Offshore wind power DR-MMC direct current sending-out converter and control method thereof

Abstract

The invention discloses an offshore wind power DR-MMC direct current sending-out converter and a control method thereof, and efficient wind power direct current sending-out is realized through a structure that a diode rectifier (DR) is connected with a direct current side of an auxiliary converter in parallel and an alternating current side is connected with a grid through a transformer. The auxiliary converter is composed of a high-voltage series submodule branch (HVSB), a low-voltage three-phase series modular multilevel converter (LV-SC-MMC) and an LC filter, the LV-SC-MMC adopts a three-phase modular full-bridge series structure, each phase comprises four bridge arms, and the alternating current side is in triangular connection through three single-phase transformers. According to the control method, variable-amplitude high-frequency voltage (the amplitude is dynamically adjusted along with a fundamental component) is injected into an LV-SC-MMC bridge arm, on the premise that the number of sub-modules is not increased, the peak value / valley value of the bridge arm voltage is not changed, and meanwhile power balance control is completed by using the superposition characteristic of the high-frequency voltage on the direct current side and combining HVSB voltage adjustment. The device effectively reduces the number of sub-modules, guarantees the quality of AC-side electric energy, and is suitable for offshore wind power low-cost and compact DC transmission scenes.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of offshore wind power transmission, and relates to a kind of offshore wind power DR-MMC direct current transmission converter and control method thereof. BACKGROUND

[0002] With the transformation of global energy structure to low carbonization, far sea wind power has become an important direction of offshore wind power development due to its advantages of rich resources, great development potential and small impact on land environment. However, the electric energy transmission of far sea wind farm faces technical challenges: due to the charging capacitor effect of submarine cable and the limitation of transmission distance, the traditional high voltage alternating current (HVAC) transmission scheme has problems of complex reactive power control, poor voltage stability and high submarine cable loss, so the high voltage direct current (HVDC) transmission technology has gradually become the mainstream choice of far sea wind power grid connection.

[0003] In the field of far sea wind power direct current transmission, modular multilevel converter (Modular Multilevel Converter, MMC) has become the preferred technology in current engineering application due to its advantages of modular design, high scalability, large number of output levels and common DC bus structure. MMC realizes low switching frequency operation under high voltage and large capacity through the cascade of sub-modules (SM), significantly reduces the harmonic content and switching loss, and has flexible reactive power regulation capability. However, the MMC scheme still has the following inherent defects: first, the number of sub-modules is huge (usually hundreds or even thousands), which greatly increases the volume and weight of the converter valve, directly increasing the construction cost and offshore operation difficulty of the offshore converter platform (such as converter station or wind turbine tower); second, the control of a large number of sub-modules in series is complex, which requires high real-time performance and reliability of the control system, further increasing the system design and maintenance cost.

[0004] To break through the economic bottleneck of MMC technology, the DC transmission scheme based on diode rectifier (DR) has been proposed and attracted attention in recent years. The DR scheme realizes power transmission from wind turbine to DC grid through uncontrolled rectification, which has simple topology, less number of devices, significantly reduces the size and weight of the converter valve, and avoids the complex sub-module voltage balancing control of MMC, thus improving the reliability of system operation. However, the introduction of DR scheme also brings new technical challenges: first, DR is a passive rectifier device, which cannot actively build an offshore AC grid, so wind turbines must use grid-forming (GFM) control strategy to achieve autonomous support of voltage and frequency, which puts higher requirements on the dynamic performance and stability of the wind turbine control system; second, DR will generate significant reactive power demand when transmitting active power, and the rectification process will introduce low-order harmonic currents (such as 6th and 12th harmonics), which requires additional reactive power compensation devices (such as SVG) and filters to be configured on the converter platform, increasing the system cost and size; finally, the one-way conduction characteristic of DR limits the self-sustaining ability of the wind farm in fault or black start scenarios, and requires external power assistance to start.

[0005] To compensate for the shortcomings of the DR scheme, existing research proposes a hybrid scheme of parallel auxiliary converters (such as voltage source converters, VSC) on the DC side: the auxiliary converter realizes reactive power compensation, harmonic suppression, active return, and grid-forming functions, thereby expanding the system operating capacity. However, the parallel design of auxiliary converters requires additional sub-modules and control systems, which may partially offset the advantages of DR in size and weight. Therefore, how to further reduce the number of sub-modules of auxiliary converters and optimize the system topology while ensuring the integrity of system functions has become a key technical requirement for realizing a more compact, more economical, and more reliable offshore wind DC transmission scheme. SUMMARY

[0006] The purpose of the present application is to solve the technical problems of complex converter structure, difficulty in further reducing volume and weight, and inability to realize reactive power compensation and harmonic suppression in the prior art, and to provide an offshore wind DR-MMC DC transmission converter and a control method thereof.

[0007] To achieve the above purpose, the following technical solutions are adopted in the present application: The present application provides an offshore wind DR-MMC DC transmission converter in a first aspect, comprising a diode rectifier and an auxiliary converter. The DC side of the diode rectifier is connected in parallel with the DC side of the auxiliary converter; the AC sides of the diode rectifier and the auxiliary converter are connected to the AC bus of the offshore wind farm through transformers, respectively; The auxiliary converter comprises an HVSB, an LV-SC-MMC and an LC filter. The DC side of the LV-SC-MMC is connected in series with the DC side of the HVSB, and the overall DC output end after the series connection of the two is connected in parallel with the LC filter. The AC side of the LV-SC-MMC is connected with the AC collection bus of the offshore wind farm through three single-phase transformers, the secondary sides of the three single-phase transformers are respectively connected with the three-phase AC output ends of the LV-SC-MMC, the primary sides of the three single-phase transformers are connected in a triangle connection structure and are connected with the power grid.

[0008] Further, the LV-SC-MMC comprises three-phase phase units and three single-phase transformers; each phase unit in the three-phase phase units comprises four bridge arms, each bridge arm is composed of N sub-modules and a bridge arm reactor L connected in series; two of the four bridge arms are connected in series to form two half-bridge structures, the midpoint of the two half-bridge structures is the AC output end of the phase, and the parallel connection of the two ends of the two half-bridges is the DC output end of the phase; the DC output ends of the three-phase phase units are connected in series to serve as the DC output end of the LV-SC-MMC.

[0009] Further, the sub-module is one of a half-bridge sub-module, a full-bridge sub-module or a hybrid sub-module.

[0010] The second aspect of the present application provides a control method of an offshore wind power DR-MMC DC transmission converter, comprising the following steps: A variable-amplitude high-frequency voltage is injected into each bridge arm of the LV-SC-MMC, the amplitude of the high-frequency voltage is dynamically adjusted according to the fundamental component of the bridge arm output voltage, and the peak value and the valley value of the bridge arm output voltage are not changed; The output voltage of the HVSB is adjusted to balance the power flow between the LV-SC-MMC and the HVSB, so as to realize stable control of the total DC voltage of the system; The LC filter is used to limit the high-frequency current in the auxiliary converter, so as to prevent the influence on the DC side of the system.

[0011] Further, the variable-amplitude high-frequency voltage satisfies:

[0012] Wherein, ; is the modulation ratio of the fundamental component; is the DC component of the DC voltage of the LV-SC-MMC; is the modulation phase of the fundamental component; is the modulation ratio of the injected high-frequency voltage; is the variable-amplitude high-frequency voltage.

[0013] Further, the output voltage of the HVSB is adjusted by the following formula:

[0014] wherein, is the adjustment value of the high-frequency voltage; is the DC component of the LV-SC-MMC DC side voltage; represents the phase of the high-frequency voltage and current; is the DC side voltage of the HVSB; is the modulation ratio of the injected high-frequency component; is the angular frequency of the high-frequency voltage.

[0015] Further, the LC filter resonant frequency and the angular frequency of the injected high-frequency voltage are consistent with the following formula:

[0016] wherein, is the filter inductance; is the filter capacitance; is the angular frequency of the high-frequency voltage.

[0017] Further, the adjustment value of the high-frequency voltage is:

[0018] wherein, is the amplitude of the high-frequency current; is the equivalent inductance.

[0019] Further, the amplitude of the high-frequency current is determined according to the DC voltage division ratio; The expression of the amplitude of the high-frequency current is:

[0020] wherein, the is the DC side current of the converter; is the DC voltage division ratio.

[0021] Further, the expression of the DC voltage division ratio is:

[0022] wherein, is the DC side voltage of the HVSB; is the total DC voltage of the converter.

[0023] Compared with the prior art, the present application has the following beneficial effects: The application discloses a marine wind power DR-MMC direct current sending converter and a control method thereof, and changes a low-voltage modular multilevel converter structure in an auxiliary converter into a low-voltage three-phase series modular multilevel converter structure, effectively reduces the number of sub-modules required by the topology while bearing the same direct current voltage. Meanwhile, in order to meet the power balance between the low-voltage three-phase series modular multilevel converter and the high-voltage series sub-module branch in the auxiliary converter, a high-frequency voltage component is injected into the output voltage of the two. Compared with the traditional fixed-amplitude high-frequency voltage injection method, the amplitude of the high-frequency voltage component injected by the variable-amplitude high-frequency voltage injection method changes with the instantaneous value of the fundamental component of the original output voltage of the bridge arm. When the amplitude of the original output voltage of the bridge arm is high, the amplitude of the injected high-frequency component is small, and vice versa. Thus, after the variable-amplitude high-frequency voltage injection method is adopted, the high-frequency voltage component is injected while the peak value of the required output voltage of the bridge arm is not increased or the valley value of the required output voltage of the bridge arm is not reduced, that is, the low-voltage three-phase series modular multilevel converter does not need to increase the number of sub-modules after injection. Compared with the traditional auxiliary converter scheme, the method effectively reduces the number of sub-modules required by the system, and further reduces the volume and weight of the system. BRIEF DESCRIPTION OF DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments. It should be understood that the following drawings only show some of the embodiments of the present application, and therefore should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can also be obtained without creative labor.

[0025] Figure 1 is a schematic diagram of a marine wind power DR-MMC direct current sending converter structure of the embodiment of the present application; Figure 2 is a schematic diagram of an auxiliary converter topology structure in the DR-MMC direct current sending converter of the embodiment of the present application; Figure 2 (a) is an auxiliary converter structure diagram; Figure 2 (b)(1) is a single-phase phase unit structure diagram; Figure 2 (b)(2) is a sub-module structure diagram in a bridge arm; Figure 3 is a schematic diagram of the LV-SC-MMC bridge arm output voltage when no high-frequency voltage is injected, the LV-SC-MMC bridge arm output voltage when the traditional fixed-amplitude high-frequency voltage is injected, and the LV-SC-MMC bridge arm output voltage when the variable-amplitude high-frequency voltage is injected; Figure 4 is an equivalent circuit diagram of the LV-SC-MMC of the present application; Figure 5 is the number of sub-modules required for each method and different DC voltage division ratio coefficient α dc the change relationship diagram; Figure 6 is the simulation waveform diagram of the system in the case of transmitting rated active power of the auxiliary converter of the DR-MMC offshore wind power transmission system in the embodiment of the application; Figure 6 (a) is the voltage waveform diagram of the converter side of the transformer, Figure 6 (b) is the current waveform diagram of the converter side of the transformer, Figure 6 (c) is the sum of the output voltage of the A1 bridge arm and the capacitor voltage, Figure 6 (d) is the sum of the output voltage of the B1 bridge arm and the capacitor voltage, Figure 6 (e) is the sum of the output voltage of the C1 bridge arm and the capacitor voltage, Figure 6 (f) is the output voltage of the HVSB, the sum of the HVSB capacitor voltage and the DC side voltage of the LV-SC-MMC, Figure 6 (g) is the A1 bridge arm current of the LV-SC-MMC and the current flowing through the HVSB. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solutions and advantages of the embodiments of the application clearer, the technical solutions in the embodiments of the application will be described clearly and completely below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. The components of the embodiments of the application described and indicated in the drawings can be arranged and designed in various different configurations.

[0027] Therefore, the following detailed description of the embodiments of the application provided in the drawings is not intended to limit the scope of the claimed application, but only represents selected embodiments of the application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the application without creative labor are within the scope of protection of the application.

[0028] It should be noted that: similar reference numbers and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0029] The application will be described in further detail below with reference to the drawings: Reference is made to Figure 1The application provides a sea wind power DR-MMC direct current sending converter, which comprises a diode rectifier DR and an auxiliary converter.

[0030] The auxiliary converter has a topology structure as shown in Figure 2 The auxiliary converter comprises a high-voltage series sub-module branch (HVSB), a low-voltage three-phase series modular multilevel converter (LV-SC-MMC) and an LC filter. The low-voltage three-phase series modular multilevel converter is connected in series with the high-voltage series sub-module branch, and the series connection is connected in parallel with the LC filter branch, as shown in Figure 2 The low-voltage three-phase series modular multilevel converter has a structure as shown in Figure 2 The low-voltage three-phase series modular multilevel converter mainly comprises three-phase units and three single-phase transformers. Figure 2 The internal detailed structure of each phase unit is shown in (1) of (b), and the phase unit comprises four bridge arms, each bridge arm is connected in series with N sub-modules and a bridge arm reactor, the two bridge arms form a half-bridge structure, the midpoint of the two bridge arms is an AC output point, and the two end points are DC output points. Figure 2 The full-bridge structure of each phase unit is shown in (1) of (b). The midpoint of the two bridge arms in the phase unit is an AC output point of the phase, and the DC end points of the two bridge arms are DC output points of the phase. Figure 2 The specific structure of the sub-module in the bridge arm is shown in (2) of (b), which comprises two IGBTs (S1 and S2) with anti-parallel diodes and a DC energy storage capacitor C The two IGBTs are connected in series with the capacitor C in parallel, and two connection ends are led out. Figure 2 As shown in (a), the DC output points of the three-phase units are connected in series to form the DC output points of the low-voltage three-phase series modular multilevel converter, and the AC output points of the phase units are connected to the power grid through a single-phase transformer.

[0031] The proposed high-frequency voltage with varying amplitude is injected into each arm of the low-voltage three-phase series modular multilevel converter. The amplitude of the injected high-frequency voltage varies with the fundamental frequency component of the output voltage of each phase arm. The injection of the high-frequency voltage does not affect the peak and valley values ​​of the arm output voltage. Therefore, there is no need to increase the number of sub-modules.

[0032] One embodiment of the present invention provides a control method for the above-mentioned offshore wind power DR-MMC DC transmission converter, specifically: Taking the first bridge arm of each phase as an example, without the injection of high-frequency voltage, the output voltage components of the first bridge arm of each phase of the low-voltage three-phase series modular multilevel converter can be expressed as:

[0033] in, It is the DC component of the DC-side voltage of LV-SC-MMC. and It refers to the fundamental frequency component modulation ratio and phase.

[0034] When using conventional fixed-amplitude high-frequency voltage injection, the output voltage components of the first bridge arm of each phase of the low-voltage three-phase series modular multilevel converter can be expressed as:

[0035] in, yes The amplitude of the high-frequency component injected into the phase, and and It refers to the modulation ratio and phase of the injected high-frequency components. It is the angular frequency of the high-frequency component.

[0036] To ensure that injecting high-frequency voltage does not affect the peak and valley values ​​of the original bridge arm output voltage, and thus the required number of sub-modules, the amplitude of the injected high-frequency component is... The following formula must be satisfied

[0037] To make the above formula easy to implement and to include a considerable amount of high-frequency components, the proposed method ultimately injects high-frequency component amplitudes. Selectable as

[0038] in, The following formula must be satisfied

[0039] It is evident that the amplitude of the high-frequency component injected by the proposed method is not constant, and varies with the instantaneous value of the fundamental frequency output component.

[0040] The output voltage of the first bridge arm of each phase of the low-voltage three-phase series modular multilevel converter after the application of the proposed variable-amplitude high-frequency voltage injection can be expressed as:

[0041] The output voltage of the first bridge arm of each phase of the low-voltage three-phase series modular multilevel converter without high-frequency voltage injection , the output voltage after the injection of the conventional fixed-amplitude high-frequency voltage , and the output voltage after the injection of the proposed variable-amplitude high-frequency voltage are shown in Figure 3 . In the figure, = 0.85, = 0.85 / 3. As can be seen from Figure 3 , in the case of the injection of high-frequency voltages of the same amplitude, the proposed variable-amplitude high-frequency voltage injection method does not increase the peak value of the required output voltage of the bridge arm or reduce the valley value of the required output voltage of the bridge arm, compared with the conventional fixed-amplitude high-frequency voltage injection method. Thus, the number of sub-modules does not need to be additionally increased.

[0042] The output voltage of the four bridge arms of phase j of the low-voltage three-phase series modular multilevel converter after the application of the proposed variable-amplitude high-frequency voltage injection method can be expressed as:

[0043] Ignoring the high-frequency components introduced by the switching process, the equivalent circuit diagram of each phase of the low-voltage three-phase series modular multilevel converter can be shown as Figure 4 . It can be seen that the variable-amplitude high-frequency voltages injected in the four bridge arms of each phase by the proposed method are completely the same, so the high-frequency voltages are canceled at the alternating current side of each phase, while appearing at the direct current side of each phase with a doubled amplitude.

[0044] Therefore, the direct current side voltage of each phase of the low-voltage three-phase series modular multilevel converter can be expressed as:

[0045] At this time, the equivalent circuit diagram of the direct current side of the corresponding auxiliary converter is shown in Figure 4 (b). Further, the three-phase addition of the low-voltage three-phase series modular multilevel converter is performed, and the total direct current side voltage of the low-voltage three-phase series modular multilevel converter is:

[0046] It can be seen that the fluctuation components of the high-frequency voltage cancel each other out, and a high-frequency voltage component with a fixed amplitude is obtained at the total DC side of the low-voltage three-phase series modular multilevel converter. At this time, the equivalent circuit diagram of the DC side of the corresponding auxiliary converter is as shown in Figure 4 (c) shown.

[0047] To ensure that the total DC side voltage of the auxiliary converter is a set value , the voltage required to be output by the high-voltage series sub-module branch can be represented as:

[0048] , wherein is the adjustment value of the high-frequency voltage, which interacts with the equivalent reactance of the DC side to control the high-frequency current component, and according to Figure 4 (c) it can be represented as:

[0049] , wherein is the high-frequency current in the auxiliary converter, and is the amplitude of the high-frequency current in the auxiliary converter. In order to minimize the amplitude of the high-frequency current, the phase of the high-frequency voltage and current is set to be the same, both being .

[0050] Further, in order to control the high-frequency current within the auxiliary converter, the LC filter branch can be designed as:

[0051] The amplitude of the high-frequency current in the auxiliary converter is directly related to the proportion of the DC side voltage of the low-voltage three-phase series modular multilevel converter. In order to facilitate the analysis of different DC side voltage proportion cases, the DC voltage division ratio coefficient is defined as follows:

[0052] Therefore, in order to maintain the power balance of the high-voltage series sub-module branch, the active power required to be transmitted by the high-frequency voltage and current can be represented as:

[0053] Therefore, the target value of the amplitude of the high-frequency current can be represented as:

[0054] Therefore, by controlling the adjustment value of the high-frequency voltage in the output voltage of the high-voltage series sub-module branch , the amplitude of the high-frequency current component can be controlled to be the target value . Finally, the power balance between the high-voltage series sub-module branch and the low-voltage three-phase series modular multilevel converter can be achieved.

[0055] Another embodiment of the present application is to illustrate the sub-module quantity reduction effect of the present application scheme. The required sub-module quantity of the MMC bearing total DC voltage, the SC-MMC bearing total DC voltage, the DR+ auxiliary converter based on MMC applying the traditional high-frequency injection method, the DR+ auxiliary converter based on SC-MMC applying the traditional high-frequency injection method, and the DR+ auxiliary converter based on SC-MMC applying the proposed variable-amplitude high-frequency voltage injection are calculated and compared.

[0056] The required sub-module quantity calculated by different methods is as follows: The required sub-module quantity of the MMC bearing total DC voltage is:

[0057] The required sub-module quantity of the SC-MMC bearing total DC voltage is:

[0058] The total required sub-module quantity of the MMC auxiliary converter bearing partial DC voltage applying the traditional high-frequency injection method is:

[0059] wherein, is the required sub-module quantity of the high-voltage series sub-module branch, is the sub-module quantity of the low-voltage three-phase series modular multilevel converter. may be expressed as:

[0060] wherein, is the modulation ratio margin of the high-frequency voltage, which is taken as =0.85, that is, 17.6% margin is left. may be expressed as:

[0061] Thus, the total required sub-module quantity of the MMC auxiliary converter bearing partial DC voltage applying the traditional high-frequency injection method is:

[0062] Similarly, it can be known that the total required sub-module quantity of the SC-MMC auxiliary converter bearing partial DC voltage applying the traditional high-frequency injection method is:

[0063] The total number of sub-modules required for the SC-MMC auxiliary converter subjected to the partial DC voltage after the application of the proposed variable-amplitude high-frequency voltage injection is:

[0064] It can be seen that the number of sub-modules required for the auxiliary converter is continuously reduced from the MMC structure to the SC-MMC structure, and further to the SC-MMC structure using the proposed injection method.

[0065] Further, the values of the amplitude of the high-frequency component injected in the auxiliary converter under different cases are derived as follows: Since the high-voltage series sub-module branch is composed of half-bridge sub-modules in series, the output voltage u dc,U should always be greater than or equal to 0. Since Δ u h is relatively small, it can be ignored. Let m h = m a / 3, u dc,U ≥ 0 can be derived as

[0066] To ensure that u dc,U ≥ 0 under the premise of increasing the amplitude of the high-frequency component as much as possible, the amplitude of the high-frequency component injected in the auxiliary converter under different α dc cases is finally selected as

[0067] Finally, the number of sub-modules of the MMC auxiliary converter subjected to the total DC voltage N MMC , the number of sub-modules of the SC-MMC auxiliary converter subjected to the total DC voltage N SCMMC , the number of sub-modules of the MMC auxiliary converter subjected to the partial DC voltage using the traditional high-frequency injection method N AXC,MMC , the number of sub-modules of the SC-MMC auxiliary converter subjected to the partial DC voltage using the traditional high-frequency injection method N AXC,SCMMC , and the number of sub-modules of the SC-MMC auxiliary converter subjected to the partial DC voltage after the application of the proposed variable-amplitude high-frequency voltage injection ​​Comparison is made. For the convenience of comparison, the number of sub-modules of each method is compared with the number of sub-modules of the total DC voltage MMC N MMC The unit is marked, and the number of sub-modules required by each method is plotted as a function of the change of the sub-module number required by the different DC voltage division ratio coefficients α dc as shown in Figure 5 It can be seen from Figure 5 that the number of sub-modules required by the auxiliary converter proposed in the method is the lowest among the five schemes, and thus the method effectively reduces the number of sub-modules required by the converter.

[0068] Figure 6 is a simulation waveform diagram of the auxiliary converter of the DR-MMC offshore wind power transmission system in an embodiment of the application under the condition of transmission of rated active power. Among them Figure 6 (a) is a voltage waveform diagram of the converter side of the transformer, Figure 6 (b) is a current waveform diagram of the converter side of the transformer, Figure 6 (c) is the sum of the output voltage of the A1 bridge arm and the capacitor voltage, Figure 6 (d) is the sum of the output voltage of the B1 bridge arm and the capacitor voltage, Figure 6 (e) is the sum of the output voltage of the C1 bridge arm and the capacitor voltage, Figure 6 (f) is the output voltage of the HVSB, the sum of the HVSB capacitor voltage and the DC side voltage of the LV-SC-MMC, Figure 6 (g) is the A1 bridge arm current of the LV-SC-MMC and the current flowing through the HVSB. From Figure 6 (a) and Figure 6 (b), it can be seen that after the proposed injection method is applied, the high-frequency component does not affect the voltage and current of the AC side of the converter. From Figure 6 (c), Figure 6 (d) and Figure 6 (e), it can be seen that after the proposed injection method is applied, the bridge arm contains a certain high-frequency voltage component, but the maximum and minimum values of the total output voltage of the bridge arm are not affected, so the number of sub-modules required by the bridge arm does not need to be increased. From Figure 6 (f), it can be seen that the total DC side voltage of the LV-SC-MMC only contains a DC component and a high-frequency component with a fixed amplitude. Due to the special structure of the SC-MMC, the fluctuation values of the high-frequency voltage components injected in each phase cancel each other out, and thus a high-frequency component with a fixed amplitude is presented in the total DC side. The output voltage of the HVSB also contains a DC component and a high-frequency component. The high-frequency components of the HVSB and the total DC of the LV-SC-MMC are opposite in phase and the same in amplitude. From Figure 6(f) It can be seen that the DC component of the HVSB output voltage is about 560 kV, while the DC component of the total DC voltage of the LV-SC-MMC is about 240 kV, which is consistent with the design value of the DC voltage division ratio being equal to 0.3. From the above, it can be seen that the DC voltage of the HVSB is about 320 kV, while the DC voltage of the LV-SC-MMC is about 240 kV, which is consistent with the design value of the DC voltage division ratio being equal to 0.75. Figure 6 (g) It can be seen that the A1 bridge arm current of the LV-SC-MMC contains a DC component, a fundamental frequency component and a high frequency component. While the current flowing through the HVSB contains only a DC component and a high frequency component. ​ It can be seen that the method of the present application does not affect the normal operation of the converter while reducing the number of sub-modules, and the improved frequency voltage injection method effectively realizes the power balance inside the converter and does not affect the operation of the AC side and the DC side of the converter.

[0069] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A marine wind power DR-MMC DC transmission converter, characterized in that, The diode rectifier and the auxiliary converter are included. The DC side of the diode rectifier is connected in parallel with the DC side of the auxiliary converter; the AC side of the diode rectifier and the AC side of the auxiliary converter are respectively connected to the AC collection bus of the offshore wind farm through transformers. The auxiliary converter includes an HVSB, an LV-SC-MMC and an LC filter. The DC side of the LV-SC-MMC is connected in series with the DC side of the HVSB, and the overall DC output end of the series connection is connected in parallel with the LC filter. The AC side of the LV-SC-MMC is connected to the AC collection bus of the offshore wind farm through three single-phase transformers, the secondary sides of the three single-phase transformers are respectively connected to the three-phase AC output ends of the LV-SC-MMC, and the primary sides of the three single-phase transformers are connected in a delta connection structure and connected to the power grid.

2. The offshore wind power DR-MMC DC transmission converter according to claim 1, characterized in that, The LV-SC-MMC includes three-phase units and three single-phase transformers; each phase unit in the three-phase units includes four bridge arms, each bridge arm is composed of N sub-modules and a bridge arm reactor L connected in series; two of the four bridge arms are connected in series to form two half-bridge structures, the midpoint of the two half-bridge structures is the AC output end of the phase, and the parallel connection of the two ends of the two half-bridges is the DC output end of the phase; the DC output ends of the three-phase units are connected in series to serve as the DC output end of the LV-SC-MMC.

3. The offshore wind power DR-MMC DC transmission converter of claim 2, characterized in that, The sub-module is one of a half-bridge sub-module, a full-bridge sub-module or a hybrid sub-module.

4. A control method of an offshore wind farm DR-MMC DC transmission converter, based on the offshore wind farm DR-MMC DC transmission converter of any one of claims 1-3, characterized in that, The method includes the following steps: A variable-amplitude high-frequency voltage is injected into each bridge arm of the LV-SC-MMC, the amplitude of the high-frequency voltage is dynamically adjusted according to the fundamental component of the bridge arm output voltage, and the peak value and the valley value of the bridge arm output voltage are not changed; The output voltage of the HVSB is adjusted to balance the power flow between the LV-SC-MMC and the HVSB, and the total DC voltage of the system is stably controlled; The LC filter is used to limit the high-frequency current within the auxiliary converter to prevent affecting the DC side of the system.

5. The control method of the offshore wind power DR-MMC DC transmission converter according to claim 4, characterized in that, The variable-amplitude high-frequency voltage satisfies: wherein, ; is the fundamental component modulation ratio, is the LV-SC-MMC DC voltage DC component; is the fundamental component modulation phase; is the modulation ratio of the injected high frequency voltage; is the variable amplitude high frequency voltage.

6. The control method of the offshore wind power DR-MMC DC transmission converter according to claim 4, characterized in that, The output voltage of the HVSB is adjusted by the following formula: wherein, is the regulated value of the high-frequency voltage; is the DC component of the LV-SC-MMC DC-side voltage; denotes the phase of the high-frequency voltage and current; is the DC-side voltage of the HVSB; is the modulation ratio of the injected high-frequency component; is the angular frequency of the high-frequency voltage.

7. The control method of the offshore wind power DR-MMC DC transmission converter according to claim 6, characterized in that, The resonance frequency of the LC filter is consistent with the angular frequency of the injected high-frequency voltage, which satisfies: wherein is a filter inductance; is a filter capacitance; is the angular frequency of the high frequency voltage.

8. The control method of the offshore wind power DR-MMC DC transmission converter according to claim 7, characterized in that, The adjustment value of the high-frequency voltage is: wherein, is the amplitude of the high frequency current; is the equivalent inductance.

9. The control method of the offshore wind power DR-MMC DC transmission converter according to claim 8, characterized in that, The amplitude of the high-frequency current is determined according to the DC voltage division ratio; The expression of the amplitude of the high-frequency current is: Among them, the is the DC side current of the converter; is the DC voltage division ratio.

10. The control method of the offshore wind power DR-MMC DC transmission converter according to claim 9, characterized in that, The expression of the DC voltage division ratio is: wherein, is the DC side voltage of the HVSB; is the total DC voltage of the converter.

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