An oscillation risk factor positioning method for a light converter sending-out system of offshore wind power

Abstract

The application discloses an offshore wind power light converter sending-out system oscillation risk factor positioning method. First, according to the interaction between the impedance of the wind turbine side and the impedance of the light converter, the multi-input multi-output impedance is converted into equivalent single-input single-output impedance, the positive sequence equivalent impedance of the wind turbine side and the positive sequence equivalent impedance of the light converter are obtained, the interaction characteristics of the equivalent impedance of the wind turbine side and the light converter are analyzed, and then the wideband oscillation mechanism is obtained. Subsequently, the equivalent impedance of the light converter is deconstructed and analyzed, and the impedance components of the DRU and the MMC are separated; it is determined that the weak damping characteristics of the light converter mainly come from the MMC, and the impedance of the MMC is processed by targeted order reduction. The application realizes the revelation of the oscillation mechanism and the accurate tracing of the oscillation risk factor; through layer-by-layer deconstruction and tracing, the weak link of system stability is determined, and a direct theoretical basis is provided for system oscillation suppression.

Description

Technical Field

[0001] This invention belongs to the field of offshore wind power DC transmission technology, specifically a method for locating oscillation risk factors in an offshore wind power transmission system via a lightweight converter. Background Technology

[0002] The development of offshore wind power continues to accelerate towards deeper and larger-scale operations, which places higher demands on the efficient and reliable collection and transmission of electricity.

[0003] Against this backdrop, high-voltage direct current (HVDC) transmission technology based on modular multilevel converters (MMCs) has become one of the mainstream technologies for transmitting power from deep-sea offshore wind farms due to its significant advantages, such as establishing a stable AC grid voltage, enabling black start of wind farms, and large-capacity, long-distance transmission. However, this solution requires the installation of a fully controlled MMC on the offshore platform, significantly increasing the platform's size, weight, and construction costs. To overcome this challenge, HVDC transmission schemes based on diode rectifier units (DRUs) have attracted widespread attention. DRUs have a simple topology, require no active control, can significantly improve engineering efficiency, and possess higher operating efficiency and reliability. However, as an uncontrolled rectifier device, the commutation of a DRU depends on a stable AC bus voltage; therefore, offshore wind turbines must be adjusted to grid-based control, which brings new challenges to the design and operation of wind farms.

[0004] To combine the advantages of MMC and DRU, researchers have proposed a lightweight converter structure. This scheme uses DRU to transmit the majority of the power while reserving a portion of the capacity of MMC to establish and stabilize the offshore AC grid voltage, achieving both technical feasibility and engineering economy. However, the cascading of DRU and MMC significantly alters the system's impedance characteristics, making its small-disturbance stability and broadband oscillation risk more prominent compared to traditional converters. This has become one of the key bottlenecks restricting the large-scale engineering application of this technology.

[0005] Most existing studies treat lightweight converters as a whole for impedance modeling and analysis, failing to clarify the respective roles of the DRU subsystem and MMC subsystem in shaping the overall impedance characteristics of the lightweight converter. Secondly, the established impedance models of lightweight converters are usually quite complex and of high order, lacking effective methods for reducing the model order, thus failing to accurately trace the source of oscillation risk factors. Summary of the Invention

[0006] In view of the technical problems existing in the prior art, the present invention provides a method for locating the oscillation risk factor of an offshore wind power transmission system via a lightweight converter. This method achieves the acquisition of the oscillation mechanism and the accurate tracing of the oscillation risk factor by deconstructing and reducing the order of the impedance of the lightweight converter.

[0007] Therefore, the present invention adopts the following technical solution: a method for locating the oscillation risk factor of an offshore wind power transmission system via a lightweight converter, comprising the following steps: 1) Based on the offshore wind power topology, establish the wind turbine-side impedance taking into account the direct-drive wind turbine and its filter, AC cable and passive filter; 2) Analyze the internal small-signal characteristics of the diode rectifier unit (DRU) and the modular multilevel converter (MMC). Based on the small-signal coupling relationship between the DRU and MMC on both the AC and DC sides, establish the impedance of the DRU-MMC lightweight converter that takes into account the frequency coupling characteristics. 3) Based on the interaction between the wind turbine side impedance and the lightweight converter impedance, the multi-input multi-output impedance is converted into an equivalent single-input single-output impedance, and the positive-sequence equivalent impedance of the wind turbine side and the lightweight converter is obtained. The interaction characteristics of the equivalent impedances of the wind turbine side and the lightweight converter are analyzed, and then the broadband oscillation mechanism of the offshore wind power transmission system via the lightweight converter is obtained. 4) Based on the broadband oscillation mechanism of offshore wind power transmission system via lightweight converter, the positive sequence equivalent impedance of lightweight converter is deconstructed, the equivalent impedances of DRU and MMC are separated, and the equivalent impedance of MMC is reduced in order to locate the oscillation risk factor.

[0008] Furthermore, taking the AC terminal of the lightweight converter as the dividing point, the offshore wind power transmission system via the lightweight converter is divided into two parts: the wind turbine side and the lightweight converter side, and their impedances are calculated separately.

[0009] Furthermore, in step 1), the wind turbine side impedance Represented as:

[0010] In the formula, 、 , , These represent the impedance matrices of the direct-drive fan, direct-drive fan filter, AC cable, and passive filter, respectively.

[0011] Furthermore, in step 2), the lightweight converter consists of a diode rectifier unit (DRU) and a modular multilevel converter (MMC), which are connected in parallel on the AC side and in series on the DC side. Its impedance includes both the DRU and the MMC, and the small-signal coupling relationship caused by the series connection of the two on the DC side must also be taken into account.

[0012] Furthermore, the DC-side DRU / MMC of the lightweight converter is connected in series, and the current disturbance components of the two are consistent, so the total voltage disturbance on the DC side is the sum of the two voltage disturbances.

[0013] Furthermore, the AC-side DRU / MMC of the lightweight converter is connected in parallel, and the voltage disturbance components of the two are consistent. The positive / negative sequence disturbance components of the total AC current are the sum of the two current disturbances.

[0014] Furthermore, the positive-sequence disturbance component of the total AC current. and negative order perturbation components The calculation formula is as follows:

[0015] In the formula, For a lightweight converter impedance matrix, and These represent the positive and negative sequence disturbance components of the total AC voltage, respectively. and These represent the positive and negative sequence components of the AC side current of the DRU, respectively. and These represent the positive and negative sequence components of the AC side current of the DRU, respectively.

[0016] Furthermore, in step 3), the positive sequence equivalent impedance on the wind turbine side... and the positive sequence equivalent impedance of lightweight converter Calculated by the following formula:

[0017] In the formula, , These represent the impedance of the wind turbine side. Negative-order coupling components and negative-order components in the text; , , , These represent the wind turbine side admittance matrix. Positive-order components, positive-order coupled components, negative-order coupled components, and negative-order components; , These represent the impedance matrices of the lightweight converter side. Negative-order coupling components and negative-order components in the text; , , , These represent the side admittance matrices of the lightweight converter. The positive-order component, positive-order coupled component, negative-order coupled component, and negative-order component in the equation.

[0018] Furthermore, in step 4), firstly, the high-frequency harmonic components in the MMC multi-harmonic coupling characteristics are ignored, and secondly, the control loop is simplified; while retaining the impedance characteristics of the oscillation neighborhood frequency band, the MMC impedance is reduced in order step by step.

[0019] Furthermore, the following principle is adopted to reduce the order of MMC impedance: In the MMC multiharmonic coupling characteristics, the harmonic order is reduced to h=1, considering only ( f p + f 0), ( f p - f 0) and f p Three harmonic components, f p Indicates the frequency of the disturbance. f 0 indicates a fundamental frequency of 50Hz; Ignore the MMC circulating current suppression part; In differential mode control, the decoupling terms of the voltage loop and current loop are directly set to zero in the impedance model.

[0020] This invention first converts the impedance of the wind turbine side and the lightweight converter into the equivalent impedance of a single-input single-output model to obtain a wideband oscillation mechanism. Then, based on the wideband oscillation mechanism, it performs a deconstructive analysis on the equivalent impedance of the lightweight converter, separating the impedance components of the DRU and MMC. After determining that the weak damping characteristics of the lightweight converter mainly originate from the MMC subsystem, it performs targeted order reduction processing on the equivalent impedance of the MMC, realizing accurate tracing of the oscillation risk factor.

[0021] This invention, through layer-by-layer deconstruction and tracing, clarifies the weak links in the stability of lightweight converters, providing a direct theoretical basis for system oscillation suppression. Attached Figure Description

[0022] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the description will be briefly introduced below. Obviously, the accompanying drawings described below are one embodiment of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a flowchart of a method for locating oscillation risk factors in an offshore wind power transmission system via a lightweight converter, according to the present invention. Figure 2 This is a topology and control structure diagram of the offshore wind power transmission system via a lightweight converter according to the present invention. Figure 3 The wind turbine side impedance of the present invention Impedance sweep frequency test diagram; Figure 4 The lightweight converter side admittance matrix of the present invention Impedance sweep frequency test diagram; Figure 5This is a system broadband oscillation analysis diagram of the present invention; Figure 6 This is the impedance breakdown diagram of the lightweight converter of the present invention; Figure 7 This is a comparison diagram of the equivalent impedance of MMC before and after the order reduction in this invention; Figure 8 This is a diagram illustrating the mechanism of the MMC voltage loop to subsynchronous / supersynchronous oscillation in this invention. Detailed Implementation

[0024] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The advantages and features of the present invention will become clearer through the following description. It should be noted that the accompanying drawings are in a simplified schematic form, and the proportions shown are not precise; they are only for auxiliary illustration and are intended to help understand the embodiments of the present invention. Please refer to the accompanying drawings for a clearer demonstration of the purpose, features, and advantages of the present invention. It should be pointed out that the structures, proportions, sizes, etc., shown in the accompanying drawings are only for illustrative purposes and do not limit the specific conditions of the embodiments of the present invention; therefore, they do not have substantial technical significance. Any modifications to the structure, adjustments to the proportions, or changes in the dimensions, without affecting the function and purpose achieved by the present invention, should be considered within the scope of the technical solution of the present invention.

[0025] This invention relates to a method for locating oscillation risk factors in offshore wind power transmitted via a lightweight converter, such as... Figure 1 As shown, the steps are as follows: (1) Based on the offshore wind power topology, establish the turbine-side impedance considering the direct-drive wind turbine and its filter, AC cable and passive filter. .

[0026] In step (1), the wind turbine side impedance includes three parts: the direct-drive wind turbine and its filter, the AC cable, and the passive filter. The DC bus voltage fluctuation of the wind turbine is small, and the turbine-side and grid-side converters are decoupled from each other. Only the dynamic characteristics of the grid-side converter are considered.

[0027] Figure 2 This diagram shows the topology and control structure of an offshore wind power transmission system via a lightweight converter. After being rectified and transmitted by the lightweight converter at the sending end, the offshore wind power is transmitted to the onshore converter via DC cable and then inverted and connected to the power grid.

[0028] Figure 2 middle, u abcf This refers to the voltage at the filter port of the direct-drive fan. u abc This refers to the AC bus voltage. i abc-D For the AC side current of the DRU; i abc-MThis refers to the AC side current of the MMC. u dc-D This refers to the DC-side voltage of the DRU. u dc-M This refers to the DC side voltage of the MMC. i dc This is the DC side current; U dc0 This is the equivalent receiving-end DC source voltage; i d-ref and i q-ref These are the reference values ​​for the d-axis and q-axis components of the port current of the direct-drive fan, respectively. i d and i q These are the actual values ​​of the d-axis and q-axis components of the port current of the direct-drive fan, respectively. u df and u qf These are the actual values ​​of the d-axis and q-axis components of the filter port voltage of the direct-drive fan, respectively. f Phase angle transformation for direct-drive fan park; u d-ref and u q-ref These are the reference values ​​for the d-axis and q-axis components of the AC bus voltage, respectively. u d and u q These are the actual values ​​of the d-axis and q-axis components of the AC bus voltage, respectively. 0 represents the reference angular velocity represented by the fundamental frequency of 50Hz; M This is the phase angle of the park transformation in MMC. i dM , i qM These represent the actual values ​​of the d-axis and q-axis components of the AC side current of the MMC, respectively. i cd , i cq These represent the actual values ​​of the d-axis and q-axis components of the circulating current on the AC side of the MMC, respectively. K WF Indicates the decoupling coefficient of the direct-drive fan's inner current control loop. K v Indicates the decoupling coefficient of the outer loop control of the MMC voltage. K i Indicates the decoupling coefficient of the MMC current inner loop control. K cThis represents the decoupling coefficient of the MMC circulating current suppression control. As can be seen from the topology, on the fan side, the direct-drive fan, the direct-drive fan filter, and the AC cable are connected in series, and then in parallel with the passive filter.

[0029] Therefore, the wind turbine side impedance Represented as:

[0030] In the formula, 、 , , These represent the impedance matrices of the direct-drive fan, direct-drive fan filter, AC cable, and passive filter, respectively.

[0031] Subsequently, an impedance frequency sweep test was performed in MATLAB / Simulink, and the test results are as follows. Figure 3 As shown, Figure 3 In , , , These represent the impedance of the wind turbine side. The positive-order component, positive-order coupled component, negative-order coupled component, and negative-order component in the equation.

[0032] (2) Analyze the small-signal characteristics of DRU and MMC, consider the small-signal coupling relationship between DRU and MMC on both AC and DC sides, and establish the impedance of DRU-MMC lightweight converter considering frequency coupling characteristics. .

[0033] Using the AC terminal of the lightweight converter as the dividing point, the offshore wind power transmission system via the lightweight converter is divided into two parts: the wind turbine side and the lightweight converter side. The impedance of each part is calculated separately.

[0034] The lightweight converter consists of a diode rectifier unit (DRU) and a modular multilevel converter (MMC), which are connected in parallel on the AC side and in series on the DC side. Its impedance includes both the DRU and the MMC, and the small-signal coupling relationship caused by the series connection of the two on the DC side must also be taken into account.

[0035] In a lightweight converter, the DC-side DRU / MMC is connected in series, and the current disturbance components of both are identical, resulting in a total DC-side voltage disturbance. It is the sum of the two voltage disturbances, i.e.:

[0036] In the formula, Z dc The DC-side equivalent impedance is composed of a DC filter inductor and a DC cable. This represents the small disturbance component of the DC-side current; This indicates the small disturbance component of the DC-side voltage of the DRU; This indicates the small disturbance component of the DC side voltage of the MMC.

[0037] The AC-side DRU / MMC of the lightweight converter is connected in parallel, and the voltage disturbance components of both are consistent, while the positive-sequence disturbance component of the total AC current is the same. and negative order perturbation components The calculation formula is as follows:

[0038] In the formula, For a lightweight converter impedance matrix, and Represents the positive / negative sequence disturbance components of the total AC voltage; and Indicates the positive / negative sequence components of the AC side current of the DRU; and This represents the positive / negative sequence components of the AC side current of the DRU.

[0039] In the DRU section, the frequency domain mapping relationship between voltage and current on both AC and DC sides is characterized by defining the switching functions of diodes in each phase. The MMC section uses an average value model, assuming that the capacitance of each submodule has been averaged and is equal at any given time, while also considering its voltage loop, current loop, circulating current suppression, and the small-signal dynamic characteristics of the main circuit. Subsequently, combining the small-signal coupling relationship between the DRU and MMC on both AC and DC sides, the impedance of the lightweight converter is calculated. And frequency scanning verification was performed, the results are as follows Figure 4 As shown.

[0040] (3) Based on the fan side impedance and lightweight converter impedance The interaction relationship between the wind turbine and the lightweight converter is analyzed to transform the multi-input multi-output system into an equivalent single-input single-output system. Specifically, the impedance of the wind turbine side and the impedance of the lightweight converter are transformed into the equivalent positive-sequence impedance of the wind turbine side and the equivalent positive-sequence impedance of the lightweight converter. The interaction characteristics of the equivalent impedances of the wind turbine side and the lightweight converter are analyzed to reveal the broadband oscillation mechanism of the system.

[0041] Positive sequence equivalent impedance of the wind turbine side and the positive sequence equivalent impedance of lightweight converter Calculated by the following formula:

[0042] In the formula, , These represent the impedance of the wind turbine side. Negative-order coupling components and negative-order components in the text; , , , These represent the wind turbine side admittance matrix. Positive-order components, positive-order coupled components, negative-order coupled components, and negative-order components; , These represent the impedance matrices of the lightweight converter side. Negative-order coupling components and negative-order components in the text; , , , These represent the side admittance matrices of the lightweight converter. The positive-order component, positive-order coupled component, negative-order coupled component, and negative-order component in the equation.

[0043] draw and The interaction situation and the results are as follows Figure 5 As shown, at 12.6Hz, the turbine side exhibits inductive negative damping, interacting with the lightly damped capacitive converter, causing subsynchronous oscillations; at 87.2Hz, the turbine side exhibits capacitive negative damping, interacting with the lightly damped inductive converter, causing supersynchronous oscillations. Therefore, the oscillations in the offshore wind power transmission system via the lightly damped converter originate from the interaction between the lightly damped converter and the negative damping of the turbine.

[0044] (4) Based on the broadband oscillation mechanism of the offshore wind power transmission system via a lightweight converter, the equivalent positive sequence impedance of the lightweight converter is... The impedance components of DRU and MMC are separated by deconstruction. The shaping effect of DRU and MMC on the equivalent positive sequence impedance characteristics of the lightweight converter is analyzed (that is, it is determined that the weak damping characteristics of the lightweight converter mainly originate from MMC). The MMC impedance is then subjected to targeted order reduction processing to locate the oscillation risk factor.

[0045] based on Figure 2 Topology, reducing the equivalent positive sequence impedance of the lightweight converter The impedances of the DRU and MMC are deconstructed and isolated to investigate their shaping effect on impedance characteristics. Focusing only on the oscillation neighborhood frequency band, the impedance characteristics of the DRU and MMC within the 100Hz range are plotted, and the results are as follows. Figure 6 As shown, the DRU did not exhibit negative damping characteristics within 100Hz; however, in the subsynchronous / supersynchronous oscillation frequency range, the MMC exhibited weak capacitive and weak inductive damping, respectively, which worsened the overall phase angle characteristics of the lightweight converter, bringing it closer to the negative damping region. Therefore, the weak damping characteristics introduced by the MMC are a key factor in triggering the system's subsynchronous / supersynchronous oscillations.

[0046] The full-order impedance of the lightweight converter fully considers the small-signal characteristics of various circuit loops and control loops of the DRU and MMC, and can accurately determine the system stability and oscillation frequency. However, its expression is complex, making it difficult to further trace the weak damping characteristics of the MMC. Further reduction of the MMC impedance order follows the principles below: 1. In the MMC multi-harmonic coupling characteristics, the harmonic order is reduced to h=1, considering only ( f p + f 0), ( f p - f 0) and f p Three harmonic components, f p Indicates the frequency of the disturbance. f 0 indicates a fundamental frequency of 50Hz; 2. Since the MMC circulation effect is not obvious in the simulation and the circulation suppression PI parameter is small, the circulation suppression part can be ignored; 3. Since the decoupling terms of the voltage loop and current loop in differential mode control are relatively small, they can be directly set to zero in the impedance model.

[0047] Based on the above order reduction principle, the simplified MMC impedance is obtained. The complete impedance with harmonic order h=4 For example Figure 7 As shown. The results indicate that by reducing the harmonic order of the MMC to 1 and simplifying the control loop, the impedance characteristics of the oscillation neighborhood frequency band are preserved. The reduced-order positive-sequence impedance of the MMC is shown. It can be represented as:

[0048] In the formula, For MMC transformer turns ratio; P i and P v Here are the PI transfer functions for the current loop and voltage loop, respectively; the specific expressions for A, B, C, and K are:

[0049] In the formula, This represents the impedance of the MMC bridge arm resistors and inductors. f p Indicates the frequency of the disturbance. This represents the DC component of the voltage of the upper bridge arm submodule. This indicates the rated DC-side voltage of the MMC. Indicates the modulation ratio of the upper bridge arm submodule. Indicates the capacitance impedance of the upper bridge arm submodule. f0 indicates a base frequency of 50Hz. This represents the 50Hz fundamental frequency component of the MMC AC side voltage. This indicates the rated value of the AC side voltage of the MMC.

[0050] Combination Figure 2 The control structure assumes the actual values ​​of the d-axis and q-axis components of the AC bus voltage. and There are small perturbation components. and The presence of the voltage outer loop causes the disturbance and Through the product of two PI components, i.e. P i· P v Small perturbation components transmitted to the corresponding modulation signal and If we further neglect the outer voltage loop, it is equivalent to P v It is always 0.

[0051] When the disturbance frequency f p When located in the subsynchronous frequency band, the perturbation frequency in the dq domain ( f p - f 0)<0, at this time P i and P v Both are emotional, and K Being a real number does not affect ( K + P i P v This property, when taken as its reciprocal, results in an overall capacitive property, making the impedance of the voltage loop more significant. The phase frequency response will lag behind the impedance of the voltage loop, which is negligible. Similarly, when the perturbation frequency... f p When located in the supersynchronous band, the perturbation frequency in the dq domain ( f p - f 0)>0, at this time P i and P v All are capacitive, taking into account the impedance of the voltage loop. The phase frequency response will lead the impedance without considering the voltage loop. Therefore, the introduction of a voltage loop will significantly alter the phase-frequency characteristics of the impedance. Subsynchronous frequencies cause phase lag, while supersynchronous frequencies cause phase lead, causing the impedance phase-frequency characteristic curve to gradually approach the negative damping region.

[0052] In summary, the weak damping characteristic of the MMC in the oscillation neighborhood frequency band originates from the voltage loop. Specifically, the voltage loop introduces the transfer function product of two PI elements into the impedance characteristics. This structure is a key factor causing the phase frequency curve shift in the sub- / super-synchronous frequency band, thus reducing the system's phase margin. Based on this, the complete influence path from the MMC voltage loop to the system's sub- / super-synchronous oscillation can be plotted, as follows: Figure 8 As shown.

[0053] 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 locating oscillation risk factors in an offshore wind power transmission system via a lightweight converter, characterized in that, Including the following steps: 1) Based on the offshore wind power topology, establish the wind turbine-side impedance taking into account the direct-drive wind turbine and its filter, AC cable and passive filter; 2) Analyze the internal small-signal characteristics of the diode rectifier unit (DRU) and the modular multilevel converter (MMC). Based on the small-signal coupling relationship between the DRU and MMC on both the AC and DC sides, establish the impedance of the DRU-MMC lightweight converter that takes into account the frequency coupling characteristics. 3) Based on the interaction between the wind turbine side impedance and the lightweight converter impedance, the multi-input multi-output impedance is converted into an equivalent single-input single-output impedance, and the positive-sequence equivalent impedance of the wind turbine side and the lightweight converter is obtained. The interaction characteristics of the equivalent impedances of the wind turbine side and the lightweight converter are analyzed, and then the broadband oscillation mechanism of the offshore wind power transmission system via the lightweight converter is obtained. 4) Based on the broadband oscillation mechanism of offshore wind power transmission system via lightweight converter, the positive sequence equivalent impedance of lightweight converter is deconstructed, the equivalent impedances of DRU and MMC are separated, and the equivalent impedance of MMC is reduced in order to locate the oscillation risk factor.

2. The method for locating oscillation risk factors according to claim 1, characterized in that, Using the AC terminal of the lightweight converter as the dividing point, the offshore wind power transmission system via the lightweight converter is divided into two parts: the wind turbine side and the lightweight converter side. The impedance of each part is calculated separately.

3. The method for locating oscillation risk factors according to claim 1, characterized in that, In step 1), the wind turbine side impedance Represented as: In the formula, 、 , , These represent the impedance matrices of the direct-drive fan, direct-drive fan filter, AC cable, and passive filter, respectively.

4. The method for locating oscillation risk factors according to claim 1, characterized in that, In step 2), the lightweight converter consists of a diode rectifier unit (DRU) and a modular multilevel converter (MMC). The two are connected in parallel on the AC side and in series on the DC side. Its impedance includes both the DRU and the MMC. At the same time, the small-signal coupling relationship caused by the series connection of the two on the DC side must be taken into account.

5. The method for locating oscillation risk factors according to claim 4, characterized in that, In the lightweight converter, the DC-side DRU / MMC is connected in series, and the current disturbance components of the two are the same. The total voltage disturbance on the DC side is the sum of the two voltage disturbances.

6. The method for locating oscillation risk factors according to claim 4, characterized in that, The AC-side DRU / MMC of the lightweight converter is connected in parallel, and the voltage disturbance components of the two are consistent. The positive / negative sequence disturbance components of the total AC current are the sum of the two current disturbances.

7. The method for locating oscillation risk factors according to claim 6, characterized in that, Positive sequence disturbance component of total AC current and negative order perturbation components The calculation formula is as follows: In the formula, For lightweight converter impedance matrix, and These represent the positive and negative sequence disturbance components of the total AC voltage, respectively. and These represent the positive and negative sequence components of the AC side current of the DRU, respectively. and These represent the positive and negative sequence components of the AC side current of the DRU, respectively.

8. The method for locating oscillation risk factors according to claim 1, characterized in that, In step 3), the positive sequence equivalent impedance on the wind turbine side and the positive sequence equivalent impedance of lightweight converter Calculated by the following formula: In the formula, , These represent the impedance of the wind turbine side. Negative-order coupling components and negative-order components in the text; , , , These represent the wind turbine side admittance matrix. Positive-order components, positive-order coupled components, negative-order coupled components, and negative-order components; , These represent the impedance matrices of the lightweight converter side. Negative-order coupling components and negative-order components in the text; , , , These represent the side admittance matrices of the lightweight converter. The positive-order component, positive-order coupled component, negative-order coupled component, and negative-order component in the equation.

9. The method for locating oscillation risk factors according to claim 1, characterized in that, In step 4), firstly, the high-frequency harmonic components in the MMC multi-harmonic coupling characteristics are ignored, and secondly, the control loop is simplified; while retaining the impedance characteristics of the oscillation neighborhood frequency band, the MMC impedance is reduced in order step by step.

10. The method for locating oscillation risk factors according to claim 9, characterized in that, The following principles are used to reduce the order of MMC impedance: In the MMC multiharmonic coupling characteristics, the harmonic order is reduced to h=1, considering only ( f p + f 0), ( f p - f 0) and f p Three harmonic components, f p Indicates the frequency of the disturbance. f 0 indicates a fundamental frequency of 50Hz; Ignore the MMC circulating current suppression part; In differential mode control, the decoupling terms of the voltage loop and current loop are directly set to zero in the impedance model.

Images