An Analysis Method for Oscillation Propagation Characteristics of Wind Power Transmitted via DRU-MMC

Abstract

This invention discloses a method for analyzing the oscillation propagation characteristics of wind power transmitted via DRU-MMC. First, a DC-side impedance model of the receiving-end MMC is constructed and aggregated with the DC line impedance to obtain the equivalent DC impedance at the receiving end. Then, the impedance matrices of the wind turbine side and the AC-side impedance matrices of the sending-end lightweight converter are constructed separately. Next, the propagation coefficients of voltage and current of each converter on both the AC and DC sides are solved to analyze the propagation characteristics of subsynchronous / supersynchronous oscillations. Based on these propagation characteristics, the oscillation risk level of different nodes in the AC / DC interconnection system can be further analyzed, accurately identifying nodes with abnormal amplification and concentrated oscillation components. This effectively improves the prediction accuracy and proactive prevention and control capabilities of the AC / DC interconnection system for subsynchronous / supersynchronous oscillation risks, providing a reliable guarantee for the safe and stable operation of large-scale wind power transmission systems via lightweight converters.

Description

Technical Field

[0001] This invention belongs to the field of wind power DC transmission technology, specifically relating to a method for analyzing the oscillation propagation characteristics of wind power transmitted via DRU-MMC. Background Technology

[0002] my country's installed wind power capacity has continued to grow rapidly, becoming an important component of the new power system. Newly developed wind power bases have long transmission distances, placing higher demands on the reliability and economy of the transmission system. Therefore, there is an urgent need to design lightweight wind power converter platforms to reduce platform weight, size, and construction and operation costs, thereby improving the overall economic efficiency of wind power development.

[0003] At the sending end of wind power flexible DC transmission projects, a lightweight converter composed of diode rectifier units (DRUs) and modular multilevel converters (MMCs) has become an effective technical approach that balances reliability and economy. In this converter, the DRU and MMC are connected in parallel on the AC side and in series on the DC side. The DRU handles most of the power transmission, possessing the advantages of high integration, low cost, and low loss characteristic of uncontrolled diode rectifiers. The MMC provides AC bus voltage construction and reactive power regulation capabilities, leveraging the flexible control and rapid response features of fully controlled converters. Working together, these two components significantly reduce the equipment capacity requirements and construction costs of the wind power converter platform while ensuring system operational flexibility, thus meeting the needs of large-scale long-distance wind power development.

[0004] However, the introduction of uncontrolled rectifiers alters the small-signal characteristics of the system. When subsynchronous / supersynchronous oscillations occur between the wind turbine and the sending-end lightweight converter due to mismatch in small-signal interaction characteristics, these oscillations propagate along a specific path. Specifically, the oscillations first propagate from the AC side of the lightweight converter to the DC side, then via the DC cable to the DC side of the receiving-end MMC, and finally from the DC side of the receiving-end MMC back to the AC side and then to the power grid. During this process, the propagation paths of voltage and current disturbance components are not consistent, and the propagation characteristics of disturbance components differ significantly under different control methods. Therefore, it is urgent to propose an analysis method for the oscillation propagation characteristics of wind power transmitted through the DRU-MMC lightweight converter to elucidate the impact mechanism of the lightweight converter after the introduction of the DRU on the system oscillation propagation characteristics, and to provide theoretical support for system oscillation suppression and stable operation.

[0005] Through research, we found that existing technologies for studying the oscillation propagation characteristics of wind power transmission systems via DRU-MMC lightweight converters are incomplete. Some literature, focusing on multi-terminal flexible DC transmission systems, uses three levels of sensitivity—port-level, impedance-level, and parameter-level—to locate the dominant variables and oscillation propagation paths. Other literature, also focusing on multi-terminal flexible DC transmission systems, analyzes the phenomenon of unequal oscillation power propagation on both the AC and DC sides of the MMC converter and proposes a method for quantifying the energy propagation characteristics of oscillations.

[0006] In summary, existing research focuses on multi-terminal flexible DC transmission systems, calculating the oscillation energy propagation coefficient based on the small-signal characteristics of the converter, and analyzing the oscillation propagation characteristics of the system. For systems that transmit wind power from DRU and MMC via lightweight converters, the voltage and current propagation characteristics of different converters differ significantly due to the introduction of uncontrolled rectifier devices, making existing methods difficult to apply directly and lacking effective methods for analyzing oscillation propagation characteristics. Summary of the Invention

[0007] In view of the above, the present invention provides a method for analyzing the oscillation propagation characteristics of wind power transmitted via DRU-MMC, which can accurately identify the nodes with abnormal amplification and concentration of oscillation components, thereby effectively improving the prediction accuracy and proactive prevention and control capability of AC / DC interconnection systems for sub / supersynchronous oscillation risks.

[0008] A method for analyzing the oscillation propagation characteristics of wind power transmitted via DRU-MMC includes the following steps: (1) For the receiving-end MMC in the wind power transmission system via DRU-MMC, consider the small-signal equations of its circuit characteristics and control characteristics, and construct the DC-side impedance model of the receiving-end MMC. (2) The DC side impedance of the receiving end MMC is combined with the impedance of the DC cable and the filter inductor in series to obtain the equivalent DC impedance of the receiving end. (3) The system is divided into two sides, the fan and the light converter, with the AC side of the light converter at the sending end as the boundary. The impedance matrix of the fan side is constructed. The small-signal characteristics of the DC side of the light converter at the sending end and the small-signal characteristics of the MMC control loop at the sending end are also taken into account. The impedance matrix of the AC side of the light converter at the sending end is constructed. (4) Inject positive-sequence disturbance voltages of different frequencies into the AC side of the light converter at the sending end, and solve the voltage and current propagation coefficients of the light converter at the sending end and the MMC at the receiving end respectively, in order to analyze the propagation characteristics of the subsynchronous / supersynchronous oscillation of the system.

[0009] The wind power transmission system via DRU-MMC includes a wind farm, an AC bus, a sending-end lightweight converter, a DC cable, and a receiving-end MMC. Wind turbines in the wind farm are connected to the AC bus via grid-side converters and converter transformers. The sending-end lightweight converter consists of a DRU and a sending-end MMC, connected in series on the DC side and then connected to one end of the DC cable via a filter inductor. Their AC sides are connected in parallel to the AC bus. The receiving-end MMC's DC side is connected to the other end of the DC cable via a filter inductor, and its AC side is connected to the onshore AC grid. When the wind turbine output power increases, the system is prone to subsynchronous / supersynchronous oscillations. These oscillations propagate from the sending-end lightweight converter through the DC cable to the receiving-end MMC, and then further propagate to the onshore AC grid.

[0010] The sending-end lightweight converter and the receiving-end MMC have significant differences in small-signal characteristics. During the propagation of subsynchronous / supersynchronous oscillations, the propagation paths of voltage and current are different, and the current and voltage transmission often differs for a single device. Therefore, it is necessary to solve the corresponding oscillation propagation coefficients to further analyze the oscillation propagation characteristics of the system.

[0011] Furthermore, the small-signal equations for the characteristics and control characteristics of the receiving-end MMC circuit in step (1) include multiple components such as phase-locked loop, DC voltage control, and main circuit. Based on this, the DC-side impedance model of the receiving-end MMC is constructed as follows, which is used to analyze the dynamic characteristics of voltage and current disturbance components propagating from the DC side to the AC side in the receiving-end MMC.

[0012] in: Z r The DC-side impedance of the receiving-end MMC, For the small-signal component of the DC-side voltage of the receiving-end MMC, Δ i dc This represents the small-signal component of the DC-side current in the system.

[0013] Furthermore, the expression for the equivalent DC impedance at the receiving end in step (2) is as follows:

[0014] in: Z r-equ The receiving end is the equivalent DC impedance. Z r The DC-side impedance of the receiving-end MMC, Z DC-line The impedance of the DC cable. Z L This is the impedance of the filter inductor.

[0015] Furthermore, the specific expression for the small-signal characteristics of the DC side of the light-duty converter at the sending end in step (3) is as follows:

[0016] in: This represents the small-signal component of the DC-side voltage of the sending-end lightweight converter. Z r-equ The equivalent DC impedance of the receiving end, Δ i dc This represents the small-signal component of the DC-side current in the system.

[0017] Furthermore, in step (3), the sending end MMC adopts... V / f (Constant voltage-frequency ratio) control, the control structure consists of a voltage outer loop and a current inner loop, and the expression for the small-signal characteristics of the MMC control loop at the sending end is as follows:

[0018] in: The upper arm modulation ratio of the sending-end MMC is the disturbance component. For the current disturbance component of the upper arm of the MMC at the sending end, Δ v hybrid This refers to the AC side voltage disturbance component of the MMC at the sending end. G i and G v These are the control effect factors for the inner current loop and the outer voltage loop, respectively.

[0019] Furthermore, the expressions for the voltage and current propagation coefficients of the light-duty converter at the sending end in step (4) are as follows: ,

[0020] in: and These are the voltage propagation coefficient and current propagation coefficient of the sending-end lightweight converter, respectively. Δ represents the small-signal component of the DC-side voltage of the sending-end lightweight converter. i dc For the small-signal component of the DC-side current of the system, Δ v p-hybrid and Δ v n-hybrid These are the small-signal components of the positive-sequence AC voltage and the negative-sequence AC voltage of the sending-end lightweight converter, Δ i p-hybrid and Δ i n-hybrid These are the small-signal components of the positive-sequence AC current and negative-sequence AC current of the sending-end lightweight converter, respectively.

[0021] Furthermore, the expressions for the voltage and current propagation coefficients of the receiving-end MMC in step (4) are as follows: ,

[0022] in: and Do not specify the voltage propagation coefficient and current propagation coefficient of the receiving-end MMC. Δ represents the small-signal component of the DC-side voltage of the receiving-end MMC. i dc For the small-signal component of the DC-side current of the system, Δ v p-rece and Δ v n-rece These are the small-signal components of the positive-sequence AC voltage and the negative-sequence AC voltage of the receiving-end MMC, respectively, Δ i p-rece and Δ i n-rece These are the small-signal components of the positive-sequence AC current and negative-sequence AC current of the receiving-end MMC, respectively.

[0023] Furthermore, the small-signal component of the DC-side voltage and the small-signal component Δ of the DC side current of the system i dc The expression is as follows: ,

[0024] in: f p The frequency of the positive-sequence perturbation voltage. f 0 is the fundamental frequency. This represents the disturbance component of the upper arm current of the receiving-end MMC. Z r This is the DC-side impedance of the receiving end MMC.

[0025] Furthermore, the small signal component Δ v p-hybrid Δ v n-hybrid Δ i p-hybrid and Δ i n-hybrid The expression is as follows:

[0026] in: Y hybird Let be the AC side admittance matrix of the sending-end lightweight converter and be Z hybird The inverse matrix, I It is the identity matrix.Z WF This is the impedance matrix on the wind turbine side. Z hybird For the AC side impedance matrix of the sending-end lightweight converter, Δ v p The injected positive-sequence perturbation voltage. inv ( ) indicates finding the inverse of a matrix.

[0027] Furthermore, the small signal component Δ v p-rece Δ v n-rece Δ i p-rece and Δ i n-rece The expression is as follows:

[0028] in: f p The frequency of the positive-sequence perturbation voltage. f 0 is the fundamental frequency. This refers to the current disturbance component of the upper arm of the receiving-end MMC. Z g This indicates the impedance of the onshore AC power grid.

[0029] A computer device includes a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the above-described method for analyzing the oscillation propagation characteristics of wind power transmitted via DRU-MMC.

[0030] A computer-readable storage medium storing a computer program, which, when executed by a processor, implements the above-described method for analyzing the oscillation propagation characteristics of wind power transmitted via DRU-MMC.

[0031] The method of this invention can quantitatively describe the propagation characteristics and attenuation amplification laws of oscillation components such as power, current, and voltage on both AC and DC sides of the wind turbine, the sending-end lightweight converter, and the receiving-end MMC, providing a theoretical basis for the oscillation analysis and suppression of the system. Based on these propagation characteristics, the degree of oscillation risk of different nodes in the AC / DC interconnection system can be further analyzed, and nodes with abnormal amplification and concentration of oscillation components can be accurately identified. This effectively improves the prediction accuracy and active prevention and control capability of the AC / DC interconnection system for sub / supersynchronous oscillation risks, providing a reliable guarantee for the safe and stable operation of large-scale wind power transmission systems via lightweight converters. Attached Figure Description

[0032] Figure 1 This is a block diagram of the topology and control structure of a wind power transmission system via a DRU-MMC lightweight converter.

[0033] Figure 2 This is a schematic diagram illustrating the variation of the current propagation coefficient of the light-duty converter at the sending end with the frequency of the disturbance voltage in an embodiment of the present invention.

[0034] Figure 3 This is a schematic diagram illustrating the variation of the voltage propagation coefficient of the light-duty converter at the sending end with the frequency of the disturbance voltage in an embodiment of the present invention.

[0035] Figure 4 This is a schematic diagram illustrating the variation of the current propagation coefficient of the receiving-end MMC converter with the frequency of the disturbance voltage in an embodiment of the present invention.

[0036] Figure 5 This is a schematic diagram illustrating the variation of the voltage propagation coefficient of the receiving-end MMC converter with the frequency of the disturbance voltage in an embodiment of the present invention.

[0037] Figure 6 This is a schematic diagram of the power output curve of the fan in an embodiment of the present invention.

[0038] Figure 7 This is a schematic diagram of the FFT decomposition results of the AC side current of the light converter at the sending end in an embodiment of the present invention.

[0039] Figure 8 This is a schematic diagram of the FFT decomposition results of the DC side current of the light converter at the sending end in an embodiment of the present invention.

[0040] Figure 9 This is a schematic diagram of the FFT decomposition results of the AC side voltage of the light converter at the sending end in an embodiment of the present invention.

[0041] Figure 10 This is a schematic diagram of the FFT decomposition results of the DC side voltage of the light converter at the sending end in an embodiment of the present invention.

[0042] Figure 11 This is a schematic diagram of the FFT decomposition results of the AC side voltage of the receiving-end MMC in an embodiment of the present invention.

[0043] Figure 12 This is a schematic diagram of the FFT decomposition results of the DC-side voltage of the receiving-end MMC in an embodiment of the present invention.

[0044] Figure 13 This is a schematic diagram of the FFT decomposition results of the AC side current of the receiving-end MMC in an embodiment of the present invention. Detailed Implementation

[0045] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0046] This embodiment provides a method for analyzing the oscillation propagation characteristics of wind power transmitted via DRU-MMC, including the following steps: (1) The receiving-end MMC operates in inverter mode and adopts DC voltage control mode. Considering the small-signal equations of the receiving-end MMC circuit characteristics and control characteristics, its DC side impedance model is constructed.

[0047] like Figure 1 As shown, the receiving-end MMC inverts and transmits the voltage and current of the DC bus to the onshore AC power grid. Its small-signal characteristics include phase-locked loop, DC voltage control, and main circuit components. The small-signal characteristics of the receiving-end MMC's DC voltage control can be expressed as:

[0048] In the formula: This represents the small-signal component of the DC-side voltage at the receiving end. and These represent the d-axis and q-axis current small-signal components of the receiving-end MMC, respectively. and These represent the modulation ratios of the small signal components on the d-axis and q-axis of the receiving-end MMC, respectively. k To control the decoupling coefficients, and P dc ( s ) represent the transfer functions of the current inner loop and the DC voltage control, respectively.

[0049] The small-signal characteristics of the receiving-end MMC's phase-locked loop can be expressed as:

[0050] In the formula: Δ θ For small angle signal components, This represents the small-signal component of the q-axis voltage. H PLL This is the transfer function for the phase-locked loop.

[0051] The small-signal characteristic equation of the receiver-side MMC circuit can be expressed as:

[0052] In the formula: Z Δl and Z Δc These represent the bridge arm impedance and the submodule capacitor impedance, respectively. , and These represent the disturbance components of the upper bridge arm current, voltage, and modulation ratio, respectively. , and This represents the corresponding steady-state component. Z g Δ represents the impedance of the onshore AC power grid. v g and Δi g This represents the voltage and current disturbance components on the AC side of the receiving-end MMC.

[0053] Combining the above control domain small-signal characteristic equation and circuit small-signal characteristic equation, the DC-side impedance model of the receiving-end MMC can finally be expressed as:

[0054] In the formula: Δ i dc This represents the small-signal component of the DC-side current in the system.

[0055] (2) Combine the DC side impedance of the receiving end MMC with the impedances of DC cable, DC filter, etc. in series to calculate the equivalent DC impedance of the receiving end.

[0056] DC filter and DC filter inductor and receiving-end MMC DC side impedance Z r Because they are in series, the equivalent DC impedance at the receiving end is... Z r-equ The expression can be defined as:

[0057] In the formula: Z DC-line and Z L These represent the impedance of the DC cable and the impedance of the DC filter inductor, respectively.

[0058] (3) Divide the system into two sides, the fan and the light converter, with the AC side of the light converter as the boundary, and construct the impedance matrix on the fan side. Z WF Simultaneously considering the small-signal characteristics of the DC side and the small-signal characteristics of the sending-end MMC control loop of the lightweight converter, an AC side impedance matrix of the lightweight converter is constructed. Z hybird .

[0059] like Figure 1 As shown, the impedance matrix on the wind turbine side Z WF It consists of components such as wind turbine generators and AC cables. The solution of its impedance matrix can be found in the reference [Xu Yunyang. Impedance Modeling and Grid-connected Stability Analysis of Wind Turbine Generators under Oscillating Frequency Coupling [D]. Zhejiang University, 2021].

[0060] The sending-end lightweight converter consists of two parts: DRU and MMC. Therefore, the AC-side impedance matrix... Z hybird The solution needs to consider its DC-side dynamic characteristics, MMC control loop small-signal characteristics, MMC main circuit characteristics, and DRU switching function characteristics.

[0061] The dynamic characteristics of the DC side of the sending-end lightweight converter can be expressed as:

[0062] In the formula: This represents the small-signal component of the DC-side voltage of the sending-end lightweight converter.

[0063] The send-end MMC in the lightweight converter adopts V / f The control loop, consisting of an outer voltage loop and an inner current loop, is used to construct the AC bus voltage amplitude and frequency. Its control method differs significantly from that of the receiving-end MMC. The small-signal characteristics of the sending-end MMC control loop can be expressed as:

[0064] In the formula: For the upper arm modulation ratio disturbance component of the sending-end MMC, Δ v hybrid This refers to the AC side voltage disturbance component of the MMC at the sending end. G i and G v These represent the control effect factors for the inner current loop and the outer voltage loop, respectively. This refers to the upper arm current disturbance component of the sending-end MMC.

[0065] For the characteristics of the MMC main circuit and the DRU switching function in the sending-end lightweight converter, please refer to the following reference: [Zhang Gan, Xiang Wen, Wen Jinyu. Impedance modeling and stability analysis of DR-MMC-based hybrid HVDC transmission system for offshore wind farm grid connection [J]. IEEE Transactions on Electric Power Transmission, 2023, Vol. 38, No. 6, pp. 4397-4409].

[0066] (4) Solve for the voltage and current propagation coefficients of the sending-end lightweight converter and the receiving-end MMC respectively, and analyze the subsynchronous / supersynchronous oscillation propagation characteristics of the wind power transmission system through the DRU-MMC lightweight converter.

[0067] Assume the frequency injected on the AC side of the light-duty converter at the sending end is . f p Positive sequence voltage small perturbation Δ v p Due to the difference in impedance characteristics between the fan and the sending-end lightweight converter, the positive and negative sequence AC voltages Δ received by the sending-end lightweight converter are different. v p-hybrid and Δ v n-hybrid and the positive and negative sequence alternating current Δ i p-hybrid and Δ i n-hybridIt can be represented as:

[0068] In the formula: Y hybird This is the AC side admittance matrix of a lightweight converter, which is related to the impedance matrix. Z hybird Mutually inverse.

[0069] After obtaining the disturbance components of the positive and negative sequence AC voltages and currents of the sending-end lightweight converter, the small-signal component of the DC-side voltage of the sending-end lightweight converter can be further calculated using the small-signal characteristics of the MMC control loop, the characteristics of the MMC main circuit, and the characteristics of the DRU switching function. and small signal component of current Δ i dc Thus, the small-signal component of the DC-side voltage of the receiving-end MMC converter station can be obtained. ,have:

[0070] Subsequently, based on the circuit and control small-signal characteristic equations of the receiving-end MMC converter, the upper arm current disturbance component of the receiving-end MMC converter can be obtained, and the positive and negative sequence voltage disturbance components Δ can be further obtained. v p-rece and Δ v n-rece and the positive and negative sequence alternating current Δ i p-rece and Δ i n-rece There will be:

[0071] in: f 0 represents the base frequency of 50Hz.

[0072] Further define the voltage and current propagation factors from AC to DC as follows: and The voltage and current propagation factors from DC to AC are: and ,have: ,

[0073] ,

[0074] To further illustrate the practical application of the aforementioned propagation factor in the abnormal amplification node and concentration node of the oscillation component in the positioning system, the parameter configuration table of the wind power transmission system via DRU-MMC in this embodiment is shown in Table 1: Table 1

[0075] Based on the above configuration, the voltage and current propagation factors of the sending-end lightweight converter and the receiving-end MMC are calculated respectively. For the lightweight converter, the disturbance propagates from the AC side to the DC side; the total current propagation coefficient is calculated, and the results are as follows. Figure 2 As shown, except for the low-frequency band <11Hz, the current component attenuates during propagation; the total voltage propagation coefficient is calculated, and the results are as follows. Figure 3 As shown, in the low-frequency band <40Hz, the voltage propagates with almost constant amplitude. Near 50Hz, there is a spike in the propagation coefficient, indicating that the voltage propagates with increased amplitude. Meanwhile, in the supersynchronous frequency band >55Hz, the voltage propagates with decreased amplitude.

[0076] For the receiving-end MMC, the disturbance propagates from the DC side to the AC side. The total current propagation coefficient is calculated, and the results are as follows: Figure 4 As shown, except near the 50Hz fundamental frequency, the current component attenuates during propagation; the total voltage propagation coefficient is calculated, and the results are as follows. Figure 5 As shown, except for the vicinity of the 50Hz fundamental frequency and the extremely low frequency band <1.5Hz, the voltage component attenuates during propagation.

[0077] In this embodiment, we plotted the wind turbine's output power curve as follows: Figure 6 As shown, the system operates stably when the wind turbine output power is 0.4 pu, but oscillations occur after the output power is increased to 0.5 pu. In this scenario, the FFT decomposition results corresponding to the AC and DC voltages and currents on both sides of the sending-end lightweight converter and the receiving-end MMC are plotted below. Figure 7 The figure shows the FFT (Fast Fourier Transform) decomposition results of the AC side current of the sending-end lightweight converter. As can be seen from the figure, there are two oscillating components at 8.3Hz and 91.7Hz, with amplitudes of 274A and 403A, respectively. 91.7Hz is the main oscillation frequency.

[0078] Figure 8 The figure shows the FFT decomposition results of the DC-side current of the sending-end lightweight converter. An oscillating component with an amplitude of 99.8 A can be seen at 41.7 Hz. Figure 2 The theoretical value of the current propagation coefficient at 91.7 Hz is less than 0, indicating attenuated propagation, which is consistent with the actual results.

[0079] Figure 9 The figure shows the FFT decomposition results of the AC side voltage of the light converter at the sending end. It can be seen from the figure that there are two oscillation components at 8.3Hz and 91.7Hz, with amplitudes of 2587V and 2635V respectively. 91.7Hz is the main oscillation frequency.

[0080] Figure 10The figure shows the FFT decomposition results of the DC-side voltage of the sending-end lightweight converter. An oscillating component with an amplitude of 544.2V can be observed at 41.7Hz. Figure 3 The theoretical value of the voltage propagation coefficient at 91.7 Hz is less than 0, indicating attenuation propagation, which is consistent with the actual results.

[0081] Figure 11 The figure shows the FFT decomposition results of the AC side voltage of the receiving end MMC. It can be seen from the figure that there are two oscillating components at 8.3Hz and 91.7Hz, but the amplitudes are relatively small, at 7V and 98.5V respectively. 91.7Hz is the main oscillation frequency.

[0082] Figure 12 The FFT decomposition results of the DC-side voltage of the receiving-end MMC are shown in the figure. An oscillating component can be seen at 41.7Hz with a relatively small amplitude of 416.2V. Combined with... Figure 5 The theoretical value of the voltage propagation coefficient at 91.7 Hz is less than 0, indicating attenuation propagation, which is consistent with the actual results.

[0083] Figure 13 The figure shows the FFT decomposition results of the AC side current of the receiving-end MMC. It can be seen from the figure that there are two oscillating components at 8.3Hz and 91.7Hz, but their amplitudes are relatively small, at 11A and 17A respectively, with 91.7Hz being the dominant oscillation frequency. Combined with... Figure 4 The theoretical value of the current propagation coefficient at 91.7 Hz is less than 0, indicating attenuated propagation, which is consistent with the actual results.

[0084] In summary, the theoretical results of the voltage and current propagation factors are consistent with the actual situation. In this oscillation scenario, the oscillating voltage and current components are concentrated on the AC side of the sending-end lightweight converter, and attenuate sequentially as they propagate towards the sending-end DC side, the receiving-end AC side, and the receiving-end DC side. Therefore, in wind power transmission systems via DRU-MMC, the system is at risk of oscillation when wind power output increases. The AC side of the sending-end lightweight converter is the concentrated node of oscillation components, and corresponding oscillation control measures need to be configured here.

[0085] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A method for analyzing the oscillation propagation characteristics of wind power transmitted via DRU-MMC, characterized in that, Includes the following steps: (1) For the receiving-end MMC in the wind power transmission system via DRU-MMC, consider the small-signal equations of its circuit characteristics and control characteristics, and construct the DC-side impedance model of the receiving-end MMC. (2) The DC side impedance of the receiving end MMC is combined with the impedance of the DC cable and the filter inductor in series to obtain the equivalent DC impedance of the receiving end. (3) The system is divided into two sides, the fan and the light converter, with the AC side of the light converter at the sending end as the boundary. The impedance matrix of the fan side is constructed. The small-signal characteristics of the DC side of the light converter at the sending end and the small-signal characteristics of the MMC control loop at the sending end are also taken into account. The impedance matrix of the AC side of the light converter at the sending end is constructed. (4) Inject positive-sequence disturbance voltages of different frequencies into the AC side of the light converter at the sending end, and solve the voltage and current propagation coefficients of the light converter at the sending end and the MMC at the receiving end respectively, in order to analyze the propagation characteristics of the subsynchronous / supersynchronous oscillation of the system.

2. The method for analyzing the oscillation propagation characteristics of wind power transmitted via DRU-MMC according to claim 1, characterized in that: The small-signal equations for the characteristics and control characteristics of the receiving-end MMC circuit in step (1) include multiple components such as phase-locked loop, DC voltage control, and main circuit. Based on this, the DC-side impedance model of the receiving-end MMC is constructed as follows, which is used to analyze the dynamic characteristics of voltage and current disturbance components propagating from the DC side to the AC side in the receiving-end MMC. in: Z r The DC-side impedance of the receiving-end MMC, For the small-signal component of the DC-side voltage of the receiving-end MMC, Δ i dc This represents the small-signal component of the DC-side current in the system.

3. The method for analyzing the oscillation propagation characteristics of wind power transmitted via DRU-MMC according to claim 1, characterized in that, The expression for the equivalent DC impedance at the receiving end in step (2) is as follows: in: Z r-equ The receiving end is the equivalent DC impedance. Z r The DC-side impedance of the receiving-end MMC, Z DC-line The impedance of the DC cable. Z L This is the impedance of the filter inductor.

4. The method for analyzing the oscillation propagation characteristics of wind power transmitted via DRU-MMC according to claim 1, characterized in that, The specific expression for the small-signal characteristics of the DC side of the light-duty converter at the sending end in step (3) is as follows: in: This represents the small-signal component of the DC-side voltage of the sending-end lightweight converter. Z r-equ The equivalent DC impedance of the receiving end, Δ i dc This represents the small-signal component of the DC-side current in the system.

5. The method for analyzing the oscillation propagation characteristics of wind power transmitted via DRU-MMC according to claim 1, characterized in that, In step (3), the sending end MMC adopts V / f The control structure consists of a voltage outer loop and a current inner loop. Therefore, the expression for the small-signal characteristics of the MMC control loop at the sending end is as follows: in: The upper arm modulation ratio of the sending-end MMC is the disturbance component. For the current disturbance component of the upper arm of the MMC at the sending end, Δ v hybrid This refers to the AC side voltage disturbance component of the MMC at the sending end. G i and G v These are the control effect factors for the inner current loop and the outer voltage loop, respectively.

6. The method for analyzing the oscillation propagation characteristics of wind power transmitted via DRU-MMC according to claim 1, characterized in that, The expressions for the voltage and current propagation coefficients of the light-duty converter at the sending end in step (4) are as follows: ， in: and These are the voltage propagation coefficient and current propagation coefficient of the sending-end lightweight converter, respectively. Δ represents the small-signal component of the DC-side voltage of the sending-end lightweight converter. i dc For the small-signal component of the DC-side current of the system, Δ v p-hybrid and Δ v n-hybrid These are the small-signal components of the positive-sequence AC voltage and the negative-sequence AC voltage of the sending-end lightweight converter, Δ i p-hybrid and Δ i n-hybrid These are the small-signal components of the positive-sequence AC current and negative-sequence AC current of the sending-end lightweight converter, respectively.

7. The method for analyzing the oscillation propagation characteristics of wind power transmitted via DRU-MMC according to claim 1, characterized in that, The expressions for the voltage and current propagation coefficients of the receiving-end MMC in step (4) are as follows: ， in: and Let's exclude the voltage propagation coefficient and current propagation coefficient of the receiving-end MMC. Δ represents the small-signal component of the DC-side voltage of the receiving-end MMC. i dc For the small-signal component of the DC-side current of the system, Δ v p-rece and Δ v n-rece These are the small-signal components of the positive-sequence AC voltage and the negative-sequence AC voltage of the receiving-end MMC, respectively, Δ i p-rece and Δ i n-rece These are the small-signal components of the positive-sequence AC current and the negative-sequence AC current of the receiving-end MMC, respectively.

8. The method for analyzing the oscillation propagation characteristics of wind power transmitted via DRU-MMC according to claim 7, characterized in that, The small-signal component of the DC-side voltage and the small-signal component Δ of the DC side current of the system i dc The expression is as follows: ， in: f p The frequency of the positive-sequence perturbation voltage. f 0 is the fundamental frequency. This represents the disturbance component of the upper arm current of the receiving-end MMC. Z r This is the DC-side impedance of the receiving end MMC.

9. The method for analyzing the oscillation propagation characteristics of wind power transmitted via DRU-MMC according to claim 6, characterized in that, The small signal component Δ v p-hybrid Δ v n-hybrid Δ i p-hybrid and Δ i n-hybrid The expression is as follows: in: Y hybird Let be the AC side admittance matrix of the sending-end lightweight converter and be Z hybird The inverse matrix, I It is the identity matrix. Z WF This is the impedance matrix on the wind turbine side. Z hybird For the AC side impedance matrix of the sending-end lightweight converter, Δ v p The injected positive-sequence perturbation voltage. inv ( ) indicates finding the inverse of a matrix.

10. The method for analyzing the oscillation propagation characteristics of wind power transmitted via DRU-MMC according to claim 7, characterized in that, The small signal component Δ v p-rece Δ v n-rece Δ i p-rece and Δ i n-rece The expression is as follows: in: f p The frequency of the positive-sequence perturbation voltage. f 0 is the fundamental frequency. This refers to the current disturbance component of the upper arm of the receiving-end MMC. Z g This indicates the impedance of the onshore AC power grid.

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