Arc quenching in wind turbines

Abstract

A switching arrangement (30) for arc quenching in a wind turbine (1), the wind turbine (1) having a generator (11) and a machine-side converter (12). The switching arrangement (30) comprises a set of switches (34) electrically coupled to power lines (13) of a connection (13) between the generator (11) and the machine-side converter (12). At least one power line (36, 38) of the connection (13) is permanently connected to multiple switches (34) of the switching arrangement (30).

Description

[0001] Arc Quenching in Wind Turbines

[0002] Technical Field

[0003] The present disclosure generally relates to arc quenching in wind turbines, and particularly to quenching arcs in wind turbines having a permanent magnet generator.

[0004] Background

[0005] In wind turbines that use a permanent magnet generator (PMG), due to the fixed nature of the magnets the output of the generator is typically dictated by the rotational speed of an armature, or ‘generator rotor’, that carries the magnets, which in turn is dictated by a speed of a main rotor of the wind turbine that drives the armature. The rotor speed, in turn, depends on wind conditions. Accordingly, the output of a PMG varies widely and stochastically in operation.

[0006] Wind turbines use power converters to modify the generator output into a form suitable for onward transmission to a power grid. A power converter typically includes a ‘machine-side converter’ that converts the AC output of the generator to DC, followed by a ‘grid-side converter’ or ‘line-side converter’ that converts the DC output of the machine-side converter into an AC output having characteristics suitable for the grid. The machine-side converter typically includes a set of power semiconductor switches that are operated to combine the phases of the output of the stator windings into a DC signal at a controllable amperage.

[0007] In a typical implementation, the permanent magnets of a PMG are carried by an armature that is disposed centrally within a stator, the stator holding one or more winding sets that surround the magnets, each winding set having windings that are grouped to create a three-phase output. When the rotor of the wind turbine drives rotation of the armature, the magnets produce a rotating flux field, or ‘rotor flux’, which in turn induces complementary sinusoidal back-EMF in each of the stator windings. In such arrangements, voltage and power will be generated in the stator windings and circuits connected to the stator windings whenever the armature is rotating.

[0008] The PMG and the machine-side converter are typically connected by one or more generator links comprising conductors, power distribution cables, power leads, bus bars, or the like. The output load of the PMG is fed through the generator link(s) to the machine-side converter for power conversion and eventual delivery to the grid. Each generator link typically carries a three-phase output from the PMG, and so includes multiple parallel power lines that each carries a respective phase. Multiple generator links may be implemented, to help manage the output load from the PMG. For example, the stator windings may be split into multiple winding sets that produce respective three-phase outputs that are carried by respective generator links, to balance the output current from the PMG for improved load sharing.

[0009] A protective device, such as a circuit breaker or fuse, is typically positioned between the PMG and the machine-side converter to disconnect the PMG should a fault arise, such as an arc. The protective device will typically be mounted in proximity to the machine-side converter, resulting in the majority of the impedance generated by the generator links between the PMG and the machineside converter being between the PMG and the protective device.

[0010] In the event of a fault, an arc may form between components of a wind turbine. An arc is ordinarily a conductive path of plasma that propagates through an ordinarily non-conductive medium such as air. An arc is sustained if the current is sufficient to evaporate material in the arc path to form plasma and the voltage is sufficient to drive enough current through the arcing impedance to sustain the arc path. Higher currents and voltages can therefore drive and sustain larger arcs.

[0011] The energy of an arc is a proportional function of voltage, current, and time. It is therefore desirable to quench an arc quickly, to reduce the energy of the arc and the associated risk of damage. The short-circuit current of a PMG is typically relatively low, for example below 2x per unit relative to normal operation. As such, it is mainly arcs that are long in time duration that can threaten significant consequences.

[0012] The generator link(s) connecting the PMG to the machine-side converter may be equipped with an arc detection system and arc relay system. Typically, in the case of arc detection, the PMG is brought to a stop as fast as possible and the machine-side converter is disconnected using the protective device. However, the portions of the generator link(s) between the PMG and the protective device cannot be disconnected and are therefore always live when the armature is rotating. A fault in the generator link(s) will therefore be fed as long as the armature rotates. Stopping a PMG to terminate a fault can take over a minute, particularly for a large PMG such as those used in megawatt wind turbines, for example. A fault that persists for such a time period may threaten damage to the wind turbine. Known methods to extinguish an arc between the PMG and the machine-side converter involve disconnecting the power source to the arc or slowing the PMG down to reduce the arcing current. These known methods can take a relatively long period of time to extinguish an arc, particularly for larger arcs, and may require additional hardware. The inherent impedance of the generator link(s) connecting the PMG and the machine-side converter can also reduce the effectiveness of known quenching systems.

[0013] Crowbar arrangements, systems, or circuits are known to be used to short-circuit or ‘clamp’ the voltage across individual conductors of a generator link between the generator and a protective device and can be composed of, for example, circuit-breakers or thyristors. An example known crowbar system can be found in US 2023 / 144976. Dedicated quenching devices may also be used for clamping the voltage across conductors of a generator link.

[0014] Known dedicated arc quenching devices are often configured for a single use, and so must be replaced each time the device is activated to quench an arc.

[0015] It is against this background that the invention has been devised.

[0016] Summary of the Invention

[0017] According to an aspect of the invention, there is provided a switching arrangement for arc quenching in a wind turbine, the wind turbine having a generator, comprising two sets of three phase windings and a machine-side converter arranged to connect to the two sets of three phase windings, wherein the switching arrangement comprises a set of four switches electrically coupled to power lines of a connection between the generator and the machine-side converter, and wherein at least one power line of the connection for each of the two sets of three phase windings is permanently connected to multiple switches of the switching arrangement, so a first two of the four switches connects to the first sets of three phase windings and a second two of the four switches connects to the second sets of three phase windings.

[0018] A power line being permanently connected to switches may entail that the power line is connected to the switches whether those switches are open or closed, for example, or that the power line is directly connected to the switches and does not connect to the switches through other switches, for example. This arrangement may advantageously entail that the switching arrangement does not require a respective switch for each power line to which the switching arrangement is connected, thereby reducing the component count and complexity of the arrangement. Especially the use of a four pole switch to connect to two sets of three phase windings, provides the benefit of low component count, it also provides the benefit of ensuring that all two times three connecting power lines are short circuited at the same time, as the four pole switch operates all four poles simultaneously.

[0019] The connection may be configured to transfer electrical power output from the generator to the machine-side converter in normal operation, and may comprise one or more conductors providing electrical connections between the generator and the machine-side converter, for example. The conductors of the connection may be in the form of power leads, cables, or busbar links, for example. An arc that is in, or on, the connection may refer to an arc that is attached to a power line of the connection, for example, in that the arc extends to or originates from the power line. An arc may, for example, extend between power lines of the connection, from one power line to another, and / or between a power line of the connection and another component of the wind turbine. Each power line of the connection may carry a respective phase of an output of the generator.

[0020] The machine-side converter may be configured to perform AC to DC conversion for power output from the generator. The machine-side converter may form part of a power converter of the wind turbine that provides AC to AC power conversion.

[0021] The switching arrangement may advantageously enable arcs to be quenched in a simple, efficient, and effective manner which reduces system complexity. The switching arrangement may define, or comprise, a crowbar arrangement.

[0022] The generator of the wind turbine may comprise a permanent magnet generator, for example.

[0023] In some embodiments, the number of power lines to which switches of the set of switches are coupled may exceed the number of switches of the set. Accordingly, advantageously the switching arrangement may have fewer individual switches than there are power lines in the connection, reducing the number of components used compared to a scenario in which each power line is associated with a respective switch, for example.

[0024] In some embodiments, the switching arrangement may be operable to create a short-circuit in or on the connection between the generator and the machine-side converter upon detection of an arc. In some embodiments, the switching arrangement may be operable to connect together electrically power lines of the connection between the generator and the machine-side converter, to create a short-circuit across the connection and help quench the arc. Optionally, the switching arrangement may be operable to connect all power lines of the connection together electrically. The switching arrangement may therefore be operable to clamp voltage in the power lines of the connection to promote quenching of an arc. The switching arrangement may be operable to connect power lines of the connection together electrically substantially simultaneously.

[0025] In some embodiments, the wind turbine may comprise a protective device disposed between the generator and the machine-side converter, in which case the set of switches may be electrically connected to power lines of a connection between the generator and the protective device. In some embodiments, the switching arrangement may be configured to quench an arc at a location between the generator and the protective device, therefore beneficially providing for the quenching of arcs in an area of the wind turbine system that is usually regarded as unprotected.

[0026] In some embodiments, the switching arrangement may be configured to quench an arc that extends to or from at least one of the power lines.

[0027] In some embodiments, the switching arrangement comprises 2n switches, wherein n is a positive integer, and wherein the 2n switches are electrically coupled to 3n power lines of the connection.

[0028] In some embodiments, the set of switches may be disposed proximal to the generator, allowing the switching arrangement to advantageously bypass the majority of impedance associated with the connection between the generator and the machine-side converter. Alternatively, the set of switches may be disposed proximal to the machine-side converter, which may facilitate installation of the switching arrangement within the wind turbine system.

[0029] In some embodiments, the set of switches may comprise one or more circuit breaker switches.

[0030] In some embodiments, the switching arrangement may comprise one or more multi-pole switches having a plurality of poles. Each pole of the, or each, multi-pole switch may be electrically connected to a respective power line. A multi-pole switch may be embodied as a switching device. The switching arrangement may comprise a switching device that comprises the set of switches. It is also possible for the set of switches to be distributed between multiple switching devices. The, or each, switching device may comprise a multi-pole switch.

[0031] The connection between the generator and the machine-side converter may comprise multiple sets of power lines, each set of power lines carrying a respective three-phase output from the generator, in use. Each set of power lines may define a generator link, for example. The connection may comprise multiple sets of power lines that are all electrically coupled to a common switching device of the switching arrangement that comprises the set of switches, such as a multi-pole switch.

[0032] The set of switches may be connected to all power lines of the connection. If the switching arrangement comprises a switching device such as a multi-pole switch that comprises the set of switches, all power lines of the connection may be connected to the switching device.

[0033] According to another aspect of the invention, there is provided a wind turbine, comprising the switching arrangement of any preceding claim. The wind turbine may comprise a generator, for example a permanent magnet generator, and a machine-side converter. The wind turbine may comprise a connection between the generator and the machine-side converter, which connection may comprise a set of power lines to which switches of the switching arrangement are electrically coupled. At least one power line of the connection may be permanently connected to multiple switches of the switching arrangement.

[0034] The first and second switches may be operated substantially simultaneously. The first and second switches may form part of a set of switches of a switching arrangement, which switching arrangement may comprise, or define, a crowbar arrangement.

[0035] The method may comprise operating additional switches associated with further power lines. For example, a third switch may be operated to connect a fourth power line of the connection to a fifth power line of the connection, and a fourth switch may be operated to connect the fourth power line of the connection to a sixth power line of the connection.

[0036] It will be appreciated that preferred and / or optional features of each aspect of the invention may be incorporated alone or in appropriate combination in the other aspects of the invention also.

[0037] Brief Description of the Drawings So that it may be more fully understood, the invention will now be described, by way of example only, with reference to the following drawings, in which like features are assigned like reference numerals, and in which:

[0038] Figure 1 is a schematic diagram of a wind turbine in which embodiments of the invention may be implemented;

[0039] Figure 2 is a simplified schematic diagram of a generator system of the wind turbine of Figure 1 ;

[0040] Figure 3 is a schematic diagram of the generator system of Figure 1 showing an implementation of an arc quenching arrangement according to an embodiment of the invention; and

[0041] Figure 4 shows a detail view of an area ‘A’ of the schematic diagram of the generator system of Figure 3.

[0042] Detailed Description

[0043] Embodiments of the invention are directed to improved arc quenching for arcs forming between a generator, for example a permanent magnet generator (PMG), and an associated machine-side converter (MSC) in a generator system, for example a generator system of a wind turbine, and in particular for quenching arcs that form between the generator and a protective device such as a circuit breaker disposed between the generator and the MSC.

[0044] More specifically, arcs to be quenched between a generator such as a PMG and a protective device may form on or in a connection between respective terminals of the PMG and the protective device, which connection may include multiple separate conductors within one or more generator links and may be referred to as a ‘PMG power interconnect’, for example. Embodiments of the invention therefore beneficially provide for quenching of arcs in an area of the system that is usually regarded as unprotected.

[0045] In some embodiments, the proposed approach involves implementing a switching arrangement that connects to a PMG power interconnect and comprises, for example, a plurality or set of switches such as circuit-breaker switches, which may be configured to clamp voltage across the conductors of the PMG power interconnect. Upon detection of an arc in the PMG power interconnect, the switching arrangement is operated to clamp the voltage and so help to quench the arc in a simple and effective manner.

[0046] The switching arrangement may be arranged such that multiple switches of the switching arrangement are permanently connected to a single common conductor in the generator link. For example, the common conductor may connect to a conductor, terminal, or node between two switches, such that the common conductor is connected to those two switches whether the switches are open or closed. It follows that the switching arrangement may be configured such that the number of power lines in the PMG power interconnect exceeds the number of switches of the plurality or set of switches in the switching arrangement, thereby reducing the complexity in the associated system and increasing the system’s reliability. For example, a switching arrangement having a set of two individual switches can be used to connect three conductors together if one of the conductors is permanently connected to both of the switches. So, in principle, in some embodiments two-pole switches may be used to provide the desired functionality.

[0047] A generator system may have multiple generator links and associated winding sets, in which case an arc quenching arrangement, such as a switching arrangement, may be employed across each generator link to enable clamping of the voltage across each respective winding set. Each winding set typically shares a portion of the overall output load from the PMG, therefore clamping only one winding set would potentially increase the portion of the overall load in each of the other winding sets. It may therefore be desirable to clamp all of the winding sets for effective arc quenching. In some embodiments, the above solution involving using two switches for an individual generator link may be scaled to an arrangement having multiple links, for example by using a single four-pole switch having four individual switches to provide an arc quenching arrangement for a pair of three- phase generator links. In this respect, a four-pole switch or other multi-pole switch may be regarded as a single switching arrangement or device that includes a set of individual switches.

[0048] Specific example arc quenching arrangements in accordance with embodiments of the invention are described herein. First, to provide context for the invention, an individual wind turbine 1 in which quenching arrangements and strategies may be implemented is described with reference to Figure 1. The example shown is based on a full-scale architecture, but it should be appreciated that the wind turbine 1 is referred to here by way of example only, and it would be possible to implement embodiments of the invention into many different types of wind turbine systems and power plant architectures. It is noted that the topology illustrated in Figure 1 is entirely illustrative, and many architectures are known that would be suitable for use with embodiments of the invention. The wind turbine 1 shown in Figure 1 is a three-bladed upwind horizontal-axis wind turbine (HAWT), which is the most common type of turbine in use. The wind turbine 1 comprises a rotor 2 having three blades 3 extending radially from, and equi-angularly spaced around, a central hub 4. It is noted that although three blades are common, different numbers of blades may be used in alternative implementations. The rotor 2 is supported by its hub 4 at the front of a nacelle 5, which in turn is mounted at the top of a support tower 6 that is secured to a foundation (not shown) that is embedded in the ground.

[0049] The nacelle 5 contains a generator system 10 (shown in Figures 2 and 3), which in this example includes a PMG that is driven by the rotor 2 to produce electrical energy. Thus, the wind turbine 1 is able to generate electrical power from a flow of wind passing through the swept area of the rotor 2 causing rotation of the blades 3. In this respect, the blades 3 cover a circular swept area that is represented in Figure 1 by a dashed circle encompassing the tips of the blades 3.

[0050] Figure 2 is a simplified schematic of part of the generator system 10 of the wind turbine 1 of Figure 1 in an embodiment of the invention. Further components of the generator system 10 are shown in Figures 3 and 4 and will be described later. This example is representative only and the skilled reader will appreciate that embodiments of the invention may be applicable to many different configurations. Moreover, some components of the wind turbine 1 and the generator system 10 shown in Figure 2 may be generally conventional and as such familiar to the skilled reader, and so will only be described briefly where appropriate.

[0051] In this embodiment, the generator system 10 includes an electrical generator in the form of a PMG 11 , which includes a central armature, or ‘generator rotor’, supporting a set of permanent magnets arranged to rotate within a stator to produce electrical power. Rotation of the armature is driven by the rotor 2 via a drivetrain (not shown) that comprises a gearbox that provides an increase in speed between the rotor 2 and the armature, the gearbox being coupled to the armature via a generator shaft. Wind turbines without a gearbox are also common, and these are often referred to as ‘direct drive’ wind turbines. In such wind turbines, the armature speed and position directly correspond to the speed and position of the main rotor.

[0052] In this example, the generator stator has multiple winding sets, each winding set comprising a set of windings that collectively produce a respective three-phase output in response to varying magnetic flux created by rotation of the armature, the output from each winding set being delivered to a respective generator link. For the purposes of illustration and to simplify the description of the overall layout of the system 10, Figure 2 represents one winding set of the stator, while Figure 3 shows the overall layout of the system and is described later.

[0053] The power produced in the stator windings is three-phase AC, but is not in a form suitable for delivery to a grid. In particular, in this example the PMG 11 is a low voltage system having an output voltage below 1000V, which is much lower than the voltage required by some electrical grids. The PMG 11 output is also typically not at the correct frequency or phase angle, because the frequency and angle of the generated power are determined, at least in part, by the speed of rotation of the rotor 2, which in turn is dependent on wind conditions.

[0054] An MSC 12 provides a first stage of power conversion for the generator system 10 to process the electrical power produced by the PMG 11 into a suitable output waveform having suitable characteristics for delivery to the grid via a line-side converter (LSC) (not shown in Figure 2). The MSC 12 carries out AC to DC conversion to produce a DC output, which is converted back to AC by the LSC, which is a DC-AC converter in series with the MSC 12. The MSC 12 comprises an array of power semiconductor switches, such as insulated gate bi-polar transistors (IGBTs), each of which outputs a voltage that is controlled using a respective control signal from an associated control system, with the output voltage being proportional to a duty cycle of the control signal. The MSC 12 may also connect to a chopper (not shown) for stabilising the DC output from the MSC 12 in case the LSC cannot consume and convert the generated power from the PMG 11 .

[0055] The PMG 11 and MSC 12 are connected by circuitry defining a three-phase generator link 13 between the PMG 11 and the MSC 12. The generator link 13 includes a set of three power lines that each carries a respective phase of the three-phase output of the stator windings, each power line comprising conductors in the form of power leads in this example. Other types of circuitry having alternative conductors and / or connections, such as power distribution cables, bus bars or the like, may be equally applicable for the generator link 13 between the PMG 11 and MSC 12.

[0056] An arc detection system (not shown) is coupled to the circuitry connecting the PMG 11 to the MSC 12.

[0057] The generator system 10 includes a protective device 14, which in this example is a circuit-breaker, positioned between the PMG 11 and the MSC 12 in the generator link 13. Accordingly, the generator link 13, and each power line of the generator link 13, has a first portion that connects the PMG 11 to the protective device 14, and a second portion that connects the protective device 14 to the MSC 12. If the generator link 13 shown in Figure 2 were the only such link in the system 10, the first portion of the generator link 13 could be regarded as a ‘PMG power interconnect’. However, as discussed below with reference to Figure 3, the generator system includes at least one further generator link and so the respective first portions of the generator links collectively define a PMG power interconnect in this example.

[0058] Each power line of the generator link 13 has a pair of individual conductors arranged in series, one on each side of the protective device 14. More specifically, in the example shown in Figure 2 a first power line of the generator link 13, shown uppermost in Figure 2, includes conductors 23a and 23d, a second power line includes conductors 23b and 23e, and a third power line includes conductors 23c and 23f.

[0059] The conductors 23a, 23b, 23c within the first portion of the generator link 13 connect to the PMG 11 via respective PMG electrical power terminals 21a, 21 b, 21 c, and connect to the protective device 14 via respective circuit-breaker terminals 24a, 24b, 24c. Conductors 23d, 23e, and 23f, within the second portion of the generator link 13, connect the protective device 14 to the MSC 12 to complete the connection between the PMG 11 and the MSC 12.

[0060] When activated and opened, the protective device 14 acts to isolate the MSC 12 from the PMG 11 electrically, by breaking the connections between the respective pairs of conductors of the power lines of the generator link 13.

[0061] The protective device 14 may be positioned proximal to the MSC 12, such that the conductors of the first portion of the generator link 13 may be comparatively longer in length than those of the second portion of the generator link 13. Accordingly, the first portion of the generator link 13 may represent the majority of the impedance 18 between the PMG 11 and the MSC 12, as represented by respective impedances 18a, 18b, 18c associated with each conductor 23a, 23b, 23c of the first portion of the generator link 13 shown in Figure 2.

[0062] The generator system 10 includes a number of sensors. These sensors include a shaft encoder 15 for providing a signal indicative of the position of the rotor 2 in this example, but in other examples an equivalent phase sensor method or the like may be used to determine the rotor position. The encoder 15 is coupled to the PMG 11 to sense rotation of the generator shaft, and therefore of the armature. Electrical phase power lead current sensors 16 may be coupled to each of the conductors 23d, 23e, 23f in the second portion of the generator link 13 as illustrated in Figure 2, enabling the current sensors 16 to monitor the currents in the generator link 13. In the event of a line-to-line arc 100 in the first portion of the generator link 13 as shown in Figure 2, the current sensors 16 allow the phase angle of the armature and therefore of the arcing current to be tracked and so improves the estimation of the arc parameters.

[0063] An MSC control module 17 controls the operation of the MSC 12, for example by implementing synchronous reference frame or ‘direct-quadrature’ (DQ) control. In normal operation, signals are generated by the sensors based on system measurements and are fed into the control module 17. Based on the received signals, the control module 17 controls the operation of the MSC 12, for example by providing a control signal, which may be in the form of a pulse-width modulated regulated sinusoidal-type current, to the MSC 12.

[0064] As the rotor of the wind turbine rotates the armature, the magnets of the PMG produce a rotating flux field, or ‘rotor flux’, which in turn induces complementary sinusoidal back-EMF in each of the stator windings. Voltage and power are therefore delivered to the conductors 23a, 23b, 23c of the first portion of the generator link 13 whenever the armature is rotating.

[0065] The arc 100 represented in Figure 2 may be detected in various ways, for example by one or more of the sensors in the system as illustrated in Figure 2. For example, the current sensors 16 may detect a spurious or abnormal current, and / or the shaft encoder 15 may determine a deviation in the PMG’s speed from normal conditions. An arc detector (not shown) may be implemented in the system 10 to detect that an arc 100 has formed via, for example, optical means or by further current and voltage analysis. The one or more sensors in the system may be configured to generate a signal indicative of an arc in or on the connection between the generator and the protective device. An associated controller may be operatively coupled to the one or more sensors and may receive the signal from the one or more sensors. The controller may be configured to carry out the arc quenching methods described herein based on the received signal. In this example, the MSC control module 17 acts as the controller that implements arc quenching, but in other embodiments a different controller may be used.

[0066] The first portion of the generator link 13 is typically considered to be unprotected by the protective device 14, in that opening the protective device 14 to disconnect the PMG 11 from the MSC 12 does not interrupt or quench any arcs that arise in the first portion of the generator link 13. A fault in the first portion of the generator link 13 may arise over a combination of conductors 23a, 23b, 23c within the generator link 13. For example, a fault that occurs in the first portion of the generator link 13 can lead to the formation of a line-to-line arc 100 as shown in Figure 2, namely an arc extending between two or more conductors of the first portion of the generator link 13. It will be appreciated that a generator link-to-component arc, between one or more conductors of the generator link 13 and other components within the wind turbine 1 , is also possible.

[0067] An arc 100 in the first portion of the generator link 13 cannot be extinguished or quenched by opening the protective device 14 to disconnect the MSC 12, therefore the PMG 11 is typically slowed and stopped upon detecting the arc 100 to help reduce the arcing current below the minimum current necessary to sustain the arc 100. In examples, the PMG 11 may have a constant and linear short-circuit contribution across the rotational speed range of the PMG 11 above approximately 3Hz. At a low speed, such as below 3Hz, a resistance of the PMG 11 may become the dominating impedance, which decreases the short-circuit contribution. The voltage back-EMF is a function of rotational speed, and decreases as the speed of the PMG 11 reduces. The short- circuit current from the PMG 11 is therefore present to sustain the arc as the speed of the PMG 11 decreases, until a point is reached at which the PMG resistance dominates such that the arc is extinguished due to the lowered voltage. However, this can take a considerable amount of time which can lead to significant consequences. Therefore, in this example an arc quenching arrangement is implemented to decrease the amount of time needed to quench the arc, as shall now be described with reference to Figure 3.

[0068] As noted above, while only one generator link 13 is shown in Figure 2, the PMG 11 of this example is relatively large and generates considerable power. For this reason, the MSC 12 is configured to process power in multiple parallel strings to balance load sharing, and the PMG 11 has multiple winding sets to reduce the output load across each individual winding set. Each winding set connects to a respective string of the MSC 12 through a respective generator link 13.

[0069] In principle, the generator system 10 could implement a separate arc quenching arrangement, such as a switching arrangement or system, for each generator link. However, this may entail a corresponding high number of switching arrangements connected to each of the generator links 13 connecting the winding sets of the PMG 11 to the MSC 12.

[0070] In this respect, Figure 3 is a wider schematic of the generator system 10 of Figure 2, which represents an overview of the generator system 10 shown in Figure 2. It shall be noted that some counterpart features of Figures 2 and 3 are assigned similar reference numbers. It shall additionally be appreciated that various changes and modifications can be made to the generator system 10 of Figures 2, 3, and 4 without departing from the scope of the present invention.

[0071] In this example, the PMG 11 is shown to comprise multiple winding sets 11a, 11 b and therefore has multiple generator links 13 connecting the PMG 11 to the MSC 12 to facilitate the conversion of power from the PMG 11 . A pair of generator links 13, namely a first generator link 13a and a second generator link 13b, are illustrated in Figure 3. Each generator link 13 connects a respective three-phase winding set to a respective pair of MSC stacks 25 in the MSC 12, to aid power conversion of the PMG output. The pairs of MSC stacks 25 therefore represent parallel strings for processing power in the MSC 12. More specifically, in the illustrated arrangement the first generator link 13a connects a first winding set 11 a to a first pair of MSC stacks 25a, 25b, and the second generator link 13b connects a second winding set 11 b to a second pair of MSC stacks 25c, 25d. It will be appreciated there may be up to n winding sets in the PMG 11 and therefore up to n corresponding generator links 13, wherein n is an integer.

[0072] Figure 3 shows an example of the PMG 11 with two set of three phase windings 11 a and 11 b, and therefore also two three phase generator links 13a and 13b, connecting to the MSC 12, with the respective number of MSC stacks 25.

[0073] Each generator link 13a, 13b comprises a set of power lines numbered 36, 38 in Figure 3 to carry the three phases, each power line 36, 38 carrying a respective phase, and to provide the connection from the PMG 11 to the MSC 12. It will be appreciated that the individual generator links 13a, 13b and the power lines 36, 38 and conductors within them may be generally the same as each other, and therefore each have the configuration of the generator link 13 and the conductors 23a, 23b, 23c, 23d, 23e, 23f shown in Figure 2, for example.

[0074] Respective protective devices in the form of circuit breakers 14a, 14b are provided for each generator link 13a, 13b, whereby each circuit breaker 14a, 14b is disposed across all of the power lines 36, 38 of the respective generator link 13a, 13b and is positioned between the PMG 11 and the MSC 12. Each generator link 13a, 13b can be separately controlled, for example by opening specific respective circuit breakers 14a, 14b to disconnect the associated generator links 13a, 13b.

[0075] In the example of Figure 3 each generator link 13, and each of the power lines 36, 38 within the generator link 13, has a first portion extending from respective PMG electrical power terminals 21 d, 21 e of the PMG 11 to respective circuit-breaker terminals 24d, 24f of the associated protective device 14, and a second portion extending from the protective device 14 to the MSC 12. The protective devices 14 are positioned proximal to the MSC 12 in this example, so that the first portion of each generator link 13 is longer than the second portion of the generator link 13.

[0076] The respective first portions of the first and second generator links 13a, 13b between the respective PMG electrical power terminals 21 d, 21 f and the respective circuit-breaker terminals 24d, 24e may be referred to collectively as a PMG power interconnect 19. As discussed above in relation to the first portion of the generator link 13 shown in Figure 2, the PMG power interconnect 19 shown in Figure 3 represents the majority of the impedance of the generator links 13a, 13b and would ordinarily be effectively unprotected from faults.

[0077] Each MSC stack 25a, 25b, 25c, 25d includes a plurality of power semiconductor switches 26 arranged in generator-side switch groups which provide AC-DC conversion. The power semiconductor switches 26 may be in the form of IGBTs, IGCTs, SiC MOSFETs, or the like. Although not shown in Figure 3, the MSC stacks 25a, 25b, 25c, 25d are connected via a DC link to corresponding line-side groups of switches of a LSC that provides DC-AC conversion for delivery to the grid. Various other switch configurations may alternatively be used. It will also be appreciated that other components such as filters, choppers, capacitors, resistors, and the like may be implemented in the configuration of the MSC 12, but are omitted for clarity.

[0078] Through appropriate control of the frequency and duty cycle of the control signals that are used to control the outputs from the power semiconductor switches 26 by an associated control module or controller (not shown in Figure 3), a desired DC output can be delivered from the MSC 12. The controller may be a dedicated controller for power conversion, or may be a system controller for controlling various components within the generator system 10.

[0079] The generator system 10 includes a switching arrangement, system, or circuit, hereafter referred to as a crowbar arrangement 30, for accelerating arc quenching. The crowbar arrangement 30 is shown in more detail in Figure 4. In the example shown in Figures 3 and 4, the crowbar arrangement 30 is connected to the PMG power interconnect 19. The number of power lines 36, 38 within the generator links 13a, 13b of the PMG power interconnect 19 exceeds the number of switches in the crowbar arrangement 30. More specifically, the crowbar arrangement 30 includes a switching arrangement or device in the form of a ‘four-pole switch’ (FPS) having a set of four individual switches 34 that are connected to the six power lines of the first and second generator links 13a and 13b, between the winding sets 11 a and 11 b and the protective devices 14a and 14b, as shown in Figures 3 and 4. The FPS 30 may be regarded as a single and discrete component, and so decreases the complexity of manufacturing the FPS 30 and of installing the FPS 30 in the generator system 10. Various other ways of implementing an FPS or other switching arrangement are possible, however.

[0080] As best shown in Figure 4, which corresponds to area ‘A’ shown in the schematic diagram of the overview of the generator system 10 shown in Figure 3, the FPS 30 is connected across conductors 36a, 36b, 36c of the first portion of the first generator link 13a and conductors 38a, 38b, 38c of the first portion of the second generator link 13b. It will be appreciated that other configurations of the FPS in the generator system 10 are equally applicable.

[0081] In the example shown in Figure 4, the four individual switches 34 of the FPS 30 are arranged in two switch pairs 40, 42. In general terms, each switch pair 40, 42 is connected to a respective generator link 13a, 13b in the PMG power interconnect 19. More specifically, a first switch pair 40 is connected to the conductors 36a, 36b, 36c of the first portion of the first generator link 13a via respective poles 44a, 44b, 44c, and a second switch pair 42 is connected to the conductors 38a, 38b, 38c of the first portion of the second generator link 13a via respective poles 46a, 46b, 46c. Poles 44b and 46b are permanently connected to respective conductors 36b and 38b of the first and second generator links 13a, 13b, so that each generator link 13 has a conductor that is permanently connected to both switches 34 of the associated pair of switches 40, 42.

[0082] Taking the example of the first pair of switches 40 of the FPS 30, conductors 36a and 36c are connected to respective switches 34 of the first pair 40 of the FPS 30, so that closing both switches 34 of the pair has the effect of connecting conductors 36a and 36c together electrically, whereas opening either switch 34 disconnects conductors 36a and 36c electrically. Conductor 36b is permanently connected to both of the switches 34 of the first pair 40. For example, conductor 36b may connect to a conductor, terminal or node between the two switches 34. Accordingly, closing the switches 34 also connects conductor 36b to each of conductors 36a and 36c and thus connects all three conductors 36a, 36b, 36c together. Similar considerations apply for the second pair of switches 42 and the associated conductors 38 of the second generator link 13b.

[0083] When activated, the FPS 30 receives power from the PMG output via the generator links 13a, 13b and is controlled by an associated controller (not shown). The FPS 30 is controlled so that its switches 34 all open and close in unison. Accordingly, the switches 34 are either all open, so that the power lines 36, 38 of the first and second generator links 13a, 13b are isolated from one another, or the switches 34 are all closed so that the power lines 36, 38 of the first and second generator links 13a, 13b are all connected together.

[0084] In normal operation of the system 10 shown in Figures 3 and 4 power is generated by the PMG 11 and delivered from the first and second winding sets 11 a, 11 b via the first and second generator links 13a, 13b to the MSC 12. The winding sets 11a, 11 b each carry a portion of the overall output load from the PMG 11. The switches 34 of the FPS 30 remain open during normal operation such that no generated current is drawn away from the MSC 12.

[0085] A line-to-line arc 100 as illustrated in Figures 3 and 4 may form in the PMG power interconnect 19. As noted above, sensors may be employed in the generator system 10 for arc detection.

[0086] Upon detection of the arc 100, the PMG 11 is operated to slow down from a current or initial speed towards a slowed speed, and optionally towards standstill, such that the current in generator link 13a reduces below the minimum current required to sustain the arc 100. The protective devices 14a, 14b are activated to isolate and protect the MSC 12 from the arc 100. Slowing the PMG 11 advantageously helps to reduce the arcing current of the arc 100 quickly, thus aiding the quenching of the arc 100.

[0087] To accelerate the arc quenching process while the PMG 11 is slowing, one of the switches 34 of the first pair of switches 40 is operated to connect a first power line 36 to a second power line 36 of the first generator link 13a, and the other one of the switches 34 is operated to connect the first power line 36 to a third power line 36 of the first generator link 13a. In this example, the ‘first power line’ corresponds to the power line that includes conductor 36b, which is permanently connected to both switches 34 of the first pair 40. Similarly, one of the switches 34 of the second pair of switches 42 is operated to connect a first power line 38 to a second power line 38 of the second generator link 13b, and the other one of the switches 34 is operated to connect the first power line 38 to a third power line 38 of the second generator link 13b. The operation of the switches 34 of the FPS 30 may occur concurrently such that the switches 34 are closed in a simultaneous manner.

[0088] More specifically, the FPS 30 is activated to create connections across the respective conductors 36, 38 of each generator link 13a and 13b in the PMG power interconnect 19, so that the voltage in each generator link 13 is clamped. Each generator link 13 is clamped separately, in that the FPS 30 does not directly connect the conductors of different generator links 13 together electrically. These connections provide short-circuits or low voltage shunt circuits of low impedance across the PMG power interconnect 19. The short-circuit current in the PMG 11 is slightly higher than nominal currents for the FPS 30 and below nominal currents for the arc 100 or, for example, other line-to- line short-circuits. The impedance of the short-circuit created by the FPS 30 is less than the impedance of the arcing path, therefore current is diverted away from the arc 100 and to the FPS 30 as the generator links 13a, 13b are clamped. The arcing current therefore reduces and the arc 100 quenches when the arcing current is below the minimum current required to sustain the arc 100.

[0089] To summarise, by employing a crowbar arrangement, such as the FPS 30 and the associated connections, across the respective generator links 13a, 13b in the PMG power interconnect 19, the generator links 13a, 13b may be advantageously clamped to reduce voltage feeding the arc 100 in a simple, efficient, and effective manner to accelerate the quenching of the arc while the PMG 11 is slowed.

[0090] In the examples shown in Figures 3 and 4, the total number of individual switches required to connect the power lines of generator links 13a, 13b is reduced when compared to other switching arrangements that could be employed. One such switching arrangement, for example, may comprise three individual poles, each pole providing a connection to a respective conductor of a generator link connecting an associated generator and MSC. Each pole is permanently connected to an associated switch such that the circuit-breaker switch, or ‘three-pole switch’, comprises three total individual switches. If a generator has multiple winding sets producing respective three-phase outputs, a respective three-pole switch may be provided for each output. Upon detection of an arc, each three-pole switch may be activated to close its three individual switches in a similar manner as described above with reference to Figures 3 and 4 for reducing the arcing current to help quench the arc while the generator is slowed.

[0091] Referring to Figures 3 and 4, employing one or more crowbar arrangements 30 in a generator system exhibits several advantages over, for example, an equivalent generator system which instead employs a number of three-pole switches, whereby each three-pole switch is disposed across a separate generator link.

[0092] For example, two three-pole switches with a total of six individual switches would be required to provide similar functionality to the crowbar arrangement 30 of Figures 3 and 4, which utilises a total of four individual switches across the two switch pairs 40, 42, implemented in a single device in the form of a four-pole switch. Each individual switch carries an associated impedance, and so the impedance of the activated FPS 30 is less than the impedance of a pair of activated three-pole switches because of the reduced quantity of switches. This increases system efficiency and reduces the length of time the arc 100 can be sustained for.

[0093] The crowbar arrangements of the above example are scalable such that a large generator system (which is larger than the generator system 10 of Figures 2 to 4) requiring n three-phase winding sets and thus n generator links 13 with 3n power lines would only require 2n individual switches, wherein n is a positive integer. In general terms, such crowbar arrangements use 33% less individual switches when compared to an equivalent generator system employing three-pole switches. The number of components in the generator system 10 is therefore advantageously reduced and which in turn reduces system complexity and cost.

[0094] Each switching arrangement requires control wiring to activate the switches of the switching arrangement. By using a crowbar arrangement 30 such as shown in Figures 3 and 4 instead of two three-pole switches, for example, system complexity and cost is reduced since the quantity of control wiring required to activate the crowbar arrangement 30 in the generator system 10 is reduced. The reduced number of components also enhances the reliability of the generator system 10, and saves space within the generator system 10.

[0095] The above switching arrangement arc quenching strategy for typically unprotected PMG power interconnects advantageously employs a crowbar arrangement configured such that a single conductor of the, or each, generator link is permanently connected to multiple switches of the crowbar arrangement. In other words, the crowbar arrangement has fewer switches than the number of power lines that the crowbar arrangement is connected to.

[0096] More generally, the crowbar arrangements of the above examples each act to create a low voltage shunt circuit which has a lower impedance than the arcing path. This advantageously reduces the arcing current and so acts to quench the arc, as the energy takes the path of lesser impedance, such that the arc does not have sufficient current to sustain itself and quenches as the current of the arc is commutated to the shunt circuit. This is achieved in a fast and efficient manner, using fewer components when compared to other arc quenching strategies.

[0097] The skilled person will appreciate that modifications may be made to the specific embodiments described above without departing from the inventive concept as defined by the claims. For example, in the above examples, a crowbar arrangement is disposed in a PMG power interconnect proximal to a machine-side converter, so that the crowbar arrangement is close to the machine-side converter and closer to the machine-side converter than to the PMG at least. This may facilitate installation of the crowbar arrangement within the generator system. However, it is also possible for the FPS shown in Figures 3 and 4 to be disposed in the PMG power interconnect in a different position, for example proximal to the generator. This may allow the crowbar arrangement to bypass the majority of the impedance associated with the PMG power interconnect, which may improve the effectiveness of the crowbar arrangement in drawing current away from an arc formed in or on the generator link(s).

Claims

Claims1. A switching arrangement for arc quenching in a wind turbine, the wind turbine having a generator, comprising two sets of three phase windings and a machine-side converter arranged to connect to the two sets of three phase windings, wherein the switching arrangement comprises a set of four switches electrically coupled to power lines of a connection between the generator and the machine-side converter, and wherein at least one power line of the connection for each of the two sets of three phase windings is permanently connected to multiple switches of the switching arrangement, so a first two of the four switches connects to the first sets of three phase windings and a second two of the four switches connects to the second sets of three phase windings.

2. The switching arrangement of claim 1 wherein the set of switches is operable to create a short-circuit in or on the connection between the generator and the machine-side converter upon detection of an arc.

3. The switching arrangement of any preceding claim, wherein the set of switches is operable to connect together electrically power lines of the connection between the generator and the machine-side converter.

4. The switching arrangement of any preceding claim, wherein the wind turbine comprises a protective device disposed between the generator and the machine-side converter, and wherein the set of switches are electrically connected to power lines of a connection between the generator and the protective device.

5. The switching arrangement of claim 4, configured to quench an arc at a location between the generator and the protective device.

6. The switching arrangement of any preceding claim, configured to quench an arc that extends to or from at least one of the power lines.

7. The switching arrangement of any preceding claim, comprising 2n switches, wherein n is a positive integer, and wherein the 2n switches are electrically coupled to 3n power lines of the connection.

8. The switching arrangement of any preceding claim, wherein the set of switches is disposed proximal to the generator.

9. The switching arrangement of any of claims 1 to 7, wherein the set of switches is disposed proximal to the machine-side converter.

10. The switching arrangement of any preceding claim, wherein the set of switches comprises one or more circuit breaker switches.

11. The switching arrangement of any preceding claim, comprising one or more multi-pole switches having a plurality of poles, and wherein each pole of the or each multi-pole switch is electrically connected to a respective power line.

12. The switching arrangement of claim 11 , comprising a switching device that comprises the set of switches.

13. The switching arrangement of any preceding claim, wherein the connection between the generator and the machine-side converter comprises multiple sets of power lines, each set of power lines carrying a respective three-phase output from the generator, in use.

14. The switching arrangement of any preceding claim, wherein the set of switches is connected to all power lines of the connection.

15. A wind turbine, comprising the switching arrangement of any preceding claim.

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