Wind turbine generator grid-connected system oscillation suppression method based on network construction type energy storage converter

Abstract

The invention discloses a wind turbine generator grid-connected system oscillation suppression method based on a network construction type energy storage transverter, which comprises the following steps of: constructing a port characteristic model of the network construction type energy storage transverter, constructing an equivalent admittance model of a wind turbine generator based on the port characteristic model, obtaining a signal flow diagram, and calculating the oscillation of a grid-connected system of the wind turbine generator according to the model and the signal flow diagram. According to the method, an equivalent admittance model of the wind turbine generator with the impedance characteristic of the constructed energy storage converter is obtained, so that a negative sequence impedance remolding control strategy based on virtual parallel impedance is designed, and finally, control parameters in the remolding control strategy are optimized, so that the oscillation phenomenon in the direct-driven wind turbine generator grid-connected system is inhibited. According to the invention, through implementing a control strategy on the network-forming type energy storage converter, impedance coupling of an interconnection system is suppressed, and through virtual parallel impedance, an impedance interaction effect between the new energy equipment and the network-forming type energy storage converter is weakened, so that effective suppression of system oscillation is realized; and it is ensured that the direct-driven wind turbine generator grid-connected system can maintain a safe and stable operation state.

Description

Technical Field

[0001] The present invention relates to the technical field of grid-connected system control for wind turbines, and particularly to a method for suppressing oscillations in a grid-connected system of a wind turbine based on a grid-forming energy storage converter. Background Art

[0002] With the rapid progress in the field of new energy, the trend of large-scale and megascale development of new energy projects has become increasingly evident. Flexible DC transmission technology has become a key technical means for current new energy grid connection. However, although high-voltage DC transmission systems are favored for their control flexibility, the control interaction between new energy power stations and converter stations has caused increasingly serious broadband oscillation problems. It is not difficult to find from multiple oscillation accidents at home and abroad that the oscillation problems encountered by the grid-connected system of direct-drive wind turbines not only span multiple frequency bands but also exhibit a new characteristic, that is, the oscillation frequency is mirror-symmetric with the fundamental frequency, and this phenomenon is called mirror frequency coupling.

[0003] Current control strategies for mirror frequency coupling mainly focus on single-converter grid-connected systems. However, in the case of grid connection of wind power through direct-drive wind turbines, both sides of the AC collection line exhibit the characteristics of power electronic controlled power sources, which is fundamentally different from the impedance coupling characteristics of traditional wind turbines connected to the AC system. Therefore, directly applying existing strategies to the grid-connected system of direct-drive wind turbines, the potential impact on the operating characteristics of the interconnected system remains to be clarified. Even if these strategies can achieve the expected results, complex control modifications to a large number of existing wind turbines will bring high labor and economic costs.

[0004] Thus, using the controllability of the grid-forming energy storage converter station to achieve oscillation suppression for grid connection of wind power through direct-drive wind turbines to ensure the safe and stable operation of the interconnected system has become an urgent problem for those skilled in the art. Summary of the Invention

[0005] Aiming at the deficiencies of the above-mentioned prior art, the present invention provides a method for suppressing oscillations in a grid-connected system of a wind turbine based on a grid-forming energy storage converter. By implementing a control strategy for the grid-forming energy storage converter, the impedance coupling of the interconnected system is suppressed, and through a virtual shunt impedance, the impedance interaction between the wind turbine and the grid-forming energy storage converter is weakened, effectively suppressing system oscillations and ensuring that the grid-connected system of the wind turbine can maintain a safe and stable operating state.

[0006] To solve the above technical problems, the present invention adopts the following technical solutions:

[0007] A method for suppressing oscillations in a grid-connected system of a wind turbine based on a grid-forming energy storage converter includes the following steps:

[0008] S1. Build the port characteristic model of the network-forming energy storage converter;

[0009] S2. Based on the port characteristic model of the network-forming energy storage converter, build the equivalent admittance model of the wind turbine and obtain the signal flow graph;

[0010] S3. Use the signal flow graph to derive the equivalent admittance model of the wind turbine with the impedance characteristics of the network-forming energy storage converter according to the port characteristic model of the network-forming energy storage converter and the equivalent admittance model of the wind turbine;

[0011] S4. Use the derived equivalent admittance model of the wind turbine with the impedance characteristics of the network-forming energy storage converter to design a negative sequence impedance reshaping control strategy based on virtual shunt impedance;

[0012] S5. Optimize the control parameters in the negative sequence impedance reshaping control strategy based on virtual shunt impedance to identify high resonance risks and suppress oscillation phenomena in the grid-connected system of direct-drive wind turbines.

[0013] As a preferred solution, in step S1, the expression of the port characteristic model of the network-forming energy storage converter is:

[0014]

[0015] In the formula, Z MIMO (s) represents the AC impedance matrix of the network-forming energy storage converter, Z pp (s) represents the positive sequence impedance at the fundamental frequency s, Δv(s) represents the disturbance change of the voltage at the fundamental frequency s, Δi(s) represents the disturbance change of the current at the fundamental frequency s, Z nn (s -2 ) represents the negative sequence impedance at the second harmonic s -2 -2 -2 ) represents the disturbance change of the voltage at the second harmonic s -2 -2 GSC ) represents the disturbance change of the current at the second harmonic s -2 GSC

[0016] As a preferred solution, in step S2, the expression of the equivalent admittance model of the wind turbine is:

[0017]

[0018] In the formula, represents the AC admittance matrix of the network-forming energy storage converter, Δi GSC (s) represents the disturbance change of the current of the wind turbine at the fundamental frequency s, Δi GSC (s -2 ) represents the disturbance change of the current at the second harmonic s-2 The disturbance change of the current of the downwind generator set, Y pp (s) represents the positive-sequence admittance at the fundamental frequency s, Y nn (s -2 ) represents the negative-sequence admittance at the second harmonic s -2 The negative-sequence admittance at the second harmonic s pn (s) represents the admittance of the positive-sequence voltage change to the negative-sequence current change at the fundamental frequency s, Y np (s -2 ) represents the admittance of the negative-sequence voltage change to the positive-sequence current change at the second harmonic s -2 The negative-sequence voltage change to the positive-sequence current change at the second harmonic s, Δv GSC (s) is the disturbance change of the voltage of the downwind generator set at the fundamental frequency s, Δv GSC (s -2 ) is the disturbance change of the voltage of the downwind generator set at the second harmonic s -2 The disturbance change of the voltage of the downwind generator set at the second harmonic s.

[0019] As a preferred solution, in step S3, the expression of the equivalent admittance model of the wind turbine with the impedance characteristic of the grid-forming energy storage converter is:

[0020]

[0021] In the formula, represents the equivalent admittance of the wind turbine, represents the current response at the frequency ω p The current response at the frequency ω represents the current disturbance at the PCC.

[0022] As a preferred solution, in step S4, the negative-sequence impedance reshaping control strategy based on virtual parallel impedance includes:

[0023] S401. Virtually connect a parallel impedance at the common node of the grid-forming energy storage converter through a control algorithm to adjust the impedance characteristic of the grid-forming converter to the power grid;

[0024] S402. Introduce a band-pass filter to weaken the interference of the negative-sequence impedance to other frequency bands;

[0025] S403. Update the control equation of the current inner loop to achieve the control of the virtual parallel impedance.

[0026] As a preferred solution, the implementation process of the negative-sequence impedance reshaping control strategy based on virtual parallel impedance specifically includes:

[0027] Subtract the dq-axis disturbance component Δv sdq-ref of the PCC point voltage from the actual dq-axis disturbance component Δv sdq , and the obtained difference passes through the AC voltage outer loop controller G u (s) to generate the reference value Δi of the current control inner loopsdq-ref ; Then, subtract Δi sdq-ref from the dq-axis disturbance components Δi sdq of the PCC point current. After the obtained difference is processed by the current inner-loop controller G i (s), it is combined with the dq-axis disturbance components Δv sdq of the PCC point voltage to generate a modulation signal Δm sdq ; Then, multiply the modulation signal Δm sdq by the modulation gain K PWM to generate an AC voltage; Finally, multiply the generated AC voltage by the virtual resistance R vir and the virtual inductor L vir respectively, and feedback the multiplied results to the current outer-loop controller, thereby updating the control equation of the current inner-loop.

[0028] As a preferred solution, the expression for updating the control equation of the current inner-loop is:

[0029]

[0030] In the formula, G bpf (s) represents the transfer function of the band-pass filter, G i (s) represents the transfer function of the current inner-loop controller, L eq represents the equivalent inductance, m vd and m vq represent the modulation signals on the d-axis and q-axis respectively, v sd and v sq represent the components of the inverter terminal voltage on the d-axis and q-axis respectively, i sd-ref and i sq-ref represent the reference values of the AC current on the d-axis and q-axis respectively, i sd and i sq represent the components of the actual AC current on the d-axis and q-axis respectively, and ω represents the angular frequency.

[0031] As a preferred solution, in step S5, the process of optimizing the control parameters in the negative-sequence impedance reshaping control strategy based on virtual parallel impedance includes:

[0032] S501. Calculate the minimum loop gain of the system;

[0033] S502. According to the minimum loop gain, calculate the maximum value of the impedance coupling degree when the minimum loop gain is less than the preset threshold;

[0034] S503. According to the maximum value of the impedance coupling degree, calculate the value ranges of the virtual resistance and virtual inductor when the system coupling degree is lower than the maximum value of the impedance coupling degree;

[0035] S504. Select the minimum values of the virtual resistance and the virtual inductance from the value ranges of the virtual resistance and the virtual inductance as the current values of the virtual resistance and the virtual inductance, and perform system dynamic and steady-state characteristic tests;

[0036] S505. According to the test results, determine whether the system dynamic and steady-state characteristics meet the requirements. If they meet the requirements, execute step S506; if they do not meet the requirements, execute S507;

[0037] S506. Directly output the current values of the virtual resistance and the virtual inductance to obtain the final values of the virtual resistance and the virtual inductance;

[0038] S507. After increasing the current values of the virtual resistance and the virtual inductance by a preset value respectively, use them as the new current values of the virtual resistance and the virtual inductance, and return to step S504 to perform system dynamic and steady-state characteristic tests until the final values of the virtual resistance and the virtual inductance are output.

[0039] As a preferred solution, in step S501, the expression for calculating the minimum loop gain of the system is:

[0040]

[0041] In the formula, T m (s) represents the minimum gain amount in the system loop, Z WFGSC (s) represents the AC impedance of the grid-forming energy storage converter, Z wf (s) represents the impedance of the wind power generation unit.

[0042] As a preferred solution, in step S502, the expression for the impedance coupling degree is:

[0043]

[0044] In the formula, β represents the impedance coupling degree, Z eq represents the AC equivalent impedance.

[0045] Compared with the prior art, the present invention has the following technical effects:

[0046] By constructing a port characteristic model of a grid-forming energy storage converter and an equivalent admittance model of a wind turbine, the present invention designs a negative-sequence impedance reshaping control strategy. This strategy utilizes virtual shunt impedance technology to significantly weaken the impedance interaction between new energy devices and the grid-forming energy storage converter, thereby achieving strong suppression of system oscillation phenomena and ensuring that the direct-drive wind turbine grid-connected system can maintain a safe and stable operating state. In addition, this method realizes impedance reshaping through control, reduces costs and system complexity, reduces energy losses, improves control accuracy, enhances the robustness of the system, and further ensures the stable operation of the power grid by identifying and controlling the high-resonance risk frequency range. Description of the Drawings

[0047] In order to make the objectives, technical solutions, and advantages of the invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings, where:

[0048] Figure 1 is a flowchart of a method for suppressing oscillations in a wind turbine grid-connected system based on a grid-forming energy storage converter disclosed in the present invention;

[0049] Figure 2 is a signal flow diagram between the AC voltage disturbance and current response of the wind turbine in this embodiment;

[0050] Figure 3 is a schematic diagram of the system transfer structure after introducing negative-sequence impedance control based on virtual shunt impedance in this embodiment;

[0051] Figure 4 is a flowchart for optimizing the control parameters in the negative-sequence impedance reshaping control strategy based on virtual shunt impedance in this embodiment. Detailed Embodiments

[0052] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. Usually, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but only represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0053] The present invention will be further described in detail below with reference to the accompanying drawings.

[0054] In view of the broadband oscillation problem encountered in the process of new energy grid connection, especially the mirror frequency coupling phenomenon in which the oscillation frequency and the fundamental frequency are mirror-symmetric in the grid-connected system of direct-drive wind turbines. The existing control strategies mainly target single-converter grid-connected systems. However, when wind power is grid-connected through a network-forming energy storage converter, due to the characteristics of the power electronic controlled power sources on both sides of the AC collection line, there are differences in the impedance coupling characteristics from those of traditional wind turbines connected to the AC system. The direct application of existing strategies has limited effect and high cost. To solve this problem, the present invention proposes a negative-sequence impedance reshaping control strategy based on a network-forming energy storage converter. By virtually connecting an impedance in parallel at the point of common coupling, the impedance characteristics of the converter are adjusted, effectively suppressing oscillations, improving system stability, and at the same time reducing the transformation cost and system complexity, meeting the high requirements of wind power grid connection for grid stability and power quality.

[0055] The present invention discloses a method for suppressing oscillations in a grid-connected system of a wind turbine based on a network-forming energy storage converter by using the proposed negative-sequence impedance reshaping control strategy based on a network-forming energy storage converter, as Figure 1 shown. The method includes:

[0056] S1. Construct a port characteristic model of the network-forming energy storage converter;

[0057] S2. Based on the port characteristic model of the network-forming energy storage converter, construct an equivalent admittance model of the wind turbine and obtain a signal flow graph;

[0058] S3. Using the signal flow graph, according to the port characteristic model of the network-forming energy storage converter and the equivalent admittance model of the wind turbine, derive an equivalent admittance model of the wind turbine with the impedance characteristics of the network-forming energy storage converter;

[0059] S4. Using the derived equivalent admittance model of the wind turbine with the impedance characteristics of the network-forming energy storage converter, design a negative-sequence impedance reshaping control strategy based on virtual parallel impedance;

[0060] S5. Optimize the control parameters in the negative-sequence impedance reshaping control strategy based on virtual parallel impedance, so as to identify high resonance risks and suppress the oscillation phenomenon in the grid-connected system of direct-drive wind turbines.

[0061] In the present invention, by constructing a port characteristic model of a network-forming energy storage converter and an equivalent admittance model of a wind turbine generator set, a negative-sequence impedance reshaping control strategy is designed. This strategy utilizes virtual parallel impedance technology to significantly weaken the impedance interaction between new energy devices and the network-forming energy storage converter, thereby achieving strong suppression of system oscillation phenomena and ensuring that the grid-connected system of direct-drive wind turbine generators can maintain a safe and stable operating state. In addition, this method realizes impedance reshaping through control, reduces costs and system complexity, reduces energy losses, improves control accuracy, enhances the robustness of the system, and further ensures the stable operation of the power grid by identifying and controlling the high-resonance risk frequency range.

[0062] During specific implementation, in the process of executing step S1, in the context of the AC interconnection between the wind turbine generator set and the network-forming energy storage converter, the impedance characteristics of the network-forming energy storage converter will have a significant impact on the port characteristics of the wind turbine generator set. Therefore, when constructing the equivalent admittance model of the wind turbine generator set, the impedance coupling relationship between the network-forming energy storage converter and the wind turbine generator set must be fully incorporated to obtain an equivalent admittance model of the wind turbine generator set that can accurately reflect the influence of the impedance characteristics of the network-forming energy storage converter. The network-forming energy storage converter usually adopts an AC voltage control strategy to provide a stable AC voltage for the wind turbine generator set. In this case, the network-forming energy storage converter does not adopt a control strategy with asymmetric dq gains. Its frequency coupling characteristics mainly come from the capacitor voltage of the sub-module and the circulating current. However, after the circulating current suppression measures are put into effect, the above-mentioned frequency coupling phenomenon will be significantly suppressed, that is, the off-diagonal elements of the two-dimensional impedance model can be ignored. Therefore, the expression of the port characteristic model of the network-forming energy storage converter can be simplified as:

[0063]

[0064] In the formula, it is defined that s = jω p , s -2 = jω p - j2ω 1 , where ω p = 2πf p , f p represents the externally applied perturbation frequency; Z MIMO (s) represents the AC impedance matrix of the network-forming energy storage converter; Z pp (s) represents the positive-sequence impedance at the fundamental frequency s, that is, when the AC side of the network-forming energy storage converter is subjected to a current perturbation Δi(s) with a frequency of f p , the response of the AC side voltage Δv(s) to this perturbation, and the response frequency is also f p ; Δv(s) represents the perturbation change of the voltage at the fundamental frequency s; Δi(s) represents the perturbation change of the current at the fundamental frequency s; Z nn (s -2) represents the negative sequence impedance at the second harmonic s -2 That is, when the disturbance frequency is f p -2f 1 of the current Δi(s -2 ) acts on the AC side of the network-forming energy storage converter, the AC side voltage Δv(s -2 )'s response to this disturbance, and the response frequency is f p -2f 1 , where f 1 represents the fundamental frequency of the system; Δv(s -2 ) represents the disturbance change of the voltage at the second harmonic s -2 ; Δi(s -2 ) represents the disturbance change of the current at the second harmonic s -2 .

[0065] Specifically, in step S2, the wind power generation unit often uses the phase-locked loop technology to track the phase information of the voltage at the point of common coupling. The phase-locked loop mainly completes the phase tracking task by regulating the q-axis voltage component. This mechanism leads to the asymmetry of the output of the control link of the wind power generation unit, and further makes its AC port exhibit the characteristics of double input-double output. The expression of the equivalent admittance model of the wind turbine is:

[0066]

[0067] In the formula, it is defined that s = jω p , s -2 = jω p -j2ω 1 , where ω p = 2πf p , f p represents the externally applied disturbance frequency; represents the AC admittance matrix of the network-forming energy storage converter, which is used to describe its multi-input multi-output characteristics; Δi GSC (s) represents the disturbance change of the current of the wind turbine at the fundamental frequency s; Δi GSC (s -2 ) represents the disturbance change of the current of the wind turbine at the second harmonic s -2 ; Y pp (s) represents the positive sequence admittance at the fundamental frequency s, that is, it describes the response of the AC side current Δi p of the network-forming energy storage converter to the AC side voltage disturbance Δv GSC (s) at the frequency of f GSC (s), and the response frequency is the same as the disturbance frequency, both are f p ; Y nn (s -2 ) represents at the second harmonic s -2Negative sequence admittance at, that is, at a frequency of f p -2f 1 Voltage disturbance Δv GSC (s -2 ) causes current response Δi GSC (s -2 ), and the response frequency is the same as the disturbance frequency, which is f p -2f 1 , where f 1 is the fundamental frequency of the system; Y pn (s) represents the admittance of the positive sequence voltage change to the negative sequence current change at the fundamental frequency s, that is, under the voltage disturbance at a frequency of f p , the transfer relationship of the current response at a frequency of f p -2f 1 ; Y np (s -2 ) represents the admittance of the negative sequence voltage change to the positive sequence current change at the second harmonic s -2 , that is, the transfer relationship of the current response at a frequency of f p -2f 1 to the voltage disturbance at a frequency of f p ; Δv GSC (s) is the disturbance change of the wind turbine generator voltage at the fundamental frequency s; Δv GSC (s -2 ) is the disturbance change of the wind turbine generator voltage at the second harmonic s -2 .

[0068] Specifically, based on the obtained port characteristic model of the grid-forming energy storage converter and the obtained equivalent admittance model of the wind turbine generator, a signal flow graph is constructed, as shown in Figure 2 , which shows the signal flow path of the AC voltage disturbance and current response of the wind power generation unit. When a voltage disturbance Δv p with an angular frequency of ω pcc,ωp is injected at the point of common coupling (PCC), this disturbance can pass through Y pp , Y pn (the angular frequency annotation is omitted here, Y pp and Y pn correspond to Y pp (s) and Y pn (s -2 )) respectively to generate a current response with an angular frequency of ω p and a coupled current response with an angular frequency of ω p -2ω 0 , as shown in paths ① and ②. Further, these two current responses will simultaneously serve as current disturbances of the grid-forming energy storage converter. After the action of Z pn , Z nn , an angular frequency of ωp -2ω 0 Coupled voltage response Δv c,ωp-2ω0 , which is then passed through Y np to generate a current response i p at frequency ω p,ωp . It can be seen from Figure 2 that there are two forward paths in Δv pcc,ωp that can excite the current response Δi p,ωp , namely path ①③⑤ and path ②, which also indicates that the relationship between the AC side port voltage and current of the grid-forming energy storage converter (GSC) in the power generation unit is no longer independent of the GSC itself, but is affected by the impedance of the grid-forming energy storage converter.

[0069] In specific implementation, in step S3, after mastering the impedance characteristics of the grid-forming energy storage converter and the wind power generation unit, with the help of the signal flow graph between the AC voltage perturbation and current response of the wind power generation unit, the dynamic interaction of each electrical quantity is intuitively revealed, and then the equivalent impedance of the wind power generation unit is derived. The expression of the equivalent admittance model of the wind turbine with the impedance characteristics of the grid-forming energy storage converter is as follows:

[0070]

[0071] In the formula, represents the equivalent admittance of the wind turbine, represents the frequency ω p under the current response, represents the current perturbation at the PCC.

[0072] In specific implementation, during the execution of step S4, by examining the signal flow graph between the AC voltage perturbation and current response of the wind power generation unit and combining the mathematical expression of its AC equivalent admittance, it can be clearly recognized that the core link of the impedance interaction between the wind power generation unit and the grid-forming energy storage converter lies in the negative sequence impedance Z nn . If measures can be taken to effectively reduce the magnitude of the negative sequence impedance Z nn , then the amplitude of the coupled voltage perturbation will be significantly constrained, thereby effectively reducing the impedance interaction phenomenon inside the entire interconnected system.

[0073] It should be noted that the positive sequence impedance Z pp of the grid-forming energy storage converter does not participate in the impedance interaction process. Further, through in-depth analysis of the positive and negative sequence phase sequence relationship, it is found that within the frequency range of 0 to 100 Hz, there is a specific relationship between the positive sequence impedance Z pp and the negative sequence impedance Z nn of the grid-forming energy storage converter, which is specifically expressed as follows:

[0074] Z nn (ωp ) = Z pp (2ω 1 - ω p )

[0075] By shunting an impedance at the AC port of the grid-forming energy storage converter, the amplitude of its AC impedance can be effectively reduced, thereby ensuring that the coupling degree β is maintained at β max as follows, thus enhancing the stability of the system. Given that it is difficult to directly introduce a passive inductive impedance in specific application embodiments, it is necessary to consider from the perspective of the control system.

[0076] In this embodiment, the negative-sequence impedance reshaping control strategy based on virtual shunt impedance includes:

[0077] S401. Virtually shunt an impedance at the common node of the grid-forming energy storage converter through a control algorithm to adjust the impedance characteristics of the grid-forming converter to the power grid;

[0078] S402. Introduce a band-pass filter to weaken the interference of the negative-sequence impedance on other frequency bands;

[0079] S403. Update the control equation of the current inner loop to achieve the control of the virtual shunt impedance.

[0080] As Figure 3 shown, the system transfer function block diagram after adding an additional parallel virtual inductive impedance controller is presented. It can be seen from Figure 3 that the implementation process of the negative-sequence impedance reshaping control strategy based on virtual shunt impedance specifically includes:

[0081] Subtract the dq-axis disturbance component Δv sdq-ref of the PCC point voltage from the actual dq-axis disturbance component Δv sdq . The obtained difference passes through the AC voltage outer loop controller G u (s) to generate the reference value Δi sdq-ref of the current control inner loop; then, subtract Δi sdq-ref from the dq-axis disturbance component Δi sdq of the PCC point current. The obtained difference is processed by the current inner loop controller G i (s) and then generates a modulation signal Δm sdq together with the dq-axis disturbance component Δv sdq of the PCC point voltage; then, the modulation signal Δm sdq is multiplied by the modulation gain K PWM to generate an AC voltage; finally, the generated AC voltage is multiplied by the virtual resistance R vir and the virtual inductor L vir respectively, and the multiplied results are fed back to the current outer loop controller, thereby updating the control equation of the current inner loop.

[0082] Figure 3 Among them, G bpf (s) represents a band-pass filter, whose function is to weaken the interference caused by negative-sequence impedance to other frequency bands. Z load represents the equivalent load on the AC side. In order to describe the system more comprehensively, the equivalent resistance R of the bridge arm is introduced arm and the equivalent inductance L arm .

[0083] Add the feedback current to the current on the parallel inductive reactance to obtain the control equation for updating the current inner loop, and its expression is:

[0084]

[0085] In the formula, G bpf (s) represents the transfer function of the band-pass filter, G i (s) represents the transfer function of the current inner loop controller, L eq represents the equivalent inductance, m vd and m vq respectively represent the modulation signals on the d-axis and q-axis, v sd and v sq respectively represent the components of the inverter port voltage on the d-axis and q-axis, i sd-ref and i sq-ref respectively represent the reference values of the AC current on the d-axis and q-axis, i sd and i sq respectively represent the components of the actual AC current on the d-axis and q-axis, and ω represents the angular frequency

[0086] In specific implementation, in step S5, the process of optimizing the control parameters in the negative-sequence impedance reshaping control strategy based on virtual parallel impedance includes:

[0087] S501. Calculate the minimum loop gain of the system;

[0088] S502. According to the minimum loop gain, calculate the maximum value of the impedance coupling degree when the minimum loop gain is less than the preset threshold;

[0089] S503. According to the maximum value of the impedance coupling degree, calculate the value ranges of the virtual resistance and virtual inductance when the system coupling degree is lower than the maximum value of the impedance coupling degree;

[0090] S504. Select the minimum values of the virtual resistance and virtual inductance from the value ranges of the virtual resistance and virtual inductance as the current values of the virtual resistance and virtual inductance, and perform system dynamic and steady-state characteristic tests;

[0091] S505. According to the test results, determine whether the dynamic and steady-state characteristics of the system meet the requirements. If they meet the requirements, execute step S506; if they do not meet the requirements, execute S507;

[0092] S506. Directly output the values of the current virtual resistance and virtual inductance to obtain the final values of the virtual resistance and virtual inductance;

[0093] S507. After increasing the values of the current virtual resistance and virtual inductance by a preset value respectively, use them as the new values of the current virtual resistance and virtual inductance, and return to step S504 to test the dynamic and steady-state characteristics of the system until the final values of the virtual resistance and virtual inductance are output.

[0094] Among them, in step S501, the expression for calculating the minimum loop gain of the system is:

[0095]

[0096] In the formula, T m (s) represents the minimum gain amount in the system loop, Z WFGSC (s) represents the AC impedance of the grid-forming energy storage converter, Z wf (s) represents the impedance of the wind power generation unit.

[0097] In step S502, the expression for the impedance coupling degree is:

[0098]

[0099] In the formula, β represents the impedance coupling degree, Z eq represents the AC equivalent impedance. It is used to quantitatively consider the difference between the AC equivalent impedance Z eq of the grid-connected converter after considering the impedance coupling effect and the impedance Z pp of the grid-connected converter when ignoring the impedance coupling. The magnitude of the β value directly reflects the strength of the impedance coupling of the interconnected system. Specifically, the larger the β value, the more significant the impedance coupling phenomenon of the interconnected system.

[0100] When specifically applying this embodiment, during the power transmission of the direct-drive wind turbine grid-connected system, the frequency band with the highest impedance coupling degree happens to be within the bandwidth frequency band of the wind turbine phase-locked loop. This frequency band is also the frequency band where the system stability performance is the worst within the specified range. The bandwidth of the wind turbine phase-locked loop is represented as f pll , in order to identify the high resonance risk of the system, set f 0 ±f pll as the high resonance risk frequency range, where f 0 represents the fundamental frequency.

[0101] Therefore, by adjusting the value of the equivalent capacitance of the adjustment sub-module, the relationship between the maximum coupling degree and the minimum loop gain at the dangerous resonance point can be revealed (the same conclusion can be drawn by adjusting other parameters). Once the threshold value of the maximum coupling degree is determined, by appropriately setting the values of the virtual resistance R vir and the virtual inductance L vir such that the impedance coupling degree after enabling the virtual shunt impedance control is lower than this threshold value, the value ranges of R vir and L vir that satisfy the coupling degree limit condition (i.e., after adopting the control strategy proposed in this embodiment, the system impedance coupling degree needs to be lower than the maximum impedance coupling degree) can be determined.

[0102] While ensuring that the R vir and L vir within this value range can satisfy the coupling degree limit, it is also necessary to consider that the virtual shunt impedance control strategy should minimize the impact on the stable operation of the system when reshaping the negative sequence impedance of the network-forming energy storage converter. To this end, it is necessary to compare the characteristics of the AC voltage before and after enabling the control strategy and eliminate those parameter combinations that cause significant fluctuations in the AC voltage; further test the change in the steady-state power step response of the system to verify the impact of the virtual shunt inductance control strategy on the dynamic performance of the system, so as to provide a basis for finally determining the optimal values of R vir and L vir . Thus, the virtual shunt impedance parameter optimization flow chart as shown in Figure 4 is formed.

[0103] To this end, the optimized virtual resistance R vir and the virtual inductance L vir are obtained through an iterative process, so that without changing the hardware configuration, the impedance characteristics of the converter can be adjusted through the control algorithm to optimize the impedance coupling degree of the system within the PLL bandwidth frequency band and reduce the risk of system oscillation.

[0104] In this embodiment, by accurately calculating and iteratively optimizing the values of the virtual resistance R vir and the virtual inductance L vir , the control of the impedance coupling degree of the network-forming energy storage converter is realized, ensuring that the system stability is significantly improved within the PLL bandwidth frequency band of the wind turbine. The adjustment of these virtual impedance parameters not only reduces the oscillation problem within the high resonance risk frequency range, but also enhances the overall stability of the system by meeting the stability requirements of the minimum loop gain T m (s). In addition, the virtual resistance and the virtual inductance, as the core of the negative sequence impedance reshaping control strategy, allow the system performance to be optimized through the control algorithm without changing the hardware configuration. These optimized parameters enable the system to effectively control the impedance coupling degree, improve the system stability, and reduce the high resonance risk when the wind power is connected to the grid through a direct-drive wind turbine.

[0105] In summary, the present invention constructs a port characteristic model of a grid-forming energy storage converter and an equivalent admittance model of a wind turbine, and designs a negative-sequence impedance reshaping control strategy. This strategy utilizes virtual parallel impedance technology to significantly weaken the impedance interaction between new energy devices and the grid-forming energy storage converter, thereby achieving a strong suppression of system oscillation phenomena and ensuring that the direct-drive wind turbine grid-connected system can maintain a safe and stable operating state. In addition, this method realizes impedance reshaping through control, reduces costs and system complexity, reduces energy losses, improves control accuracy, enhances the robustness of the system, and further ensures the stable operation of the power grid by identifying and controlling the high-resonance risk frequency range.

[0106] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described by referring to the preferred embodiments of the present invention, those of ordinary skill in the art should understand that various changes can be made in form and details without departing from the spirit and scope of the present invention defined by the appended claims.

Claims

1. A method for suppressing oscillation of a wind turbine grid-connected system based on a grid-connected energy storage converter, characterized in that: The steps include: S1. Constructing the port characteristic model of the grid-type energy storage converter; S2. Based on the port characteristic model of the grid-type energy storage converter, an equivalent admittance model of the wind turbine is constructed, and a signal flow graph is obtained; S3, using the signal flow graph, according to the port characteristic model of the grid-type energy storage converter and the equivalent admittance model of the wind turbine generator set, deriving the equivalent admittance model of the wind turbine generator set with impedance characteristics of the grid-type energy storage converter; S4. Using the derived equivalent admittance model of the wind turbine generator set with impedance characteristics of the grid-connected energy storage converter, a negative-sequence impedance reshaping control strategy based on virtual parallel impedance is designed; S5. Optimize the control parameters in the negative-sequence impedance reshaping control strategy based on virtual parallel impedance, so as to identify high resonance risks and suppress oscillation phenomena in the direct-drive wind turbine grid-connected system.

2. The method for suppressing oscillation of a wind turbine grid-connected system based on a grid-connected energy storage converter according to claim 1, characterized in that: In step S1, the expression of the port characteristic model of the grid-type energy storage converter is: In the formula, Z MIMO (s) represents the AC impedance matrix of the grid-type energy storage converter, Z pp (s) represents the positive sequence impedance at the fundamental frequency s, Δv(s) represents the disturbance change of voltage at the fundamental frequency s, Δi(s) represents the disturbance change of current at the fundamental frequency s, and Z nn (s -2 ) indicates that the second harmonic s -2 Negative sequence impedance under -2 ) indicates that the second harmonic s -2 The disturbance change of the voltage, Δi(s -2 ) indicates that the second harmonic s -2 The disturbance change of the current.

3. The method for suppressing oscillation of a wind turbine grid-connected system based on a grid-connected energy storage converter according to claim 1, characterized in that: In step S2, the expression of the equivalent admittance model of the wind turbine generator set is: In the formula, represents the AC admittance matrix of the grid-type energy storage converter, Δi GSC (s) represents the disturbance change of wind turbine current at the base frequency s, Δi GSC (s -2 ) indicates that the second harmonic s -2 The disturbance change of the wind turbine current, Y pp (s) represents the positive sequence admittance at the fundamental frequency s, Y nn (s -2 ) indicates that the second harmonic s -2 The negative sequence admittance, Y pn (s) represents the admittance of the positive sequence voltage change to the negative sequence current change under the fundamental frequency s, Y np (s -2 ) indicates that the second harmonic s -2 Admittance of negative sequence voltage change to positive sequence current change, Δv GSC (s) The disturbance change of the wind turbine voltage at the base frequency s, Δv GSC (s -2 ) at the second harmonic s -2 The disturbance change of the voltage of the wind turbine generator system.

4. The method for suppressing oscillation of a wind turbine grid-connected system based on a grid-connected energy storage converter according to claim 1, characterized in that: In step S3, the expression of the equivalent admittance model of the wind turbine generator set with the impedance characteristics of the grid-type energy storage converter is: In the formula, represents the equivalent admittance of the wind turbine, Indicates frequency ω p The current response under represents the current disturbance at the PCC.

5. The method for suppressing oscillation of a wind turbine grid-connected system based on a grid-connected energy storage converter according to claim 1, characterized in that: In step S4, the negative sequence impedance reshaping control strategy based on virtual parallel impedance includes: S401, creating a virtual parallel impedance at a common node of the grid-type energy storage converter through a control algorithm to adjust the impedance characteristics of the grid-type converter to the grid; S402, introducing a bandpass filter to weaken the interference of negative sequence impedance on other frequency bands; S403: Update the control equation of the current inner loop to achieve control of the virtual parallel impedance.

6. The method for suppressing oscillation of a wind turbine grid-connected system based on a grid-connected energy storage converter according to claim 5, characterized in that: The implementation process of the negative sequence impedance reshaping control strategy based on virtual parallel impedance specifically includes: The dq-axis disturbance component of the PCC voltage Δv sdq-ref The actual dq axis disturbance component Δv sdq Subtract the difference, and the difference is passed through the AC voltage outer loop controller G u (s), generates the reference value Δi for the inner loop of the current control sdq-ref ; Next, Δi sdq-ref The dq-axis disturbance component Δi of the PCC point current sdq Subtract the difference, and the difference is passed through the current inner loop controller G i (s) After processing, the dq-axis disturbance component Δv of the PCC point voltage sdq Generate a modulation signal Δm together sdq ; Then, the modulation signal Δm sdq With modulation gain K PWM Multiply them to generate an AC voltage; finally, the generated AC voltage is multiplied by the virtual resistance R vir and virtual inductance L vir After multiplication, the multiplication result is fed back to the current outer loop controller to update the control equation of the current inner loop.

7. The method for suppressing oscillation of a wind turbine grid-connected system based on a grid-connected energy storage converter according to claim 6, characterized in that: The expression of the control equation of the updated current inner loop is: In the formula, G bpf (s) represents the transfer function of the bandpass filter, G i (s) represents the transfer function of the current inner loop controller, L eq Represents the equivalent inductance, m vd With m vq Represent the modulation signals on the d-axis and q-axis respectively, v sd With v sq They represent the components of the inverter port voltage on the d-axis and q-axis respectively, i sd-ref with i sq-ref They represent the reference values ​​of the AC current on the d-axis and q-axis respectively, i sd with i sq They represent the components of the actual alternating current on the d-axis and q-axis respectively, and ω represents the angular frequency.

8. The method for suppressing oscillation of a wind turbine grid-connected system based on a grid-connected energy storage converter according to claim 1, characterized in that: In step S5, the process of optimizing the control parameters in the negative sequence impedance reshaping control strategy based on virtual parallel impedance includes: S501, calculating the minimum loop gain of the system; S502, calculating, according to the minimum loop gain, a maximum impedance coupling value when the minimum loop gain is less than a preset threshold; S503, calculating, according to the maximum impedance coupling value, a range of virtual resistance and virtual inductance when the system coupling degree is lower than the maximum impedance coupling value; S504, selecting the minimum value of the virtual resistance and virtual inductance from the value range of the virtual resistance and virtual inductance as the current value of the virtual resistance and virtual inductance, and performing a system dynamic and steady-state characteristic test; S505: According to the test results, determine whether the system dynamic and steady-state characteristics meet the requirements. If they do, execute step S506; if they do not, execute S507; S506, directly outputting the current values ​​of the virtual resistance and virtual inductance to obtain final values ​​of the virtual resistance and virtual inductance; S507, after increasing the current values ​​of the virtual resistance and virtual inductance by the preset values, use them as the new current values ​​of the virtual resistance and virtual inductance, and return to step S504 to perform system dynamic and steady-state characteristic tests until the final values ​​of the virtual resistance and virtual inductance are output.

9. The method for suppressing oscillation of a wind turbine grid-connected system based on a grid-connected energy storage converter according to claim 8, characterized in that: In step S501, the expression for calculating the minimum loop gain of the system is: Where, T m (s) represents the minimum gain in the system loop, Z WFGSC (s) represents the AC impedance of the grid-type energy storage converter, Z wf (s) represents the impedance of the wind power generation unit.

10. The method for suppressing oscillation of a wind turbine grid-connected system based on a grid-connected energy storage converter according to claim 8, characterized in that: In step S502, the impedance coupling degree is expressed as: Where β represents the impedance coupling, Z eq Represents the AC equivalent impedance.

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