DESCRIPTION Winding connections for a generator of a wind turbine Field of invention The invention relates to a wind turbine comprising an elec- tric generator and a power converter system for transmitting the total active power from the generator to a fixed frequen- cy AC or DC system. The power converter system comprises a plurality of power converter channels. The invention further relates to a method for operating such a wind turbine. Art Background The trend in the field of wind turbines, more particularly offshore wind turbines, has for many years been towards high- er and higher active power ratings. Increasing the active power rating of a wind turbine can (electrically) be achieved by either increasing the total current, increasing the system voltages or a combination thereof. Both approaches have drawbacks in terms of implemen- tation complexity and total system lifetime cost (i.e. costs including initial investment as well as life time operational cost and the value of inherent system losses). Furthermore, available component limitations (e.g. maximum current in a single breaker) confine the solution space for how an actual system can be implemented. If the current rating is increased, the number of components (in particular, breakers) available on the market to handle the needed current are reduced. Furthermore, designs of elec- trical connections become more challenging. This leads to de- signs where the main current is split between several paral- lel branches each carrying a fraction of the total current in the electric generator, as shown in EP 2492504. This means that for a system which consists of n parallel branches each of the branches will carry approximately an n ^th of the total turbine current. Generally, a galvanic isolation between parallel power con- verters offers a higher degree of flexibility in terms of grounding schemes and how the switching elements in the indi- vidual converters are controlled. However, increasing the re- quirements on the generator for multiple galvanically isolat- ed windings restricts the possible optimization of the gener- ator in terms of winding/conductor routing and/or redundancy in the event of partial converter system failure which is al- so undesirable. A compromise between optimization of the gen- erator and of the converter system is therefore necessary. It is therefore desirable to connect a generator more effi- ciently to a power converter system by means of a plurality of connections. Summary of the Invention This objective may be solved by the wind turbine and the method for operating a wind turbine according to the inde- pendent claims. Advantageous embodiments of the present in- vention are described by the dependent claims. According to a first aspect of the present invention a wind turbine is provided, which comprises at least one electric generator having at least one winding system, each winding system comprising a plurality of sub-winding systems, and a power converter system for connecting the plurality of wind- ing systems to a fixed frequency AC or DC system, the power converter system comprising a plurality of power converter channels. Each of the sub-winding systems is galvanically isolated from the other sub-winding systems and is connected to at least one respective power converter channel. Each winding system comprises at least one breaker for connecting one sub-winding system to another sub-winding system. The permanent magnet machine may be an electrical generator. Particularly, but not exclusively, the electrical generator may be used in a wind turbine. According to a second aspect of the present invention a meth- od of operating the above-described wind turbine is provided, the method comprising the step of equally distributing the amount of electric power generated by the electric generator to all power converter channels. With “winding system” it is meant a system of wound conduc- tors within the generator covering the complete circumference of the generator about its rotational axis. A generator com- prise of one or more winding systems and each winding system may comprise of a plurality of n sub-winding systems (e.g. two sub-winding systems, each sub-winding systems covering an angle of 180 degrees). The winding system my be a concentrat- ed or distributed winding system. With “power converter channel” it is meant a converter capa- ble of “channelling” a fraction of the total active power from the generator to a fixed frequency AC or DC system, for example a three phase AC or DC system like an electrical grid. A power converter system is a complete system capable of transmitting the total active power from the generator to a fixed frequency AC or DC system. A power converter system may include a plurality of n power converter channels, each covering 1/n ^th of the total active power. According to the present invention, the winding system (in- cluding concentrated or distributed windings) is connected to the power converter system in such way that: - balanced operation of a generator in normal operation, i.e. maintaining one or more main winding systems which each cover the entire circumference of the generator; - relatively simple connection layout (relatively low number of individual conductors and connectors); - relatively low conductor material consumption (conductor square section and length). In this embodiment, the electric system associated with the generator may have a layout which consists of a plurality of winding systems. Each winding system is divided into a plu- rality of sub-winding systems which are electrical separated from each other. There may be any number n of sub-winding systems within a winding system, each sub-winding system cov- ering (360°)/n. A plurality of parallel power converter chan- nels may be connected to the generator without being galvani- cally coupled to each other. According to an embodiment of the present invention, the pow- er converter channels are also n for each winding system, each sub-winding system being connected to only one power converter channel. According to embodiments of the present invention, each wind- ing system comprises n – 1 breakers between the n sub-winding systems for connecting the n sub-winding systems to each oth- er or separating the n sub-winding systems from each other. According to embodiments of the present invention, each wind- ing system comprises one or more breakers, each breaker con- necting a respective pair of sub-winding systems. According to embodiments of the method of the present inven- tion, if a power converter channel connected to a first wind- ing system fails, the sub-winding system connected the failed power converter channel is connected to at least another sub- winding system of the first winding system. This may be done by operating the breaker to connect the sub-winding system connected to the failed power converter channel to at least another sub-winding system of the same winding system. The electric generator can further operate even after a fail- ure of a power converter channel, at a percentage of its nom- inal power. Such percentage depends on the number power con- verter channels in the power converter system. For example, if each winding system comprises of two sub-winding systems, each sub-winding system being connected to a respective power converter channel, the electrical generator can be operated at least 50% of its nominal power, each time a power convert- er channel fails. Brief Description of the Drawings The aspects defined above, and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodi- ment but to which the invention is not limited. Figure 1 shows a schematic section of a wind turbine in- cluding an electric generator connected to a pow- er converter system according to the present in- vention. Figure 2 shows an electric schematic diagram of an elec- tric generator connected to a power converter system according to a first embodiment of the present invention. Figure 3 shows an electric schematic diagram of another embodiment of an electric generator connected to a power converter system according to the present invention, in a first operative configuration. Figure 4 shows the embodiment of figure 3, in a second op- erative configuration. Figure 5 shows an electric schematic diagram of an elec- tric generator connected to a power converter system according to a second embodiment of the present invention. Detailed Description The illustrations in the drawings are schematically. It is noted that in different figures, similar or identical ele- ments are provided with the same reference signs. Figure 1 shows a wind turbine 1 according to the invention. The wind turbine 1 comprises a tower 2, which is mounted on a non-depicted fundament. A nacelle 3 is arranged on top of the tower 2. The wind turbine 1 further comprises a wind rotor 5 having two, three or more blades 4 (in the perspective of Figure 1 only two blades 4 are visible). The wind rotor 5 is rotatable around a rotational axis Y. In general, when not differently specified, the terms axial, radial and circumfer- ential in the following are made with reference to the rota- tional axis Y. The blades 4 extend radially with respect to the rotational axis Y. The wind turbine 1 comprises an electric generator 11 con- nected to a power converter system, as better explained in the following. The power converter system comprises four electric power converter channels 51, 52, 53, 54. The wind rotor 5 is rotationally coupled with the electric generator 11 by means of a rotatable shaft. According to other possible embodiments of the present invention, the wind rotor 5 is ro- tationally coupled directly with the electric generator 11 (direct-drive generator configuration). The electric generator 11 includes a stator 20 and a rotor 30. The rotor 30 is radially external to the stator 20 and is rotatable with respect to the stator 30 about the rotational axis Y. According to other embodiments of the present inven- tion (not shown) the rotor is radially internal to the stator 11. According to other possible embodiments of the present invention (not represented in the attached figures), the pre- sent invention can be applied to an electric generator, which is not comprised in a wind turbine. Figure 2 shows schematically a first embodiment of a genera- tor winding system 100 of the electric generator 11. The gen- erator winding system 100 comprises two winding systems 41, 42. According to other embodiments of the invention (not shown), the generator winding system 100 comprises a plurali- ty of winding systems greater than two. Each winding systems 41, 42 is three-phase system of conductors wound in the sta- tor 20 and covering the complete circumference of the genera- tor about the rotational axis Y. Each winding system 41, 42 comprises two respective sub-winding systems 41a,b and 42a,b. Each sub-winding system 41a,b and 42a,b is a three-phase sys- tem of conductors wound in the stator 20 and covering a frac- tion of the complete circumference of the generator about the rotational axis Y. In the embodiments of the attached figures 2 to 4, each sub-winding system 41a,b and 42a,b covers an an- gle of 180 degrees about the rotational axis Y. According to other embodiments of the invention (not shown), each winding system 41, 42 may comprise three respective sub-winding sys- tems, each sub-winding system covering an angle of 120 de- grees about the rotational axis Y. Each winding system 41, 42 may comprise n sub-winding systems, n being an integer great- er than one, each sub-winding system covering an angle of 360°/n degrees around a rotational axis Y, in each sub- winding system being generated 1/n ^th of the electrical power generated in the respective winding system 41, 42. In the em- bodiments of the attached figures 2 to 4, in each of the sub- winding systems 41a,b; 42a,b is generated a fourth of the electrical power generated in the electric generator 11, each winding system 41, 42 generating a half of the electrical power generated in the electric generator 11. Each of the sub-winding systems 41a,b; 42a,b is galvanically isolated from the other sub-winding systems 41a,b; 42a,b. A power converter system 51, 52, 53, 54 is associated to the generator winding system 100 for connecting the plurality of winding systems 41, 42 to a fixed frequency AC or DC system 200. The fixed frequency AC or DC system 200 may be a three- phase grid 200. One or more transformers (two transformers 210 are shown in the attached figures 2 to 4) may be inter- posed between power converter system 51, 52, 53, 54 and the grid 200. The power converter system 51, 52, 53, 54 compris- ing a plurality of power converter channels (four converter channels 51, 52, 53, 54 in the embodiments of the attached figures 2 to 4). Each of the sub-winding systems 41a,b; 42a,b is electrically connected to one respective power converter channel 51, 52, 53, 54. According to other embodiments of the invention (not shown), each of the sub-winding systems 41a,b; 42a,b may be electrically connected to more than one respec- tive power converter channel 51, 52, 53, 54. Each winding system 41, 42 comprises at least a breaker 61, 62 between two sub-winding systems 41a,b; 42a,b of the re- spective winding system 41, 42. In the embodiment of the at- tached figures, each winding system 41, 42 comprises one re- spective breaker 61, 62, each breaker 61, 62 being interposed between the two sub-winding systems 41a,b; 42a,b of each winding system 41, 42. The breaker may be of the RCO (Residu- al Current Operated breaker) type. According to the embodi- ments of the invention (not shown) where each winding system 41, 42 comprises three respective sub-winding systems, two breakers may be provided for each winding system. Each break- er 61, 62 may be used in operation for connecting to each other the sub-winding systems 41a,b; 42a,b of a same winding system 41, 42 or for separating from each other the two sub- winding systems 41a,b; 42a,b of a same winding system 41, 42. According to embodiments of the present invention where each winding system 41, 42 comprises n respective sub-winding sys- tems, each winding system comprises n – 1 breakers between the n sub-winding systems for connecting the n sub-winding systems to each other or separating the n sub-winding systems from each other. With reference to Figure 3, the electric generator 11 is op- erating without faults. A method of operating the wind tur- bine 1 comprises the step of equally distributing the amount of electric power generated by the electric generator 11 to all the power converter channels 51, 52, 53, 54, i.e. a fourth of the generated power to each power converter channel 51, 52, 53, 54. With reference to Figure 4, the power converter channel 51 connected to one sub-winding system 42a is faulty. According to the method of operating the wind turbine 1, the breaker 62 in the respective winding system 42 is closed so that the sub-winding system 42a,b of the respective winding system 42 are connected to each other. Both the sub-winding system 42a,b of the respective winding system 42 are so connected to the one power converter channel 53, which is not faulty. Such power converter channel 53 is still operated to receive the same percentage of the generated power as before the faulty conditions, i.e., in the embodiment of figures 2 to 4, a fourth of the power generated before the faulty conditions. The winding system 42, connected to the faulty power convert- er channel 51, is therefore operated at a fourth of the total power generated before the faulty conditions. To balance op- eration, the other winding system 41 is also operated at the same power, i.e., in the embodiment of figures 2 to 4, a fourth of the total power generated before the faulty condi- tions. Such power is equally distributed between the power converter channels 52, 54 connected to the other winding sys- tem 41, each receiving, in the embodiment of figures 2 to 4, an eight of the total power generated before the faulty con- ditions. Figure 5 shows schematically another embodiment of a genera- tor winding system 100 of the electric generator 11. The gen- erator winding system 100 comprises one winding systems 41. The winding system 41 is three-phase system of conductors wound in the stator 20 and covering the complete circumfer- ence of the generator about the rotational axis Y. The wind- ing system 41 comprises four respective sub-winding systems 41a,b,c,d. Each sub-winding system 41a,b,c,d covers an angle of 90 degrees about the rotational axis Y. According to other embodiments of the invention (not shown), the winding system 41 may comprise m respective sub-winding systems, where m is an even integer, each sub-winding system covering an angle of 360°/m degrees around a rotational axis Y, in each sub- winding system being generated 1/m ^th of the electrical power generated in the winding system 41. Each of the sub-winding systems 41a,b,c,d is galvanically isolated from the other sub-winding systems 41a,b,c,d. Each of the sub-winding sys- tems 41a,b,c,d is electrically connected to one respective power converter channel 51, 52, 53, 54. According to other embodiments of the invention (not shown), each of the sub- winding systems 41a,b,c,d may be electrically connected to more than one respective power converter channel 51, 52, 53, 54. The winding system 41 comprises two breakers 61, 62. Each breaker 61, 62 is arranged between a respective pair of sub- winding systems 41a,c; 41b,d. The sub-winding systems 41a,c; 41b,d of each pair are oppositely arranged with respect to the rotational axis Y. Each breaker 61, 62 may be used in op- eration for connecting to each other the sub-winding systems 41a,c; 41b,d of the same pair. According to the method of op- erating the wind turbine 1, if a power converter channel con- nected to one sub-winding system 41a,b,c,d is faulty the breaker which is connected to the respective sub-winding sys- tem is closed for connecting to each other the respective pair of sub-winding systems 41a,c; 41b,d. The pair of sub- winding systems 41a,c; 41b,d connected to the faulty power converter channel is therefore operated at a fraction (one fourth) of the total power generated before the faulty condi- tions. To balance operation, the other pair of sub-winding systems 41a,c; 41b,d is also operated at the same reduced power, i.e. a fourth of the total power generated before the faulty conditions. Such power is equally distributed between the respective power converter channels connected to the oth- er (not faulty) pair of sub-winding systems 41a,c; 41b,d.