Arc quenching in wind turbines

Abstract

A method of controlling a wind turbine (1) having a generator (10) and a machine-side converter (12). The method comprises detecting an arc in or on a connection (13) between the generator (10) and the machine-side converter (12), and operating the machine-side converter (12) to quench the arc.

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 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 a set of one or more winding sets that surround the magnets and 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] 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 a short-circuit. The protective device will typically be mounted in proximity to the machine-side converter, resulting in the majority of the impedance generated by a connection between the PMG and the machine-side converter, such as a bus-bar link, power distribution cables, power leads, or the like, being between the protective device and the PMG.

[0009] 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.

[0010] The energy of an arc is a proportional function of voltage, current, and its time duration 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 duration that can threaten significant consequences.

[0011] Circuitry 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 generator is brought to stop as fast as possible and the machine-side converter is disconnected via the protective device. However, the circuit between the PMG and the protective device cannot be disconnected and is therefore always live when the armature is rotating. A fault in this circuit will therefore be fed as long as the armature rotates. Stopping a PMG to terminate a fault may take over a minute, particularly for a large PMG such as used in megawatt wind turbines, for example. A fault that persists for such a time period may threaten damage to the wind turbine.

[0012] Known methods to extinguish an arc between the PMG and the machine-side converter involve disconnecting the power source to arc, slowing the PMG down to reduce the arcing current, or implementing a crowbar system between the PMG and the machine-side converter. 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 circuitry connecting the PMG and the machine-side converter can also reduce the effectiveness of known quenching systems.

[0013] It is against this background that the invention has been devised. Summary of the Invention

[0014] According to an aspect of the present invention, there is provided a method of controlling a wind turbine having a generator and a machine-side converter. The method comprises detecting an arc in or on the connection between the generator and the machine-side converter, and operating the machine-side converter to quench the arc.

[0015] 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 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 conductor of the connection, for example, in that the arc extends to or originates from the conductor. An arc may, for example, extend between conductors of the connection, from one conductor to another, and / or between a conductor of the connection and another component of the wind turbine.

[0016] Using the machine-side converter to quench the arc may advantageously enable arcs to be quenched using existing hardware of the wind turbine, thereby avoiding a need to provide dedicated hardware for arc mitigation.

[0017] The generator of the wind turbine may comprise a permanent magnet generator, for example.

[0018] The wind turbine may comprise a protective device disposed between the generator and the machine-side converter. The protective device may be operable to connect and disconnect the machine-side converter from the generator electrically. In such embodiments, detecting an arc in or on a connection between the generator and the machine-side converter may comprise detecting an arc in or on a connection between the generator and the protective device. The connection between the generator and the protective device may form part of the connection between the generator and the machine-side converter.

[0019] In embodiments including a protective device, the method may further comprise operating the machine-side converter to quench the arc while operating the protective device to connect the machine-side converter to the generator. This sits in contrast with known approaches in which a protective device is opened when an arc is detected, to disconnect the converter from the generator. In some embodiments, the generator may be operated to slow from an initial speed towards a slowed speed concurrently with operating the machine-side converter. If the wind turbine includes a protective device, the protective device may be operated to disconnect the machine-side converter from the generator when the generator is at the slowed speed, for example by opening the protective device.

[0020] In some embodiments, the arc may extend between two or more conductors of the connection between the generator and the machine-side converter, or the connection between the generator and the protective device, if present.

[0021] Operating the machine-side converter to quench the arc may further comprise operating switches of the machine-side converter, which may advantageously use existing hardware within the machine-side converter.

[0022] The method may further comprise operating the machine-side converter to deliver a current to the connection to quench the arc. If the wind turbine includes a protective device, the current may be delivered to the connection between the generator and the protective device. Such embodiments may allow for active quenching of an arc, and may enable arc quenching to be enhanced by taking the characteristics of the arc formed into account. In such embodiments, the current may be substantially equal and / or opposite to a current of the arc, advantageously enabling effective and fast arc quenching. The method may further comprise delivering the current to the connection continuously, for example to prevent the arc from reforming once quenched.

[0023] The method may further comprise determining the arcing current in accordance with at least one of: a phase of the generator, characteristics of the generator, and at least one phase of the connection.

[0024] In some embodiments, operating the machine-side converter to quench the arc may further comprise creating a short-circuit within the machine-side converter. This may allow the arcing current to be managed within the generator-side system. The method may further comprise operating one or more switches of the machine-side converter to create the short-circuit within the machine-side converter, thus potentially allowing the arcing current to be managed by utilising existing machine-side converter hardware. Creating a short-circuit within the machine-side converter may entail creating a temporary low voltage shunt circuit. The low voltage shunt circuit may have a lower impedance than the arcing path to reduce the arcing current, and also may use existing components within the machine-side converter system.

[0025] Operating the machine-side converter to quench the arc may comprise operating the machine-side converter to direct electrical power to a dump load module. The dump load module may be defined by one or more components of a chopper connected to the machine-side converter. The method may comprise activating one or more components of the machine-side converter and one or more components of a chopper connected to the machine-side converter to dissipate, absorb, and / or dump the arcing current.

[0026] According to another aspect of the invention, there is provided a control system for a wind turbine. The wind turbine has a generator and a machine-side converter. The wind turbine may have a protective device disposed between the generator and the machine-side converter. The protective device may be operable to connect and disconnect the machine-side converter from the generator electrically. The control system is configured to detect an arc in or on a connection between the generator and the machine-side converter, and operate the machine-side converter to quench the arc. Detecting an arc in or on a connection between the generator and the machine-side converter may comprise detecting an arc in or on a connection between the generator and the protective device. In this respect, the connection between the generator and the protective device may form part of the connection between the generator and the machine-side converter. The control system may comprise one or more controllers.

[0027] The control system may comprise at least one sensor. The sensor may be configured to generate a signal indicative of an arc in or on the connection between the generator and the machine-side converter. The control system may be configured to detect an arc in or on the connection between the generator and the machine-side converter based on the signal generated by the at least one sensor. A controller of the control system may be operatively coupled to the at least one sensor.

[0028] 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. Brief Description of the Drawings

[0029] 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:

[0030] Figure 1 is a schematic diagram of a wind turbine suitable for use with the embodiments of the invention;

[0031] Figure 2 is a simplified schematic diagram of a generator system of the wind turbine of Figure 1 ;

[0032] Figure 3 is a schematic diagram of the generator system of Figure 2 in an embodiment;

[0033] Figure 4 is a schematic diagram of a machine-side converter bridge for use with the embodiments of the invention;

[0034] Figure 5 corresponds to a machine-side converter of Figure 3 and shows an implementation of an alternative arc quenching approach; and

[0035] Figure 6 corresponds to the machine-side converter of Figure 3, but shows an implementation of another alternative arc quenching approach.

[0036] Detailed Description

[0037] 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.

[0038] 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 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. The PMG power interconnect may form part of an overall connection between the generator and the MSC, and in other embodiments arcs may be detected and quenched in parts of the overall connection other than in the PMG power interconnect, for example between the protective device and the MSC.

[0039] In some embodiments, the proposed approach involves detecting an arc between the PMG and the MSC and operating the MSC to quench the arc. During normal operating periods, the MSC converts AC from the PMG to DC for subsequent delivery to the grid via a line-side converter. On detection of the arc, the MSC may be operated to reduce the arcing current of the arc to below a minimum current required to sustain the arc, for example, thereby quenching the arc. The proposed approach may use existing hardware used in typical wind turbine systems and can therefore be implemented into existing systems without the need for modification.

[0040] An active arc quenching strategy may be implemented, which allows the characteristics of the arc formed to be taken into account, for example by way of providing a counter-current to act against the arcing current. This advantageously enables effective and fast arc quenching and reduces the time an arc can be sustained for, in turn reducing the risk of significant damage to the wind turbine.

[0041] In some embodiments, the active quenching strategy may involve operating power converter switches within the MSC to deliver a counter-current. The counter-current may be configured to be substantially equal in magnitude and opposite in direction or polarity to the arcing current, so that the counter-current acts to destructively interfere with the arcing current. This strategy can be implemented in a short number of cycles causing the arc to be effectively quenched over a short timeframe.

[0042] Passive quenching approaches may also be used, in which an MSC is operated to absorb and / or divert energy that would otherwise feed an arc, but without creating a counter-current or otherwise quenching the arc actively. In such approaches, an arc may be quenched by commutating the current of the arc to a shunt temporarily created within the MSC, for example, or by diverting energy to a dump load module that is connected to the MSC. The dump load module can be formed from components of a chopper connected to the MSC, for example.

[0043] Specific example quenching strategy implementations 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 strategies may be implemented is described with reference to Figures 1 to 2. A variant of the wind turbine in which quenching strategies may also be implemented is shown in Figure 3. The examples shown in Figures 1 to 3 are 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 Figures 1 to 3 is entirely illustrative, and many architectures are known that would be suitable for use with embodiments of the invention.

[0044] 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.

[0045] The nacelle 5 contains a generator system 10 (shown in Figure 2), 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.

[0046] Figure 2 is a simplified schematic of the generator system 10 of the wind turbine 1 of Figure 1 . The examples shown herein are representative only and the skilled reader will appreciate that the methods described below may be applicable to many different configurations. Moreover, the components of the wind turbine 1 and its generator system 10 are conventional and as such familiar to the skilled reader, and so will only be described in overview.

[0047] 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. The stator contains one or more sets of three-phase windings in which electrical current is induced in response to varying magnetic flux created by rotation of the armature.

[0048] 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 most electrical grids. Also, the PMG 11 output is 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.

[0049] 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 a DC-AC converter in series with the MSC 12, as shown in Figure 4 and described herein. The MSC 12 comprises an array of 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 in Figure 2) for stabilising the DC output from the MSC 12 in case the LSC cannot consume and convert the generated power from the PMG 11 .

[0050] The PMG 11 and MSC 12 are connected by circuitry, such as power lines 13 as illustrated in Figure 2, the circuitry defining a connection between the PMG 11 and the MSC 12 and the power lines 13 representing conductors of the connection. Various types of conductors may be used in the power lines, such as power leads, power distribution cables, bus bar links, or the like, for the connection between the PMG 11 and MSC 12. Each power line 13 in Figure 2 corresponds to and carries one of the three-phases of the output AC from the PMG 11 . An arc detection system (not shown) is coupled to the circuitry connecting the PMG 11 to the MSC 12.

[0051] 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 power lines 13. The power lines 13 may comprise one or more portions for connecting the PMG 11 to the MSC 12. A first portion 13a of each power line 13 may connect the PMG 11 , via PMG electrical power terminals 11a, 11 b, 11c to the protective device 14, via respective circuit-breaker terminals 14a, 14b, 14c. A second portion 13b of the power line 13 may connect the protective device 14 to the MSC 12 to complete the connection between the PMG 11 and the MSC 12.

[0052] The three first portions 13a extending between the PMG electrical power terminals 11a, 11b, 11c and the circuit-breaker terminals 14a, 14b, 14c may be referred to collectively as a PMG power interconnect 19, as illustrated in Figure 2. More generally, the PMG power interconnect 19 may encompass one or more three-phase groups of power lines 13, for example, depending on a winding arrangement within the PMG 11. The PMG power interconnect 19 therefore represents part of the connection between the PMG 11 and the MSC 12, and more specifically represents a connection between the PMG 11 and the protective device 14.

[0053] When activated and opened, the protective device 14 acts to isolate the MSC 12 from the PMG 11 electrically by breaking the connection provided by the first and second portions 13a, 13b. The protective device 14 is positioned proximal to the MSC 12 such that the majority of the impedance 18 associated with the power lines 13 lies in the PMG power interconnect 19 as illustrated in Figure 2. The PMG power interconnect 19 is considered to be unprotected because 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 PMG power interconnect 19. A fault in the PMG power interconnect 19 may arise in a single power line 13 or over a combination of power lines 13. For example, an arc may form between power lines 13 causing a line-to-line arc 100 as shown in Figure 2.

[0054] 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 may be used to determine the rotor position, for example. The encoder 15 is coupled to the PMG 11 to sense rotation of the generator shaft, and therefore of the armature. Electrical phase power line current sensors 16 may be coupled to each of the power lines 13 as illustrated in Figure 2 and are positioned between the PMG 11 and the MSC 12, enabling them to monitor the current in the power lines 13. In the event of an arc, such as the arc 100 shown in Figure 2, as the power lines 13 each correspond to one of the three-phases of the output AC from the PMG 11 , the current sensors 16 allow the phase angle of the armature and therefore of the arcing current to be tracked, improving the estimation of the arc parameters.

[0055] An MSC control module 17 controls the operation of the MSC 12, in this 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 current, such as a pulse-width modulated regulated sinusoidal-type current, to the MSC 12.

[0056] Figure 3 is a schematic of a generator system 20 in a variant of the wind turbine 1 , in which embodiments of the invention may also be implemented, showing further details of an associated MSC 22. In this example, the PMG 21 is relatively large compared to the PMG 11 of Figure 2 and thus generates considerably more power. This system 20 therefore uses multiple links from the PMG 21 to the MSC 22 to facilitate the conversion of power from the PMG 21 . Two generator links 23 are illustrated in Figure 3, however it will be appreciated there may be up to n generator links 23, wherein n is an integer. Each generator link 23 carries three phases from a respective set of windings of the PMG 21 , and comprises a set of conductors, which in this example are power leads, to carry those phases and to provide the connection from the PMG 21 to the MSC 22, similar to the example of Figure 2.

[0057] A respective protective device in the form of a circuit breaker 24 is provided for each generator link 23, each circuit breaker 24 being located across all phases of the respective generator link 23 and positioned between the PMG 21 and the MSC 22 and proximal to the MSC 22. In this way, each generator link 23 can be separately controlled, for example by opening specific circuit breakers 24 for certain generator links 23 to turn them off.

[0058] Each generator link 23 comprises a first link portion 31a, 31b, connecting the PMG 21 via respective PMG electrical power terminals 21a, 21 b to the associated circuit breaker 24 via respective circuitbreaker terminals 24a, 24b, and a second link portion 32a, 32b, connecting the circuit breaker 24 to the MSC 22. Each link portion 31a, 31b, 32a, 32b includes a group of three conductors, each carrying a respective phase, so that each generator link 23 includes six conductors arranged in three pairs, each pair carrying a respective phase.

[0059] The first link portion 31a, 31 b may be comparatively longer in length than the second link portion 32a, 32b. The first link portions 31a, 31b shown in Figure 3 may be referred to collectively as a PMG power interconnect 29. Similar to the PMG power interconnect 19 shown in Figure 2, the PMG power interconnect 29 shown in Figure 3 represents the majority of the impedance of the generator links 23 and would ordinarily be effectively unprotected from faults. The MSC 22 includes a plurality of IGBTs arranged in generator-side IGBT groups which provide AC-DC conversion. Each second link portion 32a, 32b is connected to a separate group of IGBTs 25a, 25b as illustrated in Figure 3. The IGBT groups 25a, 25b are connected via a DC link 28 to corresponding line-side groups of IGBTs (not shown) of a LSC that provides DC-AC conversion for delivery to the grid. The skilled person would appreciate that there are multiple IGBT configurations which may be equally applicable in appropriate circumstances. The configuration of the MSC 22 in this example includes a filter 26a, 26b for each of the generator links 23.

[0060] The DC link 28 includes DC link capacitors 27a, 27b, each respectively coupled to IGBT groups 25a and 25b of the MSC 22. The DC link capacitors 27a, 27b act as intermediate DC storage devices for the converted output DC and help to smooth the output DC from the IGBTs. The DC link 28 also includes a chopper 30 for stabilising the DC output from the MSC 22. The chopper 30 includes dump load resistors 33a, 33b that are used to create dump load modules when implementing specific arc quenching strategies, as will be described in more detail herein.

[0061] Through appropriate control of the frequency and duty cycle of the control signals that are used to control the outputs from the IGBTs 25a, 25b by an associated control module, a desired DC output can be delivered to the DC link 28 from the MSC 22.

[0062] Figure 4 shows an MSC converter bridge 40 connected to a LSC 50, the MSC converter bridge 40 being suitable for use in the MSC configurations shown in Figures 1 to 3. In this example, the MSC converter bridge 40 comprises a group of six IGBTs 42 arranged in a symmetrical configuration, defining a generator-side group 41 , which provides AC-DC conversion and so defines at least part of an MSC 12, 22. The LSC 50 comprises a line-side group 51 that also includes six IGBTs 52 arranged in a symmetrical configuration, which group provides DC-AC conversion. The two groups of IGBTs 41 , 51 are connected by a DC link 48 that includes a DC link capacitor 49 which acts as an intermediate DC storage device for the converted output DC from the generator-side IGBTs 42 and helps to smooth the output DC.

[0063] Power lines 45 may correspond to the second portions 13b of the power lines 13 of Figure 2 or to individual leads of a second link portion 32a, 32b of Figure 3, for example. The power lines 45 each carry one of three-phases of AC output from the PMG 11 , 21 . Each power line 45 is connected to a pair of IGBTs 42 in the generator-side group 41 for providing AC-DC power conversion. DC is then delivered to the line-side group 51 via the DC link 48 and DC link capacitor 49. DC is converted to AC by the pairs of IGBTs 52 in the line-side group 51. Power lines 55 lead to the grid and each carry one of three-phases of AC once converted.

[0064] Similar to Figure 3, an associated control module controls the frequency and duty cycle of the control signals to control the outputs from the IGBTs 42, 52 appropriately. Sensors (not shown) may be implemented in a similar manner as described in Figure 2 for fault or arc detection.

[0065] 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 generated in the power lines between the PMG and the MSC whenever the armature is rotating.

[0066] A fault that occurs in the power lines in the PMG power interconnect 19, 29 can lead to the formation of an arc as described, such as a line-to-line arc 100 as illustrated in Figure 2. In the arrangement shown in Figure 3, a line-to-line arc may form within an individual first link portion 31a, 31b, for example between the power lines or lines of that first link portion 31a, 31b, wherein an example location of the formed arc is illustrated in Figure 3 by arcing sites 110. It will be appreciated that a link-to-link arc between adjacent links 23 is also possible if, for example, the links 23 are encapsulated into a common duct, as well as a link-to-component arc between one or more links 23 and other components within the wind turbine.

[0067] The arc 100 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 may detect a spurious or abnormal current, and / or the shaft encoder may determine a deviation in the PMG’s speed from normal conditions. An arc detector (not shown) may be implemented in the system to detect that an arc 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.

[0068] An arc in the PMG power interconnect cannot be extinguished or quenched by opening the circuit breaker or other protective device to disconnect the MSC, therefore the PMG is typically slowed and stopped upon detecting the arc to reduce the arcing current below the minimum current necessary to sustain the arc. However, this can take a considerable amount of time which can lead to significant consequences.

[0069] To address this, in some embodiments of the invention the MSC is operated to quench arcs in the PMG power interconnect while keeping the protective device closed.

[0070] In an embodiment, an active approach is implemented in which the MSC is operated to provide an opposite or counter-current to an arc upon detection. This approach is now described as implemented in the configuration shown in Figure 2. The counter-current acts as a bypass current path that will actively direct the arcing current to the MSC 12 where it can be safely managed. This may be achieved by controlling the IGBTs in the MSC 12 to output the counter-current for delivery to the PMG 11 via the power lines 13.

[0071] An arc may initially ignite as a two-phase line-to-line arc 100 across two power lines 13 of the PMG power interconnect 19, for example, and may then propagate to a three-phase line-to-line arc across all power lines 13, particularly if all power lines 13 are within one bus duct or otherwise in close proximity to one another. The arc may therefore correspond to a phase-to-phase short-circuit between two or more of the relevant power lines 13 of the PMG power interconnect 19. To quench the arc, the counter-current is delivered on all power lines 13 and therefore across all phases. The counter-current to be delivered is determined using the known generator phase angle and standard short-circuit calculations for the PMG 11. It will be appreciated that other types of arcs, such as a link-to-link arc or a link-to-component arc are equally applicable for the arc quenching methods described herein.

[0072] The counter-current may be adapted to be in accordance with the known PMG characteristics and the known phase angle of the PMG 11. For example, the counter-current may comprise a sinusoidal current of a similar amplitude and frequency. As each power line 13 carries a different phase, the counter-current in the power lines 13 may also have different phase angles in accordance with the corresponding PMG phase angle of each power line 13. The respective phase angles for the individual power lines 13 may be derived, for example, from a phase of the PMG 11 as indicated by a shaft encoder, in accordance with PMG equations for current-phase characteristics.

[0073] When the counter-current is delivered to the PMG power interconnect 19, the counter-current and the arcing current advantageously superimpose and destructively interfere. The net voltage in the power lines 13 therefore drops to substantially zero (including values below zero) which actively quenches the arc.

[0074] A short-circuit in the PMG power interconnect 19 can be well defined as to its magnitude and phase using the parameters of the PMG 11 and sensor measurements. The control module 17 may be configured to take into account the PMG 11 parameters and sensor measurements, thereby allowing the characteristics of an arcing current to be accurately determined. Advantageously, the counter-current may therefore be made to be substantially equal and opposite to the arcing current. The destructive interference caused by the counter-current can therefore be maximised, causing the arc to be quenched quickly, for example over only a short number of cycles. This allows the active arc quenching strategy to cater for the characteristics of individual arcs as and when they form, thereby improving the effectiveness of the system 10.

[0075] The counter-current may be delivered continuously to the arcing site / maintained in the generator system 10 long enough to permit the site where the arc had formed to have passively cooled enough and / or for the speed of the armature of the PMG 11 to have reduced enough such that the arc will not reignite once quenched. If the arc were to reignite during this process, the active arc quenching methods may be reactivated in the same manner as described. This enables the reaction time of the system 10 to be advantageously relatively quick, further improving the system’s effectiveness.

[0076] Upon detection of the arc 100, the PMG 11 may be slowed from a current or initial speed towards a slowed speed concurrently with the delivery of the counter-current as described above to reduce the arcing current of the arc 100, thus aiding to quench the arc 100 quickly. At the slowed speed, the output current from the PMG 11 may be below the arcing current required to sustain the arc 100. For example, the slowed speed may be substantially zero where the PMG 11 has effectively come to a stop. In other words, the PMG 11 may be slowed to, or towards, a standstill. At this point, the MSC 12 may be disconnected from the PMG 11 as the active arc quenching strategy has completed and thus is no longer required, therefore the protective device 14 may be opened to isolate the PMG 11 from the MSC 12.

[0077] Advantageously, the detection and active arc quenching of the system 10 as described is able to compensate for power line impedances 18 which are inherent in power distribution systems. In alternative embodiments for arc quenching methods in the PMG power interconnect 19, an MSC 12 such as described above may be operated to divert energy away from the fault, for example by creating a path of lower impedance, or otherwise to absorb energy from the arc and therefore accelerate quenching of the arc while the PMG 11 is slowed. Examples of such embodiments are shown in Figures 5 and 6.

[0078] Figure 5 demonstrates how an alternative quenching strategy may be implemented in the topology of the MSC 22 of Figure 3 involving activating IGBTs 25a, 25b to create a three-phased bolted short. In other embodiments, other available power semiconductors of an MSC may be used to create a three-phased bolted short, such as SiC MOSFETs or the like.

[0079] Upon arc detection, specific MSC power modules, such as IGBTs 25a, 25b, are activated to create a short-circuit in the generator side of the generator system 10. The IGBTs 25a, 25b in this embodiment are selected such that they can withstand the short-circuit currents of the associated PMG 21 in the system 20 as illustrated in Figure 5. The short-circuit current in the PMG 21 is slightly higher than nominal currents for a three-phase short-circuit and below nominal currents for a line- to-line short-circuit, for example caused by an arc. The MSC circuit breaker 24 remains closed in order to allow current to flow from the PMG 21 to the MSC IGBTs.

[0080] The IGBTs 25a, 25b are activated in a specific sequence or pattern to create the three-phased bolted short. For example, the three upper IGBTs 25a, 25b of each IGBT group may be activated as indicated in Figure 5 by dashed boxes, to define active IGBTs 250a, 250b. In other examples, the lower three IGBTs 25a, 25b of each IGBT group may be activated. The active IGBTs 250a, 250b may be turned on and off in a switch pattern instead of keeping the IGBTs 25a, 25b on (i.e. normal operation). This sequence of activation shorts the current for a short period of time, for example a few seconds. The switch pattern may increase the l2t capacity of the IGBTs such that they are able to continue operating in the switch pattern until the rotation speed of the PMG 21 has dropped to a level at which the arc will not reignite. The switch pattern may follow a sinusoidal voltage curve, whereby the IGBTs 250a, 250b are activated only when the voltage reaches high amplitudes in the curve.

[0081] The chopper 30 may follow a pattern whereby its operation is then paused and then reactivated. This pattern may be a pre-determined control strategy implemented by the controller to reduce the load on the chopper 30 and therefore increases the time in which the shorted voltage can be clamped. This pattern can be followed until the rotational speed of the PMG 21 has reduced and thus the generated back-EMF voltage has reduced below the minimum level needed to sustain an arc. Advantageously, this allows the arcing current to be managed within the generator-side system while quenching arcs that would have otherwise formed.

[0082] A third embodiment shown in Figure 6 demonstrates how the topology of the MSC 22 of Figure 3 may be used to implement an alternative quenching strategy involving controlling one or more dump load modules within the chopper 30 to activate dump-load resistors 33a, 33b to absorb some of the energy that is feeding an arc. In this example, each dump load module is composed of a subset of components of the chopper 30 that serve another purpose during normal operation, which components are activated and controlled to form the dump load module when the quenching strategy is initiated.

[0083] Figure 6 demonstrates how this quenching strategy may be implemented in the topology of the MSC 22 and DC link of Figure 3 in which the components of the chopper 30 including the dump load resistors 33a, 33b are activated to form respective dump load modules 300a, 300b, indicated in Figure 6 by dashed boxes. Indicated components 251 a, 251 b of each IGBT 25a, 25b of the MSC 22 are also activated during this method. The active component 251 a, 251b of each IGBT 25a, 25b may be a switching bridge and may provide a path of low resistance or impedance for the current from the PMG power interconnect 29 to flow through to the dump load modules 300a, 300b. Thus, the MSC 22 is operated to divert energy to the dump load modules in this example. Other suitable configurations are also applicable for use with this embodiment.

[0084] In normal operation, the chopper 30 is controlled by an associated control module for power conversion (not shown). Upon arc detection, the dump load resistors 33a, 33b are controlled to form one or more dump load modules 300a, 300b and components 251a, 251b of each IGBT 25a, 25b are activated to provide a path of low resistance or impedance to the dump load modules 300a, 300b. The dump load resistors 33a, 33b are activated and a portion of the DC energy from the DC link capacitors 27a, 27b and current from the PMG 21 is dissipated, absorbed, and / or dumped by the dump load resistors 33a, 33b. This reduces the arcing current and thus helps to extinguish the arc. During the control of the dump load modules 300a, 300b, the MSC circuit breaker 24 remains closed to allow current to flow through the generator links 23 and the IGBTs 25a, 25b to the dump load modules 300a, 300b.

[0085] The amount of time the dumping occurs for is dependent on the inherent characteristics of the dump load resistors and capacitors to ensure they can withstand the dumping. In this example, the dump load module is controlled such that dumping occurs for a short period of time, for example a few seconds, and is then paused and reactivated. This pattern can be followed until the rotational speed of the PMG 21 has reduced and thus the generated back-EMF voltage has reduced below the minimum level needed to sustain an arc. Advantageously, this allows the arcing current to be managed within the generator-side system while quenching arcs that would have otherwise formed.

[0086] Both the three-phased bolted short and the activated dump-load resistor arc quenching strategies create a low voltage shunt circuit which has a lower impedance than the arcing path using existing components within generator-side systems. This advantageously reduces the arcing current as the energy takes the path of lesser impedance, such that the arc does not have sufficient current to sustain itself and thus quenches as the current of the arc is commutated to the shunt circuit.

[0087] In the same manner as for the active arc quenching strategy described above, upon detection of the arc 100 the PMG 11 may be slowed from a current or initial speed towards a slowed speed concurrently with applying the passive arc quenching methods as described above with reference to Figures 5 and 6 to reduce the arcing current of the arc 100, thus aiding to quench the arc 100 quickly. At the slowed speed, the output current from the PMG 11 may be below the arcing current required to sustain the arc 100. For example, the slowed speed may be substantially zero where the PMG 11 has effectively come to a stop, in other words a standstill. At this point, the MSC 12 may be disconnected from the MSC 12 as the passive arc quenching strategies have completed and are no longer required, therefore the protective device 14 may be opened to isolate the PMG 11 from the MSC 12.

[0088] Although the topology shown in Figure 3 has been used to explain how passive arc quenching strategies may be implemented, it should be appreciated that active quenching strategies may also be implemented in this arrangement. Conversely, passive quenching strategies may be implemented in the arrangement shown in Figure 2. More generally, active or passive quenching strategies may be implemented in a range of other wind turbine arrangements.

[0089] 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.

Claims

Claims1 . A method of controlling a wind turbine having a generator and a machine-side converter, the method comprising: detecting an arc in or on a connection between the generator and the machine-side converter; and operating the machine-side converter to quench the arc.

2. The method of claim 1 , wherein the wind turbine comprises a protective device disposed between the generator and the machine-side converter, and wherein detecting an arc in or on a connection between the generator and the machine-side converter comprises detecting an arc in or on a connection between the generator and the protective device.

3. The method of claim 2, comprising operating the machine-side converter to quench the arc while operating the protective device to connect the machine-side converter to the generator.

4. The method of any preceding claim, comprising operating the generator to slow from an initial speed towards a slowed speed concurrently with operating the machine-side converter to quench the arc.

5. The method of claim 4 when dependent on claim 2 or claim 3, comprising operating the protective device to disconnect the machine-side converter from the generator when the generator is at the slowed speed.

6. The method of any preceding claim, wherein the arc extends between two or more conductors of the connection.

7. The method of any preceding claim, wherein operating the machine-side converter to quench the arc comprises operating switches of the machine-side converter.

8. The method of any preceding claim, comprising operating the machine-side converter to deliver a current to the connection to quench the arc.

9. The method of claim 8, wherein the current is substantially equal to a current of the arc.

10. The method of claim 8 or claim 9, wherein the current is substantially opposite to a current of the arc.

11. The method of any of claims 8 to 10, comprising delivering the current to the connection continuously.

12. The method of any of claims 8 to 11 , comprising determining the current in accordance with at least one of: a phase of the generator; characteristics of the generator; and at least one phase of the connection.

13. The method of any of claims 8 to 12, comprising delivering a current to multiple conductors of the connection in accordance with respective phases of the multiple conductors.

14. The method of any of claims 1 to 7, wherein operating the machine-side converter to quench the arc comprises creating a short-circuit within the machine-side converter.

15. The method of claim 14, comprising operating one or more switches of the machine-side converter to create the short-circuit within the machine-side converter.

16. The method of any of claims 1 to 7, wherein operating the machine-side converter to quench the arc comprises operating the machine-side converter to direct electrical power to a dump load module.

17. The method of claim 16, wherein the dump load module is defined by one or more components of a chopper connected to the machine-side converter.

18. A control system for a wind turbine, the wind turbine having a generator and a machine-side converter, the control system being configured to: detect an arc in or on a connection between the generator and the machine-side converter; and operate the machine-side converter to quench the arc.

19. The control system of claim 18, wherein the control system further comprises at least one sensor configured to generate a signal indicative of an arc in or on the connection between the generator and the machine-side converter, and wherein the control system is configuredto detect an arc in or on the connection between the generator and the machine-side converter based on the signal.

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