Comprehensive control mode flexible connection switching method applied to DFIG

Abstract

The invention discloses an integrated control mode flexible connection switching method applied to a DFIG (Distributed Feedback Induction Generator). The method comprises the following steps of: ensuring that the current limiting performance of a current limiting module can be exerted to the greatest extent in each implementation stage of a control strategy by using the current limiting module; a resistance-capacitance crowbar is introduced to realize current limiting and reactive compensation, switching impact limiting of a control mode is carried out, an input and removal reference mechanism matched with control switching is designed, and then key problems encountered by crowbar use are effectively solved; a state follow-up module is integrated in a DFIG converter control framework, parameter synchronization in the control mode switching process is ensured, the system is effectively prevented from being subjected to great switching impact, and smooth transition of the control mode is further achieved. In the state follow-up period, a network construction control strategy adopted by the RSC of the DFIG provides a reactive compensation reference for the improvement of the GSC, and it is ensured that in the switching process, a tight internal relation is established between different control modes instead of simple time sequence splicing.

Description

Technical Field

[0001] The present invention relates to the technical field of stable operation of a doubly-fed wind power generation system, and more specifically to a flexible switching method for an integrated control mode of a DFIG, so as to improve the fault ride-through capability of the DFIG under different stages and voltage drop degrees in a high-proportion new energy power system. Background Art

[0002] The core of the doubly-fed induction generator (DFIG) lies in its converter control system, which decouples the generator speed from the grid frequency and optimizes wind energy capture. The grid-following control strategy adopted earlier synchronizes the DFIG output voltage and current with the grid, but in a weak grid of new energy, 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 came into being. This strategy has significant advantages in providing voltage and frequency support, adapting to a high proportion of new energy access, simple control structure and fast response speed. As the proportion of new energy increases, the grid-forming control strategy has attracted much attention for its ability to enhance the resilience of the power system.

[0003] At present, the grid-connected converter control technologies of DFIG mainly include virtual synchronous generator (VSG) control, direct current control (DCC) and droop control. VSG control simulates the characteristics of synchronous generators to make the doubly fed wind turbine exhibit dynamic and steady-state characteristics similar to those of synchronous generators when connected to the grid. This helps to enhance the inertia and damping of the system and improve the stability of the power grid. DCC control directly controls the output current of the converter to achieve power injection and voltage regulation to the power grid. DCC control has fast response speed and high control accuracy, but it is highly dependent on power 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 needs of the power grid to achieve reasonable power distribution. However, when traditional droop control is directly applied to DFIG, it does not solve the problem that droop control cannot simulate the inertia support of synchronous generators. Some scholars have proposed low-pass filter droop control (LPF-droop) to address this problem. However, most of the current research on the application of grid-type control strategies to DFIG focuses on steady-state conditions, without considering the transient characteristics of DFIG. When a DFIG under grid-type control encounters a fault such as a voltage drop, it may not be able to respond in time and provide the necessary voltage and frequency support, resulting in an increased risk of wind turbines disconnecting from the grid or grid collapse. In particular, the overcurrent problem during fault ride-through will make it impossible to effectively control the power of the DFIG, which seriously limits the improvement of the low voltage ride-through capability of the DFIG. However, most of the current research on the above issues is to improve the grid-type control strategy simply from the control principle level, and the effect is often not significant, and it can only play a certain role in minor fault scenarios. For more serious fault conditions, the current grid-type control strategy is generally difficult to ensure the fault ride-through of the DFIG. In contrast, the traditional grid-following control strategy and its supporting hardware auxiliary measures have developed quite maturely and have demonstrated excellent fault ride-through capability when the DFIG encounters a serious fault. Therefore, some studies have proposed a method for switching between the DFIG grid-type control strategy and the grid-following control strategy to effectively improve the fault ride-through capability for more serious voltage drops. For example, Chinese patent CN118842077A “Grid-following-grid-building mode switching control method for full-power wind power converter” proposes to independently design a grid-following controller and a grid-building controller to ensure the respective operating characteristics of the grid-following mode and the grid-building mode; at the same time, the two modes do not run at the same time, which reduces the burden on the controller and saves costs.

[0004] However, this patent is mainly aimed at full-power permanent magnet direct-drive wind turbines, and its switching logic adopts a method of simply switching between two control strategies. When one strategy is running, the other strategy is in a non-working state. They only pay attention to the problem of switching shock when switching, but do not fully consider the inherent connection between the two control strategies. In fact, they only simply splice the two control methods in different time periods, and ignore the importance of accelerated recovery of fault voltage. At present, there are at least some defects in the prior art, such as different control modes are not closely connected during the switching process, the switching shock is large, the current limiting and voltage recovery effects are not obvious during fault crossing, the influencing factors such as different stages and fault degree are not considered, the overall coordination control logic is complex, and its application scope is not wide enough.

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

[0006] In view of this, the present invention provides a flexible switching method for an integrated control mode applied to DFIG, which can solve the problems in the prior art that the application occasions are not wide enough, the impact caused by parameter asynchrony during the switching process is large, the current limiting and voltage recovery effects during fault ride-through are not obvious, different control modes are not connected during fault ride-through, and the integrated control logic is complex, thereby effectively improving the fault ride-through capability of DFIG in a high-proportion new energy power system.

[0007] To achieve the above object, the technical solution adopted by the present invention is:

[0008] In a first aspect, an embodiment of the present invention provides a method for flexible switching of an integrated control mode applied to a DFIG, the method comprising:

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

[0010] When voltage drops occur, a differential current limiting module with a unified form that is seamlessly adapted to the grid-building control and grid-following vector control modes is added to the control structure of the RSC, and the control structure is intervened by switching on and off the control switch.

[0011] When the voltage drops severely, the RSC switches the control mode to the vector control mode, and uses the RC crowbar to limit the current and compensate for reactive power, as well as to suppress the switching impact of the control mode. At the same time, the state follower module synchronizes the parameters during the control mode switching process to complete a smooth transition of the control mode.

[0012] Furthermore, in the method, during the fault ride-through, the control mode of the GSC is adjusted to the improved mode, and the RSC networking control of the DFIG provides a benchmark for reactive power compensation for the improved switching control of the GSC.

[0013] Furthermore, in the method, after the fault is resolved, 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, and the control mode is switched back to the LPF-droop control mode, and the state following of the LPF-droop control to the vector control is disconnected.

[0014] Furthermore, the differential current limiting module is used to control the stator side current i after the voltage drops. s Differentiate, and the calculation formula is:

[0015]

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

[0017] Furthermore, the reference mechanism for inputting the RC crowbar is: the reference current for inputting the crowbar is set to 0.9 times the withstand current; the reference mechanism for removing the RC crowbar is: the reference current for removing the crowbar is set to 1.05 times the rated current.

[0018] Furthermore, during the fault ride-through period, the state following module enables the meshed LPF-droop control to closely follow the current output command of the vector control, and the control parameters of the LPF-droop are synchronized with the parameters of the vector control in real time.

[0019] Furthermore, in this method, when the control mode of the GSC is adjusted to the improved mode, the reactive power compensation power required to restore the voltage is determined by using the reactive power-voltage droop characteristics in the LPF-droop control, and the reactive power finally compensated by the GSC is calculated in combination with the reactive power compensated by the RC rotor crowbar. The calculation formula is:

[0020] Q last =Q nd -Q cb

[0021] In the formula, Q last It represents the reactive power finally compensated by GSC, Q nd It represents the reactive power required to be compensated according to the reactive power-voltage characteristic in LPF-droop control, Q cb Indicates the reactive power compensated by the RC rotor crowbar.

[0022] Furthermore, in this method, a specific feedforward component is added to the voltage outer loop output of the GSC control, 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 Indicates the current on the rotor side of the capacitor, U s Indicates the stator voltage.

[0025] In a second aspect, an embodiment of the present invention further provides an electronic device, comprising a processor and a memory, wherein the memory stores machine executable instructions that can be executed by the processor, and the processor executes the machine executable instructions to implement the above-mentioned integrated control mode flexible switching method applied to DFIG.

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

[0027] 1. The present invention provides a flexible switching method for an integrated control mode applied to DFIG. When a doubly-fed wind turbine generator set in a high-proportion new energy power system encounters faults of varying degrees, the flexible switching method for the integrated control mode proposed by the present invention can be used at different stages to give full play to the respective advantages of network construction and grid following control, and effectively solve the problems of limiting overcurrent and quickly restoring voltage of the doubly-fed wind turbine generator set during fault ride-through. At the same time, it also solves the problems of the prior art that the applicable occasions are not wide enough, the impact caused by the asynchronous parameters during the switching process is large, the different control modes are unconnected during fault ride-through, and the integrated control logic is complex, thereby effectively improving the fault ride-through capability of DFIG in a high-proportion new energy power system.

[0028] 2. The flexible switching method proposed in this invention cleverly combines the respective advantages of grid-forming LPF-droop control and grid-following vector control, while fully considering the impact of voltage drop and introducing a differential current limiting module. It not only effectively limits the overcurrent on the rotor side, but also realizes the seamless connection and unification of the two control modes in the current limiting compensation strategy.

[0029] 3. The flexible switching method proposed in the present invention does not simply connect the two control modes in different time periods, but builds 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 synchronization of the parameters of the two control modes; on the other hand, the reactive power-voltage droop characteristics of the LPF-droop control provide a basis for the reactive power compensation of the GSC.

[0030] 4. The present invention achieves flexible switching between different control modes by cleverly using the RC rotor crowbar, state following module and improvement of GSC control strategy. This method effectively limits the overcurrent on the rotor side, 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 invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the written description and the accompanying drawings.

[0032] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, a brief introduction will be given below to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

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

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

[0036] Figure 2 It is a logical schematic diagram of the measures taken by the present invention on the stator and rotor side of the DFIG;

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

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

[0039] Figure 5 Obtain a schematic diagram for the required reactive power compensation;

[0040] Figure 6 Improve the switching control principle block diagram for DFIG grid-side converter;

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

[0042] Figure 8 Schematic diagram of DFIG DC bus voltage waveform under different control schemes when the voltage drops by 20%;

[0043] Fig. 9 Schematic diagram of DFIG rotor current waveform under different control schemes when the voltage drops by 20%;

[0044] Fig.10 Schematic diagram of reactive power compensation waveform under different control schemes when the voltage drops by 20%;

[0045] Fig.11 Schematic diagram of DFIG stator voltage waveform under different control schemes when the voltage drops by 70%;

[0046] Fig.12 Schematic diagram of DFIG DC bus voltage waveform under different control schemes when the voltage drops by 70%;

[0047] Fig.13 Schematic diagram of DFIG rotor current waveform under different control schemes when the voltage drops by 70%;

[0048] Fig.14 Schematic diagram of reactive power compensation waveform under different control schemes when the voltage drops by 70%.

[0049] Fig.15 The present invention provides a schematic structural diagram of an electronic device. DETAILED DESCRIPTION

[0050] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments.

[0051] In the description of the present invention, it should be noted that: in some processes described in the specification and drawings of this application, multiple operations appearing in a specific order are included, but it should be clearly understood that these operations may not be performed in the order in which they appear in this document or may be performed in parallel. In addition, various serial numbers are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0052] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0053] See also Figure 1 and Figure 2As shown, the present invention provides a comprehensive control mode flexible switching method applied to DFIG, the method mainly includes:

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

[0055] 2. When voltage sag occurs, a differential current limiting module with a unified form that is seamlessly adapted to the grid construction control and grid following vector control modes is added to the control structure of the RSC, and the control structure is intervened by controlling the switching of the switch.

[0056] During the control mode switching process of the DFIG-based RSC, whether it is a grid-forming control strategy or a grid-following control strategy, it has the characteristic of the current inner loop as its core component; the current limiting module can quickly intervene in the control structure by controlling the switching of the switch when a voltage drop occurs. Considering the diversity of voltage drops and the complexity of different stages in the fault crossing process, the DFIG RSC may adopt a grid-forming or grid-following control strategy at different stages. However, no matter which control mode the RSC is in, the current limiting module has a unified form when it intervenes in the control structure. This feature ensures that when the current limiting module is put into use, there is no need to confirm in advance the specific control mode that the RSC is currently in, thereby greatly simplifying the operation process.

[0057] 3. When the voltage drops severely, the RSC switches the control mode to the vector control mode, and uses the RC crowbar to limit the current and compensate for reactive power, as well as to suppress the switching impact of the control mode; at the same time, the state follower module synchronizes the parameters during the control mode switching process to complete the smooth transition of the control mode.

[0058] 4. During the fault ride-through period, the control mode of the GSC is adjusted to the improved mode, and the RSC network control of the DFIG provides a benchmark for reactive power compensation for the improved switching control of the GSC.

[0059] The control strategy of DFIG's GSC during fault ride-through cannot continue to use the traditional operation mode, but should be able to provide an appropriate amount of reactive power to the stator side to achieve reactive balance, thereby accelerating the rapid recovery of voltage. The specific improvement method is to use the reactive-voltage characteristics in LPF-droop control to determine the reactive power Q required for compensation. nd , minus the reactive power Q compensated by the RC rotor crowbar cb (If the RC crowbar is not used, the compensation power is 0), and the reactive power reference value Q for improved control of GSC is obtained. last , that is: Q last =Q nd -Q cbUnder this mechanism, LPF-droop control is not completely idle during the fault ride-through process. It not only maintains parameter synchronization with the vector control mode, but also uses its reactive-voltage characteristics to provide a basis for GSC to perform reactive compensation analysis, so as to achieve seamless switching and rapid voltage recovery.

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

[0061] To sum up, it can be seen that the present invention adopts a grid-type control strategy in the steady-state stage to improve the active support capability of DFIG units in the current high-proportion new energy power system, and proposes an effective comprehensive control mode flexible switching scheme to solve the problem of insufficient fault ride-through capability when facing voltage drop faults, especially severe voltage drops, and finally achieves a significant improvement in the fault ride-through capability of the grid-type control DFIG units.

[0062] The method of the present invention is mainly implemented in the following ways: a. Design an innovative current limiting module, which can seamlessly adapt to two different modes of grid control and grid following control, and maintain the unity of control form, so as to ensure that its current limiting performance can be maximized in each implementation stage of the control strategy. b. In addition to realizing current limiting and reactive power compensation, the introduction of the resistor-capacitor crowbar limits the switching impact of the control mode, and designs an input and removal reference mechanism that can match the control switching of the present invention, thereby effectively solving the key problems encountered in the use of the crowbar. c. The innovatively designed state following module is cleverly integrated into the converter control architecture of the DFIG to ensure the synchronization of parameters during the control mode switching process, effectively avoid the system from suffering from major switching impacts, and thus achieve a smooth transition of the control mode. d. During the state following period, the grid control strategy adopted by the rotor-side converter (RSC) of the DFIG provides a benchmark for reactive power compensation for the improvement of the grid-side converter (GSC), ensuring that in the switching process of the present method, a close internal connection is established between different control modes, rather than a simple time series splicing.

[0063] In order to smoothly integrate the current limiting module, state following module and the switching function of the GSC control input reference value into the overall control architecture of DFIG, and finally realize the effective coordinated switching of the control mode, it is preferred to deploy the control switching switch at the key position of the DFIG control structure. In the embodiment of the present invention, a switching switch is set in the current inner loop of the control structure, which is specifically used to control the input and removal of the current limiting module to ensure rapid response in the case of voltage drop and effectively limit the current fluctuation. In the link of the current loop reference value input, a switch responsible for realizing flexible switching between two different control modes, the network construction type and the network following type, is set to adapt to the changes in the power grid state and the needs of fault crossing. In the current reference value generation link of the network control strategy, a state following control switching switch is embedded to ensure real-time synchronization of parameters. A switching switch is added to the reference value input end in the control structure of the GSC, which can quickly change the input reference value of the GSC during a fault, thereby optimizing the control strategy of the GSC.

[0064] The relevant theories and specific implementation methods of the present invention are described in detail below:

[0065] In the embodiment of the present invention, when the DFIG in a high-proportion new energy power system encounters faults of varying degrees, the integrated control mode soft-link switching method proposed by the present invention is used at different fault crossing stages to give full play to the respective advantages of network construction and grid following control, effectively solving the problem of limiting overcurrent and quickly restoring voltage of the double-fed wind turbine during fault crossing. At the same time, it also solves the problems of insufficient application occasions in the prior art, large impact caused by parameter asynchrony during switching, lack of connection between different control modes during fault crossing, and complex integrated control logic, thereby effectively improving the fault crossing capability of the DFIG in a high-proportion new energy power system. The implementation methods mainly include:

[0066] (1) A current limiting module that can be seamlessly applied to two different control modes and has a unified form:

[0067] Usually, the grid-type LPF-droop control is widely used in the rotor-side converter control of DFIG, aiming to achieve precise power regulation of DFIG. Although LPF-droop control shows good anti-frequency fluctuation ability and effective voltage support characteristics in steady-state operation, when facing fault scenarios such as voltage drop, due to the constraints of factors such as overcurrent limitation and limited regulation ability, the fault ride-through ability of LPF-droop control in severe fault conditions is obviously insufficient, and it is difficult to fully improve the fault ride-through performance of DFIG, which may cause DFIG to have to operate out of the grid during the fault, which poses a severe challenge to the stable operation of the power system. In view of this, the present invention proposes to achieve the fault ride-through of DFIG by optimizing the characteristics of the LPF-droop control strategy itself when DFIG encounters a slight voltage drop fault; and when DFIG faces a severe voltage drop fault, the control strategy of its rotor-side converter is seamlessly switched from the grid-type LPF-droop control to the grid-following vector control strategy, and at the same time cooperates with the resistor-capacitor crowbar circuit to suppress the switching impact and limit the overcurrent, thereby significantly improving the fault ride-through ability of DFIG. After the fault is cleared, the control strategy will be switched back to LPF-droop control to ensure the continuous and stable operation of the DFIG.

[0068] Whether it is the LPF-droop control strategy of the grid-building type or the traditional vector control strategy of the grid-following type, when the grid voltage drops, there will be a risk of overcurrent in the DFIG rotor-side converter. This overcurrent phenomenon will seriously interfere with the effective control of the 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, LPF-droop control and traditional DFIG vector control should be improved accordingly.

[0069] In addition, considering that the two control strategies need to maintain a certain degree of uniformity during the switching process to ensure a smooth transition of the system, the implementation of the improvement measures should adopt the same control means and methods. This requires fundamentally analyzing the mechanism of overcurrent during voltage drop and the current limiting methods that can be implemented in engineering, so as to propose a current limiting measure that can be applied to these two different control strategies at the same time.

[0070] When the stator side of the DFIG suffers a voltage drop, the stator flux will quickly generate a transient component. This transient component will further induce a transient component of the rotor open-circuit voltage on the rotor side, thereby causing an overcurrent phenomenon on the rotor side. In order to effectively limit the overcurrent and thus suppress the possible overvoltage of the DC bus, measures need to be taken to reduce the transient component of the rotor open-circuit voltage. However, in actual engineering applications, it is not convenient to directly observe the transient component of the rotor open-circuit voltage. Therefore, it is necessary to find an intermediate electrical quantity that can indirectly reflect the transient component of the rotor open-circuit voltage. By controlling the change of this intermediate electrical quantity, the transient component of the rotor open-circuit voltage can be indirectly suppressed.

[0071] In the embodiment of the present invention, the stator side current i after the voltage drop is calculated by combining the basic voltage and flux equation of DFIG. s Differentiate and transform it into the synchronous rotating coordinate system, that is:

[0072]

[0073] Where U s represents the stator side voltage of 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 number sign, and t is the 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 be expressed as:

[0077]

[0078] The transient component U of the open-circuit voltage on the DFIG rotor side is rα The conversion to the synchronous rotating coordinate system can be expressed as:

[0079]

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

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

[0082]

[0083] According to formula (5), there is a direct, proportional and unidirectional relationship between the transient component of the rotor open circuit voltage and the differential term of the stator current after the voltage fault. Given that the DFIG has a large electromechanical inertia time constant and its speed remains basically unchanged during the short period of fault ride-through, the slip rate s remains constant, and the inductance L m It also remains constant. Therefore, simply monitoring the stator current and calculating its differential term is equivalent to directly obtaining the transient component of the rotor open circuit voltage. This discovery provides a solid theoretical basis for optimizing the RSC control strategy and effectively curbing the rotor overcurrent problem. For the convenience of subsequent description, the differential term of the DFIG stator current during fault ride-through is called the differential current limiting module.

[0084] From the perspective of control structure, whether the RSC adopts meshed LPF-droop control or meshed vector control, the current inner loop is built in as its core component. Considering that the differential current limiting module can accurately reflect the transient component of the open-circuit voltage on the rotor side, 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 in the opposite direction of the transient component of the rotor open-circuit voltage, thereby achieving accurate and effective limitation of the occurrence of rotor overcurrent and achieving the control goal of applying the same control means to different control strategies to ensure uniformity and control relevance.

[0085] Further, Figure 3 The schematic diagram shows the switching of the specific control structure between the LPF-droop control and the traditional vector control of the RSC. In the figure, PI represents the PI regulator commonly used in the control system, and K P is the active power droop coefficient, K Q is the reactive droop coefficient, dq, abc, αβ represent the corresponding coordinate system parameters, P and Q represent active and reactive power, u represents voltage, i represents current, s represents the stator side, r represents the rotor side, ref represents the reference value, the prefix △ represents the change, and other parameters refer to the description in the formula. In this figure, the switching control corresponding to the differential current limiting module clearly identifies the improvement link specially added to deal with the overcurrent problem during fault ride-through. It can be seen that no matter which control mode the RSC adopts, the current limiting purpose can be achieved by putting the same differential current limiting module into operation. That is, in the normal operating mode, the switching switches K1 and K2 are placed in state 1, and the system operates stably according to the established control strategy. However, once a voltage drop fault is detected, K1 and K2 can be immediately switched to state 2 to activate the differential current limiting function, thereby effectively limiting the overcurrent.

[0086] (2) Use the resistor-capacitor crowbar to suppress the control mode switching impact and coordinate the switching timing:

[0087] When the grid voltage encounters a serious drop, the rotor side overcurrent problem of DFIG will become more and more serious. At this time, it is difficult to effectively curb such a large overcurrent by relying solely on the differential current limiting module added to the converter control strategy. In order to deal with this problem, when DFIG encounters a serious voltage drop fault, in addition to switching the control strategy of RSC to the traditional vector control strategy after differential current limiting compensation for current limiting, it is also necessary to invest in the crowbar circuit on the rotor side to assist in limiting the overcurrent and reduce the impact generated during the switching process. However, in the case of severe voltage drops, the system faces a large reactive power shortage problem. The traditional pure resistive crowbar circuit cannot provide the required reactive power compensation, which is extremely unfavorable for the rapid recovery of the fault voltage. Therefore, the switching scheme proposed by the present invention adopts a resistive-capacitive rotor crowbar circuit that can provide reactive power (that is, a pure resistive crowbar is added with a capacitor, which is a prior art and will not be repeated here). The capacitor part in the resistive-capacitive crowbar can provide the necessary reactive power compensation for the system when the crowbar is put into use, thereby accelerating the voltage recovery process.

[0088] Furthermore, the timing of the crowbar insertion and removal is a key issue in the use of the crowbar technology. The switching scheme designed in the present invention is different from other studies in this respect. Specifically:

[0089] The input of the crowbar is based on the rotor side current i r The withstand current value i wth The value is usually set to 1.3 times the rated current in engineering practice, but considering a certain margin and the hysteresis of the current limiting control after input, the present invention sets the 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 When the voltage drop encountered by DFIG is not serious, the overcurrent generated is within the tolerance range. At this time, the overcurrent can be limited by the differential current limiting module added in the LPF-droop control strategy. However, once the current exceeds i invt This indicates that the DFIG is facing a serious voltage drop fault. In this case, the control strategy of the RSC should be switched to the vector control strategy after differential current limiting compensation, and the RC rotor crowbar should be put into use at the same time to assist in current limiting and suppress the impact that may occur during the switching process.

[0092] Regarding the timing of cutting off the crowbar, the present invention also does not follow the traditional method of cutting off the crowbar only after the overcurrent completely disappears or the fault is cut off. The present invention sets a reference current i cut , whose value is 1.05 times the rated current, that is, the crowbar can be removed in advance when the overcurrent is reduced to 1.05 times the rated current. The reason for this early removal is that after the crowbar is removed, the improved RSC control strategy still has the ability to suppress overcurrent and regulate DC bus overvoltage. This improvement not only solves the problems caused by relying solely on the crowbar in traditional DFIG rotor-side control (such as premature removal may cause the overcurrent to increase again, while too late removal may make the RSC unable to smoothly control the DFIG), but also optimizes the use efficiency of the crowbar, providing a more reliable guarantee for the stable operation of the DFIG during fault ride-through.

[0093] (3) The state follower module in the control structure ensures the synchronization of parameters when the control mode is switched:

[0094] As mentioned above, in order to achieve fault ride-through of DFIG under severe voltage drop conditions, the mature traditional low voltage ride-through strategy should be combined. Specifically, when the rotor side overcurrent caused by voltage drop exceeds i invt , indicating that when DFIG is facing a serious voltage drop, the RSC control strategy of DFIG should be immediately converted to a vector control mode with differential current limiting compensation, and the RC crowbar device should be enabled synchronously to effectively suppress overcurrent and alleviate the impact during switching. Given that traditional vector control is a grid-following control strategy that can be dynamically adjusted with the grid frequency, coupled with the auxiliary role of the RC rotor crowbar, there is no need to worry about parameter synchronization or switching impact when switching from grid-building 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 synchronization of parameters must be considered to prevent significant switching shock. To solve this problem, during the fault ride-through period, the LPF-droop control of the network type closely follows the current output command of the vector control to ensure that the control parameters of the LPF-droop can be synchronized with the parameters of the vector control in real time, thereby minimizing the shock when the control mode is switched. The module that realizes the above functions is called the state follow-up module (the hardware is attached). Figure 3 Switching control module three).

[0096] It is worth noting that under this switching mechanism, LPF-droop control is not completely idle during the fault ride-through process. It not only maintains parameter synchronization with the vector control mode, but also uses its reactive-voltage characteristics to provide a basis for the subsequent reactive compensation analysis of the grid-side converter (GSC), so as to achieve seamless switching and rapid voltage recovery. This switching method is completely different from the method in traditional research where the two control modes run completely independently for a period of time and then are simply spliced ​​together. In the switching method proposed in the present invention, the two control modes always maintain a close connection throughout the entire process, ensuring the impact-free switching process, so it is called a "flexible switching method." The switching diagram of the two control modes and the control structure diagram of the LPF-droop follow-up vector control are shown in the figure. Figure 3 The control mode switching and state following module switching control part.

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

[0098] ① Under normal system operation, K3 and K4 are both set to state 1, and RSC uses LPF-droop control mode. At the same time, K5 and K6 are also in state 1, that is, there is no state following relationship between LPF-droop control and vector control.

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

[0100] ③ When the fault is removed, the status of K3 and K4 should be restored to 1 again, so that the RSC switches back to the LPF-droop control mode. Since the state follow-up 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 can be switched directly. Subsequently, K5 and K6 are also reset to state 1, disconnecting the follow-up relationship between LPF-droop control and vector control, so that the RSC can operate stably in the network-type LPF-droop control mode.

[0101] (4) DFIG's RSC network control provides reactive power compensation basis for GSC improvement during state following period:

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

[0103] By analyzing Figure 4 From the reactive power voltage droop characteristic relationship diagram in Figure 1, it can be clearly seen that when the grid voltage deviates, in order to restore the voltage to its original stable value, corresponding reactive power compensation must be performed. There is a direct and quantifiable relationship between the amount of reactive power compensated and the voltage change. This relationship is Figure 5 The specific relationship can be expressed in mathematical equations as follows:

[0104] Q nd =ΔQ=-k G ΔU (7)

[0105] Where ΔU is the expected voltage recovery change, and ΔQ is the reactive power change required for voltage recovery, which is denoted as Q for the convenience of subsequent description. nd .

[0106] In short, the reactive power-voltage droop characteristics are used to determine the reactive compensation power required to restore the voltage, which provides a reactive compensation basis for DFIG to quickly restore the voltage during fault ride-through. This method not only helps DFIG to maintain grid-connected operation under fault conditions such as voltage drops, but also significantly improves its fault ride-through capability, providing strong support for the stable operation and fault recovery of the power system.

[0107] In addition to limiting the overcurrent on the rotor side of the DFIG during fault ride-through, effective measures should be taken to compensate for reactive power in order to ensure that the voltage of the DFIG can be quickly restored during fault ride-through, and the DC bus overvoltage problem that may be caused by overcurrent on the rotor side should also be suppressed. Given that the GSC of the DFIG is mainly responsible for maintaining the stability of the DC bus voltage and ensuring that the system operates at a unity power factor during normal operation, the tasks of reactive power compensation and DC bus overvoltage suppression are undertaken by the GSC, which requires the control strategy of the GSC during fault ride-through to be optimized accordingly.

[0108] In terms of reactive power compensation, although the use of the RC rotor crowbar can provide a certain amount of reactive power and promote the recovery of the fault voltage when there is a severe voltage drop, the reactive power compensation it provides is far from enough in the face of the huge reactive power demand. In addition, in the case of a mild voltage drop, the RC rotor crowbar will not be put into operation at all, thus losing the source of this part of reactive power compensation. Therefore, the control strategy of the GSC during fault ride-through cannot continue to use the traditional unit power factor operation mode, but should have the ability to provide an appropriate amount of reactive power to the stator side to achieve reactive balance, thereby accelerating the rapid recovery of voltage. In the embodiment of the present invention, the specific control strategy improvement method is: use the reactive-voltage characteristics in the LPF-droop control to determine the reactive power Q required to be compensated nd , minus the reactive power Q compensated by the RC rotor crowbar cb (If the resistor and capacitor crowbar are not used, the compensation power is 0), and the reference value of reactive power is obtained. That is, the reactive power Q finally compensated by GSC is last It can be expressed as:

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

[0110] The optimized GSC control structure is as follows: Figure 6 As shown in Figure 2, during the fault ride-through period, 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 for reactive power in the system and the current supply. This dynamic adjustment mechanism not only accelerates the recovery speed of the stator side voltage, but also effectively ensures the voltage stability.

[0111] When the grid voltage drops, the fluctuation of the DC bus voltage will also increase significantly, and even the maximum instantaneous overvoltage that exceeds the capacitor tolerance limit may occur, which poses a potential threat to the DC side equipment. In view of this, the optimized GSC control strategy should also 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 in the face of faults.

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

[0113]

[0114] Where P c , P s and P r represents the active power flowing through the DC bus capacitor, the stator side of the capacitor, and the rotor side, respectively, while U dcis the voltage across the capacitor, C dc is the capacitance value, i dcs and i dcr The stator voltage U s and the direct axis component of the stator current i sd Indicates that: Then formula (9) can be transformed into:

[0115]

[0116] When the grid voltage fails, the fluctuation of the DC bus voltage will increase significantly. 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 is intended to achieve a balance between input and output power, thereby ensuring the stability of the bus voltage. That is, to ensure the relationship Be satisfied.

[0117] Based on this balance condition and combined with formula (10), a specific feedforward component can be added to the voltage outer loop output of the GSC control system. This feedforward component is denoted as Δi sd ,Right now:

[0118]

[0119] Introducing the feedforward component Δi sd The improved control strategy of GSC is as follows Figure 6 As shown in the figure, switch K8 is switched to state 2 during fault ride-through. 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 to the capacitor. This series of optimization measures work together to ultimately achieve constant maintenance of the DC bus voltage under grid voltage fluctuations, ensuring the stability and reliability of the system.

[0120] In an embodiment of the present invention, the improved control structure of the method proposed by the present invention is as follows: Figure 3 and Figure 6 Figure 2: Depicting the improved RSC control structure Figure 3In the present invention, switching switches K1 and K2 are set in the inner current loop, which are specifically used to control the input and removal of the current limiting module to ensure rapid response in the case of voltage drop and effectively limit current fluctuations. In the link of current loop reference value input, switches K3 and K4 are set to realize flexible switching between two different control modes, namely, network construction type and network following type, to adapt to changes in grid status and fault crossing requirements. In the current reference value generation link of the network control strategy, state follow-up control switching switches K5 and K6 are embedded to ensure real-time synchronization of parameters. In the embodiment of the improvement of GSC control structure Figure 6 In the embodiment, switching switches K7 and K8 are added to the reference value input end of the control structure of the GSC, which can quickly change the input reference value of the GSC during a fault, thereby optimizing the control strategy of the GSC.

[0121] (5) Control coordination process and switching implementation logic at different fault levels and stages:

[0122] Based on the detailed analysis of the rotor side and custom side control mentioned above, focusing on the time context before and after the fault occurs, the response strategy of DFIG under different voltage drop scenarios, combined with the use of hardware auxiliary equipment, the overall flow chart of the flexible switching solution of the present invention is as follows: Figure 1 As shown in the figure, the logic diagram of the measures taken by the stator and rotor side at different time periods and voltage drop levels is as follows Figure 2 The following combination Figure 1 , Figure 2 , Figure 3 and Figure 6 As shown, the comprehensive switching logic of the designed flexible switching solution is explained:

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

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

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

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

[0127] A simulation model corresponding to the embodiment of the present invention is constructed in the MATLAB / Simulink platform to demonstrate the effect of the present invention. The relevant parameters involved in the simulation model are shown in Table 1:

[0128] Table 1 Main simulation parameters of doubly-fed generator

[0129] Parameter name (unit) value Parameter name (unit) value Rated power (kW) 1500 Stator rated voltage(V) 690 Rated frequency(Hz) 50 Rated rotor voltage(V) 420 Slip rate(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 Pole pairs 2 Mutual inductance (pu) 3.927

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

[0131] ① Comparative analysis of DFIG fault ride-through performance during mild voltage drop:

[0132] In the case of a mild voltage drop, the GSC of the DFIG can achieve fault ride-through of the DFIG by using the improved control strategy proposed in the present invention, while the RSC only needs to adopt the LPF-droop control strategy that integrates 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 capability of the DFIG, the following three simulation experimental schemes are specially designed for comparative analysis:

[0133] Solution 1: The GSC of DFIG adopts the improved control strategy designed by the present invention, while the RSC uses the LPF-droop control strategy incorporating the differential current limiting module to fully demonstrate the performance of the method proposed by the present invention.

[0134] Solution 2: In order to highlight the importance of improving the GSC control strategy, the GSC of the DFIG is set to the traditional control strategy, while the RSC still adopts the LPF-droop control strategy including the differential current limiting module as the control group.

[0135] Solution 3: In order to evaluate the improvement effect of the differential current limiting module on the LPF-droop control strategy separately, the GSC of DFIG adopts the improved control strategy designed by the present invention, but the LPF-droop control strategy of RSC does not include the differential current limiting module, which serves as another control group.

[0136] Figure 7 The effects of three different control schemes on the stator voltage of DFIG under a mild fault scenario with a voltage drop of 20% are intuitively demonstrated. Through comparative analysis, it can be seen that the LPF-droop control (i.e., Scheme 1) using the improved GSC control designed by the present invention and RSC integrated with the differential current limiting module can quickly restore the fault voltage on the stator side of DFIG. In contrast, Scheme 2, which does not adopt the improved GSC control, has a significantly reduced reactive compensation capability and is almost unable to effectively compensate the stator voltage of DFIG. Although Scheme 3 uses the improved GSC control strategy and shows a certain voltage recovery capability, due to the lack of the assistance of the differential current limiting module, its initial voltage recovery speed is not as fast as Scheme 1.

[0137] Further observation Figure 8The DFIG DC bus voltage waveforms under the three control schemes shown clearly show that Scheme 1 corresponding to the present invention can better maintain the DC bus voltage within a constant range. On the contrary, Scheme 2, which does not adopt the improved GSC control strategy, performs poorly in maintaining the stability of the DC bus voltage. It only relies on the differential current limiting module to suppress the DC bus overvoltage, with limited effect, and even produces an overvoltage close to 1.1 times the rated voltage value. Although Scheme 3 adopts the improved GSC control strategy to effectively suppress the DC bus overvoltage, its suppression performance is still inferior to Scheme 1 due to the lack of the synergy of the differential current limiting module. In this case, an overvoltage of 1.04 times the rated voltage value is generated.

[0138] Fig. 9 The performance of three control schemes in suppressing overcurrent on the rotor side is compared in the paper. Scheme 1 corresponding to the present invention performs better and almost completely eliminates the overcurrent impact on the rotor side of the DFIG. Although Scheme 2 also has a certain rotor overcurrent limiting capability, due to the lack of coordination with the improved control of GSC, its limiting effect is not as good as Scheme 1. As for Scheme 3, without the addition of a differential current limiting module, the current limiting capability is significantly deteriorated, and an impact current of more than 2 times the rated current is generated on the rotor side, which poses a direct threat to the safe operation of the RSC and rotor side of the DFIG.

[0139] Fig.10 The reactive power compensation of the three schemes during fault ride-through is shown. Obviously, Scheme 1 corresponding to the present invention can perform stable reactive power compensation during fault ride-through. Scheme 2, which does not adopt the improved GSC control, almost loses the reactive power compensation capability. Although Scheme 3 also has a certain reactive power compensation capability, due to the lack of the cooperation of the differential current limiting module, its reactive power compensation process is not as smooth as Scheme 1, especially when the fault occurs and is removed, which produces a greater impact.

[0140] ② Comparative analysis of DFIG fault ride-through performance during severe voltage drop:

[0141] When facing severe voltage drops, it is difficult to ensure that DFIG can ride through faults only by relying on the improved GSC control strategy and the LPF-droop control strategy that integrates the differential current limiting module. At this time, it is necessary to rely on the flexible switching method proposed in the present invention and its supporting hardware auxiliary equipment. In order to clearly demonstrate the significant effect of the flexible switching method proposed in the present invention in improving the fault ride-through capability of DFIG, the following three different control schemes are specially designed for comparative analysis, and they are named Scheme 4 to Scheme 6 to distinguish them from the previous three control schemes for mild voltage drops.

[0142] Solution 4: During the fault ride-through process of DFIG, the flexible switching comprehensive solution designed by the present invention is fully adopted to verify its overall performance and effect.

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

[0144] Scheme 6: In another control group, during the fault ride-through period, the RSC of the DFIG adopts a flexible switching scheme, but the GSC does not adopt the improved control strategy, so as to evaluate the effect of the RSC flexible switching separately.

[0145] Fig.11 The effects of three different control schemes on the DFIG stator voltage under severe fault conditions with a voltage drop of 70% are shown. Through comparative analysis, it can be clearly seen that the flexible switching comprehensive scheme (Scheme 4) proposed in the present invention performs best in restoring the DFIG stator voltage. Not only is the recovery speed fast in the early stage, but it is also relatively stable during the fault crossing process. Although the withdrawal of the resistor-capacitor crowbar causes a temporary reduction in reactive power, resulting in a sag in the voltage waveform, the overall recovery effect is still significant. In contrast, although Scheme 5, in which the RSC does not adopt control switching, also shows a certain voltage recovery capability, which is due to the reactive compensation effect of the improved GSC control strategy, its voltage recovery speed is significantly slower than that of Scheme 4 due to the lack of cooperation with the resistor-capacitor rotor crowbar. As for Scheme 6, it does have a certain voltage recovery capability under the action of the resistor-capacitor crowbar in the early stage, but it loses this capability after the crowbar is withdrawn, and it also causes a voltage sag.

[0146] Further observation Fig.12 From the DFIG DC bus voltage waveforms under the three control strategies shown, it can be seen that the flexible switching comprehensive scheme (Scheme 4) proposed in the present invention is effective in suppressing DC bus overvoltage, and successfully limits the overvoltage to within 1.1 times the rated voltage. However, Scheme 5, due to the failure to adopt RSC control switching, loses the effective limitation of rotor-side overcurrent, and only relies on improved GSC control to suppress DC bus overvoltage, the effect is not ideal, and an overvoltage close to 1.3 times the rated value is generated. Although Scheme 6 has a certain inhibitory effect on DC bus overvoltage under the action of differential current limiting terms and RC crowbars, due to the lack of cooperation with improved GSC control, its overall suppression effect is still not as good as Scheme 4, and an impact overvoltage close to 1.2 times the rated voltage is generated in this case.

[0147] Fig.13The performance of three control strategies in suppressing rotor-side overcurrent is compared in the paper. The flexible switching comprehensive scheme (Scheme 4) designed by the present invention still performs well under severe voltage drops, and successfully limits the rotor-side overcurrent to within 1.15 times the rated current. However, in Scheme 5, since the RSC does not implement control switching, it is unable to effectively limit the rotor-side overcurrent, resulting in an overcurrent of more than 2.2 times the rated current. Although Scheme 6 still has a certain ability to limit the overcurrent on the rotor side and can limit the overcurrent to within 1.25 times the rated current, its current limiting performance is still inferior to Scheme 4 due to the lack of coordination with the improved GSC control.

[0148] at last, Fig.14 The reactive power compensation of the three schemes during fault ride-through is demonstrated. The flexible switching integrated scheme (Scheme 4) corresponding to the present invention performs well in reactive power compensation and effectively helps the recovery of the stator side voltage, especially in the early stage of the fault ride-through phase, the combined effect of the RC crowbar and the improved GSC control provides greater reactive power compensation. Although Scheme 5 can also provide reactive power compensation under the action of the improved GSC control, due to the lack of cooperation of the RC crowbar in the early stage, its compensation strength is not as good as that of Scheme 4, and the impact when the fault occurs and is removed is also more severe. Although Scheme 6 has a certain reactive power compensation capability in the early stage of fault ride-through, this is due to the use of the RC crowbar, but this capability is lost after the crowbar is removed.

[0149] Through the description of the above embodiments, those skilled in the art can know that: compared with the prior art, the flexible switching method of the integrated control mode applied to DFIG of the present invention can give full play to the respective advantages of the network-building LPF-droop control and the grid-following vector control at different stages during the fault ride-through period, and also considers the influence of the fault degree of the method in specific application. The introduced differential current limiting module can not only effectively limit the overcurrent on the rotor side, but also ensure the seamless connection and form unification of the two control modes in current limiting compensation. In addition, the flexible switching method designed by the present invention does not simply splice the two control modes in different time periods, but builds 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 synchronization of the parameters of the two control modes; on the other hand, the reactive-voltage droop characteristics of the LPF-droop control provide a basis for the reactive compensation of the GSC. In short, the flexible switching method of the integrated control mode applied to DFIG designed by the present invention realizes flexible switching between different control modes by cleverly using the resistor-capacitor rotor crowbar, the state following module and the improvement of the GSC control strategy. This method effectively limits the overcurrent on the rotor side, maintains the stability of the DC bus voltage, and accelerates the recovery process of the fault voltage, thereby significantly improving the fault ride-through capability of the DFIG.

[0150] Further, refer to Fig.15 As shown, an embodiment of the present invention further provides an electronic device, which may include a processor 10, a memory 11, a communication bus 12 and a communication interface 13, and may also 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 integrated control mode flexible switching method applied to DFIG in the above method embodiment.

[0151] The processor 10 may be composed of an integrated circuit in some embodiments, for example, a single packaged integrated circuit, or a plurality of packaged integrated circuits with the same or different functions, including one or more central processing units (CPUs), microprocessors, digital processing chips, graphics processors, and combinations of various control chips. The processor 10 is the control core of the electronic device, and uses various interfaces and lines to connect various components of the entire electronic device, and executes various functions of the electronic device and processes data by running or executing programs or modules stored in the memory 11, and calling data stored in the memory 11.

[0152] It should be understood by those skilled in the art that embodiments of the present invention may be provided as methods, devices, or computer program products, etc. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present invention may take 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 "comprising" does not exclude the presence of steps or components not listed in the claims. The present invention can be implemented by means of hardware comprising a number of different components and by means of a suitably programmed computer. The implementation of the communication, control and other functions described in the above embodiments can be obtained from the instructions of the relevant products or the prior art, which is a prior art means. In addition, the selection of the various device components involved can be selected and set according to actual needs, and no specific limitation is made here.

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

[0155] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for flexible switching of integrated control modes applied to DFIG, characterized in that: The method includes: When the system is in normal operation, the RSC of the DFIG adopts the LPF-droop control mode, and the GSC maintains the voltage constant and unity power factor operation control mode; When voltage drops occur, a differential current limiting module with a unified form that is seamlessly adapted to the grid-building control and grid-following vector control modes is added to the control structure of the RSC, and the control structure is intervened by switching on and off the control switch. When the voltage drops severely, the RSC switches the control mode to the vector control mode, and uses the RC crowbar to limit the current and compensate for reactive power, as well as to suppress the switching impact of the control mode. At the same time, the state follower module synchronizes the parameters during the control mode switching process to complete a smooth transition of the control mode.

2. The method for flexible switching of integrated control modes applied to DFIG according to claim 1 is characterized in that: In this method, during fault ride-through, the control mode of the GSC is adjusted to the improved mode, and the RSC network control of the DFIG provides a benchmark for reactive power compensation for the improved switching control of the GSC.

3. The method for flexible switching of integrated control modes applied to DFIG according to claim 2 is characterized in that: In this method, after the fault is resolved, 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, and the control mode is switched back to the LPF-droop control mode, and the state following of the LPF-droop control to the vector control is disconnected.

4. The method for flexible switching of integrated control modes applied to DFIG according to claim 1 is characterized in that: The differential current limiting module is used to control the stator side current i after the voltage drops. s Differentiate, and the calculation formula is: Where U rα It represents the transient component of the open-circuit voltage on the DFIG rotor side, L m It represents the mutual inductance between the stator and the rotor, and s represents the slip rate of the DFIG.

5. The method for flexible switching of integrated control modes applied to DFIG according to claim 1 is characterized in that: The reference mechanism for inputting the RC crowbar is: the reference current for inputting the crowbar is set to 0.9 times the withstand current; the reference mechanism for removing the RC crowbar is: the reference current for removing the crowbar is set to 1.05 times the rated current.

6. The method for flexible switching of integrated control modes applied to DFIG according to claim 1 is characterized in that: During the fault ride-through period, the state following module enables the meshed LPF-droop control to closely follow the current output command of the vector control, and the control parameters of the LPF-droop are synchronized with the parameters of the vector control in real time.

7. The method for flexible switching of integrated control modes applied to DFIG according to claim 2 is characterized in that: In this method, when the control mode of GSC is adjusted to the improved mode, the reactive power compensation power required to restore the voltage is determined by using the reactive power-voltage droop characteristics in LPF-droop control, and the reactive power finally compensated by GSC is calculated by combining the reactive power compensated by the RC rotor crowbar. The calculation formula is: Q last =Q nd -Q cb In the formula, Q last It represents the reactive power finally compensated by GSC, Q nd It represents the reactive power required to be compensated according to the reactive power-voltage characteristic in LPF-droop control, Q cb Indicates the reactive power compensated by the RC rotor crowbar.

8. The method for flexible switching of integrated control modes applied to DFIG according to claim 7 is characterized in that: In this method, a specific feedforward component is also added to the voltage outer loop output of the GSC control. The expression of the feedforward component is: In the formula, Δi sd represents the feedforward component, U dc represents the voltage across the capacitor, i dcr Indicates the current on the rotor side of the capacitor, U s Indicates the stator voltage.

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