MMC-CLCC novel DC power transmission system structure and control method

Abstract

The invention provides an MMC-CLCC novel DC power transmission system structure and a control method, and relates to the technical field of high-voltage DC power transmission. The method comprises the following steps: establishing an MMC-CLCC hybrid DC power transmission system topology architecture; the method comprises the following steps: determining a full-bridge and half-bridge sub-module proportion of an MMC in an MMC-CLCC hybrid direct-current power transmission system topology architecture by considering a direct-current fault ride-through demand and a possible power flow reversal demand; determining a control strategy of the MMC-CLCC hybrid direct-current power transmission system based on the topological characteristics of the topological architecture of the MMC-CLCC hybrid direct-current power transmission system; and determining an on-line power flow reversal control strategy of the MMC-CLCC hybrid direct-current power transmission system according to the actual engineering capacity power flow reversal demand. The method provided by the invention solves the problem that the applicability of large-scale new energy delivery and a scene with multiple direct current drop points in a load center area is insufficient.

Description

Technical Field

[0001] The present application relates to the technical field of high-voltage direct current transmission, and in particular to a novel MMC-CLCC direct current transmission system structure and control method. Background Art

[0002] In view of the large-scale transmission of renewable energy and the scenario of multiple DC points in the load center area, a large number of literatures have been published on the topology of DC transmission systems. The high-voltage DC transmission technology based on line-commutated converter (LCC) has a significant advantage in long-distance and large-capacity transmission scenarios due to its mature technology and good economy. However, since thyristors are semi-controlled devices, they need to rely on the connected AC power grid to provide commutation voltage for them, and cannot provide voltage support for the transmission of renewable energy. In addition, the failure of the AC system at the receiving end may cause multiple LCCs to fail to commutate, causing drastic changes in transmission power, posing a threat to the safe operation of the power grid at the transmitting and receiving ends. The high-voltage DC transmission technology based on modular multilevel converter (MMC) has the ability to independently regulate active and reactive power, can provide voltage support for renewable energy, and has significant technical advantages in the scenario of renewable energy transmission. However, compared with LCC-HVDC, MMC-HVDC uses a large number of fully controlled insulated-gate bipolar transistors (IGBT), which leads to higher cost, greater loss and insufficient economy. At present, existing literature has proposed a controllable line-commutated converter (CLCC) topology based on a hybrid of fully-controlled devices IGBT and semi-controlled devices thyristor. CLCC not only has a controllable shutdown capability to avoid commutation failure, but also inherits the advantages of LCC such as large transmission capacity, low cost and low loss.

[0003] In actual projects, in order to adapt to different application scenarios and give full play to the technical advantages of different DC transmission, hybrid DC transmission topology solutions have strong development potential. Research on hybrid DC transmission topology has achieved certain results, but the existing research is not applicable to scenarios of large-scale new energy transmission and multiple DC points in load center areas. In addition, the current flow reversal of the existing hybrid DC system involving LCC is centered on the voltage polarity reversal after the DC locking mode. When there is a demand for flow reversal in the hybrid DC transmission system, online flow reversal cannot be achieved. For this reason, it is necessary to propose a new MMC-CLCC DC transmission system structure and control method, which is applied to the scenarios of large-scale new energy transmission via DC and multiple DC points in load center areas. At the same time, considering the demand for flow reversal, an online flow reversal control strategy is proposed. Summary of the invention

[0004] The present application provides a new MMC-CLCC DC transmission system structure and control method, which solves the technical problem of insufficient applicability in scenarios of large-scale new energy transmission and multiple DC points in load center areas, and at the same time considers the demand for power flow reversal, and proposes an online power flow reversal control strategy. The present application considers the demand for DC fault ride-through and the possible demand for power flow reversal, and designs a method for selecting the proportion of MMC full half-bridge submodules; based on the topological characteristics of the MMC-CLCC hybrid DC transmission system, a control strategy for the MMC-CLCC hybrid DC transmission system is proposed; and an online power flow reversal control strategy for the MMC-CLCC hybrid DC transmission system is proposed.

[0005] In a first aspect, the present application provides a novel MMC-CLCC DC transmission system structure and control method, including:

[0006] Establish the MMC-CLCC hybrid DC transmission system topology architecture;

[0007] Considering the DC fault ride-through requirements and the possible power flow reversal requirements, determine the proportion of the MMC full half-bridge submodules in the MMC-CLCC hybrid DC transmission system topology architecture;

[0008] Determine a control strategy of the MMC-CLCC hybrid DC transmission system based on the topological characteristics of the MMC-CLCC hybrid DC transmission system topology architecture;

[0009] According to the actual project capacity flow reversal requirements, the online flow reversal control strategy of the MMC-CLCC hybrid DC transmission system is determined.

[0010] In one possible design, the MMC-CLCC hybrid DC transmission system topology architecture includes a rectifier side and an inverter side; wherein the rectifier side adopts a full half-bridge hybrid MMC, and the inverter side adopts a dual 12-pulse CLCC.

[0011] In a possible design, considering the DC fault ride-through requirement and the possible power flow reversal requirement, the proportion of the full half-bridge submodule of the MMC in the MMC-CLCC hybrid DC transmission system topology is determined, including:

[0012] For full half-bridge hybrid MMC, define m dc is the DC modulation ratio, m ac is the AC modulation ratio, then the expression for determining the full half-bridge hybrid MMC is:

[0013]

[0014] Where U dc is the DC voltage on the rectifier side during actual operation, UdcN is the rated DC voltage of MMC, U sm is the peak value of the AC voltage;

[0015] The output voltage of the upper and lower bridge arms of the full half-bridge hybrid MMC in steady state is calculated by the following formula:

[0016]

[0017] In the formula, u pj 、u nj is the output voltage of the upper and lower bridge arms of phase j, u sj represents the output voltage of the j-phase AC side, a, b, c are the three phases in the three-phase AC system, i.e., phase a, phase b, phase c;

[0018] According to equations (1) and (2), the value range of the j-phase bridge arm voltage is determined as:

[0019]

[0020] After a bipolar short-circuit fault occurs on the DC side, if the valve group is to meet the operation requirements of DC fault ride-through without blocking, the number M of full-bridge submodules should satisfy the following relationship:

[0021]

[0022] Where U c Represents the submodule capacitor voltage;

[0023] According to equations (1) and (4), the full-bridge sub-module ratio that satisfies DC fault ride-through is obtained as follows:

[0024]

[0025] Where N represents the total number of submodules of a single bridge arm;

[0026] When the DC voltage decreases, m dc <1, according to formula (3), the ratio constraint is:

[0027]

[0028] Where, t is the difference between the AC modulation ratio and the minimum DC modulation ratio during the voltage reduction process, t = m ac -m dc_min , m dc_min is the minimum DC modulation ratio.

[0029] In one possible design, based on the topological characteristics of the MMC-CLCC hybrid DC power transmission system topology architecture, a control strategy of the MMC-CLCC hybrid DC power transmission system is determined, including:

[0030] MMC adopts direct current vector control, selects capacitor voltage or DC voltage as active control quantity, and selects AC voltage or reactive power as reactive control quantity to provide voltage support for the access of new energy;

[0031] CLCC adopts constant DC current control and is equipped with a low-voltage current limiting control link. After taking the absolute value of the real-time DC voltage measured value, it is input into the low-voltage current limiting control link.

[0032] In one possible design, MMC uses direct current vector control, selects capacitor voltage or DC voltage as active control quantity, and selects AC voltage or reactive power as reactive control quantity to provide voltage support for new energy access, including:

[0033] MMC adopts the nearest level approximation modulation, and the number of sub-modules put into the upper and lower bridge arms of phase j is:

[0034]

[0035] In the formula, It indicates the number of submodules in the upper and lower bridge arms of phase j, and * indicates the per-unit value. Indicates the per unit value of the output voltage on the AC side of phase j, Indicates the closest integer;

[0036] To reverse the polarity of the DC voltage of the MMC by the first method and / or the second method without changing the direction of the DC current, so as to realize the online power flow reversal function;

[0037] The first method is to directly change the outer ring DC voltage reference value, and in the process of DC voltage polarity reversal, it is achieved by discharging the capacitor in the full bridge submodule and then charging it in the reverse direction;

[0038] The second method is to directly change m dc The full-bridge submodule is reversed to achieve the DC voltage polarity reversal; where m dc The value of m is less than 0. dc When the minimum negative value is reached, the output reverse voltage reaches its maximum value, and the active power transmitted in the reverse direction of the system reaches its maximum value.

[0039] In one possible design, the CLCC adopts constant DC current control and is equipped with a low-voltage current limiting control link. After taking the absolute value of the real-time DC voltage measured value, it is input into the low-voltage current limiting control link, including:

[0040] When the measured voltage value drops to the limit of low-voltage current limiting, the low-voltage current limiting is enabled, and the DC current reference value changes according to the limiting curve, and maintains stable operation when it drops to the minimum limit; when the DC voltage drops to negative, the DC voltage is processed by taking the absolute value, so as to ensure that at the moment of voltage polarity change, the voltage polarity flowing into the DC voltage control link remains unchanged, thereby maintaining the DC current at the minimum limit of the DC voltage link, and changing according to the current limiting curve as the voltage increases in the reverse direction.

[0041] In one possible design, based on the actual engineering capacity flow reversal requirements, the online flow reversal control strategy of the MMC-CLCC hybrid DC transmission system is determined, including:

[0042] MMC uses constant capacitor voltage / constant DC voltage control, and CLCC uses constant DC current control. After the system receives the power flow reversal command, it starts to reverse the power flow;

[0043] By reducing m dc The value of m can be used to reduce the number of submodules, thereby reducing the DC voltage. dc Decrease to 0 at a preset rate, U dc It will then decrease from the positive rated value to 0;

[0044] Inverter side DC voltage U dcI As shown in formula (8):

[0045]

[0046] Where U acI represents the AC voltage on the inverter side, X2 represents the equivalent commutation reactance on the inverter side, β is the CLCC leading trigger angle, I dc Indicates direct current;

[0047] Follow U dc When the trigger angle α is reduced to 0, the trigger angle α is reduced to 90°; wherein, α=π-β. When the trigger angle α is reduced to 90°, the low-voltage current limiting control link on the inverter side will start. After reaching the starting voltage of the low-voltage current limiting, the DC current I dc It will change according to the current limiting curve, and when the voltage reaches the minimum limit value, it will stabilize at the minimum value of the current limiting curve;

[0048] Continue to reduce m dc , so that m dc <0, at this time, the system only puts FBSM into operation to bear the bridge arm voltage, and FBSM is in negative input state, U dc The polarity is reversed and increases in the opposite direction as the number of negative inputs of FBSM increases;

[0049] According to formula (1), determine the minimum DC modulation ratio m dcmin :

[0050]

[0051] Where U dcmin It is the minimum value of the DC voltage on the rectifier side during actual operation;

[0052] When m dc Reduce to m dcmin When all FBSM inputs are negative, the maximum reverse voltage U dcmax =-MU c In this process, U dc Take the absolute value so that |U dc |Before reaching the low voltage current limit start voltage, I dc Maintain at the minimum value, and after the low voltage current limiting is started, dc The reverse increase will change according to the current limiting curve;

[0053] As the DC voltage and DC current change, the active power P = U dc I dc , the active power is reversed; the trigger angle α of CLCC gradually decreases from the rated value to below 90°, and it changes from the inverter state to the rectification state.

[0054] In a second aspect, the present application provides a direct current transmission system control device, the device comprising:

[0055] An architecture building unit, used to establish the MMC-CLCC hybrid DC transmission system topology architecture;

[0056] A ratio determination unit, used to determine the ratio of the full half-bridge submodules of the MMC in the MMC-CLCC hybrid DC transmission system topology architecture by taking into account the DC fault ride-through requirement and the possible power flow reversal requirement;

[0057] A first strategy determination unit, configured to determine a control strategy of the MMC-CLCC hybrid DC power transmission system based on the topological characteristics of the MMC-CLCC hybrid DC power transmission system topology architecture;

[0058] The second strategy determination unit is used to determine the online power flow reversal control strategy of the MMC-CLCC hybrid DC transmission system according to the actual engineering capacity power flow reversal demand.

[0059] In a third aspect, an embodiment of the present application provides an electronic device, comprising: at least one processor and a memory; the memory stores computer-executable instructions; the at least one processor executes the computer-executable instructions stored in the memory, so that the at least one processor executes the MMC-CLCC new DC transmission system structure and control method described in the first aspect and various possible designs of the first aspect.

[0060] In a fourth aspect, an embodiment of the present application provides a computer-readable storage medium, in which computer execution instructions are stored. When a processor executes the computer execution instructions, the MMC-CLCC new DC transmission system structure and control method described in the first aspect and various possible designs of the first aspect are implemented.

[0061] In a fifth aspect, an embodiment of the present application provides a computer program product, including a computer program. When the computer program is executed by a processor, it implements the MMC-CLCC new DC transmission system structure and control method as described in the first aspect and various possible designs of the first aspect.

[0062] The MMC-CLCC new DC transmission system structure and control method provided in this application is aimed at the scenarios of large-scale new energy transmission via DC and multiple DC landing point load center areas. It integrates the supporting role of MMC for new energy at the sending end and the operating advantages of CLCC, such as no phase change failure and low construction and operation costs, and proposes a new MMC-CLCC DC transmission system structure and control scheme. For actual high-voltage DC projects, the MMC-CLCC hybrid DC transmission system topology architecture is established; considering the requirements of DC fault ride-through and possible power reversal requirements, the MMC full half-bridge sub-module ratio selection method is designed; based on the topological characteristics of the MMC-CLCC hybrid DC transmission system, a MMC-CLCC hybrid DC transmission system control strategy is proposed; considering the actual engineering capacity power reversal requirements, an MMC-CLCC hybrid DC transmission system online power reversal control strategy is proposed. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0064] Figure 1 A flowchart of the MMC-CLCC novel DC transmission system structure and control method provided in the embodiment of the present application;

[0065] Figure 2 A topological structure diagram of an 800kV MMC-CLCC hybrid DC transmission system provided in an embodiment of the present application;

[0066] Figure 3 A basic topological structure diagram of the MMC converter provided in the embodiment of the present application;

[0067] Figure 4 A basic topological structure diagram of a CLCC converter provided in an embodiment of the present application;

[0068] Figure 5The CLCC operation path and the sub-valve switch timing diagram provided in the embodiment of the present application;

[0069] Figure 6 A control block diagram of an MMC converter provided in an embodiment of the present application;

[0070] Figure 7 A control block diagram of a CLCC converter provided in an embodiment of the present application;

[0071] Figure 8 A VDCOL control curve diagram after voltage startup optimization provided in an embodiment of the present application;

[0072] Fig. 9 A flow chart for implementing power flow reversal control provided in an embodiment of the present application;

[0073] Fig.10 A system response characteristic diagram of a power flow reversal strategy provided in an embodiment of the present application;

[0074] Fig.11 A schematic diagram of the structure of a DC power transmission system control device provided in an embodiment of the present application.

[0075] The above drawings have shown clear embodiments of the present application, which will be described in more detail later. These drawings and text descriptions are not intended to limit the scope of the present application in any way, but to illustrate the concept of the present application to those skilled in the art by referring to specific embodiments. DETAILED DESCRIPTION

[0076] Exemplary embodiments will be described in detail herein, examples of which are shown in the accompanying drawings. When the following description refers to the drawings, the same numbers in different drawings represent the same or similar elements unless otherwise indicated. The implementations described in the following exemplary embodiments do not represent all implementations consistent with the present application. Instead, they are merely examples of devices and methods consistent with some aspects of the present application as detailed in the appended claims.

[0077] In the technical solution of this application, the collection, storage, use, processing, transmission, provision and disclosure of information such as financial data or user data involved shall comply with the provisions of relevant laws and regulations and shall not violate public order and good morals.

[0078] It should be noted that in the embodiments of the present application, certain software, components, models and other existing solutions in the industry may be mentioned, and they should be regarded as exemplary. Their purpose is only to illustrate the feasibility of implementing the technical solution of the present application, but it does not mean that the applicant has or will necessarily use the solution.

[0079] The technical solution of the present application and how the technical solution of the present application solves the above-mentioned technical problems are described in detail below with specific embodiments. The following specific embodiments can be combined with each other, and the same or similar concepts or processes may not be repeated in some embodiments. The embodiments of the present application will be described below in conjunction with the accompanying drawings.

[0080] Glossary:

[0081] MMC: Modular Multilevel Converter;

[0082] CLCC: Controllable Commutation Converter;

[0083] FHMMC: Hybrid Modular Multilevel Converter;

[0084] STATCOM: static synchronous compensator;

[0085] FBMMC: Full-Bridge Modular Multilevel Converter;

[0086] FBSM: Full Bridge Submodule;

[0087] VDCOL: Low voltage current limiting.

[0088] The embodiment of the present application provides a new MMC-CLCC DC transmission system structure and control method. The DC transmission system control method integrates the supporting role of MMC for new energy at the sending end and the operating advantages of CLCC, such as no phase-changing failure and low construction and operation costs, to establish the topological architecture of the MMC-CLCC hybrid DC transmission system; considering the requirements of DC fault ride-through and possible power flow reversal, a method for selecting the proportion of the full half-bridge submodule of MMC is designed; based on the topological characteristics of the MMC-CLCC hybrid DC transmission system, a control strategy for the MMC-CLCC hybrid DC transmission system is proposed; considering the actual engineering capacity power flow reversal requirements, an online power flow reversal control strategy for the MMC-CLCC hybrid DC transmission system is proposed.

[0089] Figure 1 This is a flow chart of the MMC-CLCC novel DC transmission system structure and control method provided in the embodiment of the present application. Figure 1 As shown, the DC power transmission system control method includes the following steps S10 to S40.

[0090] S10. Establish the MMC-CLCC hybrid DC transmission system topology architecture.

[0091] In this embodiment, step S10 aims to establish an MMC-CLCC hybrid DC transmission system topology architecture for scenarios where large-scale new energy is transmitted via DC and multiple DC drop points are located in load center areas, combining the supporting role of MMC for new energy at the sending end and the operating advantages of CLCC, such as no phase-changing failure and low construction and operation costs. The MMC-CLCC hybrid DC transmission system adopts a full half-bridge hybrid MMC on the rectifier side and a dual 12-pulse CLCC on the inverter side. The sending-end AC system can be in the form of a conventional AC system and a large-scale new energy base. During normal operation, the sending end bears part of the load. In some special cases, there is a need for flow reversal. The receiving end is a conventional AC system, and is connected to an AC filter and a reactive compensation device at the same time to filter out harmonics and provide reactive support functions.

[0092] In some embodiments, see Figure 2 , which is a topological structure diagram of the 800kV MMC-CLCC hybrid DC transmission system provided in an embodiment of the present application. Taking the 800kV MMC-CLCC hybrid DC transmission system topology as an example of the MMC-CLCC hybrid DC transmission system topology architecture, the rectifier side adopts a full half-bridge hybrid MMC, and the inverter side adopts a dual 12-pulse CLCC. The sending-end AC system can take the form of a conventional AC system and a large-scale new energy base. During normal operation, the sending end bears part of the load. In some special cases, there is a demand for power flow reversal. The receiving end is a conventional AC system, and is connected to an AC filter and a reactive compensation device at the same time to achieve the function of filtering out harmonics and providing reactive support.

[0093] Common submodule topology types in MMC include half-bridge, full-bridge and hybrid bridge. Among them, hybrid bridge MMC has both DC fault clearing capability and economy, and has good engineering application value. The basic topology structure of FHMMC converter is as follows: Figure 3 As shown, a three-phase 6-bridge arm structure is adopted. The upper and lower bridge arms are composed of N sub-modules connected in series, including M full-bridge sub-modules and (NM) half-bridge sub-modules. At the same time, each bridge arm is equipped with a bridge arm reactance L0 to suppress the circulating current and the fault current rising rate.

[0094] The CLCC topology is similar to the traditional LCC, such as Figure 4 As shown in (a), it consists of 6 converter valves, U d The DC voltage output by the converter, I d is the DC current of the converter. Among them, the electrical topology diagram of a single valve is as follows Figure 4 As shown in (b), V11 and V12 form a main branch, and V13 and V14 form an auxiliary branch, wherein V11 and V14 are thyristor valves, and V12 and V13 are fully controlled IGBT valves. The thyristor and IGBT are connected in parallel with an RC circuit and an RCD circuit respectively to achieve dynamic voltage equalization.

[0095] The controllable commutation mode of CLCC includes two states: natural commutation operation and forced commutation operation. The flow path of its operation process and the switching timing of each device in each commutation valve are as follows: Figure 5 When the natural commutation operation is in progress, the valves V11, V12, and V13 are turned on, and the current flows through the main branch. The equivalent circuit is as follows: Figure 5 When the commutation current is less than a certain set value, the V12 valve is closed, the auxiliary branch V14 valve is turned on, and the current flows through the main and auxiliary branches. At this time, the V11 valve enters the blocking recovery period due to the reverse voltage. The equivalent circuit is as follows Figure 5 (b). When the auxiliary branch current decays to zero under the action of the external AC voltage, the natural commutation is completed. When the AC system fails in the receiving grid, CLCC is forced to commutate. The conduction sequence of each valve is consistent with that in normal operation. The difference is that after the V13 valve is closed, the current cannot drop to 0 due to the AC system failure. At this time, the current is transferred to the arrester arr13. The equivalent circuit is as follows: Figure 5 (c) Rely on the arrester action voltage to enhance the commutation voltage and complete the forced commutation process.

[0096] S20. Considering the DC fault ride-through requirements and possible power flow reversal requirements, determine the proportion of MMC full half-bridge sub-modules in the MMC-CLCC hybrid DC transmission system topology architecture.

[0097] In some embodiments, considering the DC fault ride-through requirements and possible power flow reversal requirements, the following MMC full-half-bridge sub-module ratio selection method is designed:

[0098] For FHMMC, define m dc is the DC modulation ratio, m ac is the AC modulation ratio, which is expressed as:

[0099]

[0100] Where U dc is the DC voltage on the rectifier side during actual operation, U dcN is the rated DC voltage of MMC, U sm is the peak value of the AC voltage.

[0101] The output voltage of the upper and lower bridge arms of FHMMC in steady state is:

[0102]

[0103] In the formula, u pj 、u nj is the output voltage of the upper and lower bridge arms of phase j, u sj It represents the output voltage of the j-phase AC side, and a, b, and c are the three phases in the three-phase AC system, namely, phase a, phase b, and phase c.

[0104] Combining equations (1) and (2), the value range of the j-phase bridge arm voltage is:

[0105]

[0106] ① After a bipolar short-circuit fault occurs on the DC side, in order to meet the operation requirements of the valve group without blocking the DC fault ride-through, and at the same time enable the converter to achieve fault ride-through in STATCOM mode, the number M of full-bridge submodules should satisfy the following relationship:

[0107]

[0108] Where U c Indicates the submodule capacitor voltage.

[0109] Combining equation (1) and equation (4), the full-bridge sub-module ratio that satisfies DC fault ride-through can be obtained as:

[0110]

[0111] Wherein, N represents the total number of single bridge arm submodules.

[0112] The normal operating range of the AC modulation ratio is usually around 0.9, which can ensure the stability of the system and the quality of the output voltage. ac =1, at this time, the minimum proportion of full-bridge sub-modules that meet the system DC fault clearance requirement is 50%.

[0113] ② Considering the power flow reversal demand, the DC voltage should be lowered first, then the voltage polarity of the converter station should be changed, and finally the negative level output capability of FBMMC should be used to reversely increase the DC voltage to achieve the power flow reversal of the hybrid DC system. dc <1, according to formula (3), the ratio constraint is:

[0114]

[0115] Where, t is the difference between the AC modulation ratio and the minimum DC modulation ratio during the voltage reduction process, t = m ac -m dc_min , m dc_min is the minimum DC modulation ratio.

[0116] Where, t is the difference between the AC modulation ratio and the minimum DC modulation ratio during the voltage reduction process, t = m ac -m dc_min .

[0117] When the system is running at reduced voltage until the DC voltage reaches 0, m dc=0, so if you want the system to run in reverse voltage, the configuration ratio to achieve voltage reversal should be greater than 50%.

[0118] In order to improve the economic efficiency of FHMMC, the proportion of FBSM should be reduced as much as possible on the basis of meeting the above-mentioned DC fault clearing capability and power flow reversal requirements.

[0119] S30. Determine a control strategy of the MMC-CLCC hybrid DC transmission system based on the topological characteristics of the MMC-CLCC hybrid DC transmission system topology architecture.

[0120] In this embodiment, step S30 proposes a control strategy for the MMC-CLCC hybrid DC transmission system, in which the sending end adopts a constant capacitor voltage / constant DC voltage control and the receiving end adopts a constant DC current control. This can achieve precise control of the system over the transmission power, ensure a dynamic balance of power between the sending end and the receiving end, effectively avoid system voltage and frequency fluctuations caused by power mismatch, and ensure stable operation of the power system.

[0121] In some embodiments, for the MMC-CLCC hybrid DC transmission system, in order to achieve stable operation of the system, the control strategy is designed as follows:

[0122] MMC adopts classic direct current vector control, selects capacitor voltage or DC voltage as active control quantity, selects AC voltage or reactive power as reactive control quantity, and can provide voltage support for the access of new energy. The basic control structure is as follows: Figure 6 As shown, the subscript ref represents the reference value of each electrical quantity.

[0123] CLCC adopts constant DC current control and is equipped with a low-voltage current limiting control link. In order to consider the operation needs under the subsequent power flow reversal, compared with the original LCC control, the real-time DC voltage measured value is processed by taking the absolute value and input into the low-voltage current limiting (voltage dependent current order limit, VDCOL) control link. The control block diagram is as follows Figure 7 As shown, where * indicates per-unit value.

[0124] In summary, the sending end adopts constant capacitor voltage / constant DC voltage control, and the receiving end adopts constant DC current control, which can realize the system's precise control of transmission power, ensure the dynamic balance of power between the sending and receiving ends, effectively avoid system voltage and frequency fluctuations caused by power mismatch, and ensure the stable operation of the power system.

[0125] MMC adopts the nearest level approximation modulation, and the number of sub-modules put into the upper and lower bridge arms of phase j is:

[0126]

[0127] In the formula, It indicates the number of submodules in the upper and lower bridge arms of phase j, and * indicates the per-unit value. Indicates the per unit value of the output voltage on the AC side of phase j, Indicates the closest An integer.

[0128] Due to the topological characteristics of CLCC, the DC current flowing through the MMC-CLCC hybrid DC transmission system can only be in one direction without changing the system wiring form. In order to achieve the online power flow reversal function, the DC voltage polarity of the MMC can only be reversed without changing the DC current direction.

[0129] There are two ways to reverse the DC voltage polarity of the MMC: one is to directly change the DC voltage reference value of the outer ring. During the DC voltage polarity reversal process, it is necessary to rely on the discharge of the capacitor in the full-bridge submodule and then reverse charging to achieve the function. This process takes a long time to reverse the flow; the other is to directly change the m dc The full-bridge submodule is reversed to achieve the DC voltage polarity reversal. This process does not require large-scale regulation of the submodule capacitor energy, and the power flow reversal speed is faster and smoother.

[0130] By changing m dc To achieve the trend reversal, m dc The value of should be less than 0. dc When the minimum negative value is reached, the maximum reverse voltage can be output, and the active power transmitted in the reverse direction of the system will also reach the maximum value.

[0131] In order to ensure that the DC current can be smoothly regulated during the change of DC voltage polarity, the VDCOL control link is added to the CLCC control. When the measured voltage value drops to the limit of VDCOL, VDCOL is enabled, and the DC current reference value changes according to the limiting curve, and maintains stable operation when it drops to the minimum limit; when the DC voltage drops to negative, it is processed by taking its absolute value, so as to ensure that at the moment of voltage polarity change, the voltage polarity flowing into the VDCOL control link remains unchanged, thereby maintaining the DC current at the minimum limit of the VDCOL link, and changing according to the current limiting curve as the voltage rises in the reverse direction. The VDCOL control curve of this article is as follows Figure 8 shown.

[0132] S40. Determine the online power flow reversal control strategy of the MMC-CLCC hybrid DC transmission system according to the actual project capacity power flow reversal demand.

[0133] In this embodiment, step S40 is based on the topological structure characteristics of the MMC-CLCC hybrid DC transmission system. Without changing the system wiring form, the DC current flowing through the MMC-CLCC can only be in a single direction. In order to achieve the online power flow reversal function, without changing the DC current direction, only the DC voltage polarity of the MMC can be reversed. By changing the m dc To achieve the trend reversal, m dc The value of should be less than 0. dc When the minimum negative value is reached, the maximum reverse voltage can be output, and the active power transmitted in the reverse direction of the system will also reach the maximum value. In order to ensure that the DC current can be smoothly regulated during the change of the DC voltage polarity, the VDCOL control link is added to the CLCC control. When the voltage is reduced to the VDCOL start-up link, the DC current changes according to the current limiting curve.

[0134] In some embodiments, based on the topological characteristics and control strategy of the MMC-CLCC hybrid DC transmission system, the following is formulated: Fig. 9 The flow reversal control implementation process is shown in the figure. The specific design of the flow reversal process is as follows.

[0135] S41 and MMC use constant capacitor voltage / constant DC voltage control, and CLCC uses constant DC current control. After the system receives the power flow reversal command, it starts to reverse the power flow.

[0136] S42. From formula (7), we can see that the number of sub-modules put into use is positively correlated with the DC modulation ratio. Therefore, by reducing m dc The value of m can be used to reduce the number of submodules, thereby reducing the DC voltage. dc Decrease to 0 at a certain rate, U dc The positive value will then be reduced to 0.

[0137] Inverter side DC voltage U dcI As shown in formula (8):

[0138]

[0139] Where U acI represents the AC voltage on the inverter side, X2 represents the equivalent commutation reactance on the inverter side, β is the CLCC leading trigger angle, I dc Indicates direct current.

[0140] Follow U dc When the VDCOL voltage is reached, the DC current I dc It will change according to the current limiting curve and stabilize at the minimum value of the current limiting curve when the voltage reaches the minimum limit value.

[0141] S43, continue to reduce m dc , so that m dc <0, at this time, the system only puts FBSM into operation to bear the bridge arm voltage, and FBSM is in negative input state, U dc The polarity is reversed and increases in the opposite direction as the number of negative inputs of FBSM increases.

[0142] From formula (1), we know that the minimum DC modulation ratio m dcmin :

[0143]

[0144] Where U dcmin It is the minimum value of the DC voltage on the rectifier side during actual operation.

[0145] When m dc Reduce to m dcmin When all FBSM inputs are negative, the maximum reverse voltage U dcmax =-MU c In this process, due to the VDCOL control link of the inverter side constant DC current control, U dc Take the absolute value so that |U dc |Before reaching the startup voltage of VDCOL, I dc Maintain at the minimum value, and after VDCOL starts, dc The reverse increase will change according to the current limiting curve.

[0146] As the DC voltage and DC current change, P = U dc I dc , the active power is reversed; the trigger angle α of CLCC gradually decreases from the rated value (>90°) to below 90°, and it changes from the inverter state to the rectification state.

[0147] S44, the flow reversal process ends.

[0148] The example analysis provided in the embodiment of this application is as follows Fig.10 In PSCAD / EMTDC, the following Figure 2The MMC-CLCC hybrid DC transmission system simulation platform shown in the figure. The DC rated voltage is 800kV and the rated power is 1600MW. The effective values ​​of the AC line voltages on the FHMMC and CLCC sides are 380kV and 525kV, respectively, and the ratios of the converter transformers are 380kV / 220kV and 500kV / 190kV, respectively. Two FHMMC series structures are used on the rectifier side. The submodule capacitance value and submodule capacitance voltage of the FBSM in a single FHMMC are set to 10mF and 2kV, so N=200. The inverter side adopts a dual 12-pulse CLCC structure, so there are 4 6-pulse converter valves, and the rated DC voltage of a single valve group is 200kV. The bridge arm reactance L0 is 29mH, and the DC side close to the rectifier station and the inverter station are equipped with 0.6H smoothing reactors to suppress ripples and limit the fault current rise rate.

[0149] When the full-bridge submodule accounts for 100% in the MMC, the simulation process of power flow reversal is as follows: Fig. 9 At t = 1.0s, the DC system starts to reverse the power flow, m dc The rate of decrease is 1p.u. / s, so the flow reversal rate is 1.0pu / s. The response characteristics of the system are as follows: Fig.10 As shown. Fig.10 (a) to (f) respectively give the response curves of DC voltage, DC current, trigger angle α, active power, number of bridge arm input sub-modules (>0, positive input; <0, negative input) and capacitor voltage.

[0150] from Fig.10 It can be seen that in the process of the DC voltage decreasing from the positive rated value to zero, the DC current is affected by the constant DC current control and VDCOL control links. After the voltage decreases to the starting voltage of VDCOL, it changes according to the current limiting curve, and after the voltage reaches the lower limit of VDCOL, it stabilizes at the lowest value of the current limiting curve, such as Fig.10 (b); at the same time, the trigger angle gradually decreases to 90°, such as Fig.10 (c). When the DC voltage polarity reverses, the full-bridge submodule changes to a negative input state, such as Fig.10 As shown in (e), the number of submodules put into operation is less than 0. As the number of negative submodules increases, the DC voltage increases in the reverse direction. dc Take the absolute value so that |U dc |Before reaching the startup voltage of VDCOL, I dc Maintain at the minimum value, and after VDCOL starts, dc The reverse increase will change according to the current limiting curve. During this process, the trigger angle of CLCC gradually decreases to below 90°, and changes from the inverter state to the rectification state. P=U dc I dc, the active power also changes from positive to negative, achieving reverse direction.

[0151] The present application also provides a DC power transmission system control device, Fig.11 This is a schematic diagram of the structure of the DC power transmission system control device provided in the embodiment of the present application. Fig.11 As shown, the DC transmission system control device includes:

[0152] The architecture building unit 111 is used to establish a topology architecture of the MMC-CLCC hybrid DC transmission system;

[0153] A ratio determination unit 112, configured to determine the ratio of the full half-bridge submodules of the MMC in the MMC-CLCC hybrid DC power transmission system topology architecture by taking into account the DC fault ride-through requirement and the possible power flow reversal requirement;

[0154] A first strategy determination unit 113, configured to determine a control strategy of the MMC-CLCC hybrid DC power transmission system based on the topological characteristics of the MMC-CLCC hybrid DC power transmission system topology architecture;

[0155] The second strategy determination unit 114 is used to determine the online power flow reversal control strategy of the MMC-CLCC hybrid DC power transmission system according to the actual engineering capacity power flow reversal demand.

[0156] An embodiment of the present application provides an electronic device, which may include: a processor and a memory, wherein the processor and the memory may communicate with each other; illustratively, the processor and the memory communicate with each other via a communication bus.

[0157] The processor executes the computer execution instructions stored in the memory, so that the processor executes the scheme in the above embodiment. The processor can be a general-purpose processor, including a central processing unit (CPU), a network processor (NP), etc.; it can also be a digital signal processor (DSP), an application-specific integrated circuit ASIC, a field programmable gate array FPGA or other programmable logic devices, discrete gates or transistor logic devices, and discrete hardware components.

[0158] The communication bus can be a peripheral component interconnect (PCI) bus or an extended industry standard architecture (EISA) bus. The system bus can be divided into an address bus, a data bus, a control bus, etc. The transceiver is used to realize the communication between the database access device and other computers (such as clients, read-write libraries, and read-only libraries). The memory may include random access memory (RAM) and may also include non-volatile memory.

[0159] The electronic device provided in the embodiment of the present application may be the terminal device of the above embodiment.

[0160] An embodiment of the present application also provides a computer-readable storage medium, in which computer instructions are stored. When the computer instructions are executed on a computer, the computer executes the technical solution of the MMC-CLCC new direct current transmission system structure and control method of the above-mentioned embodiment.

[0161] An embodiment of the present application also provides a computer program product, which includes a computer program stored in a computer-readable storage medium. At least one processor can read the computer program from the computer-readable storage medium. When at least one processor executes the computer program, it can implement the technical solutions of the MMC-CLCC new DC transmission system structure and control method in the above-mentioned embodiment.

[0162] In the several embodiments provided in the present application, it should be understood that the disclosed devices and methods can be implemented in other ways. For example, the device embodiments described above are only schematic, for example, the division of modules is only a logical function division, and there may be other division methods in actual implementation, such as multiple modules can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be an indirect coupling or communication connection through some interfaces, devices or modules, which can be electrical, mechanical or other forms.

[0163] The modules described as separate components may or may not be physically separated, and the components shown as modules may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the modules may be selected according to actual needs to implement the solution of this embodiment.

[0164] In addition, each functional module in each embodiment of the present application can be integrated into one processing unit, or each module can exist physically separately, or two or more modules can be integrated into one unit. The above-mentioned module-composed unit can be implemented in the form of hardware or in the form of hardware plus software functional units.

[0165] The above-mentioned integrated module implemented in the form of a software function module can be stored in a computer-readable storage medium. The above-mentioned software function module is stored in a storage medium, including a number of instructions for enabling a computer device (which can be a personal computer, a server, or a network device, etc.) or a processor to perform some steps of the methods of various embodiments of the present application.

[0166] It should be understood that the processor may be a central processing unit (CPU), or other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), etc. A general-purpose processor may be a microprocessor or any conventional processor. The steps of the method disclosed in the invention may be directly implemented as being executed by a hardware processor, or may be implemented by a combination of hardware and software modules in the processor.

[0167] The memory may include a high-speed RAM memory, and may also include a non-volatile storage NVM, such as at least one disk memory, and may also be a USB flash drive, a mobile hard disk, a read-only memory, a magnetic disk or an optical disk, etc.

[0168] The bus may be an Industry Standard Architecture (ISA) bus, a Peripheral Component Interconnect (PCI) bus, or an Extended Industry Standard Architecture (EISA) bus, etc. The bus may be divided into an address bus, a data bus, a control bus, etc. For ease of representation, the bus in the drawings of the present application is not limited to only one bus or one type of bus.

[0169] The above storage medium can be implemented by any type of volatile or non-volatile storage device or a combination thereof, such as static random access memory (SRAM), electrically erasable programmable read-only memory (EEPROM), erasable programmable read-only memory (EPROM), programmable read-only memory (PROM), read-only memory (ROM), magnetic memory, flash memory, magnetic disk or optical disk. The storage medium can be any available medium that can be accessed by a general or special purpose computer.

[0170] An exemplary storage medium is coupled to a processor so that the processor can read information from the storage medium and write information to the storage medium. Of course, the storage medium can also be a component of the processor. The processor and the storage medium can be located in an application specific integrated circuit (ASIC). Of course, the processor and the storage medium can also exist as discrete components in an electronic control unit or a main control device.

[0171] Those skilled in the art can understand that all or part of the steps of implementing the above-mentioned method embodiments can be completed by hardware related to program instructions. The aforementioned program can be stored in a computer-readable storage medium. When the program is executed, the steps of the above-mentioned method embodiments are executed; and the aforementioned storage medium includes: ROM, RAM, disk or optical disk and other media that can store program codes.

[0172] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit it. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A novel MMC-CLCC DC transmission system structure and control method, characterized in that: The method comprises: Establish the MMC-CLCC hybrid DC transmission system topology architecture; Considering the DC fault ride-through requirements and the possible power flow reversal requirements, determine the proportion of the MMC full half-bridge submodules in the MMC-CLCC hybrid DC transmission system topology architecture; Determine a control strategy of the MMC-CLCC hybrid DC transmission system based on the topological characteristics of the MMC-CLCC hybrid DC transmission system topology architecture; According to the actual project capacity flow reversal requirements, the online flow reversal control strategy of the MMC-CLCC hybrid DC transmission system is determined.

2. The method according to claim 1, characterized in that The MMC-CLCC hybrid DC transmission system topology architecture includes a rectifier side and an inverter side; wherein the rectifier side adopts a full half-bridge hybrid MMC, and the inverter side adopts a dual 12-pulse CLCC.

3. The method according to claim 2, characterized in that Considering the DC fault ride-through requirements and possible power flow reversal requirements, the proportion of the MMC full half-bridge submodules in the MMC-CLCC hybrid DC transmission system topology architecture is determined, including: For full half-bridge hybrid MMC, define m dc is the DC modulation ratio, m ac is the AC modulation ratio, then the expression for determining the full half-bridge hybrid MMC is: Where U dc is the DC voltage on the rectifier side during actual operation, U dcN is the rated DC voltage of MMC, U sm is the peak value of the AC voltage; The output voltage of the upper and lower bridge arms of the full half-bridge hybrid MMC in steady state is calculated by the following formula: In the formula, u pj 、u nj is the output voltage of the upper and lower bridge arms of phase j, u sj represents the output voltage of the j-phase AC side, a, b, c are the three phases in the three-phase AC system, i.e., phase a, phase b, phase c; According to equations (1) and (2), the value range of the j-phase bridge arm voltage is determined as: After a bipolar short-circuit fault occurs on the DC side, if the valve group is to meet the operation requirements of DC fault ride-through without blocking, the number M of full-bridge submodules should satisfy the following relationship: Where U c Represents the submodule capacitor voltage; According to equations (1) and (4), the full-bridge sub-module ratio that satisfies DC fault ride-through is obtained as follows: Where N represents the total number of submodules of a single bridge arm; When the DC voltage decreases, m dc <1, according to formula (3), the ratio constraint is: Where, t is the difference between the AC modulation ratio and the minimum DC modulation ratio during the voltage reduction process, t = m ac -m dc_min , m dc_min is the minimum DC modulation ratio.

4. The method according to claim 3, characterized in that: Based on the topological characteristics of the MMC-CLCC hybrid DC transmission system topology architecture, a control strategy of the MMC-CLCC hybrid DC transmission system is determined, including: MMC adopts direct current vector control, selects capacitor voltage or DC voltage as active control quantity, and selects AC voltage or reactive power as reactive control quantity to provide voltage support for the access of new energy; CLCC adopts constant DC current control and is equipped with a low-voltage current limiting control link. After taking the absolute value of the real-time DC voltage measured value, it is input into the low-voltage current limiting control link.

5. The method according to claim 4, characterized in that MMC adopts direct current vector control, selects capacitor voltage or DC voltage as active control quantity, and selects AC voltage or reactive power as reactive control quantity to provide voltage support for the access of new energy, including: MMC adopts the nearest level approximation modulation, and the number of sub-modules put into the upper and lower bridge arms of phase j is: In the formula, Indicates the number of submodules in the upper and lower bridge arms of phase j, respectively. * indicates the per-unit value. Indicates the per unit value of the output voltage on the AC side of phase j, Indicates the closest integer; To reverse the polarity of the DC voltage of the MMC by the first method and / or the second method without changing the direction of the DC current, so as to realize the online power flow reversal function; The first method is to directly change the outer ring DC voltage reference value, and in the process of DC voltage polarity reversal, it is achieved by discharging the capacitor in the full bridge submodule and then charging it in the reverse direction; The second method is to directly change m dc The full-bridge submodule is reversed to achieve the DC voltage polarity reversal; where m dc The value of m is less than 0. dc When the minimum negative value is reached, the output reverse voltage reaches its maximum value, and the active power transmitted in the reverse direction of the system reaches its maximum value.

6. The method according to claim 4, characterized in that CLCC adopts constant DC current control and is equipped with a low-voltage current limiting control link. After taking the absolute value of the real-time DC voltage measured value, it is input into the low-voltage current limiting control link, including: When the measured voltage value drops to the limit of low-voltage current limiting, the low-voltage current limiting is enabled, and the DC current reference value changes according to the limiting curve, and maintains stable operation when it drops to the minimum limit; when the DC voltage drops to negative, the DC voltage is processed by taking the absolute value, so as to ensure that at the moment of voltage polarity change, the voltage polarity flowing into the DC voltage control link remains unchanged, thereby maintaining the DC current at the minimum limit of the DC voltage link, and changing according to the current limiting curve as the voltage increases in the reverse direction.

7. The method according to claim 5, characterized in that According to the actual project capacity flow reversal requirements, the online flow reversal control strategy of the MMC-CLCC hybrid DC transmission system is determined, including: MMC uses constant capacitor voltage / constant DC voltage control, and CLCC uses constant DC current control. After the system receives the power flow reversal command, it starts to reverse the power flow; By reducing m dc The value of m can be used to reduce the number of submodules, thereby reducing the DC voltage. dc Decrease to 0 at a preset rate, U dc It will then decrease from the positive rated value to 0; Inverter side DC voltage U dcI As shown in formula (8): Where U acI represents the AC voltage on the inverter side, X2 represents the equivalent commutation reactance on the inverter side, β is the CLCC leading trigger angle, I dc Indicates direct current; Follow U dc When the trigger angle α is reduced to 0, the trigger angle α is reduced to 90°; wherein, α=π-β. When the trigger angle α is reduced to 90°, the low-voltage current limiting control link on the inverter side will start. After reaching the starting voltage of the low-voltage current limiting, the DC current I dc It will change according to the current limiting curve, and when the voltage reaches the minimum limit value, it will stabilize at the minimum value of the current limiting curve; Continue to reduce m dc , so that m dc <0, at this time, the system only puts FBSM into operation to bear the bridge arm voltage, and FBSM is in negative input state, U dc The polarity is reversed and increases in the opposite direction as the number of negative inputs of FBSM increases; According to formula (1), determine the minimum DC modulation ratio m dcmin : Where U dcmin It is the minimum value of the DC voltage on the rectifier side during actual operation; When m dc Reduce to m dcmin When all FBSM inputs are negative, the maximum reverse voltage U dcmax =-MU c In this process, U dc Take the absolute value so that |U dc |Before reaching the low voltage current limit start voltage, I dc Maintain at the minimum value, and after the low voltage current limiting is started, dc The reverse increase will change according to the current limiting curve; As the DC voltage and DC current change, the active power P = U dc I dc , the active power is reversed; the trigger angle α of CLCC gradually decreases from the rated value to below 90°, and it changes from the inverter state to the rectification state.

8. A DC power transmission system control device, characterized in that: The device comprises: An architecture building unit, used to establish the MMC-CLCC hybrid DC transmission system topology architecture; A ratio determination unit, used to determine the ratio of the full half-bridge submodules of the MMC in the MMC-CLCC hybrid DC transmission system topology architecture by taking into account the DC fault ride-through requirement and the possible power flow reversal requirement; A first strategy determination unit, configured to determine a control strategy of the MMC-CLCC hybrid DC power transmission system based on the topological characteristics of the MMC-CLCC hybrid DC power transmission system topology architecture; The second strategy determination unit is used to determine the online power flow reversal control strategy of the MMC-CLCC hybrid DC transmission system according to the actual engineering capacity power flow reversal demand.

9. An electronic device, characterized in that: include: A processor, and a memory communicatively connected to the processor; The memory stores computer-executable instructions; The processor executes the computer-executable instructions stored in the memory to implement the method according to any one of claims 1 to 7.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores computer-executable instructions, which are used to implement the method according to any one of claims 1 to 7 when executed by a processor.

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