Method and device for ac fault ride-through of hybrid dc transmission system

Abstract

The embodiment of the specification provides a hybrid direct current power transmission system receiving end AC fault ride-through method and device, wherein the method comprises: detecting the receiving end AC fault by using a protection device; after detecting the fault, starting a cooperative control strategy, the cooperative control strategy comprising an HBMMC station fault ride-through strategy based on bridge arm reference wave reconstruction and an LCC station fault ride-through strategy based on local measurement; after the fault is removed, switching the control mode of the receiving end flexible direct current converter station back to the constant direct current voltage mode in normal operation, and completing system reset. The characteristics of the reduction of AC voltage during the fault are used to make the direct current voltage self-adaptively reduce with the reduction of output active power, thereby reducing the overvoltage risk.

Description

Technical Field

[0001] The embodiments in this specification relate to the field of power system technology, and in particular to a method for AC fault ride-through at the receiving end of a hybrid DC transmission system. Background Technology

[0002] High-voltage direct current (HVDC) transmission offers excellent technical and economic advantages in terms of large capacity, long distance, and flexible transmission. A hybrid HVDC transmission system employing grid-commutated converters (LCCs) on the rectifier side and half-bridge modular multilevel converters (HBMMCs) on the inverter side can eliminate the commutation failure problem on the inverter side of traditional HVDC transmission and is also an effective way to resolve multiple DC feed-in issues. There are two technical approaches for the MMC side in LCC-MMC hybrid HVDC transmission systems: one is a hybrid MMC (FHMMC) composed of full / half-bridge submodules, and the other is a half-bridge MMC (HBMMC) composed of pure half-bridge submodules. To find high-performance and low-cost hybrid HVDC transmission systems, domestic and international scholars have conducted extensive research on DC fault handling in LCC-HBMMC systems. Some experts have pointed out that a high-power diode valve connected in series at the DC-side outlet of the HBMMC can be used to block fault current, and that this scheme is highly feasible because power flow reversal is generally not considered in West-to-East power transmission projects. Furthermore, some scholars have proposed an improved hybrid DC circuit breaker model suitable for this system and with lower cost through analysis of the characteristics of DC line short-circuit faults. Some scholars have also made valuable explorations in the fault nature identification of LCC-HBMMC, for example, by using LCC injection signals or proposing an adaptive restart strategy for DC faults based on the difference in LCC terminal voltage under permanent and transient fault conditions. Other literature has proposed a DC line fault location method based on the 6th harmonic component of electrical quantities during the AC feed-in stage of HBMMC in LCC-HBMMC fault location. The above research has laid a good foundation for the application of LCC-HBMMC systems. However, during faults in the AC system at the receiving end of the LCC-HBMMC system, the HBMMC experiences a high level of DC overvoltage due to the constant DC voltage control. In addition, some scholars have pointed out that voltage margin control strategies do not significantly reduce DC voltage during AC faults at the receiving end of hybrid DC transmission systems, and may even cause DC current interruption during severe faults. To address this issue, some experts have proposed relying on communication to transmit fault information. When the sending end receives a signal of an AC fault from the receiving end, the LCC reduces DC power by forcibly shifting the phase to a constant firing angle to suppress overvoltage on the inverter side. However, this strategy depends on the communication system, and for transmission lines spanning hundreds or even thousands of kilometers, communication delays are significant. Furthermore, a communication link failure would render the cross-traffic method ineffective.

[0003] The above analysis shows that there is currently a lack of effective and highly reliable fault-crossing solutions for the receiving end of LCC-HBMMC systems. A better solution is urgently needed. Summary of the Invention

[0004] In view of this, embodiments of this specification provide a method for AC fault ride-through at the receiving end of a hybrid DC transmission system. One or more embodiments of this specification also relate to an AC fault ride-through device at the receiving end of a hybrid DC transmission system, a computing device, a computer-readable storage medium, and a computer program, to address technical deficiencies in the prior art.

[0005] According to a first aspect of the embodiments of this specification, a method for AC fault ride-through at the receiving end of a hybrid DC transmission system is provided, comprising: Detect AC faults at the receiving end using protection devices; Upon detecting a fault, a collaborative control strategy is initiated, which includes a fault ride-through strategy for HBMMC stations based on bridge arm reference wave reconstruction and a fault ride-through strategy for LCC stations based on local measurements. After the fault is cleared, the control mode of the receiving-end flexible DC converter station is switched back to the constant DC voltage mode during normal operation, and the system is reset.

[0006] In one possible implementation, the HBMMC station fault ride-through strategy based on bridge arm reference wave reconstruction includes: The DC voltage reference value is designed in segments according to the AC voltage drop depth at the receiving end and the reactive power demand during the crossing stage. The DC voltage is adaptively reduced during the fault period as the output active power decreases by reconstructing the bridge arm reference wave.

[0007] In one possible implementation, the DC voltage reference value is set based on the power balance principle. When the grid-side voltage drop is greater than the critical grid-side voltage drop, the DC voltage reference value is set to be equal to the per-unit value of the valve-side output active power. When the grid-side voltage drop is less than or equal to the critical grid-side voltage drop, the DC voltage reference value is set to the minimum controllable DC voltage value corresponding to the grid-side voltage drop.

[0008] In one possible implementation, the instantaneous average voltage of the submodule capacitor is selected during modulation to dynamically allocate the number of bridge arm submodules to be engaged, so as to achieve closed-loop stable control of DC voltage.

[0009] In one possible implementation, the fault ride-through strategy for LCC stations based on local measurements includes: when the low-frequency component of the DC voltage measured by the LCC is continuously lower than a first threshold for a predetermined time, current reduction control is initiated, and a DC current command value is calculated based on the low-frequency component of the DC voltage and the mapping relationship; when the low-frequency component of the DC voltage is continuously higher than a second threshold for a predetermined time, the original current control strategy is switched back.

[0010] In one possible implementation, the first threshold is determined based on the minimum controllable DC voltage of the HBMMC and the line voltage drop during normal operation, and the second threshold is set to 0.9 per unit.

[0011] In one possible implementation, the DC current command value is calculated based on the power matching principle of the sending and receiving ends. The valve-side output active power is deduced from the initial value and mapping relationship of the low-frequency component of the DC voltage measured locally by the LCC, and the DC current command value is obtained by combining the initial value of the low-frequency component of the DC voltage.

[0012] According to a second aspect of the embodiments of this specification, a receiving-end AC fault ride-through device for a hybrid DC transmission system is provided, comprising: The fault detection module is configured to detect AC faults at the receiving end using protection devices; The control strategy module is configured to activate a cooperative control strategy after a fault is detected. The cooperative control strategy includes a fault ride-through strategy for HBMMC stations based on bridge arm reference wave reconstruction and a fault ride-through strategy for LCC stations based on local measurements. The system reset module is configured to switch the control mode of the receiving-end flexible DC converter station back to the constant DC voltage mode during normal operation after the fault is cleared, and to complete the system reset.

[0013] According to a third aspect of the embodiments of this specification, a computing device is provided, comprising: Memory and processor; The memory is used to store computer-executable instructions, and the processor is used to execute the computer-executable instructions. When the computer-executable instructions are executed by the processor, they implement the steps of the above-described hybrid DC transmission system receiving-end AC fault ride-through method.

[0014] According to a fourth aspect of the embodiments of this specification, a computer-readable storage medium is provided that stores computer-executable instructions, which, when executed by a processor, implement the steps of the above-described hybrid DC transmission system receiving-end AC fault ride-through method.

[0015] According to a fifth aspect of the embodiments of this specification, a computer program is provided, wherein when the computer program is executed in a computer, it causes the computer to perform the steps of the above-described hybrid DC transmission system receiving-end AC fault ride-through method.

[0016] This application analyzes the overvoltage mechanism on the DC side during AC faults at the receiving end of a hybrid DC system and the control capability of the HBMMC on DC voltage under different fault ride-through requirements. It attempts to address the DC overvoltage blocking problem caused by AC faults at the receiving end from the perspective of post-fault switching control, proposing a communication-independent AC fault ride-through control method. Utilizing the characteristic of AC voltage reduction during a fault, the DC voltage adaptively decreases as the output active power decreases, thereby mitigating the overvoltage risk. To address the possibility of AC output voltage distortion caused by excessive voltage reduction of the HBMMC, a corresponding LCC current reduction strategy is proposed. Attached Figure Description

[0017] Figure 1 This is a flowchart of an embodiment of a hybrid DC transmission system receiving-end AC fault ride-through method provided in this specification; Figure 2 This is a schematic diagram of the topology of a hybrid DC transmission system, which provides a hybrid DC transmission system receiving-end AC fault ride-through method according to an embodiment of this specification. Figure 3 This is a schematic diagram of the LCC station constant current control for a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification; Figure 4 This is a schematic diagram of the dual-loop decoupling control of an HBMMC station for a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification; Figure 5 This is an equivalent circuit diagram of HBMMC for a receiving-end AC fault ride-through method of a hybrid DC transmission system provided in one embodiment of this specification; Figure 6 This is a schematic diagram of the controllable minimum DC voltage and output power of HBMMC under different fault conditions in a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification; Figure 7 This is a schematic diagram of the HBMMC arm reference voltage for a receiving-end AC fault ride-through method in a hybrid DC transmission system according to one embodiment of this specification. Figure 8 This is a schematic diagram of DC voltage control based on bridge arm reference wave reconstruction for a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification. Figure 9 This is a schematic diagram of a DC current control strategy based on local measurement for a receiving-end AC fault ride-through method of a hybrid DC transmission system provided in one embodiment of this specification. Figure 10a This is a schematic diagram of the first simulation waveform of a hybrid DC transmission system receiving-end AC fault ride-through method without additional control strategy, provided in one embodiment of this specification. Figure 10b This is a second simulation waveform diagram of a hybrid DC transmission system receiving-end AC fault ride-through method without additional control strategy, provided in one embodiment of this specification. Figure 10c This is a schematic diagram of the third simulation waveform of a hybrid DC transmission system receiving-end AC fault ride-through method without additional control strategy, provided in one embodiment of this specification. Figure 10d This is a schematic diagram of the fourth simulation waveform of a hybrid DC transmission system receiving-end AC fault ride-through method without additional control strategy, provided in one embodiment of this specification. Figure 11a This is a schematic diagram of the first simulation waveform of a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification when the proposed control strategy is adopted; Figure 11b This is a second simulation waveform diagram of a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification when the proposed control strategy is adopted; Figure 11c This is a third simulation waveform diagram of a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification when the proposed control strategy is adopted; Figure 11d This is a fourth simulation waveform diagram of a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification when the proposed control strategy is adopted; Figure 11e This is a fifth simulation waveform diagram of a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification when the proposed control strategy is adopted; Figure 11f This is a sixth simulation waveform diagram of a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification when the proposed control strategy is adopted; Figure 12 This is a schematic diagram of the maximum arm current amplitude during fault ride-through of a receiving-end AC fault ride-through method for a hybrid DC transmission system provided in one embodiment of this specification. Figure 13a This is a schematic diagram of the first simulated waveform of a single-phase ground fault in a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification; Figure 13b This is a schematic diagram of the second simulated waveform for a single-phase ground fault in a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification; Figure 14aThis is a schematic diagram of the first simulation waveform during a two-phase short-circuit fault in a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification; Figure 14b This is a schematic diagram of the second simulation waveform during a two-phase short-circuit fault in a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification; Figure 15a This is a schematic diagram of the first simulated waveform during a two-phase short-circuit ground fault, according to an embodiment of the hybrid DC transmission system receiving-end AC fault ride-through method provided in this specification. Figure 15b This is a schematic diagram of the second simulation waveform during a two-phase short-circuit ground fault, provided in one embodiment of the hybrid DC transmission system receiving-end AC fault ride-through method. Figure 16a This is a schematic diagram of the first simulation waveform of a hybrid DC transmission system receiving-end AC fault ride-through method without additional control strategy, provided in one embodiment of this specification. Figure 16b This is a second simulation waveform diagram of a hybrid DC transmission system receiving-end AC fault ride-through method without additional control strategy, provided in one embodiment of this specification. Figure 16c This is a schematic diagram of the third simulation waveform of a hybrid DC transmission system receiving-end AC fault ride-through method without additional control strategy, provided in one embodiment of this specification. Figure 16d This is a schematic diagram of the fourth simulation waveform of a hybrid DC transmission system receiving-end AC fault ride-through method without additional control strategy, provided in one embodiment of this specification. Figure 17a This is a schematic diagram of the first simulation waveform of a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification when the proposed control strategy is adopted; Figure 17b This is a second simulation waveform diagram of a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification when the proposed control strategy is adopted; Figure 17c This is a third simulation waveform diagram of a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification when the proposed control strategy is adopted; Figure 17d This is a fourth simulation waveform diagram of a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification when the proposed control strategy is adopted; Figure 17eThis is a fifth simulation waveform diagram of a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification when the proposed control strategy is adopted; Figure 17f This is a sixth simulation waveform diagram of a hybrid DC transmission system receiving-end AC fault ride-through method provided in one embodiment of this specification when the proposed control strategy is adopted; Figure 18 This is a schematic diagram of the structure of a receiving-end AC fault ride-through device for a hybrid DC transmission system provided in one embodiment of this specification; Figure 19 This is a structural block diagram of a computing device provided in one embodiment of this specification. Detailed Implementation

[0018] Many specific details are set forth in the following description to provide a full understanding of this specification. However, this specification can be implemented in many other ways than those described herein, and those skilled in the art can make similar extensions without departing from the spirit of this specification. Therefore, this specification is not limited to the specific implementations disclosed below.

[0019] The terminology used in one or more embodiments of this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the one or more embodiments of this specification. The singular forms “a” and “the” as used in one or more embodiments of this specification and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used in one or more embodiments of this specification refers to and includes any or all possible combinations of one or more associated listed items.

[0020] It should be understood that although the terms first, second, etc., may be used to describe various information in one or more embodiments of this specification, such information should not be limited to these terms. These terms are only used to distinguish information of the same type from one another. For example, first may also be referred to as second without departing from the scope of one or more embodiments of this specification, and similarly, second may also be referred to as first. Depending on the context, the word "if" as used herein may be interpreted as "when," "when," or "in response to a determination."

[0021] This specification provides a method for AC fault ride-through at the receiving end of a hybrid DC transmission system. It also relates to an AC fault ride-through device for the receiving end of a hybrid DC transmission system, a computing device, and a computer-readable storage medium, which will be described in detail in the following embodiments.

[0022] See Figure 1 , Figure 1A flowchart is shown of a receiving-end AC fault ride-through method for a hybrid DC transmission system according to an embodiment of this specification, which specifically includes the following steps.

[0023] Step 101: Use the protection device to detect AC faults at the receiving end.

[0024] Among them, protection devices can refer to equipment in the power system used to monitor electrical parameters and trigger protection actions, and are used to detect abnormal conditions such as short circuits or overvoltages; receiving-end AC faults can refer to electrical abnormalities such as voltage drops or short circuits that occur in the receiving-end AC network of a hybrid DC transmission system, which can lead to system instability and DC-side overvoltages.

[0025] As a specific example: In a hybrid DC transmission system, when a three-phase short-circuit fault occurs in the receiving-end AC system, causing the grid connection voltage to drop to 65%, the protection device detects the voltage anomaly within milliseconds after the fault occurs.

[0026] In practical applications, before proceeding to this step, it is necessary to analyze the DC overvoltage mechanism under AC faults at the receiving end, the power characteristics of the HBMMC station, and its controllability of DC voltage.

[0027] Specifically, the basic structure of a hybrid DC transmission system is shown in [reference needed]. Figure 2 The diagram only shows one pole for illustration, and each converter station can be composed of multiple converters connected in series. The system uses LCC and HBMMC converters on the rectifier and inverter sides, respectively. The basic converter unit of LCC is a 12-pulse converter, and the DC-side smoothing reactor of HBMMC is equipped with DC fault isolation devices, such as diode valves or DC circuit breakers, between it and the DC line.

[0028] LCC operates as a rectifier, employing methods such as Figure 3 The constant DC current with minimum firing angle control shown is implemented using PI control. Its control logic is as follows: DC current command value... I dcref Compared with the measured value of DC current I dcr The two are compared, and the resulting deviation signal is input to the PI circuit to output the lead firing angle command. α This is further converted into a trigger delay angle. β =180°- α The resulting trigger pulse is then sent to the valve control.

[0029] HBMMC stations adopt such as Figure 4 The constant DC voltage / constant reactive power control shown is implemented using dual closed-loop decoupled control. Specifically, the outer loop controller operates according to the DC voltage command... U dcref and reactive power command Qref Calculate the active current reference value i dref and reactive current reference value i qref This data is then input to the inner-loop current controller. The inner-loop current controller adjusts the differential-mode voltage... u diffjref (j=a, b, c) make the active current i d reactive current i q Fast Tracking i dref , i qref Considering the limited overload capacity of the converter valve, a current-limiting circuit must be set after the output of the outer loop PI valve. Figure 4 middle I m This is the maximum permissible output current of the HBMMC. Furthermore, to prevent power device current from exceeding limits during AC grid voltage asymmetry faults, a negative sequence current suppression control loop is included in the inner loop control, setting the d-axis and q-axis negative sequence current reference values ​​to zero.

[0030] Furthermore, the DC overvoltage mechanism under AC faults at the receiving end is analyzed as follows.

[0031] In a hybrid DC transmission system, an AC fault at the receiving end causes a voltage drop on the grid side. The HBMMC (Hybrid Power Management Controller) increases the active current through negative feedback control in the outer loop to maintain active power balance and thus stabilize the DC voltage. However, when the fault is severe, the active current cannot increase indefinitely due to the overload capacity of the power devices. Simultaneously, the remote LCC (Limited Control Center) may not respond promptly to the receiving-end fault and will continue to transmit power. Therefore, when the active axis current reaches saturation, the HBMMC will be unable to support the power balance within the station, and excess power will be injected into the HBMMC arm. At this point, the DC power input to the HBMMC... P dci Its valve-side output active power P aci They are respectively: (1) In the formula, U diffd , U diffq These are the d-axis and q-axis components of the differential-mode voltage, respectively. I dref , I qref These are the reference values ​​for the d-axis and q-axis currents of the HBMMC during a fault in the AC system at the receiving end. Let the fault occurrence time be... t 0, existence time is Δ tThe energy change Δ in the bridge arm E for: (2) In the formula, C eq , L eq These are the equivalent capacitance and inductance of the HBMMC converter station, respectively. u dc0 , i arm0 These represent the DC voltage and bridge arm current, respectively. t The initial value at time 0. Considering that the inductive reactance of the bridge arm reactor is relatively small, the energy change inside the submodule capacitor is much greater than the energy fluctuation in the bridge arm reactor. As a result, a large amount of surplus energy will be forced to be transferred to the submodule capacitor, causing submodule overvoltage and DC line overvoltage.

[0032] Combining equations (1) and (2), the relationship between unbalanced power and DC overvoltage can be derived as follows: (3) As can be seen from equation (3), the greater the unbalanced power, the higher the DC voltage and the greater the DC voltage rise rate.

[0033] Furthermore, the power characteristics analysis of the MMC under AC fault at the receiving end is as follows.

[0034] After a fault occurs in the AC grid connected to the MMC, the MMC can independently inject or absorb active and reactive power. Currently, there are no mandatory standards requiring the active and reactive power values ​​provided by the MMC converter station. For hybrid DC transmission systems where the receiving end has multiple DC feed points, when the electrical distance between the MMC and multiple LCC feed points in the receiving system is relatively short, the dynamic reactive power support capability of the MMC can be fully utilized during a fault to provide emergency reactive power support to the receiving system, reducing the probability of commutation failure at nearby LCC converter stations and improving the voltage stability of the receiving system. However, if a large number of reactive power sources are distributed in the receiving grid, these sources can quickly provide reactive power support when the voltage drops, while active power can only be generated by generator sets or nearby converter stations. Once the power deficit exceeds the grid's active power reserve, it may lead to grid frequency collapse. Furthermore, when the short-circuit ratio of the receiving grid is large, the reactive power provided by the MMC has little effect on raising the voltage of the receiving grid and may even cause the short-circuit current level to exceed the standard. Based on actual engineering data and research, the control command setting principle of the partial MMC system under AC fault is as follows: When the AC grid is strong, it does not rely on the MMC to provide reactive power support. After the fault, the MMC can generate less or no reactive power, thereby prioritizing the transmission of active power and reducing the short-circuit current level. The control command is shown in equation (4), which is called the non-injection reactive power strategy. When the AC grid is weak, the MMC needs to provide reactive power support to reduce voltage drop. At this time, the control command should be determined according to the degree of voltage drop at the grid connection point. I qref Combined with the current limiting value, we can obtain I dref As shown in equation (5), this is called the reactive power injection strategy.

[0035] (4) (5) In the formula, U sd The d-axis component of the grid connection point voltage. I N and P N These are the rated current and rated power of the MMC, respectively.

[0036] Furthermore, the design of the AC fault ride-through control strategy at the receiving end is as follows.

[0037] As can be seen from equation (3), to suppress overvoltage caused by AC faults at the receiving end, fault ride-through strategies can be designed by focusing on shortening the duration of unbalanced power, reducing unbalanced power, and increasing the equivalent capacitance of the converter station. The duration of unbalanced power is determined by the relay protection operation time and circuit breaker tripping time of the AC system at the receiving end. The equivalent capacitance of the converter station is determined by the submodule capacitance and the number of submodules. Therefore, once the system parameters are determined, reducing unbalanced power is the most direct fault ride-through method.

[0038] Observing equation (1), it can be seen that, without increasing investment in additional energy balancing equipment, the DC power input to the MMC can be reduced by controlling the converter, thereby reducing the unbalanced power. In equation (1) U dci Determined by MMC control, reducing U dci This can be achieved by reducing the DC component of the bridge arm voltage; I dc Determined by LCC control, reducing I dc This can be achieved by increasing the LCC firing angle. Considering the long length of DC transmission lines, the LCC side has an inherent time lag in sensing faults from the receiving end; therefore, MMC should be prioritized to reduce this time lag. U dci To overcome faults. However, HBMMC cannot achieve decoupled control of AC and DC voltages, excessively reducing... U dci This could lead to distortion of the AC output voltage of the MMC. Therefore, it is necessary to first examine the control range of the HBMMC on the DC voltage during fault ride-through to provide a theoretical basis for formulating a suitable fault ride-through control strategy.

[0039] Furthermore, the controllability analysis of the HBMMC DC voltage under AC faults is as follows.

[0040] Figure 5 The diagram shows the equivalent circuit of HBMMC. In the figure, each arm of the HBMMC bridge contains... N One and a half bridge modules, i j and u diffj (j=a, b, c) represents the j-phase current and voltage at the AC outlet of the HBMMC; i p(n)j and u p(n)j For the upper (lower) bridge arm current and voltage of phase j; u sj The voltage of phase j at the grid connection point on the grid side; R and LLet be the equivalent resistance and inductance between the AC outlet of the HBMMC and the grid connection point, respectively. Based on the valve-side structure of the HBMMC, their expressions can be written as: (6) In the formula, R 0 and L 0 represents the bridge arm resistance and bridge arm inductance, respectively; L T This is the leakage inductance of the transformer.

[0041] based on Figure 5 The dynamic output equation of HBMMC in the d and q coordinate systems can be obtained by the Park transformation: (7) In the formula, the subscripts d and q represent the d-axis and q-axis components of the voltage and current, respectively; ω This is the fundamental angular frequency. The controller is based on a grid voltage vector orientation design, and when the phase-locked loop always follows the phase of the grid connection point voltage... U sq =0. Considering the fast response speed of the inner loop current control, it can be approximated that the current always follows the given value. Substituting the current commands in equations (4) and (5) into equation (7), the fault differential voltage amplitude is obtained. U diff The expression is: (8) like U diff The reference value is taken as half of the rated DC voltage. Since the modulation ratio of HBMMC does not exceed 1, and based on the definition of modulation ratio, the value calculated by equation (8) is... U diff / pu can also be numerically represented as the minimum per-unit value of the controllable DC voltage of HBMMC, based on the rated DC voltage. U dcmin / pu. Combining the analysis in Section 3, traversing the grid-side voltage from 0 to 1 p.u., substituting equations (4) and (5) into equations (1) and (8) respectively, we obtain: when HBMMC adopts a non-injected reactive power strategy, the minimum controllable DC voltage corresponding to different grid-side voltage drop levels. U dcmin and valve side output active power P ac like Figure 6 As shown in (a); when HBMMC adopts the reactive power injection strategy, the voltage drop corresponding to different grid-side voltage sags. U dcmin and P ac like Figure 6 As shown in (b). By Figure 6As can be seen from the above analysis, the drop in AC voltage leads to U diff This naturally reduces the minimum DC voltage of HBMMC.

[0042] Step 102: After a fault is detected, the cooperative control strategy is activated. The cooperative control strategy includes the HBMMC station fault ride-through strategy based on bridge arm reference wave reconstruction and the LCC station fault ride-through strategy based on local measurements.

[0043] Among them, the coordinated control strategy can refer to the combination of control methods that are coordinated and executed by the sending and receiving end converter stations after detecting an AC fault at the receiving end, in order to achieve power balance and overvoltage suppression during the fault; the arm reference waveform reconstruction can refer to the process of recalculating and generating the reference waveform of the arm voltage of the half-bridge modular multilevel converter, which can adjust the AC and DC components of the arm voltage to reduce the DC voltage; the HBMMC station fault ride-through strategy can refer to the adaptive control method based on power balance adopted by the receiving end half-bridge modular multilevel converter station, which can adaptively reduce the DC voltage as the output active power decreases; local measurement can refer to the process of collecting electrical quantities such as the low-frequency component of DC voltage locally at the converter station to provide the real-time input signal required by the control strategy; the LCC station fault ride-through strategy can refer to the current reduction control method based on local measurement adopted by the sending end grid commutation converter station, which is used to cooperate with the receiving end converter station to reduce DC power.

[0044] As a specific example: In a hybrid DC transmission system, when the protection device detects a three-phase short-circuit fault in the receiving-end AC system causing the grid connection voltage to drop to 65%, it immediately initiates a coordinated control strategy. The HBMMC station reconstructs the bridge arm reference wave to set the DC voltage reference value to 0.85 per unit value to match the active power output. At the same time, the LCC station initiates current reduction control based on the condition that the low-frequency component of the DC voltage measured locally is 0.75 per unit value for 5 milliseconds below the threshold, adjusting the DC current command value to 0.9 per unit value. After the fault lasts for 0.2 seconds, it is cleared, the system switches back to the constant DC voltage mode and completes the reset. The entire process does not require communication.

[0045] In practical applications, the design concept of the fault ride-through control strategy is as follows.

[0046] As can be seen from the structure of the MMC controller, U diff A reduction means a decrease in the AC component of the bridge arm reference voltage, while reducing the DC voltage during fault ride-through is equivalent to reducing the DC component of the bridge arm voltage, such as... Figure 7 As shown.

[0047] observe Figure 7As can be seen, when the active power output by the MMC is very small, the DC component of the bridge arm reference voltage needs to be reduced very low, which may result in the bridge arm reference voltage being below the zero axis, i.e., the blue line in the figure (corresponding to a modulation ratio greater than 1). Since the HBMMC cannot output negative voltage, the MMC will be unable to control the DC voltage at this time, which will also affect the output characteristics on the valve side. Relying solely on the voltage reduction on the MMC side is insufficient to reliably ride through the fault. To address this situation, in order to match the output power that is blocked at the receiving end, it is necessary to coordinate with the current reduction on the LCC side. In summary, this application fully utilizes the controllability of the sending and receiving end converter equipment and adopts a fault ride-through strategy based on the design concept of prioritizing voltage reduction by the HBMMC and auxiliary current reduction by the LCC.

[0048] In one possible implementation, the fault ride-through strategy of HBMMC station based on bridge arm reference wave reconstruction includes: designing DC voltage reference values ​​in segments according to the AC voltage drop depth at the receiving end and the reactive power demand during the ride-through stage, and making the DC voltage adaptively decrease as the output active power decreases during the fault by reconstructing the bridge arm reference wave.

[0049] Among them, the receiving-end AC voltage sag depth refers to the degree of voltage drop at the grid connection point relative to the rated value when the receiving-end AC system fails, used to quantify the severity of the fault and as an input parameter for the control strategy; the reactive power demand during the fault ride-through phase refers to the degree of reactive power support required by the receiving-end AC system from the half-bridge modular multilevel converter, which can determine the reactive current command based on the strength of the grid; the segmented design refers to the method of setting the DC voltage reference value to different calculation methods according to whether the voltage sag depth exceeds the critical value, which can ensure that the DC voltage command is always within the controllable range; the DC voltage reference value refers to the DC voltage target value set in the fault ride-through control loop, used to guide the arm reference wave reconstruction process to achieve power balance; the reconstructed arm reference wave refers to the process of regenerating the arm voltage reference signal containing AC and DC components, which can achieve DC voltage control by adjusting the number of sub-modules; the output active power refers to the per-unit value of active power delivered by the half-bridge modular multilevel converter to the receiving-end AC system, used to calculate the appropriate DC voltage reference value to achieve power matching.

[0050] As a specific example: When a fault occurs in the receiving-end AC system, causing the voltage to drop to 0.65 per unit, based on the condition that the voltage drop depth is greater than the critical value of 0.425 per unit, a segmented design is adopted to set the DC voltage reference value to be equal to the current output active power of 0.7 per unit. By reconstructing the bridge arm reference wave, the DC component of the bridge arm voltage is reduced accordingly, while keeping the AC component undistorted. This achieves the DC voltage adaptively decreasing from 1.0 per unit to 0.7 per unit, effectively suppressing submodule overvoltage.

[0051] By designing the DC voltage reference value in segments, the system can maintain stable operation under different fault depths. The DC voltage adaptive adjustment is achieved by using the bridge arm reference wave reconstruction, avoiding output voltage distortion caused by modulation ratio exceeding the limit, improving the fault ride-through success rate and reducing dependence on the communication system.

[0052] In one possible implementation, the DC voltage reference value is set based on the power balance principle. When the grid-side voltage drop is greater than the critical grid-side voltage drop, the DC voltage reference value is set to be equal to the per-unit value of the valve-side output active power. When the grid-side voltage drop is less than or equal to the critical grid-side voltage drop, the DC voltage reference value is set to the minimum controllable DC voltage value corresponding to the grid-side voltage drop.

[0053] Among them, the power balance principle refers to the physical principle of maintaining a balance between input and output power in a power system, used to ensure stable system operation and prevent energy accumulation; the grid-side voltage drop value refers to the degree of drop in the voltage amplitude at the receiving end AC system grid connection point relative to the rated value, which can reflect the severity of the fault and serve as the basis for determining the control strategy; the critical grid-side voltage drop value refers to the threshold voltage for classifying the DC voltage reference value setting mode in fault ride-through control, which can determine whether to enable the minimum controllable DC voltage value; the per-unit value of valve-side output active power refers to the per-unit value of the active power delivered by the valve side of the half-bridge modular multilevel converter to the AC system relative to the rated value, used as the basis for calculating the DC voltage reference value; the minimum controllable DC voltage value refers to the minimum DC voltage value that can be stably controlled under the current operating conditions of the half-bridge modular multilevel converter, used to avoid bridge arm reference voltage distortion and ensure system controllability.

[0054] As a specific example: During an AC fault at the receiving end of a hybrid DC transmission system, when the grid-side voltage drop is 0.3 per unit and less than the critical grid-side voltage drop of 0.425 per unit, the DC voltage reference value is set to the minimum controllable DC voltage value corresponding to that grid-side voltage drop of 0.5 per unit; when the grid-side voltage drop is 0.6 per unit and greater than the critical value, the DC voltage reference value is set to be equal to the valve-side output active power per unit of 0.7 per unit. The DC voltage is adaptively adjusted through the power balance principle to ensure that the system maintains power balance and suppresses overvoltage during the fault.

[0055] By setting the DC voltage reference value based on the power balance principle, the control failure caused by the modulation ratio exceeding the limit is reasonably adapted to different fault depths, thereby improving the reliability and stability of the system fault ride-through and reducing the dependence on additional communication.

[0056] In one possible implementation, the instantaneous average voltage of the submodule capacitor is selected during modulation to dynamically allocate the number of bridge arm submodules to be engaged, so as to achieve closed-loop stable control of DC voltage.

[0057] Among them, the instantaneous average voltage of the submodule capacitor can refer to the arithmetic mean of the instantaneous voltage values ​​of all submodule capacitors within the same bridge arm, used to reflect the energy storage status of the bridge arm in real time and serve as a benchmark for modulation calculation; dynamic allocation can refer to the process of adjusting the number of submodules to be put into each bridge arm according to the real-time calculated average voltage, which can ensure accurate tracking of the bridge arm voltage reference waveform; the number of bridge arm submodules put into operation can refer to the number of half-bridge submodules turned on at a specific time to generate the target bridge arm voltage, so as to achieve precise control of the bridge arm output voltage; closed-loop stable control can refer to the system mechanism that continuously adjusts the control output by feedback of the deviation between the measured value and the target value, used to maintain the stable operation of the DC voltage near the reference value.

[0058] As a specific example: When an AC fault at the receiving end causes DC voltage fluctuations, the controller collects the capacitor voltage of the three-phase bridge arm submodule in real time and calculates the instantaneous average voltage as 1.015 kV. Based on the current bridge arm voltage reference value of 20 kV and the instantaneous average voltage, the controller dynamically calculates that 19 submodules need to be activated through a formula. Subsequently, the valve control system executes the corresponding switching action to make the actual DC voltage stably track the reference voltage of 0.95 per unit, thereby achieving closed-loop stable control of the DC voltage during the fault period.

[0059] By using the instantaneous average voltage for dynamic distribution, the fluctuation characteristics of the submodule capacitor voltage can be effectively adapted, ensuring the accuracy of the bridge arm voltage output, improving the stability and response speed of DC voltage control, and preventing system instability caused by capacitor voltage imbalance.

[0060] In practical applications, the fault ride-through control strategy for HBMMC stations is as follows.

[0061] Based on the above analysis, the desired DC voltage reference value should be such that it does not change the LCC-side control strategy (i.e., it remains unchanged before and after the fault). I dc Under the premise of keeping the DC voltage constant (i.e., 1p.u. remains constant), excess power should be suppressed as much as possible, and the degree of DC voltage reduction should be positively correlated with the degree of output power obstruction. Meanwhile, considering that controllers are mostly designed in per-unit systems, based on the input / output power matching principle of the receiving-end MMC, the ideal DC voltage command in the controller during low-voltage breakdown is... U 'dcref (per unit value) is shown in equation (9): (9) In fact, the right side of the second equal sign in equation (9) is actually the active power output from the MMC valve side. P ac / pu. Therefore, equation (9) achieves power matching between the input and output of the MMC at the receiving end without affecting the LCC control strategy at the sending end, thus reducing the impact range of the fault to a certain extent. However, based on the analysis in Section 4.2 and Figure 6 , Figure 7 It can be seen that when the grid-side voltage drops significantly, it leads to... P ac / pu< U dcmin / pu time ( Figure 6 If the DC voltage command is still given according to equation (9) for the portion below the intersection of the two curves, the valve-side voltage will be distorted due to the maximum modulation ratio limitation of HBMMC. To address this situation, based on equation (9), a segmented DC voltage command as shown in equation (10) is set: (10) In the formula, U sd0 For actual fault ride-through requirements P ac = U dcmin Time corresponding U sd / pu, referred to in this application as the critical network side voltage sag. As shown in equation (8), this value can be calculated offline based on control commands and system parameters. For the model in this application, from... Figure 6 It is evident that when HBMMC adopts a non-injection reactive power strategy, U sd0 =0.283 pu; when HBMMC adopts the reactive power injection strategy, U sd0 =0.425pu; U dcmin ( U sd ) for when U sd < U sd0 hour, U sd corresponding U dcmin According to the analysis in Section 3.1, the corresponding relationship can be calculated offline by selecting a suitable benchmark value using equation (8).

[0062] Furthermore, in the traditional Nearest Level Modulation (NLM) modulation strategy, the voltage of the submodule capacitor used for modulation calculation is generally taken as its rated value. u cN(Constant value). In order to ensure that the actual DC voltage can effectively follow the desired command value and achieve closed-loop stable control of the DC voltage, the instantaneous average voltage of the submodule capacitor is selected during modulation to dynamically allocate the number of bridge arm submodules to be engaged, as shown in equation (11): (11) In the formula, N pj and N nj These represent the number of sub-modules deployed in the upper and lower arms of phase j, respectively. u c_avg This represents the average instantaneous voltage of the submodule capacitors; N This refers to the number of submodules within a single bridge arm when redundant submodules are not considered. u c This represents the instantaneous value of the capacitor voltage of a single submodule.

[0063] In summary, the following is proposed: Figure 8 The figure shows the DC voltage control strategy for the HBMMC station based on the bridge arm voltage reference waveform reconstruction. u diffjref This is the reference value for the differential voltage of phase j; u p(n)jref This is the reference value for the upper (lower) bridge arm voltage of phase j; u cirjref This is the reference value for the j-phase circulating current voltage. During normal operation, switch S is set to 0. When a fault is detected in the receiving-end AC system, control switch S is set to 1 to activate the HBMMC fault ride-through strategy. This reduces both the AC and DC components of the HBMMC arm voltage reference waveform during the low-voltage ride-through period, thereby reducing the number of submodules in operation and lowering the DC voltage. Once the fault is cleared, switch S is switched back to 0, and the HBMMC switches back to normal constant DC voltage control.

[0064] In one possible implementation, the fault ride-through strategy for LCC stations based on local measurements includes: when the low-frequency component of the DC voltage measured by the LCC is continuously lower than a first threshold for a predetermined time, current reduction control is initiated, and a DC current command value is calculated based on the low-frequency component of the DC voltage and the mapping relationship; when the low-frequency component of the DC voltage is continuously higher than a second threshold for a predetermined time, the original current control strategy is switched back.

[0065] Among them, the low-frequency component of DC voltage can refer to the slowly varying voltage component extracted from the DC line voltage measurement signal through a low-pass filter, used to reflect the stable change trend of the receiving-end voltage and avoid interference from line frequency variation effects; the first threshold can refer to the threshold value of the low-frequency component of DC voltage preset according to system parameters to start the current reduction control, which can determine whether the receiving end has entered a deep fault state; the predetermined time can refer to the duration for which the monitored quantity must continuously meet the conditions before the control strategy is activated, used to improve the anti-interference capability of the criterion and prevent false activation; the current reduction control can refer to the control mode of the grid commutator reducing DC current by increasing the firing angle to match the reduction of DC current at the receiving-end converter station. The power transmission capability; the mapping relationship can refer to the correspondence between the low-frequency component of DC voltage and the active power output on the valve side established through offline calculation, which can indirectly obtain the power information of the receiving end based on local measurement values; the DC current command value can refer to the DC current target value set in the current reduction control loop, which is used to guide the firing angle control of the grid commutator; the second threshold can refer to the preset DC voltage low-frequency component recovery threshold value used to switch back to normal control, in order to determine whether the fault has been eliminated and whether the system has returned to normal; the original current control strategy can refer to the constant DC current control strategy adopted by the grid commutator under normal operating conditions, which is used to maintain the steady-state operation of the system.

[0066] As a specific example: when the LCC station measures a DC voltage low-frequency component of 0.75 per unit value and it remains below the first threshold of 0.8 per unit value for five milliseconds, the current reduction control is immediately initiated. Based on the current DC voltage low-frequency component and the pre-stored mapping relationship, the active power output on the valve side is calculated to be 0.8 per unit value, and thus the DC current command value is 1.06 per unit value. When the fault is cleared, the DC voltage low-frequency component recovers to 0.92 per unit value and remains above the second threshold of 0.9 per unit value for five milliseconds, the control strategy automatically switches back to the original current control mode.

[0067] Local measurement enables rapid and reliable fault response without waiting for communication transmission delays. The mapping relationship is used to accurately estimate the power status of the receiving end, ensuring timely power matching between the sending and receiving ends, effectively preventing DC overvoltage and improving the system's fault ride-through success rate.

[0068] In one possible implementation, the first threshold is determined based on the minimum controllable DC voltage of the HBMMC and the line voltage drop during normal operation, and the second threshold is set to 0.9 per unit.

[0069] Among them, the line voltage drop during normal operation can refer to the voltage drop value on the DC line under the steady-state operation conditions of the hybrid DC transmission system, which is used to calculate the first threshold to ensure the accuracy of the fault ride-through strategy of LCC station.

[0070] As a specific example: In the design of a hybrid DC transmission system, the first threshold is determined as 0.8 per unit by subtracting the line voltage drop of 0.05 per unit during normal operation from the minimum controllable DC voltage of HBMMC (0.85 per unit). The second threshold is fixed at 0.9 per unit. When the LCC station measures that the low-frequency component of the DC voltage is lower than the first threshold for five milliseconds, the current reduction control is initiated. When the low-frequency component of the DC voltage is higher than the second threshold for five milliseconds, the current control is switched back to constant current control, thus achieving a fast fault response without communication.

[0071] By setting thresholds based on system parameters, the reliability of fault criteria is improved, ensuring power coordination and matching between the sending and receiving ends, effectively suppressing DC overvoltage, and enhancing the stability of the system during fault ride-through.

[0072] In one possible implementation, the DC current command value is calculated based on the power matching principle of the sending and receiving ends. The valve-side output active power is deduced from the initial value and mapping relationship of the low-frequency component of the DC voltage measured locally by the LCC, and the DC current command value is obtained by combining the initial value of the low-frequency component of the DC voltage.

[0073] Among them, the power matching principle of the sending and receiving ends can refer to the physical law that the output power of the sending end and the absorbed power of the receiving end are kept in balance in the DC transmission system. It is used to calculate the appropriate DC current command to maintain the power balance of the system. The initial value of the low-frequency component of DC voltage can refer to the value of the low-frequency component of DC voltage measured and recorded at the start of the fault ride-through control strategy of LCC station. It is used as the reference voltage value for calculating the DC current command.

[0074] As a specific example: when the LCC station detects that the low-frequency component of the DC voltage meets the current reduction start-up condition, it immediately records the initial value of the current low-frequency component of the DC voltage as 0.78 per unit. By querying the pre-stored mapping relationship, it solves out the corresponding valve-side output active power as 0.75 per unit. Then, based on the power matching principle of the sending and receiving ends, it calculates the DC current command value as 0.75 divided by 0.78, which is approximately equal to 0.96 per unit. The LCC station adjusts the firing angle according to this command value to achieve current reduction control.

[0075] By combining local measurements with mapping relationships, the operating status of the receiving end can be accurately estimated, enabling rapid self-matching of power between the sending and receiving ends, effectively suppressing the risk of DC overvoltage, and improving the system's autonomous response capability and operational reliability under fault conditions.

[0076] In practical applications, the fault ride-through control strategy for LCC stations is as follows.

[0077] Based on the foregoing analysis, it can be seen that when the active power output of HBMMC is low, the maximum drop in the controlled DC voltage is limited, and in this case, it is necessary to reduce the current on the LCC side.

[0078] First, switching criteria need to be set to determine when the LCC should engage the fault ride-through strategy and switch back to the original current control strategy. To avoid communication failures, the monitoring quantities in this criterion should be collected locally on the LCC. Given the long transmission lines, the frequency variation effect of the line cannot be ignored when the LCC responds to voltage changes on the inverter side. Based on the principle of equal transmission line transmission, it is known that the low-frequency characteristics of electrical quantities are less affected by the frequency variation effect of long lines. Therefore, the criteria shown in Equations (12) and (13) are designed as the enable signals for switching the control strategy of the LCC station: (12) (13) In the formula, T The duration for which the criterion is satisfied; U dcmin0 The voltage drop value corresponding to the critical network side U dcmin ;Δ U This represents the line voltage drop during normal operation. U dcrdp This is the low-frequency component (per unit value) of the DC voltage on the LCC side, which can be obtained by filtering the voltage measured at the DC line outlet point through a low-pass filter.

[0079] Ignoring DC line losses, according to the power matching principle of the sending and receiving ends, the DC current value that should be output by the LCC side is shown in equation (14): (14) In the formula, the output power of the MMC valve side is... P ac It cannot be measured locally by the LCC. But by Figure 6 It can be seen that, for U sd < U sd0 Part of P ac and U dcmin It is a one-to-one correspondence; if viewed U dcmin As the independent variable, P ac If it is the dependent variable, then P ac Can be seen as about U dcmin The function, and the mapping relationship between the two can be expressed as: P ac ( U dcmin (This mapping relationship can be calculated offline). And from equation (10), it can be seen that when... U sd < Usd0 At that time, the fault ride-through strategy of the MMC station makes its DC voltage exactly equal to U dcmin And because U dcrdp ≈ U dcmin -Δ U Therefore, it can be combined with local measurements of LCC. U dcrdp and mapping relationship P ac ( U dcrdp Indirectly solve for the corresponding P ac Meanwhile, given that equation (14) is in fractional form, to avoid fluctuations in the command value caused by system transient response, which would affect the stability of the controller, the expression in equation (14) is modified accordingly. U dcrdp The value measured at the start time of the fault ride-through control strategy at the LCC station is taken as U dcrdp Value, denoted as U dcrdp0 In summary, the proposed DC current control strategy for LCC stations based on local measurements is as follows: Figure 9 As shown. This strategy is essentially a low-voltage current-limiting control used in conjunction with DC buck converter on the MMC side. Its control logic is as follows: when the criterion shown in equation (12) is satisfied, record the current value at that time. U dcrdp0 The value is substituted into equation (14) to give the DC current command value to the LCC station. Then, monitoring continues. U dcrdp Once the criterion shown in equation (13) is satisfied, the original current control strategy is switched back.

[0080] Step 103: After the fault is cleared, switch the control mode of the receiving-end flexible DC converter station back to the constant DC voltage mode during normal operation, and complete the system reset.

[0081] Among these, fault clearing refers to the process of isolating or clearing AC faults at the receiving end by relay protection devices, used to terminate the fault state and allow the system to recover; receiving-end flexible DC converter station refers to a converter station at the receiving end of a hybrid DC transmission system that uses fully controlled devices, such as a half-bridge modular multilevel converter, which can achieve flexible active and reactive power control; control mode refers to the type of control strategy adopted by the converter station during operation, such as constant DC voltage or constant reactive power, used to determine the converter's operating objectives and control parameters; constant DC voltage mode refers to the control mode of the receiving-end flexible DC converter station that maintains a constant DC voltage during normal operation to ensure the stable operation of the DC transmission system; system reset refers to the process of restoring all control strategies and operating states of the hybrid DC transmission system to normal operating conditions after fault clearing, used to ensure the continuous and reliable operation of the system.

[0082] As a specific example: After the AC fault at the receiving end of the hybrid DC transmission system is cleared, the receiving end flexible DC converter station will switch the control mode from the fault ride-through strategy back to the constant DC voltage mode during normal operation. The DC voltage reference value is restored to 1.0 per unit value. At the same time, the sending end LCC station automatically resets to constant current control after detecting that the DC voltage has been restored to 0.95 per unit value. The entire system completes the reset process and restores steady-state power transmission within 200 milliseconds.

[0083] By quickly switching control modes and completing system reset, the system can be restored to stable operation in a timely manner after a fault, preventing the risk of continuous overvoltage or power imbalance, and improving the system's self-healing ability and overall reliability.

[0084] The following example, using the application of the hybrid DC transmission system receiving-end AC fault ride-through method provided in this specification in a DC transmission system renovation project, further illustrates the hybrid DC transmission system receiving-end AC fault ride-through method.

[0085] To verify the effectiveness of the proposed fault ride-through strategy, a model was constructed in PSCAD based on parameters from a DC transmission line upgrade project. Figure 2 The model shown is a hybrid DC transmission system. The DC lines in the simulation use a frequency-dependent model. The positive direction of the current (power) is defined as from the rectifier side to the inverter side. Under normal system operation, the MMC reactive power command value is 0. It is assumed that all faults occur at... t =1s, and the sampling frequency at each measuring point is 10kHz. Considering the influence of the relay protection action time and AC circuit breaker action time of the receiving end AC system, the fault duration is set to 0.2s, and the fault detection time of the MMC station is 1ms.

[0086] Furthermore, MMC adopts a non-injection reactive power strategy, as detailed below.

[0087] A three-phase short-circuit fault is set up where the grid connection point voltage drops to 65%. During the fault, the MMC does not inject reactive power into the AC system, and the dq-axis current command is shown in equation (4). The simulated waveforms of various electrical quantities under the conditions of no additional control strategy and the fault ride-through control strategy proposed in this application are shown in Figures 10 and 11. (The figures are in the original text.) P dc This refers to the DC power input to the MMC; P s and Q s These represent the active and reactive power injected into the AC system by the MMC, respectively.

[0088] As shown in Figure 10(a), without additional control strategies, the DC component of the bridge arm reference voltage in the MMC controller remains unchanged after a fault occurs, but the AC component decreases, consistent with the analysis in Section 3.1. As shown in Figure 10(b), the MMC station can follow the control commands to achieve the expected power injection target. However, because the DC power input to the MMC station is greater than the active power it outputs to the receiving-end AC system, excess energy accumulates on the bridge arm submodules, causing a rapid rise in both the submodule voltage and the DC line voltage, endangering the safety of the primary equipment, as shown in Figures 10(c) and (d). The peak DC voltage on the MMC side can reach approximately 1.898 pu, and the peak voltage on the submodule can reach approximately 1.884 pu. If no measures are taken, the system overvoltage lockout logic will be triggered, causing the entire system to shut down.

[0089] As shown in Figure 11(a), the proposed fault ride-through strategy reduces both the AC and DC components of the bridge arm voltage by reconstructing the bridge arm voltage reference waveform. The DC component decreases with the reduction of active power output on the valve side, and the bridge arm voltage reference after superimposing the AC component remains within the range of 0~1 p.u. As shown in Figure 11(b), the proposed fault ride-through strategy allows the HBMMC to reduce the DC voltage while ensuring that the AC output voltage remains undistorted. Comparing Figures 10(b) and 11(c), it can be seen that the MMC achieves the desired power control target under both control strategies. However, compared with the absence of an additional control strategy, the proposed fault ride-through control strategy reduces the peak DC voltage by approximately 43.83% and the peak submodule capacitor voltage by approximately 45.36%, demonstrating that the proposed fault ride-through method effectively reduces the risk of system overvoltage lockout. Figure 6(a) It can be seen that for the MMC adopting the non-injection reactive power strategy, the voltage drop value of the grid side of the fault is greater than the critical drop value. Therefore, theoretically, the fault ride-through control strategy of the LCC station should not be activated, and the surplus power can be smoothed out by the HBMMC station alone. As shown in Figure 11(d), when the fault is transmitted to the LCC side along the line, the voltage of the LCC side has a high-frequency component due to the traveling wave process, but its overall trend (low-frequency component) is consistent with that of the MMC side, and it does not meet the LCC side fault ride-through strategy activation criterion shown in Equation (12). The LCC is always in constant current control mode during the fault, which meets the theoretical requirements. As shown in Figure 11(f), in the initial stage of the fault, the LCC cannot quickly respond to the change of DC voltage on the MMC side and the DC current has a transient increase. Then, with the action of constant current control, the firing angle increases and the DC current drops back to the command value of 1p.u. After the AC fault at the receiving end is cleared, the hybrid DC transmission system can smoothly recover to the steady state.

[0090] In addition, to ensure that the converter does not lock out during a fault, it is also necessary to check whether the arm current is overcurrent. Therefore, three-phase short-circuit faults with different grid-side voltage sags are simulated. When the proposed fault ride-through control strategy is adopted, the maximum amplitude of the six arm currents of the MMC is measured as follows: Figure 12 As shown in the legend. I set This is the overcurrent protection threshold for the bridge arm, and its value is generally twice the rated current of the bridge arm.

[0091] Depend on Figure 12 It can be seen that for faults of different severity, when the proposed control strategy is adopted, the power electronic devices on the bridge arm will not experience overcurrent and will not trigger the converter overcurrent blocking logic.

[0092] The simulation waveforms of the control strategy proposed in this application under asymmetrical short-circuit faults are shown in Figures 13-14. As can be seen from Figures 13-14, the control strategy proposed in this application can still reliably overcome the fault under asymmetrical fault conditions.

[0093] Furthermore, MMC employs a reactive power injection strategy, as detailed below.

[0094] A three-phase short-circuit fault is set up where the grid connection point voltage drops to 65%. During the fault, the HBMMC injects a certain amount of reactive power into the AC system according to the degree of voltage drop on the grid side. The d-axis and q-axis current commands are shown in Equation (5). The simulation waveforms of each electrical quantity are shown in Figures 16 and 17 under the control strategies without additional control strategies and the control strategies proposed in this application.

[0095] As shown in Figure 17(d), after the fault occurs, the HBMMC station activates the fault ride-through control strategy. In the initial stage after activation, the DC voltage on the HBMMC side can effectively and adaptively adjust to follow its valve-side active power output. As the fault process progresses, the LCC station... tAt 1.02s, the current reduction strategy is activated, increasing the firing angle. Afterward, the DC current begins to decrease to the level shown in equation (14), as shown in Figure 17(f). After the fault is cleared, the LCC station... t At 1.21s, the system switches back to the original current control strategy. Comparing Figures 16 and 17, it can be seen that compared with the case without additional control strategy, the proposed fault ride-through control strategy can reduce the peak DC voltage on the HBMMC side by about 45.85% and the peak voltage of the submodule capacitor by about 41.14%, effectively reducing the risk of system overvoltage lockout.

[0096] This application focuses on the LCC-HBMMC hybrid DC transmission system, attempting to address the AC fault ride-through problem at the receiving end from the perspective of post-fault switching control. Based on the analysis of the overvoltage mechanism and the control capability of HBMMC, a communication-independent AC fault ride-through control strategy at the receiving end is proposed: This application fully leverages the control capability of HBMMC over DC voltage, proposing a reconstruction of the bridge arm voltage reference waveform based on the power balance principle. This allows the DC voltage to adaptively decrease during faults as the output active power drops, aiming to maximize the self-balancing of surplus power within the station. Addressing the issue that HBMMC's operating range is limited by its voltage modulation ratio, preventing it from solely relying on the station to mitigate surplus power, this application proposes a coordinated control strategy involving auxiliary current reduction at the LCC station to achieve rapid DC power reduction. Through inter-station coordination, the AC output voltage of the HBMMC is ensured to remain undistorted while the LCC promptly reduces its load to match the low-level active power output on the inverter side. Simulation results show that for different fault conditions, the proposed fault ride-through control strategy effectively and quickly reduces DC power, exhibiting significant DC overvoltage suppression and providing more time to handle AC faults at the receiving end, thus improving the system's ability to respond to AC faults at the receiving end. Furthermore, the proposed method remains applicable even in the event of communication system failures. Compared to existing fault ride-through methods that rely on communication, it offers advantages in reliability and speed.

[0097] Corresponding to the above method embodiments, this specification also provides embodiments of a receiving-end AC fault ride-through device for a hybrid DC transmission system. Figure 18 A schematic diagram of the structure of a receiving-end AC fault ride-through device for a hybrid DC transmission system according to one embodiment of this specification is shown. Figure 18 As shown, the device includes: Fault detection module 1801 is configured to detect AC faults at the receiving end using a protection device; The control strategy module 1802 is configured to activate a cooperative control strategy after a fault is detected. The cooperative control strategy includes an HBMMC station fault ride-through strategy based on bridge arm reference wave reconstruction and an LCC station fault ride-through strategy based on local measurements. The system reset module 1803 is configured to switch the control mode of the receiving-end flexible DC converter station back to the constant DC voltage mode during normal operation after the fault is cleared, and to complete the system reset.

[0098] In one possible implementation, the HBMMC station fault ride-through strategy based on bridge arm reference wave reconstruction includes: The DC voltage reference value is designed in segments according to the AC voltage drop depth at the receiving end and the reactive power demand during the crossing stage. The DC voltage is adaptively reduced during the fault period as the output active power decreases by reconstructing the bridge arm reference wave.

[0099] In one possible implementation, the DC voltage reference value is set based on the power balance principle. When the grid-side voltage drop is greater than the critical grid-side voltage drop, the DC voltage reference value is set to be equal to the per-unit value of the valve-side output active power. When the grid-side voltage drop is less than or equal to the critical grid-side voltage drop, the DC voltage reference value is set to the minimum controllable DC voltage value corresponding to the grid-side voltage drop.

[0100] In one possible implementation, the instantaneous average voltage of the submodule capacitor is selected during modulation to dynamically allocate the number of bridge arm submodules to be engaged, so as to achieve closed-loop stable control of DC voltage.

[0101] In one possible implementation, the fault ride-through strategy for LCC stations based on local measurements includes: when the low-frequency component of the DC voltage measured by the LCC is continuously lower than a first threshold for a predetermined time, current reduction control is initiated, and a DC current command value is calculated based on the low-frequency component of the DC voltage and the mapping relationship; when the low-frequency component of the DC voltage is continuously higher than a second threshold for a predetermined time, the original current control strategy is switched back.

[0102] In one possible implementation, the first threshold is determined based on the minimum controllable DC voltage of the HBMMC and the line voltage drop during normal operation, and the second threshold is set to 0.9 per unit.

[0103] In one possible implementation, the DC current command value is calculated based on the power matching principle of the sending and receiving ends. The valve-side output active power is deduced from the initial value and mapping relationship of the low-frequency component of the DC voltage measured locally by the LCC, and the DC current command value is obtained by combining the initial value of the low-frequency component of the DC voltage.

[0104] The above is a schematic scheme of a hybrid DC transmission system receiving-end AC fault ride-through device according to this embodiment. It should be noted that the technical solution of this hybrid DC transmission system receiving-end AC fault ride-through device belongs to the same concept as the technical solution of the hybrid DC transmission system receiving-end AC fault ride-through method described above. Details not described in detail in the technical solution of the hybrid DC transmission system receiving-end AC fault ride-through device can be found in the description of the technical solution of the hybrid DC transmission system receiving-end AC fault ride-through method described above.

[0105] Figure 19 A structural block diagram of a computing device 1900 according to one embodiment of this specification is shown. The components of the computing device 1900 include, but are not limited to, a memory 1910 and a processor 1920. The processor 1920 is connected to the memory 1910 via a bus 1930, and a database 1950 is used to store data.

[0106] The computing device 1900 also includes an access device 1940, which enables the computing device 1900 to communicate via one or more networks 1960. Examples of these networks include Public Switched Telephone Network (PSTN), Local Area Network (LAN), Wide Area Network (WAN), Personal Area Network (PAN), or combinations of communication networks such as the Internet. The access device 1940 may include one or more of any type of wired or wireless network interface (e.g., a network interface card (NIC)), such as an IEEE 802.11 Wireless Local Area Network (WLAN) wireless interface, a Wi-MAX (Worldwide Interoperability for Microwave Access) interface, an Ethernet interface, a Universal Serial Bus (USB) interface, a cellular network interface, a Bluetooth interface, or a Near Field Communication (NFC) interface.

[0107] In one embodiment of this specification, the above-described components of the computing device 1900 and Figure 6 Other components, not shown, can also be connected to each other, for example, via a bus. It should be understood that... Figure 6The block diagram of the computing device shown is for illustrative purposes only and is not intended to limit the scope of this specification. Those skilled in the art can add or replace other components as needed.

[0108] The computing device 1900 can be any type of stationary or mobile computing device, including mobile computers or mobile computing devices (e.g., tablet computers, personal digital assistants, laptop computers, notebook computers, netbooks, etc.), mobile phones (e.g., smartphones), wearable computing devices (e.g., smartwatches, smart glasses, etc.) or other types of mobile devices, or stationary computing devices such as desktop computers or personal computers (PCs). The computing device 1900 can also be a mobile or stationary server.

[0109] The processor 1920 executes computer-executable instructions, which, when executed by the processor, implement the steps of the above-described hybrid DC transmission system receiving-end AC fault ride-through method. The above is an illustrative scheme of a computing device according to this embodiment. It should be noted that the technical solution of this computing device and the technical solution of the above-described hybrid DC transmission system receiving-end AC fault ride-through method belong to the same concept. Details not described in detail in the technical solution of the computing device can be found in the description of the technical solution of the above-described hybrid DC transmission system receiving-end AC fault ride-through method.

[0110] An embodiment of this specification also provides a computer-readable storage medium storing computer-executable instructions that, when executed by a processor, implement the steps of the above-described hybrid DC transmission system receiving-end AC fault ride-through method.

[0111] The above is an illustrative scheme of a computer-readable storage medium according to this embodiment. It should be noted that the technical solution of this storage medium belongs to the same concept as the technical solution of the above-described hybrid DC transmission system receiving-end AC fault ride-through method. For details not described in detail in the technical solution of the storage medium, please refer to the description of the technical solution of the above-described hybrid DC transmission system receiving-end AC fault ride-through method.

[0112] An embodiment of this specification also provides a computer program, wherein when the computer program is executed in a computer, it causes the computer to perform the steps of the above-described hybrid DC transmission system receiving-end AC fault ride-through method.

[0113] The above is an illustrative scheme of a computer program according to this embodiment. It should be noted that the technical solution of this computer program belongs to the same concept as the technical solution of the above-described hybrid DC transmission system receiving-end AC fault ride-through method. For details not described in detail in the technical solution of the computer program, please refer to the description of the technical solution of the above-described hybrid DC transmission system receiving-end AC fault ride-through method.

[0114] The foregoing has described specific embodiments of this specification. Other embodiments are within the scope of the appended claims. In some cases, the actions or steps recited in the claims may be performed in a different order than that shown in the embodiments and may still achieve the desired result. Furthermore, the processes depicted in the drawings do not necessarily require the specific or sequential order shown to achieve the desired result. In some embodiments, multitasking and parallel processing are possible or may be advantageous.

[0115] The computer instructions include computer program code, which may be in the form of source code, object code, executable file, or certain intermediate forms. The computer-readable medium may include any entity or device capable of carrying the computer program code, recording media, USB flash drives, portable hard drives, magnetic disks, optical disks, computer memory, read-only memory (ROM), random access memory (RAM), electrical carrier signals, telecommunication signals, and software distribution media, etc. It should be noted that the content included in the computer-readable medium may be appropriately added to or subtracted according to the requirements of legislation and patent practice in the jurisdiction. For example, in some jurisdictions, according to legislation and patent practice, computer-readable media may not include electrical carrier signals and telecommunication signals.

[0116] It should be noted that, for the sake of simplicity, the foregoing method embodiments are all described as a series of actions. However, those skilled in the art should understand that the embodiments in this specification are not limited to the described order of actions, because according to the embodiments in this specification, some steps can be performed in other orders or simultaneously. Furthermore, those skilled in the art should also understand that the embodiments described in this specification are all preferred embodiments, and the actions and modules involved are not necessarily essential to the embodiments in this specification.

[0117] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0118] The preferred embodiments disclosed above are merely illustrative of this specification. The optional embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the embodiments described herein. These embodiments are selected and specifically described in this specification to better explain the principles and practical applications of the embodiments, thereby enabling those skilled in the art to better understand and utilize this specification. This specification is limited only by the claims and their full scope and equivalents.

Claims

1. A method for AC fault ride-through of a receiving end of a hybrid DC power transmission system, characterized in that, include: Detect AC faults at the receiving end using protection devices; Upon detecting the fault, a collaborative control strategy is initiated, which includes an HBMMC station fault ride-through strategy based on bridge arm reference wave reconstruction and an LCC station fault ride-through strategy based on local measurements. After the fault is cleared, the control mode of the receiving-end flexible DC converter station is switched back to the constant DC voltage mode during normal operation, and the system is reset.

2. The method for AC fault ride-through at the receiving end of a hybrid DC transmission system according to claim 1, characterized in that, The HBMMC station fault ride-through strategy based on bridge arm reference wave reconstruction includes: The DC voltage reference value is designed in segments according to the AC voltage drop depth at the receiving end and the reactive power demand during the crossing stage. The DC voltage is adaptively reduced during the fault period as the output active power decreases by reconstructing the bridge arm reference wave.

3. The method for AC fault ride-through at the receiving end of a hybrid DC transmission system according to claim 2, characterized in that, The DC voltage reference value is set based on the power balance principle. When the grid-side voltage drop is greater than the critical grid-side voltage drop, the DC voltage reference value is set to be equal to the per-unit value of the active power output on the valve side. When the grid-side voltage drop is less than or equal to the critical grid-side voltage drop, the DC voltage reference value is set to the minimum controllable DC voltage value corresponding to the grid-side voltage drop.

4. The method for AC fault ride-through at the receiving end of a hybrid DC transmission system according to claim 2, characterized in that, During modulation, the instantaneous average voltage of the submodule capacitor is selected to dynamically allocate the number of bridge arm submodules to be engaged, so as to achieve closed-loop stable control of DC voltage.

5. The method for AC fault ride-through at the receiving end of a hybrid DC transmission system according to claim 1, characterized in that, The fault ride-through strategy for LCC stations based on local measurements includes: when the low-frequency component of the DC voltage measured by the LCC is continuously lower than the first threshold for a predetermined time, current reduction control is initiated, and a DC current command value is calculated based on the low-frequency component of the DC voltage and the mapping relationship; when the low-frequency component of the DC voltage is continuously higher than the second threshold for a predetermined time, the original current control strategy is switched back.

6. The method for AC fault ride-through at the receiving end of a hybrid DC transmission system according to claim 5, characterized in that, The first threshold is determined based on the minimum controllable DC voltage of HBMMC and the line voltage drop during normal operation, and the second threshold is set to 0.9 per unit.

7. The method for AC fault ride-through at the receiving end of a hybrid DC transmission system according to claim 5, characterized in that, The DC current command value is calculated based on the power matching principle of the sending and receiving ends. The valve-side output active power is deduced from the initial value of the low-frequency component of the DC voltage measured locally by the LCC and the mapping relationship. The DC current command value is obtained by combining the initial value of the low-frequency component of the DC voltage.

8. A receiving-end AC fault ride-through device for a hybrid DC transmission system, characterized in that, include: The fault detection module is configured to detect AC faults at the receiving end using protection devices; The control strategy module is configured to activate a cooperative control strategy after the fault is detected. The cooperative control strategy includes an HBMMC station fault ride-through strategy based on bridge arm reference wave reconstruction and an LCC station fault ride-through strategy based on local measurements. The system reset module is configured to switch the control mode of the receiving-end flexible DC converter station back to the constant DC voltage mode during normal operation after the fault is cleared, and to complete the system reset.

9. A computing device, characterized in that, include: Memory and processor; The memory is used to store computer-executable instructions, and the processor is used to execute the computer-executable instructions, which, when executed by the processor, implement the steps of the receiving-end AC fault ride-through method of the hybrid DC transmission system according to any one of claims 1 to 7.

10. A computer-readable storage medium storing computer-executable instructions that, when executed by a processor, implement the steps of the receiving-end AC fault ride-through method for a hybrid DC transmission system according to any one of claims 1 to 7.

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