A non-blocking type DC fault ride-through control method for hybrid three-terminal DC power transmission system

Abstract

The present application relates to the technical field of high-voltage direct current transmission, and particularly relates to a non-blocking type direct current fault ride-through control method for a hybrid three-terminal direct current transmission system; comprising the following steps: S1, performing sub-module proportioning design on the hybrid three-terminal direct current transmission system; S2, performing fault detection and triggering, and when the triggering condition is met, performing S3; S3, performing hierarchical fault control strategy, including mode conversion of the LCC converter station and non-blocking control of the FHMMC converter station; S4, when the fault clearing condition is met, resetting the mode conversion of the LCC converter station and stopping the regulation of the DC current size of the FHMMC converter station by the current converter inhibitor. The technical problems of poor fault resistance and operation economy of the LCC-FHMMC hybrid three-terminal direct current transmission system are solved.

Description

Technical Field

[0001] This invention relates to the field of high voltage direct current (HVDC) transmission technology, and in particular to a non-blocking DC fault ride-through control method for a hybrid three-terminal DC transmission system. Background Technology

[0002] The LCC-FHMMC hybrid three-terminal DC transmission system, combining the advantages of LCC's large capacity and long-distance transmission with MMC's flexible control and reactive power support capabilities, has become a core technology for the inter-regional consumption of clean energy. While overhead lines are commonly used as transmission channels, they are susceptible to lightning strikes and flashovers, leading to frequent faults such as single-pole grounding and bipolar short circuits on the DC side. These fault currents rise rapidly and are highly destructive, seriously threatening the stable operation of the system.

[0003] The MMC-locked DC fault ride-through scheme aims to quickly clear DC faults by shutting down all power electronic switching devices in submodules with DC fault self-clearing capabilities. However, during the MMC lockout period, the submodule is in an uncontrolled state, causing capacitors to continue storing energy and resulting in increased capacitor voltage. Uncontrolled DC current leads to the failure of reactive power compensation and interruption of active power transmission, both of which significantly impact the system and hinder system restart and recovery.

[0004] Existing DC fault handling solutions have significant drawbacks: (1) Isolation scheme based on circuit breaker: AC circuit breaker has a slow response (requires 2-3 power frequency cycles) and is prone to expanding the fault range; DC circuit breaker (mechanical, solid-state, hybrid) has problems such as difficulty in extinguishing arc, high cost and complex structure, and is difficult to adapt to high voltage and large capacity scenarios.

[0005] (2) MMC-blocked fault ride-through scheme: After a fault, the MMC submodule is blocked. Although the fault current can be cut off, the uncontrolled charging and discharging of the submodule capacitor causes a sudden voltage rise, interruption of active power transmission, failure of reactive power compensation capability, difficulty in restarting the system, and significant impact on the power grid.

[0006] (3) Traditional non-blocking fault ride-through scheme: It relies on the negative voltage output of the full bridge submodule (FBSM) to suppress the fault current. However, the on-state loss of FBSM is twice that of half bridge submodule (HBSM). The existing scheme does not optimize the ratio of FBSM to HBSM, resulting in high system loss and high cost. At the same time, the fault current suppression speed is slow and the voltage balance of the submodule capacitor is poor, making it difficult to balance fault ride-through and steady-state economy. Summary of the Invention

[0007] The purpose of this invention is to provide: A non-blocking DC fault ride-through control method for a hybrid three-terminal DC transmission system, and related technologies, are proposed to address technical problems such as the inability to simultaneously achieve rapid suppression of fault currents underground, optimization of submodule cost losses, and uninterrupted system operation, resulting in poor fault resilience and operational economy of the LCC-FHMMC hybrid three-terminal DC transmission system, or a combination thereof.

[0008] Terminology Explanation: Unless otherwise defined, all technical terms herein have the same meanings as commonly understood by one of ordinary skill in the art to which this subject matter pertains. Unless otherwise stated, all patents, patent inventions, and disclosures referenced throughout this invention are incorporated herein by reference in their entirety. If multiple definitions exist for terms in this invention, the definitions in this chapter shall prevail.

[0009] It should be understood that the above brief description and the following detailed description are exemplary and for illustrative purposes only, and do not limit the subject matter of the invention in any way. In this invention, the singular is used in conjunction with the plural unless otherwise specifically stated. It should also be noted that, unless otherwise stated, the use of “or” or “or” means “and / or”. Furthermore, the use of the term “comprising” and other forms such as “including,” “containing,” and “contains” are not limiting.

[0010] The definition of the standard terminology can be found in the reference "Analysis and Application of DC Transmission Control and Protection System".

[0011] Unless otherwise stated, conventional methods within the scope of the art, such as fault handling based on converter control, shall be used.

[0012] Unless specifically defined, the use of various commercially available products in this invention employs standard techniques. The techniques and methods described herein can generally be implemented according to conventional methods well known in the art, based on the descriptions in the numerous summary and more specific documents cited and discussed in this specification.

[0013] The terms “optional / arbitrary” or “optionally / arbitrarily” mean that the event or situation described below may or may not occur, including both the occurrence and non-occurrence of the event or situation.

[0014] The term "LCC converter station" as used in this article refers to a converter that uses commutation voltage provided by an AC system for commutation.

[0015] In a first aspect, the present invention provides:

[0016] This invention provides a non-blocking DC fault ride-through control method for a hybrid three-terminal DC transmission system, comprising the following steps: S1. Design the sub-module ratio for the hybrid three-terminal DC transmission system, which includes an LCC converter station and an FHMMC converter station; S2. Perform fault detection and triggering. When the triggering conditions are met, execute step S3. S3. Implement a hierarchical fault control strategy, including mode switching for the LCC converter station and non-blocking control for the FHMMC converter station; in the non-blocking control of the FHMMC converter station, use a converter suppressor to adjust the magnitude of the DC current of the FHMMC converter station. S4. When the fault clearing conditions are met, control the mode switching of the LCC converter station to reset and stop the converter suppressor from adjusting the DC current of the FHMMC converter station.

[0017] The FHMMC converter station includes two MMC converter stations, namely MMC1 and MMC2. Each MMC converter station includes a three-phase FHMMC bridge arm, and each phase of the FHMMC bridge arm is composed of a hybrid HBSM and FBSM. The LCC converter station is connected to the MMC1 converter station, and the MMC1 converter station is connected to the MMC2 converter station via overhead lines. The LCC converter station, MMC1 converter station and MMC2 converter station are all equipped with converter transformers and grounding electrodes. The LCC converter station is equipped with AC filters and DC filters.

[0018] In step S3, during the mode switching of the LCC converter station, the control trigger angle is uniformly accelerated from the steady state of 15° to 150°, so that the LCC converter station enters the inverter operation state; at the same time, when the DC current decays to 0, the arc is extinguished.

[0019] In step S3, the specific method for adjusting the DC current of the FHMMC converter station using a converter suppressor is as follows: The actual value and reference value of the dq-axis component of the circulating current inside the MMC converter station are subtracted, and then the result is processed through a PI circuit and dq / abc transformation to obtain the reference value of the three-phase voltage drop. Three-phase voltage drop reference value Also the output of the commutator suppressor, in Add voltage to the base The DC current of the FHMMC converter station is adjusted by summing the results. Size.

[0020] Among them, the DC current of the FHMMC converter station With voltage The relationship is represented as: ; In the above formula, This represents the equivalent reactance on phase A bridge arm. This represents the resistance on phase A bridge arm.

[0021] In this configuration, the total number of submodules in each bridge arm of the MMC converter station is N, and the number of HBSMs is [missing information]. The number of FBSMs is , It simultaneously meets the requirements for fault ride-through and reduced-voltage operation.

[0022] Among them, under the fault ride-through requirement The voltage reduction requirement must not be lower than 50%. Therefore, when When both fault ride-through and step-down operation requirements are met, the value is 1.

[0023] The triggering condition in step S2 is: the duration of the fault characteristic is greater than or equal to 2ms.

[0024] The fault characteristics include single-pole grounding faults and double-pole short-circuit faults. A single-pole grounding fault is characterized by a sudden drop in DC voltage to 0 and a rise in voltage to twice the rated value of the other pole, with DC power fluctuations exceeding 10% of the rated value. A double-pole short-circuit fault is characterized by a sudden drop in DC voltage to 0 for both the positive and negative poles and a sudden rise in DC current exceeding twice the rated value.

[0025] The fault clearing condition in step S4 is: when the DC voltage is detected to rise back to more than 90% of the rated value, and the DC current stabilizes at the rated value. Within range Compared with the prior art, the beneficial effects of the present invention are as follows: This invention features non-blocking operation to avoid power interruption: During a fault, the FHMMC is not blocked, the submodule remains controllable, active power transmission experiences only brief fluctuations (recovering within 50ms after fault clearance), and reactive power compensation is continuously maintained, significantly reducing the impact on the power grid; rapid fault current suppression: The LCC quickly shifts to inverter mode (150° firing angle), and the FHMMC, through the superposition of the CCSC voltage and the negative voltage output of the FBSM, suppresses the fault current to zero within 10ms, preventing overcurrent damage to IGBTs and other devices; optimized submodule ratio for high economic efficiency: the ratio of FBSM to HBSM is... The 1:1 ratio satisfies both fault ride-through and step-down operation requirements while minimizing FBSM usage. Compared to the full FBSM solution, system losses are reduced by 40%-50%, and costs are reduced by 30%-35%. It is adaptable to multiple fault types: it can effectively handle typical faults such as DC-side single-pole grounding and bipolar short circuits, and the fault location (LCC outlet, MMC outlet, line mid-section) does not affect the control effect, demonstrating strong adaptability. It offers rapid recovery and high reliability: after fault clearance, there is no need for a complex restart process; the system automatically switches back to steady-state control with a short recovery time (<100ms), improving the reliability of power grid supply. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the system of the present invention.

[0027] Figure 2 This is a DC current control circuit diagram of the LLC converter station of the present invention.

[0028] Figure 3 This is a schematic diagram of the CCSC control principle of the present invention.

[0029] Figure 4 This is a schematic diagram of the voltage reference value generation of the sub-module of the MMC converter station of the present invention.

[0030] Figure 5 This is a schematic diagram of the non-blocking fault ride-through control strategy for the FHMMC converter station of the present invention.

[0031] Figure 6 This is a schematic diagram of the DC voltage change of the LCC converter station after simulation according to the present invention.

[0032] Figure 7 This is a schematic diagram of the DC current change of the LCC converter station after simulation according to the present invention.

[0033] Figure 8 This is a schematic diagram of the DC voltage change of the MMC1 converter station after simulation according to the present invention.

[0034] Figure 9 This is a schematic diagram of the DC current change of the MMC1 converter station after simulation according to the present invention.

[0035] Figure 10 This is a schematic diagram of the DC voltage change of the MMC2 converter station after simulation according to the present invention.

[0036] Figure 11 This is a schematic diagram of the DC current change of the MMC2 converter station after simulation according to the present invention.

[0037] Figure 12 This is a schematic diagram illustrating the change in the firing angle of the LCC converter station after simulation according to the present invention.

[0038] Figure 13 This is a schematic diagram of the DC power change of the LCC converter station after simulation according to the present invention.

[0039] Figure 14 This is a schematic diagram of the DC voltage change of the MMC1 converter station after simulation according to the present invention.

[0040] Figure 15 This is a schematic diagram of the DC voltage change of the MMC2 converter station after simulation according to the present invention.

[0041] Figure 16This is a schematic diagram illustrating the change in reactive power of the inverter station after simulation according to the present invention. Detailed Implementation

[0042] The technical solution of the present invention will be clearly described below with reference to the accompanying drawings. Obviously, the described embodiments are not all embodiments of the present invention. All other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0043] Example 1 like Figure 1 As shown, this invention provides a non-blocking DC fault ride-through control method for a hybrid three-terminal DC transmission system, comprising the following steps: S1. System topology and submodule configuration design: The system topology includes an LCC converter station and an FHMMC converter station. The FHMMC converter station includes two MMC converter stations, namely MMC1 and MMC2. Both MMC1 and MMC2 include three-phase FHMMC bridge arms, and each phase of the FHMMC bridge arm is composed of a hybrid connection of HBSM and FBSM. The LCC converter station is connected to the MMC1 converter station, and the MMC1 converter station is connected to the MMC2 converter station via overhead lines. The LCC converter station, MMC1 converter station, and MMC2 converter station are all equipped with converter transformers and grounding electrodes. The LCC converter station is equipped with AC filters and DC filters.

[0044] Design the configuration of each submodule in the MMC converter station, setting the total number of submodules in each bridge arm of the MMC converter station to N, where the number of HBSMs is [missing information]. The number of FBSMs is ,make The following two requirements must be met: (1) Fault ride-through requirements must be met; (2) Reduced-voltage operation requirements must be met.

[0045] (1) For meeting the fault ride-through requirements The following analysis yields the proportional configuration of the hybrid submodules for fault ride-through conditions. Taking phase A as an example, the phase A AC voltage and AC current of the MMC converter station are expressed as follows: (1) In the above formula, This represents the instantaneous AC voltage of phase A. This represents the instantaneous alternating current of phase A. This indicates the amplitude of the AC voltage in phase A. This indicates the amplitude of the A-phase current. Indicates the power frequency angular frequency. t represents the initial phase angle and t represents time.

[0046] The AC modulation ratio of the MMC converter station in steady state is obtained. ,in , is represented as: (2) (3) In the above formula, Indicates DC voltage. This represents the capacitor voltage.

[0047] Based on the circuit equations derived from KVL, the output voltages of the upper and lower arms of phase A of each submodule are obtained, expressed as: (4) In the above formula, This indicates the output voltage of the upper bridge arm of phase A of submodule A. This indicates the output voltage of the lower bridge arm of submodule A.

[0048] Because FBSM has , It has three output levels: 0, 0, and 1. Its negative output capability allows the FHMMC containing FBSM to have overmodulation capability, enabling it to operate with reduced DC voltage. The DC modulation ratio is defined as... The control range of the DC modulation ratio depends on the number of FBSMs, therefore Represented as: (5) In the above formula, , This is expressed as the rated DC voltage.

[0049] Since the MMC converter station is three-phase symmetrical, taking phase A as an example, by introducing the DC modulation ratio and combining formulas (4) and (5), the output voltages of the upper and lower bridge arm sub-modules are obtained as follows: (6) when When the system operates at its rated voltage, the operating range of the upper and lower arm voltages of the MMC converter station is expressed as follows: (7) when At that time, among them This represents the minimum per-unit DC operating voltage allowed by the system, i.e., the operating range of the upper and lower arm voltages of the MMC converter station when the system operates at a reduced DC voltage. (8) By taking the union of formulas (7) and (8), the voltage operating range of the upper and lower arms of the MMC converter station is obtained, expressed as: (9) According to formula (9), when At this time, the MMC bridge arm needs to generate a negative voltage, which can only be generated by all the connected FBSMs in the bridge arm. When a single-pole ground fault occurs, the faulty pole... When a bipolar short-circuit fault occurs, From equation (4), the output voltages of the upper and lower bridge arms at this time are: (10) During DC fault ride-through, the HBSM must be bypassed, and the arm voltage can only be supplied through the FBSM. To mitigate the impact of the fault on the AC system, the MMC needs to provide reactive power compensation to ensure the normal operation of the AC grid. In this case, the AC voltage amplitude output by the MMC needs to remain the same as under normal operating conditions. The maximum arm voltage requirement needs to be met by the output voltage of the FBSM to ensure normal operation during DC fault ride-through. From equations (1), (2), (3), and (10), we can obtain: (11) In the above formula, This represents the maximum value function, where k takes values ​​of p and n, and j takes values ​​of n. .

[0050] Pick Then the proportion of FBSM in equation (11) It should meet the following requirements: (12) Therefore, the proportion of FBSM in FHMMC must not be less than 50% to meet the requirements for ride-through of DC faults without blocking.

[0051] (2) For meeting the requirements of reduced-pressure operation The following analysis is performed: When the system voltage is reduced to 70%-80% of the rated voltage, 50% of the FBSM can maintain the AC side voltage stability through modulation, avoiding the uncontrolled charging and discharging of the capacitor caused by the lack of zero crossing point of the bridge arm current in the HBSM. Therefore, the proportion of FBSM in the HMMC needs to be equal to 50%.

[0052] S2. Fault Detection and Triggering: When the triggering condition is met, the fault control strategy in step S3 is executed. The triggering condition is: when the duration of the fault characteristic is greater than or equal to 2ms, the non-blocking fault passage control logic is triggered. The fault characteristics include single-pole grounding fault and double-pole short-circuit fault. The single-pole grounding fault is: the DC voltage of one pole suddenly drops to 0, the voltage of the other pole rises to twice the rated value, and the DC power supply fluctuation exceeds 10% of the rated value. The double-pole short-circuit fault is: the DC voltage of both the positive and negative poles suddenly drops to 0, and the DC current suddenly rises to more than twice the rated value.

[0053] S3. Implement a hierarchical fault control strategy, including LCC converter station control and FHMMC converter station non-blocking control.

[0054] (1) Control of LCC converter station.

[0055] Converting the LCC converter station from "constant DC current control" to "fast phase-shifting inverter control" is as follows: Figure 2 As shown, this specifically includes: controlling the trigger angle. From a steady state of 15°, the temperature rises uniformly to 150°. This allows the LCC converter station to enter inverter operation mode and release DC side energy; by utilizing the unidirectional conduction characteristic of the thyristors in the LCC converter station, the arc will naturally extinguish when the DC current decays to 0, thus achieving self-clearing of fault current.

[0056] (2) Non-locking control of FHMMC converter station.

[0057] After the fault is triggered, the FHMMC converter station does not lock the sub-module, but achieves rapid cross-through control through active current limiting, thereby quickly limiting the DC short-circuit current to 0. The control achieves two objectives: (1) The control strategy is used to quickly suppress and clear the fault current, so as not to cause overcurrent, so that the MMC converter station does not exceed the overcurrent blocking value and achieves uninterrupted operation; (2) The MMC converter station does not lock, so that the energy of the FBSM circuit is controllable and the reactive power compensation capability continues to be maintained.

[0058] Specifically: such as Figure 3 As shown, in order to reduce the circulating current in the bridge arm within the MMC converter station, the MMC converter station needs to be equipped with a converter suppressor (CCSC) to reduce the power consumption within the converter station. This reduces the actual value of the dq-axis component of the circulating current within the MMC converter station. , The difference between the value and the reference value is then passed through a PI controller to obtain the d-axis common-mode voltage reference value output by the circulating current suppression controller. q-axis common-mode voltage reference value output by the current suppression controller Then, the reference value of the three-phase voltage drop is obtained through dq / abc transformation. (j=a,b,c); for example Figure 4As shown, the outer loop power control output and CCSC output of the MMC converter station are calculated and used to determine the voltage reference value of the MMC submodule. , Then, as Figure 5 As shown, when a fault signal is detected, the CCSC outputs... Superimposed layers can affect control voltage The switches in the MMC converter station are switched from normal operating mode to fault ride-through mode, and a new command voltage is generated by the PI controller. ,Will Replace the original By adjusting the DC current reference value of the converter station It can realize the DC current after a fault occurs. Precise control of the magnitude allows for rapid limitation of the DC fault current to zero after a system failure.

[0059] After adopting the above-mentioned non-blocking fault ride-through strategy, each LCC converter station and MMC converter station is in a non-blocking operation state during the entire fault ride-through period. Therefore, the voltage balance of their submodule capacitors is good, the submodules are in a controllable state, and the DC voltage output can be quickly restored by switching to the operation command before the fault. At the same time, the reactive power compensation function is basically unaffected during the fault.

[0060] and The following relationship must be satisfied: ; In the above formula, This represents the equivalent reactance on phase A bridge arm. This represents the resistance on phase A bridge arm.

[0061] S5. Fault Clearing and Recovery: When the fault clearing conditions are met, the firing angle of the LCC converter station is gradually reduced from 150° to a steady-state 15°, switching back to "constant DC current control mode". The FHMMC converter removes the CCSC function, the FBSM switches back to "positive connection / disconnection" state, the HBSM resumes normal switching, and the MMC1 and MMC2 converter stations switch back to "constant DC voltage" and "constant active power" control, respectively. The system power gradually increases and recovers to the pre-fault steady-state level within 50ms. The fault clearing condition is: when the DC voltage is detected to rise to more than 90% of the rated value, and the DC current stabilizes at the rated value. Within the range.

[0062] Example 2 Based on the background of the "Kunming-Liuzhou-Longyan ±800kV DC Transmission Project", a system was built on the PSCAD / EMTDC platform as follows: Figure 1The simulation model of the LCC-FHMMC hybrid three-terminal DC transmission system shown is configured with the following fault: at t=2s, a positive ground fault occurs at the beginning of line AB (LCC outlet), the fault transition resistance is 0.01Ω, and the fault lasts for 0.2s; the control results are as follows. Figure 6-16 As shown, this supports the effectiveness and feasibility of the control strategy of the present invention.

[0063] like Figure 6 , Figure 7 , Figure 12 , Figure 13 As shown, 0-10ms after fault triggering: the LCC switches from "constant DC current control" to "fast phase-shifting inverter control". 10ms after the fault: the positive voltage and current of the LCC drop to 0, while the negative voltage and current are unaffected. At this time, the LCC outputs 50% of its original power, and the LCC firing angle rises from 15° to 150°, entering inverter mode. After the fault is cleared, the voltage recovers to 800kV, the current recovers to 5kA, and the LCC firing angle falls back to 15°. Figure 6 , Figure 7 , Figure 12 , Figure 13 .

[0064] like Figure 8 , Figure 9 , Figure 10 , Figure 11 , Figure 14 , Figure 15 As shown, 0-10ms after fault triggering: the positive voltage and current of MMC1 and MMC2 drop to 0, and MMC1 and MMC2 initiate non-blocking control. Figure 5 (This is used to clear faults. After the fault is cleared, the positive voltage, current and power of MMC1 and MMC2 return to normal values.)

[0065] like Figure 16 As shown, this is used to verify that the system still has reactive power compensation capability under fault conditions: during the fault period, the reactive power of MMC1 and MMC2 is maintained at 400Mvar, and the AC side voltage distortion rate is <2%, which will not have a significant impact on the grid side and helps to maintain grid stability.

[0066] This invention features non-blocking operation to avoid power interruption: During a fault, the FHMMC is not blocked, the submodule remains controllable, active power transmission experiences only brief fluctuations (recovering within 50ms after fault clearance), and reactive power compensation is continuously maintained, significantly reducing the impact on the power grid; rapid fault current suppression: The LCC quickly shifts to inverter mode (150° firing angle), and the FHMMC, through the superposition of the CCSC voltage and the negative voltage output of the FBSM, suppresses the fault current to zero within 10ms, preventing overcurrent damage to IGBTs and other devices; optimized submodule ratio for high economic efficiency: the ratio of FBSM to HBSM is... The 1:1 ratio satisfies both fault ride-through and step-down operation requirements while minimizing FBSM usage. Compared to the full FBSM solution, system losses are reduced by 40%-50%, and costs are reduced by 30%-35%. It is adaptable to multiple fault types: it can effectively handle typical faults such as DC-side single-pole grounding and bipolar short circuits, and the fault location (LCC outlet, MMC outlet, line mid-section) does not affect the control effect, demonstrating strong adaptability. It offers rapid recovery and high reliability: after fault clearance, there is no need for a complex restart process; the system automatically switches back to steady-state control with a short recovery time (<100ms), improving the reliability of power grid supply.

[0067] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, and is not intended to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions made by those skilled in the art to the technical solution of the present invention do not depart from the essence and scope of the technical solution of the present invention.

Claims

1. A non-blocking DC fault ride-through control method for a hybrid three-terminal DC transmission system, characterized in that, Includes the following steps: S1. Design the sub-module ratio for the hybrid three-terminal DC transmission system, which includes an LCC converter station and an FHMMC converter station; S2. Perform fault detection and triggering. When the triggering conditions are met, execute step S3. S3. Implement a hierarchical fault control strategy, including mode switching for the LCC converter station and non-blocking control for the FHMMC converter station; in the non-blocking control of the FHMMC converter station, use a converter suppressor to adjust the magnitude of the DC current of the FHMMC converter station. S4. When the fault clearing conditions are met, control the mode switching of the LCC converter station to reset and stop the converter suppressor from adjusting the DC current of the FHMMC converter station.

2. The non-blocking DC fault ride-through control method for a hybrid three-terminal DC transmission system according to claim 1, characterized in that, The FHMMC converter station includes two MMC converter stations, namely MMC1 and MMC2. Each MMC converter station includes a three-phase FHMMC bridge arm, and each phase of the FHMMC bridge arm is composed of a hybrid HBSM and FBSM. The LCC converter station is connected to the MMC1 converter station, and the MMC1 converter station is connected to the MMC2 converter station via overhead lines. The LCC converter station, MMC1 converter station and MMC2 converter station are all equipped with converter transformers and grounding electrodes. The LCC converter station is equipped with AC filters and DC filters.

3. The non-blocking DC fault ride-through control method for a hybrid three-terminal DC transmission system according to claim 2, characterized in that, In step S3, during the mode switching of the LCC converter station, the control firing angle is uniformly accelerated from the steady-state 15° to 150°, so that the LCC converter station enters the inverter operation state; at the same time, when the DC current decays to 0, the arc is extinguished.

4. The non-blocking DC fault ride-through control method for a hybrid three-terminal DC transmission system according to claim 2, characterized in that, In step S3, the specific method for adjusting the DC current of the FHMMC converter station using a converter suppressor is as follows: The actual value and reference value of the dq-axis component of the circulating current inside the MMC converter station are subtracted, and then the result is processed through a PI circuit and dq / abc transformation to obtain the reference value of the three-phase voltage drop. Three-phase voltage drop reference value Also the output of the commutator suppressor, in Add voltage to the base The DC current of the FHMMC converter station is adjusted by summing the results. Size.

5. The non-blocking DC fault ride-through control method for a hybrid three-terminal DC transmission system according to claim 4, characterized in that, DC current of FHMMC converter station With voltage The relationship is represented as: ； In the above formula, This represents the equivalent reactance on phase A bridge arm. This represents the resistance on phase A bridge arm.

6. The non-blocking DC fault ride-through control method for a hybrid three-terminal DC transmission system according to claim 2, characterized in that, The total number of submodules in each arm of the MMC converter station is set to N, where the number of HBSMs is [missing information]. The number of FBSMs is , It simultaneously meets the requirements for fault ride-through and reduced-voltage operation.

7. The non-blocking DC fault ride-through control method for a hybrid three-terminal DC transmission system according to claim 6, characterized in that, Under fault-crossing requirements, The voltage reduction requirement must not be lower than 50%. Therefore, when When both fault ride-through and step-down operation requirements are met, the value is 1.

8. The non-blocking DC fault ride-through control method for a hybrid three-terminal DC transmission system according to claim 1, characterized in that, The triggering condition in step S2 is: the duration of the fault characteristic is greater than or equal to 2ms.

9. The non-blocking DC fault ride-through control method for a hybrid three-terminal DC transmission system according to claim 8, characterized in that, The fault characteristics include single-pole grounding faults and double-pole short-circuit faults. A single-pole grounding fault is characterized by a sudden drop in DC voltage to 0 and a rise in voltage to twice the rated value of the other pole, with DC power fluctuations exceeding 10% of the rated value. A double-pole short-circuit fault is characterized by a sudden drop in DC voltage to 0 for both the positive and negative poles and a sudden rise in DC current exceeding twice the rated value.

10. The non-blocking DC fault ride-through control method for a hybrid three-terminal DC transmission system according to claim 1, characterized in that, The fault clearing condition in step S4 is: when the DC voltage is detected to rise back to more than 90% of the rated value, and the DC current stabilizes at the rated value. Within the range.

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