A method for the protection of one or more power converters

Abstract

A method (300) for the protection of one or more power converters (204) of a power converter arrangement (202) connected to a power grid (102) via a transformer (222) by usage of a circuit breaking arrangement (208a-c). The circuit breaking arrangement (208a-c) comprises first and second circuit breaking apparatuses (210a-c, 212), connected between a low voltage side of the transformer and the power converter arrangement (202). The first circuit breaking apparatus (210a-c) comprises a first circuit breaker (214) and a first element (216) having an impedance and being connected in parallel with the first circuit breaker (214). The second circuit breaking apparatus (212) comprises one or more second circuit breakers (220a, 220b). The first circuit breaking apparatus (210a-c) is connectable to the power converter arrangement (202) via the second circuit breaking apparatus (212). The second circuit breaking apparatus (212) is connectable to an electric power grid (102) via the first circuit breaking apparatus (210a-c). The method (300) comprises: when a fault is detected in the power converter arrangement (202), controlling (302) the first circuit breaker (214) to switch to an open position so as to transfer a fault current to the first element (216) while keeping the one or more second circuit breakers (220a, 220b) in a closed position; and when the first circuit breaker (214) is in the open position, controlling (303) the one or more second circuit breakers (220a, 220b) to switch to an open position so as to interrupt a fault current.

Description

[0001] A METHOD FOR THE PROTECTION OF ONE OR MORE POWER CONVERTERS

[0002] Technical field

[0003] Aspects of the present invention relate to a method for the protection of one or more power converters of a power converter arrangement by usage of a circuit breaking arrangement.

[0004] Background

[0005] In general, a wind turbine generator, or a power plant including one or more wind turbine generators or other renewable electric power generating units, includes one or more power converters for electric power conversion and one or more circuit breaking arrangements for the protection of the power converters upon a fault, such as a short circuit fault.

[0006] Summary

[0007] In general, wind turbine generators are becoming increasingly larger and more powerful while still operating at voltages below 1000 VAC at the power converter and at the terminals of the electric generator of the wind turbine generator, which results in high electric currents in operation and consequently also in high short circuit currents upon faults. These high short circuit currents are problematic to break, or interrupt, even for large circuit breakers.

[0008] The inventors of the present invention have found drawbacks in conventional solutions for protecting power converters, which may be included in a wind turbine generator or a power plant, upon a fault, such as a short circuit fault. For example, some conventional solutions do not provide a sufficiently efficient protection of the power converters.

[0009] An object of the invention is to provide a solution which mitigates or solves the drawbacks and problems of conventional solutions. The above and further objects are solved by the subject matter of the independent claims. Further advantageous embodiments of the invention can be found in the dependent claims.

[0010] According to a first aspect of the invention, the above mentioned and other objects are achieved with a method for the protection of one or more power converters of a power converter arrangement connected to a power grid via a transformer by usage of a circuit breaking arrangement, wherein the circuit breaking arrangement comprises a first circuit breaking apparatus and a second circuit breaking apparatus connected between a low voltage side of the transformer and the power converter arrangement, wherein the first circuit breaking apparatus comprises a first circuit breaker and a first element having an impedance, wherein the first element is connected in parallel with the first circuit breaker, wherein the second circuit breaking apparatus comprises one or more second circuit breakers, wherein the first circuit breaking apparatus is connectable to the power converter arrangement via the second circuit breaking apparatus, wherein the second circuit breaking apparatus is connectable to an electric power grid via the first circuit breaking apparatus, wherein the method comprises: when a fault is detected or after a fault has been detected in the power converter arrangement, controlling the first circuit breaker to switch to an open position so as to transfer a fault current to the first element while keeping the one or more second circuit breakers in a closed position so as to allow an electric current to pass; and when the first circuit breaker is in the open position, controlling the one or more second circuit breakers to switch to an open position so as to interrupt a fault current.

[0011] An advantage of the method according to the first aspect is an improved protection of the one or more power converters of a power converter arrangement upon a fault. An advantage of the method according to the first aspect is that the fault current is efficiently reduced to a suitable level by the innovative transfer of the fault current to the first element before the one or more second circuit breakers is / are switched to an open position. An advantage of the method according to the first aspect is that the short circuit (SC) level is reduced without adding losses to, or negatively impacting, the power converter PQ chart, which is disclosed in further detail in the detailed description hereinbelow. An advantage of the method according to the first aspect is that the short circuit (SC) rating of the one or more second circuit breakers can be reduced, and / or the wear on the second circuit breaker is reduced, since the second circuit breaker will break a reduced, or lower, fault current as a result of the action of the first circuit breaking apparatus. Thus, an advantage of the method according to the first aspect is that an improved overload and short circuit protection of power converters for high fault currents is provided without increased costs, or to lower costs, in relation to conventional solutions.

[0012] For some embodiments, the fault may, for example, comprise a fault current, a short circuit (SC) fault, or a short circuit (SC) current.

[0013] According to an advantageous embodiment of the method according to the first aspect, the method comprises: detecting a fault in the power converter arrangement.

[0014] According to a further advantageous embodiment of the method according to the first aspect, the first circuit breaker comprises a main contact switchable between a closed position and an open position, and wherein the second circuit breaker comprises a main contact switchable between a closed position and an open position, and wherein the method comprises: when a fault is detected or after a fault has been detected in the power converter arrangement, controlling the main contact of the first circuit breaker to switch to the open position so as to transfer a fault current to the first element while keeping the main contact of the one or more second circuit breakers in the closed position so as to allow an electric current to pass; and when the main contact of the first circuit breaker is in the open position, controlling the main contact of the one or more second circuit breakers to switch to the open position so as to interrupt a fault current. An advantage of this embodiment is a further improved protection of the one or more power converters of a power converter arrangement when a fault occurs.

[0015] According to another advantageous embodiment of the method according to the first aspect, the method comprises: when a fault above a first predetermined level is or has been detected in the power converter arrangement, controlling the first circuit breaker to switch to an open position so as to transfer a fault current to the first element while keeping the one or more second circuit breakers in a closed position so as to allow an electric current to pass. An advantage of this embodiment is a further improved protection of the one or more power converters of a power converter arrangement when a fault occurs.

[0016] According to yet another advantageous embodiment of the method according to the first aspect, the method comprises: when a fault below a first predetermined level and above a second predetermined level is or has been detected in the power converter arrangement, keeping the first circuit breaker in a closed position so as to allow an electric current to pass while controlling the one or more second circuit breakers to switch to an open position so as to interrupt a fault current. An advantage of this embodiment is a further improved protection of the one or more power converters of a power converter arrangement, since the first circuit breaker is not activated for low- level faults, which the second circuit breaker can handle on its own, and wear of the first circuit breaker is thus avoided.

[0017] According to still another advantageous embodiment of the method according to the first aspect, the method comprises: when the fault is cleared, controlling the first and second circuit breakers to switch to the closed position so as to allow an electric current to pass.

[0018] According to a second aspect of the invention, the above mentioned and other objects are achieved with a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out one or more of the method according to any one of the embodiments disclosed above or below and process according to any one of the embodiments disclosed above or below. Advantages of the computer program according to the second aspect correspond to advantages of the method according to the first aspect and its embodiments mentioned above or below.

[0019] For some embodiments, the above mentioned and other objects are achieved with a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method according to any one of the embodiments mentioned above or below.

[0020] According to a third aspect of the invention, the above mentioned and other objects are achieved with a computer-readable medium comprising instructions which, when the instructions are executed by a computer, cause the computer to carry out one or more of the method according to any one of the embodiments disclosed above or below and process according to any one of the embodiments disclosed above or below. Advantages of the computer-readable medium according to the third aspect correspond to advantages of the method according to the first aspect and its embodiments mentioned above or below.

[0021] For some embodiments, the above mentioned and other objects are achieved with a computer-readable medium comprising instructions which, when the instructions are executed by a computer, cause the computer to carry out the method according to any one of the embodiments mentioned above or below.

[0022] According to an aspect of the present invention, the above-mentioned computer program and / or the computer-readable medium are / is configured to implement the method and its embodiments described herein.

[0023] According to a fourth aspect of the invention, the above mentioned and other objects are achieved with a control arrangement for the protection of one or more power converters of a power converter arrangement connected to a power grid via a transformer by usage of a circuit breaking arrangement, wherein the circuit breaking arrangement comprises a first circuit breaking apparatus and a second circuit breaking apparatus connected between a low voltage side of the transformer and the power converter arrangement,, wherein the first circuit breaking apparatus comprises a first circuit breaker and a first element having an impedance, wherein the first element is connected in parallel with the first circuit breaker, wherein the second circuit breaking apparatus comprises one or more second circuit breakers, wherein the first circuit breaking apparatus is connectable to the power converter arrangement via the second circuit breaking apparatus, wherein the second circuit breaking apparatus is connectable to an electric power grid via the first circuit breaking apparatus, wherein the control arrangement is configured to: when a fault is detected or after a fault has been detected in the power converter arrangement, control the first circuit breaker to switch to an open position so as to transfer a fault current to the first element while the one or more second circuit breakers is / are kept in a closed position so as to allow an electric current to pass; and when the first circuit breaker is in the open position, control the one or more second circuit breakers to switch to an open position so as to interrupt a fault current.

[0024] Advantages of the control arrangement according to the fourth aspect correspond to advantages of the method according to the first aspect and its embodiments mentioned above or below.

[0025] It is to be appreciated that all the embodiments described for the method aspects of the invention are applicable also to the control arrangement aspects of the invention. Thus, all embodiments described for the method aspects of the invention may be performed by the control arrangement, which may include one or more controllers, control units, or control devices. As mentioned above, the embodiments of the control arrangement have advantages corresponding to advantages mentioned above for the method and its embodiments. According to a fifth aspect of the invention, the above mentioned and other objects are achieved with an electrical system comprising a power converter arrangement comprising one or more power converters, wherein the electrical system comprises a circuit breaking arrangement for the protection of the power converter arrangement connected to a power grid via a transformer, wherein the circuit breaking arrangement comprises a first circuit breaking apparatus and a second circuit breaking apparatus connected between a low voltage side of the transformer and the power converter arrangement,, wherein the first circuit breaking apparatus comprises a first circuit breaker and a first element having an impedance, wherein the first element is connected in parallel with the first circuit breaker for the transfer of a fault current from the first circuit breaker to the first element when the first circuit breaker is switched to an open position, wherein the second circuit breaking apparatus comprises one or more second circuit breakers, wherein the first circuit breaking apparatus is connectable to the power converter arrangement via the second circuit breaking apparatus, and wherein the second circuit breaking apparatus is connectable to an electric power grid via the first circuit breaking apparatus.

[0026] An advantage of the electrical system according to the fifth aspect is an improved protection of the one or more power converters of a power converter arrangement upon a fault. An advantage of the electrical system according to the fifth aspect is an improved circuit breaking arrangement protecting the power converter arrangement. Otherwise, advantages of the electrical system according to the fifth aspect correspond to advantages of the method according to the first aspect and its embodiments mentioned above or below.

[0027] It is to be understood that the fact that the first circuit breaking apparatus is connectable to the power converter arrangement via the second circuit breaking apparatus and that the second circuit breaking apparatus is connectable to an electric power grid via the first circuit breaking apparatus implies that the first circuit breaking apparatus is connected, or connectable, to the second circuit breaking apparatus.

[0028] According to an advantageous embodiment of the electrical system according to the fifth aspect, the first circuit breaking apparatus is connectable to the one or more power converters of the power converter arrangement via the second circuit breaking apparatus.

[0029] According to a further advantageous embodiment of the electrical system according to the fifth aspect, the first circuit breaking apparatus is configured to connect the second circuit breaking apparatus to the electric power grid, wherein the second circuit breaking apparatus is configured to connect the first circuit breaking apparatus to the power converter arrangement.

[0030] According to another advantageous embodiment of the electrical system according to the fifth aspect, the second circuit breaking apparatus is configured to connect the first circuit breaking apparatus to the one or more power converters of the power converter arrangement.

[0031] According to yet another advantageous embodiment of the electrical system according to the fifth aspect, the power converter arrangement comprises the second circuit breaking apparatus. An advantage of this embodiment is a further improved protection of the one or more power converters of a power converter arrangement and a further improved circuit breaking arrangement by the integration of the second circuit breaking apparatus in the power converter arrangement.

[0032] According to still another advantageous embodiment of the electrical system according to the fifth aspect, one or more of the first and second circuit breaking apparatuses is / are configured to detect a fault in the power converter arrangement. An advantage of this embodiment is a further improved protection of the one or more power converters of a power converter arrangement. An advantage of this embodiment is a further improved circuit breaking arrangement protecting the power converter arrangement. According to an advantageous embodiment of the electrical system according to the fifth aspect, the first circuit breaking apparatus comprises a first circuit comprising a capacitor, wherein the first circuit is connected in parallel with the first element. An advantage of this embodiment is a further improved protection of the one or more power converters of a power converter arrangement. An advantage of this embodiment is a further improved circuit breaking arrangement protecting the power converter arrangement. An advantage of this embodiment is that the capacitor efficiently absorbs high surge currents during the switching, or control, of the first circuit breaker. An advantage of this embodiment is more efficient transfer, or commutation, of a fault current from the first circuit breaker arcing to the capacitor and the first element. An advantage of this embodiment and of the capacitor is an efficient extinguishing of an arc across the first circuit breaker when the first circuit breaker opens.

[0033] According to a further advantageous embodiment of the electrical system according to the fifth aspect, the first circuit comprises a second circuit connected in series with the capacitor, wherein the second circuit comprises a diode and a second element having a resistance, and wherein the diode is connected in parallel with the second element.

[0034] An advantage of this embodiment is a further improved protection of the one or more power converters of a power converter arrangement. An advantage of this embodiment is a further improved circuit breaking arrangement protecting the power converter arrangement. An advantage of this embodiment is that the diode prevents, or counteracts, oscillations between the capacitance and the line inductance. An advantage of this embodiment is that the second element provides an efficient discharge path for the capacitor.

[0035] According to another advantageous embodiment of the electrical system according to the fifth aspect, the electrical system comprises a transformer, wherein the first circuit breaking apparatus is connectable to the electric power grid via the transformer, and wherein the second circuit breaking apparatus is connectable to the electric power grid via the transformer and via the first circuit breaking apparatus.

[0036] According to yet another advantageous embodiment of the electrical system according to the fifth aspect, the transformer is configured to connect the second circuit breaking apparatus to the electric power grid.

[0037] According to still another advantageous embodiment of the electrical system according to the fifth aspect, one or more power converters of the power converter arrangement is / are configured to convert DC power to AC power to be provided to the electric power grid.

[0038] According to an advantageous embodiment of the electrical system according to the fifth aspect, when a fault is detected or after a fault has been detected in the power converter arrangement, the first circuit breaker is configured to open so as to transfer a fault current to the first element before any one of the one or more second circuit breakers opens. An advantage of this embodiment is a further improved protection of the one or more power converters of a power converter arrangement. An advantage of this embodiment is a further improved circuit breaking arrangement protecting the power converter arrangement.

[0039] According to a further advantageous embodiment of the electrical system according to the fifth aspect, the first circuit breaker is switchable between an open position and a closed position, wherein when the first circuit breaker is in the closed position the first circuit breaker is configured to allow an electric current to pass, wherein when the first circuit breaker is in the open position the first circuit breaker is configured to interrupt an electric current through the first circuit breaker so as to transfer a fault current to the first element, wherein the second circuit breaker is switchable between an open position and a closed position, wherein when the second circuit breaker is in the closed position the second circuit breaker is configured to allow an electric current to pass, and wherein when the second circuit breaker is in the open position the second circuit breaker is configured to interrupt an electric current.

[0040] According to another advantageous embodiment of the electrical system according to the fifth aspect, the first circuit breaker comprises one or more switches switchable between the open position and the closed position, wherein the second circuit breaker comprises one or more switches switchable between the open position and the closed position.

[0041] According to yet another advantageous embodiment of the electrical system according to the fifth aspect, the first element is configured to promote the extinguishing of an arc across the first circuit breaker when the first circuit breaker opens. An advantage of this embodiment is a further improved protection of the one or more power converters of a power converter arrangement. An advantage of this embodiment is a further improved circuit breaking arrangement protecting the power converter arrangement. An advantage of this embodiment is a reduction of wear on the first circuit breaker as a result of arcing.

[0042] According to still another advantageous embodiment of the electrical system according to the fifth aspect, the electrical system comprises a control arrangement according to any one of the embodiments disclosed above or below.

[0043] According to a sixth aspect of the invention, the above mentioned and other objects are achieved with a wind turbine generator, wherein the wind turbine generator comprises one or more of the group of:

[0044] • an electrical system according to any one of the embodiments disclosed above or below; and

[0045] • a control arrangement according to any one of the embodiments disclosed above or below.

[0046] Advantages of the wind turbine generator according to the sixth aspect correspond to advantages of the method according to the first aspect and its embodiments and of the electrical system according to the fifth aspect its embodiments mentioned above or below.

[0047] According to an advantageous embodiment of the wind turbine generator according to the sixth aspect, the wind turbine generator comprises an electric generator connected to the power converter arrangement.

[0048] The above-mentioned features and embodiments of the method, the computer program, the computer-readable medium, the control arrangement, the electrical system and the wind turbine generator, respectively, may be combined in various possible ways providing further advantageous embodiments.

[0049] Further advantageous embodiments of the method, the computer program, the computer-readable medium, the control arrangement, the electrical system and the wind turbine generator according to the present invention and further advantages with the embodiments of the present invention emerge from the detailed description of embodiments.

[0050] Brief Description of the Drawings

[0051] Embodiments of the invention will now be illustrated, for exemplary purposes, in more detail by way of embodiments and with reference to the enclosed drawings, where similar references are used for similar parts, in which:

[0052] Figure 1 is a schematic diagram illustrating a power plant including one or more embodiments of the wind turbine generator according to the sixth aspect of the invention;

[0053] Figure 2 is a schematic diagram illustrating aspects of embodiments of the wind turbine generator according to the sixth aspect of the invention, which may be included in the power plant of figure 1 ;

[0054] Figure 3 is a schematic diagram illustrating aspects of embodiments of the electrical system according to the fifth aspect of the invention;

[0055] Figure 4 is a schematic diagram illustrating further aspects of embodiments of the electrical system according to the fifth aspect of the invention; Figure 5 is a schematic diagram illustrating additional aspects of embodiments of the electrical system according to the fifth aspect of the invention;

[0056] Figure 6 is a schematic flow chart illustrating aspects of embodiments of the method according to the first aspect of the invention;

[0057] Figure 7 is another schematic flow chart illustrating further aspects of embodiments of the method according to the first aspect of the invention;

[0058] Figure 8 is a schematic diagram illustrating aspects of embodiments of the method according to the first aspect of the invention; and

[0059] Figure 9 is a schematic diagram illustrating an embodiment of the control arrangement according to the fourth aspect of the invention, in which a method according to any one of the herein described embodiments may be implemented.

[0060] Detailed Description

[0061] With reference to figure 1 , a power plant 100 for providing electric power, or electrical energy, to an electric power grid 102 is schematically illustrated. The power plant 100 includes one or more electric power generating units 103, which may be referred to as renewable electric power generating units 103. One or more of the one or more electric power generating units 103 may include one or more of the group of: a wind turbine generator 104; an electric battery energy storage system 112; a photo-voltaic panel 108; and a fuel cell 110. The electric battery energy storage system 112 may include one or more electric battery units. The power plant 100 may include one or more additional power sources or power generators, such as one or more additional electric battery units. Thus, the power plant 100 may include one or more wind turbine generators 104, for example two, three or more wind turbine generators 104. Aspects of embodiments of the wind turbine generator 104 according to the sixth aspect are disclosed in further detail hereinbelow in connection with figure 2.

[0062] With reference to figure 1 , the wind turbine generator 104, the electric battery energy storage system 112, the photo-voltaic panel 108, or the fuel cell 110 may be described as a power source of the power plant 100 or as a power generator of the power plant 100. For some embodiments, the power plant 100 may be referred to as a hybrid power plant. The power plant 100 may be connected, or connectable, more specifically electrically connected / connectable, to the electric power grid 102 via a point of common coupling, PCC, 114. The electric power grid 102 may be referred to as a utility grid, an electrical grid, or an electric power network. For example, the power plant 100 may be located offshore or on land. The power plant 100 may include a control arrangement 116 for controlling the power plant 100. The control arrangement 116 may comprise, or be referred to as, a power plant controller, PPC.

[0063] With reference to figure 2, aspects of embodiments of the wind turbine generator 104 according to the sixth aspect are schematically illustrated. For some embodiments, the wind turbine generator 104 may comprise a rotor 118 including one or more blades 120, or rotor blades 120, for example two or more blades 120, such as three blades 120, or more. The wind turbine generator 104 may comprise a tower 122 and a nacelle 124 mounted to the top of the tower 122. The rotor 118 may be connected, such as rotatably connected or mounted, to the nacelle 124. The wind turbine generator 104 may comprise an electric generator 126. The rotor 118 may be connected to the electric generator 126. The rotor 118 is configured to drive the electric generator 126. The electric generator 126 may be a permanent magnet, PM, generator, or any other type of electric generator. The nacelle 124 may house the electric generator 126.

[0064] With reference to figure 2, the rotor 118 is rotatable by action of the wind. The wind- induced rotational energy of the blades 120 and rotor 118 may be transferred via a coupling 128, for exampling including one or more shafts 130, to the electric generator 126. Thus, the wind turbine generator 104 may be described to be configured to convert kinetic energy of the wind to mechanical energy, or rotational energy, by way of the blades 120 and, subsequently, to electric power by way of the electric generator 126.

[0065] With reference to figure 2, the wind turbine generator 104 may comprise a power converter arrangement 132 for electric power conversion. The electric generator 126 may be connected, such as electrically connected, to the power converter arrangement 132. For some embodiments, the power converter arrangement 132, and / or the wind turbine generator 104, may comprise one or more power converters 133a, 133b connected, more specifically electrically connected, to the electric generator 126. The wind turbine generator 104 and / or the electric generator 126 may be connected, more specifically electrically connected, to the electric power grid 102 via said one or more power converters 133a, 133b, and / or via the power converter arrangement 132. The one or more power converters 133a, 133b may comprise a first power converter 133a for converting AC power from the electric generator 126 to DC power. The one or more power converters 133a, 133b may comprise a second power converter 133b for converting DC power from the first power converter 133a to AC power to be provided to the electric power grid 102. The nacelle 124 may house the one or more power converters 133a, 133b, or the one or more power converters 133a, 133b may be located elsewhere, such in the tower 122 or outside the wind turbine generator 104. Accordingly, the nacelle 124 may house the power converter arrangement 132, or the power converter arrangement 132 may be located elsewhere.

[0066] With reference to figure 2, the wind turbine generator 104 may comprises a control arrangement 134, or controller 136, for controlling the wind turbine generator 104. The control arrangement 134 of the wind turbine generator 104 may comprise the controller 136, which may be referred to as a wind turbine generator controller 136. The control arrangement 134 of the wind turbine generator 104 may be configured to communicate with and / or be connected to, or be part of, the control arrangement 116 of the power plant 100 (see figure 1 ) comprising one or more wind turbine generators 104.

[0067] With reference to figure 2, for some embodiments, the wind turbine generator 104 may be referred to as a variable-speed wind turbine generator. It is to be understood that the wind turbine generator 104 may include further unites, components and / or devices, such as sensors, required for a wind turbine generator 104. For example, the wind turbine generator 104 may be located offshore or on land.

[0068] With reference to figures 3 to 6, aspects of embodiments of the electrical system 200a, 220b, 200c according to the fifth aspect are schematically illustrated. In figures 3 to 6, the electrical system 200a-c is shown as a single line diagram, well known to the skilled person. With reference to figure 3, the electrical system 200a comprises a power converter arrangement 202. The power converter arrangement 202 includes one or more power converters 204. For some embodiments, the power converter 204 in figure 3 may correspond to the second power converter 133b of figure 2. For some embodiments, the power converter 204 in figure 3 and the second power converter 133b of figure 2 may be referred to as a line-side converter (LSC). For some embodiments, the first power converter 133a of figure 2 may be referred to as a machine-side converter (MSC). For some embodiments, the power converter 204 may include, or be built up by, one or more power converter stacks 206a, 206b, 206c, 206d. The power converter 204 and / or the power converter stacks 206a, 206b, 206c, 206d may include circuits and semiconductor devices, such as transistors, thyristors and diodes, according to various conventional designs. For some embodiments, one or more power converters 204 of the power converter arrangement 200a may be configured to convert DC power to AC power to be provided to the electric power grid 102, for example, AC power with various voltages and / or various frequencies.

[0069] With reference to figure 3, the electrical system 200a includes a circuit breaking arrangement 208a for the protection of the power converter arrangement 202 and / or for the protection of the one or more power converters 204 of the power converter arrangement 202. The circuit breaking arrangement 208a includes a first circuit breaking apparatus 210a and a second circuit breaking apparatus 212. The first circuit breaking apparatus 210a includes a first circuit breaker 214. The first circuit breaking apparatus 210a includes a first element 216 having an impedance. For some embodiments, the first circuit breaking apparatus 210a may include two or more first circuit breakers. The first element 216 may be described as an electrical element, which, for example, may be described to be configured to pass an electric current. The first element 216 may comprise one or more of the group of: a resistor 218, such as a power resistor; an electrical component; and an electrical line or cable. More specifically, for some embodiments, the first element 216 may have a resistance. For some embodiments, the first element 216 may have an impedance or resistance above 100 milliohm, for example above 200 milliohm, such as above 300 milliohm. For some embodiments, the first element 216 may be described to have an impedance or resistance that is high enough to reduce, or lower, a fault current (or, to dampen a short circuit current) before opening the one or more second circuit breakers 220a, 220b, or before controlling the one or more second circuit breakers 220a, 220b to switch to the open position so as to interrupt the consequently reduced fault current. The reduction of the fault current is disclosed in further detail hereinbelow. For some embodiments, the first element 216 may be described to have an impedance or resistance that is low enough to efficiently extinguish an arc across the first circuit breaker 214 when the first circuit breaker 214 opens. For some embodiments, the first element 216 may be described to have an impedance or resistance that is low enough to enable a sufficient supply of electric power or current to an auxiliary power system of a wind turbine generator 104 through, or via, the first element 216, also when the first circuit breaker 214 is in the open position.

[0070] With reference to figure 3, the first element 216 is connected, more specifically electrically connected, in parallel with the first circuit breaker 214 for the transfer of a fault current from the first circuit breaker 214 to the first element 216 when the first circuit breaker 214 is switched to an open position. The second circuit breaking apparatus 212 includes one or more second circuit breakers 220a, 220b, for example, two second circuit breakers 220a, 220b. However, for other embodiments, the second circuit breaking apparatus 212 may include one second circuit breakers 220a only, or more than two second circuit breakers 220a, 220b.

[0071] With reference to figure 3, the first circuit breaking apparatus 210a is connectable or connected, more specifically electrically connectable / connected, to the power converter arrangement 202 via the second circuit breaking apparatus 212. The second circuit breaking apparatus 212 is connectable or connected, more specifically electrically connectable / connected, to an electric power grid 102 via the first circuit breaking apparatus 210a. It is to be understood that the fact that the first circuit breaking apparatus 210a is connectable to the power converter arrangement 202 via the second circuit breaking apparatus 212 and that the second circuit breaking apparatus 212 is connectable to the electric power grid 102 via the first circuit breaking apparatus 210a implies that the first circuit breaking apparatus 210a is connected, or connectable, more specifically electrically connected / connectable, to the second circuit breaking apparatus 212. For some embodiments, it may be defined that the first circuit breaking apparatus 210a is connected, or connectable, more specifically electrically connected / connectable, to the second circuit breaking apparatus 212. For some embodiments, it may be defined that the first circuit breaking apparatus 210a and the second circuit breaking apparatus 212 are connected in series to one another.

[0072] With reference to figure 3, for some embodiments, it may be defined that the when the first circuit breaker 214 is open, the first element 216 is configured to reduce, or configured to promote the reduction of, a fault current. For some embodiments, the first element 216 is configured to promote the extinguishing of an arc across the first circuit breaker 214 when the first circuit breaker 214 opens. For some embodiments, it may be defined that when a fault (for example, a fault current, or short circuit current) is detected or after a fault has been detected in the power converter arrangement 202, the first circuit breaker 210a is configured to open so as to transfer a fault current to the first element 216 before any one of the one or more second circuit breakers 220a, 220b opens. For some embodiments, it may be defined that the first element 216 connects, or is configured to connect, such as in series, the second circuit breaking apparatus 212 to the electric grid 102, for example, when the first circuit breaker 214 is open.

[0073] With reference to figure 3, for some embodiments, it may be defined that the first circuit breaking apparatus 210a is connectable, more specifically electrically connectable, to the one or more power converters 204 of the power converter arrangement 202 via the second circuit breaking apparatus 212. For some embodiments, it may be defined that the first circuit breaking apparatus 210a is configured to connect, more specifically electrically connect, the second circuit breaking apparatus 212 to the electric power grid 102 and that the second circuit breaking apparatus 212 is configured to connect, more specifically electrically connect, the first circuit breaking apparatus 210a to the power converter arrangement 202. For some embodiments, it may be defined that the second circuit breaking apparatus 212 is configured to connect, more specifically electrically connect, the first circuit breaking apparatus 210a to the one or more power converters 204 of the power converter arrangement 202. With reference to figure 3, for some embodiments, the power converter arrangement 202 may include, or incorporate, the second circuit breaking apparatus 212. However, for other embodiments, the second circuit breaking apparatus 212 may be located elsewhere, such as outside the power converter arrangement 202. For some embodiments, one or more of the first and second circuit breaking apparatuses 210a, 212 may be configured to detect a fault, for example a fault current, a short circuit (SC) fault, and / or a short circuit current, in the power converter arrangement 202, for example, in one or more power converters 204 of the power converter arrangement 202.

[0074] With reference to figure 3, for some embodiments, the electrical system 200a may include a transformer 222, for example, so as to change the AC voltage level, such as step-up, or increase, the voltage level before input of the electric power to the electric power grid 102. The first circuit breaking apparatus 210a may be connectable, more specifically electrically connectable, to the electric power grid 102 via the transformer 222. The second circuit breaking apparatus 212 may be connectable, more specifically electrically connectable, to the electric power grid 102 via the transformer 222 and via the first circuit breaking apparatus 210a. For some embodiments, it may be defined that the transformer 222 is configured to connect, more specifically electrically connect, the second circuit breaking apparatus 212 to the electric power grid 102. More specifically, for some embodiments, first circuit breaking apparatus 210a may be connected, or installed, between the low voltage terminals of the transformer 222 and the second circuit breaking apparatus 212.

[0075] With reference to figure 3, for some embodiments, it may be defined that the first circuit breaker 214 is switchable between an open position and a closed position, that when the first circuit breaker 214 is in the closed position the first circuit breaker 214 is configured to allow an electric current to pass, and that when the first circuit breaker 214 is in the open position the first circuit breaker 214 is configured to interrupt an electric current through the first circuit breaker 214 so as to transfer a fault current to the first element 216. For some embodiments, it may be defined that that the second circuit breaker 220a, 220b is switchable between an open position and a closed position, that when the second circuit breaker 220a, 220b is in the closed position the second circuit breaker 220a, 220b is configured to allow an electric current to pass, and that when the second circuit breaker 220a, 220b is in the open position the second circuit breaker 220a, 220b is configured to interrupt an electric current. For some embodiments, the first circuit breaker 214 includes one or more switches switchable between the open position and the closed position For some embodiments, the second circuit breaker 220a, 220b includes one or more switches switchable between the open position and the closed position.

[0076] With reference to figure 3, for some embodiments, the electrical system 200a may include a high voltage switchgear 224 configured to connect, more specifically electrically connect, the transformer 222 to the electric power grid 102. For some embodiments, the power converter arrangement 202 may comprise one or more harmonic filters, for example a harmonic filter section 225 including one or more harmonic filters.

[0077] With reference to figure 4, another embodiment of the electrical system 200b according to the fifth aspect is schematically illustrated. Several items or features of the electrical system 200b of figure 4 correspond to items or features of the electrical system 200a disclosed above in connection with figure 3 and are thus not repeated here. In addition to the items which the electrical system 200b of figure 4 has in common with the electrical system 200a of figure 3, the first circuit breaking apparatus 210b of the circuit breaking arrangement 208b includes a first circuit 226b. The first circuit 226b includes a capacitor 228. The first circuit 226b is connected, more specifically electrically connected, in parallel with the first element 216.

[0078] With reference to figure 5, yet another embodiment of the electrical system 200c according to the fifth aspect is schematically illustrated. Several items or features of the electrical system 200c of figure 5 correspond to items or features of the electrical systems 200a, 200b disclosed above in connection with figures 3 and 4 and are thus not repeated here. In addition to the items which the electrical system 200c of figure 5 has in common with the electrical systems 200a, 200b of figures 3 and 4, the first circuit 226c of the first circuit breaking apparatus 210c of the circuit breaking arrangement 208c includes a second circuit 230. The second circuit 230 is connected, more specifically electrically connected, in series with the capacitor 228. The second circuit 230 includes a diode 232. The second circuit 230 includes a second element 234 having a resistance. The diode 232 is connected, more specifically electrically connected, in parallel with the second element 234. The second element 234 may be described as an electrical element. The second element 234 may comprise one or more of the group of: a resistor 236, such as a power resistor; an electrical component; and an electrical line or cable. For some embodiments, the second element 234 may have a resistance above 100 milliohm, for example above 200 milliohm, such as above 300 milliohm. For some embodiments, it may be defined that diode 232 has its cathode toward the capacitor 228, such that the capacitor current can only go through in one direction, thereby avoiding oscillations.

[0079] With reference to figures 6 and 7, aspects of embodiments of the method 300 for the protection of one or more power converters 204, 133a, 133b of a power converter arrangement 202, 132 by usage of a circuit breaking arrangement 208a-c according to the first aspect of the invention are schematically illustrated. The circuit breaking arrangement 208a-c comprises a first circuit breaking apparatus 210a-c and a second circuit breaking apparatus 212. The first circuit breaking apparatus 210a-c comprises a first circuit breaker 214 and a first element 216 having an impedance. The first element 216 is connected in parallel with the first circuit breaker 214. The second circuit breaking apparatus 212 comprises one or more second circuit breakers 220a, 220b. The first circuit breaking apparatus 210a-c is connectable to the power converter arrangement 202 via the second circuit breaking apparatus 212 while the second circuit breaking apparatus 212 is connectable to an electric power grid 102 via the first circuit breaking apparatus 210a-c. Otherwise, the circuit breaking arrangement 208a- c may be configured according to any one of the embodiments disclosed above or below.

[0080] With reference to figure 6, embodiments of the method 300 include the steps of:

[0081] • when a fault (for example, a fault current, a short circuit (SC) fault, or a short circuit current) is detected or after a fault has been detected in the power converter arrangement, controlling 302 the first circuit breaker 214 to switch to an open position so as to transfer a fault current to the first element 216 while keeping the one or more second circuit breakers 220a, 220b in a closed position so as to allow an electric current to pass; and

[0082] • when the first circuit breaker 214 is in the open position, controlling 303 the one or more second circuit breakers 220a, 220b (for example, all second circuit breakers 220a, 220b) to switch to an open position so as to interrupt a fault current (for example, a reduced fault current).

[0083] For some embodiments, the step of controlling 302 may be described as controlling 302 the first circuit breaker 214 to switch to an open position so as to transfer a fault current to the first element 216 and so as to interrupt an electric current, or a fault current, through the first circuit breaker 214 while keeping the one or more second circuit breakers 220a, 220b in a closed position so as to allow an electric current to pass.

[0084] The method 300 and the electrical system 200a-c are advantageous for MW power converters with an AC system voltage above 690V, and where the short circuit level exceeds the range, or rating, of conventional off-the-shelf-power circuit breakers. Conventional low voltage (such as below 1000 VAC) grid-connected AC power circuit breakers provide the following protection of the power converter:

[0085] • Overload protection (by opening and interrupting the circuit);

[0086] • Short circuit (SC) protection (by opening and interrupting the circuit); and

[0087] • Arc flash protection (by opening and interrupting the circuit).

[0088] The method 300 and the electrical system 200a-c are advantageous for several power converter topologies, such as full-scale systems and double-feed systems. The inventors of the present invention have identified that there is an ongoing evolution of wind turbine generators toward larger wind turbine generators with increased electric power. Larger electric power requires larger power converters and larger high-voltage step-up transformers for electric grid connection. Conventionally, the power converter power rating can easily be increased by adding more parallel power module strings. The AC low voltage power circuit breakers can also be parallelly connected to increase the load current capability. However, the inventors of the present invention have identified that adding more circuit breakers in parallel will not increase the circuit breaker short circuit breaking capacity.

[0089] An explanation why the short circuit level cannot be reduced by the conventional solution disclosed above according to the inventors is as follows: The short circuit level on the low voltage side of the transformer 222 is driven by the high voltage grid impedance and the high voltage transformer impedance. Higher impedance reduces the short circuit level. In general, the high voltage grid 102 is stiff with a low damping of the short circuit level. In general, the dominating impedance that mainly defines the short circuit level on the low voltage side is the impedance of the high voltage transformer 222. The transformer impedance (Z = R + jX) can be split in resistive impedance (R) and reactive impedance (X). In general, the resistive impedance is regulated by regulations (such as, the ECO directive of the European Union) that define the maximum losses of the transformer 222. The reactance of the transformer 222 cannot be increases to limit the short circuit level as it impacts the power converter PQ chart performance in a negative way. With these limitations, conventionally, the short circuit level cannot be reduced by adding transformer impedance.

[0090] An explanation of the low-voltage circuit breaker breaking capacity limitation for conventional solutions according to the inventors is as follows: In general, the low- voltage circuit breaker methodology for MW power converters is “air circuit breakers”. That methodology, or principle, sets limitations when the system voltage increases because of the challenge of extinguishing arcing. The conventional circuit breaker main contact behavior during a fault interruption may be disclosed as follows: While the circuit breaker trips and its main contact opens and disengage, an arc is ignited because of the voltage difference across the pole in combination with electric current flow. The arc across the pole is intended to be extinguished in the arc chamber while the AC current crosses zero. For a system voltage level above 690 VAC (such as in the LV directive range), the fault current is more challenging to interrupt because of the large voltage difference across the circuit breaker main pole. By that, the circuit breaker breaking capacity goes down when the system voltage increases. Therefore, conventionally, it is difficult to find suitable circuit breaker solutions with both the system voltage and the short circuit (SC) level in the high range of the circuit breaker application. Conventionally, for a 1000 V power converter feasibility study, it is not possible to find a conventional circuit breaker rated for the required voltage and short circuit level.

[0091] By way of the method 300 and the electrical system 200a-c, and since the fault current is efficiently reduced to a suitable level by the transfer of the fault current to the first element 216 before the one or more second circuit breakers 220a, 220b is / are switched to an open position, the short circuit (SC) level is reduced without adding losses to, or negatively impacting, the power converter PQ chart. Thus, by way of the method 300 and the electrical system 200a-c, the short circuit (SC) rating of the one or more second circuit breakers 220a, 220b can be reduced, or unchanged for higher fault current levels, since the second circuit breaker 220a, 220b will break a reduced fault current as a result of the action of the first circuit breaking apparatus 210a-c.

[0092] With reference to figure 7, for some embodiments, the method 300 may comprise one or more of the steps of:

[0093] • detecting 301 a fault (for example, a fault current, a short circuit (SC) fault, or a short circuit current) in the power converter arrangement 202 (for example, in one or more power converters 204 of the power converter arrangement 202);

[0094] • when a fault above a first predetermined level (for example, 50 kA, or any other level) is detected or after a fault above a first predetermined level has been detected in the power converter arrangement, controlling 302 the first circuit breaker 214 to switch to an open position so as to transfer a fault current to the first element 216 while keeping the one or more second circuit breakers 220a, 220b in a closed position so as to allow an electric current to pass; and

[0095] • when the first circuit breaker 214 is in the open position (and when the fault current has been transferred to the first element 216), controlling 303 the one or more second circuit breakers 220a, 220b (for example, all second circuit breakers 220a, 220b) to switch to an open position so as to interrupt a fault current (for example, a reduced fault current);

[0096] • when a fault below a first predetermined level (for example, 50 kA, or any other level) and above a second predetermined level (for example, 1 -5 kA, or any other level) is or has been detected in the power converter arrangement 202, keeping the first circuit breaker 214 in a closed position so as to allow an electric current to pass while controlling 303 the one or more second circuit breakers 220a, 220b to switch to an open position so as to interrupt a fault current; and

[0097] • when the fault is, or has been, cleared, controlling 304 the first and / or second circuit breakers 214, 220a, 220b to switch to the closed position so as to allow an electric current to pass.

[0098] With reference to figure 3, for some embodiments, the first circuit breaker 214 may comprise a main contact 238 switchable between a closed position and an open position. The second circuit breaker 220a, 220b, or each second circuit breaker 220a, 220b, may comprise a main contact 240a, 240b switchable between a closed position and an open position.

[0099] With reference to figure 7, for some embodiments, the method may comprise the steps of:

[0100] • when a fault is detected or after a fault has been detected in the power converter arrangement, controlling 302a the main contact 238 of the first circuit breaker 214 to switch to the open position so as to transfer a fault current to the first element 216 while keeping the main contact 240a, 240b of the one or more second circuit breakers 220a, 220b in the closed position so as to allow an electric current to pass; and

[0101] • when the main contact 238 of the first circuit breaker 214 is in the open position, controlling 303a the main contact 240a, 240b of the one or more second circuit breakers 220a, 220b to switch to the open position so as to interrupt a fault current.

[0102] Unless disclosed otherwise, it should be noted that the method steps illustrated in figures 6 and 7 and described herein do not necessarily have to be executed in the order illustrated in figures 6 and 7. The steps may essentially be executed in any suitable order. Further, one or more steps may be added without departing from the scope of the appended claims. One or more steps may be excluded without departing from the scope of the appended claims. Method steps of the different embodiments of figures 6 and 7 may be combined in various ways so as to form additional embodiments.

[0103] Aspects of embodiments of the method 300 according to the first aspect and of embodiments of the electrical system 200a-c are further disclosed with reference to the diagram 400 of figure 8, which schematically illustrates the time sequence for embodiments of the method 300 according to the first aspect. The flow at the top, from left to right, in figure 8 represents the control and action of first circuit breaking apparatus 210a-c. The flow at the bottom, from left to right, of figure 8 represents the control and action of second circuit breaking apparatus 212. As illustrated in figure 8 and according to the embodiments of figure 8, the protection relay for the first circuit breaking apparatus 210a-c occurs for fault currents above 50 kA while the protection relay for the second circuit breaking apparatus 212 occurs for fault currents below 50 kA. However, other levels are possible. As illustrated in figure 8, there is a delay between the trip coil activation of the first circuit breaking apparatus 210a-c and the trip coil activation of the second circuit breaking apparatus 212. Said delay may be small and may be in the range of a few milliseconds, or in the range of a few seconds.

[0104] Advantageously, the impedance of the first element 216 is balanced to provide an efficient extinguishing of an arc across the first circuit breaker 214 when the first circuit breaker 214 opens and to provide an efficient reduction of the fault current. The control of the first circuit breaker 214 and of the one or more second circuit breakers 220a, 220b may be described to be coordinated to make sure that the first circuit breaker 214 opens before the one or more second circuit breakers 220a, 220b upon a fault, such as a high current fault, or a high fault current. The method 300 and the electrical system 200a-c are advantageous for very high short circuit (SC) currents, for example above 50 kA. However, other levels are also possible.

[0105] For example, advantages of embodiments of the method 300 and of the electrical system 200a-c are as follows:

[0106] • The method 300 and the circuit breaking arrangement 208a-c can handle and interrupt short circuit (SC) faults with a voltage in the range of 900-1300 VAC (such as up to the limit of the LV directive range, and also higher in case the standards allow it) and fault currents with a very high short circuit (SC) level, for example above 50 kA;

[0107] • The first circuit breaking apparatus 210a-c can handle high short circuit (SC) levels since the arc across the main contact of the first circuit breaking apparatus 210a-c is extinguished very fast because of the parallel first element 216;

[0108] • The opening process of the one or more second circuit breakers 220a, 220b can start with only a small delay after the opening process of the first circuit breaker 214, whereby a quick interruption and a fast protection mechanism upon a fault are provided;

[0109] • The total clearing time (i.e. , the time for clearing a fault) corresponds approx, to the total clearing time of a conventional second circuit breaking apparatus having a single second circuit breaker without any first circuit breaking apparatus 210a-c according to the invention, because the main contact arc is more efficiently extinguished;

[0110] • The l2t (electric current2x time) fault level in the power converter 204 is reduced;

[0111] • There is a higher likelihood that healthy power converter stacks 206a-d survive, in case of a fault in a neighbor power converter stack 206a-d;

[0112] • The arc flash level is reduced, which lowers the arc flash hazard level;

[0113] • It is easier to pass an arc fault containment test, which is a conventional standard test known to the skilled person;

[0114] • By way of the method 300 and the electrical system 200a-c, the short circuit (SC) rating of the second circuit breakers 214, 220a, 220b can be reduced, or be unchanged for higher fault currents or higher short circuit levels; and

[0115] • When the auxiliary power system (APS) of a wind turbine generator 104 is supplied with electric power or current via the first circuit breaking apparatus 210a-c, the auxiliary power system of the wind turbine generator 104 can still be supplied with electric power also when the first circuit breaker 214 is in the open position, since electric power or current it is feed through, or via, the first element 216. It is to be understood that the method 300 and the electrical system 200a-c are applicable to three-phase applications or three-phase electric power, although this is not explicitly shown in the figures for illustrative purposes. For some embodiments, it is desirable to have the same impedance value in all three phases, for example the same impedance or resistance of the first element 216 in all three phases, which provides for an efficient current shunting to interrupt a short circuit current through the first circuit breaker 214. In general, a fault in the power converter 204 often results in a three-phase fault. Thus, equal impedance values in all three phases result in a symmetric fault, which is advantageous for the management of the fault.

[0116] With reference to figures 2 and 9, aspects of embodiments of the control arrangement 134 for the protection of one or more power converters 204 of a power converter arrangement 202 by usage of a circuit breaking arrangement 208a-c according to the fourth aspect of the invention are schematically illustrated. Embodiments of the control arrangement 134 are configured to:

[0117] • when a fault is detected or after a fault has been detected in the power converter arrangement 202, control 302 the first circuit breaker 214 to switch to an open position so as to transfer a fault current to the first element 216 while the one or more second circuit breakers 220a, 220b is / are kept in a closed position so as to allow an electric current to pass; and

[0118] • when the first circuit breaker 214 is in the open position, control 303 the one or more second circuit breakers 220a, 220b to switch to an open position so as to interrupt a fault current.

[0119] With reference to figure 2, some embodiments of the control arrangement 134 may include a controller unit 134a for controlling the first circuit breaker 214 and the one or more second circuit breakers 220a, 220b in order to perform steps 302, 303, 302a, 303a and 304 in figures 6 and 7. Some embodiments of the control arrangement 134 may include a detection unit 134b for detecting a fault in order to perform step 301 in figure 7. The detection unit 134b may be configured to be connected to and / or to communicate with a sensor or detector for measuring or detecting a signal or fault current / voltage associated with the fault, for example located in the first or second circuit breaker 214, 220a, 220b, in the power converter arrangement 202, in the power converter 204, or elsewhere.

[0120] With reference to figure 2, for some embodiments, the control arrangement 134 is configured to directly or indirectly communicate, for example via signal lines (or cables or wires) or wirelessly, with one or more of the group of: the electrical system 200a-c; the power converter arrangement 202; the power converter 204; the circuit breaking arrangement 208a-c; the first and second circuit breaking apparatuses 210a-c, 212; the first and second circuit breakers 214, 220a, 220b; the power plant 100; the wind turbine generator 104; the electric power grid 102; sensors; and other devices or systems of the wind turbine generator 104, of the power plant 100, or of the electric power grid 102.

[0121] Figure 9 shows in schematic representation an embodiment of the control arrangement 134 according to the fourth aspect of the invention, which may include a control unit 700, which may correspond to or may include one or more of the above-mentioned units 134a-b of the control arrangement 134. The control unit 700 may comprise a computing unit 701 , which can be constituted by essentially any suitable type of processor or microcomputer, for example a circuit for digital signal processing (Digital Signal Processor, DSP), or a circuit having a predetermined specific function (Application Specific Integrated Circuit, ASIC). The computing unit 701 is connected to a memory unit 702 arranged in the control unit 700. The memory unit 702 provides the computing unit 701 with, for example, the stored program code and / or the stored data which the computing unit 701 requires to be able to perform computations. The computing unit 701 is also arranged to store partial or final results of computations in the memory unit 702.

[0122] With reference to figure 9, in addition, the control unit 700 may be provided with devices 711 , 712, 713, 714 for receiving and transmitting input and output signals. These input and output signals may contain waveforms, impulses, or other attributes which, by means of the devices 711 , 713 for the reception of input signals, can be detected as information and can be converted into signals which can be processed by the computing unit 701 . These signals are then made available to the computing unit 701 . The devices 712, 714 for the transmission of output signals are arranged to convert signals received from the computing unit 701 in order to create output signals by, for example, modulating the signals, which, for example, can be transmitted to other parts and / or systems of, or associated with, the electrical system 200a-c, the wind turbine generator 104, the electric power grid 102 (see figure 2) and / or the power plant 100 (see figure 1 ). Each of the connections to the devices for receiving and transmitting input and output signals can be constituted by one or more of a cable; a data bus; and a wireless connection.

[0123] Here and in this document, units are often described as being provided for performing steps of the method according to embodiments of the invention. This also includes that the units are designed to and / or configured to perform these method steps.

[0124] With reference to figure 2, the units 134a-b of the control arrangement 116 are in figure 1 illustrated as separate units. These sperate units may, however, be logically separated but physically implemented in the same unit, or can be both logically and physically arranged together. The units 134a-b may for example correspond to groups of instructions, which can be in the form of programming code, that are input into, and are utilized by a processor / computing unit 701 (see figure 9) when the units are active and / or are utilized for performing its method step.

[0125] With reference to figures 2 and 9, the control arrangement 134, which may include one or more control units 700, for example one or more devices, controllers or control devices, according to embodiments of the present invention may be arranged to perform all of the method steps mentioned above, in the claims, and in connection with the herein described embodiments. The control arrangement 134 is associated with the above-described advantages for each respective embodiment of the method.

[0126] With reference to figure 9, according to the second aspect of the invention, a computer program 703 is provided, comprising instructions which, when the program is executed by a computer, cause the computer to carry out one or more of the method according to any one of the embodiments disclosed above and process according to any one of the embodiments disclosed above. For some embodiments, a computer program 703 is provided, comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method according to any one of the embodiments disclosed above

[0127] According to the third aspect of the invention, a computer-readable medium is provided, comprising instructions which, when the instructions are executed by a computer, cause the computer to carry out one or more of the method according to any one of the embodiments disclosed above and process according to any one of the embodiments disclosed above. For some embodiments, a computer-readable medium is provided, comprising instructions which, when the instructions are executed by a computer, cause the computer to carry out the method according to any one of the embodiments disclosed above.

[0128] The person skilled in the art will appreciate that the herein described embodiments of the method according to the first aspect may be implemented in a computer program 703 (see figure 9), which, when it is executed in a computer, instructs the computer to execute the method. The computer program is usually constituted by a computer program product 703 stored on a non-transitory / non-volatile digital storage medium, in which the computer program is incorporated in the computer-readable medium of the computer program product. The computer-readable medium comprises a suitable memory, such as, for example: ROM (Read-Only Memory), PROM (Programmable Read-Only Memory), EPROM (Erasable PROM), Flash memory, EEPROM (Electrically Erasable PROM), a hard disk unit, etc.

[0129] With reference to figures 2 to 6 and 9, for some embodiments, the electrical system 200a-c may comprise a control arrangement 134 according to any one of the embodiments disclosed above. For some embodiments, the wind turbine generator 104 may comprise one or more of the group of: an electrical system 200a-c according to any one of the embodiments disclosed above; and a control arrangement 134 according to any one of the embodiments disclosed above. It is to be understood that that some embodiments of the method 300 according to the first aspect may be applied to power converter arrangements not included in or being part of, or associated with, a wind turbine generator 104. The present invention is not limited to the above-described embodiments. Instead, the present invention relates to, and encompasses all different embodiments being included within the scope of the independent claims.

Claims

Claims1 . A method (300) for the protection of one or more power converters (204) of a power converter arrangement (202) connected to a power grid (102) via a transformer (222) by usage of a circuit breaking arrangement (208a-c), wherein the circuit breaking arrangement (208a-c) comprises, a first circuit breaking apparatus (210a-c) and a second circuit breaking apparatus (212), connected between a low voltage side of the transformer and the power converter arrangement (202), wherein the first circuit breaking apparatus (210a-c) comprises a first circuit breaker (214) and a first element (216) having an impedance, wherein the first element (216) is connected in parallel with the first circuit breaker (214), wherein the second circuit breaking apparatus (212) comprises one or more second circuit breakers (220a, 220b), wherein the first circuit breaking apparatus (210a-c) is connectable to the power converter arrangement (202) via the second circuit breaking apparatus (212), wherein the second circuit breaking apparatus (212) is connectable to an electric power grid (102) via the first circuit breaking apparatus (210a-c), wherein the method (300) comprises: when a fault is detected or after a fault has been detected in the power converter arrangement (202), controlling (302) the first circuit breaker (214) to switch to an open position so as to transfer a fault current to the first element (216) while keeping the one or more second circuit breakers (220a, 220b) in a closed position so as to allow an electric current to pass; and when the first circuit breaker (214) is in the open position, controlling (303) the one or more second circuit breakers (220a, 220b) to switch to an open position so as to interrupt a fault current.

2. A method (300) according to claim 1 , wherein the method (300) comprises: detecting (301 ) a fault in the power converter arrangement (202).

3. A method (300) according to claim 1 or 2, wherein the first circuit breaker (214) comprises a main contact (238) switchable between a closed position and an open position, and wherein the second circuit breaker (220a, 220b) comprises a main contact (240a, 240b) switchable between a closed position and an open position, and wherein the method (300) comprises: when a fault is detected or after a fault has been detected in the power converter arrangement (202), controlling (302a) the main contact (238) of the first circuit breaker (214) to switch to the open position so as to transfer a fault current to the first element (216) while keeping the main contact (240a, 240b) of the one or more second circuit breakers (220a, 220b) in the closed position so as to allow an electric current to pass; and when the main contact (238) of the first circuit breaker (214) is in the open position, controlling (303a) the main contact (240a, 240b) of the one or more second circuit breakers (220a, 220b) to switch to the open position so as to interrupt a fault current.

4. A method (300) according to any one of the claims 1 to 3, wherein the method (300) comprises: when a fault above a first predetermined level is or has been detected in the power converter arrangement (202), controlling (302) the first circuit breaker (214) to switch to an open position so as to transfer a fault current to the first element (216) while keeping the one or more second circuit breakers (220a, 220b) in a closed position so as to allow an electric current to pass.

5. A method (300) according to any one of the claims 1 to 4, wherein the method (300) comprises: when a fault below a first predetermined level and above a second predetermined level is or has been detected in the power converter arrangement (202), keeping the first circuit breaker (214) in a closed position so as to allow an electric current to pass while controlling (303) the one or more second circuit breakers (220a, 220b) to switch to an open position so as to interrupt a fault current.

6. A method (300) according to any one of the claims 1 to 5, wherein the method (300) comprises: when the fault is cleared, controlling (304) the first and second circuit breakers (214, 220a, 220b) to switch to the closed position so as to allow an electric current to pass.

7. A computer program (703) or a computer-readable medium comprising instructions which, when the program or the instructions is / are executed by a computer, cause the computer to carry out the method (300) according to any one of the claims 1 to 6.

8. A control arrangement (134) for the protection of one or more power converters (204) of a power converter arrangement (202) connected to a power grid (102) via a transformer (222) by usage of a circuit breaking arrangement (208a-c), wherein the circuit breaking arrangement (208a-c) comprises a first circuit breaking apparatus (210a-c) and a second circuit breaking apparatus (212), connected between a low voltage side of the transformer and the power converter arrangement (202), wherein the first circuit breaking apparatus (210a-c) comprises a first circuit breaker (214) and a first element (216) having an impedance, wherein the first element (216) is connected in parallel with the first circuit breaker (214), wherein the second circuit breaking apparatus (212) comprises one or more second circuit breakers (220a, 220b), wherein the first circuit breaking apparatus (210a-c) is connectable to the power converter arrangement (202) via the second circuit breaking apparatus (212), wherein the second circuit breaking apparatus (212) is connectable to an electric power grid (102) via the first circuit breaking apparatus (210a-c), wherein the control arrangement (134) is configured to: when a fault is detected or after a fault has been detected in the power converter arrangement (202), control (302) the first circuit breaker (214) to switch to an open position so as to transfer a fault current to the first element (216) while the oneor more second circuit breakers (220a, 220b) is / are kept in a closed position so as to allow an electric current to pass; and when the first circuit breaker (214) is in the open position, control (303) the one or more second circuit breakers (220a, 220b) to switch to an open position so as to interrupt a fault current.

9. An electrical system (200a-c) comprising a power converter arrangement (202) comprising one or more power converters (204) connected to a power grid (102) via a transformer (222), wherein the electrical system (200a-c) comprises a circuit breaking arrangement (208a-c) for the protection of the power converter arrangement (202), connected between a low voltage side of the transformer and the power converter arrangement (202), wherein the circuit breaking arrangement (208a-c) comprises a first circuit breaking apparatus (210a-c) and a second circuit breaking apparatus (212), wherein the first circuit breaking apparatus (210a-c) comprises a first circuit breaker (214) and a first element (216) having an impedance, wherein the first element (216) is connected in parallel with the first circuit breaker (214) for the transfer of a fault current from the first circuit breaker (214) to the first element (216) when the first circuit breaker (214) is switched to an open position, wherein the second circuit breaking apparatus (212) comprises one or more second circuit breakers (220a-b), wherein the first circuit breaking apparatus (210a-c) is connectable to the power converter arrangement (202) via the second circuit breaking apparatus (212), and wherein the second circuit breaking apparatus (212) is connectable to an electric power grid (102) via the first circuit breaking apparatus (210a-c).

10. An electrical system (200a-c) according to claim 9, wherein the power converter arrangement (202) comprises the second circuit breaking apparatus (212).

11. An electrical system (200a-c) according to claim 9 or 10, wherein one or more of the first and second circuit breaking apparatuses (214, 220a, 220b) is / are configured to detect a fault in the power converter arrangement (202).

12. An electrical system (200b-c) according to any one of the claims 9 to 11 , wherein the first circuit breaking apparatus (210b-c) comprises a first circuit (226b-c) comprising a capacitor (228), and wherein the first circuit (226b-c) is connected in parallel with the first element (216).

13. An electrical system (200c) according to claim 12, wherein the first circuit (226c) comprises a second circuit (230) connected in series with the capacitor (228), wherein the second circuit (230) comprises a diode (232) and a second element (234) having a resistance, and wherein the diode (232) is connected in parallel with the second element (234).

14. An electrical system (200a-c) according to any one of the claims 9 to 13, wherein when a fault is detected or after a fault has been detected in the power converter arrangement (202), the first circuit breaker (214) is configured to open so as to transfer a fault current to the first element (216) before any one of the one or more second circuit breakers (220a, 220b) opens.

15. A wind turbine generator (104), wherein the wind turbine generator (104) comprises one or more of the group of:• an electrical system (200a-c) according to any one of the claims 9 to 14; and• a control arrangement (134) according to claim 8.

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