A fault ride-through control method applicable to long-distance power transmission systems

Abstract

This application relates to a fault ride-through control method applicable to long-distance power transmission systems, comprising the following steps: when a fault occurs in the long-distance power transmission system, the AC side voltage of the receiving-end converter station drops, while the DC side voltage of the receiving-end converter station continuously rises; when the DC side voltage of the receiving-end converter station reaches a set action threshold, the power electronic switch control resistor of the DC voltage limiting device is engaged; when the AC energy dissipation device receives a system fault signal and is activated, the DC voltage limiting device stops operating, and the AC energy dissipation device absorbs surplus power; when the AC side voltage of the receiving-end converter station gradually recovers to the grid-connected voltage, the AC energy dissipation device stops operating. By combining DC-side voltage limiting and AC-side energy dissipation, fault ride-through control of long-distance power transmission systems can be achieved without configuring an additional water-cooling system, further reducing system losses and costs. This method changes the traditional energy dissipation method that combines AC and DC energy dissipation devices.

Description

Technical Field

[0001] This application relates to the field of fault ride-through technology for long-distance power transmission systems, and specifically to a fault ride-through control method applicable to long-distance power transmission systems. Background Technology

[0002] With the rapid construction of wind and solar power bases, large-scale centralized transmission has become a trend in new energy development. Flexible direct current transmission technology (VSC-HVDC), due to its advantages such as low loss and flexible control, has become one of the ideal choices for large-scale long-distance transmission of new energy, and has broad development prospects.

[0003] However, for traditional flexible DC transmission systems, when a system fault occurs, there is usually an overvoltage problem on the DC side due to surplus power. One type occurs when a single pole of the sending-end converter station is blocked, and the renewable energy units cannot be disconnected in a short time. The intact converter then transmits all the system power, leading to overvoltage and overcurrent in the converter, causing it to block. Another type occurs when the receiving-end AC system fails or the receiving-end converter station is blocked, reducing the system's power transmission capacity. However, the power output of the renewable energy units does not change in a short time, and the surplus power continues to charge the converter capacitors, causing the DC side voltage to rise continuously, threatening the safe operation of the system. Therefore, fault ride-through in flexible DC transmission systems has become a core issue restricting the development of large-capacity renewable energy transmission systems.

[0004] Existing fault ride-through schemes are basically divided into three types: The first is to reduce the power output of wind turbines through active control methods, mainly including fast communication, frequency increase, or voltage reduction. The second is to temporarily store or transfer surplus power. The third is to install energy dissipation devices, using resistors to absorb unbalanced power during fault ride-through. Among these, installing energy dissipation devices is the most direct and effective measure to solve the problem of surplus power in the system. By absorbing energy through energy dissipation resistors, it has been widely used in practical engineering. This method can ensure that new energy units do not need to be shut down, and that non-permanent faults in the system do not affect the normal operation of the overall power transmission system. However, as flexible DC transmission systems reach thousands of kilometers, when a fault occurs at one end of the flexible DC transmission system, the voltage on the receiving end DC side continues to rise, while the voltage on the sending end DC side does not change significantly. In addition, due to the long distance between converter stations, the communication delay between converter stations is as high as tens of milliseconds, and the AC energy dissipation devices cannot act in time to absorb surplus power, while the voltage on the receiving end DC side will exceed the voltage limit within 10-15 milliseconds. Therefore, traditional long-distance flexible DC systems configure both AC and DC power dissipation devices at the sending end and the receiving end to maintain system stability. However, the simultaneous configuration of AC and DC power dissipation devices brings two problems: first, the additional DC power dissipation devices increase size and cost; second, there is limited research on the coordinated control between AC and DC power dissipation devices. Furthermore, even if existing schemes configuring AC and DC power dissipation devices reduce the manufacturing cost of DC power dissipation devices, the lack of consideration for the coordination between the devices means that DC power dissipation devices are often designed based on the system's maximum capacity, requiring high-power water-cooling systems, resulting in wasted performance and further increasing fault ride-through costs. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, this application provides a fault ride-through control method applicable to long-distance power transmission systems, specifically adopting the following technical solution:

[0006] A fault ride-through control method applicable to a long-distance power transmission system is provided for controlling fault ride-through actions when a fault occurs in the long-distance power transmission system. The long-distance power transmission system includes at least an AC energy dissipation device and a DC voltage limiting device. The method includes the following steps:

[0007] When a fault occurs in the long-distance power transmission system, the AC side voltage of the receiving-end converter station drops, and the surplus power of the system charges the capacitor of the receiving-end converter station, causing the DC side voltage of the receiving-end converter station to rise continuously.

[0008] When the DC side voltage of the receiving-end converter station reaches the set action threshold, the power electronic switch control resistor of the DC voltage limiting device is activated to balance the DC side voltage of the receiving-end converter station.

[0009] When the AC energy dissipation device receives a system fault signal and is activated, the AC energy dissipation device will send an activation signal to the DC voltage limiting device. At this time, the DC voltage limiting device will stop operating, and the remote power transmission system will use the AC energy dissipation device to absorb the surplus power.

[0010] When the AC side voltage of the receiving-end converter station gradually recovers to the grid-connected voltage, the DC side voltage of the receiving-end converter station generates a momentary fluctuation signal. When the momentary fluctuation signal is transmitted to the AC energy dissipation device, the AC energy dissipation device stops operating.

[0011] Optionally: The DC voltage limiting device adopts a voltage outer loop control strategy. When the DC side voltage of the receiving-end converter station reaches the set action threshold, the DC voltage limiting device is activated. At this time, the outer loop control adopts a DC voltage hysteresis control strategy. The number of DC voltage limiting devices is controlled by comparing the difference between the actual value of the DC side voltage and the reference value, so as to maintain the DC side voltage of the receiving-end converter station within the preset fluctuation range.

[0012] Optionally, the DC voltage limiting device also adopts an inner-loop control strategy for the number of modules to be engaged. In this case, the inner-loop control adopts a stepped engagement strategy, which calculates the number of DC voltage limiting devices to be engaged based on the given engagement parameters of the DC voltage limiting device and the surplus power of the system.

[0013] Optional: The DC voltage limiting device adopts a periodic switching strategy when it is put into operation in stages.

[0014] The number of DC voltage limiting devices to be activated is set to m;

[0015] During the i-th cycle, the m DC voltage limiting devices operate sequentially according to their activation order, where the cycle set is represented as:

[0016] Input i ={1,2,3,…,m};

[0017] Input i This represents the set of periods for m DC voltage limiting devices;

[0018] The corresponding DC voltage limiting devices are switched on and off sequentially according to the order of the period set.

[0019] Optionally: The AC energy-consuming device adopts a power outer loop control strategy. In this case, the outer loop control calculates the number of groups of AC energy-consuming devices that need to be put into operation by comparing the difference between the input power at the sending end and the output power at the receiving end of the system. The AC energy-consuming device adopts a thyristor group arrangement and a group switching strategy to ensure stable absorption of surplus power during system faults.

[0020] Optionally, the AC energy-consuming device adopts a voltage inner-loop control strategy. In this case, the number of groups of AC energy-consuming devices is controlled by comparing the difference between the actual value and the reference value of the DC side voltage of the receiving-end converter station, so as to maintain the DC side voltage of the receiving-end converter station within a preset fluctuation range.

[0021] Optionally: The number of DC voltage limiting devices and the selection of their parameters must meet the DC voltage constraint conditions;

[0022] The DC voltage constraint conditions include:

[0023] When a fault occurs in the long-distance power transmission system, the power absorption capacity of the DC voltage limiting device exceeds the system's rated power:

[0024]

[0025] Where P rate P is the system's rated power. VLM U represents the absorption power of a single DC voltage limiting device; SM is the capacitor voltage of the MMC submodule; r is the internal resistance of the MMC submodule; m is the number of DC voltage limiting devices in operation;

[0026] When the DC voltage limiting device is activated, the energy absorbed by the DC voltage limiting device is equal to the energy transmitted by the system:

[0027]

[0028] Where D is the duty cycle of the DC voltage limiting device; f VLM is the operating frequency of the DC voltage limiting device; k is the ratio of the system's transmitted power to its rated power.

[0029] When the DC voltage limiting device is disconnected, the energy relationship of the remote power transmission system is as follows:

[0030]

[0031] Where C system U is the system equivalent capacitance; dc_high This represents the upper limit of the DC side voltage at the receiving-end converter station; U dc_low This is the lower limit of the DC side voltage of the receiving-end converter station; U dc Δu represents the DC-side voltage of the receiving-end converter station; Δu represents the fluctuation range of the DC-side voltage of the receiving-end converter station.

[0032] Meanwhile, the operating frequency of the DC voltage limiting device must meet the following requirements:

[0033]

[0034] Where f VLM_max This represents the maximum operating frequency of the DC voltage limiting device.

[0035] Optional: The number and parameters of the DC voltage limiting device must meet the requirements of energy absorption limit of the energy-consuming resistor and voltage fluctuation range of the capacitor;

[0036] When the DC voltage limiting device operates, the energy absorbed by each DC voltage limiting device must not exceed the upper limit of the resistor's allowable absorption capacity.

[0037]

[0038] Where E VLM Energy is absorbed by a single DC voltage limiting device; P rate The system's rated power; T VLM_MMC The operating time of the DC voltage limiting device from activation to deactivation; m is the number of DC voltage limiting devices activated; E allow The energy that each resistor in the DC voltage limiting device is allowed to absorb;

[0039] The maximum voltage relationship between the capacitor voltage of an MMC submodule equipped with a DC voltage limiting device and an MMC submodule without a DC voltage limiting device must satisfy the following:

[0040]

[0041] Where Δu SM1 The maximum voltage drop across the capacitor voltage of the MMC submodule configured with a DC voltage limiter; Δu SM2 The maximum voltage rise of the capacitor voltage of an MMC submodule without a DC voltage limiter; ΔU SM The system specifies the capacitor voltage fluctuation range for the MMC submodule; U SM U is the capacitor voltage of the MMC submodule; dc_high This represents the upper limit of the DC side voltage at the receiving-end converter station; U dc_low C represents the lower limit of the DC-side voltage at the receiving-end converter station. SM The capacitance value of the MMC submodule capacitor; i r i is the current flowing through the resistor in the DC voltage limiting device. bridge n is the bridge arm current; n is the total number of MMC submodules; D is the duty cycle of the DC voltage limiting device; r is the internal resistance of the MMC submodule; f VLM This refers to the operating frequency of the DC voltage limiting device.

[0042] Optionally: The selection of the AC energy dissipation device must meet the group number constraint, wherein the group number constraint includes:

[0043] When system faults are overcome and crossover control is implemented, the maximum power absorbed by the AC power dissipation device is equal to the system's rated power.

[0044]

[0045] When the power absorbed by the AC energy-consuming device is greater than the system transmission power, and the maximum power difference between the power absorbed by the AC energy-consuming device and the system transmission power is equal to the power absorbed by a single branch of the AC energy-consuming device, the voltage change caused by the power difference between the power absorbed by the AC energy-consuming device and the system transmission power discharging the system capacitor within a single operating cycle is as follows:

[0046]

[0047] Where P ACC Power absorbed by AC energy-consuming devices; P ACC_single For the power absorbed by a single branch of an AC power-consuming device; U ac_k T is the secondary voltage of the AC transformer. ac The period of the AC voltage; Δu ACC The maximum DC voltage change caused by power imbalance in AC power-consuming devices; U rate The system's rated voltage; C system P is the system equivalent capacitance; rate R is the rated power of the system; R is the resistance of the AC energy-consuming device; a is the number of AC energy-consuming device groups.

[0048] Optionally: The selected AC power dissipation device must meet voltage level constraints, wherein the voltage level constraints include:

[0049] The voltage level of the AC energy-consuming device is related to the turns ratio of the AC transformer. The higher the voltage level of the AC energy-consuming device, the more thyristors need to be connected in series; the lower the voltage level of the AC energy-consuming device, the more thyristors need to be connected in parallel.

[0050] When the turns ratio of an AC transformer is determined, the total number of thyristors required is expressed as:

[0051]

[0052] Number T U represents the total number of thyristors required; a represents the number of AC power dissipation device groups; U ac_k U is the secondary voltage of the AC transformer. ac U is the AC side voltage of the receiving-end converter station; k is the ratio of the system's transmitted power to its rated power; U T This is the operating voltage of the thyristor;

[0053] The current flowing through the thyristor is expressed as:

[0054]

[0055] Where I T P is the thyristor operating current. ACC_singleThe power absorbed by a single branch of an AC energy-consuming device.

[0056] Beneficial effects

[0057] The technical solution of this application achieves the following beneficial effects:

[0058] The fault ride-through control method for remote power transmission systems proposed in this application combines DC-side voltage limiting and AC-side energy dissipation. A modular multilevel converter (MMC) module with short-time voltage limiting capability is used at the receiving end, while an AC energy dissipation device is used at the sending end. When a system fault occurs and the DC voltage at the receiving end reaches a set value, the MMC with short-time voltage limiting capability at the receiving end activates first to stabilize the DC voltage. When the AC energy dissipation device detects the fault and starts, the MMC at the receiving end stops its voltage limiting function, and the AC energy dissipation device continues to operate to absorb surplus power until the fault ends. Through this combination, fault ride-through control of remote power transmission systems can be achieved. Furthermore, since the receiving-end MMC voltage limiting module (i.e., the DC voltage limiting device) only needs to operate for tens of milliseconds and absorb a small amount of energy, it does not require an additional water-cooling system, further reducing system losses and costs. This method changes the traditional energy dissipation method that combines AC and DC energy dissipation devices. Attached Figure Description

[0059] Figure 1 This is a schematic diagram of the structure of the remote power transmission system in the embodiments of this application.

[0060] Figure 2 This is a schematic diagram of the fault ride-through control method for remote power transmission systems applicable to embodiments of this application.

[0061] Figure 3 This is a diagram illustrating the principle and control logic of the DC voltage limiting device in this embodiment of the application. Figure 3 (a) is a schematic diagram of the DC voltage limiting device. Figure 3 Figure (b) shows the control logic diagram for limiting the voltage of the DC voltage limiting device.

[0062] Figure 4 This is a diagram illustrating the principle and control logic of the AC power consumption device limiting voltage in an embodiment of this application. Figure 4 (a) is a schematic diagram of the power dissipation of an AC power dissipation device; Figure 4 (b) is the control logic diagram for the power dissipation of the AC energy-consuming device.

[0063] Figure 5 This is a comparison diagram of the structure and parameters of the embodiments of this application and existing solutions.

[0064] Figure 6 This is a schematic diagram of the structure of an electronic device according to an embodiment of this application. Detailed Implementation

[0065] The present application will now be further described with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present application and should not be construed as limiting the scope of protection of the present application. It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of the present application.

[0066] Combination Figure 1 As shown in the illustration, this application specifically discloses a fault ride-through control method applicable to remote power transmission systems, which is used to control fault ride-through actions when a fault occurs in a remote power transmission system. The remote power transmission system includes at least an AC energy-consuming device and a DC voltage limiting device. The AC energy-consuming device is a device that achieves a specific function by consuming AC power. Its working principle typically utilizes the characteristics of components such as resistors, inductors, and capacitors to convert AC power into other forms of energy, such as heat, magnetic field energy, or electric field energy, for consumption. In the dynamic stability control of power systems, the AC energy-consuming device can maintain system stability by consuming excess energy in the system. Generally, the AC energy-consuming device (ACC) is located at the sending end of the remote power transmission system. The DC voltage limiting device typically uses specific circuit structures and components, such as Zener diodes, varistors, and thyristors, to limit the DC voltage. In this application, the DC voltage limiting device (VLM) is composed of power electronic switches and resistors. By controlling the switching of the resistors through the power electronic switches, the capacitor voltage is balanced. The voltage limiting module operates independently and does not affect the normal operation of the MMC (Modular Multilevel Converter) converter module. When the voltage in a DC circuit exceeds the set limit, the DC voltage limiting device will automatically activate and limit the voltage to a safe range through shunt, clamping, or other means to protect other components in the circuit from damage by excessive voltage.

[0067] Specifically, in combination Figure 2 As shown, the fault ride-through control method for remote power transmission systems in this embodiment of the application specifically includes the following steps:

[0068] Firstly, when the remote transmission system is fault-free, i.e., before time t0, neither the DC voltage limiting device VLM nor the AC power dissipation device ACC operates, and the system operates normally and transmits power normally. However, when a fault occurs in the remote transmission system, i.e., during the time interval t0-t1, the system enters the fault ride-through process (FRT process). At this time, the AC side voltage U of the receiving-end converter station of the remote transmission system... ac When a voltage drop occurs, the system's surplus power charges the capacitors of the receiving-end converter station, and the DC-side voltage U of the receiving-end converter station... dc_out It continues to rise; it should be noted that Figure 2 Middle U ac U represents the AC side voltage of the receiving-end converter station. dc_out T represents the DC-side output voltage of the receiving-end converter station. controlRepresents the control signal; Number represents the number of DC voltage limiting devices (VLMs) activated; Group represents the number of AC power consuming devices (ACCs) grouped together.

[0069] During the period from t1 to t2, the DC side is in the voltage limiting phase, and at time t1, the DC side voltage U of the receiving-end converter station is... dc_out When the set action threshold is reached, the power electronic switch control resistor of the DC voltage limiting device is engaged to balance the DC side voltage of the receiving-end converter station. At this time, the energy transmission path of the system is as follows: Figure 3 As shown in (a). During the operation phase, the DC voltage limiting device adopts independent operation logic, which does not affect the normal operation of the MMC submodule and maintains the stability of the DC voltage.

[0070] Among them, the combination Figure 3 As shown in (b), the operation logic of the DC voltage limiting device employs an outer-loop voltage control strategy and an inner-loop control strategy for the number of activated modules. The outer-loop voltage control strategy for the DC voltage limiting device is as follows: when the DC side voltage of the receiving-end converter station reaches a set action threshold, the DC voltage limiting device is activated. At this time, the outer loop control adopts a DC voltage hysteresis control strategy, controlling the number of activated DC voltage limiting devices by comparing the difference between the actual DC side voltage value and the reference value, in order to maintain the DC side voltage of the receiving-end converter station within a preset fluctuation range. To reduce the impact of the DC voltage limiting device's operation on the DC voltage, the inner-loop control strategy for the number of activated modules is as follows: the inner loop control adopts a stepped activation strategy, calculating the number of activated DC voltage limiting devices based on given activation parameters (e.g., operating frequency, duty cycle) and system surplus power. It should be noted that... Figure 3 E in (b) ab_VLM This represents the operating signal of the DC voltage limiting device VLM; U dcN Represents the rated DC voltage; T VLM This represents the operating time of the DC voltage limiting device.

[0071] It should be noted that, in order to balance the energy absorbed by each DC voltage limiting device, the DC voltage limiting devices adopt a periodic switching strategy when they are put into operation in stages:

[0072] First, set the number of DC voltage limiting devices to be activated to be m;

[0073] During the i-th cycle, the m DC voltage limiting devices operate sequentially according to their activation order, where the cycle set is represented as:

[0074] Input i ={1,2,3,…,m};

[0075] Input i This represents the set of periods for m DC voltage limiting devices;

[0076] The corresponding DC voltage limiting devices are switched on and off sequentially according to the order of the period set.

[0077] For example, when m DC voltage limiting devices operate in the first cycle, the order in which the m DC voltage limiting devices are activated is 1, 2, ..., m; in the second cycle, the order in which the m DC voltage limiting devices are activated is 2, 3, ..., m, 1; the DC voltage limiting devices are switched on and off in the same manner. In the m-th cycle, the order in which the m DC voltage limiting devices are activated is m, 1, ..., m-1. Through the above cycle-rotation switching strategy, each VLM can absorb energy evenly, avoiding excessive energy absorption and resistor heating by a single VLM, which could affect the operation of surrounding equipment.

[0078] During the AC side energy consumption phase from t2 to t3, when the AC energy consumption device ACC is activated upon receiving a system fault signal at time t2, the ACC will send an activation signal to the DC voltage limiting device VLM. At this time, the DC voltage limiting device VLM stops operating, and the remote power transmission system uses the AC energy consumption device ACC to absorb the surplus power. The energy transmission path of the system at this time is as follows: Figure 4 As shown in (a).

[0079] like Figure 4 As shown in (b), the control logic of the AC energy-consuming device adopts a power outer-loop control strategy and a DC voltage inner-loop control strategy. The power outer-loop control strategy of the AC energy-consuming device is as follows: the outer-loop power control compares the input power P at the system's sending end. in and the output power P of the receiving end out The difference is used to calculate the number of AC power dissipation devices required. Furthermore, to achieve stable power absorption, the ACC uses a thyristor grouping arrangement, such as... Figure 1 As shown, it is divided into a group. Based on the periodic switching strategy of the DC voltage limiting device, similarly, the AC energy-consuming device adopts a grouped periodic switching strategy to ensure stable absorption of surplus power during system faults. The voltage inner-loop control strategy of the AC energy-consuming device is as follows: by comparing the difference between the actual value and the reference value of the DC side voltage of the receiving-end converter station, the number of groups of AC energy-consuming devices activated is controlled to maintain the DC side voltage of the receiving-end converter station within a preset fluctuation range. It should be noted that... Figure 4 E in (b) ab_ACC The signal representing the operation of the AC power consumption device ACC; U dcN Represents the rated DC voltage; T ACC This represents the operating time of the AC energy-consuming device.

[0080] During the fault recovery phase from time t3 to t4, when the AC side voltage of the receiving-end converter station gradually recovers to the grid-connected voltage at time t3, the grid power will impact the system, causing short-term fluctuations in the DC voltage. This results in a transient fluctuation signal in the DC side voltage of the receiving-end converter station. When this transient fluctuation signal is transmitted to the AC energy dissipation device, the AC energy dissipation device stops operating. With U... ac Once the voltage is restored to normal, the DC side voltage of the system also returns to its rated value.

[0081] It should be noted that in this embodiment, DC voltage limiting devices are evenly configured in each bridge arm circuit of the MMC submodule, and each bridge arm circuit contains m DC voltage limiting devices. The number and parameters of the DC voltage limiting devices must meet DC voltage constraints, as well as energy absorption limits for energy-consuming resistors and capacitor voltage fluctuation ranges. Under the premise of meeting these two constraints, the fewer VLMs configured, the lower the energy consumption cost.

[0082] The DC voltage constraint conditions mainly include:

[0083] For VLM (Voltage Limiting Mechanism), it needs to maintain DC voltage stability even under the most severe fault conditions. Therefore, when a fault occurs in a long-distance transmission system, the DC voltage limiting device's absorption capacity must exceed the system's rated power.

[0084]

[0085] Where P rate P is the system's rated power. VLM U represents the absorption power of a single DC voltage limiting device; SM denoted as , where is the capacitor voltage of the MMC submodule; r is the internal resistance of the MMC submodule; and m is the number of DC voltage limiting devices in operation.

[0086] When the DC voltage limiting device is activated, the DC side voltage of the receiving-end converter station continues to decrease as the VLM is gradually put into operation. Considering the most extreme case, the energy absorbed by the DC voltage limiting device must be equal to the energy transmitted by the system.

[0087]

[0088] Where D is the duty cycle of the DC voltage limiting device; f VLM The operating frequency of the DC voltage limiting device; k is the ratio of the system's transmitted power to its rated power, where 0 <k<1。

[0089] When the DC voltage limiting device is disconnected, the DC side voltage of the receiving-end converter station continues to rise as the VLM is gradually disconnected. At this time, the energy relationship of the long-distance transmission system is as follows:

[0090]

[0091] Where C system U is the system equivalent capacitance; dc_high This is the upper limit of the DC side voltage of the receiving-end converter station;

[0092] U dc_low This is the lower limit of the DC side voltage of the receiving-end converter station; U dc Δu represents the DC-side voltage of the receiving-end converter station; Δu represents the fluctuation range of the DC-side voltage of the receiving-end converter station.

[0093] By solving the above formula, the operating frequency of the DC voltage limiting device can be determined. For any k, VLM needs to maintain the stability of the system's DC voltage. When 3mP VLM =kP rate At that time, the operating frequency of the DC voltage limiting device is the maximum value f. VLM_max At this time, the operating frequency of the DC voltage limiting device must meet the following requirements:

[0094]

[0095] Furthermore, the energy absorption limit of the energy-consuming resistor and the voltage fluctuation range of the capacitor mainly include the following requirements:

[0096] When the DC voltage limiting device operates, since it employs a periodic switching strategy, it can be assumed that each VLM absorbs the same amount of energy. During operation, the energy E absorbed by each VLM is... VLM The energy absorbed must not exceed the allowable value to avoid excessive energy absorption, which could cause the resistor to generate excessive heat and affect surrounding power electronic equipment. Therefore, the energy absorbed by each DC voltage limiter must not exceed the upper limit of the resistor's allowable absorption capacity.

[0097]

[0098] Where E VLM Energy is absorbed by a single DC voltage limiting device; P rate The system's rated power; T VLM_MMC The operating time of the DC voltage limiting device from activation to deactivation; m is the number of DC voltage limiting devices activated; E allow This is the energy that each resistor in a DC voltage limiting device is allowed to absorb. This value depends on the resistor's material, temperature rise characteristics, etc., and different resistor models have different values.

[0099] Furthermore, for MMC submodules equipped with DC voltage limiting devices and those without, the relationship between their capacitor voltages must meet the following conditions to maintain DC voltage balance:

[0100]

[0101] Where Δu SM1 The maximum voltage drop across the capacitor voltage of the MMC submodule configured with a DC voltage limiter; Δu SM2 The maximum voltage rise of the capacitor voltage of an MMC submodule without a DC voltage limiter; ΔU SM The system specifies the capacitor voltage fluctuation range for the MMC submodule; U SM U is the capacitor voltage of the MMC submodule; dc_high This represents the upper limit of the DC side voltage at the receiving-end converter station; U dc_low C represents the lower limit of the DC-side voltage at the receiving-end converter station. SM The capacitance value of the MMC submodule capacitor; i r i is the current flowing through the resistor in the DC voltage limiting device. bridge n is the bridge arm current; n is the total number of MMC submodules; D is the duty cycle of the DC voltage limiting device; r is the internal resistance of the MMC submodule; f VLM This refers to the operating frequency of the DC voltage limiting device.

[0102] Furthermore, in this embodiment, the AC energy-consuming device is connected to the AC bus via a step-down transformer, and different surplus power can be absorbed by changing the number of input groups. In this case, the selection must meet constraints on the number of input groups and voltage levels.

[0103] The selection of the number of ACC groups needs to comprehensively consider both system performance requirements and economic efficiency. During fault ride-through, a higher number of groups results in a smaller difference between the power absorbed by the ACC and the system's surplus power, thus reducing the power impact on the system. However, since each branch of the ACC requires a three-phase thyristor arrangement, the cost increases linearly with the number of groups, and the floor space also increases dramatically. Specifically, the constraints on the number of groups include:

[0104] When system faults are overcome and crossover control is implemented, the maximum power absorbed by the AC power dissipation device is equal to the system's rated power.

[0105]

[0106] When the power absorbed by the AC energy-consuming device is greater than the system transmission power, and the maximum power difference between the power absorbed by the AC energy-consuming device and the system transmission power is equal to the power absorbed by a single branch of the AC energy-consuming device, the voltage change caused by the power difference between the power absorbed by the AC energy-consuming device and the system transmission power discharging the system capacitor within a single operating cycle is as follows:

[0107]

[0108] Where P ACC Power absorbed by AC energy-consuming devices; P ACC_singleFor the power absorbed by a single branch of an AC power-consuming device; U ac_k T is the secondary voltage of the AC transformer. ac The period of the AC voltage; Δu ACC The maximum DC voltage change caused by power imbalance in AC power-consuming devices; U rate The system's rated voltage; C system P is the system equivalent capacitance; rate R is the rated power of the system; R is the resistance of the AC energy-consuming device; a is the number of AC energy-consuming device groups.

[0109] Furthermore, the voltage level constraints described in the embodiments of this application include:

[0110] The voltage level of the AC energy-consuming device is related to the turns ratio of the AC transformer. The higher the voltage level of the AC energy-consuming device, the more thyristors need to be connected in series; the lower the voltage level of the AC energy-consuming device, the more thyristors need to be connected in parallel.

[0111] When the turns ratio of an AC transformer is determined, the total number of thyristors required is expressed as:

[0112]

[0113] Number T U represents the total number of thyristors required; a represents the number of AC power dissipation device groups; U ac_k U is the secondary voltage of the AC transformer; k is the ratio of the system's transmitted power to its rated power; U T This is the operating voltage of the thyristor;

[0114] The current flowing through the thyristor is expressed as:

[0115]

[0116] Among them I T P is the thyristor operating current. ACC_single The power absorbed by a single branch of an AC energy-consuming device.

[0117] Combination Figure 5 As shown, the embodiments of this application further illustrate the effectiveness of the proposed solution by comparing its performance with existing solutions. The existing solutions employed include centralized DC power consumption devices (centralized DCC), distributed DC power consumption devices (distributed DCC), and thyristor-based DC power consumption devices (thyristor-based DCC).

[0118] from Figure 5As can be seen, regarding voltage balancing, due to the synchronous operation of centralized DCC switches, dynamic voltage balancing of series switches needs to be considered. For distributed and thyristor-based DCCs, since they all have supporting capacitors, only capacitor voltage balancing needs to be considered. For DCCs with DC voltage limiting devices, they are directly connected in parallel across the capacitors of the MMC submodule. The MMC submodule itself has capacitor voltage balancing capabilities, thus having an advantage in voltage equalization.

[0119] Furthermore, regarding the impact on the MMC submodule, since the independent power consumption device absorbs surplus power independently, it does not affect the characteristics of the MMC submodule. For the DCC with DC voltage limiting device, although the charging and discharging of the MMC submodule capacitor during operation will increase the voltage fluctuation of the MMC submodule capacitor and the bridge arm current, the process is short and will not affect the normal operation of the MMC submodule. In terms of DC voltage ripple, the centralized DCC uses PWM control, and the pulse current has the greatest impact on the system, with ripple typically between 10% and 15%. Distributed and thyristor-based DCCs both use stepped switching, which reduces the rate of current change and controls the ripple to 5%-10%. The scheme in this application also uses stepped switching and has a higher number of modules than the distributed scheme, so the DC voltage ripple can be lower than 5%.

[0120] Furthermore, regarding the switching current, both centralized and distributed DCC schemes directly interrupt the DC current, resulting in a switching current of 2kA. For thyristor-based DCCs, a thyristor is used as the main switch, relying on an LC circuit to generate an oscillating current to assist the thyristor's turn-off; therefore, the peak current flowing through the thyristor is more than twice the current flowing through the resistor. However, for the scheme in this application, due to the increased number of power-consuming modules, the current required to be turned off is only 1.1kA.

[0121] The above analysis shows that the proposed solution reduces the performance requirements of the power-consuming module and significantly reduces the DC voltage ripple of the system.

[0122] Furthermore, in conjunction with Table 1, the embodiments of this application further illustrate the effectiveness of the solution by comparing the cost of the solution with existing solutions.

[0123] Table 1

[0124]

[0125] Table 1 evaluates the costs of each scheme primarily based on the switches, resistors, capacitors, power extraction, and insulation frames used. It compares the quantity and price of the main equipment required for different schemes. To visually illustrate the comparison of the total costs of each scheme, the total cost of a centralized DCC is used as 1 p.u.

[0126] Based on Table 1, the centralized DCC uses 4.5kV / 3kA IGBTs, with each IGBT having a withstand voltage of 2kV, requiring a total of 500 IGBTs. Furthermore, to reduce the impact of pulse current on the system, absorption capacitors need to be connected in parallel across the DCC, and the directly series-connected IGBTs are powered independently. The distributed DCC also uses 4.5kV / 3kA IGBTs, with each module having a withstand voltage of 2kV, requiring 500 switches and supporting capacitors. The IGBTs are powered by drawing energy from the capacitors. The thyristor-based DCC essentially replaces the IGBTs with switches composed of thyristors and LC circuits, thereby reducing the cost of fully controlled power electronic switches. Since the thyristor current turns off at zero crossing, there is no turn-off overvoltage; the rated voltage of a single module is 2.8kV, requiring 350 modules. This application's scheme uses 3.3kV / 1kA IGBTs, with 150 power-consuming modules arranged in each bridge arm, symmetrically arranged across the six bridge arms. As can be seen from Table 1, this application uses lower capacity IGBTs and saves on additional equipment such as support capacitors and insulation frames. Therefore, the total cost of DC-side power consumption is reduced by more than 70% compared with the centralized DCC solution, which has a very high cost advantage.

[0127] like Figure 6 As shown in the illustration, this application also provides an electronic device, including a processor and a memory, and a program or instructions stored in the memory and executable on the processor, which, when executed by the processor, implement as follows: Figure 1 The various processes of the method embodiments shown are all capable of achieving the same technical effect, and will not be described again here to avoid repetition.

[0128] This application embodiment also provides a readable storage medium storing a program or instructions that, when executed by a processor, implement the above-described functionality. Figure 1 The various processes described in the embodiments of the method described herein can achieve the same technical effect, and will not be repeated here to avoid repetition.

[0129] This application also provides a computer program product, including computer instructions, which, when executed by a processor, implement the above-described... Figure 1 The various processes described in the embodiments of the method described herein can achieve the same technical effect, and will not be repeated here to avoid repetition.

[0130] It should be understood that the phrase "one embodiment" or "an embodiment" throughout the specification means that a specific feature, structure, or characteristic related to the embodiment is included in at least one embodiment of this application. Therefore, "in one embodiment" or "in an embodiment" appearing throughout the specification does not necessarily refer to the same embodiment. Furthermore, these specific features, structures, or characteristics can be combined in any suitable manner in one or more embodiments. It should be understood that in the various embodiments of this application, the sequence numbers of the above-described processes do not imply a sequential order of execution; the execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application. The sequence numbers of the above-described embodiments are merely descriptive and do not represent the superiority or inferiority of the embodiments.

[0131] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element.

[0132] In the several embodiments provided in this application, it should be understood that the disclosed devices and methods can be implemented in other ways. The device embodiments described above are merely illustrative. For example, the division of units is only a logical functional division, and in actual implementation, there may be other division methods, such as: multiple units or components may be combined, or integrated into another device, or some features may be ignored or not executed. In addition, the coupling, direct coupling, or communication connection between the various components shown or discussed may be through some interfaces, and the indirect coupling or communication connection between devices or units may be electrical, mechanical, or other forms.

[0133] The units described above as separate components may or may not be physically separate. The components shown as units may or may not be physical units. They may be located in one place or distributed across multiple network units. Some or all of the units may be selected to achieve the purpose of this embodiment according to actual needs.

[0134] In addition, each functional unit in the various embodiments of this application can be integrated into one processing unit, or each unit can be a separate unit, or two or more units can be integrated into one unit; the integrated unit can be implemented in hardware or in the form of hardware plus software functional units.

[0135] Those skilled in the art will understand that all or part of the steps of the above method embodiments can be implemented by hardware related to program instructions. The aforementioned program can be stored in a computer-readable storage medium. When the program is executed, it performs the steps of the above method embodiments. The aforementioned storage medium includes various media that can store program code, such as mobile storage devices, read-only memory (ROM), magnetic disks, or optical disks.

[0136] Alternatively, if the integrated units described above are implemented as software functional modules and sold or used as independent products, they can also be stored in a computer-readable storage medium. Based on this understanding, the technical solutions of the embodiments of this application, or the parts that contribute to the prior art, 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 device (which may be a terminal or platform, etc.) to execute all or part of the methods described in the various embodiments of this application. The aforementioned storage medium includes various media capable of storing program code, such as mobile storage devices, ROMs, magnetic disks, or optical disks.

[0137] The above description is only a preferred embodiment of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of this application, and these improvements and modifications should also be considered within the scope of protection of this application.

Claims

1. A fault ride-through control method applicable to a long-distance power transmission system, used to control the fault ride-through action when a fault occurs in the long-distance power transmission system, wherein the long-distance power transmission system includes at least an AC energy-consuming device and a DC voltage-limiting device, characterized in that, The method includes the following steps: When a fault occurs in the long-distance power transmission system, the AC side voltage of the receiving-end converter station drops, and the surplus power of the system charges the capacitor of the receiving-end converter station, causing the DC side voltage of the receiving-end converter station to rise continuously. When the DC side voltage of the receiving-end converter station reaches the set action threshold, the power electronic switch control resistor of the DC voltage limiting device is activated to balance the DC side voltage of the receiving-end converter station. When the AC energy dissipation device receives a system fault signal and is activated, the AC energy dissipation device will send an activation signal to the DC voltage limiting device. At this time, the DC voltage limiting device will stop operating, and the remote power transmission system will use the AC energy dissipation device to absorb the surplus power. When the AC side voltage of the receiving-end converter station gradually recovers to the grid-connected voltage, the DC side voltage of the receiving-end converter station generates a transient fluctuation signal. When the transient fluctuation signal is transmitted to the AC energy dissipation device, the AC energy dissipation device stops operating. The number and parameters of the DC voltage limiting device must meet the DC voltage constraint conditions. The DC voltage constraint conditions include: When a fault occurs in the long-distance power transmission system, the power absorption capacity of the DC voltage limiting device exceeds the system's rated power: ； in P rate This is the system's rated power. P VLM This represents the absorption power of a single DC voltage limiting device; U SM for MMC The capacitor voltage of the submodule; r for MMC Submodule internal resistance; m The number of DC voltage limiting devices deployed; When the DC voltage limiting device is activated, the energy absorbed by the DC voltage limiting device is equal to the energy transmitted by the system: ； in D The duty cycle of the DC voltage limiting device; f VLM This refers to the operating frequency of the DC voltage limiting device. k The ratio of system transmission power to system rated power; When the DC voltage limiting device is disconnected, the energy relationship of the remote power transmission system is as follows: ； in C system This is the system's equivalent capacitance; U dc_high This is the upper limit of the DC side voltage of the receiving-end converter station; U dc_low This is the lower limit of the DC side voltage of the receiving-end converter station; U dc This refers to the DC side voltage of the receiving-end converter station. This refers to the fluctuation range of the DC side voltage at the receiving-end converter station; Meanwhile, the operating frequency of the DC voltage limiting device must meet the following requirements: ； in f VLM_max This represents the maximum operating frequency of the DC voltage limiting device.

2. The fault ride-through control method for long-distance power transmission systems according to claim 1, characterized in that, The DC voltage limiting device adopts a voltage outer loop control strategy. When the DC side voltage of the receiving-end converter station reaches the set action threshold, the DC voltage limiting device is activated. At this time, the outer loop control adopts a DC voltage hysteresis control strategy. By comparing the difference between the actual value of the DC side voltage and the reference value, the number of DC voltage limiting devices is controlled to maintain the DC side voltage of the receiving-end converter station within the preset fluctuation range.

3. The fault ride-through control method applicable to long-distance power transmission systems according to claim 2, characterized in that, The DC voltage limiting device also adopts an inner-loop control strategy for the number of modules to be put into operation. In this case, the inner-loop control adopts a stepped input strategy, which calculates the number of DC voltage limiting devices to be put into operation based on the given input parameters of the DC voltage limiting device and the surplus power of the system.

4. The fault ride-through control method applicable to long-distance power transmission systems according to claim 3, characterized in that, The DC voltage limiting device employs a periodic switching strategy during stepped activation: The number of DC voltage limiting devices to be activated is set as follows: m ; In the i When performing actions in each cycle m The DC voltage limiting devices operate sequentially according to their activation order, and the set of cycles is represented as follows: ； in express m A set of cycles for a DC voltage limiting device; The corresponding DC voltage limiting devices are switched on and off sequentially according to the order of the period set.

5. The fault ride-through control method applicable to long-distance power transmission systems according to claim 1, characterized in that, The AC energy-consuming device adopts a power outer loop control strategy. In this case, the outer loop control calculates the number of AC energy-consuming devices to be activated by comparing the difference between the input power at the sending end and the output power at the receiving end of the system. The AC energy-consuming device adopts a thyristor grouping arrangement and a group switching strategy to ensure stable absorption of surplus power during system failures.

6. The fault ride-through control method for long-distance power transmission systems according to claim 5, characterized in that, The AC energy dissipation device adopts a voltage inner loop control strategy. At this time, the number of groups of AC energy dissipation devices is controlled by comparing the difference between the actual value and the reference value of the DC side voltage of the receiving-end converter station, so as to maintain the DC side voltage of the receiving-end converter station within the preset fluctuation range.

7. The fault ride-through control method applicable to long-distance power transmission systems according to claim 1, characterized in that, The number and parameters of the DC voltage limiting device must meet the energy absorption limit of the energy-consuming resistor and the voltage fluctuation range of the capacitor. When the DC voltage limiting device operates, the energy absorbed by each DC voltage limiting device must not exceed the upper limit of the resistor's allowable absorption capacity. ； in E VLM Absorb energy for a single DC voltage limiting device; P rate This is the system's rated power. T VLM_MMC The operating time of the DC voltage limiting device from activation to deactivation; m The number of DC voltage limiting devices deployed; E allow The energy that each resistor in the DC voltage limiting device is allowed to absorb; For the configuration of DC voltage limiting device MMC Submodules and those without DC voltage limiting devices MMC The voltage relationship of the maximum capacitor voltage of the submodule must satisfy: ； in To configure a DC voltage limiting device MMC The maximum voltage drop across the capacitor of the submodule; For those without DC voltage limiting devices MMC The maximum voltage rise of the capacitor voltage in the submodule; For system specifications MMC Submodule capacitor voltage fluctuation range; U SM for MMC The capacitor voltage of the submodule; U dc_high This is the upper limit of the DC side voltage of the receiving-end converter station; U dc_low This is the lower limit of the DC side voltage of the receiving-end converter station; C SM for MMC The capacitance value of the submodule capacitor; i r The current flowing through the resistor in the DC voltage limiting device. i bridge For bridge arm current; n for MMC Total number of submodules; D The duty cycle of the DC voltage limiting device; r for MMC Internal resistance of the submodule; f VLM This refers to the operating frequency of the DC voltage limiting device.

8. The fault ride-through control method applicable to long-distance power transmission systems according to claim 1, characterized in that, The selection of the AC power dissipation device must meet the group quantity constraint, wherein the group quantity constraint includes: When system faults are overcome and crossover control is implemented, the maximum power absorbed by the AC power dissipation device is equal to the system's rated power. ; When the power absorbed by the AC energy-consuming device is greater than the system transmission power, and the maximum power difference between the power absorbed by the AC energy-consuming device and the system transmission power is equal to the power absorbed by a single branch of the AC energy-consuming device, the voltage change caused by the power difference between the power absorbed by the AC energy-consuming device and the system transmission power discharging the system capacitor within a single operating cycle is as follows: ； in P ACC Power is absorbed by AC energy-consuming devices; P ACC_single Power absorbed by a single branch of an AC power-consuming device; U ac_k This refers to the secondary voltage of an AC transformer. T ac The period of the alternating current voltage; ∆u ACC The maximum DC voltage change caused by power imbalance in AC energy-consuming devices; U rate This is the system's rated voltage; C system This is the system's equivalent capacitance; P rate This is the system's rated power. R For AC power consumption devices, the resistor is used. a The number of groups for AC energy-consuming devices.

9. The fault ride-through control method for long-distance power transmission systems according to claim 8, characterized in that, The selection of the AC power dissipation device must meet voltage level constraints, wherein the voltage level constraints include: The voltage level of the AC energy-consuming device is related to the turns ratio of the AC transformer. The higher the voltage level of the AC energy-consuming device, the more thyristors need to be connected in series; the lower the voltage level of the AC energy-consuming device, the more thyristors need to be connected in parallel. When the turns ratio of an AC transformer is determined, the total number of thyristors required is expressed as: ; in This represents the total number of thyristors required. a Number of groups for AC energy-consuming devices; U ac_k This refers to the secondary voltage of an AC transformer. U ac The AC side voltage of the receiving-end converter station; k The ratio of system transmission power to system rated power; U T This is the operating voltage of the thyristor; The current flowing through the thyristor is expressed as: ； in I T This refers to the thyristor operating current. P ACC_single The power absorbed by a single branch of an AC energy-consuming device.

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