An active support type HVDC power transmission system and an AC fault frequency control switching method at the receiving end thereof

Abstract

The application provides an active support type direct current power transmission system and an AC fault fixed frequency control switching method of a receiving end thereof. The method comprises monitoring a positive sequence component amplitude of an AC voltage of a receiving end flexible direct current station in real time; in the case that the positive sequence component amplitude of the AC voltage of the receiving end is less than a preset threshold value, switching an angular frequency generated by virtual synchronous control of the receiving end flexible direct current station to a rated angular frequency, so that the receiving end flexible direct current station operates in a fixed frequency control mode; and switching a DC directly connected over-capacity valve from a DC current control mode to an energy consumption mode, wherein in the energy consumption mode, when a DC voltage rises to be higher than a first threshold value, the DC directly connected over-capacity valve is controlled to operate at a rated current to absorb surplus power; when the DC voltage decreases to be lower than a second threshold value, the DC directly connected over-capacity valve is controlled to stop current output; and when the DC voltage is between the first threshold value and the second threshold value, an output current of the DC directly connected over-capacity valve at a previous moment is maintained, thereby solving the problems in the prior art.

Description

Technical Field

[0001] This invention belongs to the field of power transmission network technology, specifically relating to an active support type DC transmission system and its receiving-end AC fault fixed-frequency control switching method. Background Technology

[0002] With the significant increase in the proportion of installed capacity from new energy sources, traditional power systems dominated by synchronous motors are gradually shifting towards low-inertia, low-grid systems. Flexible DC transmission technology is being widely applied in large-scale clean energy transmission scenarios. Against this backdrop, the active-supported DC transmission system, as a typical flexible DC transmission architecture, has significant application value in the transmission of isolated new energy sources.

[0003] An active-supported DC transmission system is a high-voltage direct current (HVDC) transmission system based on voltage source converters. It mainly consists of a sending-end flexible DC station, a receiving-end flexible DC station, and a DC-connected overcapacity valve. The sending-end flexible DC station connects to a renewable energy base or the sending-end AC grid, employing AC power control mode to stably inject sending-end power into the active-supported DC transmission system. The receiving-end flexible DC station connects to the receiving-end AC grid, employing a control strategy that integrates DC voltage control and grid-based control. DC voltage control maintains stable DC bus voltage, while grid-based control (virtual synchronization control) provides active frequency and voltage support to the receiving-end grid, ensuring stable operation even when the receiving-end grid is weak or experiences disturbances. The DC-connected overcapacity valve is directly connected to the DC bus side, employing DC current control with the current command set to zero in steady state. Under normal operating conditions, sending-end power is transmitted to the receiving end via DC lines, and the receiving-end flexible DC station inverts the DC power into AC power to feed it into the receiving-end grid, achieving power balance.

[0004] However, when a fault occurs in the receiving-end AC grid of an actively supported DC transmission system, the AC voltage drops, limiting power transmission. Due to continuous power injection at the sending end and obstructed power output at the receiving end, surplus power on the DC side continuously charges the submodule capacitors, causing the DC voltage to gradually rise. Without effective control measures, the DC overvoltage protection will activate, leading to system shutdown. Simultaneously, the DC voltage controller at the receiving-end flexible DC station is prone to saturation during fault conditions, and the amount of power that can be transmitted is unknown. Therefore, it is impossible to adjust the frequency and phase of the flexible DC converter station's valve output voltage based on the DC voltage controller and the flexible DC converter station's output power. During steady-state operation, the DC direct-connected overcapacity valve is controlled by DC current with a zero current command, similarly failing to automatically absorb surplus power.

[0005] Several research solutions have been developed for AC fault ride-through in flexible DC transmission systems. Existing fault ride-through methods based on unloading (dynamic braking resistor, DBR) mostly use DC voltage as the switching condition, neglecting the impact of AC bus voltage and reactive power at the receiving-end converter station on DC voltage safety. This may lead to unnecessary unloading due to unreasonable switching conditions or excessive reactive power at the receiving-end converter station, causing additional impact on the AC / DC system and increasing energy loss. Other studies have proposed reducing the power injected into the DC system from renewable energy sources through voltage reduction or frequency increase control at the sending-end converter station, combined with energy-dissipating resistors to achieve fault ride-through. Still other studies have analyzed the transient characteristics of grid-type flexible DC converter stations under AC faults at the receiving end and proposed corresponding fault ride-through strategies.

[0006] For example, patent CN115224717A proposes a sending-end AC fault ride-through scheme. When an AC fault is detected at the sending end, the control system of the sending-end flexible DC converter station enters low ride-through logic, clears the integral links of the d-axis and q-axis controllers to zero, makes the input voltage reference value of the d-axis voltage controller equal to the AC voltage reference value before the fault minus the effective value of the negative sequence AC voltage after the fault, and limits the current reference values ​​of the d-axis and q-axis, keeping the output of the DC current controller at the output value before the fault. However, this scheme is aimed at the sending-end AC fault scenario, and its control objective is to suppress sending-end overvoltage and improve the coordinated control between the sending-end converter station and the new energy unit. This is fundamentally different from the receiving-end AC fault scenario addressed by this patent. Furthermore, this scheme does not address the frequency and phase adjustment issues of the receiving-end flexible DC station under DC voltage controller saturation conditions.

[0007] For example, patent CN113872243A proposes a fault ride-through scheme for power frequency AC systems. When the average voltage of the submodule capacitors of the AC-AC converter exceeds a threshold, the frequency of the low-frequency system is adjusted from f1 to f1+1, and the low-frequency voltage amplitude control mode is activated simultaneously. After the wind turbine system detects the frequency change, it activates the wind turbine power control mode or engages the DC de-energizing circuit based on the DC voltage of the wind turbine converter. This scheme transmits fault signals to the wind turbine through frequency amplification to achieve coordinated control. However, its application scenario is a low-frequency wind power transmission system, and it adjusts the frequency of the low-frequency system, which is completely different from the technical means in this patent where the receiving-end flexible DC station switches the virtual synchronous angular frequency to the rated value to avoid the influence of the saturation controller.

[0008] For example, patent CN112421669A proposes a fault severity-based fault crossing scheme. The onshore receiving-end converter station determines the severity of the AC fault: if it is a minor fault, it only consumes the excess power on the DC side through energy-dissipating resistors; if it is a severe fault, it sends control commands to the offshore sending-end converter station, causing it to use a step-down or frequency-up method to reduce the power injected into the DC system from the offshore wind farm, while simultaneously consuming the excess power through energy-dissipating resistors. Although this scheme involves AC faults at the receiving end, its core idea is fault severity classification plus step-down / frequency-up control at the offshore sending-end converter station plus energy-dissipating resistors. It relies on the actions of the sending-end converter station and the coordination of energy-dissipating resistors, without involving the active switching of the receiving-end flexible DC station's own control strategy.

[0009] Therefore, existing technologies are mostly designed for conventional flexible DC systems or sending-end fault scenarios, and adopt the design concept of dissipating surplus power, relying on energy-consuming devices to consume excess power. This approach has problems such as high investment costs and energy waste. However, for AC faults at the receiving end of actively supported DC transmission systems, the DC voltage controller at the receiving-end flexible DC station is prone to saturation, resulting in unreliable frequency and phase adjustment. The steady-state current command of the DC direct-connected overcapacity valve is zero, and it cannot automatically absorb surplus power. Currently, there is a lack of a receiving-end AC fault ride-through strategy specifically for this system. Therefore, how to solve the frequency and phase adjustment problem caused by controller saturation while maintaining receiving-end power output, and how to use overcapacity valves to actively absorb surplus power rather than passively dissipate it, are urgent technical problems that need to be solved. Summary of the Invention

[0010] This invention provides an active support type DC transmission system and its receiving-end AC fault fixed-frequency control switching method to solve the problems in the prior art.

[0011] To achieve the above objectives, the technical solution adopted in this application is to provide a method for fixed-frequency control switching of AC faults at the receiving end of an active-supported DC transmission system, the method comprising: Real-time monitoring of the positive sequence component amplitude of the AC voltage at the receiving end of the flexible DC substation; If the amplitude of the positive sequence component of the AC voltage at the receiving end is detected to be less than a preset threshold, the angular frequency generated by the virtual synchronization control of the receiving-end flexible DC station is switched to the rated angular frequency, so that the receiving-end flexible DC station operates in a fixed-frequency control mode. In this fixed-frequency control mode, the receiving-end flexible DC station automatically adjusts its power output capability according to the fault condition by adjusting the output voltage amplitude of the adjustment valve. The DC-connected overcapacity valve is switched from DC current control mode to energy consumption mode. In the energy consumption mode, when the DC voltage rises above a first threshold, the DC-connected overcapacity valve is controlled to operate at rated current to absorb surplus power; when the DC voltage drops below a second threshold, the DC-connected overcapacity valve is controlled to stop current output; when the DC voltage is between the first threshold and the second threshold, the output current of the DC-connected overcapacity valve at the previous moment is maintained.

[0012] Preferably, the method further includes: If the amplitude of the positive-sequence component of the AC voltage at the receiving end recovers to a value greater than or equal to a second preset threshold, the angular frequency of the receiving-end flexible DC station is switched from the rated angular frequency back to the angular frequency generated by the virtual synchronization control, and the DC voltage controller is reset, wherein the second preset threshold is greater than the preset threshold; and, Switch the DC direct-connected overcapacity valve back from the energy consumption mode to the DC current control mode.

[0013] Preferably, in the energy consumption mode, the DC direct-connected overcapacity valve determines the output current using the following formula: ; Among them, U dc The DC voltage; U dcref1 U is the first threshold; dcref2 The second threshold; I dcorder (t) represents the output current of the DC-connected overcapacity valve at time t; I dcorder (t-Δt) represents the output current of the DC-connected overcapacity valve at time t, preceding the previous time. N The rated current of the DC direct-connected overcapacity valve is given.

[0014] Preferably, when the amplitude of the positive-sequence component of the AC voltage at the receiving end is detected to be less than a preset threshold, the method includes determining a fault flag based on the amplitude of the positive-sequence component of the AC voltage at the receiving end and a preset hysteresis judgment condition, wherein the preset hysteresis judgment condition includes: when the amplitude of the positive-sequence component of the AC voltage at the receiving end is less than a first voltage threshold, setting the fault flag to an active state; when the amplitude of the positive-sequence component of the AC voltage at the receiving end is greater than a second voltage threshold, setting the fault flag to the state of the previous moment, wherein the second voltage threshold is greater than the first voltage threshold.

[0015] Preferably, when the amplitude of the positive-sequence component of the AC voltage at the receiving end is detected to be less than a preset threshold, a fault flag is determined using the following formula: ; Where Flag(t) is the fault flag at time t; Flag(t-Δt) is the state of the fault flag set to the state of the previous time step; U sd U is the amplitude of the positive sequence component of the AC voltage at the receiving end; sdref1 U is the first voltage threshold; sdref2 The second voltage threshold is represented by 1; 1 represents the coded value of the valid state; and 0 represents the coded value of the invalid state.

[0016] Preferably, the method further includes: initializing the DC voltage loop when the fault flag is in a valid state.

[0017] Preferably, the initialization of the DC voltage loop includes: clearing the integral element of the DC voltage loop to zero.

[0018] Preferably, the receiving-end flexible DC station adopts a control strategy that integrates DC voltage control and grid construction control under steady state. The grid construction control is achieved through virtual synchronization control, which provides active frequency and voltage support for the receiving-end AC power grid.

[0019] Preferably, the DC-connected overcapacity valve adopts a DC current control mode and the current command is set to zero in steady state.

[0020] This application also provides an active support DC transmission system, which uses the method provided in this application to perform fixed-frequency control switching for AC faults at the receiving end.

[0021] The active-supported DC transmission system receiving-end AC fault fixed-frequency control switching method provided in this application includes: real-time monitoring of the positive-sequence component amplitude of the receiving-end AC voltage at the receiving-end flexible DC station; when the positive-sequence component amplitude of the receiving-end AC voltage is detected to be less than a preset threshold, switching the angular frequency generated by the virtual synchronization control of the receiving-end flexible DC station to the rated angular frequency, so that the receiving-end flexible DC station operates in a fixed-frequency control mode, wherein, in the fixed-frequency control mode, the receiving-end flexible DC station adjusts its power output capability automatically according to the fault condition by adjusting the output voltage amplitude of the valve; and switching the DC direct-connected overcapacity valve from DC current control mode to energy consumption mode, wherein, in the energy consumption mode, when the DC voltage rises to exceed a first threshold, controlling the DC direct-connected overcapacity valve to operate at rated current to absorb surplus power; when the DC voltage drops to below a second threshold, controlling the DC direct-connected overcapacity valve to stop current output; when the DC voltage is between the first threshold and the second threshold, maintaining the output current of the DC direct-connected overcapacity valve at the previous moment. Therefore, this method switches the angular frequency generated by the virtual synchronization control of the receiving-end flexible DC substation to the rated angular frequency, enabling the substation to operate in a fixed-frequency control mode. This maintains synchronization with the AC grid during faults, avoiding system instability caused by lag in the response of the virtual synchronization control or controller saturation. Simultaneously, the DC-connected overcapacity valve switches to energy-dissipating mode, dynamically adjusting the output current to absorb surplus power on the DC side based on the hysteresis comparison results of the DC voltage with the first and second thresholds, effectively suppressing the continuous rise of the DC voltage. Thus, during faults, the DC voltage is controlled within a reasonable range, and the frequency remains stable. This verifies that the strategy combining fixed-frequency control and energy-dissipating mode can prevent overvoltage protection activation, ensure continuous system operation during AC faults at the receiving end, and quickly restore normal power supply after the fault is cleared. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the specific structure of an active-supported DC transmission system provided in the embodiments of this application; Figure 2 This is a schematic flowchart illustrating the fixed-frequency control switching method for AC faults at the receiving end of an active-supported DC transmission system, provided in an embodiment of this application. Detailed Implementation

[0023] The embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0024] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numerals in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this disclosure. Rather, they are merely examples of apparatuses and methods consistent with some aspects of this disclosure as detailed in the appended claims.

[0025] In the description of this application, it should also be noted that, unless otherwise expressly specified and limited, the terms "set up," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; 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; and they can refer to the internal communication between two components. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover a 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. Without further limitations, 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 said element.

[0026] In view of the problems in the prior art, this application provides an active-supported DC transmission system and its receiving-end AC fault fixed-frequency control switching method, which can be used to solve the problems in the prior art. For ease of understanding, the structure of the active-supported DC transmission system can be described first, such as... Figure 1 The diagram shows the specific structure of the active-supported DC transmission system. The active-supported DC transmission system includes a sending-end flexible DC station 1 (also known as a sending-end converter station), a receiving-end flexible DC station 2 (also known as a receiving-end converter station), and a DC direct-connected overcapacity valve 3. The AC side of the sending-end flexible DC station 1 is connected to power generation devices such as thermal power, photovoltaic arrays, and wind farms. The AC side of the receiving-end flexible DC station 2 is connected to the AC power grid. The DC side of both the sending-end flexible DC station 1 and the receiving-end flexible DC station 2 is connected to the DC direct-connected overcapacity valve 3.

[0027] like Figure 2 The diagram shown is a schematic flowchart of a fixed-frequency control switching method for AC faults at the receiving end of an active-supported DC transmission system, provided in an embodiment of this application. The method includes the following steps: Step S41: Monitor the positive sequence component amplitude of the AC voltage at the receiving end of the flexible DC station in real time.

[0028] Specifically, step S41 aims to acquire key electrical quantities reflecting the operating status of the receiving-end flexible DC substation, serving as the triggering basis for subsequent fault diagnosis and control strategy switching. In active-supported DC transmission systems, the receiving-end flexible DC substation is typically equipped with voltage transformers or measurement units, enabling the acquisition of instantaneous three-phase AC voltage values ​​at the grid connection point (as the measurement point). This monitoring process first acquires sampling data of the instantaneous three-phase AC voltage values ​​from the measurement point. The sampling frequency can be set, for example, between 1kHz and 10kHz, depending on the system's control bandwidth requirements. Subsequently, the acquired three-phase instantaneous voltage can be decomposed using a phase-locked loop or a positive-negative sequence separation algorithm to extract the positive-sequence component of the voltage, which is the amplitude of the positive-sequence component of the receiving-end AC voltage in step S41.

[0029] For example, a synchronous rotating coordinate system filtering algorithm based on dq coordinate transformation can be used to transform the three-phase voltage to a rotating coordinate system and then obtain the DC component through a low-pass filter, thereby calculating the amplitude of the positive sequence component of the AC voltage at the receiving end; or, an improved notch filter method based on instantaneous power theory can be used to quickly filter out the negative sequence and zero sequence components and directly calculate the amplitude of the positive sequence component of the AC voltage at the receiving end.

[0030] In this embodiment, the amplitude of the positive-sequence component of the receiving-end AC voltage can be used as the core indicator for determining whether a fault has occurred in the receiving-end AC power grid. If the amplitude drops significantly, it means that a short-circuit fault or other disturbances causing voltage reduction may have occurred in the receiving-end AC power grid. By monitoring this electrical quantity in real time, the system can promptly capture the moment of fault occurrence, providing an accurate time reference for subsequent switching of fixed-frequency control and activation of energy consumption modes.

[0031] Step S42: When the amplitude of the positive sequence component of the AC voltage at the receiving end is less than the preset threshold, the angular frequency generated by the virtual synchronization control of the receiving end flexible DC station is switched to the rated angular frequency so that the receiving end flexible DC station operates in the fixed frequency control mode.

[0032] In the fixed-frequency control mode, the receiving-end flexible DC station adjusts its power output capability automatically according to the fault conditions by adjusting the output voltage amplitude of the adjustment valve.

[0033] Specifically, step S42 is used to quickly change the control strategy of the receiving-end flexible DC station after detecting an AC fault at the receiving end (that is, after detecting that the amplitude of the positive sequence component of the AC voltage at the receiving end is less than a preset threshold) in order to adapt to the operating environment where power transmission is limited under fault conditions.

[0034] This preset threshold is a voltage criterion used to determine the occurrence of a fault. It can be determined, for example, based on the voltage level and normal fluctuation range of the receiving-end AC grid, or through statistical analysis of historical fault data. For instance, for a 220kV AC system, this preset threshold can be set to 0.7 to 0.9 times the rated voltage. When the amplitude of the positive sequence component of the receiving-end AC voltage is less than this preset threshold, it indicates a fault has occurred in the receiving-end AC grid, and control switching can be executed immediately.

[0035] In practical applications, during steady-state operation, the receiving-end flexible DC substation typically employs a control strategy that integrates DC voltage control and grid-based control. Grid-based control is achieved through virtual synchronous control, simulating the inertia and damping characteristics of a synchronous generator to provide active frequency and voltage support to the receiving-end AC grid. However, during fault periods, the DC voltage controller is prone to saturation, and its power transmission capability becomes uncertain due to AC voltage drops. Continuing to use the dynamic angular frequency generated by virtual synchronous control could lead to system frequency instability. Therefore, this step forcibly switches the angular frequency generated by the virtual synchronous control of the receiving-end flexible DC substation to the rated angular frequency (e.g., the angular velocity corresponding to 50Hz or 60Hz), putting the receiving-end flexible DC substation into a fixed-frequency control mode. A switching switch locks the angular frequency reference value to the rated value, thereby cutting off the frequency regulation effect of the virtual synchronous generator rotor motion equation.

[0036] It is important to note that in the fixed-frequency control mode of this application, the receiving-end flexible DC station no longer responds to power balance by adjusting the frequency, but instead controls power output by adjusting the amplitude of the valve output voltage. Specifically, the controller dynamically adjusts the modulation ratio or inner-loop voltage reference value based on the DC voltage deviation or power command, thereby changing the amplitude of the converter valve output voltage. Due to the reduction in AC voltage, while maintaining fixed-frequency operation, the phase difference between the converter valve output voltage and the AC grid voltage automatically changes with the fault condition, thus automatically adjusting the power output capability to follow the fault condition. This control method avoids oscillations or instability caused by controller saturation during a fault, ensuring that the receiving-end flexible DC station can maintain synchronous connection with the grid during a fault and quickly restore power transmission when the AC voltage recovers.

[0037] Step S43: Switch the DC direct-connected overcapacity valve from DC current control mode to energy consumption mode.

[0038] Specifically, step S43 aims to solve the problem of absorbing surplus power on the DC side during AC faults at the receiving end by switching the control mode of the DC direct-connected overcapacity valve, and to prevent the DC voltage from continuously rising.

[0039] Under steady-state conditions, the DC direct-connected overcapacity valve can typically operate in DC current control mode with the current command set to zero, primarily used to maintain DC voltage stability or smooth power output. However, when a fault occurs in the receiving-end AC grid, causing a voltage drop, the power injected from the sending end cannot be fully delivered to the receiving-end AC grid due to power transmission obstruction. This surplus power continuously charges the DC submodule capacitor, leading to a gradual increase in DC voltage. In this situation, without intervention, the DC overvoltage protection will activate, causing the system to shut down.

[0040] Therefore, upon detecting a fault, step S43 switches the DC-connected overcapacity valve from DC current control mode to energy dissipation mode. In energy dissipation mode, the DC-connected overcapacity valve is configured as an energy-dissipating resistor or virtual load with hysteresis control based on the DC voltage level, suppressing DC voltage rise by absorbing surplus power. In this energy dissipation mode, when the DC voltage rises above a first threshold, the DC-connected overcapacity valve is controlled to operate at its rated current to absorb surplus power; when the DC voltage drops below a second threshold, the DC-connected overcapacity valve stops current output; and when the DC voltage is between the first and second thresholds, the output current of the DC-connected overcapacity valve is maintained as it was at the previous moment.

[0041] It is important to note that the control logic of the energy consumption mode in this application is based on the comparison between the DC voltage and a first threshold and a second threshold. The first and second thresholds are boundary parameters defining the DC voltage hysteresis control range. They can be determined, for example, based on the insulation level of the DC system, the capacitor's withstand voltage, and the protection action settings. For instance, the first threshold can be set to 1.05 times the rated DC voltage, and the second threshold can be set to 1.0 times the rated DC voltage, with the first threshold being greater than the second threshold.

[0042] In this specific control process, when the DC voltage rises above the first threshold, it indicates that there is a significant accumulation of surplus power on the DC side, and the voltage rise trend is obvious. At this time, the DC-connected overcapacity valve can be controlled to operate at its rated current to absorb the surplus power to the maximum extent. The rated current is the maximum long-term operating current designed for the DC-connected overcapacity valve, which can be set to 300A to 500A depending on the heat dissipation capacity of the IGBT module. When the DC voltage drops below the second threshold, it indicates that the surplus power has been consumed or the system power has returned to balance. At this time, the DC-connected overcapacity valve is controlled to stop current output, that is, the output current drops to zero to avoid unnecessary energy loss or excessively low voltage. When the DC voltage is between the first and second thresholds, it indicates that the DC voltage is within the hysteresis range. To prevent frequent switching of control modes at the threshold boundaries from causing system oscillation, the output current of the DC-connected overcapacity valve is maintained unchanged from the previous moment, that is, the current operating state is maintained. Through this hysteresis control strategy, the DC-connected overcapacity valve can quickly respond to changes in DC voltage, forming an effective energy discharge channel during faults, ensuring that the DC voltage is maintained within a safe range, thereby achieving fault ride-through of the system.

[0043] It should be further noted that, in the energy consumption mode, the output current of the DC direct-connected overcapacity valve can be determined by the following formula: Formula 1; In Formula 1, U dc DC voltage; U dcref1 U is the first threshold; dcref2 The second threshold; I dcorder (t) represents the output current of the DC-connected overcapacity valve at time t; I dcorder (t-Δt) represents the output current of the DC-connected overcapacity valve at time t, preceding the previous time step; I N This refers to the rated current of the DC direct-connected overcapacity valve.

[0044] Specifically, for example, Formula 1 above constructs the hysteresis current control logic for a DC-connected overcapacitance valve in energy-consuming mode. At this time, the DC voltage U... dc As a control variable, it is related to the preset first threshold U. dcref1 Second threshold U dcref2 Real-time comparisons are performed to determine the current output current command. First threshold U dcref1 Second threshold U dcref2 Together, they constitute the upper and lower boundaries of the hysteresis control. Their values ​​are set based on the allowable voltage fluctuation range of the DC system and the rated power parameters of the overcapacitance valve, for example, by referring to the withstand voltage level of the DC side submodule capacitor. Among them, the first threshold U... dcref1 A voltage higher than the system's normal operating voltage is used to trigger energy absorption; the second threshold U dcref2Slightly below the first threshold U dcref1 It is used to provide control hysteresis to prevent frequent oscillations at the action boundary caused by signal noise or small fluctuations.

[0045] Thus, when the monitored DC voltage Exceeding the first threshold At this time, it is determined that there is excess power accumulation on the DC side, resulting in excessively high voltage. At this point, the output current... Set to rated current The DC direct-connected overcapacity valve is controlled to operate at full load capacity, quickly absorbing excess power to suppress further rise in DC voltage. If the DC voltage... Falling below the second threshold This indicates that the DC side energy has been restored to balance or is in an undervoltage state. At this time, the output current... When set to 0, the DC direct-connected overcapacity valve stops current output, thereby avoiding unnecessary losses or reverse surges to the system. When the DC voltage... At the first threshold With the second threshold When the voltage fluctuates within a certain range, in order to maintain the stability of the control system and avoid current command jumps caused by small voltage fluctuations, the output current is adjusted accordingly. Maintain the output current from the previous moment. Unchanged. By constructing a system based on DC voltage A robust hysteresis controller is formed by combining a three-stage logic with a double threshold comparison and a state-holding mechanism for the output current from the previous time step. This controller can automatically adjust the output current of the DC-connected overcapacitance valve according to fluctuations in the DC voltage. When the voltage is too high, it actively intervenes to consume energy, and when the voltage returns to the normal range, it maintains its original state or stops working, thereby ensuring that the DC voltage remains stable within a safe range during AC faults at the receiving end, and realizing precise control and automatic adjustment of the energy consumption mode.

[0046] The technical solution provided in this application includes steps S41 to S43 as described above. Therefore, this method can jointly solve the stability problem faced by actively supported DC transmission systems under AC faults at the receiving end. After detecting a voltage drop, the receiving-end flexible DC station switches to a fixed-frequency control mode. By locking the angular frequency and adjusting the valve output voltage amplitude, it maintains synchronous operation with the AC grid during the fault, avoiding system instability caused by lag in virtual synchronization control response or controller saturation. Simultaneously, the DC-connected overcapacity valve switches to energy-consuming mode. Based on the hysteresis comparison results of the DC voltage with the first and second thresholds, it dynamically adjusts the output current to absorb surplus power on the DC side, effectively suppressing the continuous rise of DC voltage. Thus, during the fault, the DC voltage is controlled within a reasonable range, and the frequency remains stable. This verifies that the strategy combining fixed-frequency control and energy-consuming mode can prevent overvoltage protection activation, ensure continuous system operation during AC faults at the receiving end, and quickly restore normal power supply after the fault is cleared.

[0047] In addition, regarding the specific execution order of steps S41 to S43 in this application, after step S41 is executed, steps S42 and S43 can be executed simultaneously, or steps S42 and S43 can be executed sequentially.

[0048] Of course, in another embodiment of this application, the method may further include: when the amplitude of the positive sequence component of the AC voltage at the receiving end recovers to a level greater than or equal to a second preset threshold, switching the angular frequency of the receiving end flexible DC station from the rated angular frequency back to the angular frequency generated by the virtual synchronization control, and resetting the DC voltage controller, wherein the second preset threshold is greater than the preset threshold mentioned in step S42 above; and switching the DC direct-connected overcapacity valve from the energy consumption mode back to the DC current control mode.

[0049] Specifically, for example, if the amplitude of the positive-sequence component of the AC voltage at the receiving end recovers to a level greater than or equal to a second preset threshold, the angular frequency of the receiving-end flexible DC station is switched from the rated angular frequency back to the angular frequency generated by the virtual synchronization control, and the DC voltage controller is reset. The second preset threshold is greater than a preset threshold. This second preset threshold serves as the voltage criterion for determining whether the AC fault has been cleared to trigger the control strategy recovery. It is set based on hysteresis comparison logic, and its value is strictly greater than the preset threshold used to trigger the fault ride-through strategy. This setting provides a certain hysteresis margin for the fault clearing criterion, effectively preventing frequent switching between fault ride-through mode and normal steady-state mode due to voltage fluctuations during AC voltage recovery, thus ensuring the stability of the control state. The specific value of the second preset threshold can be determined comprehensively based on the voltage fluctuation characteristics of the receiving-end AC grid and the system's requirements for recovery response sensitivity; for example, it can be set to 0.9 to 0.95 times the rated voltage.

[0050] When the positive sequence component amplitude of the AC voltage at the receiving end is detected to have recovered and reached or exceeded the second preset threshold, it indicates that the fault on the AC side of the receiving end has been cleared and the power grid is capable of receiving power again. At this time, the angular frequency control of the receiving-end flexible DC substation is returned to virtual synchronization control from the fixed rated angular frequency during the fault period. This process involves a seamless switching of control commands, that is, changing the angular frequency setpoint from a constant (such as...) The frequency (rad / s) is switched back to the dynamic angular frequency calculated in real time by the virtual synchronous machine algorithm based on power balance and grid inertia. By switching back to virtual synchronous control, the receiving-end flexible DC station can regain its active support capability of providing inertia and damping support, thereby enhancing the stability of the weak grid at the receiving end.

[0051] Resetting the DC voltage controller aims to eliminate the negative impact of the controller's internal state during a fault (such as accumulated errors in the integral stage) on the recovery process. Since the DC voltage controller may be in saturation or abnormal operating conditions during fault ride-through mode, directly introducing its internally stored integral values ​​into the recovery phase could cause power overshoot or regulation oscillations at the moment of recovery. A reset operation could, for example, clear the controller's integral accumulator to zero or reset the internal state variables to their initial values, allowing the controller to re-engage with a clean initial state, thus smoothly transitioning to steady state and avoiding the impact on the system caused by residual controller states.

[0052] Furthermore, the DC-connected overcapacity valve is switched from energy dissipation mode back to DC current control mode. Switching the DC-connected overcapacity valve from energy dissipation mode back to DC current control mode is a necessary step to restore the system to standby state after fault clearance. During fault ride-through, the DC-connected overcapacity valve operates in energy dissipation mode, actively absorbing surplus power on the DC side. Once the AC fault is cleared and the system's power transmission capacity is restored, it is no longer necessary to rely on energy dissipation mode to balance DC-side energy; therefore, its control logic must be switched back to steady-state operation mode. In DC current control mode, the DC-connected overcapacity valve is typically used to maintain DC current stability or to maintain a zero-current command in steady state for standby.

[0053] During the switching process, the controller's input reference value switches from the dynamic current command generated by the hysteresis current command generator back to the preset DC current command (e.g., 0A), and the control loop structure is correspondingly restored to a steady-state configuration. This switching ensures that the DC-connected overcapacity valve does not generate unnecessary power loss during non-fault periods and restores its function as a regular regulating unit to smooth DC fluctuations, preparing for the next possible fault ride-through. Through the above recovery process, using the second preset threshold criterion based on hysteresis logic, the control of the receiving-end flexible DC station is restored to virtual synchronous control in an orderly manner, and the power surge at the moment of recovery is suppressed by resetting the DC voltage controller. At the same time, the DC-connected overcapacity valve is restored to the standby state, thereby achieving a smooth switching of the control strategy and stable operation of the system.

[0054] It should be further explained that the method may also include determining the fault flag based on the amplitude of the positive-sequence component of the receiving-end AC voltage and a preset hysteresis judgment condition. Specifically, for example, if the amplitude of the positive-sequence component of the receiving-end AC voltage is detected to be less than a preset threshold, the state of the fault flag is updated based on the comparison result between the amplitude of the positive-sequence component of the receiving-end AC voltage and the preset hysteresis judgment condition.

[0055] The preset hysteresis judgment condition sets a first voltage threshold and a second voltage threshold, wherein the second voltage threshold is greater than the first voltage threshold. Specifically, the judgment process is as follows: when the amplitude of the positive sequence component of the AC voltage at the receiving end is less than the first voltage threshold, the fault flag is set to an active state; when the amplitude of the positive sequence component of the AC voltage at the receiving end is greater than the second voltage threshold, the fault flag is set to an inactive state; when the amplitude of the positive sequence component of the AC voltage at the receiving end is between the first and second voltage thresholds (including boundary values), the fault flag remains in the state of the previous moment.

[0056] In this embodiment, the fault flag serves as a binary logic variable characterizing whether the system is in a fault state. Its state directly determines whether subsequent fixed-frequency control switching and energy consumption mode activation are triggered. A preset hysteresis judgment condition with hysteresis characteristics is introduced to address the misjudgment problem caused by voltage signal fluctuations, noise interference, or non-monotonic changes during AC grid fault processes at the receiving end.

[0057] Compared to the traditional single-threshold comparison method, this judgment condition constructs a judgment interval including a first voltage threshold and a second voltage threshold, forming a hysteresis loop region, thereby improving the robustness of the criterion. The amplitude of the positive-sequence component of the receiving-end AC voltage is usually calculated by a phase-locked loop (PLL) or a positive-to-negative-sequence separation algorithm and is used as real-time input data in the judgment. When the amplitude is detected to be less than the first voltage threshold, it indicates that the voltage drop has exceeded the set fault initiation threshold. At this time, a fault can be determined to have occurred, and the fault flag is set to an active state (e.g., logic 1), thereby triggering the fault ride-through control strategy.

[0058] As the AC grid fault at the receiving end is cleared, if the positive-sequence component amplitude of the AC voltage at the receiving end recovers and rises above the second voltage threshold, it indicates that the voltage level has returned to a safe range. At this time, the fault flag is set to an invalid state (e.g., logic 0), providing a basis for exiting the fault ride-through strategy and restoring steady-state control. When the positive-sequence component amplitude of the AC voltage at the receiving end is within the range formed by the first and second voltage thresholds, a memory mechanism is used to maintain the fault flag in the state of the previous moment. This means that once the fault flag is set to an valid state, it will not be immediately reset due to small voltage fluctuations within the threshold range; it must wait for the voltage to rise above a higher second voltage threshold before it is cleared. Conversely, if the current state is normal, a brief voltage drop that does not fall below the first voltage threshold will not falsely trigger the fault flag. By setting two voltage thresholds with a difference, a buffer zone is introduced between the fault detection and recovery criteria, effectively eliminating frequent switching of the control strategy caused by voltage jitter near a single threshold critical point. For example, the first voltage threshold can be set to 0.8 to 0.9 times the rated voltage, and the second voltage threshold can be set to 0.92 to 0.95 times the rated voltage. This process ensures the uniqueness and stability of fault determination, so that the state of the fault flag will only reverse when the grid voltage experiences a substantial drop or substantial recovery. This provides a reliable trigger signal for AC fault ride-through at the receiving end of the active-supported DC transmission system, ensuring the smooth operation of the system during fault transients.

[0059] In this embodiment of the application, when the amplitude of the positive sequence component of the AC voltage at the receiving end is detected to be less than a preset threshold, a fault flag can be determined using the following formula: Formula 2; In Formula 2, Flag(t) is the fault flag at time t; Flag(t-Δt) is the state of the fault flag set to the state of the previous time step; U sd U is the amplitude of the positive sequence component of the AC voltage at the receiving end; sdref1 U is the first voltage threshold; sdref2 The second voltage threshold is 1; 1 indicates the code value for a valid fault flag; 0 indicates the code value for an invalid fault flag.

[0060] Specifically, Formula 2 above constructs a fault state determination model based on the hysteresis comparison principle. This model is based on the amplitude of the positive sequence component of the receiving-end AC voltage. With the first voltage threshold Second voltage threshold The relative size relationship is used to dynamically determine the current moment. Fault signs .when Below the first voltage threshold When the system is deemed to have entered a fault state, a fault flag is set. Set to 1, i.e., the valid state code value; when Above the second voltage threshold When the system is determined to be in normal condition or the fault has been cleared, the fault flag is set. Set to 0, which is an invalid status code value. (For...) At the first voltage threshold With the second voltage threshold The interval between, fault signs Keep as the previous moment The previous state This state-maintaining mechanism introduces a time-dimensional memory function, effectively filtering out interference caused by voltage signal fluctuations near the threshold and preventing frequent transitions between valid and invalid fault flags. Through the above segmented logic processing, the stability of fault determination is achieved, providing a reliable logical trigger basis for the smooth switching of subsequent control strategies.

[0061] Furthermore, when the fault flag is active, the method provided in this application embodiment can also initialize the DC voltage loop. Specifically, for example, when the fault flag is active, it indicates that the AC grid at the receiving end of the active-supported DC transmission system is in a fault-prolonged period. During this period, the DC voltage typically cannot be maintained at its rated value. Without corresponding intervention measures, the integral element in the DC voltage controller will continuously accumulate error signals, causing the controller to enter a deep saturation state. Once the fault is cleared and the steady-state control strategy is directly switched back, the controller output in the saturated state will cause the system power command to oscillate violently or overshoot at the moment of recovery, thereby causing secondary disturbances in the system.

[0062] Therefore, when the fault flag is detected as active, the DC voltage loop can be initialized immediately. The initialization of the DC voltage loop aims to reset the internal state variables of the DC voltage controller to preset initial values. This process primarily targets the integrator and output limiting components within the controller. In specific implementation, during the discretized execution cycle of the control system, when the fault flag (Flag) is detected as 1, the logic control unit triggers the control processor to write a reset instruction to the controller register, forcibly clearing the integral accumulation term of the DC voltage controller to zero and setting its output instruction value to a preset safe initial value. This safe initial value can be zero or a value near the steady-state operating point corresponding to the current system condition. By actively resetting the controller's internal state during the fault duration, deep saturation of the DC voltage controller due to prolonged deviation can be effectively avoided. This allows the controller to readjust from a zero initial state or a preset equilibrium state when the fault is cleared and the system switches back to a steady-state control strategy, thereby preventing power overshoot or oscillation during the recovery process caused by controller saturation and ensuring a smooth transition from fault ride-through mode to steady-state operation mode.

[0063] One specific implementation method for initializing the DC voltage loop can be to reset the integral component of the DC voltage loop to zero. For example, the DC voltage loop typically uses a proportional-integral (PI) controller or a proportional-integral-derivative (PID) controller, where the integral component accumulates historical errors to eliminate the system's steady-state error. During the occurrence and ride-through of a fault in the receiving-end AC grid, the DC voltage deviates significantly from its rated value, causing a large accumulated error in the integral component, which in turn leads to deep controller saturation. If this accumulated error is not addressed, when the fault is cleared and the system attempts to resume steady-state control, the residual integral quantity will output incorrect control commands, causing control overshoot, resulting in severe power oscillations in the system or triggering protection actions again.

[0064] Therefore, resetting the integral terminator is a crucial operation for eliminating integral saturation and ensuring smooth control transitions. When a fault flag is detected as valid or preset initialization trigger conditions are met, the control system forcibly resets the output value of the integral terminator or its internal accumulation register to 0. This reset is achieved through assignment instructions in digital control systems, and through shorting the integral capacitor or switching the reset circuit in analog control circuits. Performing the resetting operation completely eliminates the impact of accumulated errors from historical fault conditions on the current control, providing a clean initial state for the DC voltage controller to re-intervene after fault clearance. The controller can then restart adjustment based on the current actual DC voltage deviation, thus ensuring system stability when switching back from fixed-frequency control mode to steady-state control mode and avoiding control failures or equipment shocks caused by integral saturation.

[0065] It should also be noted that in practical applications, the control strategy for the receiving-end flexible DC substation under steady-state operation is typically configured to integrate DC voltage control and grid control. For example, DC voltage control is a core component in maintaining the power balance of the DC transmission system. During steady-state operation, the DC voltage control loop regulates the active power absorption or emission of the receiving-end flexible DC substation, keeping the DC line voltage near a preset reference value, thereby ensuring stable energy transmission between the sending and receiving ends. This control loop collects DC side voltage samples and compares them with the DC voltage reference value to obtain an error signal. This error signal is then input to a proportional-integral regulator for calculation, outputting an active power command or a reference value for the inner current loop, thereby achieving precise regulation of the DC voltage through closed-loop feedback.

[0066] Grid-based control endows the flexible DC substation with operating characteristics similar to a traditional synchronous generator, enabling it to actively establish and provide AC voltage support, rather than passively following the frequency and voltage of the AC grid. In this embodiment, grid-based control is implemented through virtual synchronization control. Virtual synchronization control simulates the rotor motion equations (i.e., the swing equations) and stator electrical characteristics of a synchronous generator at the algorithm level. By introducing virtual inertia and damping elements, the output of the receiving-end flexible DC substation possesses inertial response characteristics. In specific implementation, virtual synchronization control includes an active-frequency control loop and a reactive-voltage control loop. The active-frequency control loop simulates rotor inertia and dynamically adjusts the output power according to the grid frequency deviation, providing active frequency support to the receiving-end AC grid. The reactive-voltage control loop simulates excitation regulation characteristics and dynamically adjusts the reactive power output according to the AC voltage amplitude deviation, providing active voltage support to the receiving-end AC grid. This control strategy enables the receiving-end flexible DC substation to not only maintain a constant DC voltage in steady state but also exhibit external voltage source characteristics, actively participating in the regulation of the AC grid. By integrating DC voltage control with grid control based on virtual synchronization control, the receiving-end flexible DC station achieves an organic combination of power balance in the DC transmission channel and synchronous support function in the AC grid under steady state. This integration strategy provides a clear starting control benchmark for switching from this steady-state control mode to the fixed-frequency control mode when an AC fault is detected, ensuring a smooth transition and logical consistency of the system state before and after the control mode switch.

[0067] Of course, the specific configuration of the DC direct-connected overcapacity valve under steady-state operation conditions is as follows: During steady-state operation, the DC direct-connected overcapacity valve adopts a DC current control mode and sets the current command to zero. For example, the DC current control mode can refer to a control strategy that uses the DC side current as the controlled variable and adjusts the actual output current to track the current command through closed-loop feedback. In this mode, the DC direct-connected overcapacity valve is in standby or hot standby state, neither injecting active power into the external power grid nor absorbing active power from the external power grid. Setting the current command to zero means that during normal system operation and without faults, the reference current input signal of the DC direct-connected overcapacity valve is constantly set to 0 amperes. This configuration is usually completed during system initialization or power-on phase and executed by the upper-level scheduling logic of the control system. In this state, the power switching devices such as the insulated-gate bipolar transistor inside the DC direct-connected overcapacity valve respond to the zero current command by performing corresponding modulation actions, making its equivalent impedance exhibit a blocking state or maintaining minimal circulating current loss, thereby ensuring that the DC direct-connected overcapacity valve does not participate in the balance regulation of DC system power transmission. The above configuration establishes the initial state reference of the DC direct-connected overcapacity valve, creating conditions for rapid switching to energy-consuming mode in the event of a subsequent fault, and effectively avoiding transient impacts caused by the presence of initial current commands during control mode switching.

[0068] The above is a detailed description of the method provided in the embodiments of this application. Based on this method, the embodiments of this application can also provide an active support DC transmission system. The active support DC transmission system performs fixed-frequency control switching of AC faults at the receiving end through the method provided in the embodiments of this application. Since this method can solve the problems of the prior art, the active support DC transmission system can also solve the problems in the prior art, which will not be elaborated here.

[0069] This application is not limited to the above-described embodiments. Those skilled in the art can make various improvements and modifications without departing from the principles of this application, and such improvements and modifications are also considered within the scope of protection of this application. Content not described in detail in this specification belongs to prior art known to those skilled in the art.

Claims

1. A method for fixed-frequency control switching of AC faults at the receiving end of an active-supported DC transmission system, characterized in that, The method includes: Real-time monitoring of the positive sequence component amplitude of the AC voltage at the receiving end of the flexible DC substation; If the amplitude of the positive sequence component of the AC voltage at the receiving end is detected to be less than a preset threshold, the angular frequency generated by the virtual synchronization control of the receiving-end flexible DC station is switched to the rated angular frequency, so that the receiving-end flexible DC station operates in a fixed-frequency control mode. In this fixed-frequency control mode, the receiving-end flexible DC station automatically adjusts its power output capability according to the fault condition by adjusting the output voltage amplitude of the adjustment valve. The DC-connected overcapacity valve is switched from DC current control mode to energy consumption mode. In the energy consumption mode, when the DC voltage rises above a first threshold, the DC-connected overcapacity valve is controlled to operate at rated current to absorb surplus power; when the DC voltage drops below a second threshold, the DC-connected overcapacity valve is controlled to stop current output; when the DC voltage is between the first threshold and the second threshold, the output current of the DC-connected overcapacity valve at the previous moment is maintained.

2. The method according to claim 1, characterized in that, The method further includes: If the amplitude of the positive-sequence component of the AC voltage at the receiving end recovers to a value greater than or equal to a second preset threshold, the angular frequency of the receiving-end flexible DC station is switched from the rated angular frequency back to the angular frequency generated by the virtual synchronization control, and the DC voltage controller is reset, wherein the second preset threshold is greater than the preset threshold; and, Switch the DC direct-connected overcapacity valve back from the energy consumption mode to the DC current control mode.

3. The method according to claim 1, characterized in that, In the energy consumption mode, the DC direct-connected overcapacity valve determines the output current using the following formula: ； Among them, U dc The DC voltage; U dcref1 U is the first threshold; dcref2 The second threshold; I dcorder (t) represents the output current of the DC-connected overcapacity valve at time t; I dcorder (t-Δt) represents the output current of the DC-connected overcapacity valve at time t, preceding the previous time. N The rated current of the DC direct-connected overcapacity valve is given.

4. The method according to claim 1, characterized in that, When the amplitude of the positive-sequence component of the AC voltage at the receiving end is detected to be less than a preset threshold, the method includes determining a fault flag based on the amplitude of the positive-sequence component of the AC voltage at the receiving end and a preset hysteresis judgment condition. The preset hysteresis judgment condition includes: when the amplitude of the positive-sequence component of the AC voltage at the receiving end is less than a first voltage threshold, setting the fault flag to an active state; when the amplitude of the positive-sequence component of the AC voltage at the receiving end is greater than a second voltage threshold, setting the fault flag to the state of the previous moment, where the second voltage threshold is greater than the first voltage threshold.

5. The method according to claim 4, characterized in that, If the amplitude of the positive-sequence component of the AC voltage at the receiving end is detected to be less than a preset threshold, the fault flag is determined by the following formula: ； Where Flag(t) is the fault flag at time t; Flag(t-Δt) is the state of the fault flag set to the state of the previous time step; U sd U is the amplitude of the positive sequence component of the AC voltage at the receiving end; sdref1 U is the first voltage threshold; sdref2 The second voltage threshold is represented by 1; 1 represents the coded value of the valid state; and 0 represents the coded value of the invalid state.

6. The method according to claim 4 or 5, characterized in that, The method further includes initializing the DC voltage loop when the fault flag is in an active state.

7. The method according to claim 6, characterized in that, The initialization of the DC voltage loop includes: clearing the integral element of the DC voltage loop to zero.

8. The method according to claim 1, characterized in that, The receiving-end flexible DC station adopts a control strategy that integrates DC voltage control and grid construction control under steady state. The grid construction control is realized through virtual synchronization control, which provides active frequency and voltage support for the receiving-end AC power grid.

9. The method according to claim 1, characterized in that, The DC direct-connected overcapacity valve adopts DC current control mode and the current command is set to zero in steady state.

10. An active-supported DC transmission system, characterized in that, The active support type DC transmission system uses the method described in any one of claims 1 to 9 to perform fixed-frequency control switching for AC faults at the receiving end.

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