Control method and control system of converter, converter and wind generating set

Abstract

The invention provides a control method and system of a converter, the converter and a wind generating set. The method comprises the steps that the current rotor speed of a doubly-fed wind generator is determined; in response to the fact that the difference between the rotor speed and the synchronous speed of the doubly-fed wind generator is smaller than a preset threshold value, for each three-level bridge arm, the state of a switching tube is controlled in the following mode that according to a modulation signal used for switching the output level of the three-level bridge arm from the positive level or the negative level to the midpoint level, the output level of the three-level bridge arm is converted into the midpoint level; and turning off the outer tube which is turned on in the current state, and turning on the clamping tubes and the inner tubes under the condition that the turned-on outer tube is turned off. The problem that the thermal stress of the power device near the synchronous speed cannot be properly solved is solved, and the thermal stress of the inner tube can be effectively dispersed by optimizing modulation logic, so that the thermal distribution of the power device of the switch tube is more uniform, and the problem that the thermal stress of the power device is too large is relieved.

Description

Technical Field

[0001] The present disclosure relates to the field of wind power generation, and more specifically, to a control method and control system of a converter, a converter and a wind power generator set. Background Art

[0002] A doubly-fed induction generator (DFIG) is a commonly used wind turbine. The stator winding of a doubly-fed wind turbine can be directly connected to the power grid, and the rotor winding can be connected to the converter through a slip ring. The converter can control the speed and output power of the wind turbine by precisely controlling the frequency and amplitude of the rotor current.

[0003] The converter used for the doubly-fed wind turbine generator can be a three-level active neutral point clamped (ANPC) topology. Compared with the two-level converter, the three-level converter can provide a higher voltage level, lower switching loss and smaller harmonic distortion. Therefore, the three-level converter used in the doubly-fed wind turbine generator has higher reliability and more advantages. Summary of the invention

[0004] For a doubly-fed wind generator, the torque capacity near the synchronous speed of the generator will affect the overall performance of the doubly-fed wind generator system. Therefore, unlike general converter control schemes, the present disclosure recognizes that converter control near the synchronous speed of the generator may become one of the keys to improving the performance of the power generation system.

[0005] Specifically, when the synchronous speed of the doubly-fed wind turbine generator is near, the rotor speed is close to the synchronous speed, the slip is very small, and the rotor current frequency is very low; when the rotor speed is equal to the synchronous speed, the slip is 0, and the rotor current is DC. In this regard, the present disclosure recognizes that when the rotor speed is close to the synchronous speed or the rotor speed is equal to the synchronous speed, the zero level duration of the converter during modulation will be longer. In this case, if the power devices of the converter are still modulated according to the traditional normal modulation method such as for the ANPC topology, some specific power devices will not be able to change phase in time and will be subjected to excessive thermal stress. The wind turbine generator set has to be operated with reduced torque to solve the problem of excessive thermal stress of the power devices near the synchronous speed, which will inevitably affect the power generation.

[0006] In view of the fact that the existing control scheme of the converter in the doubly-fed wind generator cannot properly solve the problem of excessive thermal stress of the power devices near the synchronous speed, the present disclosure proposes a control method, a control system, a converter and a wind generator set for the converter to solve or at least alleviate the above problem.

[0007] A first aspect of the present disclosure provides a control method for a converter, wherein the converter is applied to a doubly-fed wind generator, the converter comprises a plurality of three-level bridge arms, each of the three-level bridge arms comprises a plurality of switch tubes, the plurality of switch tubes comprise two inner tubes, two outer tubes and two clamp tubes, wherein the control method comprises: determining a current rotor speed of the doubly-fed wind generator; in response to a difference between the rotor speed and a synchronous speed of the doubly-fed wind generator being less than a preset threshold, for each three-level bridge arm, controlling the state of the switch tube in the following manner: according to a modulation signal for switching the output level of the three-level bridge arm from a positive level or a negative level to a midpoint level, turning off the outer tube that is turned on in the current state, and turning on each clamp tube and each inner tube when the outer tube that is turned on has been turned off.

[0008] Optionally, the three-level bridge arm includes an upper half bridge arm and a lower half bridge arm, wherein the control method further includes: in response to the difference between the rotor speed and the synchronous speed of the doubly fed wind generator being greater than the preset threshold, for each three-level bridge arm, controlling the state of the switching tube in the following manner: according to a modulation signal used to switch the output level of the three-level bridge arm from a positive level or a negative level to a midpoint level, turning off the outer tube that is turned on in the current state, and when the turned-on outer tube is turned off, turning on the clamping tube in the same half bridge arm as the turned-on outer tube; when the clamping tube in the same half bridge arm is turned on, turning on the other clamping tube and each inner tube of the two clamping tubes.

[0009] Optionally, the control method also includes: in response to the difference between the rotor speed and the synchronous speed of the doubly fed wind generator being greater than the preset threshold, for each three-level bridge arm, controlling the state of the switching tube in the following manner: according to a modulation signal for switching the output level of the three-level bridge arm from a positive level or a negative level to a midpoint level, turning off the outer tube that is turned on in the current state, and when the turned-on outer tube is turned off, turning on each clamping tube and each inner tube.

[0010] Optionally, in response to the difference being less than a preset threshold, for each three-level bridge arm, the state of the switching tube is also controlled in the following manner: according to a modulation signal used to switch the output level of the three-level bridge arm from a midpoint level to a positive level, the inner tube and each clamping tube of the lower half bridge arm of the three-level bridge arm are turned off, and when the inner tube and each clamping tube of the lower half bridge arm are turned off, the outer tube of the upper half bridge arm of the three-level bridge arm is turned on; according to a modulation signal used to switch the output level of the three-level bridge arm from a midpoint level to a negative level, the inner tube and each clamping tube of the upper half bridge arm of the three-level bridge arm are turned off, and when the inner tube and each clamping tube of the upper half bridge arm are turned off, the outer tube of the lower half bridge arm of the three-level bridge arm is turned on.

[0011] Optionally, the three-level bridge arm includes a first inner tube, a first outer tube and a first clamping tube located in the upper half bridge arm, and a second inner tube, a second outer tube and a second clamping tube located in the lower half bridge arm, wherein, when the difference is less than the preset threshold and the output level of the three-level bridge arm is the midpoint level, each switch tube is controlled to be in the first state, and when the difference is greater than the preset threshold and the output level of the three-level bridge arm is the midpoint level, each switch tube is controlled to be in the first state or the second state, wherein, in the first state, the first inner tube, the second inner tube, the first clamping tube and the second clamping tube are in the on state, and the first outer tube and the second outer tube are in the off state, and in the second state, the first inner tube and the first clamping tube are in the on state, and the other switch tubes except the first inner tube and the first clamping tube are in the off state; or, the second inner tube and the second clamping tube are in the on state, and the other switch tubes except the second inner tube and the second clamping tube are in the off state.

[0012] Optionally, when the difference is smaller than a preset threshold, the state of each three-level bridge arm is controlled based on a first control mode, and when the difference is greater than a preset threshold, the state of each three-level bridge arm is controlled based on a second control mode, wherein the state of the three-level bridge arm refers to a combination of states of all switching tubes in the three-level bridge arm, wherein the number of states switched by each three-level bridge arm under the first control mode is different from the number of states switched under the second control mode.

[0013] Optionally, the rotor speed is determined by: receiving a sensing signal from an encoder provided in the doubly-fed wind generator; and determining the rotor speed based on the sensing signal.

[0014] Optionally, the three-level bridge arm includes a first inner tube, a first outer tube and a first clamping tube located in the upper half bridge arm, and a second inner tube, a second outer tube and a second clamping tube located in the lower half bridge arm, the modulation signal is generated based on a three-phase modulation wave and a carrier, and each switch tube is controlled by a PWM waveform, wherein the PWM waveform for controlling each switch tube is generated in the following manner: according to the comparison result of the three-phase modulation wave and the carrier, a PWM waveform for controlling the first outer tube in the positive half cycle of the three-phase modulation wave and a PWM waveform for controlling the first inner tube in the positive half cycle are generated; based on the PWM waveform for controlling the first outer tube in the positive half cycle, a PWM waveform for controlling the second inner tube in the positive half cycle is generated; based on the PWM waveform for controlling the first inner tube in the positive half cycle, a PWM waveform for controlling the second outer tube in the positive half cycle is generated; based on the PWM waveform for controlling the second inner tube in the positive half cycle, a PWM waveform for controlling the first clamping tube in the positive half cycle and a PWM waveform for controlling the second clamping tube in the positive half cycle are generated.

[0015] Optionally, a PWM waveform for controlling each switching tube is also generated in the following manner: based on the comparison result of the three-phase modulation wave and the carrier, a PWM waveform for controlling the second outer tube in the negative half-cycle of the three-phase modulation wave and a PWM waveform for controlling the second inner tube in the negative half-cycle are generated; based on the PWM waveform for controlling the second outer tube in the negative half-cycle, a PWM waveform for controlling the first inner tube in the negative half-cycle is generated; based on the PWM waveform for controlling the second inner tube in the negative half-cycle, a PWM waveform for controlling the first outer tube in the negative half-cycle is generated; based on the PWM waveform for controlling the first inner tube in the negative half-cycle, a PWM waveform for controlling the first clamp tube in the negative half-cycle is generated.

[0016] A second aspect of the present disclosure provides a control system for a converter, the control system comprising: a processor; and a memory for storing instructions executable by the processor, wherein the instructions executable by the processor, when executed by the processor, prompt the processor to execute a control method for the converter according to an embodiment of the present disclosure.

[0017] A third aspect of the present disclosure provides a converter, comprising a control system of the converter according to an embodiment of the present disclosure.

[0018] A fourth aspect of the present disclosure provides a wind turbine generator set, comprising a doubly-fed wind turbine generator and a converter according to an embodiment of the present disclosure.

[0019] A fifth aspect of the present disclosure provides a computer-readable storage medium. When instructions in the computer-readable storage medium are executed by a processor of an electronic device, the electronic device is enabled to execute the control method of the converter according to an embodiment of the present disclosure.

[0020] A sixth aspect of the present disclosure provides a computer program product, comprising computer executable instructions, which, when executed by at least one processor, implement the control method of the converter according to the embodiment of the present disclosure.

[0021] According to the control scheme of the converter disclosed in the present invention, the current rotor speed of the doubly fed wind generator can be determined, and when the difference between the rotor speed and the synchronous speed of the doubly fed wind generator is less than a preset threshold, for each three-level bridge arm, according to the modulation signal for switching the output level of the three-level bridge arm from a positive level or a negative level to a midpoint level, the outer tube that is turned on in the current state is turned off, and when the turned-on outer tube is turned off, each clamping tube and each inner tube are turned on, so that by optimizing the control algorithm of the converter, under the modulation signal of the midpoint level, the current stress borne by a single inner tube and a single clamping tube in the related art can be controlled to be borne jointly by two clamping tubes and two inner tubes. In this way, even if the midpoint level such as zero level lasts for a long time when close to the synchronous speed, the problem of excessive thermal stress of the power devices can be alleviated, the thermal stress of the inner tube can be effectively dispersed, and the thermal distribution of the power devices of the switching tube can be more uniform. This solution can solve the problem of excessive thermal stress of the power devices of the converter without losing power generation by optimizing the modulation logic. It is universal and suitable for the design and application of new units, and is also convenient for improving units in service. It has good control effect and high application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is a schematic structural diagram showing a doubly-fed wind turbine generator according to an exemplary embodiment of the present disclosure.

[0023] Figure 2 is a schematic diagram showing the topology of a three-level bridge arm of a converter according to an exemplary embodiment of the present disclosure.

[0024] Figure 3 is a schematic diagram showing an example of a switching state switching manner of a three-level bridge arm of a converter according to an exemplary embodiment of the present disclosure.

[0025] FIG. 4A to FIG. 4G is a schematic diagram showing current paths of three-level bridge arms of a converter in different states according to an exemplary embodiment of the present disclosure.

[0026] Figure 51 is a flow chart showing a method for controlling a converter near a synchronous speed according to an exemplary embodiment of the present disclosure.

[0027] Figure 6 is a schematic diagram showing another example of a switching state switching manner of a three-level bridge arm of a converter according to an exemplary embodiment of the present disclosure.

[0028] Figure 7 is an example of a PWM waveform diagram showing a converter according to an exemplary embodiment of the present disclosure.

[0029] Figure 8 FIG. 1 is a flowchart illustrating an example of generating a PWM waveform in a method for controlling a converter according to an exemplary embodiment of the present disclosure.

[0030] Fig. 9 is another example of a PWM waveform diagram showing a converter according to an exemplary embodiment of the present disclosure.

[0031] Fig.10 FIG. 1 is a flowchart illustrating another example of generating a PWM waveform in a method for controlling a converter according to an exemplary embodiment of the present disclosure.

[0032] Fig.11A and Fig. 11B It is a schematic diagram showing the effect of using the control method of the converter according to the exemplary embodiment of the present disclosure. DETAILED DESCRIPTION

[0033] The following specific embodiments are provided to help the reader obtain a comprehensive understanding of the methods, devices and / or systems described herein. However, after understanding the disclosure of the present application, various changes, modifications and equivalents of the methods, devices and / or systems described herein will be clear. For example, the order of operations described herein is only an example and is not limited to those orders set forth herein, but can be changed as will be clear after understanding the disclosure of the present application, except for operations that must occur in a specific order. In addition, for greater clarity and simplicity, the description of features known in the art may be omitted.

[0034] The features described herein can be implemented in different forms and should not be construed as being limited to the examples described herein. Rather, the examples described herein have been provided to illustrate only some of the many possible ways to implement the methods, devices, and / or systems described herein, which will be clear after understanding the disclosure of the present application.

[0035] As used herein, the term "and / or" includes any one of the associated listed items and any combination of any two or more.

[0036] Although terms such as "first", "second", and "third" may be used herein to describe various members, components, regions, layers, or portions, these members, components, regions, layers, or portions should not be limited by these terms. Instead, these terms are only used to distinguish one member, component, region, layer, or portion from another member, component, region, layer, or portion. Therefore, without departing from the teachings of the examples described herein, the first member, first component, first region, first layer, or first portion referred to in the examples may also be referred to as the second member, second component, second region, second layer, or second portion.

[0037] In the specification, when an element (such as a layer, a region, or a substrate) is described as being “on”, “connected to”, or “coupled to” another element, the element may be directly “on”, “connected to”, or “coupled to” another element, or one or more other elements may be present therebetween. Conversely, when an element is described as being “directly on”, “directly connected to”, or “directly coupled to” another element, there may be no other elements present therebetween.

[0038] The terms used herein are only used to describe various examples and are not intended to limit the disclosure. Unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. The terms "comprise", "include" and "have" indicate the presence of the described features, quantities, operations, components, elements and / or combinations thereof, but do not exclude the presence or addition of one or more other features, quantities, operations, components, elements and / or combinations thereof.

[0039] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as those generally understood by a person of ordinary skill in the art to which the present disclosure belongs after understanding the present disclosure. Unless explicitly defined as such herein, terms (such as those defined in a general dictionary) should be interpreted as having a meaning consistent with their meaning in the context of the relevant art and the present disclosure, and should not be interpreted in an idealized or overly formal manner.

[0040] Furthermore, in the description of examples, when it is considered that a detailed description of a well-known related structure or function would cause vague interpretation of the present disclosure, such a detailed description will be omitted.

[0041] As mentioned above, for a doubly-fed wind turbine, the zero-level duration of the converter during modulation will be relatively long, which will cause certain power devices to not switch in time and be subjected to excessive thermal stress. However, existing control schemes cannot properly solve the problem of excessive thermal stress of power devices near the synchronous speed.

[0042] In view of such problems, the present disclosure provides a control method of a converter, a control system of a converter, a converter, a wind turbine generator set, a computer-readable storage medium and a computer program product, so as to solve or at least alleviate the above problems.

[0043] In order to better understand the embodiments of the present disclosure, before describing the control scheme of the converter, first refer to Figures 1 to 4G A topology example of a converter of a doubly-fed wind turbine generator, an example of switching its switch states, and examples of current paths in different states are described.

[0044] like Figure 1 As shown, the doubly-fed wind turbine generator set may include a wind turbine 110, two machine-side filter reactors 120 connected in parallel, two machine-side converters 130 connected in parallel, a grid-side converter 140, a grid-side filter 150, and a transformer 160. The electric energy output by the wind turbine 110 can enter the corresponding machine-side converter 130 through the two machine-side filter reactors 120, enter the grid-side converter 140 after rectification by the machine-side converter, and the current inverted by the grid-side converter 140 is connected to the low-voltage side of the transformer 160 through the grid-side filter 150 (for example, an LCL filter) to be integrated into the power grid 170. Although reference is made here to Figure 1 A topology example in which two machine-side converters are connected in parallel is described, but the topology of the converter is not limited thereto. For example, the embodiments of the present disclosure are also applicable to a topology in which one machine-side converter is used.

[0045] In the above example, the two machine-side converters and the grid-side converter may be controlled by the same control system, which may include a converter controller, a power interface board, and a drive control board.

[0046] Here, the converter controller can be used to collect converter data, implement control algorithms, implement fault protection, control power modules, and upload status information.

[0047] The power interface board can convert the modulation signal such as pulse width modulation (PWM) output by the converter controller into a driving signal suitable for the power module and the braking unit on the drive control board, and can convert the status signals of the power module and the braking unit (such as fault signals, sensing signals of negative temperature coefficient (NTC) thermistor sensors, etc.) into level signals suitable for the converter controller. The power interface board can also isolate the power module from the intelligent core component signals, improve the anti-interference ability of the control system, and improve the reliability of the control system.

[0048] The drive control board can convert the modulation signal output by the power interface board into a drive signal of an insulated-gate bipolar transistor (IGBT), can monitor the short circuit of the IGBT, and upload the internal temperature of the IGBT in real time.

[0049] According to an embodiment of the present disclosure, the converter is applied to a doubly-fed wind turbine generator, and the converter includes a plurality of three-level bridge arms, each of which includes a plurality of switch tubes, and the plurality of switch tubes include two inner tubes, two outer tubes, and two clamp tubes. In addition, each three-level bridge arm may include an upper half bridge arm and a lower half bridge arm, and each of the upper half bridge arm and the lower half bridge arm may include an inner tube, an outer tube, and a clamp tube.

[0050] As an example, the converter may be a grid-connected ANPC (active clamped) three-level converter. For example, the converter may be used as a grid-connected converter in photovoltaic, wind power, energy storage, hydrogen production and other fields.

[0051] Specifically, the converter may include a plurality of three-level bridge arms with the same structure, such as an ANPC type three-level circuit topology. Figure 2 A topology example of a three-level bridge arm of a converter according to an exemplary embodiment of the present disclosure is shown.

[0052] like Figure 2 As shown, each three-level bridge arm may include multiple switching tubes (IGBT), and the switching tubes may include a first outer tube T1, a first inner tube T2, a second inner tube T3, a second outer tube T4, a first clamping tube T5 and a second clamping tube T6, wherein the first inner tube T2, the first outer tube T1 and the first clamping tube T5 may be located in the upper half bridge arm, and the second inner tube T3, the second outer tube T4 and the second clamping tube T6 may be located in the lower half bridge arm.

[0053] In addition, each three-level bridge arm may also include a fast recovery freewheeling diode reversely connected in parallel with each switch tube, specifically including: a diode D1 reversely connected in parallel with the first outer tube T1, a diode D2 reversely connected in parallel with the first inner tube T2, a diode D3 reversely connected in parallel with the second inner tube T3, a diode D4 reversely connected in parallel with the second outer tube T4, a diode D5 reversely connected in parallel with the first clamp tube T5, and a diode D6 reversely connected in parallel with the first clamp tube T6. In addition, each three-level bridge arm may also include a first DC capacitor C1 and a second DC capacitor C2 respectively having a voltage Udc. The DC bus of the three-level bridge arm is respectively composed of upper and lower DC capacitors C1 and C2, and the upper and lower DC capacitors have equal voltages and are both Udc, forming three voltages of DC+, NP, and DC-. When different current paths are turned on, the AC terminal can output positive level, negative level, and midpoint level. Specifically, the AC terminal can output 1-level, 1+level, 0+level, and 0-level.

[0054] According to the analysis of the commutation process of the positive level, negative level and midpoint level, and in accordance with the commutation principle of the "shortest commutation loop", the modulation of the three-level bridge arm topology can be divided into seven switching states, namely, positive 1 state, positive 1 dead zone state, positive 0 state, positive 0 - 0 state, negative 0 state, negative 1 dead zone state and negative 1 state.

[0055] Figure 3 An example of switching between the seven switching states of the three-level bridge arm is shown in FIG. Figure 3 As shown, the switch state switching mode of the three-level bridge arm may include: switching between the positive 1 state and the positive 1 dead zone state, switching between the positive 1 dead zone state and the positive 0 state, switching between the positive 0 state and the positive 0 negative 0 state, switching between the negative 1 state and the negative 1 dead zone state, switching between the negative 1 dead zone state and the negative 0 state, and switching between the negative 0 state and the positive 0 negative 0 state.

[0056] FIG. 4A to FIG. 4G The current paths in each switching state are shown separately.

[0057] like Figure 4A As shown, in the positive 1 state, the AC end can output a positive bus voltage, the first outer tube T1 and the first inner tube T2 are turned on, and the other switch tubes are turned off.

[0058] like Figure 4B As shown, in the positive 1 dead zone state, the first outer tube T1 and the first clamping tube T5 are both turned off to prevent short circuit from occurring during the switching process between the positive 1 state and the positive 0 state.

[0059] like Figure 4C As shown, in the positive 0 state, the AC terminal can output a midpoint level, the first inner tube T2 and the first clamping tube T5 are turned on, and the other switch tubes are turned off.

[0060] like Figure 4D As shown, the 0 positive 0 negative state can be an intermediate state of switching from the positive 0 state to the negative 0 state or from the negative 0 state to the positive 0 state. In this state, the first inner tube T2, the second inner tube T3, the first clamping tube T5 and the second clamping tube T6 are all turned on, thereby increasing the current carrying capacity of the midpoint level (zero level), and the other switching tubes are turned off.

[0061] like Figure 4E As shown, in the negative 0 state, the AC terminal can output a midpoint level, the second inner tube T3 and the second clamping tube T6 are turned on, and the other switch tubes are turned off.

[0062] like Figure 4F As shown, in the negative 1 dead zone state, the second clamping tube T6 and the second outer tube T4 are both turned off to prevent short circuit from occurring during the switching process between the negative 1 state and the negative 0 state.

[0063] like Figure 4G As shown, in the negative 1 state, the AC terminal can output a negative bus voltage, the second inner tube T3 and the second outer tube T4 are turned on, and the other switch tubes are turned off.

[0064] The on-off state of each switch tube in the above-mentioned states can be shown in the following Table 1, where "1" represents on and "0" represents off.

[0065] Table 1

[0066]

[0067] In the above example, the modulation logic of the converter can switch between the positive state and the negative state according to the positive and negative states of the modulation wave sent by the control algorithm. When the modulation wave is positive, the modulation logic can switch between the positive 1 state, the positive 1 dead zone state and the positive 0 state; when the modulation wave is negative, the modulation logic can switch between the negative 0 state, the negative 1 dead zone state and the negative 1 state, where the zero level as the midpoint level can include three states: the positive 0 state, the positive 0 negative 0 state and the negative 0 state.

[0068] In this regard, the present disclosure finds that when the rotor speed is close to the synchronous speed, the slip is very small and the rotor current frequency is very low; when the rotor speed is equal to the synchronous speed, the slip is 0 and the rotor current is DC. In order to solve the problem of excessive thermal stress of power devices, when the rotor speed is close to the synchronous speed, the modulation optimization strategy near the synchronous speed can be turned on to solve the problem of excessive thermal stress of power devices without reducing the torque.

[0069] like Figure 5 As shown, the control method of the converter according to the embodiment of the present disclosure may include the following steps:

[0070] In step S510, the current rotor speed of the doubly-fed wind generator may be determined.

[0071] In step S520, in response to the difference between the rotor speed and the synchronous speed of the doubly fed wind generator being less than a preset threshold, the state of the switch tube can be controlled in the following manner for each three-level bridge arm: according to a modulation signal for switching the output level of the three-level bridge arm from a positive level or a negative level to a midpoint level, the outer tube that is turned on in the current state is turned off, and when the outer tube that is turned on is turned off, each clamp tube and each inner tube are turned on.

[0072] Specifically, when the rotor speed of the generator is close to or equal to the synchronous speed, the frequency of each phase current of the machine-side doubly fed converter is low or all are DC currents, and the modulation wave of the machine-side converter is much smaller than that of the asynchronous speed. Accordingly, the duration of the midpoint level in the modulation state of the three-level bridge arm topology is longer, and the midpoint level includes positive 0 state, positive 0-negative 0 state and negative 0 state.

[0073] In conventional modulation logic, when entering synchronous speed, if the machine side current is positive, the positive 0 state lasts for a long time, the current stress of the first inner tube T2 and the first clamp tube T5 increases, and the heat is large. If the time lasts too long, the power device will overheat; when entering synchronous speed, if the machine side current is negative, the negative 0 state lasts for a long time, the current stress of the second inner tube T3 and the second clamp tube T6 increases, and the heat is large. If the time lasts too long, the power device will also overheat.

[0074] In the above step S520, in response to the difference between the rotor speed and the synchronous speed of the doubly fed wind generator being less than a preset threshold, for each three-level bridge arm, the outer tube (for example, the first outer tube T1 corresponding to the positive level and the positive 1 state or the second outer tube T4 corresponding to the negative level and the negative 1 state) that is turned on in the current state (for example, the positive 1 state corresponding to the positive level or the negative 1 state corresponding to the negative level) can be turned off according to the modulation signal for switching from the positive level or the negative level to the midpoint level, and when the turned-on outer tube has been turned off, for example, after passing through the positive 1 dead zone state or the negative 1 dead zone state, then each clamp tube (for example, the first clamp tube T5 and the second clamp tube T6) and each inner tube (for example, the first inner tube T2 and the second inner tube T3) can be turned on.

[0075] In the above process, the preset threshold value may be set according to actual needs, for example, it may be determined based on the measurement error of the rotor speed.

[0076] Through the above methods, such as Figure 6 As shown, when the positive level or negative level switches to the midpoint level, it can switch directly from the positive 1 state through the positive 1 dead zone state to the positive 0 negative 0 state, or from the negative 1 state through the negative 1 dead zone state to the positive 0 negative 0 state, instead of Figure 3 As shown in , it switches from the positive 1 state to the positive 0 state through the positive 1 dead zone state, or switches from the negative 1 state to the negative 0 state through the negative 1 dead zone state. This makes the zero level only include the positive 0 and negative 0 states near the synchronous speed, and will not experience the positive 0 state and the negative 0 state. In the positive 0 and negative 0 states, the four switching tubes, two clamping tubes and two inner tubes, can jointly bear the current stress, which is the same as Figure 3Compared with the solution shown in which two switching tubes, a clamping tube and an inner tube, bear current stress, the overheating problem of power devices can be solved by optimizing the modulation logic alone, and the thermal stress of the switching tubes can be effectively dispersed, making the heat distribution of the power devices of each switching tube more uniform.

[0077] In addition, if Figure 6 As shown, in the embodiment of the present disclosure, near the synchronization speed, it is also possible to switch from the midpoint level to the positive level or the negative level.

[0078] Specifically, in response to the difference between the rotor speed and the synchronous speed of the doubly-fed wind turbine generator being less than a preset threshold, for each three-level bridge arm, the state of the switch tube can also be controlled in the following manner:

[0079] According to the modulation signal for switching the output level of the three-level bridge arm from the midpoint level to the positive level, the inner tube (for example, the second inner tube T3) and each clamping tube (for example, the first clamping tube T1 and the second clamping tube T2) of the lower half bridge arm of the three-level bridge arm can be turned off, and when the inner tube and each clamping tube of the lower half bridge arm have been turned off, for example, after passing through the positive 1 dead zone state, the outer tube (for example, the first outer tube T1) of the upper half bridge arm of the three-level bridge arm is turned on.

[0080] According to the modulation signal used to switch the output level of the three-level bridge arm from the midpoint level to the negative level, the inner tube (for example, the first inner tube T2) and each clamping tube (for example, the first clamping tube T1 and the second clamping tube T2) of the upper half bridge arm of the three-level bridge arm can be turned off, and when the inner tube and each clamping tube of the upper half bridge arm have been turned off, for example, after passing through the negative 1 dead zone state, the outer tube (for example, the second outer tube T4) of the lower half bridge arm of the three-level bridge arm is turned on.

[0081] In the above manner, when the difference between the rotor speed and the synchronous speed of the doubly-fed wind turbine generator is less than a preset threshold (or near the synchronous speed), as Figure 6 As shown in Table 2 below, the modulation of the three-level bridge arm topology can be divided into five switching states, namely, positive 1 state, positive 1 dead zone state, positive 0 negative 0 state, negative 1 dead zone state and negative 1 state.

[0082] like Figure 6 As shown, the switch state switching mode of the three-level bridge arm may include: switching between the positive 1 state and the positive 1 dead zone state, switching between the positive 1 dead zone state and the positive 0 negative 0 state, switching between the negative 1 state and the negative 1 dead zone state, and switching between the negative 1 dead zone state and the positive 0 negative 0 state.

[0083] Table 2

[0084] state T1 T2 T3 T4 T5 T6 1 Positive 1 1 1 0 0 0 0 2 Positive 1 Dead Zone 0 1 0 0 0 0 3 Positive 0 Negative 0 0 1 1 0 1 1 4 Negative 1 dead zone 0 0 1 0 0 0 5 Negative 1 0 0 1 1 0 0

[0085] In addition, in the control method of the converter of the embodiment of the present disclosure, when the rotor speed is a non-synchronous speed, different control modes can be used as needed, for example, Figure 3 The model shown or the Figure 6 The mode shown.

[0086] Specifically, in an example, the control method of the converter may further include: in response to the difference between the rotor speed and the synchronous speed of the doubly-fed wind turbine generator being greater than the preset threshold, for each three-level bridge arm, the state of the switch tube may be controlled in the following manner:

[0087] According to the modulation signal for switching the output level of the three-level bridge arm from the positive level or the negative level to the midpoint level, the outer tube (for example, the first outer tube T1 corresponding to the positive level and the positive 1 state or the second outer tube T4 corresponding to the negative level and the negative 1 state) that is turned on in the current state (for example, the positive 1 state corresponding to the positive level or the negative 1 state corresponding to the negative level) is turned off, and when the turned-on outer tube has been turned off, for example, after the positive 1 dead zone state or the negative 1 dead zone state, then each clamping tube (for example, the first clamping tube T5 and the second clamping tube T6) and each inner tube (for example, the first inner tube T2 and the second inner tube T3) can be turned on.

[0088] Specifically, in this example, for both the rotor speed being near the synchronous speed and the rotor speed being at the non-synchronous speed, the following can be used: Figure 6 In the control mode shown, the modulation of the three-level bridge leg topology switches between five switching states.

[0089] In another example, the control method of the converter may further include: in response to the difference between the rotor speed and the synchronous speed of the doubly-fed wind turbine being greater than a preset threshold, for each three-level bridge arm, controlling the state of the switch tube in the following manner:

[0090] According to the modulation signal for switching the output level of the three-level bridge arm from the positive level or the negative level to the midpoint level, the outer tube that is turned on in the current state is turned off, and when the outer tube that is turned on is turned off, each clamp tube and each inner tube are turned on.

[0091] In this example, when the difference is less than the preset threshold, the state of each three-level bridge arm can be controlled based on the first control mode; when the difference is greater than the preset threshold, the state of each three-level bridge arm can be controlled based on the second control mode. The state of the three-level bridge arm may refer to the combination of the states of all the switch tubes in the three-level bridge arm.

[0092] Here, the number of states switched by each three-level bridge arm in the first control mode may be different from the number of states switched in the second control mode. In the first control mode, when the modulation wave is positive, the modulation logic of the converter switches between the positive 1 state, the positive 1 dead zone state, and the positive 0 negative 0 state; when the modulation wave is negative, the modulation logic of the converter switches between the positive 0 negative 0 state, the negative 1 dead zone state, and the negative 1 state, wherein the zero level may only include the positive 0 negative 0 state. In the second control mode, when the modulation wave is positive, the modulation logic may switch between the positive 1 state, the positive 1 dead zone state, and the positive 0 state; when the modulation wave is negative, the modulation logic may switch between the negative 0 state, the negative 1 dead zone state, and the negative 1 state, wherein the zero level may include the positive 0 state, the positive 0 negative 0 state, and the negative 0 state.

[0093] Specifically, different control modes can be used for the two speed states of rotor speed near synchronous speed and rotor speed at non-synchronous speed. When the rotor speed is close to or equal to synchronous speed, the modulation of the three-level bridge arm topology can be as follows: Figure 6 When the rotor speed is non-synchronous, the modulation of the three-level bridge arm topology can be as follows: Figure 3 Switch between the 7 switch states shown.

[0094] Specifically, when the difference is smaller than the preset threshold and the output level of the three-level bridge arm is the midpoint level, each switch tube can be controlled to be in the first state; when the difference is greater than the preset threshold and the output level of the three-level bridge arm is the midpoint level, each switch tube can be controlled to be in the first state or the second state.

[0095] In the first state, the first inner tube T2, the second inner tube T3, the first clamp tube T5, and the second clamp tube T6 may be in the on state, and the first outer tube T1 and the second outer tube T4 may be in the off state. In the second state, the first inner tube and the first clamp tube may be in the on state, and the switch tubes other than the first inner tube T2 and the first clamp tube T5 may be in the off state; or, the second inner tube T3 and the second clamp tube T6 may be in the on state, and the switch tubes other than the second inner tube T3 and the second clamp tube T6 may be in the off state.

[0096] In addition, in the embodiments of the present disclosure, the rotor speed can be determined in the following manner: a sensing signal can be received from an encoder provided in the doubly-fed wind turbine generator; and the rotor speed can be determined based on the sensing signal. Specifically, the encoder can sense the rotation speed of the wind turbine generator, and the control system of the converter can be connected to the encoder in communication to receive the sensed signal from the encoder and determine the current rotor speed, so that the corresponding control mode can be switched according to the rotor speed.

[0097] The control logic of the converter at different rotor speeds according to the embodiment of the present disclosure is described above. Figures 7 to 10 Describe the specific process example of implementing the on-off control of each switch tube.

[0098] During the control process of the converter, a three-phase modulation wave corresponding to each three-level bridge arm may be generated based on the operation data of the three-level converter.

[0099] As an example, the parameters of the three-phase modulation wave corresponding to the three-level bridge arm can be determined based on the operating data of the three-level converter, so as to generate the corresponding three-phase modulation wave based on these parameters. Here, the operating data of the three-level converter can include, for example, but not limited to, at least one of the following items: DC bus voltage, machine-side AC voltage, grid-side AC voltage, and output current of the three-level bridge arm.

[0100] In addition, the three-phase modulation wave may include, but is not limited to, a sine wave, and other types of three-phase modulation waves may also be used. The parameters of the three-phase modulation wave may include, for example, amplitude, phase, frequency, and the like.

[0101] A modulation signal for controlling each switch tube of the three-level bridge arm may be generated based on the three-phase modulation wave and the carrier wave corresponding to each three-level bridge arm. For example, each switch tube may be controlled by a PWM waveform.

[0102] Here, the corresponding carrier wave can be generated based on the parameters of the carrier wave. For example, the parameters of the carrier wave may include: amplitude, phase, frequency, etc. The type of carrier wave may include, but is not limited to, upper and lower layers of triangular carrier waves with the same frequency and phase, and other types of carrier waves may also be used.

[0103] As an example, Figure 7 As shown, the three-phase modulation wave Us can be generated based on the grid-side voltage by using coordinate transformation such as the dq coordinate system. Specifically, the DC voltage Udc, the AC voltage Uabc on the machine side and the grid side, and the module current Iabc of the converter can be collected by an analog-to-digital converter, and the three-phase modulation wave data can be generated after calculation by a preset internal control algorithm.

[0104] In addition, the same-direction carrier modulation method can be used to obtain the driving pulse of the power device. When the amplitude of the positive half-wave of the modulated wave is greater than the upper carrier, a positive level is output; when the amplitude of the negative half-wave of the modulated wave is less than the lower carrier, a negative level is output, and in other cases, a 0 level is output.

[0105] For example, after comparing the modulated wave with the carrier wave, the initial PWM waveforms of the first outer tube T1, the first inner tube T2 and the second clamp tube T6 can be generated, and after adding the dead zone, the PWM waveforms of the first clamp tube T5, the second inner tube T3 and the second outer tube T4 can be generated, wherein the pulses of the first outer tube T1 and the first clamp tube T5 are complementary, the pulses of the second clamp tube T6 and the second outer tube T4 are complementary, and the pulses of the first inner tube T2 and the second inner tube T3 are complementary.

[0106] Specifically, the pulse generation method of the three-level converter of the exemplary embodiment of the present disclosure is: in the positive half cycle of the modulation wave, when the modulation wave is greater than the triangular carrier, a positive level is output, the switch tubes T1 and T2 are turned on, and the other switch tubes are turned off; when the modulation wave is less than the triangular carrier, a 0 level is output, the switch tubes T2, T3, T5, and T6 are turned on, and the other switch tubes are turned off. In the negative half cycle of the modulation wave, when the modulation wave is less than the triangular carrier, a negative level is output, the switch tubes T3 and T4 are turned on, and the other switch tubes are turned off; when the modulation wave is greater than the triangular carrier, a 0 level is output, the switch tubes T2, T3, T5, and T6 are turned on, and the other switch tubes are turned off.

[0107] As an example, Figure 8 As shown, the PWM waveform used to control each switch tube can be generated in the following way:

[0108] In step S810, a PWM waveform for controlling the first outer tube T1 in the positive half cycle of the three-phase modulation wave and a PWM waveform for controlling the first inner tube T2 in the positive half cycle can be generated according to the comparison result of the three-phase modulation wave and the carrier.

[0109] In step S820 , a PWM waveform for controlling the first clamping tube T5 in the positive half cycle may be generated based on the PWM waveform for controlling the first outer tube T1 in the positive half cycle.

[0110] In step S830, a PWM waveform for controlling the second inner tube T3 in the positive half cycle may be generated based on the PWM waveform for controlling the first inner tube T2 in the positive half cycle.

[0111] In step S840, a PWM waveform for turning off the second outer tube T4 in the positive half cycle and a PWM waveform for turning off the second clamp tube T6 in the positive half cycle may be generated.

[0112] As an example, the PWM waveform used to control the first outer tube T1 is complementary to the PWM waveform used to control the first clamp tube T5; the PWM waveform used to control the first inner tube T2 is complementary to the PWM waveform used to control the second inner tube T3. In addition, the PWM wave used to control the second outer tube T4 and the PWM wave used to control the second clamp tube T6 are turned off in the positive half cycle and complementary in the negative half cycle.

[0113] like Fig. 9 As shown, S1, S2, S3, S4, S5 and S6 represent the PWM waveforms of the switch tubes T1, T2, T3, T4, T5 and T6 respectively, V Ao It represents the voltage waveform of phase A, wherein in the positive half cycle of the modulation wave, the first inner tube T2 can be always turned on according to the comparison result of the modulation wave and the upper half carrier, and the first outer tube T1 can be turned on and off according to the comparison result of the modulation wave and the upper half carrier, that is, the initial PWM waveforms of T1 and T2 are generated after the modulation wave is compared with the carrier, and the PWM waveforms of the first clamp tube T5 and the second inner tube T3 are generated after the dead zone is added, wherein the tube pulses of the first outer tube T1 and the first clamp tube T5 are complementary, and the tube pulses of the first inner tube T2 and the second inner tube T3 are complementary, thereby forming the control pulses of T1 / T2 / T3 / T5.

[0114] As an example, Fig.10 As shown, the PWM waveform used to control each switch tube can also be generated in the following way:

[0115] In step S1010, based on the comparison result of the three-phase modulation wave and the carrier wave, a PWM waveform for controlling the second clamp tube T6 in the negative half cycle of the three-phase modulation wave and a PWM waveform for controlling the second inner tube T3 in the negative half cycle can be generated.

[0116] In step S1020, a PWM waveform for controlling the second outer tube T4 in the negative half cycle may be generated based on the PWM waveform for controlling the second clamp tube T6 in the negative half cycle.

[0117] In step S1030, a PWM waveform for controlling the first inner tube T2 in the negative half cycle may be generated based on the PWM waveform for controlling the second inner tube T3 in the negative half cycle.

[0118] In step S1040 , a PWM waveform for turning off the first outer tube T1 in the negative half cycle and a PWM waveform for turning off the first clamp tube T5 in the negative half cycle may be generated.

[0119] As an example, the PWM waveform used to control the second outer tube T4 is complementary to the PWM waveform used to control the second clamp tube T6; the PWM waveform used to control the second inner tube T3 is complementary to the PWM waveform used to control the first inner tube T2.

[0120] Fig.11A and Fig. 11B 3 is a schematic diagram showing the effect of the control method of the converter according to the exemplary embodiment of the present disclosure, wherein the abscissa represents time (in seconds) and the ordinate represents temperature (in degrees).

[0121] like Fig.11AAs shown in FIG. 1 , in the modulation method of the related art, the highest temperature of the switch tube temperature of the three-level bridge arm can reach 73.6 degrees, which is close to the preset power device over-temperature protection value of 75 degrees. Fig. 11B As shown, when the control method of the exemplary embodiment of the present disclosure is adopted, the highest temperature of the switch tube temperature of the three-level bridge arm is reduced to 58 degrees. It can be seen that according to the above control method of the embodiment of the present disclosure, the problem of overheating of the local switch tube at synchronous speed can be solved by optimizing the modulation logic of the three-level bridge arm.

[0122] According to a second aspect of an embodiment of the present disclosure, a control system of a converter is provided, the control system comprising: a processor; and a memory for storing processor executable instructions, wherein the processor executable instructions, when executed by the processor, prompt the processor to execute the control method of the converter according to the embodiment of the present disclosure.

[0123] As an example, the control system may include an electronic device having the above-mentioned processor and memory, which may be a PC computer, a tablet device, a personal digital assistant, a smart phone, or other device capable of executing the above-mentioned instruction set. Here, the electronic device is not necessarily a single electronic device, but may also be any device or circuit capable of executing the above-mentioned instructions (or instruction sets) individually or in combination. The electronic device may also be part of an integrated control system or system manager, or may be configured as a portable electronic device interconnected with a local or remote (e.g., via wireless transmission) interface.

[0124] In electronic devices, the processor may include a central processing unit (CPU), a graphics processing unit (GPU), a programmable logic device, a dedicated processor system, a microcontroller or a microprocessor. As an example and not limitation, the processor may also include an analog processor, a digital processor, a microprocessor, a multi-core processor, a processor array, a network processor, etc.

[0125] The processor can execute instructions or codes stored in the memory, wherein the memory can also store data. Instructions and data can also be sent and received through the network via the network interface device, wherein the network interface device can adopt any known transmission protocol.

[0126] The memory may be integrated with the processor, for example, RAM or flash memory is arranged within an integrated circuit microprocessor or the like. In addition, the memory may include a separate device, such as an external disk drive, a storage array, or any other storage device that can be used by a database system. The memory and the processor may be operatively coupled, or may communicate with each other, such as through an I / O port, a network connection, etc., so that the processor can read files stored in the memory.

[0127] In addition, the electronic device may also include a video display (such as a liquid crystal display) and a user interaction interface (such as a keyboard, a mouse, a touch input device, etc.) All components of the electronic device may be connected to each other via a bus and / or a network.

[0128] According to a third aspect of an embodiment of the present disclosure, a converter is provided. The converter includes a control system of the converter according to an embodiment of the present disclosure.

[0129] According to a fourth aspect of the embodiments of the present disclosure, a wind turbine generator set is provided. The wind turbine generator set includes a doubly-fed wind turbine generator and the converter according to the embodiments of the present disclosure.

[0130] According to a fifth aspect of an embodiment of the present disclosure, a computer-readable storage medium is provided. When instructions in the computer-readable storage medium are executed by a processor of an electronic device, the electronic device is enabled to execute the control method of the converter according to the embodiment of the present disclosure.

[0131] Specifically, the control method of the converter according to the embodiment of the present disclosure can be written as a computer program and stored on a computer-readable storage medium. When the instructions in the computer-readable storage medium are executed by at least one processor, the at least one processor is prompted to perform the control method of the converter according to the exemplary embodiment of the present disclosure. Examples of computer-readable storage media include: read-only memory (ROM), random access programmable read-only memory (PROM), electrically erasable programmable read-only memory (EEPROM), random access memory (RAM), dynamic random access memory (DRAM), static random access memory (SRAM), flash memory, non-volatile memory, CD-ROM, CD-R, CD+R, CD-RW, CD+RW, DVD-ROM, DVD-R, DVD+R, DVD-RW, DVD+RW, DVD-RAM, BD-ROM, BD-R, BD-R LTH, BD-RE, Blu-ray or optical disk storage, hard disk drive (HDD), solid state drive (SSD), card storage (such as, multimedia card, secure digital (SD) card or extreme digital (XD) card), magnetic tape, floppy disk, magneto-optical data storage device, optical data storage device, hard disk, solid state disk and any other device, any other device is configured to store computer program and any associated data, data files and data structures in a non-transitory manner and provide the computer program and any associated data, data files and data structures to a processor or computer so that the processor or computer can execute the computer program. In one example, the computer program and any associated data, data files and data structures are distributed on a networked computer system so that the computer program and any associated data, data files and data structures are stored, accessed and executed in a distributed manner by one or more processors or computers.

[0132] According to a sixth aspect of an embodiment of the present disclosure, a computer program product is provided, comprising computer executable instructions, which, when executed by at least one processor, implement the control method of the converter according to the embodiment of the present disclosure.

[0133] The specific implementation methods of the present disclosure have been described in detail above. Although some embodiments have been shown and described, those skilled in the art should understand that these embodiments may be modified and varied without departing from the principles and spirit of the present disclosure, the scope of which is defined by the claims and their equivalents. These modifications and variations should also be within the scope of protection of the claims of the present disclosure.

Claims

1. A method for controlling a converter, characterized in that: The converter is applied to a doubly-fed wind turbine generator, the converter comprises a plurality of three-level bridge arms, each of the three-level bridge arms comprises a plurality of switch tubes, the plurality of switch tubes comprises two inner tubes, two outer tubes and two clamp tubes, wherein the control method comprises: Determining a current rotor speed of the doubly-fed wind turbine; In response to the difference between the rotor speed and the synchronous speed of the doubly-fed wind turbine generator being less than a preset threshold, for each three-level bridge arm, the state of the switch tube is controlled in the following manner: According to the modulation signal for switching the output level of the three-level bridge arm from a positive level or a negative level to a midpoint level, the outer tube that is turned on in the current state is turned off, and when the outer tube that is turned on is turned off, each clamping tube and each inner tube are turned on.

2. The control method according to claim 1, characterized in that: The three-level bridge arm includes an upper half bridge arm and a lower half bridge arm, wherein the control method further includes: In response to the difference between the rotor speed and the synchronous speed of the doubly-fed wind turbine generator being greater than the preset threshold, for each three-level bridge arm, the state of the switch tube is controlled in the following manner: According to the modulation signal for switching the output level of the three-level bridge arm from the positive level or the negative level to the midpoint level, the external tube that is turned on in the current state is turned off, and when the external tube that is turned on is turned off, the clamping tube in the same half bridge arm as the external tube that is turned on is turned on; When the clamping tube in the same half bridge arm is turned on, the other clamping tube and each inner tube of the two clamping tubes are turned on.

3. The control method according to claim 1, characterized in that: The control method further comprises: In response to the difference between the rotor speed and the synchronous speed of the doubly-fed wind turbine generator being greater than the preset threshold, for each three-level bridge arm, the state of the switch tube is controlled in the following manner: According to the modulation signal for switching the output level of the three-level bridge arm from a positive level or a negative level to a midpoint level, the outer tube that is turned on in the current state is turned off, and when the outer tube that is turned on is turned off, each clamping tube and each inner tube are turned on.

4. The control method according to any one of claims 1 to 3, characterized in that: In response to the difference being less than a preset threshold, for each three-level bridge arm, the state of the switch tube is further controlled in the following manner: According to a modulation signal for switching the output level of the three-level bridge arm from a midpoint level to a positive level, turning off the inner tube and each clamping tube of the lower half bridge arm of the three-level bridge arm, and turning on the outer tube of the upper half bridge arm of the three-level bridge arm when the inner tube and each clamping tube of the lower half bridge arm are turned off; According to a modulation signal for switching the output level of the three-level bridge arm from a midpoint level to a negative level, the inner tube and each clamping tube of the upper half bridge arm of the three-level bridge arm are turned off, and when the inner tube and each clamping tube of the upper half bridge arm are turned off, the outer tube of the lower half bridge arm of the three-level bridge arm is turned on.

5. The control method according to claim 1, characterized in that: The three-level bridge arm includes a first inner tube, a first outer tube and a first clamping tube located in the upper half bridge arm, and a second inner tube, a second outer tube and a second clamping tube located in the lower half bridge arm. Wherein, when the difference is less than the preset threshold value and the output level of the three-level bridge arm is the midpoint level, each switch tube is controlled to be in the first state, When the difference is greater than the preset threshold and the output level of the three-level bridge arm is a midpoint level, each switch tube is controlled to be in the first state or the second state, Wherein, in the first state, the first inner tube, the second inner tube, the first clamp tube and the second clamp tube are in a conducting state, and the first outer tube and the second outer tube are in a shutoff state, In the second state, the first inner tube and the first clamp tube are in the on state, and the other switch tubes except the first inner tube and the first clamp tube are in the off state; or, the second inner tube and the second clamp tube are in the on state, and the other switch tubes except the second inner tube and the second clamp tube are in the off state.

6. The control method according to claim 1, characterized in that: When the difference is less than the preset threshold, the state of each three-level bridge arm is controlled based on the first control mode, When the difference is greater than a preset threshold, the state of each three-level bridge arm is controlled based on the second control mode, where the state of the three-level bridge arm refers to a combination of states of all switch tubes in the three-level bridge arm. The number of states that each three-level bridge arm switches in the first control mode is different from the number of states that each three-level bridge arm switches in the second control mode.

7. The control method according to claim 1, characterized in that: The rotor speed is determined by: receiving a sensing signal from an encoder disposed on the doubly-fed wind turbine generator; Based on the sensed signal, the rotor speed is determined.

8. The control method according to claim 1, characterized in that: The three-level bridge arm includes a first inner tube, a first outer tube and a first clamping tube located in the upper half bridge arm, and a second inner tube, a second outer tube and a second clamping tube located in the lower half bridge arm. The modulation signal is generated based on a three-phase modulation wave and a carrier wave, and each switch tube is controlled by a PWM waveform. The PWM waveform used to control each switch tube is generated in the following manner: Generate a PWM waveform for controlling the first outer tube in a positive half cycle of the three-phase modulation wave and a PWM waveform for controlling the first inner tube in the positive half cycle according to a comparison result of the three-phase modulation wave and the carrier wave; generating a PWM waveform for controlling the first clamping tube in the positive half cycle based on the PWM waveform for controlling the first outer tube in the positive half cycle; generating a PWM waveform for controlling the second inner tube in the positive half cycle based on the PWM waveform for controlling the first inner tube in the positive half cycle; A PWM waveform is generated for turning off the second outer tube and the second clamp tube in the positive half cycle.

9. The control method according to claim 8, characterized in that: The PWM waveform used to control each switch tube is also generated in the following way: Based on the comparison result of the three-phase modulation wave and the carrier wave, a PWM waveform for controlling the second clamping tube in the negative half cycle of the three-phase modulation wave and a PWM waveform for controlling the second inner tube in the negative half cycle are generated; generating a PWM waveform for controlling the second outer tube in the negative half cycle based on the PWM waveform for controlling the second clamp tube in the negative half cycle; generating a PWM waveform for controlling the first inner tube in the negative half cycle based on the PWM waveform for controlling the second inner tube in the negative half cycle; A PWM waveform for turning off the first outer tube in the negative half cycle and a PWM waveform for turning off the first clamp tube in the negative half cycle are generated.

10. A control system for a converter, characterized in that: The control system comprises: processor; a memory for storing processor executable instructions, Wherein, when the processor executable instructions are executed by the processor, the processor is prompted to execute the control method of the converter according to any one of claims 1 to 9.

11. A converter, characterized in that: The converter comprises a control system for the converter according to claim 10 .

12. A wind turbine generator set, characterized in that: The wind turbine generator set comprises a doubly-fed wind turbine generator and the converter according to claim 11.

13. A computer-readable storage medium, characterized in that: When the instructions in the computer-readable storage medium are executed by a processor of an electronic device, the electronic device is enabled to execute the control method of the converter according to any one of claims 1 to 9.

14. A computer program product comprising computer executable instructions, characterized in that: When the computer executable instructions are executed by at least one processor, the control method of the converter according to any one of claims 1 to 9 is implemented.

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