A positive and negative sequence current cooperative control method and system for a doubly-fed networked unit

Abstract

The application discloses a kind of double-fed network unit positive and negative sequence current collaborative control method and system, it is related to double-fed unit network control technical field, the method includes: S1.data acquisition;S2.coordinate transformation and positive and negative sequence separation;S3.negative sequence current collaborative enabling logic design and negative sequence current instruction distribution;S4.network loop control;S5.positive and negative sequence reference voltage synthesis and generation rotor reference voltage.The application aims to solve how to improve positive and negative sequence separation method to adapt to double-fed unit variable speed operation characteristics and combined with network control, and how to design the collaborative control strategy of machine network side negative sequence current ring, to realize the high stability, high reliability operation of double-fed unit under the condition of weak power grid and asymmetric power grid.

Description

Technical Field

[0001] This invention relates to the field of grid control technology for doubly-fed induction generators, specifically to a method and system for coordinated control of positive and negative sequence currents in doubly-fed induction generators. Background Technology

[0002] With the increasing penetration rate of renewable energy, doubly-fed induction generators (DFIGs) have been widely used in the wind power sector due to their cost and performance advantages. Under weak grid conditions, DFIGs employing grid-connected control can effectively reduce the interaction with grid impedance by adjusting their equivalent output impedance, thereby significantly improving system stability. Compared to conventional grid-connected control, grid-connected control typically only requires modification to the turbine-side converter of the DFIG, while the grid-side converter can still maintain the DC voltage control target unchanged. It has the advantages of simple structure and ease of implementation, and has therefore attracted widespread attention.

[0003] Current national standards specify performance requirements for doubly-fed induction generators (DFIGs) under asymmetrical grid operation conditions. Specifically, the units must possess independent negative-sequence control or suppression capabilities and be able to provide negative-sequence current support to the grid during three-phase imbalance or asymmetrical faults. However, traditional methods for separating positive and negative sequence currents, such as those based on second-order generalized integrators and dual dq synchronous coordinate systems, while achieving accurate separation, rely on multiple integration operations, resulting in a heavy computational burden and significant dynamic response delays. In contrast, positive and negative sequence separation methods based on the T / 4 period delay cancellation method are widely used due to their lower computational load and faster response. For example, patent document CN121124194A discloses its application to the fault voltage ride-through process of energy storage converters. However, careful analysis revealed that the T / 4 period delay cancellation method, suitable for energy storage converters, cannot be directly applied to doubly-fed induction generators (DFIGs). Its application to DFIGs faces a key obstacle: the rotor speed of a DFIG varies with wind speed, and its rotor current frequency is not a fixed power frequency, rendering the traditional delay cancellation method based on a fixed period T unsuitable. Forcing its application would increase the positive and negative sequence separation error due to frequency mismatch, thereby affecting control accuracy and system stability.

[0004] Furthermore, both the generator-grid side and grid-side converters in a doubly-fed induction generator (DFIG) possess negative-sequence current control capabilities, but they differ in current capacity and dynamic characteristics. If the negative-sequence current loops on both sides are activated simultaneously during operation, even slight system state changes (such as wind speed fluctuations, sudden changes in grid voltage amplitude or phase, load switching, etc.) will cause significant fluctuations in the output of the negative-sequence current loops, thus deteriorating the quality of the output voltage and current. Although existing research indicates that the negative-sequence current loop should only be activated when necessary, there is currently a lack of in-depth design and clear strategies for the enabling logic, coordination timing, and capacity coordination allocation of the generator-side and grid-side negative-sequence current loops. This results in the negative-sequence current support capability of the DFIG not being maximized, and also limits its overall operating performance under asymmetrical grid conditions.

[0005] In summary, current doubly-fed induction generator (DFIG) units still face two key challenges in improving grid control and asymmetrical operation capabilities: First, there is a lack of efficient positive and negative sequence separation methods suitable for the variable-speed operation characteristics of DFIG units and compatible with grid control, limiting performance improvement under weak grid conditions and asymmetrical operating conditions. Second, the lack of a coordinated control strategy for the negative sequence current loop on both the generator and grid sides results in the underutilization of negative sequence current support capabilities, making it difficult to meet support requirements during grid asymmetrical faults. Therefore, it is urgent to improve existing technologies to achieve high-performance and high-reliability operation of DFIG units under weak grid conditions and asymmetrical operating conditions. Summary of the Invention

[0006] To address the aforementioned technical problems in the prior art, this invention provides a method and system for coordinated control of positive and negative sequence currents in doubly-fed grid-connected generator units. The aim is to solve how to improve the positive and negative sequence separation method to adapt to the variable speed operation characteristics of doubly-fed generator units and integrate it with grid-connected control, and how to design a coordinated control strategy for the negative sequence current loop on the generator-grid side, so as to achieve high stability and high reliability operation of doubly-fed generator units under weak grid and asymmetrical grid conditions.

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

[0008] A method for coordinated control of positive and negative sequence currents in a doubly-fed grid-connected generating unit includes the following steps:

[0009] S1. Data Acquisition

[0010] Collect the stator three-phase AC voltage, stator three-phase current, rotor three-phase current and rotor electrical angle of the doubly fed motor, and calculate the active power and reactive power of the stator;

[0011] S2. Coordinate transformation and separation of positive and negative order

[0012] Coordinate transformations were performed on the three-phase AC voltage of the stator and the three-phase current of the rotor to obtain the stator voltage components and rotor current components in the αβ coordinate system. The rotor current components were then subjected to an inverse Park transformation according to the rotor electrical angle to reduce the rotor current components to the power frequency or the frequency consistent with the stator current. Subsequently, the positive and negative sequences of the stator voltage components and the reduced rotor current components were separated based on the T / 4 delay cancellation method. The measured values ​​of the positive and negative sequence components of the stator voltage and the rotor current in the dq axis were obtained by Park transformation.

[0013] S3. Negative-sequence current collaborative enable logic design and negative-sequence current instruction allocation

[0014] Based on the detection results of three-phase current imbalance and grid voltage imbalance, it is determined whether the grid voltage has experienced a three-phase symmetrical drop or an asymmetrical drop. Based on the judgment results, the negative sequence current collaborative enable logic and negative sequence current command allocation of the generator-grid side converter are designed, and the negative sequence current setpoint value of the rotor dq axis is calculated.

[0015] S4. Network Loop Control

[0016] Based on the stator's active power, the active power loop of the grid is controlled by combining the simulated rotor motion equation, and the generation angle of the active power loop is output. Based on the stator's reactive power, the reactive power loop of the grid is controlled by combining the simulated reactive power-voltage droop equation, and the positive sequence setpoint of the stator voltage on the d-axis is output. Based on the measured value of the positive sequence component of the stator voltage on the dq-axis and the positive sequence setpoint of the stator voltage on the d-axis, the stator dq-axis voltage orientation in the grid voltage loop is achieved by PI control of the measured value of the positive sequence component of the stator voltage, and the rotor dq-axis positive sequence current setpoint is output. Based on the measured value of the positive and negative sequence components of the rotor current on the dq-axis, the positive sequence current setpoint of the rotor dq-axis, and the negative sequence current setpoint of the rotor dq-axis, the negative sequence current in the grid current loop is suppressed by separate PI control of the positive and negative sequence current setpoints of the rotor dq-axis, and the reference values ​​of the positive and negative sequence voltages of the rotor dq-axis are output.

[0017] S5. Synthesize positive and negative sequence reference voltages to generate rotor reference voltage.

[0018] The positive and negative sequence voltage reference values ​​of the rotor dq axis are sequentially transformed into the αβ coordinate system, merged, reduced, and transformed into the abc coordinate system to obtain the modulation voltage reference value of the rotor-side converter. Based on the modulation voltage reference value, a PWM wave is generated to realize the closed-loop control of the doubly fed grid unit.

[0019] In S1, the formulas for calculating the active power and reactive power of the stator are as follows:

[0020] (1)

[0021] In equation (1), Pmeas and Q meas These are the active power and reactive power of the stator, respectively. U s_a , U s_b and U s_c This refers to the three-phase AC voltage of the stator. I s_a , I s_b and I s_c This refers to the three-phase stator current.

[0022] In S2, the formula for coordinate transformation of the three-phase AC voltage of the stator is:

[0023] (2)

[0024] In equation (2), U s_α and U s_β These are the stator voltage components in the αβ coordinate system, respectively; U s_a , U s_b and U s_c The stator three-phase AC voltage; t )express t The value at any given moment.

[0025] The formula for separating the positive and negative sequences of stator voltage components based on the T / 4 delay cancellation method is as follows:

[0026] (3)

[0027] In equation (3), , , , They are respectively U s_α and U s_β Positive and negative order components; U s_α ( t )and U s_β ( t )Delay T / 4 cycles are denoted as U s_α ( tT / 4) and U s_β ( tT / 4), TIndicates the power frequency period.

[0028] Obtaining the measured values ​​of the positive and negative sequence components of the stator voltage along the dq axis through Park transformation involves applying Park transformation to the positive sequence component of the voltage after separation of positive and negative sequences, and applying inverse Park transformation to the negative sequence component. The transformation formula is as follows:

[0029] (4)

[0030] In equation (4), , , , These are the measured values ​​of the positive and negative sequence components of the stator voltage along the dq axis, respectively. θ GFM The angle for generating the active power loop in the network.

[0031] In S2, the formula for coordinate transformation of the rotor three-phase current is:

[0032] (5)

[0033] In equation (5), I r_α and I r_β These are the rotor current components in the αβ coordinate system, respectively; I r_a , I r_b , I r_c For the rotor three-phase current; t )express t The value at any given moment.

[0034] The formula for performing the Park inverse transformation of the rotor current component based on the rotor electrical angle is as follows:

[0035] (6)

[0036] In equation (6), I r_α_m and I r_β_m These are the reduced rotor current components; θ r The rotor electrical angle.

[0037] The formula for separating the positive and negative sequences of the reduced rotor current components based on the T / 4 delay cancellation method is as follows:

[0038] (7)

[0039] In equation (7), , , , They are respectively I r_α_m and I r_β_m Positive and negative order components; I r_α_m ( t )and I r_β_m ( t )Delay T / 4 cycles are denoted as I r_α_m ( tT / 4) and I r_α_m ( tT / 4), T Indicates the power frequency period.

[0040] The measured values ​​of the positive and negative sequence components of the rotor current under the dq axis obtained by Park transformation refer to applying Park transformation to the positive sequence component of the current after separation of positive and negative sequences, and applying inverse Park transformation to the negative sequence component of the current. The transformation formula is as follows:

[0041] (8)

[0042] In equation (8), , , , These are the measured values ​​of the positive and negative sequence components of the rotor current under the dq axis, respectively. θ GFM The angle for generating the active power loop in the network.

[0043] In S3, the formula for calculating the unbalance is as follows:

[0044] (9)

[0045] In formula (9), ɛ U_grid For voltage imbalance, and These are the positive and negative sequence components of the grid voltage along the d-axis, respectively; ɛ I_stator andɛ I_grid These are the unbalance of the stator output current of the doubly-fed induction generator and the unbalance of the output current of the grid-side converter, respectively. and These are the positive and negative sequence components of the stator's d-axis current, respectively. and These are the positive and negative sequence components of the d-axis current on the grid side, respectively.

[0046] In S3, the method for determining whether the grid voltage has experienced a three-phase symmetrical drop or an asymmetrical drop is as follows:

[0047] (10)

[0048] In equation (10), LVRT_B_Flag and LVRT_A_Flag are flags indicating whether the grid voltage has experienced a three-phase symmetrical or asymmetrical drop. When the flag is 1, it indicates that a drop has occurred and has exceeded the undervoltage design threshold. U L_Limit Otherwise, it is 0; U L_Limit As per the lower voltage limit requirement, if this limit is exceeded, low-voltage protection control must be activated; U_max This is the highest threshold for voltage imbalance. When the grid voltage drops below the lower limit and the voltage imbalance also exceeds the highest threshold, the grid voltage is determined to be an asymmetrical drop. U_grid This refers to the voltage imbalance.

[0049] In S3, the specific process of designing the negative sequence current collaborative enable logic and negative sequence current command allocation based on the judgment result, and calculating the given value of the negative sequence current of the dq axis of the grid-side converter is as follows:

[0050] S31. When the grid voltage experiences a three-phase symmetrical drop, the negative sequence current loop of the generator-grid side converter does not start. The doubly fed generator responds to the voltage drop through the reactive power droop link in the grid control loop, providing reactive power support to the grid. When an asymmetrical drop occurs, the negative sequence current loop on the generator-grid side and the generator-side negative sequence current q-axis command and the grid-side negative sequence current q-axis command are coordinated through the grid-side q-axis negative sequence current limit and the low-through negative sequence reactive current demand value.

[0051] S32. When LVRT_B_Flag=LVRT_A_Flag=0, it indicates that the grid voltage has not experienced a low-voltage breakdown. Determine whether the current imbalance between the generator and the grid exceeds the threshold α. I_max , ifɛ I_stator >ɛ I_max or ɛ I_grid >ɛ I_max If so, the corresponding machine-grid side negative sequence current loop is started, and both the machine-side negative sequence current q-axis command and the grid-side negative sequence current q-axis command are 0;

[0052] S33. When LVRT_B_Flag=1, it indicates that the grid voltage has a three-phase symmetrical voltage drop. At this time, the negative sequence current loop on the grid side will not start. When LVRT_A_Flag=1, it indicates that the grid voltage has a three-phase asymmetrical voltage drop. At this time, the current loop enable sequence and negative sequence current command allocation are determined according to the negative sequence current demand, and the negative sequence current setpoint value of the rotor dq shaft is calculated.

[0053] In S31, the formula for calculating the grid-side q-axis negative sequence current limit is as follows:

[0054] (11)

[0055] In equation (11), This is the limit for the negative sequence q-axis current on the grid side; I g_max This refers to the current limit value for the grid-side converter; and These are the measured values ​​of the positive and negative sequence components of the d-axis current, respectively. This is the positive sequence measurement of the q-axis current.

[0056] In S31, the formula for calculating the low-pass negative sequence reactive current demand is:

[0057] (12)

[0058] In equation (12), This is the value of the low-pass negative sequence reactive current requirement; The negative sequence reactive current coefficient; I N This refers to the rated current of the doubly-fed generator unit. This represents the negative sequence component of the grid voltage along the d-axis.

[0059] In S32, the start-up judgment formula for the negative sequence current loop on the grid side is:

[0060] (13)

[0061] In equation (13), and These are the given values ​​for the negative sequence current of the rotor's dq axis, respectively. and These are the dq-axis negative sequence current setpoints for the grid-side converter, respectively; ɛ I_max The threshold for current imbalance on the grid side; ɛ I_stator andɛ I_grid These are the unbalance of the stator output current of the doubly fed motor and the unbalance of the output current of the grid-side converter, respectively.

[0062] In S33, the current loop enable sequence and negative sequence current command allocation are determined based on the negative sequence current requirement, and the formula for the negative sequence current setpoint of the dq axis on the computer network side is as follows:

[0063] (14)

[0064] In equation (14), and These are the given values ​​for the negative sequence current of the rotor's dq axis, respectively. K grid_In A separate flag enable signal is set for the negative sequence current loop of the grid-side converter. K rotor_InA separate flag enable signal is set for the negative sequence current loop of the machine-side converter. When the flag is 0, it means that the negative sequence current loop is disabled, and when it is 1, it means that the negative sequence current loop is enabled. This is the value of the low-pass negative sequence reactive current requirement; This is the limit for the negative sequence q-axis current on the grid side.

[0065] In S4, based on the stator's active power and combined with the simulated rotor motion equations, the control of the grid-connected active power loop is realized, and the calculation formula for the grid-connected active power loop generation angle is output as follows:

[0066] (15)

[0067] In equation (15), θ GFM To construct the active power loop generation angle; P meas The active power of the stator; P ref The active power is a given value, manually set; P f_p This represents the incremental increase in the downward working power. K f_p The active power-frequency droop factor; ω 0 and ω meas These are the rated angular frequency of the power grid and the output angular frequency of the active power loop, respectively; 1 / s represents the integral operation. J The virtual inertia coefficient for the active power loop of the network.

[0068] In S4, based on the stator's reactive power and combined with the simulated reactive power-voltage droop equation, the control of the grid reactive power loop is realized, and the calculation formula for the positive sequence setpoint of the d-axis stator voltage is as follows:

[0069] (16)

[0070] In equation (16), U s_ref and U 0 represents the positive sequence setpoint of the d-axis stator voltage and the rated stator voltage, respectively. K u_q This is the reactive power-voltage droop factor; Q meas The reactive power of the stator; Q ref The reactive power setpoint is manually set.

[0071] In S4, the calculation formula for the stator dq-axis voltage orientation in the grid voltage loop and the output rotor dq-axis positive sequence current setpoint is achieved by PI control of the measured stator voltage positive sequence component.

[0072] (17)

[0073] In equation (17), , , , These are the given and measured values ​​of the positive and negative sequence voltages of the stator dq axis, respectively; K p_sd , K i_sd , K p_sq , K i_sq These are the PI parameters for the positive and negative sequence voltage control loops along the dq axis, respectively. and These are the given values ​​for the positive sequence current of the dq axis rotor, respectively.

[0074] In S4, the negative sequence current in the grid current loop is suppressed by separately controlling the positive and negative sequence current setpoints of the rotor dq axis, and the calculation formula for the reference values ​​of the positive and negative sequence voltages of the rotor dq axis is as follows:

[0075] (18)

[0076] In equation (18), , , , These are the reference values ​​for the positive and negative sequence voltages of the rotor's dq axis, respectively. , , and are the given and measured values ​​of the negative sequence current of the rotor dq shaft, respectively; and These are the measured values ​​of the positive sequence current along the dq axis, respectively. , , and These are the PI control parameters for the positive and negative sequence current loops along the d-axis, respectively. , , and These are the PI control parameters for the positive and negative sequence current loops on the q-axis, respectively, and S represents the integral operation.

[0077] A doubly-fed grid-connected unit positive and negative sequence current coordinated control system includes:

[0078] The data acquisition unit is used to acquire the stator three-phase AC voltage, stator three-phase current, rotor three-phase current and rotor electrical angle of the doubly fed motor, and to calculate the active power and reactive power of the stator.

[0079] The coordinate transformation and positive / negative sequence separation unit is used to perform coordinate transformation on the stator three-phase AC voltage and the rotor three-phase current respectively to obtain the stator voltage component and rotor current component in the αβ coordinate system. Then, the rotor current component is subjected to Park inverse transformation according to the rotor electrical angle to reduce the rotor current component to the power frequency or the same frequency as the stator current. After that, the positive and negative sequence of the stator voltage component and the reduced rotor current component are separated based on the T / 4 delay cancellation method. The measured values ​​of the stator voltage positive and negative sequence components and the rotor current positive and negative sequence components in the dq axis are obtained by Park transformation.

[0080] The negative sequence current collaborative enable logic design and negative sequence current command allocation unit are used to determine whether the grid voltage has experienced a three-phase symmetrical drop or an asymmetrical drop based on the detection results of the three-phase current imbalance and grid voltage imbalance. Based on the judgment results, the negative sequence current collaborative enable logic and negative sequence current command allocation of the generator-grid side converter are designed, and the negative sequence current setpoint of the rotor dq axis is calculated.

[0081] The network loop control unit is used to control the active power loop of the network based on the stator's active power and the simulated rotor motion equation, and outputs the generation angle of the active power loop. It is also used to control the reactive power loop of the network based on the stator's reactive power and the simulated reactive power-voltage droop equation, and outputs the d-axis stator voltage positive sequence setpoint. Furthermore, based on the measured values ​​of the stator voltage positive sequence component under the dq axis and the d-axis stator voltage positive sequence setpoint, it uses PI control of the measured values ​​of the stator voltage positive sequence component to orient the stator dq axis voltage in the network voltage loop and outputs the rotor dq axis positive sequence current setpoint. Finally, based on the measured values ​​of the rotor current positive and negative sequence components under the dq axis, the rotor dq axis positive sequence current setpoint, and the rotor dq axis negative sequence current setpoint, it uses separate PI control of the rotor dq axis positive and negative sequence current setpoints to suppress the negative sequence current in the network current loop and outputs reference values ​​for the rotor dq axis positive and negative sequence voltages.

[0082] Positive and negative sequence reference voltages are synthesized and a rotor reference voltage unit is generated. This unit is used to perform αβ coordinate system transformation, merging, reduction and abc coordinate system transformation on the positive and negative sequence voltage reference values ​​of the rotor dq axis in sequence to obtain the modulation voltage reference value of the rotor-side converter. Based on the modulation voltage reference value, a PWM wave is generated to realize the closed-loop control of the doubly fed grid unit.

[0083] Compared with the prior art, the beneficial effects of the present invention are:

[0084] 1. This invention is suitable for the grid-connected control of doubly-fed induction generator (DFIG) units and exhibits strong adaptability to positive and negative sequence separation. By innovatively combining the T / 4 period delay cancellation method with the DFIG grid-connected control loop, and particularly by normalizing the rotor current component to the power frequency or the stator current frequency, it overcomes the technical bottleneck of the traditional T / 4 delay method, which is only applicable to fixed-frequency equipment and cannot adapt to the non-power frequency rotor current caused by the rotor speed changing with wind speed. This invention perfectly adapts to the variable-speed operation characteristics of DFIG units. Furthermore, compared to traditional positive and negative sequence separation schemes that rely on multiple integral operations, this invention eliminates the need for complex integration and direction-locking calculations. Accurate separation can be achieved through simple addition and subtraction operations and coordinate transformations, significantly reducing the system's computational burden and saving computational resources. It also boasts advantages such as low latency and rapid response, effectively avoiding separation errors caused by frequency mismatch, ensuring grid-connected control accuracy and system stability, and significantly improving the operating performance of DFIG units under weak grid conditions and asymmetrical operating conditions.

[0085] 2. The grid control of this invention has excellent compatibility, achieving seamless integration of the T / 4 delay method and the grid control algorithm. Without significantly modifying the core architecture of the doubly-fed generator grid control, it endows the grid algorithm with independent control of positive and negative sequence currents and negative sequence suppression capabilities through simple additional calculations. Compared to the limitations of traditional grid control, which can only handle symmetrical operating conditions, it eliminates the need for additional complex control modules, retaining the advantages of simple structure and ease of implementation of the original grid control. Simultaneously, it expands the adaptability of grid control under asymmetrical grid operating conditions, enabling the doubly-fed generator to reduce the interaction with weak grid impedance and improve system stability through grid control, while also meeting the national standard requirements for negative sequence control under asymmetrical operating conditions.

[0086] 3. This invention achieves high efficiency in grid-machine side coordination and maximizes negative-sequence support capability. By specifically designing the enable logic, coordination timing, and instruction optimization allocation method for the negative-sequence current loop of the grid-machine side converter of the doubly-fed generator unit, it effectively solves the problems in existing technologies where simultaneous activation of the grid-machine side negative-sequence current loop easily leads to output fluctuations and deterioration of power quality, and the lack of a clear coordination strategy results in the underutilization of negative-sequence support capability. Through an on-demand enable mechanism and optimized allocation based on the differences in current capacity and dynamic characteristics of the grid-machine side converters, precise coordinated control of the grid-machine side negative-sequence current loop is achieved. This avoids control fluctuations caused by system state changes and maximizes the overall negative-sequence current support capability of the doubly-fed system, significantly improving the unit's support performance under three-phase unbalanced or asymmetrical grid faults, and enhancing the overall operational reliability and adaptability of the doubly-fed generator unit under asymmetrical grid conditions.

[0087] 4. The improved T / 4 delay cancellation method of this invention eliminates the multiple integration operations and direction-locking operations in the traditional positive-negative sequence separation scheme, retaining only simple addition and subtraction operations and basic coordinate transformations. This forms a specific technical connection with the embedded hardware architecture of the doubly-fed generator control system. From a hardware execution perspective, this algorithm can directly adapt to the characteristics of the controller's CPU's computing unit, avoiding the high hardware computing power consumption of complex integration, significantly reducing the CPU's computing load and instruction execution cycle. This allows the hardware to simultaneously handle multiple tasks such as network control, positive-negative sequence separation, and machine-network side collaborative control, solving the hardware processing bottleneck problem caused by the excessive computing power requirements of traditional algorithms. This achieves a substantial improvement in hardware computing efficiency, meeting the technical requirements for improving the internal performance of computer systems.

[0088] 5. The algorithm employed in this invention is simple, requiring no storage of complex intermediate computation data. It only needs to cache the basic current signal and delay period parameters, significantly reducing data storage requirements. Simultaneously, the simplified computational logic reduces the frequency of data interaction, alleviating the load on the hardware transmission link and avoiding control synchronization issues caused by transmission delays. This algorithm design's adaptability to hardware storage and transmission structures enables efficient utilization of hardware resources, reduces unnecessary hardware resource consumption, and further improves the overall system operating efficiency.

[0089] 6. The low latency and lack of complex iterations of the algorithm in this invention are highly compatible with the real-time processing architecture of the controller hardware, shortening the entire chain time from current signal acquisition to control command output and improving hardware processing speed. It meets the stringent requirements for the real-time response capability of the control system hardware under the dynamic changes in rotor current frequency during the variable-speed operation of doubly-fed generator units. Compared to the command lag caused by dynamic response delays in traditional algorithms, this invention enables the hardware to quickly track rotor frequency changes and promptly adjust the positive and negative sequence separation parameters and the coordinated commands from the grid side, avoiding control deviations caused by untimely hardware response. This ensures accurate execution of control commands under scenarios such as wind speed fluctuations and sudden changes in grid operating conditions, significantly optimizing the hardware execution effect.

[0090] 7. The on-demand enabling logic and optimized allocation strategy of the negative sequence current loop on the grid side designed in this invention not only solves the control coordination problem at the software level, but also forms a technical linkage with the hardware topology and current capacity characteristics of the grid-side converter. By avoiding output fluctuations caused by simultaneous activation of the negative sequence current loop on both the grid and mechatronics sides, it reduces frequent start-stop and current surges of hardware switching devices, lowers hardware losses and heat accumulation, and extends the service life of the converter power modules. Simultaneously, the stable control logic can prevent abnormal states such as overcurrent and overvoltage caused by sudden changes in operating conditions, reducing the risk of hardware failure and improving the long-term operational stability of the doubly-fed generator system from a hardware reliability perspective. Attached Figure Description

[0091] Figure 1This is a flowchart of the present invention;

[0092] Figure 2 The diagram shows the positive and negative sequence separation control of a doubly fed grid unit based on the T / 4 delay cancellation method.

[0093] Figure 3 Here is the flowchart for S3;

[0094] Figure 4 This is a system block diagram of the present invention;

[0095] Figure 5 The experimental waveforms show a comparison between the positive and negative order separation method of this invention and the conventional double second-order generalized integrator (DSOGI).

[0096] Figure 6 This is a comparison curve of the negative sequence current response of the present invention and conventional methods under the condition of asymmetrical voltage drop in the power grid. Detailed Implementation

[0097] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0098] Example 1

[0099] See Figures 1-2 This embodiment provides a method for coordinated control of positive and negative sequence currents in a doubly-fed grid-connected turbine unit. This method is applicable to various doubly-fed converter applications, such as doubly-fed flywheel energy storage systems, doubly-fed wind turbine units, and doubly-fed pumped storage. It includes five steps: data acquisition, coordinate transformation and positive / negative sequence separation, negative sequence current coordinated enable logic design and negative sequence current command allocation, grid-connected loop control, and synthesis of positive and negative sequence reference voltages to generate a rotor reference voltage. The specific implementation flow of each step is as follows:

[0100] S1. Data Acquisition

[0101] Collect the stator three-phase AC voltage, stator three-phase current, rotor three-phase current, and rotor electrical angle of the doubly fed motor, and calculate the active power and reactive power of the stator based on the collected stator three-phase AC voltage and stator three-phase current.

[0102] The formulas for calculating the active power and reactive power of the stator are as follows:

[0103] (1)

[0104] In equation (1), P meas andQ meas These are the active power and reactive power of the stator, respectively. U s_a , U s_b and U s_c This refers to the three-phase AC voltage of the stator. I s_a , I s_b and I s_c This refers to the three-phase stator current.

[0105] S2. Coordinate transformation and separation of positive and negative order

[0106] First, coordinate transformations are performed on the stator three-phase AC voltage and rotor three-phase current to convert them from the abc coordinate system to the αβ coordinate system, obtaining the stator voltage components and rotor current components in the αβ coordinate system. Since the rotor speed changes in real time, to adapt it to the T / 4 delay cancellation method, a Park inverse transformation is also needed on the rotor current components according to the rotor electrical angle, reducing the rotor current components to the power frequency or the frequency consistent with the stator current. Then, based on the T / 4 delay cancellation method, the positive and negative sequence separations of the stator voltage components and the reduced rotor current components are performed, and the measured values ​​of the positive and negative sequence components of the stator voltage and rotor current under the dq axis are obtained through the Park transformation.

[0107] The specific processing procedure for the three-phase AC voltage of the stator in this step is as follows:

[0108] First of all, t The stator three-phase AC voltage at time t is transformed into the αβ coordinate system. After the transformation, the stator voltage components in the αβ coordinate system are obtained. The transformation formula is as follows:

[0109] (2)

[0110] In equation (2), U s_α and U s_β These are the stator voltage components in the αβ coordinate system, respectively; U s_a , U s_b and U s_c The stator three-phase AC voltage; t )express t The value at any given moment.

[0111] Secondly, the stator voltage components are separated into positive and negative sequences based on the T / 4 delay cancellation method, resulting in the positive and negative sequence components of the stator voltage components. The calculation formula for the positive and negative sequence separation is as follows:

[0112] (3)

[0113] In equation (3), , , , They are respectively U s_α and U s_β Positive and negative order components; U s_α ( t )and U s_β ( t )Delay T / 4 cycles are denoted as U s_α ( tT / 4) and U s_β ( tT / 4), T This indicates the power frequency cycle, typically 50Hz.

[0114] Furthermore, obtaining the measured values ​​of the stator voltage positive and negative sequence components under the dq axis through Park transformation involves applying Park transformation to the positive sequence component of the voltage after separation of positive and negative sequences, and applying inverse Park transformation to the negative sequence component. The transformation formula is as follows:

[0115] (4)

[0116] In equation (4), , , , , These are the measured values ​​of the positive and negative sequence components of the stator voltage along the dq axis, respectively. θ GFM The angle for generating the active power loop in the network.

[0117] The specific processing procedure for the rotor three-phase current in this step is as follows:

[0118] First of all, t The rotor three-phase current at time t (abc coordinate system) is transformed into the αβ coordinate system. After the transformation, the rotor current components in the αβ coordinate system are obtained. The transformation formula is as follows:

[0119] (5)

[0120] In equation (5),I r_α and I r_β These are the rotor current components in the αβ coordinate system, respectively; I r_a , I r_b , I r_c For the rotor three-phase current; t )express t The value at any given moment.

[0121] Secondly, since the rotor speed changes in real time, in order to adapt it to the T / 4 delay cancellation method, it is also necessary to perform a Park inverse transformation on the rotor current component according to the rotor electrical angle, and reduce it to the power frequency or the frequency consistent with the stator current. The reduction formula is as follows:

[0122] (6)

[0123] In equation (6), I r_α_m and I r_β_m These are the reduced rotor current components; θ r The rotor electrical angle.

[0124] Finally, based on the T / 4 delay cancellation method, the positive and negative sequence components of the reduced rotor current are separated to obtain the positive and negative sequence components of the rotor current. The calculation formula for the positive and negative sequence separation is as follows:

[0125] (7)

[0126] In equation (7), , , , They are respectively I r_α_m and I r_β_m Positive and negative order components; I r_α_m ( t )and I r_β_m ( t )Delay T / 4 cycles are denoted as I r_α_m ( tT / 4) and I r_α_m ( tT / 4), T Indicates the power frequency period.

[0127] Furthermore, obtaining the measured values ​​of the positive and negative sequence components of the rotor current under the dq axis through Park transformation involves applying Park transformation to the positive sequence component of the current after separation of positive and negative sequences, and applying inverse Park transformation to the negative sequence component of the current. The transformation formula is as follows:

[0128] (8)

[0129] In equation (8), , , , These are the measured values ​​of the positive and negative sequence components of the rotor current under the dq axis, respectively. θ GFM The angle for generating the active power loop in the network.

[0130] S3. Negative-sequence current collaborative enable logic design and negative-sequence current instruction allocation

[0131] First, based on the detection results of three-phase current imbalance and grid voltage imbalance, it is determined whether the grid voltage has experienced a three-phase symmetrical drop or an asymmetrical drop.

[0132] The formula for calculating the degree of imbalance is as follows:

[0133] (9)

[0134] In formula (9), ɛ U_grid For voltage imbalance, and These are the positive and negative sequence components of the grid voltage along the d-axis, respectively; ɛ I_stator andɛ I_grid These are the unbalance of the stator output current of the doubly-fed induction generator and the unbalance of the output current of the grid-side converter, respectively. and These are the positive and negative sequence components of the stator's d-axis current, respectively. and These represent the positive and negative sequence components of the d-axis current on the grid side, respectively; where, , , and The control of the grid-side converter is also obtained by separating the positive and negative sequences using the T / 4 delay cancellation method, which will not be elaborated further.

[0135] When determining whether a three-phase symmetrical or asymmetrical voltage drop has occurred based on the test results, first set a flag indicating whether a three-phase symmetrical or asymmetrical voltage drop has occurred, and then combine this with the current imbalance and voltage imbalance for judgment. The specific judgment method is as follows:

[0136] (10)

[0137] In equation (10), LVRT_B_Flag and LVRT_A_Flag are flags indicating whether the grid voltage has experienced a three-phase symmetrical or asymmetrical drop. When the flag is 1, it indicates that a drop has occurred and has exceeded the undervoltage design threshold. U L_Limit Otherwise, it is 0; U L_Limit As per the lower voltage limit requirement, if this limit is exceeded, low-voltage protection control must be activated; U_max This is the highest threshold for voltage imbalance. When the grid voltage drops below the lower limit and the voltage imbalance also exceeds the highest threshold, the grid voltage is determined to be an asymmetrical drop. U_grid This refers to the voltage imbalance.

[0138] Secondly, based on the judgment results, the negative sequence current collaborative enable logic and negative sequence current command allocation for the grid-connected converter are designed, and the rotor dq-axis negative sequence current setpoint value is calculated. The specific process is as follows:

[0139] S31. When the grid voltage experiences a three-phase symmetrical drop, the negative sequence current loop of the generator-grid side converter does not start. The doubly fed generator responds to the voltage drop through the reactive power droop link in the grid control loop, providing reactive power support to the grid. When an asymmetrical drop occurs, the negative sequence current loop on the generator-grid side and the generator-side negative sequence current q-axis command and the grid-side negative sequence current q-axis command are coordinated through the grid-side q-axis negative sequence current limit and the low-through negative sequence reactive current demand value.

[0140] The formula for calculating the grid-side q-axis negative sequence current limit is as follows:

[0141] (11)

[0142] In equation (11), This is the limit for the negative sequence q-axis current on the grid side. I g_max This refers to the current limit value for the grid-side converter; and These are the measured values ​​of the positive and negative sequence components of the d-axis current, respectively. This is the positive sequence measurement of the q-axis current.

[0143] The formula for calculating the low-pass negative sequence reactive current demand is:

[0144] (12)

[0145] In equation (12), This is the value of the low-pass negative sequence reactive current requirement; The negative sequence reactive current coefficient; I N This refers to the rated current of the doubly-fed generator unit. This represents the negative sequence component of the grid voltage along the d-axis.

[0146] S32. When LVRT_B_Flag=LVRT_A_Flag=0, it indicates that the grid voltage has not experienced a low-voltage breakdown. At this time, it is necessary to determine whether the current imbalance between the generator and the grid exceeds the threshold α. I_max , ifɛ I_stator >ɛ I_max or ɛ I_grid >ɛ I_max If so, the corresponding negative sequence current loop on the machine-grid side will be activated, and both the negative sequence current q-axis command on the machine side and the negative sequence current q-axis command on the grid side will be 0.

[0147] The start-up judgment formula for the negative sequence current loop on the grid side is as follows:

[0148] (13)

[0149] In equation (13), and These are the given values ​​for the negative sequence current of the rotor's dq axis, respectively. and These are the dq-axis negative sequence current setpoints for the grid-side converter, respectively; ɛ I_max The threshold for current imbalance on the grid side; ɛ I_stator andɛ I_grid These are the unbalance of the stator output current of the doubly fed motor and the unbalance of the output current of the grid-side converter, respectively.

[0150] S33. When LVRT_B_Flag=1, it indicates that the grid voltage has a three-phase symmetrical voltage drop. At this time, the negative sequence current loop on the grid side will not start. When LVRT_A_Flag=1, it indicates that the grid voltage has a three-phase asymmetrical voltage drop. At this time, the current loop enable sequence and negative sequence current command allocation are determined according to the negative sequence current demand, and the negative sequence current setpoint value of the rotor dq shaft is calculated.

[0151] The formula for determining the current loop enable sequence and negative sequence current command allocation based on the negative sequence current requirement, and the formula for setting the negative sequence current value of the dq axis on the computer network side, is as follows:

[0152] (14)

[0153] In equation (14), and These are the given values ​​for the negative sequence current of the rotor's dq axis, respectively. K grid_In A separate flag enable signal is set for the negative sequence current loop of the grid-side converter. K rotor_InA separate flag enable signal is set for the negative sequence current loop of the machine-side converter. When the flag is 0, it means that the negative sequence current loop is disabled, and when it is 1, it means that the negative sequence current loop is enabled. This is the value of the low-pass negative sequence reactive current requirement; This is the limit for the negative sequence q-axis current on the grid side.

[0154] See Figure 3 In practice, this step requires initializing the variables, enable signals, and flags, including setting a threshold for the low-breakdown voltage of the variables. U L_Limit The highest threshold for voltage imbalance ɛ U_max and the highest threshold value of current imbalance ɛ I_max Assignment, set the flag bit and negative sequence loop enable signal to zero: Set K grid_In = K rotor_In =LVRT_B_Flag=LVRT_A_Flag=0; then calculate the grid voltage imbalance ɛ in real time. U_grid The unbalance of the stator output current of a doubly-fed motor ɛ I_stator and the imbalance of the output current of the grid-side converter ɛ I_grid The system calculates the flags LVRT_B_Flag and LVRT_A_Flag based on real-time voltage, current, and unbalance. Then, based on the calculation results, when LVRT_B_Flag = LVRT_A_Flag = 0, it indicates that the grid voltage has not dropped or the drop has not exceeded the threshold. At this time, the system determines whether to enable the negative sequence current loop of the grid-side converter based on the balance of the stator current of the doubly-fed induction generator and the output current of the grid-side converter. If enabled, the corresponding dq-axis negative sequence current setpoints are all 0. If LVRT_B_Flag = 1, it indicates that the grid voltage has experienced a symmetrical drop, and the negative sequence current loop is disabled. If LVRT_A_Flag = 1, it indicates that the grid voltage has experienced an asymmetrical drop, and the system determines whether to enable the negative sequence current loop based on the maximum current limit of the grid-side converter. and low-pass negative sequence reactive current demand value Make a comprehensive judgment when > At that time, only the negative sequence current loop of the grid-side converter is activated, the d-axis current setpoint of the negative sequence current loop is 0, and the setpoint of the negative sequence q-axis current loop is [value missing]. .when < At the same time, the grid-side negative sequence current control loop is enabled, and the grid-side converter q-axis negative sequence current command is... The negative sequence current command for the q-axis of the rotor-side converter is: - .

[0155] S4. Network Loop Control

[0156] Grid loop control includes the control of the grid active power loop, grid reactive power loop, grid voltage loop, and grid current loop.

[0157] The control process for the active power loop of the power grid is as follows: Based on the active power of the stator, the control of the active power loop is realized by combining the simulated rotor motion equation, and the generation angle of the active power loop is output. Its calculation formula is (to simplify the description, (t) is omitted from subsequent variable characters):

[0158] (15)

[0159] In equation (15), θ GFM To construct the active power loop generation angle; P meas The active power of the stator; P ref The active power is a given value, manually set; P f_p This represents the incremental increase in the downward working power. K f_p The active power-frequency droop factor; ω 0 and ω meas These are the rated angular frequency of the power grid and the output angular frequency of the active power loop, respectively; 1 / s represents the integral operation. J The virtual inertia coefficient for the active power loop of the network.

[0160] It should be noted that the calculation in equation (15) θ GFM In equation (8) applied to S2, due to the delay in the actual controller, the coordinate transformation used above... θ GFM This is the result of the calculation of formula (15) in the previous running cycle, i.e., S4.

[0161] The control process for the reactive power loop in the power grid is as follows: Based on the stator reactive power and combined with the simulated reactive power-voltage droop equation, the control of the reactive power loop is realized, and the positive sequence setpoint of the stator voltage on the d-axis is output. The calculation formula is:

[0162] (16)

[0163] In equation (16), U s_ref and U 0 represents the positive sequence setpoint of the d-axis stator voltage and the rated stator voltage, respectively. K u_q This is the reactive power-voltage droop factor; Q measThe reactive power of the stator; Q ref The reactive power setpoint is manually set.

[0164] The control flow for the grid voltage loop is as follows: Based on the measured values ​​of the stator voltage positive sequence components along the dq axis and the given value of the stator voltage positive sequence along the d axis, PI control of the measured values ​​of the stator voltage positive sequence components is used to orient the stator dq axis voltage in the grid voltage loop and output the given value of the rotor dq axis positive sequence current. The calculation formula is:

[0165] (17)

[0166] In equation (17), , , and are the given and measured values ​​of the positive and negative sequence voltages of the stator dq axis, respectively; K p_sd , K i_sd , K p_sq , K i_sq These are the PI parameters for the positive and negative sequence voltage control loops along the dq axis, respectively. and These are the given values ​​for the positive sequence current of the dq axis rotor, respectively.

[0167] The control process for the grid current loop is as follows: Based on the measured values ​​of the positive and negative sequence components of the rotor current under the dq axis, the given values ​​of the positive and negative sequence currents of the rotor dq axis, the negative sequence current in the grid current loop is suppressed by individually controlling the given values ​​of the positive and negative sequence currents of the rotor dq axis, and reference values ​​of the positive and negative sequence voltages of the rotor dq axis are output. The calculation formula is as follows:

[0168] (18)

[0169] In equation (18), , , , These are the reference values ​​for the positive and negative sequence voltages of the rotor's dq axis, respectively. , , , These are the given and measured values ​​of the negative sequence current of the rotor dq shaft, respectively. and These are the measured values ​​of the positive sequence current along the dq axis, respectively. , , and These are the PI control parameters for the positive and negative sequence current loops along the d-axis, respectively. , , and These are the PI control parameters for the positive and negative sequence current loops on the q-axis, respectively, and S represents the integral operation.

[0170] S5. Synthesize positive and negative sequence reference voltages to generate rotor reference voltage.

[0171] The positive and negative sequence voltage reference values ​​of the rotor dq axis are sequentially transformed into the αβ coordinate system, merged, reduced, and transformed into the abc coordinate system to obtain the modulation voltage reference value of the rotor-side converter. A PWM wave is then generated based on the modulation voltage reference value to achieve closed-loop control of the doubly-fed grid-connected unit. The specific process is as follows:

[0172] After outputting the reference values ​​of the positive and negative sequence voltages of the rotor dq axis from the grid current loop, these values ​​are transformed into the αβ coordinate system to obtain the positive and negative sequence components in the αβ coordinate system. The transformation formula is as follows:

[0173] (19)

[0174] In equation (19), , , , These are the positive and negative sequence components of the rotor dq axis voltage reference values ​​in the αβ coordinate system.

[0175] Then, the positive and negative sequence components obtained from equation (19) are combined to obtain the rotor voltage reference value in the αβ coordinate system. The combined formula is as follows:

[0176] (20)

[0177] In equation (20), U r_α_ref and U r_β_ref These are the rotor voltage reference values ​​in the αβ coordinate system, respectively.

[0178] Since the aforementioned coordinate transformation reduced the rotor current frequency to be consistent with the power frequency, it is also necessary to convert it back to the original rotor electrical angle, as shown below:

[0179] (twenty one)

[0180] In equation (21), U r_α_θr and U r_β_θr These are the rotor voltage reference values ​​under rotor electrical angle and in the αβ coordinate system, respectively; U r_α_ref and U r_β_ref These are the rotor voltage reference values ​​in the αβ coordinate system, respectively.

[0181] By converting the rotor voltage reference value under the rotor electrical angle and in the αβ coordinate system to the abc coordinate system, the modulation voltage reference value of the rotor-side converter can be obtained. The conversion formula is as follows:

[0182] (twenty two)

[0183] In equation (22), U r_a_ref , U r_b_ref and U r_c_ref These are the modulation voltage reference values ​​for the rotor-side converter in the ADC coordinate system.

[0184] Finally, the modulation voltage reference value of the rotor-side converter is calculated using SPWM or SVPWM to generate a PWM wave, thereby realizing the closed-loop control of the doubly-fed grid unit.

[0185] Example 2

[0186] See Figure 4 This embodiment provides a coordinated control system for positive and negative sequence currents of a doubly-fed grid-connected unit, including:

[0187] The data acquisition unit is used to collect the stator three-phase AC voltage, stator three-phase current, rotor three-phase current and rotor electrical angle of the doubly fed motor, and to calculate the active power and reactive power of the stator.

[0188] The coordinate transformation and positive / negative sequence separation unit is used to perform coordinate transformation on the stator three-phase AC voltage and the rotor three-phase current respectively to obtain the stator voltage component and rotor current component in the αβ coordinate system. Then, the rotor current component is subjected to Park inverse transformation according to the rotor electrical angle to reduce the rotor current component to the power frequency or the same frequency as the stator current. After that, the positive and negative sequence of the stator voltage component and the reduced rotor current component are separated based on the T / 4 delay cancellation method, and the measured values ​​of the stator voltage positive and negative sequence components and the rotor current positive and negative sequence components in the dq axis are obtained through Park transformation.

[0189] The negative sequence current collaborative enable logic design and negative sequence current command allocation unit are used to determine whether the grid voltage has experienced a three-phase symmetrical drop or asymmetrical drop based on the detection results of the three-phase current imbalance and grid voltage imbalance. Based on the judgment results, the negative sequence current collaborative enable logic and negative sequence current command allocation of the generator-grid side converter are designed, and the negative sequence current setpoint of the rotor dq axis is calculated.

[0190] The network loop control unit is used to control the active power loop of the network based on the stator's active power and the simulated rotor motion equation, and outputs the generation angle of the active power loop. It is also used to control the reactive power loop of the network based on the stator's reactive power and the simulated reactive power-voltage droop equation, and outputs the d-axis stator voltage positive sequence setpoint. Furthermore, based on the measured values ​​of the stator voltage positive sequence component under the dq axis and the d-axis stator voltage positive sequence setpoint, it uses PI control of the measured values ​​of the stator voltage positive sequence component to orient the stator dq axis voltage in the network voltage loop and outputs the rotor dq axis positive sequence current setpoint. Finally, based on the measured values ​​of the rotor current positive and negative sequence components under the dq axis, the rotor dq axis positive sequence current setpoint, and the rotor dq axis negative sequence current setpoint, it uses separate PI control of the rotor dq axis positive and negative sequence current setpoints to suppress the negative sequence current in the network current loop and outputs reference values ​​for the rotor dq axis positive and negative sequence voltages.

[0191] Positive and negative sequence reference voltages are synthesized and a rotor reference voltage unit is generated. This unit is used to perform αβ coordinate system transformation, merging, reduction and abc coordinate system transformation on the positive and negative sequence voltage reference values ​​of the rotor dq axis in sequence to obtain the modulation voltage reference value of the rotor-side converter. Based on the modulation voltage reference value, a PWM wave is generated to realize the closed-loop control of the doubly fed grid unit.

[0192] In detail, based on the same innovative concept, each module in the system described in this embodiment adopts the same technical means as in Embodiment 1 when used, and can produce the same technical effect, which will not be repeated here.

[0193] Example 3

[0194] This embodiment tested the method provided in Embodiment 1. A 1.5MW doubly-fed wind turbine model was built based on the YuanKuan semi-physical simulation platform. The main parameters are as follows: grid voltage and frequency: 690V, 50Hz; wind turbine rated power: 1.5MW; terminal voltage: 690V; number of pole pairs: 3; main parameters of the doubly-fed motor: stator resistance: 0.03pu; rotor resistance: 0.006pu; stator leakage inductance: 0.04pu; rotor leakage inductance: 0.005pu; DC voltage: 1150V; the main control loop parameters are as follows: the generator-side converter control parameters include the grid active power loop virtual inertia parameter of 2kg*m2, and the reactive power loop PI parameters of 7 and 1.2 respectively; the grid voltage loop parameters of 20 and 1 respectively; the generator-side positive and negative sequence current loop PI parameters are the same, of 5 and 2.5 respectively; the grid-side converter DC voltage loop PI parameters are 400 and 8, and the grid-side positive and negative sequence current loop PI parameters are the same, of 7 and 2.6 respectively.

[0195] Based on the above parameters, the control method provided in Example 1 and the conventional method were run, and the results are as follows: Figure 5 , 6 As shown, where,

[0196] Figure 5 The diagram shows a comparative experimental waveform of the positive and negative sequence separation method of the present invention with that of a conventional dual second-order generalized integrator (DSOGI). The voltage drop of phase A is set to 0.6 pu. Figure 5 As shown, the method of the present invention can quickly separate the positive and negative sequence components at the moment of voltage drop, which is 10ms faster than the conventional DSOGI method, thus verifying the effectiveness of the method of the present invention.

[0197] Figure 6 The diagram shows a comparison of the negative sequence current response curves of the method of this invention and the conventional method (which only uses grid-side converters for negative sequence current support) under grid voltage asymmetry drop conditions. It can be seen that the conventional method, relying solely on grid-side negative sequence current supplementation, has a negative sequence current support capability of only about 0.2 pu, while the method of this invention, through coordination between the grid and machine-side converters and the design of the logic enable sequence, can achieve a negative sequence current of about 0.6 pu. Compared to the conventional method, this invention effectively improves the controllability of negative sequence current, which is more conducive to stable system operation.

[0198] The above description is merely a specific embodiment of the present invention. Any feature disclosed in this specification may be replaced by other equivalent or similar features unless otherwise specified. All features or steps in the disclosed methods or processes may be combined in any way, except for mutually exclusive features and / or steps.

Claims

1. A method for coordinated control of positive and negative sequence currents in a doubly-fed grid-connected generating unit, characterized in that... Includes the following steps: S1. Data Acquisition: Collect the stator three-phase AC voltage, stator three-phase current, rotor three-phase current and rotor electrical angle of the doubly fed motor, and calculate the active power and reactive power of the stator; S2. Coordinate transformation and separation of positive and negative order: Coordinate transformations were performed on the three-phase AC voltage of the stator and the three-phase current of the rotor to obtain the stator voltage components and rotor current components in the αβ coordinate system. The rotor current components were then subjected to an inverse Park transformation according to the rotor electrical angle to reduce the rotor current components to the power frequency or the frequency consistent with the stator current. Subsequently, the positive and negative sequences of the stator voltage components and the reduced rotor current components were separated based on the T / 4 delay cancellation method. The measured values ​​of the positive and negative sequence components of the stator voltage and the rotor current in the dq axis were obtained by Park transformation. S3. Negative sequence current co-enabling logic design and negative sequence current instruction allocation: Based on the detection results of three-phase current imbalance and grid voltage imbalance, it is determined whether the grid voltage has experienced a three-phase symmetrical drop or an asymmetrical drop. Based on the judgment results, the negative sequence current collaborative enable logic and negative sequence current command allocation of the generator-grid side converter are designed, and the negative sequence current setpoint value of the rotor dq axis is calculated. S4. Network loop control: Based on the stator's active power, the active power loop of the grid is controlled by combining the simulated rotor motion equation, and the generation angle of the active power loop is output. Based on the stator's reactive power, the reactive power loop of the grid is controlled by combining the simulated reactive power-voltage droop equation, and the positive sequence setpoint of the stator voltage on the d-axis is output. Based on the measured value of the positive sequence component of the stator voltage on the dq-axis and the positive sequence setpoint of the stator voltage on the d-axis, the stator dq-axis voltage orientation in the grid voltage loop is realized by PI control of the measured value of the positive sequence component of the stator voltage, and the positive sequence current setpoint of the rotor dq-axis is output. Based on the measured values ​​of the positive and negative sequence components of the rotor current under the dq axis, the given values ​​of the positive and negative sequence currents of the rotor dq axis, the negative sequence current in the grid current loop is suppressed by performing separate PI control on the given values ​​of the positive and negative sequence currents of the rotor dq axis, and the reference values ​​of the positive and negative sequence voltages of the rotor dq axis are output. S5. Positive and negative sequence reference voltages are synthesized to generate rotor reference voltage: The positive and negative sequence voltage reference values ​​of the rotor dq axis are sequentially transformed into the αβ coordinate system, merged, reduced, and transformed into the abc coordinate system to obtain the modulation voltage reference value of the rotor-side converter. Based on the modulation voltage reference value, a PWM wave is generated to realize the closed-loop control of the doubly fed grid unit.

2. The method for coordinated control of positive and negative sequence currents in a doubly-fed grid-connected unit according to claim 1, characterized in that: In S1, the formulas for calculating the active power and reactive power of the stator are as follows: （1） In equation (1), P meas and Q meas These are the active power and reactive power of the stator, respectively. U s_a , U s_b and U s_c This refers to the three-phase AC voltage of the stator. I s_a , I s_b and I s_c This refers to the three-phase stator current.

3. The method for coordinated control of positive and negative sequence currents in a doubly-fed grid-connected unit according to claim 1, characterized in that: In S2, the formula for coordinate transformation of the three-phase AC voltage of the stator is: （2） In equation (2), U s_α and U s_β These are the stator voltage components in the αβ coordinate system, respectively; U s_a , U s_b and U s_c The stator three-phase AC voltage; t )express t The value at time; The formula for separating the positive and negative sequences of stator voltage components based on the T / 4 delay cancellation method is as follows: （3） In equation (3), , , , They are respectively U s_α and U s_β Positive and negative order components; U s_α ( t )and U s_β ( t )Delay T / 4 cycles are denoted as U s_α ( tT / 4) and U s_β ( tT / 4), T Indicates the power frequency period; Obtaining the measured values ​​of the positive and negative sequence components of the stator voltage along the dq axis through Park transformation involves applying Park transformation to the positive sequence component of the voltage after separation of positive and negative sequences, and applying inverse Park transformation to the negative sequence component. The transformation formula is as follows: （4） In equation (4), , , , These are the measured values ​​of the positive and negative sequence components of the stator voltage along the dq axis, respectively. θ GFM The angle for generating the active power loop in the network.

4. The method for coordinated control of positive and negative sequence currents in a doubly-fed grid-connected unit according to claim 1, characterized in that: In S2, the formula for coordinate transformation of the rotor three-phase current is: （5） In equation (5), I r_α and I r_β These are the rotor current components in the αβ coordinate system, respectively; I r_a , I r_b , I r_c For the rotor three-phase current; t )express t The value at time; The formula for performing the Park inverse transformation of the rotor current component based on the rotor electrical angle is as follows: （6） In equation (6), I r_α_m and I r_β_m These are the reduced rotor current components; θ r The rotor electrical angle; The formula for separating the positive and negative sequences of the reduced rotor current components based on the T / 4 delay cancellation method is as follows: （7） In equation (7), , , , They are respectively I r_α_m and I r_β_m Positive and negative order components; I r_α_m ( t )and I r_β_m ( t )Delay T / 4 cycles are denoted as I r_α_m ( tT / 4) and I r_α_m ( tT / 4), T Indicates the power frequency period; The measured values ​​of the positive and negative sequence components of the rotor current under the dq axis obtained by Park transformation refer to applying Park transformation to the positive sequence component of the current after separation of positive and negative sequences, and applying inverse Park transformation to the negative sequence component of the current. The transformation formula is as follows: （8） In equation (8), , , , These are the measured values ​​of the positive and negative sequence components of the rotor current under the dq axis, respectively. θ GFM The angle for generating the active power loop in the network.

5. The method for coordinated control of positive and negative sequence currents in a doubly-fed grid-connected unit according to claim 1, characterized in that: In S3, the formula for calculating the unbalance is as follows: （9） In formula (9), ɛ U_grid For voltage imbalance, and These are the positive and negative sequence components of the grid voltage along the d-axis, respectively; ɛ I_stator andɛ I_grid These are the unbalance of the stator output current of the doubly-fed induction generator and the unbalance of the output current of the grid-side converter, respectively. and These are the positive and negative sequence components of the stator's d-axis current, respectively. and These are the positive and negative sequence components of the d-axis current on the grid side, respectively.

6. The method for coordinated control of positive and negative sequence currents in a doubly-fed grid-connected unit according to claim 5, characterized in that: In S3, the method for determining whether the grid voltage has experienced a three-phase symmetrical drop or an asymmetrical drop is as follows: （10） In equation (10), LVRT_B_Flag and LVRT_A_Flag are flags indicating whether the grid voltage has experienced a three-phase symmetrical or asymmetrical drop. When the flag is 1, it indicates that a drop has occurred and has exceeded the undervoltage design threshold. U L_Limit Otherwise, it is 0; U L_Limit As per the lower voltage limit requirement, if this limit is exceeded, low-voltage protection control must be activated; U_max This is the highest threshold for voltage imbalance. When the grid voltage drops below the lower limit and the voltage imbalance also exceeds the highest threshold, the grid voltage is determined to be an asymmetrical drop. U_grid This refers to the voltage imbalance.

7. The method for coordinated control of positive and negative sequence currents in a doubly-fed grid-connected unit according to claim 6, characterized in that: In S3, the specific process of designing the negative sequence current collaborative enable logic and negative sequence current command allocation based on the judgment result, and calculating the given value of the negative sequence current of the dq axis of the grid-side converter is as follows: S31. When the grid voltage experiences a three-phase symmetrical drop, the negative sequence current loop of the generator-grid side converter does not start. The doubly fed generator responds to the voltage drop through the reactive power droop link in the grid control loop, providing reactive power support to the grid. When an asymmetrical drop occurs, the negative sequence current loop on the generator-grid side and the generator-side negative sequence current q-axis command and the grid-side negative sequence current q-axis command are coordinated through the grid-side q-axis negative sequence current limit and the low-through negative sequence reactive current demand value. S32. When LVRT_B_Flag=LVRT_A_Flag=0, it indicates that the grid voltage has not experienced a low-voltage breakdown. Determine whether the current imbalance between the generator and the grid exceeds the threshold α. I_max , ifɛ I_stator >ɛ I_max or ɛ I_grid >ɛ I_max If so, the corresponding machine-grid side negative sequence current loop is started, and both the machine-side negative sequence current q-axis command and the grid-side negative sequence current q-axis command are 0; S33. When LVRT_B_Flag=1, it indicates that the grid voltage has a three-phase symmetrical voltage drop. At this time, the negative sequence current loop on the grid side will not start. When LVRT_A_Flag=1, it indicates that the grid voltage has a three-phase asymmetrical voltage drop. At this time, the current loop enable sequence and negative sequence current command allocation are determined according to the negative sequence current demand, and the negative sequence current setpoint value of the rotor dq shaft is calculated.

8. The method for coordinated control of positive and negative sequence currents in a doubly-fed grid-connected unit according to claim 7, characterized in that: In S31, the formula for calculating the grid-side q-axis negative sequence current limit is as follows: （11） In equation (11), This is the limit for the negative sequence q-axis current on the grid side; I g_max This refers to the current limit value for the grid-side converter; and These are the measured values ​​of the positive and negative sequence components of the d-axis current, respectively. This is the positive sequence measurement of the q-axis current.

9. The method for coordinated control of positive and negative sequence currents in a doubly-fed grid-connected unit according to claim 8, characterized in that: In S31, the formula for calculating the low-pass negative sequence reactive current demand is: （12） In equation (12), This is the value of the low-pass negative sequence reactive current requirement; The negative sequence reactive current coefficient; I N This refers to the rated current of the doubly-fed generator unit. This represents the negative sequence component of the grid voltage along the d-axis.

10. The method for coordinated control of positive and negative sequence currents in a doubly-fed grid-connected unit according to claim 9, characterized in that: In S32, the start-up judgment formula for the negative sequence current loop on the grid side is: （13） In equation (13), K grid_In A separate flag enable signal is set for the negative sequence current loop of the grid-side converter. K rotor_In A separate flag enable signal is set for the negative sequence current loop of the machine-side converter. When the flag is 0, it means that the negative sequence current loop is disabled, and when it is 1, it means that the negative sequence current loop is enabled. and These are the given values ​​for the negative sequence current of the rotor's dq axis, respectively. and These are the dq-axis negative sequence current setpoints for the grid-side converter, respectively. ɛ I_max The threshold for current imbalance on the grid side; ɛ I_stator andɛ I_grid These are the unbalance of the stator output current of the doubly fed motor and the unbalance of the output current of the grid-side converter, respectively.

11. The method for coordinated control of positive and negative sequence currents in a doubly-fed grid-connected unit according to claim 10, characterized in that: In S33, the current loop enable sequence and negative sequence current command allocation are determined based on the negative sequence current requirement, and the formula for the negative sequence current setpoint of the dq axis on the computer network side is as follows: （14） In equation (14), and These are the given values ​​for the negative sequence current of the rotor's dq axis, respectively. K grid_In A separate flag enable signal is set for the negative sequence current loop of the grid-side converter. K rotor_In A separate flag enable signal is set for the negative sequence current loop of the machine-side converter. When the flag is 0, it means that the negative sequence current loop is disabled, and when it is 1, it means that the negative sequence current loop is enabled. This is the value of the low-pass negative sequence reactive current requirement; This is the limit for the negative sequence q-axis current on the grid side.

12. A method for coordinated control of positive and negative sequence currents in a doubly-fed grid-connected unit according to any one of claims 1-11, characterized in that: In S4, based on the stator's active power and combined with the simulated rotor motion equations, the control of the grid-connected active power loop is realized, and the calculation formula for the grid-connected active power loop generation angle is output as follows: （15） In equation (15), θ GFM To construct the active power loop generation angle; P meas The active power of the stator; P ref The active power is a given value, manually set; P f_p This represents the incremental increase in the downward working power. K f_p The active power-frequency droop factor; ω 0 and ω meas These are the rated angular frequency of the power grid and the output angular frequency of the active power loop, respectively; 1 / s represents the integral operation. J The virtual inertia coefficient for the active power loop of the network.

13. The method for coordinated control of positive and negative sequence currents in a doubly-fed grid-connected unit according to claim 12, characterized in that: In S4, based on the stator's reactive power and combined with the simulated reactive power-voltage droop equation, the control of the grid reactive power loop is realized, and the calculation formula for the positive sequence setpoint of the d-axis stator voltage is as follows: （16） In equation (16), U s_ref and U 0 represents the positive sequence setpoint of the d-axis stator voltage and the rated stator voltage, respectively. K u_q The reactive power-voltage droop factor; Q meas The reactive power of the stator; Q ref The reactive power setpoint is manually set.

14. The method for coordinated control of positive and negative sequence currents in a doubly-fed grid-connected unit according to claim 13, characterized in that: In S4, the calculation formula for the stator dq-axis voltage orientation in the grid voltage loop and the output rotor dq-axis positive sequence current setpoint is achieved by PI control of the measured stator voltage positive sequence component. （17） In equation (17), , , , These are the given and measured values ​​of the positive and negative sequence voltages of the stator dq axis, respectively. K p_sd , K i_sd , K p_sq , K i_sq These are the PI parameters for the positive and negative sequence voltage control loops of the dq axis, respectively. and These are the given values ​​for the positive sequence current of the dq axis rotor, respectively.

15. The method for coordinated control of positive and negative sequence currents in a doubly-fed grid-connected unit according to claim 14, characterized in that: In S4, the negative sequence current in the grid current loop is suppressed by separately controlling the positive and negative sequence current setpoints of the rotor dq axis, and the calculation formula for the reference values ​​of the positive and negative sequence voltages of the rotor dq axis is as follows: （18） In equation (18), , , , These are the reference values ​​for the positive and negative sequence voltages of the rotor's dq axis, respectively. , , , These are the given and measured values ​​of the negative sequence current of the rotor dq shaft, respectively. and These are the measured values ​​of the positive sequence current along the dq axis, respectively. , , and These are the PI control parameters for the positive and negative sequence current loops along the d-axis, respectively. , , and These are the PI control parameters for the positive and negative sequence current loops on the q-axis, respectively, and S represents the integral operation.

16. A doubly-fed grid-connected unit positive and negative sequence current coordinated control system, characterized in that... include: The data acquisition unit is used to acquire the stator three-phase AC voltage, stator three-phase current, rotor three-phase current and rotor electrical angle of the doubly fed motor, and to calculate the active power and reactive power of the stator. The coordinate transformation and positive / negative sequence separation unit is used to perform coordinate transformation on the stator three-phase AC voltage and the rotor three-phase current respectively to obtain the stator voltage component and rotor current component in the αβ coordinate system. Then, the rotor current component is subjected to Park inverse transformation according to the rotor electrical angle to reduce the rotor current component to the power frequency or the same frequency as the stator current. After that, the positive and negative sequence of the stator voltage component and the reduced rotor current component are separated based on the T / 4 delay cancellation method. The measured values ​​of the stator voltage positive and negative sequence components and the rotor current positive and negative sequence components in the dq axis are obtained by Park transformation. The negative sequence current collaborative enable logic design and negative sequence current command allocation unit are used to determine whether the grid voltage has experienced a three-phase symmetrical drop or an asymmetrical drop based on the detection results of the three-phase current imbalance and grid voltage imbalance. Based on the judgment results, the negative sequence current collaborative enable logic and negative sequence current command allocation of the generator-grid side converter are designed, and the negative sequence current setpoint of the rotor dq axis is calculated. The network loop control unit is used to control the active power loop of the network based on the stator's active power and the simulated rotor motion equation, and outputs the generation angle of the active power loop of the network; it is used to control the reactive power loop of the network based on the reactive power of the stator and the simulated reactive power-voltage droop equation, and outputs the d-axis stator voltage positive sequence setpoint; it is used to orient the stator dq-axis voltage in the network voltage loop and output the rotor dq-axis positive sequence current setpoint by PI control of the measured stator voltage positive sequence component value under the dq-axis and the d-axis stator voltage positive sequence setpoint. It is used to suppress the negative sequence current in the grid current loop by separately controlling the positive and negative sequence current of the rotor dq axis based on the measured values ​​of the positive and negative sequence components of the rotor current under the dq axis, the positive sequence current of the rotor dq axis and the negative sequence current of the rotor dq axis, and output the reference values ​​of the positive and negative sequence voltages of the rotor dq axis. Positive and negative sequence reference voltages are synthesized and a rotor reference voltage unit is generated. This unit is used to perform αβ coordinate system transformation, merging, reduction and abc coordinate system transformation on the positive and negative sequence voltage reference values ​​of the rotor dq axis in sequence to obtain the modulation voltage reference value of the rotor-side converter. Based on the modulation voltage reference value, a PWM wave is generated to realize the closed-loop control of the doubly fed grid unit.

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