converter controller to the slave-converter controller, used to provide a “communication link” between the master-converter controller and the slave-converter controller. The interface of the slave-converter controller to the master-converter controller is, for example, an interface of the same type as the interface of the master-converter controller to the slave-converter controller. To provide an example, these interfaces are both high speed Ethernet interfaces or CAN interfaces. 
         [0027]    Thereby, the master-converter controller is arranged to receive the superordinate control commands via the interface to the wind-turbine controller, and to provide the string-control commands, derived on the basis of the superordinate control commands by the master-converter-controller, as mentioned above, to the slave-converter controller via the interface to the slave-converter controller. 
         [0028]    The slave-converter controller is arranged to receive the string-control commands from the master-converter controller via the interface to the master-converter controller. The at least one string-operation control module, comprised by the slave-converter controller, is, for example, arranged to carry out these string-control commands. 
         [0029]    In these embodiments, the slave-converter controller does not need to have an interface to the wind-turbine controller and is therefore not arranged to receive commands from, or send status reports to, the wind turbine controller. 
         [0030]    In some embodiments, the master-control module and the at least one string-operation-control module of the master-converter controller are integrated into a common master-converter-controller unit. If there is a plurality of string-operation control modules, at least one of these string-operation control modules is integrated, meaning that not all string-operation control modules are necessarily integrated into the master-converter control unit. To provide an example, the string-operation control module, arranged to control the generator side conversion from AC current provided by the generator to DC current, is integrated, while the string operation control module, arranged to control the grid side conversion from DC current to AC current is not integrated. Another example would be, that string-operation control units arranged to control energy dissipation of the corresponding converter strings are not integrated, while string operation control units, arranged to select switching states and switching times of the inverters of the corresponding string, are integrated into this unit. 
         [0031]    The master-converter-controller unit provides an internal communication link for string-control commands from the master-control module to the at least one string-operation-control module of the master-converter controller. If these modules are realized as separate electronic circuits/processors, which are physically integrated in the master-converter controller, the internal communication link is, for example, a physical communication link between the electronic circuit/processor representing the master-controller module and the electronic circuit(s)/processor(s) representing the at least one string-operation-control module, e.g. an Ethernet connection or a common bus connection between those separated electronic circuits/processors. 
         [0032]    If these modules are realized as different processes performed concurrently by the same processors integrated into the master-converter controller, the internal communication link is, for example, a program interface between the processes provided by the underlying operating system and enabling inter-process communication. 
         [0033]    In some embodiments, the converter strings comprise grid-side and generator-side inverters. The generator-side inverter convers variable frequency AC-current into DC current, whereas the grid-side inverter converts DC current into AC current. 
         [0034]    In these embodiments the master-converter controller and the slave-converter controller comprise a grid-side string operation-control module to control the conversion operation of the grid-side string inverter belonging to their associated string. To provide an example, the switching states and switching times of the switches of the grid-side string inverter, being responsible for the DC-current to fixed frequency AC-current conversion part the strings conversion operation, are controlled by the grid-side string operation-control module of the master-converter controller. 
         [0035]    The master-converter controller and the slave-converter controller further comprise generator-side string-operation-control module(s) to control the conversion operation of the generator-side inverter belonging to their associated string. To stick with the example provided above, for example, also the switching states and switching times of the switches of the generator-side string inverter, being responsible for the variable frequency AC current to DC current conversion are controlled by the generator-side string operation-control module of the master-converter controller. 
         [0036]    The grid-side operation-control module and the generator-side string operation control module of the respective master-converter controller or slave-converter controller can be separated from each other, as the grid-side operation control module and the generator-side operation control module are associated with different tasks, namely controlling the grid-side or generator-side inverter(s) of a respective converter string. Therefore, the grid-side operation control module is, for example, located near the grid-side inverter(s) of the converter string and the generator-side operation control module is, for example, located near the generator-side inverter(s). Hence, these operation control modules can, for example, be directly connected to the respective generator-side or grid-side inverters, leading to the result that no costly/lengthy couplings from these operation control modules to their respective inverters are needed, as would be needed if these control modules were remotely located from the inverters they control. 
         [0037]    In some embodiments, the at least one string-operation-control module, integrated along with the master control module in the common master converter-controller unit is the grid-side string-operation control module. This grid-side string-operation-control module, for example, receives string commands from the master control module via the internal communication link and derives, also in dependence on status reports received from the generator-side string-operation-control module, (i) control commands that the grid-side-string-operation control module applies itself to the grid side inverters and (ii) control commands for the generator-side string-operation-control module. 
         [0038]    Alternatively, the generator-side string-operation-control module is integrated along with the master-control module in the master converter controller unit. Then, for example, the generator-side-operation-control module derives control commands for the grid-side string-operation-control module and receives status reports from the grid-side module, in analogy to the example described above. 
         [0039]    This arrangement of generator-side and grid-side string-operation-control modules makes further splitting of control commands for generator-side/grid-side string-operation-control modules possible. Thereby, with this arrangement it is possible to control the grid-side/generator-side inverters by the generator-side or grid-side string-operation-control module in dependence on the controls received from the respective superordinate string-operation-control module (either the generator-side string-operation-control module or the grid-side string-operation-control module). 
         [0040]    In another alternative embodiment, both the grid-side and the at least generator-side string-operation-control modules are integrated along with the master-control module into the common master-converter controller unit. 
         [0041]    In some embodiments, the slave-converter controller is also equipped with a master-control module and is arranged to continue master-control operation in place of the failed master-converter controller, upon a failure of the master-converter controller. 
         [0042]    A failure of the master-control module of the master-converter controller does not imply that the entire master-converter controller suffers a fault, but rather that only its master-control module is faulty and the string-operation control module of said master-converter controller is still capable of performing normal string-control operation. 
         [0043]    Such a capability of providing full operation of the converter control system, regardless of a fault of one of its subcomponents, namely the master-converter controller, is also referred to as “failover”. The master-converter controller module and the slave-converter controller module include each at least both a master control module and at least one string-operation-control module, as well as internal communication links between these master-controller modules and the string-operation-control modules of the respective master or slave-converter controller module. 
         [0044]    Furthermore, the slave-converter controller and the master-converter controller are each equipped with an interface as described above, so that these controllers are able to communicate with each other. In these embodiments, the slave-converter controller also has an interface to the superordinate wind turbine controller. Therefore, the slave converter controller is also arranged to receive superordinate control commands from the wind-turbine converter system, to provide the failed master-converter controller with string-control commands on the basis of the superordinate control commands, and to control the conversion operation of its associated converter string on the basis of the superordinate control commands. The failed master-converter controller is further arranged to receive the string-control commands from the slave-converter controller now acting as master-converter controller and to control the conversion operation of its associated converter string on the basis of the string-control commands received, in the event of a fault of its master-converter control module. 
         [0045]    During normal operation, i.e. if the master control module of the master-converter controller operates without a fault, the master control module of the slave-converter controller is, for example, as a default setting, inactive. In the event of a fault of the master-converter controllers master control module this module sends, for example, an automatically issued emergency hand-over request to the slave-converter controller module over the interfaces coupling them. In response to this signal, the first inactive master control module of the slave-converter controller becomes active, and thereby the former slave-controller module continues the master controller operation. 
         [0046]    Alternatively, the master-converter controller&#39;s master control module, for example, sends “alive signals” continuously to the slave converter-controller module. If these alive signals cease, upon a fault of the master control module, the master control module of the slave-converter controller becomes active. 
         [0047]    The provision of such a failover functionality, it is ensures that the converter system continues normal control operation, even in the event of a fault of a master control module. 
         [0048]    In some embodiments in which the converter strings comprise grid-side string inverters and generator-side string inverters, the generator-side string inverters of the plurality of converter strings are connected in series. Furthermore, in these embodiments also the grid-side string inverters of the plurality of converter strings are electrically connected in series. 
         [0049]    In these embodiments, each converter string, for example, has a plurality of grid-side string inverters that are electrically connected in parallel and/or a plurality of generator-side string inverters that are electrically connected in parallel. These parallel connected grid-side or generator-side inverters are further referred to as “inverter threads”. Such inverter threads reduce the maximum current passing through each of the electrically parallel connected inverters, and thereby reduce stress on the semiconductor switches of these inverters. 
         [0050]    Due to the series connection of the generator-side and/or grid-side string inverters or corresponding inverter threads the total DC voltage outputted by the generator-side string inverters (or inverter threads) is increased, and the DC voltage being inverted to AC voltage by the grid side by each grid-side string inverter (or each grid-side inverter thread) is reduced for each grid-side string inverter (or inverter thread). To provide an example, due to the series connection of two simple generator-side string inverters the overall output voltage provided by these two inverters is increased (here doubled) compared to the output voltage of a single generator-side string inverter. On the other hand, at a given DC voltage level two series connected grid-side string inverters only see a voltage drop among them that is half the given DC voltage level. 
         [0051]    In some embodiments, the series connection of the generator-side string inverters and the series connection of the grid-side string inverters are connected back-to-back with their DC sides, thereby forming a common DC link. 
         [0052]    In these embodiments, the common DC voltage level outputted by the series connection of the generator-side string inverters or corresponding generator-side inverter threads of different converter strings, is fed to the series connected grid-side string inverters or corresponding grid-side inverter threads of different converter strings. 
         [0053]    To achieve such a series connection of generator-side string inverters and grid-side string inverters on both sides (generator-side and grid-side), the generator-side string inverters are, for example, connected to separate armature windings of the generator that are isolated from one another and the grid-side string inverters are, for example, connected to separated transformer windings that are isolated against one another. 
         [0054]    By providing the common DC link with increased DC output voltage, it is possible to transport power from the grid-side string inverters to the generator-side inverters with conductors, such as copper cables with increased electrical resistivity, and hence, lower diameter and greater length. This is the case, as the transmitted voltage is increased and therefore, to yield the same transmitted power, the current can be decreased accordingly, corresponding to a respective higher electrical resistivity of the conductor. Hence the amount of conductive material needed to transmit power from those generator-side inverters to those grid-side inverters is reduced and, moreover, also current-dependent power losses are reduced. 
         [0055]    In some embodiments, the series connection of the generator-side string inverters and the series connection of the grid-side string inverters each form a neutral-connection point and the neutral-connection points of the generator-side and of the grid-side are connected to each other by a center-connection line and these connection points are clamped to neutral potential. 
         [0056]    In these embodiments, the positive DC level output of the generator-side string inverter(s) of a certain string, for example the first string, is applied to a positive DC conductor, whereas the negative DC level output level of the generator-side string inverter(s) of this string is applied together with a positive DC level output of the generator-side string inverter(s) of, for example the second string, to the center-connection line that in turn is clamped to neutral potential. The negative DC level output of the second string is applied to a negative DC conductor. 
         [0057]    Thereby, a series connection of grid-side string inverters is achieved, forming a neutral connection point as mentioned above. 
         [0058]    The positive DC level input of the grid-side string inverter of, for example the first string, is connected to the positive DC conductor. The negative DC level input of this grid-side string inverter is together with the positive DC level input of the grid-side inverter of the second string connected to the center connection line. The negative DC level input of the grid-side inverter of the second string is connected to the negative DC conductor. 
         [0059]    Since the current generated by the wind turbine generator is split into two current paths, i.e. the positive DC conductor and the negative DC conductor, the current dependent power losses within each path are reduced. Provided that the positive DC conductor and the negative DC conductor have the same absolute potential, for example 1 kV between the positive DC conductor and the center-connection line and −1 kV between the center-connection line and the negative DC conductor, no current flows on this center-connection line. However, should the absolute value of these potentials differ from each other, an (undesired) compensating current will flow between the two converter strings. 
         [0060]    In some embodiments, the grid-side inverters of a string are located in the tower of the wind turbine near to the transformer windings that couple the wind turbine to the grid and the generator-side inverters of a string are located in the nacelle of the wind turbine, in these embodiments the string-operation controller and the master-converter control module comprised by a common master-converter-controller unit, are for example, also located in the tower of the wind turbine, near to the transformer windings. In this way fewer components of the converter system are located in the nacelle, since only the grid-side inverters of the converter strings are located in the nacelle and maintenance either of the master-converter-controller unit, for example, by reprogramming this unit and also maintenance of the grid-side inverters is eased, as the tower of a wind turbine is easier to access than the wind turbines nacelle. Furthermore, the overall weight of converter system components located in the nacelle is reduced, leading to reduced stress on a yaw bearing, connecting the nacelle to the tower and enabling the nacelle to rotate about an axis given by the tower of the wind turbine. In addition, less heat is produced in the nacelle, as only the generator-side inverters, producing heat during the conversion operation, are located in the nacelle. This leads to further reduction of thermal stress on the generator-side as well as the grid-side inverters. 
         [0061]    In these embodiments the center-connection line, the positive DC conductor and the negative DC conductor as well as the neutral center connection line are guided from the nacelle to the tower via a common DC connection line, electrically isolated from each other. The length of these conductors and the DC connection line corresponds to the distance between the grid-side inverters (tower) and the generator-side inverters (nacelle). These conductors extend therefore over a distance from, e.g. 5 to 300 m. 
         [0062]    In some embodiments, the master-converter controller is arranged to combine operation electrical data relating to the converter string associated with the master-converter controller with operation electrical data relating to the converter string associated with the slave-converter controller and obtained from the slave-converter controller. The master-converter controller is arranged to provide the combined operational electrical data to the wind-turbine controller. 
         [0063]    Operational electrical data relating to a converter string may include, for example, the amplitudes and phases of the voltage and currents at the generator and the grid side of the individual strings, active/reactive power output of the individual strings, a momentary voltage of the string, e.g. the voltage between the center connection line and the DC voltage in the DC link associated with the individual string, for example, measured in the converter string associated with the master-converter controller as well as in the converter string associated with the slave-converter controller. 
         [0064]    In these embodiments, the master-converter controller combines, for example, the currents and/or active/reactive powers and/or the DC link voltages of the first string—associated with the master-converter controller—and the second string—associated with the slave-converter controller. 
         [0065]    In embodiments in which the combined operational electrical data represents first of all electrical quantities, the master-converter controller combines for example, the momentary power active and reactive power output of the aforementioned first converter string and the aforementioned second converter string is determined. These active/reactive power outputs of the strings are, for example, combined by the master-converter-controller to obtain, a total active power output for the converter strings and/or a total reactive power output of the strings. To provide another example, also a combined power factor for the power output of all the strings together is determined by the master controller on the basis of these total active and reactive power outputs. 
         [0066]    This exemplary operational electrical data is provided to the wind turbine controller, which then adjusts the commands provided to the master-converter-controller according to the combined operational electrical data received from the master-converter-controller. 
         [0067]    In some embodiments the string control commands provided by the master-converter controller comprise at least one of active-power and reactive-power targets for the electrical power to be fed into the grid by the first and second converter strings. As already mentioned above and also hereinafter, on the basis of total active/reactive power commands received by the master-converter controller from the wind turbine controller, string-control commands regarding the active/reactive power production of the respective strings are derived, for example, by the master convert control module and then distributed to the string operation module of the master-converter controller (controlling the first converter string associated with the master-converter controller) and the string operation control module of the slave converter controller (controlling the second converter string—associated with the slave-converter controller). The active power and reactive power targets are, for example, provided as target values in various mathematically equivalent forms. For example if n converter strings are controlled by the master-converter controller(s) and the slave-converter controller(s), a set of target values provided by the wind turbine controller is “x MW” and “z MVAR”, and the targets for the converter strings, i.e. the string-operation commands are then given by “x/n MW” and “z/n MVAR”. 
         [0068]    These target values for the converter strings may, however, be expressed in other mathematical equivalent forms, such as an apparent power multiplied by the power factor, i.e. the cosine of a given phase angle between current and voltage produced by a converter string, yielding the active power target in MW, and the apparent power multiplied by the sine of a given phase angle between current and voltage, yielding the reactive power target in MVAR. 
         [0069]    Also the overall active/reactive power targets provided by the superordinate wind turbine controller may be provided in these mathematical equivalent forms. 
         [0070]    The first and second converter-string controllers are arranged to control their associated converter strings according to the at least one of the active-power and reactive-power targets independently of each other. 
         [0071]    In some embodiments, in which the series connection of the generator-side string inverters and the series connection of the grid-side string inverters are connected back-to-back with their DC sides and the series connection of the generator-side string inverters and the series connection of the grid-side string inverters each form a neutral-connection point and the neutral-connections points of the generator-side and of the grid-side are connected to each other by the aforementioned center-connection line, the master-converter controller is arranged to measure current in the center-connection line and is arranged to limit the current flowing in the center-connection line. Measuring the current on the center-connection line is, for example, achieved by current-sensor(s), integrated within the master-converter-controller unit and connected to the master-control module. 
         [0072]    The current flowing on the center-connection line is, for example, reduced by adjusting active and reactive power targets for the converter strings, the neutral connections points of which are connected by this center-connection line, e.g. the converter-string associated with the master-converter controller and the converter-string associated with the slave-converter controller. If the active and/or reactive power targets are unequal for these strings, the absolute of the positive DC voltage between the positive DC conductor and the grounded center-connection line and the absolute of the negative DC voltage between the negative DC conductor and the grounded center-connection line are, in general, not equal. This is the case, as the generator-side inverters of the string feed active and/or reactive power into the common DC link according to these target values for the strings and the grid-side inverters so to say “extract” power from this common DC link according to these target values for the strings. When the active or reactive power fed to or extracted from a potential difference between the positive DC conductor and the center connection line is unequal to the power fed or extracted from a potential difference between the negative DC conductor and the center connection line, the voltage levels between the respective positive or negative DC conductors and the center connection line will differ, leading to said current on the center line. 
         [0073]    Therefore, in normal operation mode, the master-converter-controller, for example, derives approximately equal active/reactive power target values for the string operation control module of the master-converter-controller, arranged to control the converter string associated with the master-converter controller, and for the string operation control module of the slave-converter controller, arranged to control the converter string associated with the slave-converter controller. This is achieved, as explained above, for example by either directly dividing the active/reactive power targets received from the wind turbine controller by two, or by first adding the internal power consumption of the wind turbine to these target values received and then dividing the result of this addition by two. These target values are, for example, transmitted to the string-power control modules via the aforementioned interfaces or internal communication links. 
         [0074]    To compensate for component differences in the converter strings, as may arise if the inverters of different converter strings do not have exactly equal switching properties, or if there are cable property deviations between the negative DC conductor and the positive DC conductor, the active power/reactive power targets provided to the string-operation controllers are, for example, unbalanced on purpose. The active/reactive power targets are, for example, adjusted by a proportional integral controller (being part of the master-converter control module) that modifies the active/reactive power targets for each string in order to minimize the current flowing in the center-connection line. 
         [0075]    Alternatively, the master-converter controller stops the converter operation if the current flowing in the center-connection line is above a given limit. As the current flowing in the center connection line is usually not higher than ten per cent of current flowing in the positive DC conductor or the negative DC conductor, the diameter of, for example, a copper cable used as the center connection line, is reduced accordingly. Hence, the electrical resistance of the center connection line is high compared to the resistivity of the positive or negative DC conductors. Therefore, less current can be transmitted over the center connection line compared with these two conductors. Hence, the current limit, that leads to a stop of converter operation is, for example, chosen so that a maximum transmittable current over the center connection is not exceeded. 
         [0076]    An increased current flowing in the center connection line may also be used, among other measured parameters, as an indicator for a converter fault, as in the case of a converter fault, the absolutes of the voltage levels between the positive DC conductors/negative are inevitably not equal. Thus, for example, if the current in the center line exceeds such a limit, indicating a converter fault, the converter operation is stopped. 
         [0077]    When the converter operation is stopped, the current flow on the center-connection line is automatically limited. 
         [0078]    In some embodiments each of the converter strings is equipped with at least one energy dissipator, wherein the converter system is arranged to control energy dissipation by means of the energy dissipators individually for each string. 
         [0079]    The function of these dissipators is to dissipate active power by converting the active power into heat. Such dissipators are typically Ohmic resistors with a high thermal capacity. The energy dissipators of a string are, for example, activated if the grid cannot absorb sufficient active power, as for example, in the event that the active power at a point of common coupling of a wind park in which the wind turbine having these converter strings is operated, is already at its active power limit, hence further pushing active power into the grid might damage conductors/transformers at the point of common coupling. These energy dissipators are, for example, also activated if the absorption of active power by the grid is too slow. This may be the case in embodiments with generator-side inverters located in the nacelle and grid-side inverters located in the tower of the wind turbine, as the conductors (representing the extended DC link) between these inverters are extended over a range of 5-300 m, an overvoltage might occur in this extended DC link that cannot be transferred quickly to the grid, due to the inductance of these extended conductors. Hence, in this case a temporary overvoltage in the DC link occurs, that is, for example, dissipated by these energy dissipators in order to prevent damage to the DC link&#39;s conductors. 
         [0080]    The energy dissipators are, for example, located in this common DC link, more precisely the energy dissipator of the first converter string is, for example, connectable to the positive DC conductor and the center-connection line, whereas the energy dissipator of the second converter string is connectable to the negative DC conductor and the center-connection line. There are, for example, two energy dissipators for each converter-string, one located in the DC link near the generator-side inverters, i.e. in the nacelle, and the other also located in the DC link, near the grid-side inverters, i.e. in the tower of the wind turbine. The nacelle-side energy dissipators are, for example, capable of dissipating a greater amount of active power than the grid-side energy dissipator. The grid-side energy dissipators may therefore also be heavier than their nacelle side counterparts. Since they are located in the tower, the overall weight of power conversion equipment located in the nacelle is further reduced. 
         [0081]    The energy dissipators, are, for example, activated by closing motorized chopper-switches that establish an electric connection between the energy dissipators and those conductors. In other words, a series connection between a chopper-switch and an energy dissipator sets up an energy dissipation arrangement. 
         [0082]    The activation of the energy dissipation elements and the amount of active power dissipated is controlled independently for each converter string by the string-operation controllers that control the conversion operation of the converter string associated with them. These string-operation controllers are arranged to receive commands from the master converter control module that requests the use of the energy dissipators of at least one string. The master converter controller issues that request, for example, in dependence on a DC-link voltage based criterion, such as the DC voltage of a converter string exceeding a given threshold by more than a given over-voltage. When considering the exemplary converter system with two converter strings and the common DC link, the energy dissipators of a converter-string are, for example, activated if the voltage between the positive DC conductor and the center-connection line exceeds a given threshold of 500V by more than the given over-voltage of 30V. 
         [0083]    Energy dissipation is, for example, controlled by a dissipation related string-operation control module, whereas the switching states and switching times of a string are controlled by a different string operation control module, as there are, for example, a plurality of these string operation control models controlled by a single master-converter control unit. 
         [0084]    In some embodiments, the converter system is arranged to provide a low-voltage ride-through function controlled individually for each converter string. The low-voltage ride through function enables the wind turbine to continue operation even when transient low voltage events, also referred to as “voltage dips”, occur in the grid. The converter strings, for example, stop feeding power into the grid when the voltage dip is very deep; in the event of a very deep dip (typically to a voltage less than 15% of the nominal voltage) the converter strings may even loose synchronization, but they can resynchronize to the grid phase when the grid voltage comes up again towards nominal voltage. In the event of a voltage dip the energy dissipators are actuated independently for each string to dissipate the produced power that cannot be fed to the grid. The string controllers may keep sensing the current phase angle and amplitude of the grid voltage. 
         [0085]    When the grid-voltage returns the two string controllers return to normal operation, and will output the commanded power given by the master controller, using the grid voltage angle provided by the individual string controllers. 
         [0086]    If there is a voltage peak in the grid, following the low-voltage event, due to self-induction, the energy dissipators are, for example, actuated independently for each string to prevent this voltage peak from damaging converter components, as they are activated when an excess voltage in the DC link is sensed. 
         [0087]    The converter system is, for example, also equipped with an uninterruptable power supply, e.g. a battery, which serves as the internal power supply, when the actual internal power supply of the wind turbine is offline due to the desynchronization of the wind turbine. 
         [0088]    If such low voltage events also an increased amount of reactive voltage may be fed to the grid, by the respective converter strings. The voltage of each converter-string is, for example, sensed by the string operation control module of the respective string and reactive voltage is fed to the grid by the converter-strings independently from each other, on the basis of the sensings obtained by the respective string operation control module and the active/reactive power targets received from the wind turbine controller. 
         [0089]    All the low voltage ride-through activities may also be performed as preprogrammed procedures of the string-operation controllers, independently for each string. For example, a low voltage ride through mode is initiated by the master-converter controller, activating these preprogrammed procedures, such as activating the energy dissipators or feeding voltage and current of a certain phase (angle) to the grid via the separated transformer windings. 
         [0090]    According to a second aspect, a wind turbine having a nacelle mounted on a tower and housing a generator is equipped with any embodiment of a converter system previously described. 
         [0091]    The generator-side string-inverters of the converter system are arranged in the nacelle of the wind turbine, near the wind turbine generator and each phase input of these inverters is, for example, electrically coupled to phase outputs of a corresponding isolated generator winding. The grid-side string-inverters of the converter system are arranged at a lower part of the tower, inside or outside the tower. The grid-side string operation control module is as mentioned above, for example, integrated along with the master-converter control module in a master-converter control unit. This master-converter control unit is, for example, located at the lower part of the tower. 
         [0092]    The master-converter controller has, for example, also a generator-side string operation control module, located in the nacelle, and thereby close to the generator-side inverters to which it is connected. The grid-side and generator side string-operation control modules are connected to each other, wherein the grid-side string operation control module is arranged to provide the generator-side string control modules with commands. The grid-side string-operation control module, for example, derives these commands on the basis of commands of the master-converter control module received for this string that is associated with the master controller. In this example, the generator-side string operation controller was subordinate to the grid-side string operation controller. However, this subordinate relation can also be interchanged. 
         [0093]    As mentioned above, by locating the master-converter controller and its components at the lower part of the tower, it is easy to access for reprogramming said master-converter controller and its components or carrying out maintenance procedures. Furthermore, the overall weight of the power conversion components in the nacelle is reduced and thermal stress on the power conversion components located in the nacelle is also reduced. 
         [0094]    The generator-side and grid-side inverters are connected by the aforementioned DC connection line along the tower. This DC connection line may be realized as a duct for the positive DC conductor, the center connection line and the negative DC conductor, as mentioned above, in embodiments with a center connection line and a common DC link. 
         [0095]    In alternative embodiments, without such a center connection line but with a common DC link, only the positive DC conductor and the negative DC conductor may be part of the DC connection line. 
         [0096]    In embodiments without a common DC link, meaning that there is a plurality of converter strings each having a separate DC link, a plurality of positive and negative DC conductors are part of the DC connection line. 
         [0097]    According to a third aspect, a method of controlling a converter for converting variable-frequency electrical power produced by a variable-speed wind turbine into fixed-frequency electrical power to be fed into an electricity grid is provided. The operation of the wind turbine is controlled by a turbine controller. The method for controlling the converter is carried out by performing the following activities:
       performing conversion operation by a converter with a plurality of converter strings, including at least a first converter string and a second converter string.   controlling the converter by a converter controller that is equipped with a plurality of converter-string controllers associated with the converter strings, wherein at least a first converter-string controller and a second converter-string controller are provided to control the conversion operation of the first and second converter strings, respectively.       
 
         [0100]    The first and second converter-string controllers operating in a master-slave relation relative to each other. The first converter-string controller operates as a master-converter controller, and the second converter-string controller operates as a slave-converter controller. This master-slave relation between the converter-string controllers has already been described above in conjunction with the converter-system of a wind turbine. 
         [0101]    The master-converter controller receives superordinate control commands from the wind-turbine controller and provides the slave-converter controller with string-control commands on the basis of the superordinate control commands, such as an active power or reactive power target value to be produced by the wind turbine ore one of the plurality of commands already discussed above. 
         [0102]    The master-converter controller controls the conversion operation of the first converter string on the basis of the superordinate control commands. Hence, commands received from the wind turbine controller are, for example, directly transformed into commands controlling the conversion operation of the first string directly, such as commands including switching angles or switching times of the inverters of the first converter string that is associated to the master-converter controller. 
         [0103]    Alternatively, string-control commands are derived by the master-converter-controller for the first string, and these are received by a string-operation control module, which in turn controls the conversion operation of the first string associated to the master-converter controller. 
         [0104]    The commands controlling the conversion operation of the first string directly or the string-control commands for that string are derived by the master-converter controller so that the such that the superordinate control commands of the wind turbine controller are respected, e.g. the overall active power production target value, demanded by the wind turbine controller, is respected. 
         [0105]    The master-converter controller provides the slave-converter controller with string-operation control commands derived by the master-converter controller on the basis of the superordinate control commands received from the wind turbine controller. The master-converter controller derives the string commands for the second string, which are carried out, by e.g. a string-operation control module of the slave-converter controller—associated with the second string. The commands are derived in such a way that, that when the string control commands provided to the slave-converter controller are carried out following the commands derived, and also the conversion operation of the first string is carried out following the commands derived on the basis of the superordinate control commands, the superordinate control commands by the wind turbine controller are respected. 
         [0106]    The slave-converter controller receives the string-control commands from the master-converter controller and then controls the conversion operation of the second converter string on the basis of the string-control commands received. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0107]    Exemplary embodiments of the invention are now described, also with reference to the accompanying drawings, wherein 
           [0108]      FIG. 1  schematically illustrates a wind turbine equipped with a converter system including a generator-side inverter located in the nacelle and a grid-side inverter located in the tower of the wind turbine, 
           [0109]      FIG. 2  schematically illustrates a converter control system with no redundant components, with a generator-side string operation control module arranged inside and a grid-side operation control module arranged outside of a master-converter controller unit, 
           [0110]      FIG. 3 a    illustrates the schematic converter control system of  FIG. 2  further equipped with redundant components for providing failover functionality, in the moment when a hand-over request is transmitted from a faulty master-control module to a redundant master-control module of the slave-converter controller. 
           [0111]      FIG. 3 b    illustrates the schematic converter control system of  FIG. 3 a   , after the handover request, with the slave-converter controller continuing master-operation, 
           [0112]      FIG. 4  illustrates the schematic converter control system of  FIG. 2 , wherein the generator-side string operation control module is integrated into the master-converter controller unit, and the grid-side operation control module is located outside this unit, 
           [0113]      FIG. 5  illustrates a circuitry diagram, representing a converter system with grid-side and generator-side inverter threads, forming a first and a second converter string, wherein the first and second converter string are coupled by a common centre connection line, 
           [0114]      FIG. 6 a    illustrates a circuitry diagram with a series connection of generator-side string inverters, 
           [0115]      FIG. 6 b    illustrates a circuitry diagram with a series connection of grid-side string inverters, 
           [0116]      FIG. 7  illustrates a connection of the circuitry diagrams of  FIG. 6 a    and  FIG. 6 b    by a common DC link, thereby forming two converter strings that are coupled by this common DC link, 
           [0117]      FIG. 8  illustrates a circuitry diagram with electrically separated converter strings. 
           [0118]      FIG. 9  illustrates a block diagram of a method of controlling the converter system, wherein the master-control module derives string control commands for the converter strings, 
           [0119]      FIG. 10  illustrates a block diagram of a derivation of active power target and reactive power target values for the converter strings, related to a converter topology as illustrated by  FIG. 5  or  FIG. 7 . 
           [0120]      FIG. 11  illustrates a flow chart of the provision of operational electrical data to a wind turbine controller that adjusts the active/reactive power targets according to this data received. 
       
    
    
       [0121]    The drawings and the description of the drawings are of examples of the invention and are not of the invention itself. 
       DESCRIPTION OF EMBODIMENTS 
       [0122]    A wind turbine  1  has a nacelle  7  mounted atop a tower  8 . The wind turbine  1 , schematically illustrated by  FIG. 1 , is driven by a rotor  50  that is coupled to a generator  2 , for example over a gear (not shown), to feed fixed frequency electrical power to an electricity grid  10 . The generator  2  generates variable frequency AC current, wherein the frequency of the AC current is depends on wind speed. The variable frequency AC current is fed to a converter system  3 , including generator-side inverters  4  connected via a DC connection line  6  to a grid-side inverters  5 . The grid-side and generator-side inverters are represented each by a symbol for a single rectifier and a symbol for a single inverter in  FIG. 1 , for the sake of clarity. The converter system includes at least two converter strings  24 ,  25  (not shown in  FIG. 1 ), each string is hereby equipped with respective generator-side and grid-side inverters. The generator-side inverters  4  are located in the nacelle  7  of the wind turbine  1 , whereas the grid-side inverters  5  are located in the tower  8  of the wind turbine  1 . 
         [0123]    The generator-side inverters  4  rectify the variable frequency AC current produced by the generator, the resulting DC current is transmitted via the DC connection line  6  and is subsequently converted into fixed frequency AC current by the grid-side inverters  5 . This conversion operation of the at least two converter strings  24 ,  25  is controlled by a slave-converter controller  12  and a master-converter controller  13  associated with a first converter string  25  and a second converter string  24  (not shown), respectively. The slave-converter controller  12  as well as the master-converter controller  13  include a generator-side control module  14 ′,  14  and a grid-side control module  15 ′,  15 , respectively. The fixed frequency AC power produced by the grid-side inverters is fed to the electricity grid  10  by a transformer  9 . 
         [0124]    A method of controlling a converter  3  by a converter controller  11  is illustrated by  FIG. 2 . A turbine controller  21  sends commands  110  to a master-converter controller  13  and receives status reports  114 , including combined operational electrical data  114 ′, from the master-converter controller  13  that controls a first converter string  25 . 
         [0125]    This communication between the master-converter controller  13  and the turbine controller  21  is handled over an interface to the wind turbine controller  19  that is in this example realized as an Ethernet interface. The master-converter controller comprises a master converter control unit  20 , in which a master control module  16  and a grid-side string operation control module  15 , realized as separate but connected electronic circuits, are physically integrated. The master-control module  16  derives string operation control commands for the grid-side string operation control module  101  which like the master-converter controller is associated with the first converter string  25 . These string operation control commands for the grid-side string-operation-control module  101  are transmitted from the master-control module  13  to that string-operation-control module  15  via a common bus bar connecting the two modules that corresponds to an internal communication link  18 . The grid-side string operation control module  15  either derives control commands  103  for a generator-side string-operation-control module  14 —associated with the first converter string  25  on the basis of the received string-operation control commands received  101  and transmits these derived commands to the generator-side string operation control module  14 . The grid-side and generator-side string-operation control modules  15 ,  14  associated with the first converter string  25  control the conversion operation of the first string  25  in accordance with the commands received  101 , by selecting switching states and switching times of their semiconductor switches, so as to carry out the commands received  101 . 
         [0126]    Hence, the grid-side string operation control module  15  selects switching times and switching states so as to carry out the string control commands received from the master-control module  101 , and the generator-side string operation control module  14  selects switching times and switching states so as to carry out the commands received from the grid-side operation control module  103  that were so derived by the grid-side operation control module that the string-operation commands of the master control module are fulfilled. 
         [0127]    Furthermore, the grid-side operation control module  15  continuously or on occasion sends status reports  105  to the master control module  16  and the generator-side string operation control module  14  continuously or on occasion sends status reports  107  to the grid-side operation control module  15 , too. The status reports include operational electrical data of the respective converter strings. 
         [0128]    The master-control module also derives string operation commands for a slave-converter controller  102 , i.e. the converter-controller associated with a second converter string  24 , on the basis of the commands  110  received by the turbine controller  21 . The string control commands for the slave-converter controller  12  are transmitted to this controller  12  via an interface from the master-converter controller to the slave-converter controller  17  and an interface from the slave-converter controller to the master-converter controller  17 ′, both realized as Ethernet interfaces in this example. The slave-converter controller  12 —associated with the second converter string  24 —is also equipped with a grid-side string operation control module  15 ′, wherein this grid-side string operation control module  15 ′ is arranged to control the second converter string  24 . This grid-side operation control module derives control commands for the generator-side string operation control module  103 ′—associated with the second converter string—on the basis of the string operation commands for the slave-converter controller  102 , received from the master-control module  13  and transmits these commands  103 ′ to the generator-side string-operation control module  14 ′. The grid-side string-operation control module  15 ′ and the generator-side string-operation control module  14 ′ control the conversion operation of the second string in accordance with the commands they receive, hence, by selecting switching states and switching times of their semiconductor switches, so as to carry out the commands received, in analogy to the control of the first converter string. 
         [0129]    Furthermore, the grid-side operation control module  15 ′—associated with the second converter string  24 —continuously or on occasion also sends status reports  106  to the master control module  13  and the generator-side string operation control module  14 ′ continuously or on occasion sends status reports  107 ′ to the grid-side operation control module  15 ′, too. The status reports include operational electrical data of the respective converter strings. 
         [0130]    As the converter controller  11  including the control modules  13 ,  14 ,  15 ,  14 ′,  15 ′ and the turbine controller  21  mentioned above are carrying out these activities, they are also arranged to/programmed to carry out these activities. 
         [0131]    The converter controller  11 ′ and the method of controlling the converter  3  illustrated by FIG.  3   a  is in principle the same as the method and converter controller  11  illustrated by  FIG. 2 , except that the converter controller  11 ′, illustrated by  FIG. 3 a   , is equipped with a redundant master control module  16 ′ for providing failover functionality. 
         [0132]    The converter controller  11 ′ illustrated by  FIG. 3 a    has a slave converter-controller  12 ′ that is arranged to carry out the tasks of the faulty master-converter controller  13 ′, if the master-control module  16 , integrated in the master converter control unit  20  of the master-converter controller  13 ′, suffers a fault. 
         [0133]    The slave-converter controller  12 ′ is therefore also equipped with a master control module  16 ′ and an internal communication link  18 ′ between the master-control module  16 ′ and the grid-side string-operation control module  15 ′. In the normal operation mode, the master control module  16 ′ is inactive. Furthermore, the slave-converter controller  12 ′ also has an interface to the turbine controller  19 ′, to be able to receive commands from and send status reports to the turbine controller in the event of a fault of the master control module  16  of the master-converter controller. 
         [0134]    If the master control module  16  fails, an emergency handover request  110  is transmitted to the slave-converter controller  12 ′, or to be more precise to the master control module  16 ′ of the slave-converter controller  12 ′. 
         [0135]    After receiving the emergency-handover request  110 , the previously inactive master-control module  16 ′ becomes active, as illustrated in  FIG. 3 b   . Thereby the former slave-converter controller  12 ′ becomes the new master controller  13 ′. The master-control module  16 ′ now receives the commands from the wind turbine controller  104  via the interface to the wind turbine controller  19 ′. The master-control module  16 ′ derives string control commands  101 ′ for the grid-side operation control module  15 ′ and for the grid-side string operation control module  15  of the former master converter controller  13 ′. Thereby, the tasks of the former master-controller  13 ′ are completely assumed by the former slave controller  12 ′ and the converter control system  11 ′ continues operation even with a faulty master control module  16 . 
         [0136]    This failover functionality can also be applied in other embodiments. 
         [0137]    The converter controller  11 ″ illustrated by  FIG. 4  is in principle identical to the master-control system of  FIG. 2 , except for the fact that the roles of the grid-side string operation control modules and the generator-side string-operation-control modules of the respective master-converter controller and the respective slave-converter controller are interchanged: The generator-side string operation control module is now integrated along with the master control module  16  into the master converter control unit  20  of the master-converter controller  13 , whereas the grid-side string-operation control module  15  is now not integrated in this unit. Thereby the generator-side string-operation-control modules  14 ,  14 ′ of the master-converter controller  13  and slave-converter controller  12  now derive commands for the grid-side string-operation-control modules  15 ,  15 ′ of these controllers  12 ,  13  on the basis of the string operation control commands  101  they receive from the master-control module  16 . Only the generator-side string operation control modules  14 ,  14 ′ are now arranged to receive commands and send status reports to the master control module  16 . 
         [0138]    The subordination relation between the generator-side and grid-side string operation can also be interchanged in other embodiments. 
         [0139]    An electronic circuitry diagram of an exemplary converter  3 A, controlled by one of the converter controllers  11 ,  11 ′, or  11 ″ of  FIGS. 1 to 4  is illustrated by  FIG. 5 . The generator  2  has two separate generator windings, a first generator winding  22  and a second generator winding  23  that are electrically isolated from each other. The first generator winding  22  is coupled to a first generator-side inverter thread  40  that comprises at least two string inverters electrically connected in parallel. The second generator winding  23  is coupled to at least one second generator-side inverter thread  40 ′ that also comprises at least two string inverters electrically coupled in parallel. The first and second generator-side inverter threads  40 ,  40 ′ are electrically coupled to each other in series, by connecting the negative DC voltage output of the first generator-side inverter thread  40  to the positive DC voltage output of the second generator-side inverter thread  40 ′, thereby forming a neutral connection point. The positive DC potential of this series connection is coupled to a positive DC conductor  61 , whereas the negative DC potential of this series connection is coupled to a negative DC conductor  63 . The neutral connection point of the generator-side inverter threads  40 ,  40 ′ is coupled to a centre connection line  62  that lies on neutral potential. 
         [0140]    The positive DC conductor  61  is coupled to a grid-side inverter thread  50  on the grid side. This grid-side inverter thread also comprises at least two grid-side string inverters electrically connected in parallel. The grid-side inverter thread  50  that is coupled to a first separate transformer winding  92 , is electrically connected in series to a second separate transformer winding  93 , wherein the separated transformer windings  92 ,  93  are electrically isolated from each other. The two series connected grid-side inverter threads  50 ,  50 ′ like the generator-side string inverter threads  40 ,  40 ′ connected to the neutral center connection line  62 . To achieve this, the negative potential input of the first grid-side inverter thread  50  is coupled to the positive potential input of the second grid-side inverter thread  50 ′ to form a neutral connection point for the centre connection line  62 . The negative DC conductor  63  is electrically coupled to the negative input of the second grid-side inverter thread  50 ′. 
         [0141]    A positive converter string  25 A, representing the first converter string of this converter  3 A is given by the first generator-side inverter thread  40  and the first grid-side inverter thread  50 , both coupled to the center connection line as well as to the positive DC conductor. The positive converter string  25 A is confined by a dashed line box in  FIG. 5 . 
         [0142]    A negative converter string  24 A, representing the second converter string of this converter  3 A is given by the second generator-side inverter thread  40 ′ and the second grid-side inverter thread  50 ′, both coupled to the center connection line as well as to the negative DC conductor. The negative converter string  24 A is also confined by a dashed line box in  FIG. 5 . 
         [0143]    The positive converter string  25 A is controlled by the generator-side string-operation controller  14  and grid-side string-operation controller  15  of the master-converter controller  13  that is associated with said positive converter string  25 A. The negative converter string is controlled by the generator-side-string operation controller  14 ′ and grid-side string-operation controller  15 ′ of the slave-converter controller  12  that is associated with said negative converter string  24 A. 
         [0144]    Each of the generator-side  40 ,  40 ′ and grid-side  50 ,  50 ′ inverter threads is further equipped with energy dissipation elements  30 ,  30 ′ and  31 ,  31 ′, respectively. As can be seen from the circuitry diagram illustrated by  FIG. 5 , those energy dissipation elements are a series connection of a chopper switch and a resistor with high thermal capacity, wherein there are more energy dissipation elements on the grid-side than on the generator side. Therefore, a higher amount of active power can be dissipated on the grid-side than on the generator-side. The chopper switches comprised by the energy dissipation elements are also controlled by the corresponding string operation controllers  14 ,  14 ′,  15 ,  15 ′. 
         [0145]    The potential difference (voltage level) between the positive DC conductor  61  and the center connection line  62  can be measured by means of a first voltage sensor  28   a , coupled to the positive DC conductor  61 . The potential difference (voltage level) between the negative DC conductor  63  and the center connection line can be measured by means a second voltage sensor  28   c , coupled to the negative DC conductor  63 . The amount of current flowing along the centre line can be measured by means of a current sensor  28   c , coupled to the centre connection line  62 . 
         [0146]    The circuitry diagram of  FIG. 6 a    illustrates generator-side string inverters  4   a ,  4   b  coupled to respective separate generator windings  22 ,  23 . These inverters  4   a ,  4   b  are rectifiers, with DC outputs connected in series as described in conjunction with  FIG. 5 . 
         [0147]    The circuitry diagram of  FIG. 6 b    illustrates grid-side string inverters  5   a ,  5   b  the AC output of which is coupled to corresponding separate transformer windings  92 ,  93 . The DC inputs of the string inverters  5   a ,  5   b  are connected in series as described in conjunction with  FIG. 5 . The transformer  9  is connected to the electricity grid  10 . 
         [0148]    The circuitry diagram of  FIG. 7  shows a back-to-back connection of the circuits of  FIG. 6 a    and  FIG. 6 b   , thereby establishing a common DC link between the series-connected generator-side inverters  4   a  and  4   b  and the series-connected grid-side inverters  5   a  and  5   b.  The circuitry represents another exemplary converter system  3 B, with a first converter string  25  and a second converter string  26 , marked by the dashed-line boxes of  FIG. 7 . The common DC link, representing the DC connection  6 , is, as described in conjunction with  FIG. 5 , established by the centre connection line  62 , the positive DC conductor  61  and the negative DC conductor  63 , wherein the centre connection line couples the neutral connection point of the generator-side inverters  4   a,    4   b  to the neutral connection point of the grid-side inverters  5   a ,  5   b.    
         [0149]    An electronic circuitry diagram of a further exemplary converter system  3 C shown in  FIG. 8 . In contrast to the exemplary converter systems of  FIG. 5  and  FIG. 7 , this converter system does not have a common DC link, coupling the generator side string inverters  4 ′ a ,  4 ′ b  and the grid-side string inverters  5 ′ a ,  5 ′ b . This converter system  3 C rather has two separated and mutually insulated converter strings, namely a first converter string  25 C and a second converter string  24 C having separate DC links. The first converter string  25 C is established by the generator-side string inverter  4 ′ a  coupled via a first DC link to the grid-side string inverter  5 ′ a . This converter string connects a first set of separate generator windings  22  to a first set of separate transformer windings  92 . The second converter string  24 C is established by the generator-side string inverter  4 ′ b  coupled via a second DC link to the grid-side string inverter  5 ′ b . This converter string connects a second set of separated generator windings  23  to a second set of separate transformer windings  93 . The generator windings  23 ,  22  and the transformer windings  92 ,  93  are electrically insulated from each other. 
         [0150]    The first converter string  25 C is controlled by the generator-side string-operation controller  14  and grid-side string operation controller  15  of the master-converter controller  13  (not shown) that is associated with the first converter string  25 C. The second converter string  24 C is controlled by the generator-side string-operation controller  14 ′ and grid-side string-operation controller  15 ′ of the slave-converter controller  12  that is associated with the second converter string  24 C. 
         [0151]    An exemplary method of controlling the wind turbine  50  by means of a converter controller  11 ,  11 ′,  11 ″ comprising the master-converter controller  13 ,  13 ′ and the slave-converter controller  12 ,  12 ′ mentioned above, wherein the master and slave-converter controller comprises at least one string-operation control module  14 ,  14 ′,  15 ,  15 ′ and the master-converter controller further comprises a master-control module  16  is illustrated by the block diagram of  FIG. 9  in a high-level approach. 
         [0152]    Turbine controller targets for the master-converter control module  110  are received by the master-control module  16 . At box D 1 , the master-control module  16  derives string control commands  101  for the string-operation control module of the master-converter controller  13  that is associated with the first converter string  25 , in the block diagram of  FIG. 9  referred to as string  1 . In the activities at box D 2  to Dn, the master-control module derives string control commands for the slave-converter control modules  102  associated with the respective converter strings  2  to n. 
         [0153]    In the activities at boxes C 1  to Cn, the string-operation control modules associated with the respective converter strings  1  to n receive the corresponding string-operation-control commands from the master-control module  16  and control the conversion operation of the respective strings  1  to n accordingly. Hence the string-operation-control module of the master-converter controller  14 ,  15  follows the control commands derived by the master-control module  16  for the first converter string  25 , associated with it. The string operation control modules of the slave converter controllers (in this example, one slave converter controller for each converter string  2  to n is provided) follow the control commands  102  derived for the converter strings, associated with them. 
         [0154]    Furthermore, in the activities at boxes S 1  to Sn, the string operation control modules associated with their respective string sense operational electrical data of their associated string. The operational electrical data of the respective strings is combined by the master-control module  16  at box M 1 , which results in combined operational electrical data of the converter strings  114 ′. This combined electrical data is transmitted to the turbine controller  21 . The turbine controller  21  adjusts the target for the master-controller  100  at box A 1 , if, for example, given limits for the operational electrical data have been exceeded. 
         [0155]    An exemplary derivation of active and reactive target values for the first and second converter strings  24 ,  25  of the converters  3 A and  3 B with a common DC link and a common centre connection line (illustrated by  FIG. 5  and  FIG. 7 ) is given by  FIG. 10 . 
         [0156]    This exemplary derivation is carried out by the master-control module  16  upon receipt of turbine controller target values for active/reactive power  120 ,  121 . 
         [0157]    An internal active power consumption P  125  is added to the active power target value  120  received from the wind turbine controller. The result of this addition is divided by two. Hence, the active power target is in a first activity distributed equally among the converter strings. This reduces, per default, the current flowing in the centre line  62  of converters  3 A,  3 B, as already discussed in the “general description” part. 
         [0158]    However, to compensate for the influence of unequal component quality in the first and second converter string, the centre line current  130  is continuously measured, for example by a current sensor  68  (shown in  FIG. 5 ) and the active power targets for the first and second converter string  120 ′,  120 ″ are provided “unbalanced” on purpose in order to bring this centre line current  10  to a set point value, e.g. zero. 
         [0159]    To achieve this, the equal active power targets for the two converter strings are adjusted by a proportional integral controller  150 . The proportional integral controller for example calculates the difference between the centre line current at any given time and the set point. Then, the proportional integral controller, for example, derives the active power correction value corresponding to the deviation from the set point and divides that active power correction value by two. This correction value is subtracted from the active power target for the first string and added to the active power target—in order to unbalance the target values such that the centre line current reaches the setpoint value. 
         [0160]    To derive the reactive power target values for the first and second converter string, the internal reactive power consumption Q  126  is added to the reactive power target from the wind turbine controller and the result of this addition is divided by two. Hence, the reactive power targets are distributed equally on the first and second converter string. 
         [0161]    In the block diagram of  FIG. 11  operational electrical data, represented by the DC link voltage of the respective converter strings DC 1  to DCn is combined by adding these voltage up to a total DC voltage  116  of the converter system. Subsequently, the total DC link voltage, representing the combined operational electrical data  115 , is provided as an overall status report concerning DC voltage to the wind turbine controller  21 . If the total DC voltage is higher than a given threshold, e.g. 1200 V in total, the turbine controller  21  reduces active/reactive power targets and/or commands the use of energy dissipators at box R 1 . Alternatively if the total DC voltage  116  is within a given range, i.e. does not exceed a given threshold, the turbine controller keeps its active/reactive power target at box R 2 .