A comprehensive control mode soft connection switching method applied to a DFIG

Abstract

The application discloses a comprehensive control mode flexible connection switching method applied to a DFIG, and the method comprises the following steps: using a current limiting module to ensure that the current limiting performance of each implementation stage of a control strategy can be maximized; introducing a resistance-capacitance crowbar to realize the switching impact limitation of the control mode, and designing an input and cut-off reference mechanism matched with the control switching of the application, so that the key problems encountered in the use of the crowbar are effectively solved; and integrating a state follow-up module into the converter control architecture of the DFIG to ensure the synchronization of parameters in the control mode switching process, so that the system can be effectively prevented from suffering from a major switching impact, and the smooth transition of the control mode is realized. During the state follow-up, the network configuration control strategy adopted by the RSC of the DFIG provides a reactive power compensation reference for the improvement of the GSC, so that a close internal relationship between different control modes is established in the switching process, instead of a simple time sequence splicing.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of stable operation of a doubly-fed wind power generation system, and more particularly to a comprehensive control mode soft-link switching method applied to a DFIG to improve the fault ride-through capability of the DFIG in different stages and voltage drop degrees in a high-proportion new energy power system. BACKGROUND

[0002] The core of a doubly-fed induction generator (DFIG) lies in its converter control system, which realizes the decoupling of the generator speed and the grid frequency and optimizes wind energy capture. The early grid-following control strategy makes the output voltage and current of the DFIG synchronized with the grid, but in a weak new energy grid, its regulation is limited, which may lead to unstable output power or even disconnection from the grid, threatening the stability of the power system. To solve this problem, the grid-forming control strategy emerged as the times require. This strategy has significant advantages in providing voltage and frequency support, adapting to high-proportion new energy access, having a simple control structure and fast response speed, etc. With the increase of the proportion of new energy, the grid-forming control strategy has attracted much attention because it can enhance the resilience of the power system.

[0003] Currently, the grid-forming converter control techniques for DFIG mainly include virtual synchronous generator (VSG) control, direct current control (DCC) and droop control. VSG control simulates the characteristics of synchronous generators, making DFIG exhibit similar dynamic and steady-state characteristics to synchronous generators when operating in grid-connected mode. This helps to enhance the inertia and damping of the system, improving the stability of the grid. DCC controls the output current of the converter directly, achieving power injection and voltage regulation of the grid. DCC has fast response speed and high control accuracy, but it is highly dependent on grid parameters. Droop control is a control strategy based on voltage and frequency droop characteristics. It adjusts the amplitude and frequency of the output voltage according to the grid requirements to achieve reasonable power distribution. However, traditional droop control applied directly to DFIG does not solve the problem of simulating the inertia support of synchronous generators. Some scholars have proposed low-pass filter droop control (LPF-droop) to address this issue. However, most current research on grid-forming control strategies applied to DFIG focuses on steady-state conditions and does not consider transient characteristics. DFIG under grid-forming control may not respond in time and provide necessary voltage and frequency support when encountering voltage drops and other faults, increasing the risk of wind turbine disconnection or grid collapse. In particular, the overcurrent problem during fault ride-through can prevent effective power control of DFIG, severely limiting the improvement of DFIG low-voltage ride-through capability. However, most current research on these issues simply improves the control principle of grid-forming control strategies, and the effect is often not significant, only playing a certain role in light fault scenarios. For more severe fault conditions, current grid-forming control strategies generally struggle to ensure DFIG fault ride-through. In contrast, traditional grid-following control strategies and their supporting hardware assistance measures have been well developed and have shown excellent fault ride-through capability when DFIG encounters severe faults. Therefore, some research proposes switching between grid-forming and grid-following control strategies for DFIG to effectively improve fault ride-through capability in the presence of severe voltage drops. For example, Chinese patent CN118842077A "Grid-following-grid-forming mode switching control method for full-power wind power converter" proposes independent design of grid-following and grid-forming controllers, ensuring the operation characteristics of grid-following and grid-forming modes. At the same time, the two modes do not operate simultaneously, reducing the burden on the controller and saving costs.

[0004] However, the patent is mainly aimed at full-power permanent magnet direct drive wind power generator, and its switching logic adopts a method of simply switching two control strategies, one of which is in a non-working state when the other is running. They only pay attention to the switching impact at the time of switching, but do not fully consider the internal relationship between the two control strategies. In fact, they are only simply spliced in different time periods, and more importantly, they ignore the importance of fault voltage acceleration recovery. At present, there are at least the following defects in the prior art: the different control modes are not closely related during the switching process, the switching impact is large, the current limiting and voltage recovery effect is not obvious during the fault ride-through period, the influence factors such as different stages and fault degree are not considered, the overall coordinated control logic is complex, and the application range is not wide enough.

[0005] Therefore, how to design a switching method that can effectively suppress overcurrent and switching impact, ensure that the two control strategies remain closely related during the switching process, and achieve reactive power compensation, thereby significantly improving the fault ride-through capability of DFIG, has far-reaching engineering significance. SUMMARY

[0006] Therefore, the present application provides a comprehensive control mode soft switching method applied to DFIG, which can solve the problems of insufficient application occasions, large impact caused by asynchronous parameters during the switching process, poor current limiting and voltage recovery effect during the fault ride-through period, no relationship between different control modes during the fault ride-through period, and complex comprehensive control logic in the prior art, thereby effectively improving the fault ride-through capability of DFIG in high-proportion new energy power systems.

[0007] To achieve the above-mentioned purpose, the technical solution adopted by the present application is as follows:

[0008] In the first aspect, the embodiment of the present application provides a comprehensive control mode soft switching method applied to DFIG, which comprises:

[0009] When the system is in a normal operating state, the RSC of the DFIG adopts the LPF-droop control mode, and the GSC maintains the voltage constant and unit power factor operation control mode;

[0010] When voltage drop occurs, a differential current limiting module that seamlessly adapts to the grid forming control and grid following vector control mode and has unified form is added to the control structure of the RSC, and the control structure is intervened by controlling the switching of the switch;

[0011] When severe voltage drop occurs, the RSC switches the control mode to the vector control mode, and uses the resistance-capacitance crowbar for current limiting and reactive power compensation, and suppresses the switching impact of the control mode; at the same time, the state follow-up module controls the synchronization of the parameters during the control mode switching process, and completes the smooth transition of the control mode.

[0012] Further, in the method, during the fault ride-through, the control mode of the GSC is adjusted to an improved mode, and the grid-forming control of the RSC provides a reference for reactive power compensation of the improved switching control of the GSC.

[0013] Further, in the method, after the fault is over, the control mode of the GSC is switched back to the normal mode, the differential current limiting module is removed from the RSC control structure, the LPF-droop control mode is switched back, and the state following of the LPF-droop control to the vector control is disconnected.

[0014] Further, the differential current limiting module is used to perform differential on the stator side current i s after the voltage drop, and the calculation formula is as follows:

[0015]

[0016] In the formula, U rα represents a transient component of the open circuit voltage on the rotor side of the DFIG, L m represents the mutual inductance between the stator and the rotor, and s represents the slip of the DFIG.

[0017] Further, the reference mechanism for the input of the resistance-capacitance crowbar is that the reference current for the input of the crowbar is set to 0.9 times the withstand current; and the reference mechanism for the removal of the resistance-capacitance crowbar is that the reference current for the removal of the crowbar is set to 1.05 times the rated current.

[0018] Further, the state following module makes the grid-forming LPF-droop control closely follow the current output instruction of the vector control during the fault ride-through, and the control parameters of the LPF-droop are kept in real time synchronization with the parameters of the vector control.

[0019] Further, in the method, when the control mode of the GSC is adjusted to the improved mode, the reactive power compensation power required for restoring the voltage is determined by using the reactive-voltage droop characteristic in the LPF-droop control, and the final reactive power compensated by the GSC is calculated in combination with the reactive power already compensated by the resistance-capacitance rotor crowbar, and the calculation formula is as follows:

[0020] Q last = Q nd - Q cb

[0021] In the formula, Q last represents the final reactive power compensated by the GSC, Q nd represents the reactive power required for compensation determined by the reactive-voltage characteristic in the LPF-droop control, and Q cb represents the reactive power already compensated by the resistance-capacitance rotor crowbar.

[0022] Further, in the method, a specific feedforward component is added to the voltage outer loop output controlled by the GSC, and the expression of the feedforward component is:

[0023]

[0024] In the formula, Δi sd represents the feedforward component, U dc represents the voltage across the capacitor, i dcr represents the current on the rotor side of the capacitor, U s represents the stator voltage.

[0025] In a second aspect, the embodiments of the present application also provide an electronic device, including a processor and a memory, the memory stores machine executable instructions capable of being executed by the processor, and the processor executes the machine executable instructions to implement the application for DFIG comprehensive control mode soft connection switching method.

[0026] Compared with the prior art, the present application has at least the following beneficial effects:

[0027] 1. The application provides a comprehensive control mode soft connection switching method applied to a DFIG, which can fully exert the respective advantages of grid forming and grid following control in different stages through the comprehensive control mode soft connection switching method proposed by the application when a doubly-fed wind turbine generator set in a high-proportion new energy power system encounters different degrees of faults, effectively solve the problems of limiting overcurrent and quickly restoring voltage of the doubly-fed wind turbine generator during fault ride-through, and also solve the problems of insufficient application occasions in the prior art, large impact caused by different parameters in the switching process, no connection between different control modes during fault ride-through, and complex comprehensive control logic, thereby effectively improving the fault ride-through capability of the DFIG in the high-proportion new energy power system.

[0028] 2. The soft connection switching method proposed by the application ingeniously combines the respective advantages of grid forming LPF-droop control and grid following vector control, fully considers the influence of voltage drop degree, and introduces a differential current limiting module, which not only effectively limits the overcurrent on the rotor side, but also realizes seamless connection and unification of the two control modes in the current limiting compensation strategy.

[0029] 3. The soft connection switching method proposed by the application is not simply spliced in different time periods, but constructs a close connection between them. On the one hand, when the RSC is in the vector control mode, the LPF-droop control can follow the state of the vector control in real time, ensuring the parameter synchronization of the two control modes; on the other hand, the reactive power-voltage droop characteristic of the LPF-droop control provides a basis for the reactive power compensation of the GSC.

[0030] 4.The application realizes flexible switching between different control modes by skillfully using the resistance-capacitance rotor crowbar, state following module and improvement of GSC control strategy, which effectively limits the rotor side overcurrent, maintains the stability of the DC bus voltage, accelerates the recovery process of the fault voltage, and thus significantly improves the fault ride-through capability of the DFIG.

[0031] Other features and advantages of the present application will be set forth in the following description, and in part will be apparent from the description, or can be learned by practice of the application. The objects and other advantages of the present application will be realized and attained by the structure particularly pointed out in the written description and claims thereof.

[0032] The technical solutions of the present application will be further described in detail below with the help of the accompanying drawings and examples. BRIEF DESCRIPTION OF DRAWINGS

[0033] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the accompanying drawings needed to be used in the embodiments or prior art description will be briefly introduced. Obviously, the accompanying drawings in the following description are some embodiments of the present application, and those skilled in the art can also obtain other accompanying drawings according to these accompanying drawings without any creative effort.

[0034] The accompanying drawings are used to provide a further understanding of the present application, and constitute a part of the specification, which are used together with the embodiments of the present application to explain the present application, and do not constitute a limitation to the present application.

[0035] Figure 1 It is a schematic diagram of the overall process of the integrated control mode flexible switching method;

[0036] Figure 2 It is a logic diagram of the measures taken by the present application on the stator and rotor sides of the DFIG;

[0037] Figure 3 It is a schematic diagram of the integrated control mode flexible switching control structure of the rotor side converter of the DFIG;

[0038] Figure 4 It is a schematic diagram of the reactive-voltage droop characteristic;

[0039] Figure 5 It is a schematic diagram of the required compensation reactive power acquisition;

[0040] Figure 6 It is a schematic diagram of the improved switching control principle diagram of the grid side converter of the DFIG;

[0041] Figure 7 It is a schematic diagram of the DFIG stator voltage waveform under different control schemes when the voltage drops by 20%;

[0042] Figure 8 DFIG stator voltage waveform diagram under different control schemes when voltage drop is 20%;

[0043] Figure 9 DFIG rotor current waveform diagram under different control schemes when voltage drop is 20%;

[0044] Figure 10 Reactive power compensation waveform diagram under different control schemes when voltage drop is 20%;

[0045] Figure 11 DFIG stator voltage waveform diagram under different control schemes when voltage drop is 70%;

[0046] Figure 12 DFIG DC bus voltage waveform diagram under different control schemes when voltage drop is 70%;

[0047] Figure 13 DFIG rotor current waveform diagram under different control schemes when voltage drop is 70%;

[0048] Figure 14 Reactive power compensation waveform diagram under different control schemes when voltage drop is 70%.

[0049] Figure 15 A structure schematic diagram of an electronic device provided by the application. DETAILED DESCRIPTION

[0050] In order to make the purpose, technical scheme and advantages of the embodiments of the present application clearer, the technical scheme in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some of the embodiments of the present application, but not all the embodiments of the present application.

[0051] In the description of the present application, it should be noted that in some processes described in the present application specification and drawings, a plurality of operations appearing in a specific order are included, but it should be clearly understood that these operations can be executed or performed in parallel without the order appearing in the text. In addition, various serial numbers and the like are only for the purpose of description, and cannot be understood as indicating or implying relative importance.

[0052] Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor are within the scope of protection of the present application.

[0053] Reference is made to Figure 1 and Figure 2As shown, the application provides a comprehensive control mode soft-link switching method applied to DFIG, which mainly includes:

[0054] 1. When the system is in normal operation state, the RSC of DFIG adopts LPF-droop control mode, and the GSC maintains voltage constant and unit power factor operation control mode.

[0055] 2. When voltage drop occurs, a differential current limiting module which is seamlessly adapted to grid-forming control and grid-following vector control mode and has unified form is added to the control structure of RSC, and the control structure is intervened by controlling the switching of switches.

[0056] During the control mode switching process of the RSC of DFIG, whether the grid-forming control strategy or the grid-following control strategy is adopted, the current inner loop is the core component; the current limiting module can quickly intervene the control structure by controlling the switching of switches when voltage drop occurs. Considering the diversity of voltage drop and the complexity of different stages in fault ride-through process, the RSC of DFIG may adopt grid-forming or grid-following control strategy in different stages. However, no matter which control mode the RSC is in, the form of the current limiting module when it intervenes the control structure is unified. This feature ensures that the current limiting module does not need to confirm the specific control mode of the RSC when it is put into use, thereby greatly simplifying the operation process.

[0057] 3. When severe voltage drop occurs, the RSC switches the control mode to vector control mode, and uses the resistance-capacitance crowbar for current limiting and reactive power compensation, and suppresses the switching impact of the control mode; at the same time, the state following module controls the synchronization of parameters in the control mode switching process, and completes the smooth transition of the control mode.

[0058] 4. During fault ride-through, the control mode of the GSC is adjusted to the improved mode, and the grid-forming control of the RSC of DFIG provides a reference for the improved switching control of the GSC.

[0059] The control strategy of the GSC of DFIG during fault ride-through cannot continue to follow the traditional operation mode, and should have the ability to provide appropriate reactive power to the stator side to achieve reactive power balance, thereby accelerating the rapid recovery of voltage. The specific improvement method is: using the reactive power-voltage characteristic in the LPF-droop control to determine the required compensation reactive power Q nd , and subtracting the reactive power Q cb compensated by the resistance-capacitance rotor crowbar (if the resistance-capacitance crowbar is not put into use, the compensation power is 0), to obtain the reactive power reference value Q last of the improved control of the GSC, that is: Q last = Q nd -Q cbIn this mechanism, the LPF-droop control is not completely idle during fault ride-through, it not only maintains synchronization with the parameters of the vector control mode, but also uses its reactive-voltage characteristics to provide a basis for reactive power compensation analysis for GSC, to achieve seamless switching and rapid recovery of voltage.

[0060] 5. After the fault is over, the control mode of GSC is switched back to the normal mode, the differential current limiting module in the RSC control structure is removed, and the LPF-droop control mode is switched back, and the state following of the LPF-droop control to the vector control is disconnected.

[0061] In summary, it can be known that the application aims at the problem that the DFIG unit in the current high-proportion new energy power system adopts the network-forming control strategy in the steady state stage to improve the active support capability, and the fault ride-through capability is insufficient when facing voltage drop fault, especially serious voltage drop, an effective comprehensive control mode flexible switching scheme is proposed, and finally the fault ride-through capability of the network-forming control DFIG unit is significantly improved.

[0062] The method of the application is mainly realized by the following ways: a, an innovative current limiting module is designed, which can seamlessly adapt to two different modes of network-forming control and network-following control, and keep the unity in control form, so as to ensure that the current limiting performance can be maximized in each implementation stage of the control strategy. b, the resistance-capacitance crowbar is introduced to realize the current limiting and reactive power compensation, limit the switching impact of the control mode, and design the input and removal reference mechanism that can match the control switching of the application, and then effectively solve the key problems encountered in the use of the crowbar. c, the state following module designed innovatively is ingeniously integrated into the converter control architecture of the DFIG, which ensures the synchronization of parameters in the control mode switching process, effectively avoids the system from suffering from major switching impact, and then realizes the smooth transition of the control mode. d, during the state following, the network-forming control strategy adopted by the rotor side converter (RSC) of the DFIG provides a basis for the improvement of the grid side converter (GSC), which ensures that the different control modes are closely related in the switching process, rather than simply spliced in time sequence.

[0063] In order to smoothly integrate the current limiting module, the state following module and the switching function of the GSC control input reference value in the overall control architecture of the DFIG, and finally realize the effective coordinated switching of the control mode, the control switching switch is preferably deployed at the key position of the DFIG control structure. In the embodiment of the application, the switching switch is arranged in the current inner loop of the control structure, which is specially used for controlling the input and removal of the current limiting module, ensuring that the current fluctuation can be effectively limited under voltage drop. In the link of the current loop reference value input, the switch responsible for realizing the flexible switching between the grid forming type and the grid following type is arranged to adapt to the change of the power grid state and the fault ride-through requirement. In the current reference value generation link of the grid forming control strategy, the state following control switching switch is embedded to ensure the real-time synchronization of the parameters. The reference value input end of the GSC control structure is additionally provided with the switching switch, which can quickly change the input reference value of the GSC during the fault, so as to optimize the control strategy of the GSC.

[0064] The related theories and specific embodiments related to the application will be described in detail as follows:

[0065] In the embodiment of the application, when the DFIG in the high proportion new energy power system encounters different degrees of faults, the integrated control mode flexible switching method proposed in the application is used in different fault ride-through stages, the respective advantages of the grid forming and grid following control are fully utilized, the problems of limiting overcurrent and quickly restoring voltage of the doubly-fed wind power generator during the fault ride-through period are effectively solved, and the problems of insufficient application occasions in the prior art, large impact caused by different parameters in the switching process, no connection between different control modes during the fault ride-through period and complex integrated control logic are also solved, so that the fault ride-through capability of the DFIG in the high proportion new energy power system is effectively improved. The implementation mainly includes:

[0066] (1) The current limiting module capable of being seamlessly applied to two different control modes and unified in form:

[0067] Generally, grid-forming LPF-droop control is widely used in the control of rotor-side converter of DFIG, aiming to achieve accurate power regulation of DFIG. Although LPF-droop control exhibits good anti-frequency fluctuation ability and effective voltage support characteristics in steady-state operation, it is obviously insufficient in fault ride-through ability in the case of severe fault, such as voltage sag, due to the constraints of overcurrent limitation and limited regulation ability, which makes it difficult to fully improve the fault ride-through performance of DFIG, and thus may lead to DFIG having to be disconnected from the grid during the fault, which poses a serious challenge to the stable operation of the power system. In view of this, the present application proposes to optimize the characteristics of the LPF-droop control strategy itself to achieve fault ride-through of DFIG when it encounters a slight voltage sag fault, and when DFIG faces a severe voltage sag fault, the control strategy of the rotor-side converter of DFIG is seamlessly switched from grid-forming LPF-droop control to grid-following vector control strategy, and at the same time, a resistance-capacitance crowbar circuit is used to suppress switching impact and limit overcurrent, thereby significantly improving the fault ride-through ability of DFIG. After the fault is cleared, the control strategy is switched back to LPF-droop control to ensure the continuous and stable operation of DFIG.

[0068] Both grid-forming LPF-droop control strategy and grid-following traditional vector control strategy will face the risk of overcurrent of DFIG rotor-side converter when the grid voltage drops. This overcurrent phenomenon will seriously interfere with the effective control of DFIG power by the control strategy, making it difficult for the system to maintain stable operation. Therefore, in order to deal with this overcurrent problem, the LPF-droop control and the traditional DFIG vector control should be improved accordingly.

[0069] In addition, considering that the two control strategies need to maintain a certain unity during switching to ensure smooth transition of the system, the implementation of the improvement measures should use the same control means and methods. This requires a fundamental analysis of the mechanism of overcurrent caused by voltage sag and the current limiting methods that can be achieved in engineering, so as to propose a current limiting measure that can be applied to both control strategies.

[0070] When the stator side of a DFIG suffers from a voltage dip, a transient component of the stator flux linkage is generated rapidly. This transient component further induces a transient component of the open-circuit rotor voltage at the rotor side, which leads to an overcurrent phenomenon at the rotor side. In order to effectively limit the overcurrent and further suppress the overvoltage that may occur at the DC bus, it is necessary to take measures to reduce the transient component of the open-circuit rotor voltage. However, in practical engineering applications, it is not convenient to directly observe the transient component of the open-circuit rotor voltage. Therefore, it is necessary to find an intermediate electrical quantity that can indirectly reflect the transient component of the open-circuit rotor voltage. By controlling the change of the intermediate electrical quantity, the suppression of the transient component of the open-circuit rotor voltage is indirectly achieved.

[0071] In the embodiment of the present application, the stator side current i s after a voltage dip is differentiated and converted to a synchronous rotating coordinate system, i.e.:

[0072]

[0073] In the formula, U s represents the stator side voltage of the DFIG during fault ride-through, and U s|0| represents the stator voltage before the fault, L s is the stator inductance, T s is the time constant, ω s is the stator angular frequency, j is the imaginary symbol, and t is time.

[0074] In order to reflect the degree of voltage drop, the voltage drop coefficient h is defined as:

[0075]

[0076] Then equation (1) can also be expressed as:

[0077]

[0078] And the transient component U rα of the open-circuit rotor voltage of the DFIG converted to the synchronous rotating coordinate system can be expressed as:

[0079]

[0080] In the formula, L m is the mutual inductance between the stator and the rotor, and s is the slip of the DFIG.

[0081] Dividing equation (3) by equation (4), we obtain:

[0082]

[0083] According to the disclosure of formula (5), there is a direct, proportional and same direction relationship between the transient component of the rotor open circuit voltage and the differential term of the stator current after the voltage fault. In view of the fact that the DFIG has a large electromechanical inertia time constant, and the rotor speed remains basically unchanged during the short period of fault ride-through, the slip s is maintained as a constant, and the inductance L m also remains constant. Therefore, only by monitoring the stator current and calculating the differential term thereof, the transient component of the rotor open circuit voltage can be directly obtained. This discovery provides a solid theoretical basis for optimizing the RSC control strategy and effectively preventing rotor overcurrent problems. For the convenience of subsequent description, the differential term of the DFIG stator current during the fault ride-through is referred to as the differential current limiting module.

[0084] From the perspective of control structure, whether the RSC adopts the network type LPF-droop control or the network type vector control, the current inner loop is built in as a core component. Considering the feature that the differential current limiting module can accurately reflect the transient component of the rotor open circuit voltage, the differential current limiting module is introduced as a compensation element into the rotor voltage reference value calculation link of the control system. This measure enables the rotor voltage controlled by the RSC to generate a control voltage opposite to the direction of the transient component of the rotor open circuit voltage, thereby achieving the control objectives of accurately and effectively limiting the occurrence of rotor overcurrent and applying the same control means to different control strategies to ensure the uniformity and control correlation.

[0085] Further, Figure 3 The switching schematic diagram of the specific control structure between the LPF-droop control and the traditional vector control of the RSC is shown, in which PI represents the PI regulator commonly used in the control system, K P is the active droop coefficient, K Q is the reactive droop coefficient, dq, abc and αβ represent the corresponding coordinate system parameters, P and Q are the active and reactive power, u is the voltage, i is the current, the subscript letter s represents the stator side, r represents the rotor side, ref represents the reference value, the prefix Δ represents the change amount, and other parameters are described in the formula. In the diagram, the switching control corresponding to the differential current limiting module clearly identifies the improved link specially added to cope with the overcurrent problem during the fault ride-through. It can be seen that regardless of which control mode the RSC operates, the same differential current limiting module can be used to achieve the current limiting purpose. That is, in the normal operation mode, the switching switches K1 and K2 are placed in state 1, and the system operates stably according to the established control strategy. However, as soon as a voltage drop fault is detected, K1 and K2 can be switched to state 2 to activate the differential current limiting function, thereby effectively limiting the overcurrent.

[0086] (2) Suppression of control mode switching impact using RC crowbars and coordination of switching timing:

[0087] When the grid voltage experiences a severe drop, the overcurrent problem on the rotor side of the DFIG becomes increasingly severe. At this point, relying solely on the differential current limiting module added to the converter control strategy is insufficient to effectively curb such a large overcurrent. To address this issue, when the DFIG encounters a severe voltage drop fault, in addition to switching the RSC control strategy to a traditional vector control strategy with differential current limiting compensation for current limiting, a rotor-side crowbar circuit is also required to assist in limiting the overcurrent and mitigate the impact generated during the switching process. However, under severe voltage drops, the system faces a significant reactive power deficit. Traditional purely resistive crowbar circuits cannot provide the necessary reactive power compensation, which is extremely detrimental to the rapid recovery of the fault voltage. Therefore, the switching scheme proposed in this invention employs a resistive-capacitive rotor crowbar circuit capable of providing reactive power (i.e., a purely resistive crowbar with added capacitors, which is existing technology and will not be elaborated further). The capacitor portion in the resistive-capacitive crowbar can provide necessary reactive power compensation to the system when the crowbar is put into use, thereby accelerating the voltage recovery process.

[0088] Furthermore, the timing of inserting and removing the crowbar is a key issue in crowbar technology. The switching scheme designed in this invention differs from other studies in this respect. Specifically:

[0089] The insertion of the crowbar is based on the rotor-side current i r Withstand current value i wth The value is determined by the reference value i. In engineering practice, this value is usually set to 1.3 times the rated current. However, considering a certain margin and the lag of the current limiting control after it is put into operation, this invention will use a reference value i. invt Set to 0.9 times the withstand current. That is:

[0090] i invt =0.9i wth (6)

[0091] When the rotor-side current is less than i invt At this point, it means the voltage drop encountered by the DFIG is not severe, and the resulting overcurrent is within the tolerance range. In this case, the overcurrent can be limited entirely by the differential current limiting module added to the LPF-droop control strategy. However, once the current exceeds i... invt This indicates that the DFIG is facing a relatively serious voltage drop fault. In this situation, the RSC control strategy should be switched to a vector control strategy with differential current limiting compensation, and the RC rotor crowbar should be engaged simultaneously to assist in current limiting and suppress any shocks that may occur during the switching process.

[0092] As to the disconnection timing of crowbar, the present application also does not follow the traditional method of waiting for the overcurrent to completely disappear or the fault to be disconnected before disconnecting the crowbar. The present application sets a crowbar disconnection reference current i cut , which is 1.05 times the rated current, that is, the crowbar is disconnected in advance when the overcurrent decreases to 1.05 times the rated current. The reason for taking this kind of advance disconnection is that after the crowbar is disconnected, the improved RSC control strategy still has the ability to suppress overcurrent and regulate DC bus overvoltage. This improvement not only solves the problem brought by simply relying on crowbar in traditional DFIG rotor-side control (such as over-disconnection that may lead to an increase in overcurrent, and under-disconnection that may make the RSC unable to smoothly control the DFIG), but also optimizes the use efficiency of the crowbar, providing more reliable protection for the stable operation of the DFIG during fault ride-through.

[0093] (3) The state follow-up module in the control structure ensures the synchronization of parameters during control mode switching:

[0094] As mentioned earlier, in order to realize fault ride-through of DFIG under severe voltage sag, the mature traditional low voltage ride-through strategy should be combined and used. Specifically, when the rotor-side overcurrent caused by voltage sag exceeds i invt , it indicates that the DFIG is facing a severe voltage sag, the RSC control strategy of the DFIG should be immediately converted to the vector control mode with differential current limiting compensation, and the resistance-capacitance crowbar device should be simultaneously enabled to effectively suppress overcurrent and relieve the impact during switching. Since traditional vector control is a grid-following control strategy, it can dynamically adjust with the grid frequency, and combined with the auxiliary role of resistance-capacitance rotor crowbar, there is no need to worry about the problem of parameter synchronization or switching impact when switching from grid-forming LPF-droop control to grid-following vector control.

[0095] However, after the fault is cleared, when the control mode needs to be switched back to the LPF-droop control mode, the problem of parameter synchronization must be considered to prevent significant switching impact. To solve this problem, during fault ride-through, the grid-forming LPF-droop control closely follows the current output command of the vector control to ensure that the control parameters of the LPF-droop can be synchronized in real time with the parameters of the vector control, thereby minimizing the impact during control mode switching. The module corresponding to the realization of the above function is called the state follow-up module (hardware is attached Figure 3 Switching control module three).

[0096] It is worth noting that under this switching mechanism, the LPF-droop control is not completely idle during fault ride-through, it not only maintains the parameters synchronization with the vector control mode, but also uses its reactive-voltage characteristics to provide the basis for subsequent reactive power compensation analysis of the grid-side converter (GSC) to achieve seamless switching and rapid recovery of voltage. This switching method is completely different from the traditional method of two control modes running independently for a period of time and then simply splicing. In the switching method proposed in this invention, the two control modes are always closely linked throughout the process, ensuring the non-impact of the switching process, so it is called "soft switching mode". The switching of the two control modes and the control structure diagram of the LPF-droop servo vector control are shown in Figure 3 .

[0097] In the control system shown in Figure 3 , the conversion between the two control modes is realized by the state of the switching switches K3 and K4, while K5 and K6 are responsible for establishing the state servo relationship between LPF-droop control and vector control during fault ride-through. The specific process is described as follows:

[0098] ①In the normal state of the system, K3 and K4 are set to state 1, at this time the RSC adopts the LPF-droop control mode. At the same time, K5 and K6 are also in state 1, that is, there is no state servo relationship between LPF-droop control and vector control at this time.

[0099] ②Once the system encounters a serious voltage drop, causing the rotor-side overcurrent to exceed i invt , K3 and K4 should be immediately switched to state 2, at this time the control mode of the RSC will change to the traditional vector control. At the same time, K5 and K6 are also switched to state 2, so as to ensure that the LPF-droop control can closely follow the state change of the vector control during fault ride-through, and realize the state servo.

[0100] ③When the fault is removed, K3 and K4 should be set to state 1 again to make the RSC switch back to the LPF-droop control mode. Since the state servo module ensures that the parameters of the two control modes remain synchronized during this process, there is no need to worry about the impact that may occur during the switching process, and the mode switching can be directly performed. Subsequently, K5 and K6 are also set back to state 1, disconnecting the servo relationship between LPF-droop control and vector control, so that the RSC can stably operate in the grid-forming LPF-droop control mode.

[0101] (4) The RSC grid-forming control of the DFIG provides reactive power compensation basis for the GSC improvement during the state servo:

[0102] When grid voltage drops and immediate reactive power compensation is needed to quickly restore voltage, the reactive-voltage droop characteristic of LPF-droop control can be fully utilized. This characteristic can determine the amount of reactive power compensation required to restore the voltage to the desired normal level. A visual representation of this characteristic is as follows: Figure 4 As shown, U ref and U PCC These represent the reference voltage and the voltage at the grid connection point, respectively, while Q... G k represents the corresponding reactive power. G This is the droop coefficient.

[0103] Through analysis Figure 4 The reactive power-voltage droop characteristic diagram clearly shows that when the grid voltage deviates, corresponding reactive power compensation is necessary to restore the voltage to its original stable value. There is a direct and quantifiable relationship between this reactive power compensation and the voltage change. Figure 5 This was further demonstrated in detail, and the specific relationship can be expressed by a mathematical equation as follows:

[0104] Q nd =ΔQ=-k G ΔU (7)

[0105] In the formula, ΔU is the desired voltage recovery change, and ΔQ is the reactive power change required for voltage recovery, which is denoted as Q for convenience in subsequent descriptions. nd .

[0106] In summary, utilizing the reactive power-voltage droop characteristic to determine the reactive power compensation required for voltage restoration provides a basis for reactive power compensation in the rapid voltage restoration of DFIGs during fault ride-through. This method not only helps DFIGs maintain grid-connected operation under fault conditions such as voltage dips, but also significantly improves their fault ride-through capability, providing strong support for the stable operation and fault recovery of the power system.

[0107] In addition to limiting overcurrent on the DFIG rotor side during fault ride-through, effective measures should be taken for reactive power compensation to ensure rapid voltage recovery during this period. Furthermore, DC bus overvoltage issues that may be caused by rotor-side overcurrent should be suppressed. Since the DFIG's gas control system (GSC) is primarily responsible for maintaining DC bus voltage stability and ensuring the system operates at unity power factor during normal operation, the tasks of reactive power compensation and DC bus overvoltage suppression fall on the GSC. This necessitates optimizing the GSC's control strategy during fault ride-through.

[0108] In the aspect of reactive power compensation, although the capacitor-resistor rotor bar can provide certain reactive power and promote the recovery of fault voltage when the voltage drops seriously, it is far from enough to meet the huge demand of reactive power. In addition, the capacitor-resistor rotor bar will not be put into operation in the case of light voltage drop, thus losing the source of reactive power compensation. Therefore, the control strategy of GSC during fault ride-through cannot continue to follow the traditional unit power factor operation mode, and should have the ability to provide appropriate amount of reactive power to the stator side to achieve reactive power balance, so as to accelerate the rapid recovery of voltage. In the embodiment of the application, the specific control strategy improvement method is: using the reactive-voltage characteristic in the LPF-droop control to determine the required compensation reactive power Q nd , and subtracting the reactive power Q cb compensated by the capacitor-resistor rotor bar (if the capacitor-resistor bar is not put into operation, the compensation power is 0), to obtain the reference value of the reactive power. That is, the final reactive power Q last compensated by the GSC can be expressed as:

[0109] Q last = Q nd - Q cb (8)

[0110] The optimized GSC control structure is shown in Figure 6 , and during the fault ride-through, the state of switch K7 is switched to state 2, and the GSC can flexibly adjust the amount of reactive power delivered to the stator side according to the difference between the actual demand and the current supply of the system reactive power. This dynamic adjustment mechanism not only accelerates the recovery speed of the stator side voltage, but also effectively ensures the stability of the voltage.

[0111] When the grid voltage drops, the fluctuation range of the DC bus voltage will also be significantly intensified, and even the maximum transient overvoltage exceeding the withstand limit of the capacitor may occur, which poses a potential threat to the DC side equipment. In view of this, the optimized GSC control strategy should also particularly strengthen the ability to maintain the stability of the DC bus voltage, aiming to ensure that the entire system can still operate safely and stably when facing faults.

[0112] Considering that the losses on the switch and the rotor side line are relatively small, they are often treated as negligible factors in conventional analysis. Therefore, the mathematical model of the DFIG DC link can be simplified as follows:

[0113]

[0114] In the formula, P c , P s and P r represent the active power flowing through the DC bus capacitor, the capacitor stator side and the rotor side respectively, and U dcVc is the voltage across the capacitor, C dc C is the capacitance value, i dcs and i dcr are the stator-side and rotor-side currents of the capacitor, respectively. The stator-side active power can also be expressed as the product of the stator voltage U s and the direct-axis component of the stator current i sd , i.e.: Then, equation (9) can be rewritten as:

[0115]

[0116] When the grid voltage fails, the fluctuation of the DC bus voltage will be significantly enhanced. In order to effectively suppress this voltage fluctuation, a feedforward compensation term is added to the control loop of the GSC to optimize the reference value of the GSC active current. This reference value of the active current aims to achieve the balance between the input and output power, thereby ensuring the stability of the bus voltage. That is, to ensure that the relationship is satisfied.

[0117] Based on this balance condition and in combination with equation (10), a specific feedforward component can be added to the voltage outer loop output of the GSC control system. Denote this feedforward component as Δi sd , i.e.:

[0118]

[0119] The improved control strategy of the GSC after introducing the feedforward component Δi sd is shown in Figure 6 . During the fault ride-through period, switch K8 is switched to state 2. This strategy significantly improves the regulation performance of the DC bus voltage by adjusting the reference value of the active current. Specifically, the addition of the feedforward component not only changes the set value of the active current, but also accelerates the response speed of the DC bus voltage, effectively reducing the transient current peak flowing into the capacitor. This series of optimization measures work together to ultimately achieve the constant maintenance of the DC bus voltage under grid voltage fluctuation, ensuring the stability and reliability of the system.

[0120] In the embodiments of the present application, the improved control structure embodying the method proposed by the present application is shown in Figure 3 and Figure 6 . The improvement of the RSC control structure is depicted in Figure 3In the current inner loop, the switching switches K1 and K2 are arranged, which are specially used for controlling the input and removal of the current limiting module, so as to ensure that the current fluctuation can be effectively limited under the condition of voltage drop. In the input link of the current loop reference value, the switches K3 and K4 responsible for realizing the flexible switching between the two different control modes of network construction type and network following type are arranged, so as to adapt to the change of power grid state and the demand of fault ride-through. In the current reference value generation link of the network construction control strategy, the state follow-up control switching switches K5 and K6 are embedded to ensure the real-time synchronization of parameters. In the control structure improvement part of the GSC Figure 6 , the reference value input end in the control structure of the GSC is additionally provided with the switching switches K7 and K8, which can quickly change the input reference value of the GSC during the fault, so as to optimize the control strategy of the GSC.

[0121] (5) Control cooperation process and switching implementation logic under different fault degrees and different stages:

[0122] Based on the detailed analysis of the rotor side control and the stator side control, around the time context before and after the fault, aiming at the response strategy of the DFIG under different voltage drop situations, combined with the use of hardware auxiliary equipment, the overall flow chart of the soft-link switching scheme of the application is as shown in Figure 1 , and the logic diagram of the measures taken by the stator side and the rotor side under different time periods and voltage drop degrees is as shown in Figure 2 . The following is combined with Figure 1 , Figure 2 , Figure 3 and Figure 6 to describe the comprehensive switching logic of the designed soft-link switching scheme:

[0123] ① When the system is in a normal operating state, the RSC adopts the LPF-droop control mode, and the GSC maintains the traditional voltage constant and unit power factor operation control mode, that is, the switches K1 to K8 in the control structure as shown in Figure 6 are in state 1.

[0124] ② When the DFIG encounters slight voltage drop, the differential current limiting module needs to be first added in the control structure of the RSC, that is, the control switches K1 and K2 are switched to state 2, and the control structure of the GSC needs to be changed to the improved mode, that is, the control switches K7 and K8 are switched to state 2. Since the overcurrent value caused by slight voltage drop does not reach the switching threshold i invtTherefore, there is no need to switch to the grid-type vector control strategy, and the switches K3 to K6 in the control structure should remain unchanged in the original state 1. In addition, there is no need to put in the resistance-capacitance type rotor crowbar, and only the differential current limiting term in the LPF-droop control structure is used to limit the relatively small overcurrent. After the fault is eliminated, the differential current limiting term in the RSC control structure needs to be removed, and the GSC control mode returns to the normal mode, that is, K3 to K6 remain unchanged, and K1, K2, K7 and K8 are switched back to state 1.

[0125] ③In the face of more serious voltage drop, a differential current limiting module needs to be added to the control structure of the RSC, but given that the overcurrent situation is more serious at this time, the control mode needs to be switched to the vector control mode, and the LPF-droop control needs to follow the state of the vector control during fault ride-through, that is, all control switches K1 to K6 are switched to state 2. In order to effectively limit the overcurrent and suppress the impact in the switching process, the resistance-capacitance type rotor crowbar needs to be put in at this time. Similarly, the control structure of the GSC should also be adjusted to the improved mode, that is, K7 and K8 are switched to state 2. In the later stage of fault ride-through, as the voltage gradually recovers, the overcurrent on the rotor side will be lower than the removal threshold i cut of the resistance-capacitance type crowbar at this time, and the differential current limiting term in the control structure will continue to undertake the current limiting task after the resistance-capacitance type crowbar is removed. After the fault is over, the control mode of the GSC switches back to the normal mode, the differential current limiting module in the control structure of the RSC is removed, and the LPF-droop control mode is switched back, and the state following of the LPF-droop control to the vector control is disconnected, that is, all switches K1 to K8 are switched back to state 1.

[0126] The following is an example of a specific application of the present application:

[0127] A simulation model corresponding to the embodiment of the present application is built in the MATLAB / Simulink platform to show the effect of the present application. The related parameters involved in the simulation model are shown in Table 1:

[0128] Table 1 Main simulation parameters of DFIG

[0129] Parameter name (unit) Value Parameter name (unit) Value Rated power (kW) 1500 Stator rated voltage (V) 690 Rated frequency (Hz) 50 Rotor rated voltage (V) 420 Slip (s) 1.662 Rated speed (r / min) 1780 Stator resistance (pu) 0.0048 Stator leakage inductance (pu) 0.0924 Rotor resistance (pu) 0.0055 Rotor leakage inductance (pu) 0.0995 Number of pole pairs 2 Mutual inductance (pu) 3.927

[0130] In order to fully demonstrate that the method proposed in the present application can be widely adapted to various voltage drop scenarios, two typical voltage drop situations are particularly selected as representatives for in-depth analysis: one is a mild situation with a voltage drop amplitude of 20%, and the other is a severe situation with a voltage drop amplitude of 70%. In order to ensure the consistency and comparability of the simulation experiment comparative analysis, in all different scheme experiments, the DFIG encounters voltage drop at 3s, and the fault state lasts for 0.5s, until the fault is removed at 3.5s.

[0131] ① Comparative analysis of DFIG fault ride-through performance under light voltage dip:

[0132] Under the light voltage dip scenario, the GSC of the DFIG can achieve fault ride-through by virtue of the improved control strategy proposed in the application, and the RSC only needs to adopt the LPF-droop control strategy fused with the differential current limiting module. Therefore, in this case, the RSC does not need to switch the control strategy. However, in order to verify the effectiveness of the LPF-droop control with the differential current limiting module and the improved GSC control in improving the fault ride-through ability of the DFIG, the following three simulation experiment schemes are designed for comparative analysis:

[0133] Scheme 1: The GSC of the DFIG adopts the improved control strategy designed in the application, and the RSC uses the LPF-droop control strategy fused with the differential current limiting module, so as to fully exhibit the performance of the method proposed in the application.

[0134] Scheme 2: In order to highlight the importance of the improvement of the GSC control strategy, the GSC of the DFIG is set to the traditional control strategy, and the RSC still adopts the LPF-droop control strategy containing the differential current limiting module, which is used as a control group.

[0135] Scheme 3: In order to evaluate the improvement effect of the differential current limiting module on the LPF-droop control strategy, the GSC of the DFIG adopts the improved control strategy designed in the application, but the LPF-droop control strategy of the RSC does not contain the differential current limiting module, which is used as another control group.

[0136] Figure 7 The figure directly shows the influence of the three different control schemes on the DFIG stator voltage under the light fault situation of 20% voltage dip. Through comparative analysis, it can be seen that the improved GSC control designed in the application and the LPF-droop control of the RSC fused with the differential current limiting module (i.e. scheme 1) can quickly promote the recovery of the fault voltage on the DFIG stator side. In contrast, scheme 2 without the improved GSC control has a significant decrease in reactive power compensation ability and almost cannot effectively compensate the DFIG stator voltage. Although scheme 3 uses the improved GSC control strategy and exhibits a certain voltage recovery ability, it lacks the assistance of the differential current limiting module, and its voltage recovery speed in the initial stage is slower than that of scheme 1.

[0137] Further observation Figure 8The DFIG DC bus voltage waveform under the three control schemes is shown. It can be clearly seen that the scheme 1 corresponding to the application can better maintain the DC bus voltage in a constant range. On the contrary, the scheme 2 without the improved GSC control strategy performs poorly in maintaining the stability of the DC bus voltage, and only relies on the differential current limiting module to suppress the overvoltage of the DC bus, which has limited effect and even produces an overvoltage close to 1.1 times the rated voltage value. Although the scheme 3 uses the improved GSC control strategy to effectively suppress the overvoltage of the DC bus, its suppression performance is still inferior to that of the scheme 1 due to the lack of the synergistic effect of the differential current limiting module, and an overvoltage of 1.04 times the rated voltage value is generated in this case.

[0138] Figure 9 The performance of the three control schemes in suppressing rotor-side overcurrent is compared. The scheme 1 corresponding to the application performs well and almost completely eliminates the overcurrent impact on the rotor side of the DFIG. Although the scheme 2 also has certain rotor overcurrent limiting capability, its limiting effect is inferior to that of the scheme 1 due to the lack of the synergistic effect of the improved GSC control. The scheme 3, without the differential current limiting module, has a significantly deteriorated current limiting capability, and an impact current exceeding 2 times the rated current is generated on the rotor side, which directly threatens the safe operation of the RSC and the rotor side of the DFIG.

[0139] Figure 10 The reactive power compensation of the three schemes during fault ride-through is shown. Obviously, the scheme 1 corresponding to the application can perform smooth reactive power compensation during fault ride-through. The scheme 2 without the improved GSC control almost loses the ability to compensate reactive power. The scheme 3 also has certain reactive power compensation capability, but its reactive power compensation process is not as smooth as that of the scheme 1 due to the lack of the cooperation of the differential current limiting module, especially during fault occurrence and removal, which produces a large impact.

[0140] ②Comparison and analysis of DFIG fault ride-through performance under severe voltage sag:

[0141] When facing severe voltage sag, it is difficult to ensure that the DFIG can achieve fault ride-through only by relying on the improved GSC control strategy and the LPF-droop control strategy integrated with the differential current limiting module. At this time, the soft-link switching method proposed in the application and the supporting hardware auxiliary equipment must be relied on. In order to clearly show the significant effect of the soft-link switching method proposed in the application in improving the fault ride-through capability of the DFIG, the following three different control schemes are specially designed for comparison and analysis, and they are named as schemes 4 to 6 to distinguish from the three control schemes for light voltage sag.

[0142] Scheme 4: During the fault ride-through process of the DFIG, the comprehensive soft-link switching scheme designed in the application is used to verify its overall performance and effect.

[0143] Scheme 5: As a control group, the DFIG in the fault ride-through period, its GSC uses the improved control strategy, but the RSC does not use the control switching, so as to evaluate the independent effect of the improved GSC control strategy.

[0144] Scheme 6: In another control group, the DFIG in the fault ride-through period, its RSC uses the soft-link switching scheme, but the GSC does not use the improved control strategy, so as to evaluate the effect of the RSC soft-link switching alone.

[0145] Figure 11 The influence of three different control schemes on the DFIG stator voltage under the condition of severe fault with 70% voltage drop is shown. Through comparative analysis, it can be seen that the soft-link switching comprehensive scheme (Scheme 4) proposed in the application performs best in restoring the DFIG stator voltage. Not only is the recovery speed fast in the early stage, but also the voltage is relatively stable during the fault ride-through process. Although the voltage waveform is concave due to the temporary decrease of reactive power caused by the exit of the resistance-capacitance type crowbar, the overall recovery effect is still significant. In contrast, Scheme 5 in which the RSC does not use control switching also exhibits certain voltage recovery ability, which is due to the reactive power compensation effect of the improved GSC control strategy, but its voltage recovery speed is significantly inferior to that of Scheme 4 due to the lack of cooperation of the resistance-capacitance type rotor crowbar. Scheme 6 has certain voltage recovery ability in the early stage due to the action of the resistance-capacitance crowbar, but loses this ability after the exit of the crowbar, and also causes voltage sag.

[0146] Further observation Figure 12 The DFIG DC bus voltage waveform under the three control strategies shown can be seen that the soft-link switching comprehensive scheme (Scheme 4) proposed in the application has a significant effect in suppressing the DC bus overvoltage, and successfully limits the overvoltage to within 1.1 times the rated voltage. Scheme 5, due to the lack of RSC control switching, loses the effective limitation of the rotor-side overcurrent, and only relies on the improved GSC control to suppress the DC bus overvoltage, which is not ideal, and produces an overvoltage close to 1.3 times the rated value. Scheme 6 has a certain inhibitory effect on the DC bus overvoltage under the action of the differential current limiting term and the resistance-capacitance crowbar, but due to the lack of cooperation of the improved GSC control, its overall inhibitory effect is still inferior to that of Scheme 4, which produces an impact overvoltage close to 1.2 times the rated voltage.

[0147] Figure 13The paper compares the performance of three control strategies in suppressing rotor-side overcurrent. The flexible switching integrated scheme (Scheme 4) designed in this invention performs excellently even under severe voltage drops, successfully limiting the rotor-side overcurrent to within 1.15 times the rated current. Scheme 5, however, fails to effectively limit the rotor-side overcurrent due to the lack of RSC control switching, resulting in an overcurrent exceeding 2.2 times the rated current. While Scheme 6 still possesses some ability to limit rotor-side overcurrent, restricting it to within 1.25 times the rated current, its current-limiting performance is still inferior to Scheme 4 due to the lack of improved GSC control.

[0148] at last, Figure 14 The reactive power compensation of three schemes during fault ride-through is demonstrated. The flexible switching integrated scheme (Scheme 4) corresponding to this invention performs well in reactive power compensation, effectively aiding in the recovery of stator-side voltage, especially in the early stage of fault ride-through, where the combined effect of the RC crowbar and improved GSC control provides significant reactive power compensation. Scheme 5, while also providing reactive power compensation under the improved GSC control, suffers from less compensation than Scheme 4 due to the lack of RC crowbar support in the early stage, and the impact during fault occurrence and clearance is more severe. Scheme 6, while possessing some reactive power compensation capability in the early stage of fault ride-through due to the deployment of the RC crowbar, loses this capability after the crowbar is removed.

[0149] As described in the above embodiments, those skilled in the art will understand that, compared with the prior art, the integrated control mode flexible switching method for DFIG of the present invention can fully leverage the respective advantages of grid-based LPF-droop control and grid-following vector control at different stages during fault ride-through. It also considers the impact of fault severity in specific applications. The introduced differential current limiting module not only effectively limits overcurrent on the rotor side but also ensures seamless integration and uniformity of the two control modes in current limiting compensation. Furthermore, the flexible switching method designed in this invention does not simply splice the two control methods at different time periods but establishes a close connection between them. On the one hand, when RSC is in vector control mode, LPF-droop control can follow the state of vector control in real time, ensuring parameter synchronization between the two control modes. On the other hand, the reactive power-voltage droop characteristic of LPF-droop control provides a basis for reactive power compensation of GSC. In summary, the integrated control mode flexible switching method for DFIG designed in this invention, through the ingenious use of RC rotor crowbars, state following modules, and improvements to the GSC control strategy, achieves flexible switching between different control modes. This method effectively limits rotor-side overcurrent, maintains the stability of DC bus voltage, and accelerates the recovery process of fault voltage, thereby significantly improving the fault ride-through capability of DFIG.

[0150] Further, referring to Figure 15 The electronic device can include a processor 10, a memory 11, a communication bus 12, and a communication interface 13, and can further include a computer program stored in the memory 11 and executable on the processor 10, and the processor executes the computer program to implement the application of the integrated control mode soft-link switching method for DFIG in the above method embodiments.

[0151] In some embodiments, the processor 10 can be composed of integrated circuits, for example, can be composed of a single packaged integrated circuit, or can be composed of multiple packaged integrated circuits with the same function or different functions, including one or more combinations of central processing units (CPUs), microprocessors, digital processing chips, graphics processors, and various control chips. The processor 10 is the control core of the electronic device, which connects various components of the electronic device through various interfaces and lines, and executes programs or modules stored in the memory 11 and calls data stored in the memory 11 to perform various functions and process data of the electronic device.

[0152] Those skilled in the art should understand that the embodiments of the present application can be provided as methods, devices or computer program products, etc. Therefore, the present application can be in the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can be in the form of a computer program product implemented on one or more storage media containing computer usable program code.

[0153] It should be noted that the word "comprise" does not exclude the presence of steps or components not listed in the claims. The present application can be implemented by means of hardware comprising several distinct components, and by means of a suitably programmed computer. The implementation described in the above embodiments of communication, control, etc. can be obtained from the description of the relevant products or from the prior art, which is a means of prior art, and the selection of the components of each device involved can be selected and set according to the actual needs, which is not specifically limited here.

[0154] Each embodiment in the specification is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between each embodiment can be referred to each other.

[0155] The foregoing description of the disclosed embodiments enables a person skilled in the art to make or use the application. Modifications of these embodiments will occur to persons of skill in the art, and that the appended claims are intended to cover all such modifications that do not depart from the true spirit and scope of the application. Therefore, the application is not limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for integrated control mode soft grid switching applied to a DFIG, characterized in that, The method comprises: When the system is in normal operation state, the RSC of the DFIG adopts an LPF-droop control mode, and the GSC maintains a voltage constant and unit power factor operation control mode; When the voltage dip occurs, a differential current limiting module that is seamlessly adapted to the grid-forming control and grid-following vector control mode and has unified forms is added to the control structure of the RSC, and the switching of switches is intervened into the control structure; When the voltage occurs severe dip, the RSC switches the control mode to the vector control mode, and utilizes the resistance-capacitance crowbar to limit current and compensate reactive power, and suppresses the switching impact of the control mode; meanwhile, the state following module controls the synchronization of parameters in the process of mode switching, and completes the smooth transition of the control mode; In the method, during the fault ride-through, the control mode of the GSC is adjusted to an improved mode, and the grid-forming control of the RSC provides a reference for reactive power compensation of the improved switching control of the GSC. The state following module makes the grid-forming LPF-droop control closely follow the current output instruction of the vector control during the fault ride-through, and the control parameters of the LPF-droop are kept in real time synchronization with the parameters of the vector control.

2. A method for integrated control mode soft grid switching for DFIG as claimed in claim 1, wherein, In the method, after the fault ends, the control mode of the GSC is switched back to the normal mode, the differential current limiting module is removed from the control structure of the RSC, and the LPF-droop control mode is switched back, and the state following of the LPF-droop control to the vector control is disconnected.

3. A method for integrated control mode soft grid switching for DFIG as claimed in claim 1, wherein, The differential current limiting module is used to limit the stator side current after voltage drop The differential is carried out, and the calculation formula is: wherein represents the transient component of the open circuit voltage on the rotor side of the DFIG, represents the mutual inductance between the stator and the rotor, represents the slip of the DFIG.

4. The method of integrated control mode soft grid switching for DFIG as claimed in claim 1, wherein, The reference mechanism for the investment of the resistance-capacitance crowbar is that the reference current of the crowbar investment is set to 0.9 times the withstand current; the removal reference mechanism of the resistance-capacitance crowbar is that the reference current of the crowbar removal is set to 1.05 times the rated current.

5. The method of integrated control mode soft grid switching for DFIG as claimed in claim 1, wherein, In the method, when the control mode of the GSC is adjusted to the improved mode, the reactive power compensation power required for recovering the voltage is determined by using the reactive power-voltage droop characteristic in the LPF-droop control, and the final reactive power compensation power of the GSC is calculated in combination with the compensated reactive power of the resistance-capacitance rotor crowbar, and the calculation formula is: wherein Qcomp, final represents the final reactive power compensated by the GSC, Qcomp, req represents the required compensation of reactive power determined by the reactive-voltage characteristic in the LPF-droop control, Qcomp, cap represents the reactive power compensated by the capacitive rotor crowbar.

6. A method for integrated control mode soft grid switching for DFIG as claimed in claim 5 wherein, In the method, a specific feedforward component is further added to the voltage outer loop output of the GSC control, and the expression of the feedforward component is: wherein represents a feedforward component, represents a capacitor voltage across, represents a capacitor rotor-side current, represents a stator voltage.

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