Multi-receiving-end direct current power transmission system receiving-end unbalanced fault ride-through control method, device and equipment

Abstract

This application relates to a receiving-end unbalanced fault ride-through control method, apparatus, and equipment for a multi-receiving-end DC transmission system. The method includes obtaining a determination of the AC unbalanced fault occurring at the receiving end; if the determination method is to detect an AC unbalanced fault at the receiving-end converter station through a fault detection device, obtaining the capacitor voltage and unbalanced fault conduction threshold of each MMC submodule, and setting a preset conduction threshold according to the unbalanced fault conduction threshold using an unbalanced rule; if the capacitor voltage is greater than the preset conduction threshold, controlling the corresponding power electronic switch to conduct, thereby dissipating excess energy; if the determination method is to detect an AC unbalanced fault at another receiving-end converter station through a healthy receiving-end converter station, obtaining current limiting adjustment data using a bridge arm DC voltage suppression rule and inputting it into the converter valve controller of the healthy receiving-end converter station to generate trigger control signals to control the operation of the MMC submodules of each bridge arm, thereby dissipating excess energy.

Description

Technical Field

[0001] This application relates to the field of power transmission and distribution technology, and in particular to a receiving-end unbalanced fault ride-through control method, apparatus and equipment for a multi-receiving-end DC transmission system. Background Technology

[0002] Against the backdrop of global energy transformation, new energy sources are being integrated into the power grid on a large scale. However, most new energy bases are located in remote areas with low load levels and weak grid structures, creating a significant demand for stable transmission of isolated new energy sources. Flexible DC transmission, with its flexibility, controllability, and high efficiency, is a crucial means of power transmission for new energy. When flexible DC transmission is used to connect isolated power generation systems and the receiving-end AC grid, a fault in the receiving-end AC grid can lead to a large surplus of power in the DC transmission system if the sending-end power generation system is not disconnected. This surplus power can cause severe overvoltages and jeopardize the safe operation of the DC transmission system. In engineering, two main schemes are used to dissipate this surplus power. The first scheme involves deploying DC energy dissipation devices on the DC side of the receiving-end converter station to consume excess power during a fault, allowing fault ride-through without disconnecting the sending-end isolated power generation system. However, this method involves complex DC energy dissipation devices with numerous controllable power components, resulting in high costs. The second scheme involves installing AC energy dissipation devices in the AC lines of the sending-end converter station, which has a simpler topology and lower cost. However, since the AC energy dissipation device is installed at the sending-end converter station, when the receiving-end converter station fails, it is necessary to notify the sending end to activate the energy dissipation device through communication means. For ultra-long-distance power transmission systems, the communication delay is relatively long, which may cause a large amount of surplus power to continuously flow into the flexible DC valve during the fault period, thereby causing overvoltage lockout.

[0003] The technology of converter valves with integrated energy dissipation functions is still in its infancy. Existing technologies, such as the AC fault ride-through control method and system for the receiving end of a flexible DC transmission system disclosed in CN120582110A, involve real-time acquisition of the capacitor voltage values ​​of each energy self-balancing submodule, averaging the capacitor voltage values ​​associated with each bridge arm to obtain the average module voltage of each bridge arm, adjusting each energy self-balancing submodule based on the capacitor voltage values ​​and the average module voltages, and determining the trigger command corresponding to each bridge arm based on the average module voltages and the preset converter valve control model when an AC fault signal is received, and using the trigger commands to regulate the associated energy self-balancing submodules. However, when unbalanced faults such as single-phase or phase-to-phase faults occur in AC systems, the three-phase surplus energy distribution of the converter valve becomes unbalanced. Typically, the faulty phase experiences a greater energy impact, and the capacitor voltage of the energy self-balancing submodule rises faster and higher than that of the non-faulty phase. Using the existing converter valve energy discharge triggering technology, the faulty phase submodule bears most of the energy discharge pressure during unbalanced faults, while the non-faulty phase hardly participates in energy discharge. Therefore, the faulty phase submodule needs to be equipped with a larger volume and higher heat capacity energy discharge resistor. Considering the randomness of the fault occurrence, all three-phase submodules of the converter valve need to be equipped with larger volume and higher heat capacity energy discharge resistors, which significantly increases the size, footprint, and cost of the converter valve.

[0004] Secondly, for multi-receiving-end DC transmission systems, when an AC system imbalance fault occurs at one receiving-end converter station, a large amount of surplus power from new energy sources cannot be dissipated in time and flows into nearby healthy receiving-end converter stations, causing the arm current of the healthy receiving-end converter station to rise rapidly, resulting in overcurrent blocking and failure of fault ride-through. Effective measures need to be taken to suppress the arm current of the healthy receiving-end converter station and improve the safety and reliability of multi-receiving-end DC transmission systems. Summary of the Invention

[0005] This application provides a receiving-end unbalanced fault ride-through control method, apparatus, and equipment for a multi-receiving-end DC transmission system, which is used to solve the technical problem that the energy discharge of the energy self-balancing circuit of each bridge arm submodule in the converter station is unbalanced when an AC unbalanced fault occurs in the existing multi-receiving-end DC transmission system.

[0006] To achieve the above objectives, this application provides the following technical solution:

[0007] On the one hand, a receiving-end unbalanced fault ride-through control method is provided for a multi-receiving-end DC transmission system. This method is applied to such a system, which includes a sending-end converter station and at least two receiving-end converter stations connected to the sending-end converter station via DC transmission lines. Each receiving-end converter station employs an energy-self-balancing DC converter valve. Each arm of the DC converter valve is composed of several MMC sub-modules with energy-self-balancing branches connected in series. The receiving-end unbalanced fault ride-through control method includes the following steps:

[0008] Obtain the method for determining the occurrence of AC imbalance faults at the receiving end of the multi-receiving-end DC transmission system;

[0009] If the determination method involves detecting an AC imbalance fault in the receiving-end converter station using a fault detection device, then the capacitor voltage and imbalance fault conduction threshold of each MMC submodule are obtained, and a preset conduction threshold for each MMC submodule is set according to the imbalance fault conduction threshold; if the capacitor voltage of the MMC submodule is greater than its preset conduction threshold, then the MMC submodule is controlled to perform energy discharge operation to dissipate excess energy.

[0010] If the determination method involves detecting an AC imbalance fault in another receiving-end converter station through the healthy receiving-end converter station, then the current limit value of the outer loop controller in the healthy receiving-end converter station is adjusted to obtain current limit adjustment data; the current limit adjustment data is input into the converter valve controller of the healthy receiving-end converter station to generate trigger control signals corresponding to each bridge arm, and each trigger control signal is used to control the operation of the corresponding MMC submodule to dissipate surplus energy.

[0011] Preferably, the receiving-end unbalanced fault ride-through control method of the multi-receiving-end DC transmission system further includes: setting a preset conduction threshold for each of the MMC sub-modules according to the unbalanced fault conduction threshold using an unbalanced rule, wherein the content of the unbalanced rule includes:

[0012] Obtain the faulty phase and non-faulty phase of the receiving-end converter station where the AC imbalance fault occurs;

[0013] Set the preset on-threshold of each MMC submodule in the non-faulty phase of the DC converter valve to the unbalanced fault on-threshold, and keep the preset on-threshold of each MMC submodule in the faulty phase of the DC converter valve unchanged.

[0014] Preferably, the receiving-end unbalanced fault ride-through control method of the multi-receiving-end DC transmission system further includes: adjusting the current limiting value of the outer loop controller in the healthy receiving-end converter station using a bridge arm DC voltage suppression rule to obtain current limiting adjustment data; the bridge arm DC voltage suppression rule includes:

[0015] Obtain the limiting margin of the receiving-end converter station and the current reference value output by the outer loop controller of the receiving-end converter station before the fault.

[0016] The maximum and minimum current limit values ​​of the outer loop controller are calculated based on the limiting margin and the current reference value.

[0017] The current limiting adjustment data includes the maximum current limiting value and the minimum current limiting value.

[0018] Preferably, the receiving-end unbalanced fault ride-through control method for the multi-receiving-end DC transmission system further includes: calculating, based on the limiting margin and the current reference value, the maximum and minimum current limiting values ​​of the outer loop controller using a current limiting adjustment formula; the current limiting adjustment formula is:

[0019]

[0020] In the formula, I sdref ' is the current reference value, τ is the limiting margin, I max I is the maximum current limiting value. min This is the minimum current limit.

[0021] Preferably, the value of the limiting margin is no greater than 5%.

[0022] On the other hand, a receiving-end unbalanced fault ride-through control device for a multi-receiving-end DC transmission system is provided, which is applied to the multi-receiving-end DC transmission system. The multi-receiving-end DC transmission system includes a sending-end converter station and at least two receiving-end converter stations connected to the sending-end converter station via DC transmission lines. Each receiving-end converter station adopts an energy self-balancing DC converter valve. Each arm of the DC converter valve is composed of several MMC sub-modules with energy self-balancing branches connected in series. The receiving-end unbalanced fault ride-through control device includes a mode acquisition module, a first fault ride-through module, and a second fault ride-through module.

[0023] The method acquisition module is used to acquire the determination method for AC imbalance faults occurring at the receiving end of the multi-receiving-end DC transmission system;

[0024] The first fault ride-through module is used to determine the AC imbalance fault detected by the fault detection device at the receiving-end converter station, and then obtain the capacitor voltage and imbalance fault conduction threshold of each MMC submodule, and set a preset conduction threshold for each MMC submodule according to the imbalance fault conduction threshold; if the capacitor voltage of the MMC submodule is greater than its preset conduction threshold, then control the MMC submodule to perform energy discharge operation to dissipate excess energy;

[0025] The second fault ride-through module is used to adjust the current limit value of the outer loop controller in the healthy receiving-end converter station according to the determination method, which is to detect an AC imbalance fault in another receiving-end converter station through the healthy receiving-end converter station, thereby obtaining current limit adjustment data; inputting the current limit adjustment data into the converter valve controller of the healthy receiving-end converter station to generate trigger control signals corresponding to each bridge arm, and using each trigger control signal to control the operation of the corresponding MMC submodule to achieve surplus energy dissipation.

[0026] Preferably, the first fault ride-through module is further configured to set a preset conduction threshold for each of the MMC sub-modules according to the unbalanced fault conduction threshold using an unbalanced rule. The unbalanced rule includes: obtaining the faulty phase and non-faulty phase of the receiving-end converter station where an AC unbalanced fault occurs; setting the preset conduction threshold of each MMC sub-module of the non-faulty phase in the DC converter valve as the unbalanced fault conduction threshold; and keeping the preset conduction threshold of each MMC sub-module of the faulty phase in the DC converter valve unchanged.

[0027] Preferably, the second fault ride-through module is further configured to adjust the current limiting value of the outer loop controller in the healthy receiving-end converter station using a bridge arm DC voltage suppression rule, thereby obtaining current limiting adjustment data; the bridge arm DC voltage suppression rule includes:

[0028] Obtain the limiting margin of the receiving-end converter station and the current reference value output by the outer loop controller of the receiving-end converter station before the fault.

[0029] The maximum and minimum current limits of the outer loop controller are calculated using the current limit adjustment formula based on the current limit margin and the current reference value.

[0030] The current limiting adjustment formula is as follows:

[0031]

[0032] In the formula, I sdref ' is the current reference value, τ is the limiting margin, I max I is the maximum current limiting value. min The current limit is the minimum value; wherein, the current limit adjustment data includes the maximum current limit and the minimum current limit.

[0033] Preferably, the value of the limiting margin is no greater than 5%.

[0034] On the other hand, a terminal device is provided, including a processor and a memory;

[0035] The memory is used to store program code and transmit the program code to the processor;

[0036] The processor is configured to execute the above-described receiving-end unbalanced fault ride-through control method for multi-receiving-end DC transmission systems according to the instructions in the program code.

[0037] The receiving-end unbalanced fault ride-through control method, apparatus, and equipment for a multi-receiving-end DC transmission system are applied to such systems. The multi-receiving-end DC transmission system includes a sending-end converter station and at least two receiving-end converter stations connected to the sending-end converter station via DC transmission lines. Each receiving-end converter station employs an energy-self-balancing DC converter valve. Each arm of the DC converter valve is composed of several MMC sub-modules with energy-self-balancing branches connected in series. The receiving-end unbalanced fault ride-through control method includes determining the method for detecting an AC unbalanced fault at the receiving end of the multi-receiving-end DC transmission system; if the determination method involves detecting an AC unbalanced fault at the receiving-end converter station using a fault detection device, obtaining the capacitor voltage and unbalanced voltage of each MMC sub-module. The system balances the conduction threshold for faults and sets preset conduction thresholds for each MMC submodule based on the unbalanced fault conduction threshold. If the capacitor voltage of an MMC submodule exceeds its preset conduction threshold, the module is controlled to perform energy discharge operations to dissipate excess energy. If the method determines that an AC unbalanced fault is detected in another receiving-end converter station through a healthy receiving-end converter station, the current limit value of the outer loop controller in the healthy receiving-end converter station is adjusted to obtain current limit adjustment data. This current limit adjustment data is then input into the converter valve controller of the healthy receiving-end converter station to generate trigger control signals for each bridge arm. These trigger control signals are used to control the operation of the corresponding MMC submodules to dissipate excess energy.

[0038] As can be seen from the above technical solutions, this application has the following advantages: The receiving-end unbalanced fault ride-through control method of this multi-receiving-end DC transmission system adopts different fault ride-through strategies by determining the AC unbalanced fault occurring in the receiving-end converter station, dissipating the surplus energy generated by the unbalanced fault, and realizing the unbalanced fault ride-through. This avoids the problem of uneven energy discharge in the energy self-balancing circuits of each bridge arm submodule of the receiving-end converter station, and solves the technical problem of uneven energy discharge in the energy self-balancing circuits of each bridge arm submodule of the existing multi-receiving-end DC transmission system when an AC unbalanced fault occurs.

[0039] The receiving-end unbalanced fault ride-through control device of the multi-receiving-end DC transmission system can adopt different fault ride-through strategies based on the determination of AC unbalanced faults occurring in the receiving-end converter station through the mode acquisition module, the first fault ride-through module and the second fault ride-through module. This dissipates the surplus energy generated by the unbalanced fault, realizes unbalanced fault ride-through, and avoids the problem of uneven energy discharge in the energy self-balancing circuit of each bridge arm submodule of the receiving-end converter station. Attached Figure Description

[0040] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0041] Figure 1 This is a flowchart illustrating the steps of the receiving-end unbalanced fault ride-through control method for a multi-receiving-end DC transmission system according to an embodiment of this application.

[0042] Figure 2 This is a topology diagram of the DC converter valve in the receiving-end unbalanced fault ride-through control method of the multi-receiving-end DC transmission system described in the embodiments of this application;

[0043] Figure 3 This is a control block diagram of the converter valve controller of the receiving-end converter station in the receiving-end unbalanced fault ride-through control method of the multi-receiving-end DC transmission system described in the embodiments of this application.

[0044] Figure 4 This is a schematic diagram comparing simulation results of the receiving-end unbalanced fault ride-through control method for a multi-receiving-end DC transmission system described in the embodiments of this application;

[0045] Figure 5 This is a schematic diagram of the frame of the receiving-end unbalanced fault ride-through control device for a multi-receiving-end DC transmission system as described in the embodiments of this application;

[0046] Figure 6 This is a schematic diagram of the terminal device described in an embodiment of this application. Detailed Implementation

[0047] To make the inventive objectives, features, and advantages of this application more apparent and understandable, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0048] In the description of the embodiments of this application, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0049] In the embodiments of this application, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in the embodiments of this application according to the specific circumstances.

[0050] Patent terminology used in this application:

[0051] Turning off refers to the process of switching the power electronic switch IGBT from the ON state to the OFF state.

[0052] The transmission line fault monitoring device identifies fault types by monitoring parameters such as current, voltage, and temperature, and uses the traveling wave method to locate fault points. It supports the identification of various types of faults, including short circuits, open circuits, and mixed faults.

[0053] Switch testing equipment includes high-voltage switch characteristic testers, which are used to evaluate the electrical performance of switchgear.

[0054] This application provides a receiving-end unbalanced fault ride-through control method, apparatus, and equipment for a multi-receiving-end DC transmission system, which solves the technical problem that the energy discharge of the energy self-balancing circuit of each bridge arm submodule in the converter station is unbalanced when an AC unbalanced fault occurs in the existing multi-receiving-end DC transmission system.

[0055] Example 1:

[0056] Figure 1 This is a flowchart illustrating the steps of the receiving-end unbalanced fault ride-through control method for a multi-receiving-end DC transmission system according to an embodiment of this application. Figure 2 This is a topology diagram of the DC converter valve in the receiving-end unbalanced fault ride-through control method of the multi-receiving-end DC transmission system described in the embodiments of this application.

[0057] like Figure 2As shown, this application provides a receiving-end unbalanced fault ride-through control method for a multi-receiving-end DC transmission system, which is applied to the multi-receiving-end DC transmission system. The multi-receiving-end DC transmission system includes a sending-end converter station and at least two receiving-end converter stations connected to the sending-end converter station via DC transmission lines. Each receiving-end converter station adopts an energy-self-balancing DC converter valve. Each arm of the DC converter valve is composed of several MMC sub-modules with energy-self-balancing branches connected in series. Each MMC sub-module includes a capacitor and a discharge component connected in parallel with the capacitor. The discharge component includes a power electronic switch and a discharge element connected in series.

[0058] It should be noted that the energy dissipation element can be a resistor. The power electronic switch can be a transistor, field-effect transistor, or thyristor, etc., to control the circuit switching. MMC stands for Modular Multilevel Converter. When a fault occurs in a multi-receiving-end DC transmission system, the power generation capacity of the new energy source fails to change in time, resulting in an imbalance in power transmission between the sending-end converter station and the receiving-end converter station. To reduce the waste of surplus power as heat, the surplus power / energy in the system is recovered by the capacitors of the MMC sub-modules in the full-bridge or half-bridge of the DC converter station. At this time, the capacitor voltage of the MMC sub-module continues to rise. This application adopts the receiving-end unbalanced fault ride-through control method of the multi-receiving-end DC transmission system to realize the dissipation of surplus energy.

[0059] like Figure 1 As shown in the figure, this application provides a receiving-end unbalanced fault ride-through control method for a multi-receiving-end DC transmission system, including the following steps:

[0060] S1. Obtain the method for determining AC imbalance faults occurring at the receiving end of a multi-receiving-end DC transmission system.

[0061] It should be noted that the method of obtaining information about an AC imbalance fault at a receiving-end converter station in a multi-receiving-end DC transmission system in step S1 can be as follows: the fault detection device at the receiving-end converter station detects the asymmetrical fault, or the fault detection device at a non-healthy receiving-end converter station detects the imbalance fault and then transmits the fault information to other healthy receiving-end converter stations via communication. The fault detection device can be selected as a transmission line fault monitoring device or other device capable of detecting faults or abnormal operating conditions in the power system. Fault detection is a relatively mature technology in this field.

[0062] S2. If the method is to detect an AC imbalance fault in the receiving-end converter station through a fault detection device, obtain the capacitor voltage and imbalance fault conduction threshold of each MMC submodule, and set the preset conduction threshold of each MMC submodule according to the imbalance fault conduction threshold; if the capacitor voltage of the MMC submodule is greater than its preset conduction threshold, then control the MMC submodule to perform energy discharge operation to dissipate the surplus energy.

[0063] It should be noted that step S2 is based on the method determined in step S1, which involves detecting an AC imbalance fault in the receiving-end converter station using a fault detection device. This involves obtaining the capacitor voltage and imbalance fault conduction threshold of each MMC submodule. Then, a preset conduction threshold is set for each MMC submodule based on its imbalance fault conduction threshold. Next, the obtained capacitor voltage of each MMC submodule is compared with its corresponding preset conduction threshold to determine whether fault ride-through should be performed. If the capacitor voltage of an MMC submodule is greater than its preset conduction threshold, the power electronic switch of that MMC submodule is turned on to perform energy dissipation or fault ride-through, thus dissipating excess energy. If the capacitor voltage of an MMC submodule is not greater than its preset conduction threshold, fault ride-through is not performed.

[0064] In this embodiment of the application, the receiving-end unbalanced fault ride-through control method further includes:

[0065] Obtain the first margin k, second margin h, third margin m, steady-state operating voltage U0, and single module latch-up voltage U for each MMC submodule. cut ;

[0066] Based on the single module lockout voltage U of each MMC submodule cut The first margin k is calculated using the balanced fault conduction formula to obtain the balanced fault conduction threshold of this MMC submodule. ;

[0067] Based on the balanced fault conduction threshold of each MMC submodule The second margin h is calculated using the unbalanced fault conduction formula to obtain the unbalanced fault conduction threshold of this MMC submodule. ;

[0068] The turn-off threshold of each MMC submodule is calculated using the turn-off formula based on the third margin m and the steady-state operating voltage U0. ;

[0069] The formula for balanced fault conduction is:

[0070]

[0071] The formula for conducting an unbalanced fault is:

[0072]

[0073] The turn-off formula is: .

[0074] In this embodiment of the application, the receiving-end unbalanced fault ride-through control method further includes: based on the power electronic switch of the control MMC submodule being turned on, the capacitor update voltage of the MMC submodule is also acquired in real time; when the capacitor update voltage is lower than the turn-off threshold of the MMC submodule, the power electronic switch of the MMC submodule is turned off to terminate the energy dissipation operation.

[0075] It should be noted that when the power electronic switch controlling the MMC submodule is turned off, the corresponding DC converter valve only performs the energy exchange function.

[0076] Figure 3 This is a control block diagram of the converter valve controller of the receiving-end converter station in the receiving-end unbalanced fault ride-through control method of the multi-receiving-end DC transmission system described in the embodiments of this application.

[0077] S3. If the method is determined to be that an AC imbalance fault is detected in other receiving-end converter stations through a healthy receiving-end converter station, then the current limit value of the outer loop controller in the healthy receiving-end converter station is adjusted to obtain current limit adjustment data; the current limit adjustment data is input into the converter valve controller of the healthy receiving-end converter station to generate trigger control signals corresponding to each bridge arm, and the corresponding MMC submodule is controlled by each trigger control signal to achieve surplus energy dissipation.

[0078] It should be noted that in step S3, based on the method determined in step S1 where an AC imbalance fault is detected in another receiving-end converter station (another receiving-end converter station) through a healthy receiving-end converter station, the current limit value of the outer loop controller in the healthy receiving-end converter station is adjusted to obtain current limit adjustment data. This current limit adjustment data is input into the converter valve controller of the healthy receiving-end converter station to generate trigger control signals corresponding to each bridge arm. These trigger control signals control the operation of the corresponding MMC submodule, thereby dissipating excess energy. In this embodiment, during a fault, the energy imbalance between the sending-end converter station and the receiving-end converter station will cause a large influx of excess energy into the healthy receiving-end converter station, resulting in a transient and rapid increase in the bridge arm current and saturation of the outer loop controller. Therefore, the receiving-end imbalance fault ride-through control method of this multi-receiving-end DC transmission system employs a bridge arm DC voltage suppression rule to reduce the adjustment of the outer loop controller saturation value during the fault period, thereby suppressing the bridge arm current.

[0079] In the embodiments of this application, such as Figure 3 As shown, the controller of the DC converter valve includes an outer loop controller, an inner loop controller, an abc-dq controller, a dq-abc controller, a summing arithmetic unit, and a PWM modulator. The DC voltage command value, the actual DC voltage value, the reactive power command value, and the actual reactive power value are input to the outer loop controller to obtain the current reference values ​​(i.e., i) along the d-axis and q-axis in the synchronous rotating coordinate system.dq ), current reference value i dq Current limiting adjustment data i is obtained by using the bridge arm DC voltage suppression rule for limiting adjustment. sdq_ref The three-phase grid voltage u at the AC output terminal of the energy-self-balancing DC converter valve in the three-phase stationary coordinate system. sabc and current i sabc Inputting the abc-dq controller, the three-phase output current in the three-phase stationary coordinate system is converted to the d-axis and q-axis output current values ​​in the synchronous rotating coordinate system (i.e., i... sdq The three-phase output voltage in the three-phase stationary coordinate system is converted to the d-axis and q-axis output voltage values ​​in the synchronous rotating coordinate system (i.e., u). sdq ) and the synchronous angular frequency (i.e., θ) of the three-phase AC power grid m The three-phase output current in the three-phase stationary coordinate system is converted to the d-axis and q-axis output current values ​​in the synchronous rotating coordinate system (i.e., i...). sdq The three-phase output voltage in the three-phase stationary coordinate system is converted to the d-axis and q-axis output voltage values ​​in the synchronous rotating coordinate system (i.e., u). sdq The input to the inner loop controller yields the reference values ​​of the d-axis and q-axis voltages in the synchronous rotating coordinate system (i.e., u). cdq_ref The voltage reference values ​​u of the d-axis and q-axis in the synchronous rotating coordinate system will be used. cdq_ref Synchronous angular frequency θ of a three-phase AC power grid m Inputting the bias voltage into the dq-abc controller, the bias voltage and each DC bias coefficient are input into the summing arithmetic unit to obtain the reference value of the three-phase AC port voltage (i.e., u) of the energy self-balancing flexible DC converter valve in the three-phase stationary coordinate system corresponding to each bridge arm. cabc_ref The reference values ​​of the three-phase voltage at the AC port of each energy self-balancing flexible DC converter valve in the three-phase stationary coordinate system are input to the PWM modulator to obtain the trigger control signal corresponding to each bridge arm.

[0080] In this embodiment, the current limiting adjustment data is input into the converter valve controller of a healthy receiving-end converter station to generate trigger control signals corresponding to each bridge arm. Each trigger control signal is used to control the operation of the corresponding MMC submodule. After the surplus energy is dissipated, as the surplus energy is gradually dissipated by the energy self-balancing branch of the receiving-end converter station, the receiving-end unbalanced fault ride-through control method of the multi-receiving-end DC transmission system further includes: obtaining the rated current limiting value of a healthy receiving-end converter station during normal operation and obtaining the bridge arm current of a healthy receiving-end converter station in real time. If the bridge arm current of a healthy receiving-end converter station reaches its peak value and then gradually decreases, the current limiting value of the outer loop controller in the healthy receiving-end converter station is set to the rated current limiting value.

[0081] In this embodiment, the receiving-end unbalanced fault ride-through control method for multi-receiving-end DC transmission systems is applicable to the AC unbalanced fault ride-through process of flexible DC transmission systems. Considering the uneven energy transmission between phases of the converter valve during an asymmetrical fault, resulting in a higher capacitor voltage in the faulty phase than in the non-faulty phase, the differentiated unbalanced rule triggering method effectively avoids uneven distribution of energy discharge pressure across the bridge arms due to the faulty phase's participation in energy discharge. Therefore, given a fixed total discharged energy, this method can maximize the balance of the discharged energy required by the MMC submodule discharge resistors of each bridge arm, reducing the volume and cost of the discharge resistors and effectively improving the economic efficiency of the energy self-balancing converter valves. Furthermore, it effectively controls the DC line voltage rise caused by energy imbalance in the multi-receiving-end DC transmission system during the fault period, achieving no surge arrester operation during fault ride-through and improving the safety and reliability of the multi-receiving-end DC transmission system.

[0082] It should be noted that when an AC imbalance fault occurs at one converter station in a multi-receiving-end DC transmission system, the other healthy receiving-end converter stations are subjected to transient energy impacts, causing the bridge arm current to rise rapidly and triggering overcurrent blocking. The receiving-end imbalance fault ride-through control method of this multi-receiving-end DC transmission system uses the bridge arm DC voltage suppression rule to adjust the current limit value of the outer loop controller in the receiving-end converter station, which can further improve the safety and reliability of the multi-receiving-end DC transmission system.

[0083] This application provides a receiving-end unbalanced fault ride-through control method for a multi-receiving-end DC transmission system. The method is applied to such a system, which includes a sending-end converter station and at least two receiving-end converter stations connected to it via DC transmission lines. Each receiving-end converter station employs an energy-self-balancing DC converter valve. Each arm of the DC converter valve consists of several MMC sub-modules with energy-self-balancing branches connected in series. The receiving-end unbalanced fault ride-through control method includes determining the method for detecting an AC unbalanced fault at the receiving end of the multi-receiving-end DC transmission system; if the determination method involves detecting an AC unbalanced fault at the receiving-end converter station using a fault detection device, then acquiring the capacitor voltage and capacitance of each MMC sub-module. An unbalanced fault conduction threshold is set, and a preset conduction threshold is set for each MMC submodule. If the capacitor voltage of an MMC submodule is greater than its preset conduction threshold, the MMC submodule is controlled to perform energy discharge to dissipate excess energy. If the determination method is that an AC unbalanced fault is detected in another receiving-end converter station through a healthy receiving-end converter station, the current limit value of the outer loop controller in the healthy receiving-end converter station is adjusted to obtain current limit adjustment data. The current limit adjustment data is input into the converter valve controller of the healthy receiving-end converter station to generate trigger control signals corresponding to each bridge arm. Each trigger control signal is used to control the operation of the corresponding MMC submodule to dissipate excess energy. The receiving-end unbalanced fault ride-through control method of this multi-receiving-end DC transmission system adopts different fault ride-through strategies based on the determination of AC unbalanced faults in the receiving-end converter station. It dissipates the surplus energy generated by the unbalanced fault, realizes unbalanced fault ride-through, avoids the problem of uneven energy discharge in the energy self-balancing circuits of each bridge arm submodule of the receiving-end converter station, and solves the technical problem of uneven energy discharge in the energy self-balancing circuits of each bridge arm submodule of the existing multi-receiving-end DC transmission system when AC unbalanced faults occur.

[0084] In one embodiment of this application, the receiving-end unbalanced fault ride-through control method of the multi-receiving-end DC transmission system further includes: setting a preset conduction threshold for each MMC submodule according to an unbalanced fault conduction threshold using an unbalanced rule, wherein the unbalanced rule includes:

[0085] Obtain the faulty and non-faulty phases of the receiving-end converter station where an AC imbalance fault occurs;

[0086] Set the preset on-threshold of each MMC submodule in the non-faulty phase of the DC converter valve to the unbalanced fault on-threshold, and keep the preset on-threshold of each MMC submodule in the faulty phase of the DC converter valve unchanged.

[0087] It should be noted that the receiving-end unbalanced fault ride-through control method of this multi-receiving-end DC transmission system is based on detecting AC unbalanced faults in the receiving-end converter station through a fault detection device. It uses unbalance rules to set preset conduction thresholds for MMC submodules of the fault phase and preset conduction thresholds for MMC submodules of the non-fault phase, which facilitates the receiving-end unbalanced fault ride-through control method of the multi-receiving-end DC transmission system to achieve unbalanced fault ride-through.

[0088] In one embodiment of this application, the receiving-end unbalanced fault ride-through control method of the multi-receiving-end DC transmission system further includes: adjusting the current limiting value of the outer loop controller in the healthy receiving-end converter station using a bridge arm DC voltage suppression rule to obtain current limiting adjustment data; the bridge arm DC voltage suppression rule includes:

[0089] Obtain the limiting margin of the receiving-end converter station and the current reference value output by the outer loop controller of the receiving-end converter station before the fault.

[0090] Based on the limiting margin and the current reference value, the maximum and minimum current limiting values ​​of the outer loop controller are calculated.

[0091] The current limit adjustment data includes the maximum current limit value and the minimum current limit value.

[0092] It should be noted that the maximum and minimum current limits of the outer loop controller are calculated using the current limiting margin and the current reference value according to the current limiting adjustment formula. The current limiting adjustment formula is as follows:

[0093]

[0094] In the formula, I sdref ' is the current reference value, τ is the limiting margin, I max I is the maximum current limiting value. min This is the minimum current limiting value. In this embodiment, the limiting margin is generally no greater than 5%.

[0095] Figure 4 This is a schematic diagram comparing simulation results of the receiving-end unbalanced fault ride-through control method for a multi-receiving-end DC transmission system described in the embodiments of this application. Figure 4 (a) shows the simulation results of the maximum energy discharged by the energy discharge resistors of each bridge arm submodule when a single-phase ground fault is used, employing the existing triggering method (such as application number 202311478671.4). Figure 4 (b) is a simulation result of the receiving-end unbalanced fault ride-through control method for the multi-receiving-end DC transmission system.

[0096] In the embodiments of this application, such as Figure 4As shown, the maximum energy of existing technologies is approximately 165 kJ. The maximum energy of the receiving-end unbalanced fault ride-through control method adopted in this multi-receiving-end DC transmission system is approximately 68 kJ. The energy discharged by the energy-discharging element of the MMC submodule is reduced by approximately 59% compared with existing technologies, resulting in significant economic benefits.

[0097] Example 2:

[0098] Figure 5 This is a schematic diagram of the frame of the receiving-end unbalanced fault ride-through control device for a multi-receiving-end DC transmission system as described in the embodiments of this application.

[0099] like Figure 5 As shown, this application embodiment provides a receiving-end unbalanced fault ride-through control device for a multi-receiving-end DC transmission system, which is applied to the multi-receiving-end DC transmission system. The multi-receiving-end DC transmission system includes a sending-end converter station and at least two receiving-end converter stations connected to the sending-end converter station via DC transmission lines. Each receiving-end converter station adopts an energy self-balancing DC converter valve. Each arm of the DC converter valve is composed of several MMC sub-modules with energy self-balancing branches connected in series. The receiving-end unbalanced fault ride-through control device includes a mode acquisition module 10, a first fault ride-through module 20, and a second fault ride-through module 30.

[0100] The method acquisition module 10 is used to acquire the determination method for AC imbalance faults occurring at the receiving end of a multi-receiving-end DC transmission system;

[0101] The first fault ride-through module 20 is used to obtain the capacitor voltage and unbalanced fault conduction threshold of each MMC submodule according to the determined method of detecting AC unbalanced fault in the receiving-end converter station through a fault detection device, and set the preset conduction threshold of each MMC submodule according to the unbalanced fault conduction threshold; if the capacitor voltage of the MMC submodule is greater than its preset conduction threshold, the module is controlled to perform energy discharge operation to dissipate excess energy.

[0102] The second fault ride-through module 30 is used to adjust the current limit value of the outer loop controller in the healthy receiving-end converter station when an AC imbalance fault is detected in other receiving-end converter stations through a determined method, in order to obtain current limit adjustment data. The current limit adjustment data is then input into the converter valve controller of the healthy receiving-end converter station to generate trigger control signals corresponding to each bridge arm. Each trigger control signal is used to control the operation of the corresponding MMC submodule to dissipate excess energy.

[0103] It should be noted that the content of the modules in the device of Embodiment 2 has already been described in the steps of the method of Embodiment 1. Therefore, the content of the receiving-end unbalanced fault ride-through control device module for the multi-receiving-end DC transmission system will not be repeated in this embodiment. In this embodiment, the receiving-end unbalanced fault ride-through control device of the multi-receiving-end DC transmission system, through the mode acquisition module, the first fault ride-through module, and the second fault ride-through module, can employ different fault ride-through strategies based on the determination method of AC unbalanced faults occurring in the receiving-end converter station. This dissipates the surplus energy generated by the unbalanced fault, achieving unbalanced fault ride-through and avoiding the problem of uneven energy discharge from the energy self-balancing circuits of each bridge arm submodule in the receiving-end converter station.

[0104] In this embodiment, the first fault ride-through module is further configured to set the preset conduction threshold of each MMC submodule according to the unbalanced fault conduction threshold using unbalance rules. The unbalance rules include: obtaining the faulty phase and non-faulty phase of the receiving-end converter station where the AC unbalanced fault occurs; setting the preset conduction threshold of each MMC submodule of the non-faulty phase in the DC converter valve as the unbalanced fault conduction threshold; and keeping the preset conduction threshold of each MMC submodule of the faulty phase in the DC converter valve unchanged.

[0105] In this embodiment, the second fault ride-through module is further configured to adjust the current limiting value of the outer loop controller in the healthy receiving-end converter station using a bridge arm DC voltage suppression rule, thereby obtaining current limiting adjustment data; the bridge arm DC voltage suppression rule includes:

[0106] Obtain the limiting margin of the receiving-end converter station and the current reference value output by the outer loop controller of the receiving-end converter station before the fault.

[0107] The maximum and minimum current limits of the outer loop controller are calculated using the current limit adjustment formula based on the current limit margin and the current reference value.

[0108] The current limiting adjustment formula is:

[0109]

[0110] In the formula, I sdref ' is the current reference value, τ is the limiting margin, I max I is the maximum current limiting value. min This represents the minimum current limit; the current limit adjustment data includes the maximum and minimum current limit values.

[0111] In the embodiments of this application, the value of the limiting margin is no greater than 5%.

[0112] Example 3:

[0113] Figure 6This is a schematic diagram of the terminal device described in an embodiment of this application.

[0114] like Figure 6 As shown, this application provides a terminal device, including a processor and a memory;

[0115] Memory is used to store program code and transfer the program code to the processor;

[0116] The processor is used to execute the above-mentioned receiving-end unbalanced fault ride-through control method for multi-receiving-end DC transmission systems according to the instructions in the program code.

[0117] It should be noted that the processor is used to execute the steps in the above-described embodiment of a receiving-end unbalanced fault ride-through control method for a multi-receiving-end DC transmission system according to the instructions in the program code. Alternatively, when the processor executes the computer program, it implements the functions of each module / unit in the above-described system / device embodiments.

[0118] For example, a computer program can be divided into one or more modules / units, one or more of which are stored in memory and executed by a processor to complete this application. One or more modules / units can be a series of computer program instruction segments capable of performing a specific function, which describe the execution process of the computer program in a terminal device.

[0119] Terminal devices can be computing devices such as desktop computers, laptops, handheld computers, and cloud servers. Terminal devices may include, but are not limited to, processors and memory. Those skilled in the art will understand that this does not constitute a limitation on the terminal device, which may include more or fewer components than illustrated, or combinations of certain components, or different components. For example, a terminal device may also include input / output devices, network access devices, buses, etc.

[0120] The processor can be a Central Processing Unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. A general-purpose processor can be a microprocessor or any conventional processor.

[0121] Memory can be an internal storage unit of a terminal device, such as a hard drive or RAM. Memory can also be an external storage device, such as a plug-in hard drive, SmartMedia Card (SMC), Secure Digital (SD) card, or Flash Card. Furthermore, memory can include both internal and external storage units. Memory is used to store computer programs and other programs and data required by the terminal device. Memory can also be used to temporarily store data that has been output or will be output.

[0122] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working processes of the systems, devices, and units described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0123] In the several embodiments provided in this application, it should be understood that the disclosed systems, apparatuses, and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative; for instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be an indirect coupling or communication connection between apparatuses or units through some interfaces, and may be electrical, mechanical, or other forms.

[0124] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0125] Furthermore, the functional units in the various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0126] If the integrated unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of the present invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0127] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A receiving-end unbalanced fault ride-through control method for a multi-receiving-end DC transmission system, applied to the multi-receiving-end DC transmission system, wherein the multi-receiving-end DC transmission system includes a sending-end converter station and at least two receiving-end converter stations connected to the sending-end converter station via DC transmission lines, each receiving-end converter station employs an energy-self-balancing DC converter valve, and each arm of the DC converter valve is composed of several MMC sub-modules with energy-self-balancing branches connected in series, characterized in that... The received-end unbalanced fault ride-through control method includes the following steps: Obtain the method for determining the occurrence of AC imbalance faults at the receiving end of the multi-receiving-end DC transmission system; If the determination method involves detecting an AC imbalance fault in the receiving-end converter station using a fault detection device, then the capacitor voltage and imbalance fault conduction threshold of each MMC submodule are obtained, and a preset conduction threshold for each MMC submodule is set according to the imbalance fault conduction threshold; if the capacitor voltage of the MMC submodule is greater than its preset conduction threshold, then the MMC submodule is controlled to perform energy discharge operation to dissipate excess energy. If the determination method involves detecting an AC imbalance fault in another receiving-end converter station through the healthy receiving-end converter station, then the current limit value of the outer loop controller in the healthy receiving-end converter station is adjusted to obtain current limit adjustment data; the current limit adjustment data is input into the converter valve controller of the healthy receiving-end converter station to generate trigger control signals corresponding to each bridge arm, and each trigger control signal is used to control the operation of the corresponding MMC submodule to dissipate surplus energy.

2. The receiving-end unbalanced fault ride-through control method for a multi-receiving-end DC transmission system according to claim 1, characterized in that, Also includes: Based on the unbalanced fault conduction threshold, preset conduction thresholds for each MMC submodule are set using unbalanced rules. The unbalanced rules include: Obtain the faulty phase and non-faulty phase of the receiving-end converter station where the AC imbalance fault occurs; Set the preset on-threshold of each MMC submodule in the non-faulty phase of the DC converter valve to the unbalanced fault on-threshold, and keep the preset on-threshold of each MMC submodule in the faulty phase of the DC converter valve unchanged.

3. The receiving-end unbalanced fault ride-through control method for a multi-receiving-end DC transmission system according to claim 1, characterized in that, Also includes: The current limit value of the outer loop controller in the healthy receiving-end converter station is adjusted by using the bridge arm DC voltage suppression rule to obtain current limit adjustment data; The bridge arm DC voltage suppression rules include: Obtain the limiting margin of the receiving-end converter station and the current reference value output by the outer loop controller of the receiving-end converter station before the fault. The maximum and minimum current limit values ​​of the outer loop controller are calculated based on the limiting margin and the current reference value. The current limiting adjustment data includes the maximum current limiting value and the minimum current limiting value.

4. The receiving-end unbalanced fault ride-through control method for a multi-receiving-end DC transmission system according to claim 3, characterized in that, Also includes: The maximum and minimum current limits of the outer loop controller are calculated using the current limit adjustment formula based on the current limit margin and the current reference value. The current limiting adjustment formula is as follows: In the formula, I sdref ' is the current reference value, τ is the limiting margin, I max I is the maximum current limiting value. min This is the minimum current limit.

5. The receiving-end unbalanced fault ride-through control method for a multi-receiving-end DC transmission system according to claim 3, characterized in that, The value of the limiting margin is no greater than 5%.

6. A receiving-end unbalanced fault ride-through control device for a multi-receiving-end DC transmission system, applied to the multi-receiving-end DC transmission system, wherein the multi-receiving-end DC transmission system includes a sending-end converter station and at least two receiving-end converter stations connected to the sending-end converter station via DC transmission lines, each receiving-end converter station employs an energy-self-balancing DC converter valve, and each arm of the DC converter valve is composed of several MMC sub-modules with energy-self-balancing branches connected in series, characterized in that... The receiving-end unbalanced fault ride-through control device includes: a mode acquisition module, a first fault ride-through module, and a second fault ride-through module; The method acquisition module is used to acquire the determination method for AC imbalance faults occurring at the receiving end of the multi-receiving-end DC transmission system; The first fault ride-through module is used to determine the AC imbalance fault detected by the fault detection device at the receiving-end converter station, and then obtain the capacitor voltage and imbalance fault conduction threshold of each MMC submodule, and set a preset conduction threshold for each MMC submodule according to the imbalance fault conduction threshold; if the capacitor voltage of the MMC submodule is greater than its preset conduction threshold, then control the MMC submodule to perform energy discharge operation to dissipate excess energy; The second fault ride-through module is used to adjust the current limit value of the outer loop controller in the healthy receiving-end converter station according to the determination method, which is to detect an AC imbalance fault in another receiving-end converter station through the healthy receiving-end converter station, thereby obtaining current limit adjustment data; inputting the current limit adjustment data into the converter valve controller of the healthy receiving-end converter station to generate trigger control signals corresponding to each bridge arm, and using each trigger control signal to control the operation of the corresponding MMC submodule to achieve surplus energy dissipation.

7. The receiving-end unbalanced fault ride-through control device for a multi-receiving-end DC transmission system according to claim 6, characterized in that, The first fault ride-through module is further configured to set the preset conduction threshold of each MMC submodule according to the unbalanced fault conduction threshold using unbalance rules. The unbalance rules include: obtaining the faulty phase and non-faulty phase of the receiving-end converter station where the AC unbalanced fault occurs; setting the preset conduction threshold of each MMC submodule of the non-faulty phase in the DC converter valve as the unbalanced fault conduction threshold; and keeping the preset conduction threshold of each MMC submodule of the faulty phase in the DC converter valve unchanged.

8. The receiving-end unbalanced fault ride-through control device for a multi-receiving-end DC transmission system according to claim 6, characterized in that, The second fault ride-through module is also used to adjust the current limit value of the outer loop controller in the healthy receiving-end converter station using the bridge arm DC voltage suppression rule, so as to obtain current limit adjustment data; The bridge arm DC voltage suppression rules include: Obtain the limiting margin of the receiving-end converter station and the current reference value output by the outer loop controller of the receiving-end converter station before the fault. The maximum and minimum current limits of the outer loop controller are calculated using the current limit adjustment formula based on the current limit margin and the current reference value. The current limiting adjustment formula is as follows: In the formula, I sdref ' is the current reference value, τ is the limiting margin, I max I is the maximum current limiting value. min The current limit is the minimum value; wherein, the current limit adjustment data includes the maximum current limit and the minimum current limit.

9. The receiving-end unbalanced fault ride-through control device for a multi-receiving-end DC transmission system according to claim 8, characterized in that, The value of the limiting margin is no greater than 5%.

10. A terminal device, characterized in that, Including the processor and memory; The memory is used to store program code and transmit the program code to the processor; The processor is configured to execute the receiving-end unbalanced fault ride-through control method for a multi-receiving-end DC transmission system as described in any one of claims 1-5, according to the instructions in the program code.

Images