Bipolar flexible DC power transmission system emergency control method based on mode switching

Abstract

The invention discloses a bipolar flexible DC power transmission system emergency control method based on mode switching, and the method comprises the steps: building a converter control mode switching mechanism based on a variable DC voltage threshold and a priority list, and automatically appointing a standby converter to take over DC voltage control when a converter responsible for constant voltage control trips. The interelectrode voltage is kept stable; an inter-electrode active power dynamic scheduling algorithm is provided, and the power of a fault electrode is transferred to a plurality of current converters of a healthy electrode according to a set proportion by introducing a power guarantee coefficient and combining the rated capacity of each current converter. According to the method, the overload of the converter caused by the single-pole fault can be effectively prevented, and the interruption of the transmission power is reduced to the minimum, so that the power supply reliability and the operation safety of the bipolar flexible direct-current power transmission system are improved.

Description

Technical Field

[0001] This invention relates to the field of operation and control technology of flexible DC transmission systems, and specifically to an emergency control method for a bipolar flexible DC transmission system based on mode switching. Background Technology

[0002] Multi-terminal DC (MTDC) transmission technology based on voltage source converters (VSCs), especially bipolar structures, has significant advantages in cross-regional grid interconnection and large-scale renewable energy grid integration due to its ability to achieve independent power control, flexible operation modes, and high power supply reliability. However, in the event of a unipolar fault in a bipolar VSC-MTDC system, the interruption of power transmission on the faulty pole will lead to a power surplus, causing power overload and DC voltage instability risks to the healthy pole converter, seriously threatening the safe and stable operation of the entire AC / DC hybrid power grid.

[0003] Control technology is crucial for ensuring the continued stable operation of bipolar VSC-MTDC systems after a fault. Existing control strategies mainly rely on traditional power-voltage droop control or master-slave control. These methods have significant limitations in the event of a unipolar fault: firstly, they fail to systematically address the voltage stability maintenance problem after the tripping of the constant DC voltage controlled converter, lacking a rapid and adaptive control mode switching mechanism; secondly, they fail to fully utilize the advantages of the bipolar structure to achieve coordinated and optimized power scheduling between the faulty and healthy poles, leading to a sharp drop in system transmission capacity and causing a significant power impact on the connected AC grid. Given the rapid development of DC faults, how to quickly reconstruct the system control architecture and achieve balanced power transfer within a very short time after a unipolar fault has become a key technical challenge for improving the resilience and reliability of bipolar VSC-MTDC systems. Summary of the Invention

[0004] This invention provides an emergency control method for a bipolar flexible DC transmission system based on mode switching, in order to solve the problem of stable system operation under unipolar faults and improve its fault ride-through capability.

[0005] According to a first aspect, one embodiment provides an emergency control method for a bipolar flexible DC transmission system based on mode switching, the method comprising: Real-time monitoring of DC voltage in the positive and negative polarity networks and converter operating status; If a single-pole constant DC voltage control converter trips or the DC voltage of its pole network exceeds a preset threshold due to the constant DC voltage control converter exiting operation, a pre-established converter mode switching mechanism is activated. According to the preset priority list of constant DC voltage control converters for each pole, the original constant DC voltage control converter is automatically switched to constant active power control mode, and the converter with the next priority is switched to constant DC voltage control mode. In this case, the positive and negative pole networks establish independent constant DC voltage control converter priority lists, which are used to set the constant voltage control takeover sequence within their respective pole networks. After the mode switch is completed, the pre-established mode switch malfunction detection mechanism is activated. The actual operating status of the original constant DC voltage control converter is checked through the communication network. If it is confirmed that the original constant DC voltage control converter is still operating normally, the mode switch is determined to be a malfunction, and an order is immediately issued to make the converter that has undergone mode switch return to its original mode. If it is confirmed that the original constant DC voltage control converter has been taken out of operation, the mode switch is considered to be effective, and the pre-established inter-pole active power dynamic allocation strategy is activated. By introducing a specific power allocation coefficient and comprehensively considering the capacity of each converter, the active power command of the healthy pole converter is automatically calculated and updated to realize the transfer of power from the faulty pole to the healthy pole.

[0006] Furthermore, the converter mode switching mechanism specifically includes: Only one constant DC voltage controlled converter is retained in each pole network. That is, only one converter in each positive or negative pole network operates in constant DC voltage control mode, while all other converters in the corresponding pole network operate in constant active power control mode.

[0007] Furthermore, the converter mode switching mechanism specifically includes: During the operation of the bipolar VSC-MTDC system, the DC voltage of each pole network is collected in real time by the local controller of each converter terminal. Where m represents polarity, m=p represents positive pole, m=n represents negative pole, and i represents the converter terminal number of the bipolar VSC-MTDC system. Each converter terminal contains a positive converter and a negative converter. Assume that the converter terminal T is currently responsible for constant DC voltage control. j Positive converter VSC j + and negative converter VSC j - Under steady-state conditions, both operate in a constant DC voltage control mode. To determine whether the next priority terminal needs to take over the constant voltage control, the next priority converter terminal T is first calculated based on the bipolar VSC-MTDC system circuit equations.j+1 DC voltage reference value of the polar network Then, based on the DC voltage reference value Calculate the value for terminal T j+1 Variable DC voltage threshold ; Then the real-time measured DC voltage With variable DC voltage threshold If a comparison is made, Exceed If this triggers a mode switch, the original constant DC voltage control converter will be switched to constant active power control mode, and the candidate converter will be switched to constant DC voltage control mode. Not exceeding If so, the original pattern remains unchanged.

[0008] Furthermore, based on the DC voltage reference value Calculate the value for terminal T j+1 Variable DC voltage threshold Specifically, it includes:

[0009] In the formula: When the polarity is m, the variable DC voltage threshold of the (j+1)th candidate converter in the priority sequence; This is the DC voltage reference value of the (j+1)th candidate converter calculated based on the bipolar VSC-MTDC system circuit equations, assuming all terminal converters are operating normally. α is the DC voltage margin constant, used to suppress frequent mode switching caused by small measurement fluctuations. Its value is preset by the transmission system operator (TSO). The subscript j+1 indicates that the converter terminal T has the highest priority in the constant DC voltage controlled converter priority list. j The next priority converter terminal number is then assigned, and the priority list of constant DC voltage controlled converters is pre-set by the transmission system operator (TSO).

[0010] Furthermore, the mode switching malfunction detection mechanism specifically includes: The control center communicates with the original constant DC voltage control converter VSC via the communication network. j Send a status query request and receive the returned running status information; If communication confirms VSC j If the system is still operating normally, then the mode switch is determined to be a malfunction, and the system will then switch to the next lower priority converter VSC. (j+1) Issue a recovery command to VSC(j+1) The operating mode is restored from constant DC voltage control mode to constant active power control mode, while maintaining VSC. j Continue executing the constant DC voltage control mode; If communication confirms VSC j If the program has exited, then the mode switch is considered a valid action, and VSC will be maintained. (j+1) The constant DC voltage control mode is locked through self-holding logic until a recovery signal is received.

[0011] Furthermore, the dynamic allocation strategy for inter-pole active power specifically includes: Calculate the power increment of a robust polarity converter Capacity-related increments And combined with the power guarantee factor and converter terminal weighting coefficient Update its active power reference value This achieves a smooth power transfer from the faulty pole to the healthy pole, as shown in the following formula:

[0012]

[0013]

[0014] In the formula: The subscript l, l = 1, 2, ..., N, l ≠ j, indicates the converter terminal number T that uses constant active power control. l ; The total number of converter terminals in the bipolar VSC-MTDC system; The superscript m indicates polarity; m=p represents a positive polarity network, and m=n represents a negative polarity network. For converter terminal T l Active power reference value during steady-state operation; For converter terminal T l The reference value of the DC voltage for the corresponding polarity network; For converter terminal T l The measured DC voltage of the corresponding polarity network; This is a reference value of active power obtained through adjustment after a unipolar tripping event occurs; For converter terminal T l The increase in active power allocated due to the tripping of the faulty pole; To further allocate power to the converter terminal T after considering the capacity margin of the constant DC voltage control converter. l The active power correction amount; This refers to the active power output of the faulty pole converter before tripping. For single-pole tripping flag variables, when the converter terminal T with polarity m... l When running normally When a trip occurs ; For converter terminal T l Active power-DC voltage droop control coefficient on polarity m, and ≠0; The power guarantee factor is a pre-set value by the transmission system operator (TSO) to adjust the power share transferred from the faulty pole to the healthy pole, satisfying 0 ≤ ≤ 1; when When, it indicates that the converter terminal T l When a single-pole trip occurs in the network of the affected pole, the active power reference value of the constant active power control converter in the healthy pole, which is not affected by DC faults or routine maintenance, is as follows: Based on , as well as Update; When the polarity is m, the converter terminal T l A weighting factor related to the transmission power scale under unipolar tripping conditions is used to distribute the active power transferred from the faulty pole among multiple converter terminals according to the rated capacity ratio.

[0015] Furthermore, weighting coefficients The calculation formula is as follows:

[0016]

[0017]

[0018] In the formula: For converter terminal T l Rated active power capacity of the converter with polarity m; For converter terminal T i The rated active power capacity of the converter with polarity m, and its actual output during normal operation. Must meet ; For a constant DC voltage controlled converter VSC with polarity m j The capacity utilization-related indicator variable, when hour A value of 0 indicates VSC j The output power has not yet reached 90% of the rated capacity, when hour A value of 1 indicates VSC j The power output is nearing its rated limit. At this point, it is necessary to allocate some power to other converter terminals according to the power allocation mechanism, thereby limiting the VSC. j To mitigate overload risks and achieve robust power redistribution within the electrode.

[0019] Furthermore, the method also includes: During the activation of emergency control and power scheduling, the current operating mode and power command are maintained without frequent switching through self-holding logic until a return signal indicating that the converter has resumed operation or an exit command issued by the maintenance personnel is received. At this time, the controller gradually releases the emergency control according to the preset strategy, restores to the normal operating mode, and the emergency control process ends.

[0020] According to a second aspect, one embodiment provides an emergency control device for a bipolar flexible DC transmission system based on mode switching, the device comprising: The real-time monitoring module is used to monitor the DC voltage and converter operating status in real time, and to detect faults. The mode switching module is used to activate a pre-established converter mode switching mechanism if a tripping of a single-pole constant DC voltage control converter is detected or the DC voltage of the pole network where the constant DC voltage control converter is located exceeds a preset threshold due to the constant DC voltage control converter's shutdown. According to the preset priority list of constant DC voltage control converters for each pole, the module automatically switches the original constant DC voltage control converter to constant active power control mode and switches the converter of the next priority to constant DC voltage control mode. The positive and negative pole networks each establish independent priority lists of constant DC voltage control converters, which are used to set the constant voltage control takeover sequence within their respective pole networks. The malfunction detection module is used to activate a pre-established malfunction detection mechanism after the working mode switch is completed. It checks the actual status of the original converter that was judged to have tripped through the communication network. If it is confirmed that the original converter is still operating normally, it determines that the switch is a malfunction and immediately orders the alternative converter that has undergone mode switch to return to its original working mode. The power redistribution module is used to determine that the mode switch is effective if the original converter has been confirmed to have tripped. It then initiates a pre-established dynamic active power allocation strategy between poles. By introducing a specific power allocation coefficient and taking into account the capacity of each converter, it automatically calculates and updates the active power command of the viable pole converter to complete the power redistribution between poles and achieve steady-state operation.

[0021] This invention provides an emergency control method for a bipolar flexible DC transmission system based on mode switching, which has the following advantages: (1) An automatic switching mechanism for control modes based on local electrical quantity measurement was established. This method presets control role priorities for each converter in the system and triggers switching by real-time monitoring of whether the DC voltage exceeds the set action threshold. When a failure is detected in the key converter responsible for maintaining the voltage, the system will automatically switch the next priority converter from its normal operation mode to the DC voltage control mode in a predetermined order to quickly restore and maintain grid voltage stability.

[0022] (2) A safety verification mechanism to prevent erroneous switching of control modes is set up. After the mode switching command is issued and executed, the system will check the actual status of the original converter through the communication network. If it is confirmed that the original converter is still operating normally, the switching is determined to be a erroneous action, and the system will immediately order the switched converter to return to its initial operating mode, thereby avoiding unnecessary changes in the system operation mode and improving the reliability of control.

[0023] (3) A strategy for dynamically allocating active power between the two poles is proposed. This strategy introduces a specific power allocation coefficient and comprehensively considers the capacity of each converter to automatically calculate the power command that the healthy pole converter needs to increase. By updating these power commands, the system can smoothly and reasonably transfer the power interrupted by the faulty pole to the healthy pole, effectively prevent equipment overload, minimize the power impact on the connected AC power grid, and ensure the continuous and stable power supply of the system.

[0024] The method of the present invention can effectively prevent converter overload caused by unipolar faults and minimize the interruption of transmission power, thereby improving the power supply reliability and operational safety of bipolar flexible DC transmission systems. Attached Figure Description

[0025] Figure 1 A bipolar VSC-MTDC system topology diagram in an emergency control method for a bipolar flexible DC transmission system based on mode switching, provided in an embodiment of the present invention; Figure 2 A converter station structure diagram in an emergency control method for a bipolar flexible DC transmission system based on mode switching, provided in an embodiment of the present invention; Figure 3A flowchart illustrating mode switching and malfunction detection in an emergency control method for a bipolar flexible DC transmission system based on mode switching, provided as an embodiment of the present invention; Figure 4 The following is a flowchart illustrating the specific implementation of an emergency control method for a bipolar flexible DC transmission system based on mode switching, as provided in one embodiment of the present invention. Detailed Implementation

[0026] The present invention will now be described in further detail with reference to specific embodiments and accompanying drawings. Similar elements in different embodiments are referred to by associated similar element reference numerals. In the following embodiments, many details are described to facilitate a better understanding of the invention. However, those skilled in the art will readily recognize that some features may be omitted in different situations, or may be replaced by other elements, materials, or methods. In some cases, certain operations related to the present invention are not shown or described in the specification. This is to avoid obscuring the core parts of the invention with excessive description. For those skilled in the art, detailed description of these related operations is not necessary; they can fully understand the related operations based on the description in the specification and general technical knowledge in the art.

[0027] Furthermore, the features, operations, or characteristics described in the specification can be combined in any suitable manner to form various embodiments. At the same time, the steps or actions in the method description can be rearranged or adjusted in a manner obvious to those skilled in the art. Therefore, the various orders in the specification and drawings are only for the clear description of a particular embodiment and do not imply a necessary order, unless otherwise stated that a particular order must be followed.

[0028] The first embodiment of the present invention provides an emergency control method for a bipolar flexible DC transmission system based on mode switching, the main contents of which are as follows: 1. Analyze the operating characteristics of the bipolar flexible DC transmission system under the tripping condition of the single-pole converter and its impact on DC voltage stability and power distribution, and establish the topology of the bipolar VSC-MTDC system.

[0029] Specifically, the bipolar VSC-MTDC system topology is shown in the attached figure. Figure 1 As shown in the attached diagram, the structure of the converter station (converter terminal) is as follows. Figure 2As shown. Each terminal of the bipolar VSC-MTDC system consists of a positive and a negative network. A metal conductor connects the positive and negative networks, serving as a neutral line unaffected by grounding current and providing a return loop on the DC side. This configuration allows for independent control of the active power of the positive and negative networks under fault conditions, unaffected by grounding current. When a single-pole trip occurs at an MTDC terminal, the faulty converter stops operating, but the other healthy converter, unaffected by the DC fault, can continue to operate stably. However, fluctuations in DC power flow may occur in the faulty network due to the converter's shutdown, requiring active power control to maintain the stability of the MTDC system under fault conditions. For example, as shown in the attached diagram. Figure 1 As shown, when VSCi+ trips, its active power output (indicated by the black arrow) will be suddenly interrupted. At this time, the active power output of other normal converters (indicated by the red arrow) needs to be increased significantly to make up for the shortfall.

[0030] Furthermore, the total power transmitted to the AC grid through the terminals (including both positive and negative VSCs) may be lower than the level before the VSC tripped. Changes in total transmitted power can negatively impact the stability of the AC grid. When the HVDC system, which serves as the backbone transmission line in the grid, shuts down or experiences sudden fluctuations in transmitted power, it can trigger a risk of frequency descent. This risk can spread to other power plants and equipment in the grid, potentially leading to widespread blackouts in the worst-case scenario. Therefore, bipolar MTDC systems must reduce sudden increases or decreases in transmitted power to mitigate the negative impact on AC system stability during unipolar tripping.

[0031] 2. Establish a control mode switching criterion based on local DC voltage monitoring, determine the constant voltage control role of each converter according to the preset priority order and limit each pole to retain only one constant voltage control converter, and set the DC voltage threshold for triggering mode switching.

[0032] Specifically, in the proposed method, only one converter in each of the positive and negative networks uses a constant DC voltage. V dc The system operates in constant active power (P) control mode to prevent overvoltage or voltage drop. All other converters in the MTDC system operate in constant active power (P) control mode to maintain sufficient DC power flow. This mode-switching method aims to rapidly switch the converter operating mode (constant P) to prevent overvoltage or voltage drop. V dc (Control mode and constant P control mode) can stabilize the MTDC system even in the event of a converter trip.

[0033] Use subscripts i ( i =1,2,... N ) represents the terminal number of the MTDC system ( T iThe operating mode of each VSC in steady state is determined by the transmission system operator (TSO). Constant DC voltage controlled converters are selected according to a preset priority order, which is determined by the TSO based on the rated capacity of the VSC and the size of the AC grid. This embodiment assumes the positive VSC (VSC...) j +) and negative electrode VSC (VSC) j Under steady-state conditions, all systems operate in a constant DC voltage control mode. In other words, T j These converters are configured as the highest priority terminals.

[0034] If VSC j +with VSC j If operation stops, the DC voltage of the bipolar MTDC system will not be able to remain stable. At this time, the other converter will switch to constant voltage response based on the DC voltage deviation. V dc Control mode. Specifically, when the DC voltage... Exceeding the variable threshold calculated by formula (1) At that time, the mode switching mechanism is triggered. Note: Superscript m Poles: In a positive pole network m = p In the negative polarity network m = n .

[0035] (1) In the formula, For polarity m ( m = p Indicates the positive electrode. m = n When representing the negative pole, the first in the priority sequence j +1 variable DC voltage threshold for candidate terminals; The DC voltage reference value of the (j+1)th candidate terminal is calculated based on the bipolar VSC-MTDC system circuit equations, assuming that all terminal converters are operating normally. α This is the DC voltage margin constant, used to suppress frequent mode switching caused by minor measurement fluctuations. Its value is preset by the transmission system operator (TSO), generally satisfying α≠0 and α≤0.1; subscript j This indicates the terminal number currently responsible for constant DC voltage control. T j ; Subscript j +1 indicates that it is ranked in the priority list of constant DC voltage controlled converters. T j Afterwards, it is planned to beT j Exiting runtime, the next priority terminal to take over the constant DC voltage control responsibility, the subscripts and superscripts on both sides of the equals sign. m , j The +1 maintains the same meaning.

[0036] 3. Introduce a mode switching malfunction detection mechanism. After the mode switching action, the status of the original constant pressure control converter is confirmed through communication. If it is a malfunction, the converter after switching will be returned to the original control mode.

[0037] In this embodiment, a mode switching malfunction detection mechanism is introduced. After a mode switching action, the state of the original constant voltage control converter is confirmed via communication. If the malfunction is found, the switched converter is instructed to return to the original control mode. Specifically: In this invention, the converter mode switching mechanism is mainly designed for the scenario where the converter responsible for constant DC voltage control is out of operation. When only the converter with constant active power control trips, while the converter with constant DC voltage control on that pole continues to operate normally, the system DC voltage is maintained by the original constant voltage converter. At this time, the mode switching mechanism is not triggered, and the active power commands of each terminal are redistributed only through the subsequent inter-pole active power dynamic scheduling strategy.

[0038] Despite constant V dc The controlled converter operates stably, but the measured DC voltage may fluctuate due to output fluctuations from the offshore wind farm and other factors. DC voltage reference value Significant deviations may trigger the proposed mode-switching method. In such cases, if the constant is used... V dc Any cause other than a tripped converter that triggers the proposed mode switching method is considered a fault. Therefore, this mode switching method is equipped with a fault determination mechanism to address the possibility of faults caused by DC voltage fluctuations (as shown in the attached diagram). Figure 3 (As shown by the red rectangle). According to the flowchart, in VSC (j+1) +with VSC (j+1) After completing the mode switch, the second priority converter for mode switching confirms the VSC by communicating with the central controller. j + and VSC j It is running normally. If you confirm VSC... j + and VSC j If it runs normally, then VSC is considered to be functioning correctly. (j+1) +with VSC (j+1) The mode switching is a fault, and the operating state of these converters will be restored to constant P control mode.

[0039] Appendix Figure 3 Demonstrated targeting N The proposed mode switching algorithm for the bipolar VSC-MTDC system considers the case of unipolar tripping. (See attached...) Figure 3 As shown, the mode switching and malfunction detection process includes the following steps, with each step connected sequentially in terms of signals and control quantities.

[0040] 1) Parameter initialization and priority setting: The transmission system operator (TSO) pre-inputs the rated capacity of each terminal converter, the DC voltage reference value, and the mode switching priority list for the positive and negative poles, and simultaneously sets the margin parameter for the variable DC voltage threshold. α The output of this step is: the priority sequence of the constant voltage control terminal for each pole network, the corresponding DC voltage reference value, and the threshold calculation parameters used for subsequent comparisons.

[0041] 2) Local Measurement and Data Acquisition: During system operation, the local controller of each terminal acquires the DC voltage of its respective pole network in real time. (where m represents polarity,) i (Indicates the terminal number), and sends the measurement results to the mode switching criterion calculation module. The input of this step is the local electrical quantity such as the DC voltage of each terminal, and the output is the real-time DC voltage measurement value used for subsequent threshold judgment.

[0042] 3) Variable threshold calculation: Assuming all converters are operating normally, calculate the DC voltage reference value of the current candidate constant voltage terminal based on the bipolar MTDC circuit equation. And obtain the corresponding variable threshold according to formula (1). The inputs for this step are circuit parameters, topology information, and the TSO's preset margin. α The output is the same as the terminal. T j+1 The corresponding variable voltage threshold provides a basis for subsequent mode switching judgment.

[0043] 4) Mode switching trigger determination: The real-time measured DC voltage will be used as the trigger. With threshold Comparison. When When the threshold range is exceeded, a "trigger mode switching" logic signal is output; if the threshold is not exceeded, the existing control mode remains unchanged. The inputs to this step are the measured voltage output in step 2) and the voltage threshold output in step 3), and the output is the mode switching trigger flag.

[0044] 5) Mode switching execution: When the trigger flag is "Yes", the controller will switch the original constant mode according to the priority list.V dc Controlled converter VSC j Switch to constant P control mode, and simultaneously switch the secondary priority converter VSC (j+1) Switch to constant V dc Control Mode. The inputs to this step are the mode switching trigger signal and the preset priority list, and the output is the new operating mode command for each terminal converter, which is sent to the corresponding converter through the terminal communication network.

[0045] 6) Malfunction Detection Trigger and Status Confirmation: After the mode switch is completed, the malfunction detection process begins (see attached diagram). Figure 3 (The red rectangle in the middle). The control center communicates with the original constant voltage control converter VSC via the communication network. j +、VSC j Send a status query request and receive the returned running status information. The input to this step is the mode switch execution signal, and the output is the response to the VSC. j The result of determining the current working status.

[0046] 7) Malfunction detection and mode recovery / maintenance: If communication confirms VSC j If the system is still operating normally, then the mode switch is considered a malfunction, and a request is sent to VSC. (j+1) Issue a recovery command to change its operating mode from constant. V dc Control is restored to constant P control, while VSC is maintained. j Continue to perform constant voltage control; if VSC is confirmed. j If the program has exited, the mode switch is considered a valid action, and VSC is maintained. (j+1) constant V dc The system controls the mode and locks it in place using self-holding logic until a recovery signal is received. The input to this step is the status confirmation result, and the output is the final effective control mode combination, thus completing a full mode switching and malfunction detection process.

[0047] 4. A dynamic scheduling algorithm for active power between poles is proposed. By introducing a power guarantee coefficient and a weighting coefficient that considers the rated capacity of the converter, the active power command of the healthy pole converter is calculated and updated to realize the transfer of power from the faulty pole to the healthy pole.

[0048] The objective of the active power dispatch control method for bipolar MTDC systems is to regulate the active power of the VSC (Voltage Control System) by considering the inter-pole power distribution between the faulty and normal pole networks, thereby solving the power overload problem caused by unipolar tripping. In the proposed method, the active power reference values ​​for the positive and negative converters under constant P control are... and The results are obtained by formulas (2)-(4) respectively.

[0049] (2) (3) (4) In the formula, the subscript l (l = 1, 2, ..., N, l ≠ j) represents the terminal number Tl using constant active power control. This represents the total number of terminals in the bipolar VSC-MTDC system; the superscript m indicates the polarity, where m=p represents the positive network and m=n represents the negative network. This is the reference value for the active power of terminal Tl during steady-state operation. This is the reference value for the DC voltage of the corresponding polarity network. The actual DC voltage measured at terminal Tl; This is the active power reference value updated according to the proposed scheduling algorithm after a unipolar tripping event. This refers to the increase in active power allocated to terminal Tl due to a faulty pole tripping. The active power correction amount allocated to terminal Tl after taking into account the capacity margin of the constant voltage control converter. This represents the active power output of the faulty pole VSC before tripping; physical quantities with the same subscripts and superscripts on both sides of the equal sign maintain consistency in polarity and terminal numbering. This is a single-pole trip flag variable; when the terminal Ti converter with polarity m is operating normally... When the converter trips in this pole network ; Let be the active power – DC voltage (P-Vdc) droop control coefficient at terminal Tl on pole m, and ≠0; The power guarantee factor is a pre-set value by the transmission system operator (TSO) to adjust the power transfer ratio from the faulty pole to the healthy pole, satisfying 0 ≤ ≤ 1. When When this occurs, it indicates a single-pole tripping at the terminal Tl. At this time, the active power reference value of the constant P-controlled converter in the healthy pole, unaffected by DC faults or routine maintenance, is... Based on , as well as Update.

[0050] also, The weighting coefficient related to the transmission power scale of terminal Tl under unipolar tripping condition when polarity is m is calculated according to formula (5) based on the rated capacity of each terminal VSC, satisfying the following conditions. Furthermore, equation (6) provides the normalization constraint condition; where the numerator is the rated capacity of terminal Tl at pole m. The denominator is the weighted sum of the rated capacities of all terminals involved in single-pole tripping. When l≠j and hour, Used to distribute active power transferred from a faulty pole among multiple terminals according to the rated capacity ratio.

[0051] (5) (6) (7) In the formula, When the polarity is m, the terminal T i The rated active power capacity of the connected converter, and the actual output during normal operation Must meet ; For a constant voltage control converter VSC with polarity m j Capacity utilization-related indicator variables, when A value of 0 indicates that the converter's output power has not yet reached 90% of its rated capacity; when... When the value is 1, it means that the converter is close to the rated output limit. At this time, the power needs to be distributed to other terminals through the power dispatch mechanism shown in formula (4)–(6), thereby limiting the overload risk of VSCj and realizing the power redistribution within the healthy pole.

[0052] When the proposed emergency operation and active power dispatch control are activated, the self-holding circuit starts working and maintains this operating mode until a return signal from the VSC is received, thereby avoiding oscillation. By updating the active power reference value of the healthy pole VSC according to equations (2)-(7), the total power fluctuation transmitted from the terminal to the AC grid before and after the VSC trips can be reduced. Therefore, during DC faults, this active power dispatch mechanism implemented between the healthy pole and the faulty pole network can effectively mitigate the impact on the stability of the AC grid.

[0053] In this embodiment, a "faulty pole" refers to the DC polarity network on the side where a single-pole converter trips. For example, when a terminal converter on the positive pole trips, the entire positive pole is considered a faulty pole, and the corresponding DC polarity network on the side that did not trip during the fault event is defined as a "healthy pole." It should be noted that a "faulty pole" only indicates that the power transmission capability of that pole is weakened, and does not mean that all converters on that pole have failed; subsequent inter-pole active power scheduling only compensates for the power imbalance caused by the fault event.

[0054] As attached Figure 4As shown, the overall process of the emergency control method for the bipolar flexible DC transmission system in this embodiment includes the following stages: 1) System initialization phase: First, based on the established bipolar VSC-MTDC system topology, read the rated capacity, DC voltage reference value, and inter-pole connection relationship of each terminal converter; simultaneously, input the mode switching priority list and variable threshold margin from the TSO. α and power guarantee factor These parameters are recorded. The output of this stage provides the system structure data and global control parameters required for subsequent control.

[0055] 2) Real-time monitoring and fault detection phase: During normal system operation, the local controllers of each terminal continuously collect operating data such as DC voltage, DC current, and active power of the positive and negative networks, and generate a single-pole trip flag through fault detection logic. The input for this stage is real-time measurement data, and the output is the operating status flags of each pole and terminal, providing trigger signals for mode switching and power scheduling.

[0056] 3) Mode switching judgment and execution phase: When it is detected that the constant voltage control converter of a certain pole may exit operation or the DC voltage deviates from the reference value, the following steps are called: Figure 3 The mode switching algorithm shown first calculates the reference voltage based on the circuit equations. And generate variable thresholds Then compare the real-time measured DC voltage The system uses a threshold value to determine whether to initiate a mode switch. If the condition is met, a new operating mode command is output, switching the alternative converter to constant voltage control. The inputs for this stage are real-time measurement values ​​and parameters given in the initialization stage, and the output is the currently active operating mode for each terminal.

[0057] 4) Malfunction Detection and Mode Confirmation Phase: After mode switching is executed, the central controller communicates with each terminal to verify the actual operating status of the original constant pressure control converter. If the original converter is confirmed to be operating normally, a "mode switching malfunction" judgment result is output, and the process is initiated accordingly. Figure 3 The process restores the converter mode to the configuration before the fault; if it is confirmed that the original converter has tripped, it outputs a "mode switching valid" result, keeping the new constant voltage control configuration unchanged. The output of this stage is the final mode combination after malfunction verification.

[0058] 5) Active power dispatch determination phase: After the mode switch takes effect, monitor the output power of the stable constant voltage control converter. And according to its rated capacity Relationship update marker When a healthy polarity converter is detected to be close to or exceeding its rated capacity (e.g., When the mode switch occurs, the inter-pole active power dynamic scheduling algorithm is triggered. The inputs are the operating status after the mode switch and real-time power data, and the output is a logical flag indicating whether power scheduling needs to be initiated.

[0059] 6) Inter-pole active power scheduling execution stage: According to the inter-pole active power scheduling algorithm, the power increment of each constant P control converter in the normal pole is calculated using formulas (2)–(4). Capacity-related increments And combined with the power guarantee factor and terminal weight coefficient Update its active power reference value This achieves a smooth power transfer from the faulty pole to the healthy pole. The input for this stage is a single-pole trip flag. Capacity utilization sign The system operation data is output as the updated active power command for each terminal.

[0060] 7) Self-holding and Exit Phase: During emergency control and power dispatch activation, the current operating mode and power commands are maintained through self-holding logic to prevent frequent switching until a return signal indicating that the converter has resumed operation or an exit command is issued by maintenance personnel. At this point, the controller gradually releases emergency control according to a preset strategy, returning to the normal operating mode, and the overall process ends. The inputs to this phase are the recovery signal and the current control state, and the output is the steady-state operating condition after system recovery.

[0061] Corresponding to the above-disclosed emergency control method for a bipolar flexible DC transmission system based on mode switching, this invention also discloses an emergency control device for a bipolar flexible DC transmission system based on mode switching, which specifically includes: The real-time monitoring module is used to monitor the DC voltage and converter operating status in real time, and to detect faults. The mode switching module is used to activate a pre-established converter mode switching mechanism if a single-stage converter trips or DC voltage exceeds the limit. According to the preset constant DC voltage control converter priority list, the module automatically switches the operating mode of the next priority alternative converter in the tripped converter to the constant DC voltage control mode. The malfunction detection module is used to activate a pre-established malfunction detection mechanism after the working mode switch is completed. It checks the actual status of the original converter that was judged to have tripped through the communication network. If it is confirmed that the original converter is still operating normally, it determines that the switch is a malfunction and immediately orders the alternative converter that has undergone mode switch to return to its original working mode. The power redistribution module is used to determine that the mode switch is effective if the original converter has been confirmed to have tripped. It then initiates a pre-established dynamic active power allocation strategy between poles. By introducing a specific power allocation coefficient and taking into account the capacity of each converter, it automatically calculates and updates the active power command of the viable pole converter to complete the power redistribution between poles and achieve steady-state operation.

[0062] It should be noted that for a detailed description of the emergency control system for a bipolar flexible DC transmission system based on mode switching provided in the embodiments of the present invention, please refer to the relevant description of the emergency control method for a bipolar flexible DC transmission system based on mode switching provided in the embodiments of the present invention, which will not be repeated here.

[0063] In addition, embodiments of the present invention also provide an electronic device, the device comprising: a processor and a memory; the memory being used to store one or more program instructions; the processor being used to execute one or more program instructions to perform the steps of an emergency control method for a mode-switching-based bipolar flexible DC transmission system as described in any of the preceding embodiments.

[0064] It should be noted that for a detailed description of an electronic device provided in the embodiments of the present invention, please refer to the relevant description of an emergency control method for a bipolar flexible DC transmission system based on mode switching provided in the embodiments of this application, which will not be repeated here.

[0065] In addition, embodiments of the present invention also provide a computer-readable storage medium storing a computer program, which, when executed by a processor, implements the steps of an emergency control method for a mode-switching bipolar flexible DC transmission system as described in any of the preceding claims.

[0066] It should be noted that for a detailed description of the computer-readable storage medium provided in the embodiments of the present invention, please refer to the relevant description of the emergency control method for a bipolar flexible DC transmission system based on mode switching provided in the embodiments of this application, which will not be repeated here.

[0067] The above examples illustrate the present invention only to aid in understanding it and are not intended to limit the scope of the invention. Those skilled in the art can make various simple deductions, modifications, or substitutions based on the principles of this invention.

Claims

1. An emergency control method for a bipolar flexible DC transmission system based on mode switching, characterized in that, The method includes: Real-time monitoring of the DC voltage of the positive and negative polarity networks and the operating status of the converter; If a single-pole constant DC voltage control converter trips or the DC voltage of its pole network exceeds a preset threshold due to the constant DC voltage control converter exiting operation, a pre-established converter mode switching mechanism is activated. According to the preset priority list of constant DC voltage control converters for each pole, the original constant DC voltage control converter is automatically switched to constant active power control mode, and the converter with the next priority is switched to constant DC voltage control mode. In this case, the positive and negative pole networks establish independent constant DC voltage control converter priority lists, which are used to set the constant voltage control takeover sequence within their respective pole networks. After the mode switch is completed, the pre-established mode switch malfunction detection mechanism is activated. The actual operating status of the original constant DC voltage control converter is checked through the communication network. If it is confirmed that the original constant DC voltage control converter is still operating normally, the mode switch is determined to be a malfunction, and an order is immediately issued to make the converter that has undergone mode switch return to its original mode. If it is confirmed that the original constant DC voltage control converter has been taken out of operation, the mode switch is considered to be effective, and the pre-established inter-pole active power dynamic allocation strategy is activated. By introducing a specific power allocation coefficient and comprehensively considering the capacity of each converter, the active power command of the healthy pole converter is automatically calculated and updated to realize the transfer of power from the faulty pole to the healthy pole.

2. The emergency control method for a bipolar flexible DC transmission system based on mode switching as described in claim 1, characterized in that, The converter mode switching mechanism specifically includes: Only one constant DC voltage controlled converter is retained in each pole network. That is, only one converter in each positive or negative pole network operates in constant DC voltage control mode, while all other converters in the corresponding pole network operate in constant active power control mode.

3. The emergency control method for a bipolar flexible DC transmission system based on mode switching as described in claim 2, characterized in that, The converter mode switching mechanism specifically includes: During the operation of the bipolar VSC-MTDC system, the DC voltage of each pole network is collected in real time by the local controller of each converter terminal. Where m represents polarity, m=p represents positive pole, m=n represents negative pole, and i represents the converter terminal number of the bipolar VSC-MTDC system. Each converter terminal contains a positive converter and a negative converter. Assume that the converter terminal T is currently responsible for constant DC voltage control. j Positive converter VSC j + and negative converter VSC j - Under steady-state conditions, both operate in a constant DC voltage control mode. To determine whether the next priority terminal needs to take over the constant voltage control, the next priority converter terminal T is first calculated based on the bipolar VSC-MTDC system circuit equations. j+1 DC voltage reference value of the polar network Then, based on the DC voltage reference value Calculate the value for terminal T j+1 Variable DC voltage threshold ; Then the real-time measured DC voltage With variable DC voltage threshold If a comparison is made, Exceed If this triggers a mode switch, the original constant DC voltage control converter will be switched to constant active power control mode, and the candidate converter will be switched to constant DC voltage control mode. Not exceeding If so, the original pattern remains unchanged.

4. The emergency control method for a bipolar flexible DC transmission system based on mode switching as described in claim 3, characterized in that, Based on DC voltage reference value Calculate the value for terminal T j+1 Variable DC voltage threshold Specifically, it includes: In the formula: When the polarity is m, the variable DC voltage threshold of the (j+1)th candidate converter in the priority sequence; This is the DC voltage reference value of the (j+1)th candidate converter calculated based on the bipolar VSC-MTDC system circuit equations, assuming all terminal converters are operating normally. α is the DC voltage margin constant, used to suppress frequent mode switching caused by small measurement fluctuations. Its value is preset by the transmission system operator (TSO). The subscript j+1 indicates that the converter terminal T has the highest priority in the constant DC voltage controlled converter priority list. j The next priority converter terminal number is then assigned, and the priority list of constant DC voltage controlled converters is pre-set by the transmission system operator (TSO).

5. The emergency control method for a bipolar flexible DC transmission system based on mode switching as described in claim 1, characterized in that, The mode switching malfunction detection mechanism specifically includes: The control center communicates with the original constant DC voltage control converter VSC via the communication network. j Send a status query request and receive the returned running status information; If communication confirms VSC j If the system is still operating normally, then the mode switch is determined to be a malfunction, and the system will then switch to the next lower priority converter VSC. (j+1) Issue a recovery command to VSC (j+1) The operating mode is restored from constant DC voltage control mode to constant active power control mode, while maintaining VSC. j Continue executing the constant DC voltage control mode; If communication confirms VSC j If the program has exited, then the mode switch is considered a valid action, and VSC will be maintained. (j+1) The constant DC voltage control mode is locked through self-holding logic until a recovery signal is received.

6. The emergency control method for a bipolar flexible DC transmission system based on mode switching as described in claim 1, characterized in that, The inter-pole active power dynamic allocation strategy specifically includes: Calculate the power increment of a robust polarity converter Capacity-related increments And combined with the power guarantee factor and converter terminal weighting coefficient Update its active power reference value This achieves a smooth power transfer from the faulty pole to the healthy pole, as shown in the following formula: In the formula: The subscript l, l = 1, 2, ..., N, l ≠ j, indicates the converter terminal number T that uses constant active power control. l ; The total number of converter terminals in a bipolar VSC-MTDC system; The superscript m indicates polarity; m=p represents a positive polarity network, and m=n represents a negative polarity network. For converter terminal T l Active power reference value during steady-state operation; For converter terminal T l The reference value of the DC voltage for the corresponding polarity network; For converter terminal T l The measured DC voltage of the corresponding polarity network; This is a reference value of active power obtained through adjustment after a unipolar tripping event occurs; For converter terminal T l The increase in active power allocated due to the tripping of the faulty pole; To further allocate power to the converter terminal T after considering the capacity margin of the constant DC voltage control converter. l The active power correction amount; This refers to the active power output of the faulty pole converter before tripping. For single-pole tripping flag variables, when the converter terminal T with polarity m... l When running normally When a trip occurs ; For converter terminal T l Active power-DC voltage droop control coefficient on polarity m, and ≠0; The power guarantee factor is a pre-set value by the transmission system operator (TSO) to adjust the power share transferred from the faulty pole to the healthy pole, satisfying 0 ≤ ≤ 1; when When, it indicates that the converter terminal T l When a single-pole trip occurs in the network of the affected pole, the active power reference value of the constant active power control converter in the healthy pole, which is not affected by DC faults or routine maintenance, is as follows: Based on , as well as Update; When the polarity is m, the converter terminal T l A weighting factor related to the transmission power scale under unipolar tripping conditions is used to distribute the active power transferred from the faulty pole among multiple converter terminals according to the rated capacity ratio.

7. The emergency control method for a bipolar flexible DC transmission system based on mode switching as described in claim 6, characterized in that, Weighting coefficient The calculation formula is as follows: In the formula: For converter terminal T l Rated active power capacity of the converter with polarity m; For converter terminal T i The rated active power capacity of the converter with polarity m, and its actual output during normal operation. Must meet ; For a constant DC voltage controlled converter VSC with polarity m j The capacity utilization-related indicator variable, when hour A value of 0 indicates VSC j The output power has not yet reached 90% of the rated capacity, when hour A value of 1 indicates VSC j The power output is nearing its rated limit. At this point, it is necessary to allocate some power to other converter terminals according to the power allocation mechanism, thereby limiting the VSC. j To mitigate overload risks and achieve robust power redistribution within the electrode.

8. The emergency control method for a bipolar flexible DC transmission system based on mode switching as described in claim 1, characterized in that, The method further includes: During the activation of emergency control and power scheduling, the current operating mode and power command are maintained without frequent switching through self-holding logic until a return signal indicating that the converter has resumed operation or an exit command issued by the maintenance personnel is received. At this time, the controller gradually releases the emergency control according to the preset strategy, restores to the normal operating mode, and the emergency control process ends.

9. An emergency control device for a bipolar flexible DC transmission system based on mode switching, characterized in that, The device includes: The real-time monitoring module is used to monitor the DC voltage and converter operating status in real time, and to detect faults. The mode switching module is used to activate a pre-established converter mode switching mechanism if a tripping of a single-pole constant DC voltage control converter is detected or the DC voltage of the pole network where the constant DC voltage control converter is located exceeds a preset threshold due to the constant DC voltage control converter exiting operation. According to the preset priority list of constant DC voltage control converters for each pole, the module automatically switches the original constant DC voltage control converter to constant active power control mode, and switches the converter of the next priority to constant DC voltage control mode. The positive and negative pole networks establish independent constant DC voltage control converter priority lists, which are used to set the constant voltage control takeover sequence within their respective pole networks. The malfunction detection module is used to activate a pre-established malfunction detection mechanism after the working mode switch is completed. It checks the actual status of the original converter that was judged to have tripped through the communication network. If it is confirmed that the original converter is still operating normally, it determines that the switch is a malfunction and immediately orders the alternative converter that has undergone mode switch to return to its original working mode. The power redistribution module is used to determine that the mode switch is effective if the original converter has been confirmed to have tripped. It then initiates a pre-established dynamic active power allocation strategy between poles. By introducing a specific power allocation coefficient and taking into account the capacity of each converter, it automatically calculates and updates the active power command of the viable pole converter to complete the power redistribution between poles and achieve steady-state operation.

Images