A design method for a main circuit of a multi-AC port flexible DC converter, a terminal device, a storage medium, and a main circuit of a multi-AC port flexible DC converter

Abstract

The present invention discloses a design method, terminal device, storage medium, and multi-AC port flexible DC converter main circuit. The method includes calculating MMC submodule design parameters based on rated parameters, wherein the MMC submodule design parameters include: MMC DC side voltage value, number of MMC submodules, MMC bridge arm inductance parameter, and MMC submodule capacitance value; calculating eACPFC submodule design parameters based on the rated parameters, wherein the eACPFC submodule design parameters include: number of eACPFC submodules and eACPFC submodule capacitance value; and constructing the multi-AC port flexible DC converter main circuit based on the MMC submodule design parameters and the eACPFC submodule design parameters. The present invention can configure the MMC submodule and the eACPFC submodule, thereby ensuring the performance of the multi-port flexible DC converter.

Description

Technical Field

[0001] The present invention relates to the technical field of converters, and in particular to a design method, terminal equipment, storage medium and main circuit of a multi-AC port flexible DC converter. Background Art

[0002] Offshore wind power transmission solutions have evolved from AC transmission to DC transmission. AC transmission is the conventional solution for offshore wind power transmission, boasting advantages such as mature technology and simple structure. However, with the increasing size and distance from shore of offshore wind farms, the AC convergence and transmission systems of wind farms have become increasingly complex, and the resulting overvoltage and reactive power losses caused by the ground capacitance of submarine cables have become increasingly severe. This has reduced the effective load capacity of the cables, necessitating the deployment of a large number of reactive power compensation devices, limiting their application in deep-sea wind power transmission.

[0003] To solve the above problems, researchers have proposed a multi-AC port flexible DC converter based on double frequency circulating current injection. The multi-port flexible DC converter consists of a modular multi-level DC converter and an AC power flow controller (eACPFC) embedded in the modular multi-level DC converter. The AC power flow controller can interconnect multiple AC lines and has the ability to independently decouple and control the active and reactive power of each line, thereby realizing the joint absorption of DC-transmitted sea wind power by multiple lines. However, there is no design scheme for the main circuit parameters of multi-port flexible DC converters based on double frequency circulating current injection at home and abroad, which leads to poor performance of multi-port flexible DC converters in application.

[0004] Therefore, a design strategy for the main circuit of a multi-AC port flexible DC converter is urgently needed to solve the problem of poor performance of the multi-port flexible DC converter in application. Summary of the Invention

[0005] The embodiments of the present invention provide a design method for a multi-AC port flexible DC converter main circuit, a terminal device, a storage medium and a multi-AC port flexible DC converter main circuit to solve the problem of poor performance of the multi-port flexible DC converter in application.

[0006] To solve the above problem, an embodiment of the present invention provides a design method for a main circuit of a multi-AC port flexible DC converter, comprising:

[0007] Obtain the rated parameters of the main circuit of the multi-AC port flexible DC converter;

[0008] Based on the rated parameters, calculating the MMC submodule design parameters; wherein the MMC submodule design parameters include: MMC DC side voltage value, number of MMC submodules, MMC bridge arm inductance parameter, and MMC submodule capacitance value;

[0009] Calculating eACPFC submodule design parameters based on the rated parameters; wherein the eACPFC submodule design parameters include: the number of eACPFC submodules and the capacitance value of the eACPFC submodules;

[0010] The multi-AC port flexible DC converter main circuit is constructed according to the MMC submodule design parameters and the eACPFC submodule design parameters.

[0011] As an improvement to the above solution, the rated parameters include: the effective value of the AC port output voltage, the effective value of the double frequency common-mode voltage injected by the eACPFC submodule, and the effective value of the common-mode voltage required for the double frequency current injected by the MMC submodule; the calculation of the MMC DC side voltage value includes:

[0012] Substitute the effective value of the AC port output voltage, the effective value of the double-frequency common-mode voltage injected by the eACPFC submodule, and the effective value of the common-mode voltage required for the double-frequency current injected by the MMC submodule into the MMC DC side voltage calculation formula to obtain the MMC DC side voltage value. The MMC DC side voltage calculation formula satisfies the following conditions:

[0013]

[0014] Where V dc is the DC side voltage of MMC, V diff is the effective value of the AC port output voltage, V c_com V is the RMS value of the double frequency common mode voltage injected into the eACPFC submodule, cir2 The RMS common-mode voltage required to inject double-frequency current into the MMC submodule.

[0015] As an improvement to the above solution, the rated parameters also include: the rated capacitance voltage of the MMC submodule, the withstand voltage of the IGBT device, the withstand current of the IGBT device, the line rated current, the maximum injected double frequency current, and the DC side rated current; the calculation of the number of the MMC submodules includes:

[0016] Substitute the MMC DC side voltage value, the MMC submodule rated capacitance voltage, the IGBT device withstand voltage, the IGBT device withstand current, the line rated current, the maximum injected double frequency current, and the DC side rated current into the submodule number calculation formula to obtain the number of MMC submodules; wherein, the submodule number calculation formula satisfies the following conditions:

[0017]

[0018] Where N MMC is the number of MMC submodules, V c * is the rated capacitance voltage of the MMC submodule, V IGBT is the withstand voltage of the IGBT device, I IGBT is the current carrying capacity of the IGBT device, I N is the line rated current, I cir2 is the maximum injected double frequency current, I dcN is the rated current of the DC side.

[0019] As an improvement to the above solution, the rated parameters further include: the phase difference between the grid voltage and the converter output voltage, and the rate of rise of the bridge arm current; the calculation of the MMC bridge arm inductance parameters includes:

[0020] Substitute the MMC DC side voltage, the phase difference between the grid voltage and the converter output voltage, and the bridge arm current rise rate into the bridge arm inductance parameter calculation formula to obtain the MMC bridge arm inductance parameters. The MMC bridge arm inductance parameters include: equivalent reactance value and bridge arm inductance value. The bridge arm inductance calculation formula meets the following conditions:

[0021]

[0022] Where S LN (pu) is the equivalent reactance value, δ is the phase difference between the grid voltage and the converter output voltage, L is the bridge arm inductance value, and α is the rising rate of the bridge arm current.

[0023] As an improvement to the above solution, the rated parameters also include: the maximum fluctuation rate of the MMC submodule capacitor voltage, the effective value of the AC port output current, the effective value of the AC port output voltage, and the MMC DC side current value; the calculation of the MMC submodule capacitance value includes:

[0024] Substitute the rated capacitor voltage of the MMC submodule, the maximum fluctuation rate of the capacitor voltage of the MMC submodule, the effective value of the AC port output current, the effective value of the AC port output voltage, the MMC DC side current value, the MMC DC side voltage value, and the effective value of the doubled frequency common mode voltage injected by the eACPFC submodule into the first capacitance value calculation formula to obtain the MMC submodule capacitance value. The first capacitance value calculation formula satisfies the following conditions:

[0025]

[0026] Where C MMC is the capacitance value of the MMC submodule, ω is the grid angular frequency, is the maximum amplitude of the frequency fluctuation component of the MMC submodule, I diff is the effective value of the AC port output current, is the maximum amplitude of the double frequency fluctuation component of the capacitor voltage of the MMC submodule, I dc is the MMC DC side current value, and is the maximum fluctuation rate of the MMC submodule capacitor voltage.

[0027] As an improvement to the above solution, the rated parameters further include: the rated value of the eACPFC phase voltage and the capacitor voltage of the eACPFC submodule; and the calculation of the number of the eACPFC submodules includes:

[0028] Substitute the eACPFC phase voltage rating and the eACPFC submodule capacitor voltage into the eACPFC submodule number calculation formula to obtain the eACPFC submodule number. The eACPFC submodule number calculation formula satisfies the following conditions:

[0029]

[0030] Where N eMMC is the number of eACPFC submodules, V p h ase is the eACPFC phase voltage rating, V c * is the capacitor voltage of the eACPFC submodule.

[0031] As an improvement to the above solution, the rated parameters also include: the maximum fluctuation rate of the submodule capacitor voltage, the effective value of the fundamental frequency differential mode output voltage of the eACPFC bridge arm, the number of AC lines connected to the eACPFC, and the effective value of the current of the kth line; the calculation of the eACPFC submodule capacitance value includes:

[0032] Substitute the maximum fluctuation rate of the submodule capacitor voltage, the effective value of the fundamental frequency differential mode output voltage of the eACPFC bridge arm, the MMC DC side current value, the number of AC lines connected to the eACPFC, the effective value of the current of the kth line, and the effective value of the doubled frequency common mode voltage injected by the eACPFC submodule into the second capacitance value calculation formula to obtain the eACPFC submodule capacitance value. The second capacitance value calculation formula satisfies the following conditions:

[0033]

[0034] Where C eMMC is the capacitance value of the eACPFC submodule, ε max is the maximum fluctuation rate of the submodule capacitor voltage, ω is the grid angular frequency, n eMMC is the number of submodules in a single bridge arm of eACPFC, V c_refis the rated capacitor voltage of the eACPFC submodule, V ck_diff is the effective value of the fundamental frequency differential mode output voltage of the eACPFC bridge arm, I dc is the DC side current value of MMC, n feeder is the number of AC lines connected to the eACPFC, I k is the effective value of the current in the kth line, V c_com It is the RMS value of the double frequency common mode voltage injected by the eACPFC submodule.

[0035] Accordingly, an embodiment of the present invention further provides a multi-AC port flexible DC converter main circuit, comprising: an MMC submodule and an eACPFC submodule, wherein the MMC submodule includes three MMC phase units, each MMC phase unit including an MMC upper bridge arm and an MMC lower bridge arm; the eACPFC submodule includes three eACPFC phase units, each eACPFC phase unit including an eACPFC bridge arm; wherein, in the same phase, an eACPFC bridge arm is connected to an MMC upper bridge arm and an MMC lower bridge arm respectively; the eACPFC bridge arms are connected in parallel with each other and connected to the AC line through a differential mode output port;

[0036] The design parameters of the main circuit of the multi-AC port flexible DC converter include: MMC submodule design parameters and eACPFC submodule design parameters; the design parameters of the main circuit of the multi-AC port flexible DC converter are determined by the design method of the main circuit of the multi-AC port flexible DC converter described in the present invention.

[0037] Correspondingly, an embodiment of the present invention also provides a computer terminal device, including a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor. When the processor executes the computer program, it implements a design method for the main circuit of a multi-AC port flexible DC converter as described in the present invention.

[0038] Correspondingly, an embodiment of the present invention further provides a computer-readable storage medium, which includes a stored computer program, wherein when the computer program is running, the device where the computer-readable storage medium is located is controlled to execute a design method for the main circuit of a multi-AC port flexible DC converter as described in the present invention.

[0039] As can be seen from the above, the present invention has the following beneficial effects:

[0040] The present invention provides a design method for a main circuit of a multi-AC port flexible DC converter. The method comprises: calculating MMC submodule design parameters based on rated parameters; wherein the MMC submodule design parameters include: an MMC DC side voltage value, the number of MMC submodules, an MMC bridge arm inductance parameter, and an MMC submodule capacitance value; calculating eACPFC submodule design parameters based on the rated parameters; wherein the eACPFC submodule design parameters include: the number of eACPFC submodules and an eACPFC submodule capacitance value; and constructing the main circuit of the multi-AC port flexible DC converter based on the MMC submodule design parameters and the eACPFC submodule design parameters. Based on rated parameters, the present invention calculates the MMC DC side voltage value, the number of MMC submodules, the MMC bridge arm inductance parameter, the MMC submodule capacitance value, the number of eACPFC submodules, and the eACPFC submodule capacitance value. Based on the calculation results, the MMC submodules and eACPFC submodules can be set, so that the main circuit of the multi-port flexible DC converter can operate based on preset parameters, thereby ensuring the performance of the multi-port flexible DC converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 This is a flow chart of a design method for a main circuit of a multi-AC port flexible DC converter provided by one embodiment of the present invention;

[0042] Figure 2 This is a structural diagram of a main circuit of a multi-AC port flexible DC converter provided by an embodiment of the present invention;

[0043] Figure 3 This is a schematic diagram of the structure of a terminal device provided by one embodiment of the present invention;

[0044] Figure 4 This is a schematic diagram of a system in which a multi-AC port flexible DC converter provided by an embodiment of the present invention is connected to three transmission lines;

[0045] Figure 5 1 is a capacitor voltage waveform of a multi-AC port flexible DC converter MMC submodule provided by an embodiment of the present invention;

[0046] Figure 6 1 is a capacitor voltage waveform of an eACPFC submodule of a multi-AC port flexible DC converter provided by an embodiment of the present invention;

[0047] Figure 7 This is the current waveform of line 1 connected to the multi-AC port flexible DC converter provided by one embodiment of the present invention;

[0048] Figure 8 This is the current waveform of line 2 connected to the multi-AC port flexible DC converter provided by one embodiment of the present invention;

[0049] Figure 9 This is the current waveform of line 3 connected to the multi-AC port flexible DC converter provided by one embodiment of the present invention;

[0050] Figure 10 The voltage and current waveforms of the DC line connected to the multi-AC port flexible DC converter provided by one embodiment of the present invention are as follows;

[0051] Figure 11 This is the active power waveform of each line connected to the multi-AC port flexible DC converter provided by one embodiment of the present invention;

[0052] Figure 12 This is the reactive power waveform of each line connected to the multi-AC port flexible DC converter provided by one embodiment of the present invention. DETAILED DESCRIPTION

[0053] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0054] Example 1

[0055] See also Figure 1 , Figure 1 FIG. 1 is a flow chart of a design method for a main circuit of a multi-AC port flexible DC converter provided by an embodiment of the present invention. Figure 1 As shown, this embodiment includes steps 101 to 104, and each step is specifically as follows:

[0056] Step 101: Obtain rated parameters of the main circuit of the multi-AC port flexible DC converter.

[0057] In this embodiment, according to the actual application scenario requirements, the effective value of the line voltage of the AC line is selected as V line , the feeder rated capacity is S N , then the effective value of the feeder rated current can be recorded as I N The rated capacity of MMC is the same as the rated capacity of the line, which is S N .

[0058] Step 102: Calculate MMC submodule design parameters based on the rated parameters. The MMC submodule design parameters include: MMC DC link voltage value, number of MMC submodules, MMC bridge arm inductance parameter, and MMC submodule capacitance value.

[0059] In this embodiment, the rated parameters include: the effective value of the AC port output voltage, the effective value of the double frequency common-mode voltage injected by the eACPFC submodule, and the effective value of the common-mode voltage required for the double frequency current injected by the MMC submodule; the calculation of the MMC DC side voltage value includes:

[0060] Substitute the effective value of the AC port output voltage, the effective value of the double-frequency common-mode voltage injected by the eACPFC submodule, and the effective value of the common-mode voltage required for the double-frequency current injected by the MMC submodule into the MMC DC side voltage calculation formula to obtain the MMC DC side voltage value. The MMC DC side voltage calculation formula satisfies the following conditions:

[0061]

[0062] Where V dc is the DC side voltage of MMC, V diff is the effective value of the AC port output voltage, V c_com V is the RMS value of the double frequency common mode voltage injected into the eACPFC submodule, cir2 The RMS common-mode voltage required to inject double-frequency current into the MMC submodule.

[0063] In a specific embodiment, in order to ensure that the MMC bridge arm outputs the required voltage, it is first necessary to analyze the components of the MMC bridge arm voltage. The components of the upper and lower bridge arm voltages of the MMC can be expressed as

[0064]

[0065] where v p and v n are the voltage modulation waves of the upper and lower bridge arms of MMC respectively; V dc is the DC side voltage; v diff is the AC port output voltage; v c_com The double frequency common mode voltage injected into the eACPFC part is compensated in the MMC bridge arm, thereby preventing the introduction of additional circulating current in the MMC; cir2 The common-mode voltage required to inject the doubled frequency current into the MMC is calculated. To ensure that the MMC bridge arm can modulate the above voltage components, the DC side voltage of the MMC is calculated as shown in the MMC DC side voltage calculation formula. Furthermore, the selection of the MMC DC side voltage also needs to consider the rated voltage of the DC transmission line.

[0066] In this embodiment, the rated parameters also include: the rated capacitance voltage of the MMC submodule, the withstand voltage of the IGBT device, the withstand current of the IGBT device, the line rated current, the maximum injected double frequency current, and the DC side rated current; the calculation of the number of the MMC submodules includes:

[0067] Substitute the MMC DC side voltage value, the MMC submodule rated capacitance voltage, the IGBT device withstand voltage, the IGBT device withstand current, the line rated current, the maximum injected double frequency current, and the DC side rated current into the submodule number calculation formula to obtain the number of MMC submodules; wherein, the submodule number calculation formula satisfies the following conditions:

[0068]

[0069] Where N MMC is the number of MMC submodules, V c * is the rated capacitance voltage of the MMC submodule, V IGBT is the withstand voltage of the IGBT device, I IGBT is the current carrying capacity of the IGBT device, I N is the line rated current, I cir2 is the maximum injected double frequency current, I dcN is the rated current of the DC side.

[0070] In a specific embodiment, the bridge arm of the MMC is a cascaded half-bridge structure. The number of sub-modules of the MMC directly depends on the selection of the sub-module capacitor voltage and the DC side voltage. In order to ensure the reliability of the device, a certain margin is usually retained when selecting the number of sub-modules. The DC side voltage of the MMC has been explained above, so the selection of the sub-module capacitor voltage is analyzed below. When the selected sub-module capacitor voltage value is too large, a switch device with a higher withstand voltage value is required, and the corresponding manufacturing cost will also increase sharply. In addition, the output voltage level of a smaller number of sub-modules is low, and the harmonic characteristics are poor. If the selected sub-module capacitor voltage value is too low, although the manufacturing cost of the required switch device is greatly reduced, too many sub-modules make the control complex and the overall volume of the device larger. Therefore, when selecting the number of sub-modules and the sub-module capacitor voltage value, it is also necessary to comprehensively consider performance, reliability, cost and technical feasibility in combination with the actual application scenario. According to the selected sub-module capacitor voltage value, in order to ensure that the switch device meets its withstand voltage V IGBT He Nai Liu I IGBT According to engineering experience, the operating voltage of IGBT is generally in the range of 50% to 70% of its rated voltage. When the operating voltage is too high, the device loss will increase greatly, and when the operating voltage is too low, the device utilization rate will be insufficient. Therefore, 0.5·V IGBT ≤V c * ≤0.7·V IGBT , where V c * is the rated capacitance voltage of the MMC submodule.

[0071] In summary, the number of sub-modules N of MMCMMC Need to meet:

[0072]

[0073] The operating current of the switching device in the MMC is consistent with the bridge arm current, which mainly includes the fundamental frequency, double frequency AC and DC. Therefore,

[0074] It should be noted that according to 0.5·V IGBT ≤V c * ≤0.7·V IGBT as well as After selecting a switching device that meets the requirements, select the appropriate switching frequency according to the device's product data sheet, denoted as f PFCM .

[0075] In this embodiment, the rated parameters further include: a phase difference between the grid voltage and the converter output voltage, and a rise rate of the bridge arm current; and the calculation of the MMC bridge arm inductance parameter includes:

[0076] Substitute the MMC DC side voltage, the phase difference between the grid voltage and the converter output voltage, and the bridge arm current rise rate into the bridge arm inductance parameter calculation formula to obtain the MMC bridge arm inductance parameters. The MMC bridge arm inductance parameters include: equivalent reactance value and bridge arm inductance value. The bridge arm inductance calculation formula meets the following conditions:

[0077]

[0078] In a specific embodiment, the bridge arm inductor serves as a link for power transmission between the MMC and the AC power grid, and plays a role in regulating power transmission and suppressing output current fluctuations. The bridge arm inductor constitutes a part of the internal circulation loop of the MMC and has a good suppressing effect on the internal circulation. In the present invention, the MMC needs to actively inject a double frequency circulating current, and the double frequency circulating current component has been controlled, so it is sufficient to consider the suppressing effect on the high frequency component. And because the high frequency component in the MMC's circulating current is very small, the circulating current suppression constraint is not considered in the present invention. The design of the bridge arm inductor is of great significance to the stable operation of the MMC part and the performance of the system. Therefore, the parameter design needs to be comprehensively considered from multiple aspects such as meeting the system power operation requirements and fault current suppression.

[0079] First, the bridge arm inductance of the MMC should meet the system power operation requirements. Assuming the capacity of the converter is S N , based on this capacity, the per-unit value of the equivalent reactance is:

[0080]

[0081] The capacity of the equivalent reactance is related to the phase difference δ between the grid voltage and the converter output voltage. In steady state, δ is generally between 6 and 19. The capacity of the equivalent reactance S LN (pu) takes 0.1 to 0.3.

[0082] In addition, the MMC's bridge arm inductance should also be able to limit the inrush current in the event of a system fault. Considering the most severe DC side positive and negative busbar short-circuit fault, the short-circuit current can form a loop through the same phase unit. Based on Kirchhoff's voltage theorem, the bridge arm current rise rate can be obtained as:

[0083]

[0084] In the case of limiting the bridge arm current rise rate a under transient conditions, the bridge arm inductance can be selected as:

[0085]

[0086] Therefore, the selection of the MMC bridge arm inductance should be based on the above three principles, meeting the system power operation requirements, circulating current suppression, and fault current suppression principles. On this basis, the larger one should be selected and back-checked.

[0087] In this embodiment, the rated parameters further include: the maximum fluctuation rate of the MMC submodule capacitor voltage, the effective value of the AC port output current, the effective value of the AC port output voltage, and the MMC DC side current value; the calculation of the MMC submodule capacitance value includes:

[0088] Substitute the rated capacitor voltage of the MMC submodule, the maximum fluctuation rate of the capacitor voltage of the MMC submodule, the effective value of the AC port output current, the effective value of the AC port output voltage, the MMC DC side current value, the MMC DC side voltage value, and the effective value of the doubled frequency common mode voltage injected by the eACPFC submodule into the first capacitance value calculation formula to obtain the MMC submodule capacitance value. The first capacitance value calculation formula satisfies the following conditions:

[0089]

[0090] Where C MMC is the capacitance value of the MMC submodule, ω is the grid angular frequency, is the maximum amplitude of the frequency fluctuation component of the MMC submodule, I diff is the effective value of the AC port output current, is the maximum amplitude of the double frequency fluctuation component of the capacitor voltage of the MMC submodule, I dc is the MMC DC side current value, and is the maximum fluctuation rate of the MMC submodule capacitor voltage.

[0091] In a specific embodiment, the design concept of the capacitance value of the MMC submodule is mainly based on the capacitor's suppression effect on DC voltage ripple, and its capacitor voltage fluctuation is constrained within a specified range to limit the voltage stress of the device.

[0092] The bridge arm voltage of MMC is shown as the components of the upper and lower bridge arm voltages of MMC. Since the common mode voltage v required for MMC to inject the double frequency current is cir2 The value is very small, so it is ignored in subsequent analysis. The MMC bridge arm current can be expressed as:

[0093]

[0094] Among them I dc is the DC side current, i diff is the MMC differential mode output current, i cir2 It is the MMC double frequency circulating current.

[0095] Among them, the instantaneous expression of the fundamental frequency component is:

[0096]

[0097] Among them, V diff and δ diff is the effective value and phase of the AC port output voltage, I diff is and ρ diff AC port output current RMS value and phase. In order to fully utilize the modulation index of the MMC bridge arm, the phase of the injected double frequency voltage and the phase of the fundamental frequency differential mode voltage satisfy the relationship:

[0098] δ c_com =2*δ diff

[0099] Among them, δ c_com is the phase of the double frequency voltage injected into the eACPFC. To ensure maximum power interaction, the double frequency current injected by the MMC is reduced, so the phase of the double frequency current injected by the MMC is consistent with it. Therefore, the instantaneous expression of the double frequency component is:

[0100]

[0101] Among them, V c_com is the effective value of the double frequency common mode voltage injected into the eACPFC part, i cir2 is the injected double frequency circulating current. The current flowing into the MMC submodule capacitor can be expressed as follows:

[0102]

[0103] Due to power balance constraints, the DC component of the submodule capacitor current is 0. Taking the upper bridge arm as an example, the fluctuation components are expressed according to frequency. The fundamental frequency fluctuation expression is:

[0104]

[0105] The double frequency fluctuation expression is:

[0106]

[0107] In addition, the capacitor voltage fluctuation also includes triple and quadruple frequency fluctuation components. Since the triple and quadruple frequency fluctuation components have little impact on the overall capacitor voltage fluctuation, they are ignored when designing the capacitor value.

[0108] For the fundamental frequency fluctuation component of the capacitor voltage, when the preset constraint relationship shown in the following equation is satisfied:

[0109]

[0110] The maximum amplitude of the fundamental frequency fluctuation component is:

[0111]

[0112] Similarly, under the constraints of the preset constraints, ignoring the elements with less influence, the maximum amplitude of the capacitor voltage double frequency fluctuation component is:

[0113]

[0114] Given that the maximum fluctuation rate of the submodule capacitor voltage is ε, the MMC submodule capacitor value satisfies the first capacitance value calculation formula:

[0115] When the capacitance value satisfies the first capacitance value calculation formula, the voltage fluctuation of the MMC submodule will be maintained within a required range.

[0116] Step 103: Calculate eACPFC submodule design parameters based on the rated parameters; wherein the eACPFC submodule design parameters include: the number of eACPFC submodules and the capacitance value of the eACPFC submodules.

[0117] In this embodiment, the rated parameters further include: the rated value of the eACPFC phase voltage and the capacitor voltage of the eACPFC submodule; and the calculation of the number of eACPFC submodules includes:

[0118] Substitute the eACPFC phase voltage rating and the eACPFC submodule capacitor voltage into the eACPFC submodule number calculation formula to obtain the eACPFC submodule number. The eACPFC submodule number calculation formula satisfies the following conditions:

[0119]

[0120] Where N eMMC is the number of eACPFC submodules, V p h ase is the eACPFC phase voltage rating, V c * is the capacitor voltage of the eACPFC submodule.

[0121] In a specific embodiment, since the bridge arm of the eACPFC does not need to output a DC voltage, the submodules of the eACPFC adopt a full-bridge structure. The number of submodules in the eACPFC is directly determined by the selected submodule capacitor voltage and the bridge arm output voltage amplitude. To ensure the reliability of the device, a certain margin is usually retained when selecting the number of submodules. Since the bridge arm current rating of the eACPFC is the same as the bridge arm current rating of the MMC, the submodule capacitor voltage of the eACPFC is set to be the same as the submodule capacitor voltage of the MMC, which is V c * At this point, the switching device parameters of the eACPFC are the same as those of the MMC, ensuring the consistency of the device and facilitating early manufacturing and later maintenance. Next, the bridge arm output voltage amplitude is analyzed. The eACPFC bridge arm voltage includes the following components:

[0122]

[0123] Among them, v cpk and v cnk is the output voltage of the upper and lower bridge arms of the kth cluster of eACPFC; v ck_diff is the fundamental frequency differential mode output voltage of the eACPFC bridge arm, and is the equivalent series voltage between lines, used to regulate the power between lines; v c_com Δv is the double frequency common mode voltage actively injected by eACPFC; ck_com The common mode voltage required to inject the double frequency circulating current between clusters into the kth cluster of eACPFC. In the voltage component of the bridge arm of eACPFC, Δv ck_com It is relatively small compared to the other two components and therefore ignored in subsequent analyses.

[0124] Since the eACPFC regulates the line power flow by controlling the differential mode output voltage, its regulation principle is similar to that of the series part of the traditional UPFC. The AC component amplitude of the series voltage is selected to be 0.1 times the phase voltage amplitude, that is:

[0125]

[0126] Where V ck_diff_max is the maximum differential mode output voltage of the eACPFC bridge arm, Vphase Considering that the maximum power interaction between eACPFC and feeder is 0.1pu, the double frequency common mode voltage actively injected by eACPFC is set to 0.2pu. At this time, the maximum value of the double frequency circulating current to be injected is 0.25pu.

[0127] Based on the sub-module capacitor voltages selected above and the analysis of the bridge arm output voltage amplitude, the number of eACPFC sub-modules meets the eACPFC sub-module number calculation formula.

[0128] In this embodiment, the rated parameters also include: the maximum fluctuation rate of the submodule capacitor voltage, the effective value of the fundamental frequency differential mode output voltage of the eACPFC bridge arm, the number of AC lines connected to the eACPFC, and the effective value of the current of the kth line. The calculation of the eACPFC submodule capacitance value includes:

[0129] Substitute the maximum fluctuation rate of the submodule capacitor voltage, the effective value of the fundamental frequency differential mode output voltage of the eACPFC bridge arm, the MMC DC side current value, the number of AC lines connected to the eACPFC, the effective value of the current of the kth line, and the effective value of the doubled frequency common mode voltage injected by the eACPFC submodule into the second capacitance value calculation formula to obtain the eACPFC submodule capacitance value. The second capacitance value calculation formula satisfies the following conditions:

[0130]

[0131] Where C eMMC is the capacitance value of the eACPFC submodule, ε max is the maximum fluctuation rate of the submodule capacitor voltage, ω is the grid angular frequency, n eMMC is the number of submodules in a single bridge arm of eACPFC, V c_ref is the rated capacitor voltage of the eACPFC submodule, V ck_diff is the effective value of the fundamental frequency differential mode output voltage of the eACPFC bridge arm, I dc is the DC side current value of MMC, n feeder is the number of AC lines connected to the eACPFC, I k is the effective value of the current in the kth line, V c_com It is the RMS value of the double frequency common mode voltage injected by the eACPFC submodule.

[0132] In a specific embodiment, the design concept of the capacitance value of the eACPFC submodule is mainly based on the capacitor's suppression effect on voltage ripple, and its capacitor voltage fluctuation is constrained within a specified range to limit the voltage stress of the device.

[0133] It is known that the eACPFC bridge arm voltage contains a component, due to the common mode voltage Δv required to inject the double frequency circulating current between clusters ck_com The value is very small, so it is ignored in the subsequent analysis. The bridge arm current of eACPFC can be expressed as:

[0134]

[0135] Among them, i pk and i nk is the upper and lower arm current of the kth cluster of eACPFC, I dc is the DC side current, n feeder is the number of AC lines connected to the eACPFC, i k is the current of the kth circuit, i cir2k is the double frequency circulating current of the kth bridge arm of the eACPFC. The instantaneous expressions of the feeder current, the differential mode output voltage of the eACPFC bridge arm and the double frequency circulating current of the kth bridge arm are:

[0136]

[0137] Where V ck_diff and δ ck_diff is the effective value and phase of the fundamental frequency differential mode output voltage of the eACPFC bridge arm, I k and ρ k is the effective value and phase of the kth line current, I cir2k is the effective value of the double frequency circulating current of the k-th bridge arm of the eACPFC. Combining the eACPFC bridge arm voltage component, the number of eACPFC sub-modules, the eACPFC bridge arm differential mode output voltage, and the instantaneous value expression of the double frequency circulating current of the k-th bridge arm, the current flowing into the eACPFC sub-module capacitor can be expressed as:

[0138]

[0139] Due to power balance constraints, the DC component of the submodule capacitor current is 0. Taking the upper bridge arm of the kth cluster of eACPFC as an example, the fluctuation components are expressed according to frequency. Among them, the fundamental frequency fluctuation can be expressed as:

[0140]

[0141] The double frequency fluctuation can be expressed as:

[0142]

[0143] The triple frequency fluctuation can be expressed as:

[0144]

[0145] In addition, there is a quadruple frequency fluctuation component in the capacitor voltage fluctuation. However, since the quadruple frequency fluctuation component has a small impact on the overall capacitor voltage fluctuation, it is ignored when designing the capacitor value.

[0146] Further ignoring the less influential terms in the fundamental frequency and double frequency fluctuations, when the relationship shown in the following formula is satisfied, the fundamental frequency fluctuation and double frequency fluctuation reach their maximum values.

[0147]

[0148] Combining the fundamental frequency fluctuation, the second frequency fluctuation, the third frequency fluctuation, and the relationship between the fundamental frequency fluctuation and the second frequency fluctuation to achieve the maximum value, given that the maximum fluctuation rate of the submodule capacitor voltage is ε, the eACPFC submodule capacitor value satisfies the second capacitance value calculation formula; when the capacitor value satisfies the second capacitance value calculation formula, the eACPFC submodule voltage fluctuation will be maintained within the required range.

[0149] Step 104: constructing the multi-AC port flexible DC converter main circuit according to the MMC submodule design parameters and the eACPFC submodule design parameters.

[0150] In a specific embodiment, see Figure 4 This embodiment provides a design method for the main circuit of a multi-AC port flexible DC converter. In this embodiment, the multi-AC port flexible DC converter is connected to three 220kV transmission lines. The specific method is as follows:

[0151] Considering the rated voltage of general DC transmission lines, the DC side voltage of the MMC is selected to be 500kV. Next is the design of the number of MMC submodules. When selecting the number of submodules and the submodule capacitor voltage value, it is also necessary to consider the performance, reliability, cost and technical feasibility in combination with the actual application scenario. Based on the selected submodule capacitor voltage value, in order to ensure that the switching device meets its withstand voltage V IGBT He Nai Liu I IGBTAccording to engineering experience, the operating voltage of IGBT is generally in the range of 50% to 70% of its rated voltage. When the operating voltage is too high, the device loss will increase greatly, and when the operating voltage is too low, the device utilization rate is insufficient. Taking all factors into consideration, the submodule capacitor voltage is selected to be 2300V and the number of submodules is 220. Then comes the design of the MMC bridge arm inductor. The bridge arm inductor serves as the link for power transmission between the MMC and the power grid, and plays a role in regulating power transmission and suppressing output current fluctuations. At the same time, the bridge arm inductor, as part of the internal circulation loop of the MMC, has a significant effect on suppressing internal circulation. Therefore, the design of the bridge arm inductor is of great significance to the stable operation of the MMC part and the performance of the system. In the present invention, the MMC needs to actively inject a double frequency circulating current, and the double frequency circulating current component has been controlled, so it is sufficient to consider the suppression effect of the high frequency component. And because the high frequency component in the circulating current of the MMC is very small, the circulation suppression constraint is not considered in the present invention. When designing the bridge arm inductance, parameters must be comprehensively considered, taking into account system power requirements and fault current suppression. Considering system power requirements, the equivalent inductance is set at 0.1 to 0.3 pu, and the line equivalent filter inductance is set at 0.1 pu. Therefore, the bridge arm inductance should be less than 30 mH. Considering that the fault current rise rate is no greater than 0.1 kA / μs, the inductance should be greater than 2.5 mF, and the final bridge arm inductance is 7 mH. Finally, the capacitance of the MMC submodule capacitor is designed. The design of the submodule capacitor takes into account the suppression of the fundamental and double frequency ripple by the capacitor voltage. When suppressing the ripple to less than 10%, the capacitance of the submodule capacitor can be designed to be 22 mF.

[0152] Regarding the eACPFC parameter design, the first step is to determine the number of eACPFC submodules. Since the bridge arm current rating of the eACPFC is the same as that of the MMC, the capacitor voltage of the eACPFC submodule is set to the same value as that of the MMC, 2300V. At this point, the switching device parameters of the eACPFC and MMC are also the same, ensuring device consistency and facilitating both initial manufacturing and subsequent maintenance. The number of submodules is 28. Next, the capacitance of the eACPFC submodule capacitors is designed. The design of the submodule capacitors takes into account the suppression of fundamental, double, and triple frequency ripple by the capacitor voltage. To suppress ripple to within 10%, the capacitance of the submodule capacitors can be set to 22mF. The following further illustrates the application of the above structure and method using a specific simulation example. In conjunction with the above embodiment, the system was simulated and verified using MATLAB / Simulink software. The power flows of each line in the simulation are shown in Table 1, and the simulation parameters are shown in Table 2.

[0153] Table 1

[0154]

[0155] Table 2

[0156]

[0157] Figure 5 is the capacitor voltage of the sub-module of the MMC part, and its capacitor voltage fluctuation is about 9.91%, which meets the design requirements. Figure 6 is the capacitor voltage of the eACPFC submodule. Its maximum capacitor voltage fluctuation is about 4.2%, which is 10% less than the design value. This is because the actual injected double frequency circulating current is smaller than the estimated value, and the superposition of capacitor voltage fluctuation amplitudes of different frequencies is considered when designing the capacitor value. However, this constraint relationship may not be met in actual operation. Therefore, it also meets the design requirements. Figures 7 to 9 They are the current waveforms of each AC line respectively. Figure 10 is the DC line voltage and current waveform. Figure 11 and Figure 12 The waveforms of active power and reactive power of each line are shown respectively. The voltage and current waveforms of each line meet the requirements.

[0158] The simulation results show that the proposed main loop parameter design method based on MTUPFC can achieve efficient and stable operation of the device, and the effectiveness of the proposed design method is verified in the results.

[0159] Accordingly, see Figure 2 , Figure 2 An embodiment of the present invention further provides a structural schematic diagram of a main circuit of a multi-AC port flexible DC converter, comprising: an MMC submodule 201 and an eACPFC submodule 202, wherein the MMC submodule comprises three MMC phase units, each MMC phase unit comprising an MMC upper bridge arm 2011 and an MMC lower bridge arm 2012; the eACPFC submodule comprises three eACPFC phase units, each eACPFC phase unit comprising an eACPFC bridge arm; wherein, in the same phase, an eACPFC bridge arm is connected to an MMC upper bridge arm and an MMC lower bridge arm respectively; the eACPFC bridge arms are connected in parallel with each other and connected to the AC line through a differential mode output port;

[0160] The design parameters of the main circuit of the multi-AC port flexible DC converter include: MMC submodule design parameters and eACPFC submodule design parameters; the design parameters of the main circuit of the multi-AC port flexible DC converter are determined by the design method of the main circuit of the multi-AC port flexible DC converter described in the present invention.

[0161] This embodiment calculates MMC submodule design parameters based on the rated parameters; wherein the MMC submodule design parameters include: MMC DC side voltage value, number of MMC submodules, MMC bridge arm inductance parameter, and MMC submodule capacitance value; calculates eACPFC submodule design parameters based on the rated parameters; wherein the eACPFC submodule design parameters include: number of eACPFC submodules and eACPFC submodule capacitance value; and constructs the multi-AC port flexible DC converter main circuit based on the MMC submodule design parameters and the eACPFC submodule design parameters. The present invention calculates the MMC DC side voltage value, the number of MMC submodules, the MMC bridge arm inductance parameter, the MMC submodule capacitance value, the number of eACPFC submodules, and the eACPFC submodule capacitance value based on the rated parameters, and can configure the MMC submodules and eACPFC submodules based on the calculation results, so that the multi-AC port flexible DC converter main circuit can operate based on preset parameters, thereby ensuring the performance of the multi-port flexible DC converter.

[0162] Example 2

[0163] See also Figure 3 , Figure 3 It is a schematic diagram of the terminal device structure provided by one embodiment of the present invention.

[0164] A terminal device of this embodiment includes: a processor 301, a memory 302, and a computer program stored in the memory 302 and executable on the processor 301. When the processor 301 executes the computer program, the steps of the above-mentioned method for designing the main circuit of a multi-AC port flexible DC converter in the embodiment are implemented, for example: Figure 1 Alternatively, when the processor executes the computer program, the functions of the modules in the above-mentioned device embodiments are realized, for example: Figure 2 All modules of the design device of the main circuit of the multi-AC port flexible DC converter are shown.

[0165] In addition, an embodiment of the present invention also provides a computer-readable storage medium, which includes a stored computer program, wherein when the computer program is running, the device where the computer-readable storage medium is located is controlled to execute the design method of the main circuit of the multi-AC port flexible DC converter as described in any of the above embodiments.

[0166] It should be noted that the device embodiments described above are merely illustrative, wherein the units described as separate components may or may not be physically separated, and the components displayed as units may or may not be physical units, that is, they may be located in one place, or they may be distributed across multiple network units. Some or all of the modules may be selected according to actual needs to achieve the purpose of the present embodiment. In addition, in the drawings of the device embodiments provided by the present invention, the connection relationship between the modules indicates that there is a communication connection between them, which may be specifically implemented as one or more communication buses or signal lines. A person of ordinary skill in the art can understand and implement the present invention without inventive effort.

[0167] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A design method for a main circuit of a multi-AC port flexible DC converter, characterized in that: include: Obtain the rated parameters of the main circuit of the multi-AC port flexible DC converter; Based on the rated parameters, the MMC submodule design parameters are calculated; wherein the MMC submodule design parameters include: the MMC DC side voltage value, the number of MMC submodules, the MMC bridge arm inductance parameter, and the MMC submodule capacitance value; the rated parameters include: the effective value of the AC port output voltage, the effective value of the double frequency common mode voltage injected by the eACPFC submodule, and the effective value of the common mode voltage required for the MMC submodule to inject the double frequency current; it is characterized in that the calculation of the MMC DC side voltage value includes: substituting the effective value of the AC port output voltage, the effective value of the double frequency common mode voltage injected by the eACPFC submodule, and the effective value of the common mode voltage required for the MMC submodule to inject the double frequency current into the MMC DC side voltage calculation formula to obtain the MMC DC side voltage value; wherein the MMC DC side voltage calculation formula satisfies the following conditions: Where V dc is the DC side voltage of MMC, V diff is the effective value of the AC port output voltage, V c_com V is the RMS value of the double frequency common mode voltage injected into the eACPFC submodule, cir2 The RMS common-mode voltage required to inject double-frequency current into the MMC submodule; Calculating eACPFC submodule design parameters based on the rated parameters; wherein the eACPFC submodule design parameters include: the number of eACPFC submodules and the capacitance value of the eACPFC submodules; The multi-AC port flexible DC converter main circuit is constructed according to the MMC submodule design parameters and the eACPFC submodule design parameters.

2. The design method of the main circuit of a multi-AC port flexible DC converter according to claim 1, characterized in that: The rated parameters also include: the rated capacitance voltage of the MMC submodule, the withstand voltage of the IGBT device, the withstand current of the IGBT device, the line rated current, the maximum injected double frequency current, and the DC side rated current; the calculation of the number of the MMC submodules includes: Substitute the MMC DC side voltage value, the MMC submodule rated capacitance voltage, the IGBT device withstand voltage, the IGBT device withstand current, the line rated current, the maximum injected double frequency current, and the DC side rated current into the submodule number calculation formula to obtain the number of MMC submodules; wherein, the submodule number calculation formula satisfies the following conditions: Where N MMC is the number of MMC submodules, V c * is the rated capacitance voltage of the MMC submodule, V IGBT is the withstand voltage of the IGBT device, I IGBT is the current carrying capacity of the IGBT device, I N is the line rated current, I cir2 is the maximum injected double frequency current, I dcN is the rated current of the DC side.

3. The design method of the main circuit of a multi-AC port flexible DC converter according to claim 2, characterized in that: The rated parameters also include: the phase difference between the grid voltage and the converter output voltage, and the rate of rise of the bridge arm current; the calculation of the MMC bridge arm inductance parameters includes: Substitute the MMC DC side voltage, the phase difference between the grid voltage and the converter output voltage, and the bridge arm current rise rate into the bridge arm inductance parameter calculation formula to obtain the MMC bridge arm inductance parameters. The MMC bridge arm inductance parameters include: equivalent reactance value and bridge arm inductance value. The bridge arm inductance calculation formula meets the following conditions: Where S LN (pu) is the equivalent reactance value, δ is the phase difference between the grid voltage and the converter output voltage, L is the bridge arm inductance value, and α is the rising rate of the bridge arm current.

4. The design method of the main circuit of a multi-AC port flexible DC converter according to claim 3, characterized in that: The rated parameters also include: the maximum fluctuation rate of the capacitor voltage of the MMC submodule, the effective value of the AC port output current, the effective value of the AC port output voltage, and the MMC DC side current value; the calculation of the MMC submodule capacitance value includes: Substitute the rated capacitor voltage of the MMC submodule, the maximum fluctuation rate of the capacitor voltage of the MMC submodule, the effective value of the AC port output current, the effective value of the AC port output voltage, the MMC DC side current value, the MMC DC side voltage value, and the effective value of the doubled frequency common mode voltage injected by the eACPFC submodule into the first capacitance value calculation formula to obtain the MMC submodule capacitance value. The first capacitance value calculation formula satisfies the following conditions: Where C MMC is the capacitance value of the MMC submodule, ω is the grid angular frequency, is the maximum amplitude of the frequency fluctuation component of the MMC submodule, I diff is the effective value of the AC port output current, is the maximum amplitude of the double frequency fluctuation component of the capacitor voltage of the MMC submodule, I dc is the MMC DC side current value, and is the maximum fluctuation rate of the MMC submodule capacitor voltage.

5. The design method of the main circuit of a multi-AC port flexible DC converter according to claim 1, characterized in that: The rated parameters also include: the rated value of the eACPFC phase voltage and the capacitor voltage of the eACPFC submodule; the calculation of the number of the eACPFC submodules includes: Substitute the eACPFC phase voltage rating and the eACPFC submodule capacitor voltage into the eACPFC submodule number calculation formula to obtain the eACPFC submodule number. The eACPFC submodule number calculation formula satisfies the following conditions: Where N eMMC is the number of eACPFC submodules, V phase is the eACPFC phase voltage rating, V c * is the capacitor voltage of the eACPFC submodule.

6. The design method for the main circuit of a multi-AC port flexible DC converter according to claim 5, characterized in that: The rated parameters also include: the maximum fluctuation rate of the submodule capacitor voltage, the effective value of the fundamental frequency differential mode output voltage of the eACPFC bridge arm, the number of AC lines connected to the eACPFC, and the effective value of the current of the kth line; the calculation of the eACPFC submodule capacitance value includes: Substitute the maximum fluctuation rate of the submodule capacitor voltage, the effective value of the fundamental frequency differential mode output voltage of the eACPFC bridge arm, the MMC DC side current value, the number of AC lines connected to the eACPFC, the effective value of the current of the kth line, and the effective value of the doubled frequency common mode voltage injected by the eACPFC submodule into the second capacitance value calculation formula to obtain the eACPFC submodule capacitance value. The second capacitance value calculation formula satisfies the following conditions: Where C eMMC is the capacitance value of the eACPFC submodule, ε max is the maximum fluctuation rate of the submodule capacitor voltage, ω is the grid angular frequency, n eMMC is the number of submodules in a single bridge arm of eACPFC, V c_ref is the rated capacitor voltage of the eACPFC submodule, V ck_diff is the effective value of the fundamental frequency differential mode output voltage of the eACPFC bridge arm, I dc is the DC side current value of MMC, n feeder is the number of AC lines connected to the eACPFC, I k is the effective value of the current in the kth line, V c_com It is the RMS value of the double frequency common mode voltage injected by the eACPFC submodule.

7. A computer terminal device, characterized in that: The method comprises a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor, wherein when the processor executes the computer program, the method for designing a main circuit of a multi-AC port flexible DC converter according to any one of claims 1 to 6 is implemented.

8. A computer-readable storage medium, characterized in that The computer-readable storage medium includes a stored computer program, wherein when the computer program is running, the device where the computer-readable storage medium is located is controlled to execute the design method for the main circuit of a multi-AC port flexible DC converter according to any one of claims 1 to 6.

9. A multi-AC port flexible DC converter main circuit, characterized in that: include: An MMC submodule and an eACPFC submodule, wherein the MMC submodule includes three MMC phase units, each MMC phase unit including an MMC upper bridge arm and an MMC lower bridge arm; the eACPFC submodule includes three eACPFC phase units, each eACPFC phase unit including an eACPFC bridge arm; wherein, in the same phase, an eACPFC bridge arm is respectively connected to an MMC upper bridge arm and an MMC lower bridge arm; the eACPFC bridge arms are connected in parallel and connected to the AC line through a differential mode output port; The design parameters of the main circuit of the multi-AC port flexible DC converter include: MMC submodule design parameters and eACPFC submodule design parameters; the design parameters of the main circuit of the multi-AC port flexible DC converter are determined by the design method of the main circuit of the multi-AC port flexible DC converter according to any one of claims 1 to 6.

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