Excitation current control method for doubly-fed wind turbines under synchronous conditions

Abstract

This invention provides a method for controlling the excitation current of a doubly-fed induction generator (DFIG) in synchronous mode. The method includes: dividing the entire DFIG operation into two modes: synchronous speed control mode and asynchronous speed control mode; when the DFIG operates near synchronous speed, it enters the synchronous speed excitation current control mode, employing optimized excitation current control; after entering synchronous speed, based on the combined judgment of the rotational speed n and the instantaneous values ​​of the rotor three-phase currents, the method ensures that the duty cycle of the devices in the memory is the same or similar for multiple cycles, serving as a condition for starting the rotor current; and when the per-unit value of the three-phase rotor current is less than i = k, a preset triangular carrier wave or sinusoidal current is applied to the three-phase rotor current to ensure dynamic adjustment of the device duty cycle. This invention can improve the problems of heat generation / fatigue caused by the switching devices of the DFIG rotor converter maintaining the same duty cycle for a long time under synchronous speed, thus improving the reliability of the converter.

Description

Technical Field

[0001] This invention relates to the fields of new energy applications and power generation technology in oil fields, and in particular to a method for controlling the excitation current of a doubly fed wind turbine in synchronous mode. Background Technology

[0002] Doubly-fed induction generators (DFIGs) are among the most commonly used generator types, widely applied due to their advantages such as small excitation converter capacity and flexible operation control. At synchronous speed, the rotor current of a DFIG exhibits a DC current, reaching extremely low frequencies near synchronous speed. This results in the converter's power electronic components operating at a certain duty cycle for extended periods, leading to uneven heating and, over time, overheating-related shutdowns. Furthermore, excessive DC current at synchronous speed can trigger hardware protection mechanisms, also causing shutdowns. Therefore, addressing this problem is a pressing issue for large-scale wind turbine converters and requires immediate resolution. Summary of the Invention

[0003] To address the problems in the prior art, embodiments of the present invention provide a method for controlling the excitation current of a doubly-fed wind turbine in synchronous state.

[0004] Specifically, the embodiments of the present invention provide the following technical solutions:

[0005] In a first aspect, embodiments of the present invention provide an excitation current control method for a doubly-fed wind turbine in synchronous state, comprising:

[0006] The entire operating state of the doubly fed wind turbine is divided into two types: synchronous speed control mode and asynchronous speed control mode.

[0007] When the doubly-fed wind turbine is operating near synchronous speed, it enters the synchronous speed excitation current control mode, adopting optimized excitation current control, specifically including:

[0008] S1. Calculate the three-phase rotor excitation current i a i b i c Instantaneous value, and select peak current i max As the base value, and calculate the per-unit value i corresponding to the three-phase rotor current. a_pu =i a / i max i b_pu =i b / i max i c_pu =i c / i max And read the duty cycle of the converter switching devices at this time, and set i a_pu i b_pu ic_pu The duty cycle is stored in memory;

[0009] S2. After entering synchronous speed by jointly judging the rotational speed n and the instantaneous value of the rotor three-phase current, on the one hand, the duty cycle of the device in the memory is the same or similar for multiple cycles, which serves as the starting condition for the rotor to be connected to the current. On the other hand, when the per-unit value of the three-phase rotor current is less than i=k, a preset triangular carrier or sine wave current is connected to the three-phase rotor current to ensure dynamic adjustment of the device duty cycle.

[0010] S3. When the doubly fed wind turbine is operating in asynchronous speed mode, a non-optimized excitation current control mode is adopted.

[0011] Furthermore, the criteria for determining whether a doubly-fed wind turbine is operating near its synchronous speed are as follows:

[0012] n = (1 ± M%) * n sy

[0013] In the formula, n is the actual rotational speed, n sy M is the synchronous speed, and M is the selected deviation coefficient. M is determined with reference to the rotational inertia and synchronous speed of the doubly-fed wind turbine.

[0014] Furthermore, after reaching synchronous speed, the duty cycle of the converter switching devices and i a_pu i b_pu i c_pu All of these need to be stored in memory as conditions for calculating the parallel current.

[0015] Furthermore, the duty cycle read is used as a condition for judging the device's operating state within the switching cycle. When the duty cycle remains in the same state within a preset number of cycles, it is used as a condition for current inflow to start.

[0016] Furthermore, when the per-unit values ​​of the three-phase rotor currents are all less than i = k, a certain triangular carrier wave or sinusoidal wave current is applied in parallel with the three-phase rotor currents to ensure dynamic adjustment of the device duty cycle; wherein, the triangular carrier wave is selected as the central symmetrical carrier wave, and the frequencies of the triangular carrier wave and the sinusoidal wave are:

[0017] f in =3*n sy *P / 60

[0018] In the formula, f in It is the rotor current frequency under synchronous conditions, n sy It is the synchronous speed, and P is the number of pole pairs of the doubly-fed wind turbine.

[0019] Furthermore, when the doubly-fed wind turbine operates in asynchronous speed mode, a non-optimized excitation current control mode is adopted, including:

[0020] In the asynchronous speed mode, a non-optimized excitation current control mode is adopted; the relationship between the dq-axis current in the synchronous rotating coordinate system and the rotor three-phase current in the stationary coordinate system is as follows:

[0021]

[0022] That is, in a synchronously rotating coordinate system:

[0023]

[0024] In the formula, i a i b i c These are the rotor phases a, b, and c currents in the stationary coordinate system, respectively; i rd i rq It is the dq-axis current in the dq-axis coordinate system; i rd or i rq The maximum value is the peak value of the instantaneous current.

[0025] Furthermore, when the doubly-fed wind turbine is operating in synchronous speed control mode, the given i rd or i rq The current transformation relationship after considering the parallel current is as follows:

[0026]

[0027] In the formula, i inrd i inrq These are the two-phase rotor currents in the synchronous rotating coordinate system after considering the input current; i inra i inrb i inrc These are the rotor phases a, b, and c currents in the stationary coordinates after considering the current in parallel.

[0028] As can be seen from the above technical solution, the excitation current control method for a doubly-fed wind turbine in synchronous state provided by the embodiments of the present invention divides the entire operating state of the doubly-fed wind turbine into two types: synchronous speed control mode and asynchronous speed control mode. When the doubly-fed wind turbine is operating near the synchronous speed, it enters the excitation current control mode under synchronous speed, and adopts optimized excitation current control, specifically including: S1, calculating the three-phase rotor excitation current i a i b i c Instantaneous value, and select peak current i max As the base value, and calculate the per-unit value i corresponding to the three-phase rotor current. a_pu =i a / i max i b_pu =i b / i max ic_pu =i c / i max And read the duty cycle of the converter switching devices at this time, and set i a_pu i b_pu i c_pu S2. After determining the synchronous speed by combining the rotational speed n and the instantaneous values ​​of the rotor three-phase current, the duty cycle of the devices in the memory is the same or similar for multiple cycles, which can be used as the starting condition for the rotor to be connected to the current. When the per-unit value of the three-phase rotor current is less than i = k (k < 1), a certain triangular carrier wave or sine wave current is connected in parallel with the three-phase rotor current to ensure the dynamic adjustment of the device duty cycle. S3. When the doubly-fed wind turbine is operating in the non-synchronous speed mode, a non-optimized (limited) excitation current control mode is adopted. The control method provided by the embodiments of the present invention can improve the problems of heat generation / fatigue caused by the switching devices of the rotor converter of the doubly-fed wind turbine maintaining the same duty cycle for a long time under synchronous speed, thereby improving the reliability of the converter. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a control block diagram of an excitation current control method for a doubly fed wind turbine in synchronous state according to an embodiment of the present invention;

[0031] Figure 2 This is a flowchart of an excitation current control method for a doubly fed wind turbine in synchronous state, provided by an embodiment of the present invention. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] As described in the background section, doubly-fed induction generators (DFIGs) are among the most commonly used generator types, widely applied due to their advantages such as small excitation converter capacity and flexible operation control. At synchronous speed, the rotor current of a DFIG exhibits a DC current, reaching extremely low frequencies near synchronous speed. This results in the converter's power electronic components operating at a certain duty cycle for extended periods, leading to uneven heating and, over time, overheating-related shutdowns. Furthermore, excessive DC current at synchronous speed may trigger hardware protection mechanisms, also causing shutdowns. Therefore, addressing this problem is a pressing issue for large-scale wind turbine converters and requires immediate resolution.

[0034] In this invention, it should be noted that a reasonable excitation current control method can reduce the device stress of the converter in a doubly-fed induction generator (DFIG) wind turbine at synchronous speed. However, existing technologies do not optimize the control method for DFIG wind turbines at synchronous speed, but only consider it as a general approach. Furthermore, in the case of large wind turbine converters, poor synchronous excitation current quality can lead to shutdown failures, causing significant losses and jeopardizing unit safety. Therefore, this invention provides an excitation current control method for a DFIG wind turbine in synchronous mode. This invention can improve the control performance of the wind turbine and is suitable for applications such as oilfield new energy and power generation. The following will explain and illustrate the solution provided by this invention in detail through specific embodiments.

[0035] Figure 1 This is a control block diagram of an excitation current control method for a doubly fed wind turbine in synchronous state according to an embodiment of the present invention; Figure 2 This is a flowchart of an embodiment of the excitation current control method for a doubly fed wind turbine in synchronous state.

[0036] Among them, for the attached Figure 1 This document describes a scenario of the embodiment and the connection method of each unit: The PLL is a phase-locked loop unit that obtains grid voltage information; the current setpoint multiplexed port is used to switch between asynchronous speed control mode and synchronous speed control mode; the mode control selector obtains the control mode type based on the speed and rotor current information; ω slip ω1 and ω2 are the slip angular frequency and synchronous speed, respectively; It is the leakage magnetic coefficient of the generator.

[0037] like Figure 1 and Figure 2 As shown in the embodiment of the present invention, the excitation current control method for a doubly-fed wind turbine in synchronous state includes the following steps:

[0038] The entire operating state of the doubly-fed wind turbine is divided into two modes: synchronous speed control mode and asynchronous speed control mode. When the doubly-fed wind turbine is operating near the synchronous speed, it enters the synchronous speed excitation current control mode, adopting optimized excitation current control, specifically including:

[0039] S1. Calculate the three-phase rotor excitation current i a i b i c Instantaneous value, and select peak current i max As the base value, and calculate the per-unit value i corresponding to the three-phase rotor current. a_pu =i a / i max i b_pu =i b / i max i c_pu =i c / i max And read the duty cycle of the converter switching devices at this time, and set i a_pu i b_pu i c_pu The duty cycle is stored in memory;

[0040] S2. After entering the synchronous speed by jointly judging the speed n and the instantaneous value of the rotor three-phase current, on the one hand, the duty cycle of the device in the memory is the same or similar for multiple cycles, which can be used as the starting condition for the rotor to be connected to the current. On the other hand, when the per-unit value of the three-phase rotor current is less than i = k (k takes a value < 1), a certain triangular carrier or sine wave current is connected in parallel on the three-phase rotor current to ensure the dynamic adjustment of the device duty cycle.

[0041] S3. When the doubly fed wind turbine is operating in asynchronous speed mode, a non-optimized (limited) excitation current control mode is adopted.

[0042] It should be noted that the control method provided in this embodiment can improve the problems of heat generation / fatigue caused by the switching devices of the doubly fed wind turbine rotor converter maintaining the same duty cycle for a long time under synchronous speed, thereby improving the reliability of the converter.

[0043] As can be seen from the above technical solution, the excitation current control method for a doubly-fed wind turbine in synchronous state provided by the embodiments of the present invention divides the entire operating state of the doubly-fed wind turbine into two types: synchronous speed control mode and asynchronous speed control mode. When the doubly-fed wind turbine is operating near the synchronous speed, it enters the excitation current control mode under synchronous speed, and adopts optimized excitation current control, specifically including: S1, calculating the three-phase rotor excitation current i a i b i c Instantaneous value, and select peak current i maxAs the base value, and calculate the per-unit value i corresponding to the three-phase rotor current. a_pu =i a / i max i b_pu =i b / i max i c_pu =i c / i max And read the duty cycle of the converter switching devices at this time, and set i a_pu i b_pu i c_pu S2. After determining the synchronous speed by combining the rotational speed n and the instantaneous values ​​of the rotor three-phase current, the duty cycle of the devices in the memory is the same or similar for multiple cycles, which can be used as the starting condition for the rotor to be connected to the current. When the per-unit value of the three-phase rotor current is less than i = k (k < 1), a certain triangular carrier wave or sine wave current is connected in parallel with the three-phase rotor current to ensure the dynamic adjustment of the device duty cycle. S3. When the doubly-fed wind turbine is operating in the non-synchronous speed mode, a non-optimized (limited) excitation current control mode is adopted. The control method provided by the embodiments of the present invention can improve the problems of heat generation / fatigue caused by the switching devices of the rotor converter of the doubly-fed wind turbine maintaining the same duty cycle for a long time under synchronous speed, thereby improving the reliability of the converter.

[0044] Furthermore, based on the above embodiments, in this embodiment, the criterion for determining whether a doubly-fed wind turbine has entered the vicinity of synchronous speed is:

[0045] n = (1 ± M%) * n sy

[0046] In the formula, n is the actual rotational speed, n sy M is the synchronous speed, and M is the selected deviation coefficient. M is determined with reference to the moment of inertia and synchronous speed of the doubly-fed wind turbine.

[0047] Furthermore, based on the above embodiments, in this embodiment, after reaching synchronous speed, the duty cycle of the converter switching devices and i a_pu i b_pu i c_pu All of these need to be stored in memory as conditions for calculating the parallel current.

[0048] Furthermore, based on the above embodiments, in this embodiment, the read duty cycle is used as a condition for determining the device's operating state within a switching cycle. When the duty cycle remains in the same state across multiple cycles, it serves as a current-injection start-up condition. When the per-unit values ​​of the three-phase rotor currents are all less than i = k (k < 1), a certain triangular carrier or sine wave current is injected into the three-phase rotor currents to ensure dynamic adjustment of the device's duty cycle. The triangular carrier is a centrally symmetrical carrier, and the frequencies of the triangular carrier and the sine wave are:

[0049] f in =3*n sy *P / 60

[0050] In the formula, f in It is the rotor current frequency in the synchronous state, and P is the number of pole pairs of the doubly-fed wind turbine.

[0051] Furthermore, based on the above embodiments, in this embodiment, a non-optimized (limited) excitation current control mode is adopted in the asynchronous speed mode. The relationship between the dq-axis current in the synchronous rotating coordinate system and the rotor three-phase current in the stationary coordinate system is as follows:

[0052]

[0053] That is, in a synchronously rotating coordinate system:

[0054]

[0055] In the formula, i a i b i c These are the rotor phases a, b, and c currents in the stationary coordinate system, respectively; i rd i rq It is the dq-axis current in the dq-axis coordinate system; i rd or i rq The maximum value is the peak instantaneous current. When the doubly-fed wind turbine is operating in synchronous mode, given i... rd or i rq The current transformation relationship after the parallel current is considered is as follows:

[0056]

[0057] In the formula, i inrd i inrq These are the two-phase rotor currents in the synchronous rotating coordinate system after considering the input current; i inra i inrb i inrc These are the rotor phases a, b, and c currents in the stationary coordinates after considering the current in parallel.

[0058] Therefore, the control method provided in this embodiment can improve the problems of heat generation / fatigue caused by the switching devices of the doubly fed wind turbine rotor converter maintaining the same duty cycle for a long time under synchronous speed, thereby improving the reliability of the converter.

[0059] The excitation current control method for a doubly fed wind turbine in synchronous state provided in this invention can be applied to the fields of oilfield new energy applications and power generation technology.

[0060] As can be seen from the above technical solution, the excitation current control method for a doubly-fed wind turbine in synchronous state provided in this embodiment includes: the entire operating state of the doubly-fed wind turbine is divided into two modes: synchronous speed control mode and asynchronous speed control mode. When the doubly-fed wind turbine is operating near the synchronous speed, it enters the excitation current control mode under synchronous speed, and adopts optimized excitation current control, specifically including: S1, calculating the three-phase rotor excitation current i a i b i c Instantaneous value, and select peak current i max As the base value, and calculate the per-unit value i corresponding to the three-phase rotor current. a_pu =i a / i max i b_pu =i b / i max i c_pu =i c / i max And read the duty cycle of the converter switching devices at this time, and set i a_pu i b_pu i c_pu S2. After reaching synchronous speed, the duty cycle is stored in the memory. The same or similar duty cycles of the devices in the memory can be used as a starting condition for rotor current input. Furthermore, when the per-unit value of the three-phase rotor current is less than i = k (k < 1), a certain triangular carrier wave or sinusoidal current is applied to the three-phase rotor current to ensure dynamic adjustment of the device duty cycle. S3. When the doubly-fed wind turbine is operating in asynchronous speed mode, a non-optimized (limited) excitation current control mode is adopted. This control method can improve the heating / fatigue problems caused by the switching devices of the doubly-fed wind turbine rotor converter maintaining the same duty cycle for a long time under synchronous speed, thereby improving the reliability of the converter.

[0061] Furthermore, in this invention, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0062] Furthermore, in this invention, the use of terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refers to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Furthermore, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0063] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method of controlling excitation current in a doubly-fed wind generator in a synchronous state, characterized by, Comprise: The whole double-fed wind power generator operating state is divided into synchronous speed control mode and non-synchronous speed control mode; When the double-fed wind power generator operates near the synchronous speed, that is, enters the excitation current control mode under the synchronous speed, the optimized excitation current control is adopted, specifically including: S1, calculate three-phase rotor excitation current i a , i b , i c instantaneous value, and take the peak current i max as the base value, and calculate the three-phase rotor current corresponding to the per unit value i a_pu =i a / i max , i b_pu = i b / i max , i c_pu = i c / i max , and read the inverter switch device duty cycle at this time, and take i a_pu , i b_pu , i c_pu and the duty cycle into the memory; S2, when judging that the doubly-fed wind power generator enters the synchronous speed according to the rotating speed n, detecting the duty cycle of the converter switching device in the memory, if the duty cycle is same or similar in multiple control periods, determining that the rotor incorporation current starting condition is satisfied; after the rotor incorporation current starting condition is satisfied, if the three-phase rotor current standard value i a_pu 、 i b_pu 、 i c_pu are all less than k, incorporating the preset triangular carrier current or sinusoidal wave current in the given value of the three-phase rotor current, so that the duty cycle of the converter switching device is dynamically adjusted; wherein, k<1; S3, when the double-fed wind power generator operates in non-synchronous speed mode, non-optimized excitation current control mode is adopted.

2. The method of claim 1, wherein the method is characterized by: The judgment basis for judging whether the double-fed wind power generator operates near the synchronous speed is: ； In the formula, n is the actual rotational speed, n sy is the synchronous rotational speed, M is the selected deviation coefficient, M Reference the size of the inertia and the synchronous rotational speed of the doubly-fed wind generator.

3. The method of claim 1 or 2, wherein, The converter switching device duty cycle and i a_pu , i b_pu , i c_pu all need to be stored in memory as conditions for calculating the current.

4. The method of claim 1, wherein the method further comprises: The read duty ratio is used as the judgment condition of the device action state in the switching cycle, and when the duty ratio remains the same state in the preset cycle, it is used as the current merging start condition.

5. The method of controlling the field current in the synchronous state of the doubly-fed wind power generator according to claim 1, characterized by, When the three-phase rotor current standard values are all less than I=k The preset frequency triangular carrier wave or sinusoidal wave current is incorporated on the three-phase rotor current to ensure dynamic adjustment of the device duty ratio, wherein the triangular carrier wave selects a central symmetrical carrier wave, and the frequency of the triangular carrier wave and the sinusoidal wave is: ； wherein f in is the rotor current frequency in the synchronized state, n sy is the synchronous speed, P is the pole pair number of the doubly-fed wind generator.

6. The method of excitation current control in a synchronous state of a doubly-fed wind power generator according to claim 1, characterized by, When the double-fed wind power generator operates in non-synchronous speed mode, non-optimized excitation current control mode is adopted, including: Non-optimized excitation current control mode is adopted under non-synchronous speed mode; wherein the relationship between dq axis current in synchronous rotating coordinate system and rotor three-phase current in stationary coordinate system is: ， ； That is, in the synchronous rotating coordinate system: ； wherein, i a , i b , i c are the currents in the stationary frame of reference of the rotor a, b, c phases, respectively; i rd , i rq are the currents in the dq axis of the dq axis coordinate system; i rd or i rq the maximum value is the instantaneous current peak value.

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