Grid-connected circulating current suppression method and device of grid-forming type wind turbine generator

Abstract

The invention provides a grid-connected circulating current suppression method and device for a grid-forming type wind turbine generator. The grid-connected circulating current suppression method comprises the following steps: acquiring an active power reference value, an active power actual value, a reactive power reference value and a reactive power actual value of the grid-forming type wind turbine generator and a three-phase output voltage and a three-phase output current of the grid-forming type wind turbine generator; the method comprises the following steps: performing proportional integral operation on an active power reference value, an active power actual value, a reactive power reference value and a reactive power actual value of a network construction type wind turbine generator, and calculating to obtain a first reference voltage; inputting a three-phase output voltage and a three-phase output current of the grid-forming type wind turbine generator set into a virtual impedance module, and calculating to obtain a second reference voltage; and providing the difference between the second reference voltage and the first reference voltage as a voltage reference to a voltage and current loop of the grid-forming type wind turbine generator so as to suppress the grid-connected circulation of the grid-forming type wind turbine generator.

Description

Technical Field

[0001] The present disclosure generally relates to the field of wind power, and more specifically, to a method and device for suppressing grid-connected circulating current of a grid-forming wind turbine generator set. Background Art

[0002] Currently, in the scenario of grid-connected power generation using grid-forming wind turbine generator sets, when all wind turbine generator sets operate in parallel in the grid-connected mode, it is beneficial to increase the power capacity of the entire power generation system and improve reliability and efficiency. However, when multiple grid-connected converters are used in the entire power generation system, circulating current may be generated. This circulating current may lead to increased losses, reduced efficiency, and even may cause the grid to collapse.

[0003] Therefore, a method capable of suppressing this circulating current is needed. Summary of the Invention

[0004] An exemplary embodiment of the present disclosure is to provide a method and device for suppressing grid-connected circulating current of a grid-forming wind turbine generator set, which can achieve the suppression of grid-connected circulating current of a grid-forming wind turbine generator set under small disturbances.

[0005] According to an aspect of an embodiment of the present disclosure, a method for suppressing grid-connected circulating current of a grid-forming wind turbine generator set is provided. The method for suppressing grid-connected circulating current includes: obtaining the active power reference value, the actual active power value, the reactive power reference value, the actual reactive power value of the grid-forming wind turbine generator set, and the three-phase output voltage and three-phase output current of the grid-forming wind turbine generator set; calculating a first reference voltage through proportional-integral operation on the active power reference value, the actual active power value, the reactive power reference value, and the actual reactive power value of the grid-forming wind turbine generator set; calculating a second reference voltage by inputting the three-phase output voltage and three-phase output current of the grid-forming wind turbine generator set into a virtual impedance module; using the difference between the second reference voltage and the first reference voltage as a voltage reference to be provided to the voltage-current loop of the grid-forming wind turbine generator set to suppress the grid-connected circulating current of the grid-forming wind turbine generator set, where the grid-connected circulating current includes the circulating current generated under small disturbances in multiple parallel grid-forming wind turbine generator sets in the case of applying a diode rectifier unit.

[0006] Optionally, when the grid-connected circulating current includes the grid-connected circulating current caused by the deviation between the carrier phases of the grid-forming wind turbine generator set and other grid-forming wind turbine generator sets, the virtual impedance module may include a virtual inductor and a band-pass filter.

[0007] Optionally, the step of calculating the second reference voltage may include: inputting the three-phase output current into the band-pass filter to calculate the filtered output current; calculating the second reference voltage based on the filtered output current after the filter, the virtual inductor, and the first reference voltage.

[0008] Optionally, when the grid-connected circulating current includes the grid-connected circulating current caused by the deviation between the dead-time voltage of the inverter of the grid-forming wind turbine and that of the inverters of other grid-forming wind turbines, the virtual impedance module may include a damped band-pass filter.

[0009] Optionally, the step of calculating the second reference voltage may include: inputting the three-phase output current into the damped band-pass filter to calculate the filtered output current; and calculating the second reference voltage based on the filtered output current and the first reference voltage.

[0010] Optionally, when the grid-connected circulating current includes the grid-connected circulating current caused by the three-phase load imbalance between the grid-forming wind turbine and other grid-forming wind turbines, the virtual impedance module may include a preset integrator and a negative-sequence resistor.

[0011] Optionally, the step of calculating the second reference voltage may include: extracting the negative-sequence current component from the three-phase output current by using a first preset integrator; calculating the negative-sequence voltage drop generated by the negative-sequence current component on the negative-sequence impedance based on the negative-sequence current component and the negative-sequence impedance generated by using a second preset integrator and the negative-sequence resistor; and calculating the second reference voltage based on the negative-sequence voltage drop and the first reference voltage.

[0012] According to another aspect of the embodiments of the present disclosure, there is provided a grid-connected circulating current suppression device for a grid-forming wind turbine. The grid-connected circulating current suppression device includes: an acquisition unit configured to acquire the reference value of the active power, the actual value of the active power, the reference value of the reactive power, the actual value of the reactive power, the three-phase output voltage and the three-phase output current of the grid-forming wind turbine; a reference voltage calculation unit configured to calculate a first reference voltage by performing proportional-integral operation on the reference value of the active power, the actual value of the active power, the reference value of the reactive power and the actual value of the reactive power of the grid-forming wind turbine; a virtual impedance control unit configured to calculate a second reference voltage by inputting the three-phase output voltage and the three-phase output current of the grid-forming wind turbine into a virtual impedance module; and a voltage control unit configured to provide the difference between the second reference voltage and the first reference voltage as a voltage reference to the voltage-current loop of the grid-forming wind turbine to suppress the grid-connected circulating current of the grid-forming wind turbine, where the grid-connected circulating current includes the circulating current generated by multiple parallel grid-forming wind turbines under small disturbances in the case of applying a diode rectifier unit.

[0013] Optionally, when the grid-connected circulating current includes the grid-connected circulating current caused by the deviation between the carrier phases of the grid-forming wind turbine and other grid-forming wind turbines, the virtual impedance module may include a virtual inductor and a band-pass filter.

[0014] Optionally, the process of the virtual impedance control unit calculating the second reference voltage may include: inputting the three-phase output current into the band-pass filter to calculate the filtered output current; calculating the second reference voltage based on the filtered output current after the filter, the virtual inductor, and the first reference voltage.

[0015] Optionally, when the grid-connected circulating current includes the grid-connected circulating current caused by the deviation between the dead-time voltage of the inverter of the network-forming wind turbine and that of the inverters of other network-forming wind turbines, the virtual impedance module may include a band-pass filter with damping.

[0016] Optionally, the operation of the virtual impedance control unit calculating the second reference voltage may include: inputting the three-phase output current into the band-pass filter with damping to calculate the filtered output current; calculating the second reference voltage based on the filtered output current after the filter and the first reference voltage.

[0017] Optionally, when the grid-connected circulating current includes the grid-connected circulating current caused by the three-phase load imbalance between the network-forming wind turbine and other network-forming wind turbines, the virtual impedance module may include a preset integrator and a negative-sequence resistor.

[0018] Optionally, the operation of the virtual impedance control unit calculating the second reference voltage may include: extracting the negative-sequence current component from the three-phase output current by using a first preset integrator; calculating the negative-sequence voltage drop generated by the negative-sequence current component on the negative-sequence impedance based on the negative-sequence current component and the negative-sequence impedance generated by using a second preset integrator and the negative-sequence resistor; calculating the second reference voltage based on the negative-sequence voltage drop and the first reference voltage.

[0019] According to another aspect of the embodiments of the present disclosure, a network-forming wind turbine is provided, and the network-forming wind turbine includes the grid-connected circulating current suppression device as described above.

[0020] According to another aspect of the embodiments of the present disclosure, a network-forming wind power system is provided, and the network-forming wind power system includes: a wind farm including a plurality of network-forming wind turbines connected in parallel, each of the plurality of network-forming wind turbines including the grid-connected circulating current suppression device as described above; a step-up transformer connected to the wind farm via a grid connection point and configured to perform a step-up process on the electric energy received from the wind farm via the grid connection point; an LFAC cable configured to transmit the electric energy received from the step-up transformer to a DRU rectifier station by using LFAC technology; and a DRU rectifier station configured to perform a rectification process on the electric energy received from the LFAC cable and transmit the rectified electric energy to the power grid.

[0021] According to another aspect of the embodiments of the present disclosure, there is provided a computer-readable storage medium. When the instructions in the computer-readable storage medium are run by at least one processor, the at least one processor is caused to execute the grid-connected circulating current suppression method as described above.

[0022] According to another aspect of the embodiments of the present disclosure, there is provided a computer device. The computer device includes: at least one processor; at least one memory storing computer-executable instructions, wherein when the computer-executable instructions are run by the at least one processor, the at least one processor is caused to execute the grid-connected circulating current suppression method as described above.

[0023] For the grid-connected circulating current suppression method and device of a grid-forming wind turbine according to an exemplary embodiment of the present disclosure, a method is proposed to comprehensively solve the problem of circulating current suppression in the case of small disturbances by introducing virtual impedance into the control strategy of the grid-forming wind turbine, and good circulating current suppression effects can be achieved for various situations that generate small disturbances.

[0024] In addition, through the grid-connected circulating current suppression method and device of the grid-forming wind turbine of the present disclosure, it is also possible to effectively suppress the circulating current in the case of parallel small disturbances of the grid-forming wind turbine based on the DRU scenario, and still be able to conveniently and effectively achieve grid-connected circulating current suppression without increasing costs.

[0025] Some other aspects and / or advantages of the general concept of the present disclosure will be described in part in the following description, some of which will be clear from the description, or can be learned through the implementation of the general concept of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Through the following description with reference to the drawings of exemplary embodiments shown, the above and other objects and features of the exemplary embodiments of the present disclosure will become clearer, wherein:

[0027] Figure 1 is a flowchart showing the grid-connected circulating current suppression method of a grid-forming wind turbine according to an exemplary embodiment of the present disclosure;

[0028] Figure 2 is a schematic diagram showing an example application scenario of the grid-connected circulating current suppression method and device according to an exemplary embodiment of the present disclosure;

[0029] Figure 3 shows a schematic diagram of the grid-side control strategy of a grid-forming wind turbine according to an exemplary embodiment of the present disclosure;

[0030] Figure 4 is an equivalent circuit diagram of two parallel wind turbines according to an exemplary embodiment of the present disclosure;

[0031] Figure 5 is a system transfer function diagram of a wind turbine according to an exemplary embodiment of the present disclosure;

[0032] Figure 6 is a system transfer function diagram of introducing a virtual inductor under different carrier phases according to an exemplary embodiment of the present disclosure;

[0033] Figure 7 is a schematic diagram of achieving circulating current suppression under different dead zones according to an exemplary embodiment of the present disclosure;

[0034] Figure 8 is a schematic diagram for extracting positive-sequence components and negative-sequence components according to an exemplary embodiment of the present disclosure;

[0035] Figure 9 is a schematic diagram for constructing a negative-sequence impedance according to an exemplary embodiment of the present disclosure;

[0036] Figures 10 to 13 is a simulation result diagram showing an example grid-connected circulating current suppression method according to an exemplary embodiment of the present disclosure;

[0037] Figure 14 is a block diagram showing a grid-connected circulating current suppression device of a grid-forming wind turbine according to an exemplary embodiment of the present disclosure;

[0038] Figure 15 is a block diagram showing a computer device according to an exemplary embodiment of the present disclosure. Detailed Embodiments

[0039] Reference will now be made in detail to the embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings, wherein the same reference numerals always refer to the same components. The following embodiments will be described with reference to the accompanying drawings to explain the present disclosure.

[0040] At present, offshore wind power generation is developing rapidly as a clean energy generation method. The alternating current generated by an offshore wind farm (OWF) is transmitted to the onshore power grid. The common transmission methods are power frequency high-voltage AC transmission and flexible DC transmission. The flexible DC transmission method is relatively advanced in terms of capacity, loss, and cost for long-distance power transmission far from the coast. Among different OWF-HVDC (High Voltage Direct Current) transmission technologies, compared with voltage source converters and line converters, the DRU (Diode Rectifier Unit) can significantly reduce costs and improve reliability. For example, a system using DRU can reduce power loss by 30%, reduce the scale of the offshore converter station to 20% of the original, and reduce the total cost by 30%. Therefore, DRU has received great attention in recent years.

[0041] To further reduce the cost of offshore DRU converters, the present disclosure uses an LFAC (Low frequency alternating current) transmission scheme between the OWF and the DRU. Different from the VSC-HVDC (Voltage Source Converter based High Voltage Direct Current) system, in the LFAC transmission scheme, the OWF operates in a grid-following mode, and the commutation voltage of the DRU needs to be provided by an external AC voltage source (for example, a grid-forming wind turbine). Therefore, the present disclosure proposes a grid-forming mode of the OWF based on the global positioning system (GPS) power voltage (P-V) grid, and actively establishes a voltage from the OWF side to commutate the DRU. In addition, to avoid dependence on a strong communication link, a reactive power frequency (Q-f) control is also proposed to achieve synchronization.

[0042] When multiple grid-connected converters are used in a GFM-OWF (Grid-Forming Offshore wind farm) DRU-HVDC system, circulating current may be generated. This circulating current may lead to increased loss, reduced efficiency, and even may cause the collapse of the power grid. In response to this situation, the present disclosure proposes a method for suppressing circulating current under small disturbances. Here, the "small disturbance circulating current" refers to a small-amplitude circulating current generated due to different carrier phases, different dead zones, and different three-phase filter inductances between two wind turbines.

[0043] The above-mentioned small disturbance circulating current is relative to the large disturbance circulating current. Here, regarding the circulating current, the circulating current is closely related to the amplitude, phase, output impedance, and frequency of the output voltages of two wind turbines. Therefore, the generation mechanism of the circulating current can be analyzed from four aspects: output voltage amplitude imbalance, phase imbalance, output impedance imbalance, and frequency imbalance. When the output voltages, impedances, and frequencies of two wind turbines are the same, there is no circulating current. Regarding the large disturbance circulating current (i.e., large signal circulating current), the circulating current generated under the above four unbalanced conditions is the fundamental frequency circulating current, and the circulating current with a relatively large amplitude caused by a large difference between two wind turbines is defined as the large signal circulating current. For example, the value of an example of the large disturbance circulating current caused by load mismatch can be 2000 A (amperes). In contrast, the value of an example of the above-mentioned small disturbance circulating current can be several amperes, more than a dozen amperes, or dozens of amperes, etc., but these are only examples, and the present disclosure is not limited thereto.

[0044] The circulating current suppression method of the present disclosure mainly relates to the suppression of the above-mentioned small disturbance circulating current in the grid-connected type wind turbines connected in parallel in the DRU scenario.

[0045] Currently, the existing methods for suppressing these small disturbance circulating currents mainly include: suppressing the circulating current generated due to inconsistent carrier phases by adjusting the carrier phase; suppressing the circulating current generated due to inconsistent dead zones by providing a harmonic channel; suppressing the circulating current of unbalanced three-phase filter inductors by adopting PR control (i.e., proportional resonance control) to replace PI control to reduce the amplitude of the circulating current.

[0046] In contrast, the circulating current suppression method of the present disclosure mainly adopts the method of introducing virtual impedance into the control strategy of grid-connected type wind turbines to comprehensively suppress the circulating current generated in the above three cases, so as to have general applicability for the suppression of circulating current in different scenarios, and can achieve good circulating current suppression effects for various situations generating small disturbances. In addition, the circulating current suppression method of the present disclosure also fully considers that the P-f and Q-U controls adopted by the grid-side converters of wind turbines in a high-voltage environment are not applicable to the DRU scenario, and by adopting the method of introducing virtual impedance in the DRU scenario to achieve circulating current suppression, it can effectively suppress the circulating current for the parallel small disturbance situation of grid-connected type wind turbines based on the DRU scenario, and conveniently and effectively achieve grid-connected circulating current suppression without increasing costs.

[0047] The following refers to Figures 1 to 14 Specifically elaborate on the grid-connected circulating current suppression method and device for grid-connected type wind turbines according to the present disclosure.

[0048] As an example, before specifically elaborating on the grid-connected circulating current suppression method and device for grid-connected type wind turbines according to the present disclosure, first refer to Figure 2 Describe the DRU scenario to which the grid-connected circulating current suppression method and device are applied. Figure 2It is a schematic diagram showing an example application scenario of a grid-connected circulating current suppression method and device according to an exemplary embodiment of the present disclosure. For example, Figure 2 It is a simplified schematic diagram of a DRU-HVDC transmission scenario.

[0049] Referring to Figure 2 , a grid-forming wind power system based on DRU is shown, and the system includes: a wind farm, a booster station (i.e., a booster transformer), a three-phase low-frequency AC submarine cable (i.e., an LFAC cable / submarine cable), and a DRU rectifier station (including a DRU rectifier). In addition, the system may further include a reactive power compensation circuit and a converter transformer. Here, both the wind farm and the booster station are located offshore, the LFAC cable is located underwater, and the DRU rectifier station, the reactive power compensation circuit, and the converter transformer are all located onshore.

[0050] For example, the reactive power compensation circuit may include a compensation capacitor C F and a compensation impedance Z F , and the currents involved in the circuit may include the submarine cable current I F and the AC side bus current I R as well as the intermediate current I Rac (whose value is I F minus the current flowing through C F ).

[0051] Specifically, the low-frequency AC signal generated by the wind farm (including multiple wind turbines) is boosted to an AC signal with a higher voltage by the booster transformer, and the AC signal with the higher voltage is transmitted to the DRU rectifier station through the LFAC cable and converted into an HVDC signal at the DRU rectifier station, and the DRU rectifier station transmits the HVDC signal to the onshore power grid.

[0052] Hereinafter, the control strategies involved in the grid-forming wind power system based on DRU will be specifically elaborated from three aspects: the LFAC submarine cable, the DRU rectifier station, and the grid-side control strategy of the grid-forming wind turbine network.

[0053] For the LFAC submarine cable, by applying the LFAC technology to the transmission process from offshore wind power to onshore, the transmission capacity can be improved, the transmission distance can be extended, thereby reducing costs and being able to solve the problem of space charge accumulation.

[0054] Specifically, with the rapid development of offshore wind power, the HVAC (High Voltage Alternating Current) power transmission technology has been widely adopted to achieve the power transmission from offshore power generation systems to onshore AC systems. The HVAC power transmission technology is relatively mature and has rich relevant operation experience. However, the transmission capacity and distance of the HVAC power transmission technology are affected by the charging current. The LFAC technology adopted in this disclosure can well solve this problem, which will be specifically illustrated by the following formulas (1) and (2): Formulas (1) and (2) respectively show the active power P transmitted by the AC cable and the charging current I of the transmission line C .

[0055]

[0056] where E1 and E2 respectively represent the amplitudes of the starting voltage and the terminal voltage of the transmission line, δ represents the transmission angle of the transmission line, L represents the inductance of the transmission line, and f represents the frequency of the alternating current transmitted on the transmission line.

[0057] I C = 2πfCU (2)

[0058] where C represents the capacitance to ground of the transmission line, U represents the AC voltage across the capacitance to ground, and f represents the frequency of the alternating current transmitted on the transmission line.

[0059] According to Formulas (1) and (2), when low-frequency AC power transmission is adopted, the transmission capacity of the transmission line can be increased, and the transmission ability of the transmission line can be greatly improved. In addition, the reduction of the charging current can make long-distance power transmission feasible, enable more current to flow to the load, and reduce unnecessary losses. In addition, by using the low-frequency AC power transmission technology, the need for building an offshore converter platform is avoided, the cost is greatly reduced, and the space charge accumulation effect is effectively avoided, thus solving the technical problems of offshore converters.

[0060] For the DRU rectifier station, compared with the scheme adopting the traditional voltage source converter, the scheme adopting the DRU rectifier station requires fewer devices, has a simpler structure, is more convenient to use, and has a lower cost, and also has higher reliability compared with the complex voltage source converter platform. The advantages of adopting the DRU technology will be illustrated below by taking a 12-pulse rectifier as an example.

[0061] Specifically, according to the quasi-steady-state model of the 12-pulse rectifier, the expression of the DC (Direct Current) terminal voltage V shown in Formula (1) is obtained Rdc :

[0062]

[0063] where VF represents the bus voltage on the AC side of the DRU, X C represents the leakage reactance of the commutation transformer.

[0064] In addition, since in Figure 2 , the above constant voltage U (such as Figure 2 the value of V shown dI ) is controlled by an onshore MMC (Modular Multilevel Converter) (not shown in the figure), the formula (4) can be obtained.

[0065] V Rdc = U (4)

[0066] According to formulas (3) and (4), the expression of the DC side current I Rdc can be obtained:

[0067]

[0068] Therefore, the active power P on the DC side of the DRU in the following formula (6) can be calculated based on formula (5) d :

[0069]

[0070] where, L c represents the leakage inductance of the commutation transformer.

[0071]

[0072]

[0073] where, represents the power factor of the DRU rectifier, Q d represents the reactive power on the DC side of the DRU rectifier, and μ represents the commutation angle of the DRU. In addition, Figure 2 the R shown d represents the equivalent output resistance of the DRU rectifier.

[0074] According to formulas (6) to (8), it can be concluded that the DC side power of the DRU is positively correlated with the AC bus voltage. The DC side power of the DRU comes from the total power generated by the wind farm. Since the AC bus voltage is positively correlated with the outlet voltage on the grid side of the wind turbine, the amplitude of the outlet voltage can be controlled by controlling the output power of the wind turbine.

[0075] Regarding the grid-forming wind turbine network-side control strategy, since the above-mentioned DRU is an uncontrollable passive device, the OWF AC system needs to be regulated by the wind turbine itself. The voltage and frequency of its equivalent grid-forming system are established through the grid-side converter of the wind turbine to form a power grid. The specific grid-forming wind turbine network-side control strategy can be as follows Figure 3 as shown.

[0076] Figure 3 shows a schematic diagram of the grid-forming wind turbine network-side control strategy according to an exemplary embodiment of the present disclosure. Referring to Figure 3 , the voltage amplitude is determined through the active loop, and the phase angle is determined through the reactive loop, specifically as shown in the following equation (9):

[0077]

[0078] where represents the amplitude of the reference voltage, δ * represents the phase angle, K pp and K qp respectively represent the proportionality coefficients of the active loop and the reactive loop, K pi and K qi respectively represent the integral coefficients of the active loop and the reactive loop, and respectively represent the reference values of the active power and the reactive power, P w and Q w respectively represent the actual values of the active power and the reactive power, and s represents the Laplace operator.

[0079] In addition, in Figure 3 , L f , I L , C f respectively represent the filter inductance, the filter current, and the filter capacitance of the filter, U o represents the output voltage, U od and U oq respectively represent the d-axis and q-axis components of the output voltage, I Ld and I Lq respectively represent the d-axis and q-axis components of I L , and respectively represent the reference components corresponding to the d-axis and q-axis components of I L (obtained through the voltage loop calculation), and respectively represent the output voltages of the current loop, K vp and K vi respectively represent the proportionality coefficient and the integral coefficient of the voltage loop, K ip and K iiThey represent the proportional coefficient and integral coefficient of the current loop, u dc and i dc They represent the DC voltage and current used to control the inverter, and u w Indicates the inverter output voltage.

[0080] The voltage amplitude and phase angle generated by the power loop (including the active loop and the reactive loop) are calculated to obtain the d-axis and q-axis voltage reference values ​​(such as Figure 3 shown and ), and then through the voltage and current double closed loop (ie, Figure 3 The voltage loop and current loop shown in FIG. 1 ) control and the predetermined modulation module (eg, Figure 3 The PWM (Pulse Width Modulation) module shown, but not limited to this, can also be an SPWM (Sinusoidal PWM) module, SVPWM (Space Vector PWM) module, etc.) to generate a modulation signal to control the on and off of a switching device (for example, an IGBT (Insulated-Gate Bipolar Transistor), which can be used as a grid-connected inverter for a wind turbine).

[0081] Refer to the following Figure 1 The method 100 for suppressing grid-connected circulating current of a grid-connected wind turbine generator system according to the present disclosure is specifically described. Figure 1 1 is a flow chart showing a method 100 for suppressing grid-connected circulating current of a grid-connected wind turbine according to an exemplary embodiment of the present disclosure.

[0082] Here, the grid-connected circulating current includes the circulating current generated by multiple parallel-connected grid-connected wind turbines under small disturbances when DRU is applied.

[0083] Below, we take two wind turbines connected in parallel as an example to analyze the process of small disturbance grid-connected circulation. Figure 4 , Figure 4 is an equivalent circuit diagram of two wind turbines connected in parallel according to an exemplary embodiment of the present disclosure.

[0084] exist Figure 4 In the equation, E1, δ1 and E2, δ2 represent the output voltage amplitude and phase angle of the two wind turbines, respectively. and represents the equivalent output impedance of the two wind turbines, and Respectively represent the output currents of the two wind turbines (their expressions are shown in the following formula (10)), represents the load impedance, represents the load current, represents the load voltage, and δ0 represents the phase angle of the load voltage, represents the AC bus voltage on the AC side of the DRU, represents the impedance of the circulating current path. V Rdc and I Rdc respectively represent the voltage and current at the DC terminal of the DRU, V dI represents a constant DC voltage source controlled by the MMC, and represents the circulating current (whose expression is shown in Equation (11) below).

[0085]

[0086]

[0087] Referring back Figure 1 , in step S101, obtain the active power reference value, actual active power value, reactive power reference value, actual reactive power value of the network-forming wind turbine, as well as the three-phase output voltage and three-phase output current of the network-forming wind turbine.

[0088] In step S102, by performing proportional-integral operations on the active power reference value, actual active power value, reactive power reference value, and actual reactive power value of the network-forming wind turbine, calculate the first reference voltage.

[0089] In step S103, by inputting the three-phase output voltage and three-phase output current of the network-forming wind turbine into the virtual impedance module, calculate the second reference voltage.

[0090] In step S104, use the difference between the second reference voltage and the first reference voltage as the voltage reference to be provided to the voltage-current loop of the network-forming wind turbine to suppress the grid-connected circulating current of the network-forming wind turbine.

[0091] As a first example, when the grid-connected circulating current includes the grid-connected circulating current caused by the deviation between the carrier phases of the network-forming wind turbine and other network-forming wind turbines, the virtual impedance module may include a virtual inductor and a band-pass filter.

[0092] Here, still taking two parallel wind turbines as an example, analyze the process of generating the circulating current when their carrier phases are different and the suppression process for this circulating current.

[0093] Specifically, when the modulation waves of each wind turbine are the same but the carrier phases deviate, it will affect the generation of the modulation signal, making the on-off actions of the switching devices inconsistent, thus resulting in different output voltages, and therefore may cause high-frequency circulating current to be generated. The specific process of generating the circulating current can be referred to as shown in Equations (12) to (16):

[0094] First, according toFigure 4 The following formula (12) can be obtained:

[0095]

[0096] Then, let L1 = L2 = L, and subtract the two formulas in formula (12) to obtain formula (13):

[0097]

[0098] According to formula (13), it can be obtained that the slope of the change in the circulating current between two wind turbines is

[0099] When the carrier phases of two wind turbines are different, the circulating current consists of the lower envelope and the trapezoidal component , specifically as shown in formulas (14) to (16):

[0100]

[0101]

[0102]

[0103] where E1 - E2 = u d , θ represents the phase difference between the inverter carriers of two wind turbines, ω c represents the angular frequency of the triangular carrier, ω s represents the angular frequency of the modulation wave, M represents the modulation ratio, and n takes values from 1 to infinity.

[0104] According to formulas (15) and (16), it can be obtained that the DC component of the circulating current is u d θ / 4Lω c , which is proportional to the DC voltage and the carrier phase difference, and inversely proportional to the output inductance and the carrier angular frequency.

[0105] In addition, adding the fundamental frequency component in the lower envelope of the circulating current to the rectangular component (for example, using the previous trapezoidal component to be equivalent to the rectangular component here) can obtain that there is no fundamental frequency component in the circulating current. By analyzing the harmonic components, it can be obtained that the harmonic components are mainly distributed at frequencies (nω c ± kω s ), and the magnitude of its amplitude can be calculated.

[0106] ​In the above first example, the step of calculating the second reference voltage in step S103 may include: inputting the three-phase output current into a band-pass filter to calculate the filtered output current; calculating the second reference voltage based on the filtered output current, the virtual inductor, and the first reference voltage.

[0107] Specifically, as analyzed above, when the carrier phases are different, the generated circulating current is a high-frequency circulating current. If the traditional method of calculating the carrier phase difference according to the magnitude of the circulating current to correct the carrier phase difference between two wind turbines online is adopted, there are high requirements for the calculation of the circulating current magnitude, and there are a large number of harmonics in the circulating current, making it difficult to calculate the circulating current magnitude. Therefore, the present disclosure adopts a suppression strategy of introducing a virtual inductor. The way of introducing the virtual inductor is as Figure 6 shown.

[0108] Referring to Figure 6 , L V represents the introduced virtual inductor, ω0 is the center frequency of the band-pass filter described in the first example, and k s is the amplification gain of the band-pass filter described in the first example. Specifically, the way of introducing the virtual inductor is as follows: filter the output current, obtain the required frequency through filtering, then obtain the voltage drop across the virtual inductor through the virtual inductor, and introduce this voltage drop into the original reference voltage (i.e., the first reference voltage) (i.e., subtract this voltage drop from the original reference voltage) to obtain a new reference voltage value (i.e., the second reference voltage), that is, it is equivalent to introducing a virtual inductor.

[0109] By introducing the virtual inductor L V , it has a good suppression effect on the high-frequency circulating current, and since no actual inductor is added, there is no need to increase the cost and only the control strategy needs to be adjusted.

[0110] As a second example, when the grid-connected circulating current includes the grid-connected circulating current caused by the deviation between the dead-time voltages of the inverters of the network-forming wind turbines and other network-forming wind turbines, the virtual impedance module may include a band-pass filter with damping.

[0111] Here, still taking two parallel wind turbines as an example, the process of generating the circulating current when their dead-time voltages are different and the suppression process of this circulating current are analyzed.

[0112] Specifically, to prevent the simultaneous turn-on of the switching devices in the same bridge arm (e.g., IGBT) so that the current becomes too large and burns out the IGBT, it is necessary to introduce a dead zone. The commonly used method to introduce a dead zone is to delay for a period of time before the trigger pulses of the upper and lower switching devices to ensure that one of the switching devices is in the off state before turning on the other switching device. Regarding the deviation voltage of the modulation wave caused by factors such as the dead zone as a kind of harmonic disturbance, based on the transfer function of the system (here, the system refers to the entire wind turbine side "power grid" before the grid connection point of a single wind turbine), adding the disturbance of the harmonic voltage can obtain the influence of the harmonic voltage on the output voltage.

[0113] For example, when considering the circulating current generated by the inconsistent dead zones of the inverters of two wind turbines, it is first necessary to make the other hardware parameters such as the filter inductors and capacitors of the inverters of the two wind turbines the same. Then, the relationship between the dead zone voltage difference and the circulating current of the inverters of the two wind turbines can refer to Figure 5 and equations (17) to (19).

[0114] In Figure 5 where represents the d-axis component of the reference output voltage, u od represents the d-axis component of the actual output voltage, represents the d-axis component of the filter inductor current, i Ld represents the d-axis component of the actual filter inductor current, L and C represent the filter inductor and filter capacitor respectively, i o and u o represent the output current and output voltage of the inverter respectively, G V represents the transfer function of the voltage loop, G I represents the transfer function of the current loop, G PWM represents the transfer function of the PWM modulation module, u e represents the dead zone voltage (harmonic voltage), and K b represents the equivalent gain of the inverter bridge.

[0115] First, let G1 = G V , G2 = G I G PWM K b ,

[0116] Then, find the transfer functions for the three inputs respectively:

[0117]

[0118]

[0119]

[0120] Among them,

[0121] 1 + G2G3 + G3G4 + G1G2G3G4 = P(s), and

[0122] 1 + G2G3 + G3G4 + G2G3 2 G4 + G1G2G3G4 = N(s).

[0123] From the above formula, the output voltage of the inverter can be obtained as formula (20):

[0124]

[0125] After that, subtract the output voltages of the two wind turbine generators to obtain formula (21):

[0126]

[0127] After that, let u e1 - u e2 = Δu e , combining formulas (10), (11) and (21), the circulating current as shown in formula (22) can be obtained:

[0128]

[0129] Among them, the output impedance values of the two wind turbine generators are equal (both are Z), that is, Z1 = Z2 = Z.

[0130] From formula (22), it can be concluded that: when the reference voltages of the inverters of the two wind turbine generators are the same and the hardware circuit parameters are consistent, the difference between the inverter drive signals will cause a voltage difference between the two, and the magnitude of the circulating current is proportional to the voltage difference of the dead zone, that is, the larger the voltage difference, the larger the circulating current.

[0131] In the above second example, the step of calculating the second reference voltage in step S103 may include: inputting the three-phase output current into a band-pass filter with damping, and calculating the filtered output current; calculating the second reference voltage based on the filtered output current and the first reference voltage.

[0132] Specifically, as analyzed before, since the deviation voltage of the modulation signal caused by different dead zones is a kind of harmonic disturbance, harmonic suppression is required while suppressing the circulating current. The circulating current suppression method of the present disclosure uses a band-pass filter with damping (the expression of its transfer function can be, for example, formula (23) below).

[0133]

[0134] Among them, K is the amplification gain of the band-pass filter with damping, ξ is the damping ratio, and ωc1 is the cut-off frequency. For example, K can be ξ can be 0.707.

[0135] Specifically, referring to Figure 7 , the grid-connected current i is extracted by the band-pass filter with damping, abc and then the harmonic components are equivalent to virtual impedance by using the property of additional damping of the band-pass filter. The current signal (i.e., the grid-connected current i in abc coordinates abc is converted to the components i d , i q ) in the dq coordinate system and converted into a voltage signal to obtain the voltage drop of the harmonic voltage components (u d , u q ). The voltage reference values (u dref , u qref ) output by the power loop (i.e., the first reference voltage) are subtracted from the voltage drop of the harmonic voltage components to obtain new voltage reference values (u′ dref , u′ qref ) (i.e., the second reference voltage) for input into the voltage-current double closed-loop control.

[0136] As a third example, when the grid-connected circulating current includes the grid-connected circulating current caused by the three-phase load imbalance between the grid-forming wind turbines and other grid-forming wind turbines, the virtual impedance module may include a preset integrator and a negative sequence resistor.

[0137] Here, still taking two parallel wind turbines as an example, the process of generating the circulating current under the condition of their three-phase load imbalance and the suppression treatment of the circulating current are analyzed.

[0138] Specifically, when the three-phase grid is balanced, only positive sequence components exist in the voltage and current. When the three-phase grid is unbalanced, in addition to the positive sequence components, negative sequence components and zero sequence components will also exist. For example, the three-phase symmetrical component method can be used for the mathematical analysis of the three-phase unbalanced state of the grid. Specifically, assuming that the output three-phase voltages of the inverter are U oa , U ob and U oc , they are decomposed into the superposition form of positive sequence components, negative sequence components and zero sequence components, specifically as shown in the following formula (24):

[0139]

[0140] Among them, and respectively represent the voltage amplitudes of the positive sequence components, negative sequence components and zero sequence components, and α P , α N and α0 respectively represent the initial phases of the positive sequence components, negative sequence components and zero sequence voltages.

[0141] Here, both the positive-sequence component and the negative-sequence component are three-phase symmetrical components, and the three-phase directions of the zero-sequence component are the same. In actual control, generally, the alternating quantity is converted into a direct current quantity in the positive-sequence synchronous coordinate system and controlled via a PI controller (i.e., a proportional-integral controller). Here, U in Equation (24) oa , U ob and U oc are transformed into the components U d and U q in the dq coordinate system through the Park transformation, as specifically shown in Equation (25):

[0142]

[0143] It can be obtained through Equation (25) that: the positive-sequence voltage after the Park transformation shows a direct-current component, and the negative-sequence voltage shows a double-frequency component. Since the PI control cannot perform static-error-free control on the alternating quantity, it is necessary to suppress the negative-sequence component. In the case of unbalanced three-phase loads, corresponding control strategies need to be adopted to balance the three phases in order to ensure the stable operation of the system and ultimately achieve the purpose of three-phase balance of the output voltage.

[0144] Specifically, when the three-phase voltages are balanced, the expression of the output voltage is Equation (26):

[0145]

[0146] When the three-phase loads are unbalanced, the expression of the three-phase currents is Equation (27):

[0147]

[0148] In Equation (27), I oa , I ob and I oc represent the output three-phase currents, and respectively represent the amplitudes of the positive-sequence, negative-sequence, and zero-sequence currents, and θ P , θ N and θ 0 respectively represent the initial phases of the positive-sequence, negative-sequence, and zero-sequence currents. When the three-phase loads are unbalanced, the expressions of the active power and reactive power output by the converter are as shown in Equation (28):

[0149]

[0150] Substituting Equations (26) and (27) into Equation (28), Equation (29) can be obtained:

[0151]

[0152] According to Equation (29), it can be obtained that: in the case of unbalanced three-phase load, both the active power and the reactive power consist of two parts, namely, the DC component and the second-harmonic component. Here, the DC component is the result of the combined action of the positive-sequence voltage and the positive-sequence current, while the second-harmonic component is the result of the combined action of the positive-sequence voltage and the negative-sequence current. The zero-sequence current component does not generate power when acting with the positive-sequence voltage because the positive-sequence voltage is three-phase symmetrical and the zero-sequence current has the same phase for all three phases. Since the power exhibits second-harmonic fluctuations, an accurate reference voltage amplitude and phase cannot be generated through the power loop. Therefore, it is necessary to suppress the negative-sequence component. Since the zero-sequence circulating current component is much lower than the positive-sequence and negative-sequence circulating current components, the zero-sequence current component can be ignored when performing circulating current suppression later.

[0153] In the above-mentioned third example, the step of calculating the second reference voltage in step S103 may include: extracting the negative-sequence current component from the three-phase output current using a first preset integrator; calculating the negative-sequence voltage drop generated by the negative-sequence current component on the negative-sequence impedance based on the negative-sequence current component and the negative-sequence impedance generated using a second preset integrator and a negative-sequence resistor; and calculating the second reference voltage based on the negative-sequence voltage drop and the first reference voltage.

[0154] Specifically, as analyzed above, when the three-phase load is unbalanced, from the perspective of the phase sequence of the circulating current, the circulating current between the parallel wind turbines can be divided into zero-sequence circulating current (which can be ignored as described above), positive-sequence circulating current, and negative-sequence circulating current. Since the amplitude of the zero-sequence circulating current component is very small compared to the positive-sequence and negative-sequence circulating current components as described above, only the positive-sequence and negative-sequence components of the circulating current need to be extracted and suppressed.

[0155] For example, the positive-sequence and negative-sequence components of the output voltage and output current of the inverters of two wind turbines can be extracted through, for example, a second-order generalized integrator (SOGI). For example, Figure 8 shows the extracted positive-sequence components of the output currents i a 、i b and i c and and the negative-sequence components and

[0156] Here, for the SOGI, its input is a current with an expected frequency f (which includes the fundamental wave and harmonics at this time), and its output is the part of the current with the expected frequency f that does not include harmonics. And the od term and oq term corresponding to when using the SOGI do not represent the d-axis term and q-axis term, but rather the oq term lags the od term by 90 degrees in phase.

[0157] In addition, on the one hand, for the extracted positive sequence components, the positive sequence voltage and positive sequence current are input into the power loop to achieve equal sharing of the positive sequence power, and at the same time, an accurate voltage amplitude and phase are generated for subsequent voltage-current double closed-loop control.

[0158] On the other hand, for the extracted negative sequence components, a second-order generalized integrator is used to construct a negative sequence virtual impedance (for example, its corresponding negative sequence resistance is shown as R Figure 9 in V ), to suppress the negative sequence circulating current components. Specifically, the positive sequence reference voltage (i.e., the first reference voltage) generated by the power loop is subtracted from the voltage drop generated on the negative sequence virtual impedance (for example, Figure 9 shown as ), and the obtained voltage reference values on the d-axis and q-axis are used as the new compensated voltage reference values (i.e., the second reference voltage) for input into the voltage-current double closed-loop control.

[0159] By introducing the above negative sequence virtual impedance, the suppression of the circulating current in the case of unbalanced three-phase loads is achieved, and the problem of double-frequency power fluctuations caused by the negative sequence components is solved.

[0160] The following refers to Figures 10 to 13 to elaborate on the simulation results of the example grid-connected circulating current suppression method according to the embodiments of the present disclosure. Figures 10 to 13 FIG. is a diagram showing the simulation results of the example grid-connected circulating current suppression method according to an exemplary embodiment of the present disclosure.

[0161] Specifically, taking two parallel wind turbines as an example, the suppression effect of the introduced virtual impedance on the circulating current generated by the grid-forming wind turbines in the above three cases is illustrated. For example, the following three cases can be set to verify the circulating current suppression effect of the introduced virtual impedance:

[0162] Case 1), the carrier phase difference between the two wind turbines is π / 4;

[0163] Case 2), the dead time settings are different, the dead time of the first wind turbine among the two wind turbines is set to 1 μs, and the dead time of the second wind turbine among the two wind turbines is set to 6 μs;

[0164] Case 3), the three-phase filter inductances of the first wind turbine are 0.0007 H, 0.0001 H, and 0.0001 H respectively, and the three-phase filter inductances of the second wind turbine are 0.0003 H, 0.0001 H, and 0.0001 H respectively.

[0165] Referring to Figure 10, showing the schematic diagram of FFT (Fast Fourier Transform) analysis of the circulating current generated under different carrier phases. Specifically, in an example where the entire system operates at a low frequency of 20 Hz, with 20 Hz as the fundamental frequency and the carrier frequency set to 10,000 Hz, Figure 10 Diagrams (a), (b), and (c) in

[0166] show the FFT analysis of the circulating current generated for carrier phase differences of π / 4, π / 2, and π respectively. Figure 10 It can be concluded that: in this case, there is a DC component in the circulating current, and the harmonic orders are distributed around 500 and 1000, that is, the harmonic frequencies are distributed around 10,000 Hz and 20,000 Hz. By comparing diagrams (a), (b), and (c), it can be concluded that: as the carrier phase difference increases, the percentage of harmonics (i.e., THD) also increases (in line with relevant theoretical analysis).

[0167] Referring to Figure 11 , showing the comparison diagram before and after suppressing the circulating current under different carrier phases. Specifically, Figure 11 Diagrams (a1), (b1), (c1), and (d1) in Figure 11 show the waveform diagrams of the output voltage, output current, active / reactive power, and the circulating current between two wind turbines of the first wind turbine before implementing the virtual impedance circulating current suppression, and

[0168] Diagrams (a2), (b2), (c2), and (d2) in Figure 11 show the waveform diagrams of the output voltage, output current, active / reactive power, and the circulating current between two wind turbines of the first wind turbine after implementing the virtual impedance circulating current suppression.

[0169] Referring to Figure 12 , showing the comparison diagram before and after suppressing the circulating current under different dead zones. Specifically, Figure 12 Diagrams (a) and (c) in Figure 12 show the waveform diagrams of the circulating current amplitude and THD before implementing the virtual impedance circulating current suppression, and

[0170] Diagrams (b) and (d) inFigure 12 It can be obtained that before suppressing the circulating current through the virtual impedance, the amplitude of the circulating current reaches 45 A (as shown in diagram (a) in Figure 12 ). When detecting the THD by taking 5 cycles at 1.5 s and the power frequency is 20 Hz, the THD can be obtained as 113.73% (as shown in diagram (c) in Figure 12 ). After suppressing the circulating current through the band - pass filter with damping, the amplitude of the circulating current is 20 A (as shown in diagram (b) in Figure 12 ), and the corresponding THD drops to 53.09% (as shown in diagram (d) in Figure 12 ). Therefore, the effect of suppressing the circulating current in different cases of the dead zone in the present disclosure is remarkable.

[0171] Referring to Figure 13 , a comparison diagram before and after suppressing the circulating current in the case of unbalanced three - phase filter inductors is shown. Specifically, Figure 13 Diagrams (a), (c), (e), (g) and (i) in Figure 13 respectively show the waveform diagrams of the output voltage, output current, active / reactive power and the circulating current between two wind turbines of the first wind turbine before implementing the virtual impedance circulating current suppression, and

[0172] Diagrams (b), (d), (f), (h) and (j) in Figure 13 respectively show the waveform diagrams of the output voltage, output current, active / reactive power and the circulating current between two wind turbines of the first wind turbine after implementing the virtual impedance circulating current suppression. In addition, the simulation results of the second wind turbine are the same as the above.

[0172] From Figure 13 Diagrams (a) and (c), it can be obtained that when the three - phase filter inductors are unbalanced, both the three - phase voltage and the three - phase current are unbalanced. By measuring the active power and the reactive power, it can be obtained that their fluctuations are mainly double - frequency fluctuations (this verifies the conclusion that the negative - sequence component will generate a double - frequency component when the three - phase filter inductors are unbalanced) (as shown in diagram (e) in Figure 13 ). Specifically, before suppressing the circulating current through the virtual impedance, the amplitude of the circulating current is about 35 A (as shown in diagram (g) in Figure 13 ). After suppressing the circulating current by constructing the virtual impedance, both the voltage and the current are smooth three - phase sine waves (as shown in diagrams (b) and (d) in Figure 13 ), the degree of imbalance is greatly reduced, and the double - frequency fluctuations of the active power and the reactive power are also greatly reduced (as shown in diagram (f) in Figure 13 ), the amplitude of the circulating current is reduced from 35 A to 6 A (as shown in diagram (h) in Figure 13 ), and the THD of the circulating current measured in five cycles after 3 s is reduced from 58.94% (as shown in diagram (i) in Figure 13 ) to 10% (as shown in diagram (j) in Figure 13as shown in the diagram (j). Therefore, the present disclosure has a significant effect on suppressing the circulating current in the case of unbalanced three-phase filter inductors.

[0173] According to an embodiment of the present disclosure, through a grid-connected circulating current suppression method for a grid-forming wind turbine, a method of comprehensively solving the problem of circulating current suppression in the case of small disturbances is proposed by introducing virtual impedance into the control strategy of the grid-forming wind turbine, and a good circulating current suppression effect can be achieved for various situations that generate small disturbances.

[0174] In addition, through the grid-connected circulating current suppression method of the grid-forming wind turbine of the present disclosure, it is also possible to effectively suppress the circulating current in the case of parallel small disturbances of the grid-forming wind turbine based on the DRU scenario, and still be able to conveniently and effectively achieve grid-connected circulating current suppression without increasing costs.

[0175] In addition, through the grid-connected circulating current suppression method of the grid-forming wind turbine of the present disclosure, it is also possible to classify according to the causes and characteristics of the circulating current, and be able to obtain the commonalities of the circulating current generated in different situations through classification, so as to achieve the problem of circulating current caused by different factors in a unified manner, greatly reducing the complexity of realizing circulating current suppression. In addition, compared with most methods for suppressing parallel circulating current of onshore inverters (without considering the DRU transmission scenario), the present disclosure fully considers the suppression of circulating current in the scenario of offshore wind turbines transmitted through DRU.

[0176] Figure 14 is a block diagram showing a grid-connected circulating current suppression device 1400 of a grid-forming wind turbine according to an exemplary embodiment of the present disclosure.

[0177] Here, the grid-connected circulating current includes the circulating current generated by multiple parallel grid-forming wind turbines under small disturbances in the case of applying a diode rectifier unit.

[0178] Referring to Figure 14 , the grid-connected circulating current suppression device 1400 of the grid-forming wind turbine includes an acquisition unit 1410, a reference voltage calculation unit 1420, a virtual impedance control unit 1430, and a voltage control unit 1440.

[0179] According to an embodiment of the present disclosure, the acquisition unit 1410 is configured to: acquire the reference value of the active power, the actual value of the active power, the reference value of the reactive power, the actual value of the reactive power of the grid-forming wind turbine, and the three-phase output voltage and three-phase output current of the grid-forming wind turbine.

[0180] According to an embodiment of the present disclosure, the reference voltage calculation unit 1420 is configured to: calculate a first reference voltage through proportional-integral operation on the reference value of the active power, the actual value of the active power, the reference value of the reactive power, and the actual value of the reactive power of the grid-forming wind turbine.

[0181] According to an embodiment of the present disclosure, the virtual impedance control unit 1430 is configured to calculate a second reference voltage by inputting the three-phase output voltage and three-phase output current of the grid-forming wind turbine into the virtual impedance module.

[0182] As an example, when the grid-connected circulating current includes the grid-connected circulating current caused by the deviation between the carrier phases of the grid-forming wind turbine and other grid-forming wind turbines, the virtual impedance module may include a virtual inductor and a band-pass filter.

[0183] In this example, the process of the virtual impedance control unit 1430 calculating the second reference voltage may include: inputting the three-phase output current into the band-pass filter to calculate the filtered output current; calculating the second reference voltage based on the filtered output current, the virtual inductor, and the first reference voltage.

[0184] As an example, when the grid-connected circulating current includes the grid-connected circulating current caused by the deviation between the dead-time voltages of the inverters of the grid-forming wind turbine and other grid-forming wind turbines, the virtual impedance module may include a damped band-pass filter.

[0185] In this example, the operation of the virtual impedance control unit 1430 calculating the second reference voltage may include: inputting the three-phase output current into the damped band-pass filter to calculate the filtered output current; calculating the second reference voltage based on the filtered output current and the first reference voltage.

[0186] As an example, when the grid-connected circulating current includes the grid-connected circulating current caused by the three-phase load imbalance between the grid-forming wind turbine and other grid-forming wind turbines, the virtual impedance module may include a preset integrator and a negative-sequence resistor.

[0187] In this example, the operation of the virtual impedance control unit 1430 calculating the second reference voltage may include: extracting the negative-sequence current component from the three-phase output current using a first preset integrator; calculating the negative-sequence voltage drop generated by the negative-sequence current component on the negative-sequence impedance based on the negative-sequence current component and the negative-sequence impedance generated using a second preset integrator and the negative-sequence resistor; calculating the second reference voltage based on the negative-sequence voltage drop and the first reference voltage.

[0188] According to an embodiment of the present disclosure, the voltage control unit 1440 is configured to provide the difference between the second reference voltage and the first reference voltage as a voltage reference to the voltage-current loop of the grid-forming wind turbine to suppress the grid-connected circulating current of the grid-forming wind turbine.

[0189] According to another aspect of the embodiments of the present disclosure, a grid-forming wind turbine is provided, and the grid-forming wind turbine includes the grid-connected circulating current suppression device 1400 as described above.

[0190] According to another aspect of the embodiments of the present disclosure, a grid-forming wind power system is provided. The grid-forming wind power system includes: a wind farm including a plurality of grid-forming wind turbines connected in parallel, each of the plurality of grid-forming wind turbines including the grid-connected circulating current suppression device 1400 as described above; a step-up transformer connected to the wind farm via a point of common coupling and configured to perform a step-up process on the electric energy received from the wind farm via the point of common coupling; an LFAC cable configured to transmit the electric energy received from the step-up transformer to a DRU rectifier station using LFAC technology; and a DRU rectifier station configured to perform a rectification process on the electric energy received from the LFAC cable and transmit the rectified electric energy to the power grid.

[0191] It should be understood that the specific processes performed by the grid-connected circulating current suppression device, the grid-forming wind turbine, and the grid-forming wind power system according to the exemplary embodiments of the present disclosure have been described in detail with reference to Figures 1 to 13 and will not be repeated here.

[0192] It should be understood that each unit in the grid-connected circulating current suppression device of the grid-forming wind turbine according to the exemplary embodiments of the present disclosure can be implemented as a hardware component and / or a software component. Those skilled in the art can implement each unit using, for example, a field-programmable gate array (FPGA) or an application-specific integrated circuit (ASIC) according to the processes performed by the defined units.

[0193] The exemplary embodiments of the present disclosure provide a computer-readable storage medium that, when instructions in the computer-readable storage medium are run by at least one processor, cause the at least one processor to execute the grid-connected circulating current suppression method as described above. The computer-readable storage medium is any data storage device that can store data readable by a computer system. Examples of computer-readable storage media include: read-only memory, random access memory, compact disc read-only memory, magnetic tape, floppy disk, optical data storage device, and carrier waves (such as data transmission via the Internet through a wired or wireless transmission path).

[0194] Figure 15 is a block diagram showing a computer device 1500 according to an exemplary embodiment of the present disclosure.

[0195] The computer device 1500 according to the exemplary embodiments of the present disclosure includes: at least one processor 1510 and at least one memory 1520. The memory 1520 stores computer-executable instructions that, when run by the at least one processor 1510, cause the at least one processor 1510 to execute the grid-connected circulating current suppression method as described above.

[0196] Although some exemplary embodiments of the present disclosure have been shown and described, those skilled in the art should understand that these embodiments can be modified without departing from the principles and spirit of the present disclosure, which is defined by the claims and their equivalents.

Claims

1. A grid-connected circulating current suppression method for a grid-forming wind turbine, characterized in that Including: Obtaining the active power reference value, actual active power value, reactive power reference value, actual reactive power value of the grid-forming wind turbine, as well as the three-phase output voltage and three-phase output current of the grid-forming wind turbine; Calculating a first reference voltage by performing proportional-integral operation on the active power reference value, actual active power value, reactive power reference value, and actual reactive power value of the grid-forming wind turbine; Calculating a second reference voltage by inputting the three-phase output voltage and three-phase output current of the grid-forming wind turbine into a virtual impedance module; Using the difference between the second reference voltage and the first reference voltage as a voltage reference to be provided to the voltage-current loop of the grid-forming wind turbine to suppress the grid-connected circulating current of the grid-forming wind turbine, wherein the grid-connected circulating current includes the circulating current generated by multiple parallel grid-forming wind turbines under small disturbances in the case of applying a diode rectifier unit.

2. The grid-connected circulating current suppression method according to claim 1, characterized in that When the grid-connected circulating current includes the grid-connected circulating current caused by the deviation between the carrier phases of the grid-forming wind turbine and other grid-forming wind turbines, the virtual impedance module includes a virtual inductor and a band-pass filter.

3. The grid-connected circulating current suppression method according to claim 2, characterized in that The step of calculating the second reference voltage includes: Inputting the three-phase output current into the band-pass filter to calculate the filtered output current; Calculating the second reference voltage based on the filtered output current, the virtual inductor, and the first reference voltage.

4. The grid-connected circulating current suppression method according to claim 1, wherein When the grid-connected circulating current includes the grid-connected circulating current caused by the deviation between the dead-time voltages of the inverters of the grid-forming wind turbine and other grid-forming wind turbines, the virtual impedance module includes a band-pass filter with damping.

5. The grid-connected circulating current suppression method according to claim 4, wherein The step of calculating the second reference voltage includes: Inputting the three-phase output current into the band-pass filter with damping to calculate the filtered output current; Calculating the second reference voltage based on the filtered output current and the first reference voltage.

6. The grid-connected circulating current suppression method according to claim 1, characterized in that When the grid-connected circulating current includes the grid-connected circulating current caused by the unbalanced three-phase load of the grid-forming wind turbine and other grid-forming wind turbines, the virtual impedance module includes a preset integrator and a negative-sequence resistor.

7. The grid-connected circulating current suppression method according to claim 6, characterized in that, The step of calculating the second reference voltage includes: Extracting the negative-sequence current component from the three-phase output current by using a first preset integrator; Calculating the negative-sequence voltage drop generated by the negative-sequence current component on the negative-sequence impedance based on the negative-sequence current component and the negative-sequence impedance generated by using a second preset integrator and the negative-sequence resistor; Calculating the second reference voltage based on the negative-sequence voltage drop and the first reference voltage.

8. A grid-connected circulating current suppression device for a grid-forming wind turbine, characterized in that, Including: An acquisition unit configured to: obtain the active power reference value, actual active power value, reactive power reference value, actual reactive power value of the grid-forming wind turbine, as well as the three-phase output voltage and three-phase output current of the grid-forming wind turbine; A reference voltage calculation unit configured to: calculate a first reference voltage by performing proportional-integral operation on the active power reference value, actual active power value, reactive power reference value, and actual reactive power value of the grid-forming wind turbine; A virtual impedance control unit, configured to: calculate a second reference voltage by inputting the three-phase output voltage and three-phase output current of a network-forming wind turbine into a virtual impedance module; A voltage control unit, configured to: use the difference between the second reference voltage and the first reference voltage as a voltage reference to be provided to the voltage-current loop of the network-forming wind turbine, so as to suppress the grid-connected circulating current of the network-forming wind turbine; wherein, the grid-connected circulating current includes the circulating current generated by multiple parallel network-forming wind turbines under small disturbances in the case of applying a diode rectifier unit.

9. A network-forming wind turbine, characterized in that, Comprising the grid-connected circulating current suppression device according to claim 8.

10. A grid-forming wind power system, characterized in that, Comprising: A wind farm, including multiple parallel network-forming wind turbines, each of the multiple network-forming wind turbines including the grid-connected circulating current suppression device according to claim 8; A step-up transformer, connected to the wind farm via a point of common coupling, and configured to perform a step-up process on the electric energy received from the wind farm via the point of common coupling; An LFAC cable, configured to transmit the electric energy received from the step-up transformer to a DRU rectification station by using LFAC technology; A DRU rectification station, configured to perform a rectification process on the electric energy received from the LFAC cable, and transmit the rectified electric energy to the power grid.

11. A computer-readable storage medium, characterized in that, When the instructions in the computer-readable storage medium are run by at least one processor, the at least one processor is caused to execute the grid-connected circulating current suppression method according to any one of claims 1 to 7.

12. A computer device, characterized in that, Comprising: At least one processor; At least one memory storing computer-executable instructions, wherein, when the computer-executable instructions are run by the at least one processor, the at least one processor is caused to execute the grid-connected circulating current suppression method according to any one of claims 1 to 7.

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