An active support type high voltage direct current transmission system and an ac fault power recovery control method for a receiving end thereof

Abstract

The application provides an active support type direct current power transmission system and an AC fault power recovery control method for a receiving end thereof. The method comprises the following steps: obtaining a receiving end active power difference of an AC side of a receiving end converter station; calculating a current instruction correction amount of a DC directly connected overcapacity valve according to the receiving end active power difference, a first rated power of the receiving end converter station and a second rated power of the DC directly connected overcapacity valve; and controlling the DC directly connected overcapacity valve to compensate power for the receiving end converter station according to the corrected DC current instruction. Since the receiving end active power difference is directly taken as a control target and the DC directly connected overcapacity valve actively sends out power for compensation, the power recovery of the receiving end converter station is no longer limited by the slow power recovery rate of a sending end wind power plant, but the overcapacity valve actively supplements the power difference in time after the fault is cleared, so that the power recovery speed after the AC fault of the receiving end can be effectively improved, and the technical problem in the prior art is solved.

Description

Technical Field

[0001] This invention belongs to the field of power transmission fault recovery technology, specifically relating to an active support type DC transmission system and its receiving-end AC fault power recovery control 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 new energy island transmission projects.

[0003] An active-supported DC transmission system mainly includes a sending-end converter station, a receiving-end converter station, DC transmission lines, and a DC direct-connected overcapacity valve. The sending-end converter station's AC side is connected to a wind farm (usually in an islanded configuration) and uses AC voltage control to establish AC voltage on the wind farm side. The receiving-end converter station's AC side is connected to the AC grid and uses a combination of DC voltage control and grid-connected control to control DC voltage and provide inertia and reactive power support during system disturbances. The DC direct-connected overcapacity valve is connected to the DC line and uses DC current control to provide rapid power support during transient processes. In this active-supported DC transmission system, when a serious fault occurs on the AC side of the receiving-end converter station, power transmission is interrupted, and the sending-end converter station must actively reduce its power output. After the fault is cleared, because the wind farm transmits power in an islanded configuration, its active power recovery is slow, causing the receiving-end converter station to passively follow the sending-end converter station in slowly restoring its power.

[0004] For the aforementioned fault scenarios, existing technologies have several control solutions. For example, patent CN120262519A proposes a flexible DC topology with inertia support performance and its control method. This provides inertia support through grid control and directly connected overcapacity branches. However, this solution targets frequency step events; the overcapacity branches respond to frequency deviation signals rather than the power difference at the receiving end, and it does not address rapid power recovery after fault clearance. Patent CN117498334A proposes a receiving-end AC fault ride-through control method, absorbing surplus power from the islanded grid through AC energy dissipation devices. However, this solution only addresses the power surplus during faults, and the AC energy dissipation devices used are purely energy-consuming components, lacking the ability to actively compensate for power differences. Patent CN116865332A proposes a flexible DC fault ride-through control method, improving fault ride-through stability by adjusting the integral coefficient of the voltage outer loop controller, but it does not involve the utilization of energy storage devices, nor does it propose a solution to improve the power recovery speed after fault clearance.

[0005] In summary, none of the existing technologies address how to improve the power recovery speed after the AC fault at the receiving-end converter station is cleared. Currently, there is a lack of a control strategy that can effectively improve the power recovery speed after an AC fault at the receiving-end converter station. Summary of the Invention

[0006] This invention provides an active support type DC transmission system and its receiving-end AC fault power recovery control method to solve the problems in the prior art.

[0007] To achieve the above objectives, the technical solution adopted in this application is to provide a method for power recovery control during AC faults at the receiving end of an active-supported DC transmission system, the method comprising: To obtain the difference in active power at the AC side of the receiving-end converter station in an active-supported DC transmission system; Based on the difference in active power at the receiving end, the first rated power of the receiving end converter station, and the second rated power of the DC direct-connected overcapacity valve in the active support type DC transmission system, the current command correction amount of the DC direct-connected overcapacity valve is calculated and used to correct the DC current command of the DC direct-connected overcapacity valve. The DC direct-connected overcapacity valve is controlled to perform power compensation to the receiving-end converter station according to the corrected DC current command.

[0008] Preferably, obtaining the active power difference at the AC side of the receiving-end converter station in an active-supported DC transmission system includes: Obtain the instantaneous value of active power on the AC side of the receiving-end converter station; The difference in active power at the receiving end is calculated based on the rated active power and the instantaneous value of the active power, wherein the rated active power is the reference value of the first rated power in the per-unit system.

[0009] Preferably, the current command correction amount of the DC-connected overcapacity valve is calculated based on the active power difference at the receiving end, the first rated power of the receiving-end converter station, and the second rated power of the DC-connected overcapacity valve in the active-supported DC transmission system, including: The current command correction amount is calculated by a proportional controller based on the difference in active power at the receiving end, the first rated power, and the second rated power.

[0010] Preferably, the current command correction amount is calculated by a proportional controller based on the difference in active power at the receiving end, the first rated power, and the second rated power, including: The proportional gain of the proportional controller is determined based on the ratio of the second rated power to the first rated power. The active power difference at the receiving end is used as the input of the proportional controller, and after being amplified by the proportional gain, the current command correction is output. The current command correction is expressed in per-unit value to represent the current value required to be output by the DC direct-connected overcapacity valve. The product of the current command correction and the rated power of the receiving-end converter station is equal to the product of the active power difference at the receiving end and the rated power of the DC direct-connected overcapacity valve.

[0011] Preferably, the proportional gain of the proportional controller is determined according to the following formula: K=P superC / P MMC ; Among them, P superC This is the second rated power; P MMC K represents the first rated power; K represents the proportional gain.

[0012] Preferably, the method further includes: Check whether the AC side voltage of the receiving-end converter station has recovered to the rated value; The proportional controller is enabled when the AC side voltage of the receiving-end converter station returns to its rated value.

[0013] Preferably, the DC direct-connected overcapacity valve is equipped with an enable switch, which is connected to the output terminal of the proportional controller. When the AC side voltage of the receiving-end converter station recovers to the rated value, the enable switch closes, allowing the current command correction quantity to pass and participate in subsequent control. When the AC side voltage of the receiving-end converter station does not recover to the rated value, the enable switch opens, blocking the current command correction quantity.

[0014] Preferably, the method further includes: inverting the current command correction amount and superimposing it onto the DC current command to obtain the corrected DC current command.

[0015] Preferably, the DC direct-connected overcapacity valve is further equipped with a DC current controller, and the method further includes: The corrected DC current command is input into the DC current controller; Obtain the actual current of the DC direct-connected overcapacity valve, compare the corrected DC current command with the actual current, and obtain the current deviation signal; The voltage reference value is generated by proportional-integral adjustment of the current deviation signal through the DC current controller. A switching pulse is generated based on the voltage reference value to control the output current of the DC-connected overcapacity valve to track the corrected DC current command.

[0016] This application also provides an active support DC transmission system, which uses the method provided in this application to control the recovery of AC fault power at the receiving end.

[0017] The receiving-end AC fault power recovery control method provided in the embodiments of this application includes: firstly, obtaining the receiving-end active power difference on the AC side of the receiving-end converter station in the active-supported DC transmission system; then, calculating the current command correction amount of the DC direct-connected overcapacity valve based on the receiving-end active power difference, the first rated power of the receiving-end converter station, and the second rated power of the DC direct-connected overcapacity valve in the active-supported DC transmission system, for correcting the DC current command of the DC direct-connected overcapacity valve; and then controlling the DC direct-connected overcapacity valve to perform power compensation to the receiving-end converter station based on the corrected DC current command. This method directly uses the difference in active power at the receiving end as the control target and utilizes the DC direct-connected overcapacity valve to actively generate power for precise compensation. Therefore, the power recovery of the receiving-end converter station is no longer limited by the slow power recovery rate of the sending-end wind farm. Instead, the overcapacity valve actively replenishes the power difference in a timely manner after the fault is cleared, which can effectively improve the power recovery speed after an AC fault at the receiving end and solve the technical problem in the prior art that the receiving end can only passively follow the slow power recovery of the sending end. Attached Figure Description

[0018] 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 A schematic flowchart of the active support type DC transmission system and its receiving-end AC fault power recovery control method provided in the embodiments of this application; Figure 3 This is a schematic diagram of the specific structure of the DC direct-connected overcapacity valve in the active-supported DC transmission system provided in the embodiments of this application. Figure 4 This is a comparative diagram showing the use of the method provided in the embodiments of this application and the use of the method in a real-world example. Detailed Implementation

[0019] The embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0020] 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.

[0021] 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.

[0022] In view of the problems in the prior art, this application provides an active-supported DC transmission system and its receiving-end AC fault power recovery control 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 converter station 1, a receiving-end converter station 2, and a DC direct-connected overcapacity valve 3. The AC side of the sending-end converter station 1 is connected to power generation devices such as thermal power plants, photovoltaic arrays, and wind farms. The AC side of the receiving-end converter station 2 is connected to the AC power grid. The DC side of both the sending-end converter station 1 and the receiving-end converter station 2 is connected to the DC direct-connected overcapacity valve 3.

[0023] like Figure 2 The diagram shown is a schematic flowchart of a receiving-end AC fault power recovery control method in an active-supported DC transmission system provided by an embodiment of this application. The method includes the following steps: Step S41: Obtain the difference in active power at the AC side of the receiving-end converter station in the active-supported DC transmission system.

[0024] The active power difference at the receiving end represents the deviation between the actual active power currently transmitted by the receiving-end converter station and the rated active power level that the system is expected to recover to after a system fault occurs and is cleared. The magnitude of this active power difference directly reflects the scale of the system power gap. Specifically, the first implementation method can be to sample voltage and current signals in real time through voltage transformers and current transformers deployed at the AC bus of the receiving-end converter station, process them through the instantaneous power calculation module to obtain the instantaneous value of active power, and then subtract it from the preset rated active power to obtain the difference between the two. The second implementation method can be to directly read the calculated power deviation data from the real-time database of the internal operation monitoring system of the receiving-end converter station. In step S41, obtaining this active power difference at the receiving end aims to quantify the amount of power support that the current system needs to supplement, providing a data basis for subsequently formulating a precise power compensation strategy.

[0025] In practical applications, the first implementation method mentioned above may specifically include: firstly obtaining the instantaneous value of the active power on the AC side of the receiving-end converter station, and then calculating the difference in active power at the receiving end based on the rated active power and the instantaneous value of active power. Here, the rated active power is the reference value of the first rated power in the per-unit system. The structured data sequence obtained thereby can reflect the dynamic fluctuation of the electrical quantity on the AC side with high fidelity.

[0026] For example, the instantaneous active power value calculated in real time by the controller can be directly obtained through the communication link. This value can be used as the basic input data for calculating the active power difference at the receiving end. Then, the rated active power is compared with the instantaneous active power value to obtain the active power difference at the receiving end. The active power difference at the receiving end refers to the deviation between the actual power received by the receiving converter station and the rated power that the system should transmit. This deviation determines the amount of power that needs to be compensated in the future. By quantifying the current power gap of the system, a precise input variable is provided for the control strategy.

[0027] For example, the arithmetic operation of subtracting the instantaneous value of active power from the rated active power can be performed to calculate the difference in active power at the receiving end. Here, the rated active power is the benchmark value of the first rated power in the per-unit system. The per-unit system is a commonly used normalized measurement system in power system analysis. Its core principle is to divide the actual physical quantity (such as voltage, current, and power) by its corresponding benchmark value to obtain a dimensionless relative value. Therefore, this rated active power is a benchmark quantity determined based on the first rated power of the receiving-end converter station. In the per-unit system, it is usually taken as 1.0 or other standard set values ​​to unify the measurement standards between systems of different capacities.

[0028] Therefore, the calculation process for the active power difference at the receiving end can be formally expressed as: In the formula, This indicates the difference in active power at the receiving end. Indicates the rated active power. This represents the instantaneous value of active power, and all the above quantities are per-unit values. Based on the above acquisition and calculation steps, the power gap after an AC fault at the receiving end can be accurately quantified. Using a per-unit system for difference calculation makes this method universally applicable to DC transmission systems of different capacity levels, avoiding parameter tuning difficulties caused by differences in physical quantities. The obtained active power difference at the receiving end serves as feedback input for closed-loop control, sensitively tracking the power recovery progress on the AC side of the receiving end, thus providing an accurate quantitative basis for subsequent power compensation control.

[0029] Step S42: Based on the difference in active power at the receiving end, the first rated power of the receiving end converter station, and the second rated power of the DC direct-connected overcapacity valve in the active-supported DC transmission system, calculate the current command correction amount of the DC direct-connected overcapacity valve, which is used to correct the DC current command of the DC direct-connected overcapacity valve.

[0030] Among them, the current command correction amount is an incremental signal superimposed on the original DC current control command of the DC direct-connected overcapacity valve. Its physical meaning represents the additional current component that the DC direct-connected overcapacity valve needs to provide in order to fill the power gap at the receiving end. Therefore, after calculating the current command correction amount, it can be used to correct the DC current command of the DC direct-connected overcapacity valve, thereby obtaining the corrected DC current command.

[0031] In the specific implementation of step S42, a functional relationship model can be constructed based on the difference in active power at the receiving end, the first rated power, and the second rated power to determine the current adjustment value that can accurately match the power gap, thereby transforming the power dimension demand into the current dimension control command, ensuring that the electrical power output of the DC direct-connected overcapacity valve can accurately offset the difference in active power at the receiving end.

[0032] Specifically, for example, the current command correction amount can be calculated using a proportional controller based on the difference in active power at the receiving end, the first rated power, and the second rated power. This proportional controller maps the input power deviation signal into an output current adjustment command, aiming to achieve power reference matching between converter stations of different capacity levels and DC direct-connected overcapacity valves. The parameters of this proportional controller depend on the ratio of the first rated power to the second rated power.

[0033] For example, based on the principle of energy conservation, the compensation power output of the DC direct-connected overcapacity valve should be equal to the power difference required by the receiving-end converter station, thereby establishing an equal relationship between power and current. The core of this control logic lies in converting the active power difference at the receiving end from the power reference value of the receiving-end converter station to the power reference value of the DC direct-connected overcapacity valve, and then converting the power quantity into a current control quantity.

[0034] Therefore, the active power difference at the receiving end obtained in the steps can be used as the input to the controller. To convert the power deviation, normalized based on the first rated power, into a current command suitable for the DC-connected overcapacity valve side, the proportional controller's proportional coefficient can be set to the ratio of the second rated power to the second rated power. This conversion mechanism ensures that when a power deficit occurs at the receiving end, the DC-connected overcapacity valve can output a precisely matched current according to its own capacity characteristics, thereby achieving equivalent power support at the electrical level. The calculation process of the current command correction can be formally expressed as: In the formula, This represents the calculated current command correction amount for the DC direct-connected overcapacity valve; This indicates the difference in active power at the receiving end; This represents the first rated power of the receiving-end converter station, which is a named value; This represents the second rated power of the DC-connected overcapacity valve, which is also a named value. This formula establishes a mathematical mapping relationship between the per-unit power deviation of the receiving-end converter station and the current command of the DC-connected overcapacity valve. At this point, when the active power difference at the receiving end is zero, the current command correction is also zero; as the power difference increases, the current command correction increases linearly according to a fixed proportional coefficient. This proportional control strategy based on the rated power ratio enables the DC-connected overcapacity valve to dynamically and accurately adjust the current output according to the magnitude of the power deficit at the receiving end, thereby effectively supporting the rapid recovery of power at the receiving end.

[0035] In practical applications, the proportional gain of the proportional controller can be determined based on the ratio of the second rated power to the first rated power. This proportional gain is a key parameter determining the response strength of the current command correction to the difference in active power at the receiving end. Its setting aims to match the different power capacity levels between the receiving-end converter station and the DC-connected overcapacity valve. For example, during system initialization or controller configuration, the first rated power of the receiving-end converter station can be obtained as the system power reference, and the second rated power of the DC-connected overcapacity valve can be obtained as the execution unit reference. After obtaining the specific values ​​of these two power values ​​by reading the system parameter configuration table or equipment nameplate data, their ratio can be calculated to determine the proportional gain. Specifically, a division operation can be performed to obtain the proportional gain. .

[0036] The determined proportional gain K ensures that the current command correction and the power difference remain consistent in physical dimensions under the per-unit control system, providing an accurate gain coefficient for subsequent real-time control. The active power difference at the receiving end is used as the input to the proportional controller, and after amplification by the proportional gain, the current command correction is output. In the real-time control phase, the real-time acquired active power difference at the receiving end is input to the proportional controller with the configured proportional gain. The proportional controller performs a linear amplification operation on this power difference to calculate the current command correction. This current command correction is represented in per-unit form, aiming to standardize the current control command for DC-DC overcapacity valves of different capacity levels, providing a unified control benchmark and eliminating the control level mismatch problem caused by equipment capacity differences. Specifically, the current command correction represents the required output current value of the DC-DC overcapacity valve in per-unit form, and the product of the current command correction and the rated power of the receiving-end converter station is equal to the product of the active power difference at the receiving end and the rated power of the DC-DC overcapacity valve.

[0037] The above calculation process follows a strict power equivalence relation, ensuring that the control commands achieve precise power balance at the physical level. This process can be formally represented as the following decision function: In the formula, The current command correction amount is represented in per-unit value form; The rated power (nominal value) of the DC direct-connected overcapacity valve; The rated active power of the receiving-end converter station (the reference value in the per-unit system). This represents the instantaneous (per unit) value of the active power on the AC side of the receiving-end converter station. This represents the rated power (nominal value) of the receiving-end converter station. This equivalent relationship shows that the actual power change generated by the current command correction after being adjusted by the proportional gain and executed by the DC direct-connected overcapacity valve strictly corresponds to the power deficit on the receiving end side, thus ensuring the closed-loop consistency of the control strategy in both mathematical logic and engineering physics.

[0038] In addition, the current command correction amount can be inverted and then superimposed onto the DC current command I. dcrefThis process is repeated to obtain the corrected DC current command. For example, after obtaining the current command correction amount calculated in the aforementioned steps, this correction amount can be inverted and superimposed on the DC current command. The reason for this is that in the conventional control logic definition of an active-supported DC transmission system, when the DC current command of the DC direct-connected overcapacity valve is positive, it corresponds to the action of the DC direct-connected overcapacity valve absorbing power from the DC network; however, under the condition of AC fault recovery at the receiving end, when the active power difference at the receiving end is positive, it means that there is a power deficit at the receiving end converter station, and the DC direct-connected overcapacity valve needs to output power to make up for the deficit. At this time, if the current command correction amount is directly superimposed on the original DC current command, it will lead to malfunction of the control system because the polarity direction of the control command is opposite to the actual energy flow direction required.

[0039] Therefore, addressing the aforementioned polarity inconsistency issue, an inversion operation can be performed on the current command correction value—converting positive values ​​to negative values ​​or vice versa—ensuring the numerical sign correctly guides the DC-connected overcapacity valve into either discharge or power generation mode. Subsequently, the inverted current command correction value is algebraically superimposed on the original DC current command to generate the final corrected DC current command. This corrected DC current command is directly sent to the underlying control loop of the DC-connected overcapacity valve to drive the operation of the power electronic switching devices, thereby precisely controlling the DC-connected overcapacity valve to provide power compensation to the receiving-end converter station. Through this inversion and superposition operation, the control signal logic and physical power flow are unified, ensuring the accuracy of the power compensation process.

[0040] Furthermore, the control strategy of the receiving-end converter station itself can remain unchanged. The calculation of the current command correction and the control of the DC-connected overcapacity valve are both completed on the DC-connected overcapacity valve side. The overall control architecture design can follow the principle of minimizing intrusion. Thus, when implementing the aforementioned power restoration control method, the original controller configuration, parameter settings, and operating logic of the receiving-end converter station remain in their existing state, without requiring hardware replacement or software modifications. This design ensures the stability and reliability of the already mature and operational receiving-end converter station control system, effectively avoiding potential compatibility risks caused by changes in the control strategy.

[0041] It should be noted that the calculation logic for the current command correction and the control function for the DC-connected overcapacity valve involved in this application are both deployed on the DC-connected overcapacity valve side. Specifically, as... Figure 3The diagram shows the structure of the DC-connected overcapacity valve in the active-supported DC transmission system of this application. A second controller can be installed on the side of the DC-connected overcapacity valve. This second controller can handle all data processing and command generation, including obtaining the active power difference at the receiving end from the aforementioned steps, calculating the current command correction amount based on a preset proportional gain, and generating the final corrected DC current command. By decoupling the new control function from the existing main control system, the power fast recovery function operates as an independent add-on module, requiring only the input of necessary external electrical signals to execute autonomously. This architecture significantly reduces the complexity of system modification and the difficulty of engineering implementation, improving the scalability and engineering applicability of this technical solution in different DC transmission projects.

[0042] The second controller can receive the rated active power of the receiving-end converter station. Instantaneous active power on the AC side of the receiving-end converter station The difference in active power at the receiving end is then calculated using both methods. Then, the current command correction amount is calculated by combining the proportional gain K. Then the current command correction amount can be adjusted. The inversion operation is performed, and then algebraically superimposed with the original DC current command to generate the final corrected DC current command. Where I... dc It is the DC current of the DC-connected overcapacity valve, N rated This refers to the rated number of sub-modules for DC direct-connected overcapacity valves.

[0043] Step S43: Control the DC direct-connected overcapacity valve to perform power compensation to the receiving-end converter station according to the corrected DC current command.

[0044] The corrected DC current command is generated by superimposing the calculated correction amount (first inverted) with the original DC current command of the DC-connected overcapacity valve to produce the final control reference value. The switching state of the internal power electronic devices of the DC-connected overcapacity valve can be adjusted to ensure its output current tracks the corrected DC current command in real time. This control process utilizes the electrical energy stored in the DC-connected overcapacity valve to rapidly inject active power into the receiving-end converter station, thereby achieving power compensation for the receiving-end converter station and effectively shortening the power recovery time after an AC fault at the receiving end.

[0045] Specifically, for example, the corrected DC current command can be input to the closed-loop current controller of the DC direct-connected overcapacity valve, and the modulated wave can be output through the proportional-integral (PI) regulator to drive the operation of switching devices such as insulated gate bipolar transistors (IGBTs) to achieve precise control of the output current; or, a hysteresis comparison control method can be used to directly control the switching transistor based on the difference between the corrected DC current command and the actual feedback current to quickly track command changes.

[0046] It should be further explained that the DC direct-connected overcapacity valve can also be equipped with a DC current controller. Therefore, the method can also include inputting the corrected DC current command into the DC current controller to obtain the actual current of the DC direct-connected overcapacity valve, comparing the corrected DC current command with the actual current to obtain a current deviation signal, then using the DC current controller to perform proportional-integral adjustment on the deviation signal to generate a voltage reference value, and then generating a switching pulse based on the voltage reference value to control the output current of the DC direct-connected overcapacity valve to track the corrected DC current command.

[0047] After sending the corrected DC current command to the control loop of the DC-connected overcapacity valve, the DC current controller executes closed-loop control logic. First, it acquires the actual current of the DC-connected overcapacity valve, typically collected by a Hall sensor or shunt installed on the DC side, and fed back to the controller in real time via an analog-to-digital converter. Then, it compares the acquired actual current with the corrected DC current command obtained in the previous steps, calculating the deviation signal between the two. This deviation signal characterizes the difference between the current output current and the target command, serving as the direct basis for subsequent adjustment actions. The proportional-integral (PI) control loop within the DC current controller processes this deviation signal. This PI control loop can consist of a proportional unit and an integral unit. The proportional unit responds quickly according to the magnitude of the current deviation signal, using a proportional coefficient to suppress instantaneous changes in the deviation; the integral unit accumulates historical deviations to eliminate static errors. Therefore, by combining the magnitude and duration of the deviation, the control output is dynamically adjusted to generate a voltage reference value.

[0048] This voltage reference value typically exists in the form of a modulated wave, representing the target terminal voltage that the DC-DC overcapacitance valve should output at the next moment, aiming to guide the actual current towards the command value. Based on the generated voltage reference value, switching pulses are further generated. This process usually involves pulse width modulation (PWM) technology, which compares the voltage reference value with a high-frequency carrier signal to obtain a series of switching pulse sequences with specific duty cycles. These switching pulses are transmitted to the gate drive circuit of power devices such as insulated-gate bipolar transistors (IGBTs) or integrated gate commutated thyristors in the DC-DC overcapacitance valve. By controlling the on / off state of the power devices, the output current of the DC-DC overcapacitance valve is adjusted in real time to accurately track the corrected DC current command, thereby achieving rapid power compensation and stable output.

[0049] In practical applications, the DC direct-connected overcapacity valve is equipped with an enable switch connected to the output terminal of the proportional controller. This enable switch controls the passage or blocking of the current command correction. For example, when the AC side voltage of the receiving-end converter station recovers to its rated value, the enable switch closes, allowing the current command correction to pass and participate in subsequent control. When the AC side voltage of the receiving-end converter station has not recovered to its rated value, the enable switch opens, blocking the current command correction. Therefore, detecting whether the AC side voltage of the receiving-end converter station has recovered to its rated value, and enabling the proportional controller when the AC side voltage of the receiving-end converter station has recovered to its rated value, includes: closing the enable switch when the AC side voltage of the receiving-end converter station recovers to its rated value, allowing the current command correction to pass and participate in subsequent control; and opening the enable switch when the AC side voltage of the receiving-end converter station has not recovered to its rated value, blocking the current command correction.

[0050] The enable switch, as a logic switching component configured in the internal control loop of the DC-DC connected overcapacity valve, is physically connected between the output of the proportional controller and the subsequent signal superposition node. The formation process of this enable switch can be, for example, by configuring a logic control unit inside the second controller of the DC-DC connected overcapacity valve. This unit receives the real-time monitoring signal of the AC side voltage of the receiving-end converter station as input, and determines the logic to generate a switch control signal based on a preset voltage threshold, thereby forming a virtual switch at the software level or a physical switch action command at the hardware driver level. The internal structure of the enable switch can consist of logic gate circuits or solid-state relays, and its state output directly determines whether the current command correction amount generated by the proportional controller can be transmitted to the superposition point of the DC current command.

[0051] This configuration ensures that the fast power recovery control function intervenes only when the receiving-end AC system has recovery capability, preventing unnecessary adjustment interference during periods of unresolved faults or system instability, and guaranteeing the safe operation of the DC transmission system. When the AC side voltage of the receiving-end converter station recovers to its rated value, the control logic triggers a closing action to activate the power compensation channel. The voltage monitoring unit continuously collects voltage data from the AC bus of the receiving-end converter station. When the voltage amplitude is detected to be stable within the allowable deviation range of the rated voltage (e.g., ±5%) within a preset time window, it is determined that the voltage has recovered. At this time, the control unit sends a closing command to the enable switch, which changes from an open state to a closed state. The current command correction output by the proportional controller then passes through this switch and is superimposed on the DC current command of the DC-connected overcapacity valve. This action of immediately closing the switch after voltage recovery can instantly utilize the rapid response capability of the DC-connected overcapacity valve to fill the power gap. When the AC side voltage of the receiving-end converter station has not recovered to its rated value, the control logic maintains or executes a disconnect action to block the current command correction. If the monitored voltage amplitude is below the rated voltage threshold or fluctuates drastically, it indicates that the AC system is still in the early stages of fault or recovery and does not meet the conditions for accepting power supplementation. In this case, the control unit maintains or issues a disconnect command, keeping the enable switch in the open state, isolating the proportional controller from subsequent control loops. Regardless of the current command correction value calculated by the proportional controller due to the power difference, it is intercepted by the enable switch and cannot be input into the DC current control loop. By blocking the command correction value under abnormal operating conditions, improper power regulation at weak points in the system is avoided, preventing system instability or fault expansion, thus ensuring the robustness of the control strategy.

[0052] It is important to note that the second controller internally includes a power fast recovery controller. This controller generates a power compensation current command based on the active power difference at the receiving end and superimposes it onto the DC current command of the DC-connected overcapacity valve. As a functional module integrated within the second controller, the power fast recovery controller is essentially a control logic unit embedded in the DC-connected overcapacity valve control program. In this embodiment, the power fast recovery controller is not a physical device independent of the existing control system, but rather consists of algorithm code and related registers and computing units stored in the second controller's memory. It is responsible for signal conversion and command generation, transforming macroscopic power imbalance into microscopic current control commands.

[0053] The internal architecture of this power fast recovery controller mainly consists of an input signal processing stage, a proportional operation stage, and a command output stage. The construction process of the power fast recovery controller within the second controller is based on the analytical requirements of the power balance mechanism of the DC transmission system. During system initialization or control logic configuration, the first rated power of the receiving-end converter station and the second rated power of the DC direct-connected overcapacity valve are obtained as parameter inputs. By analyzing the conversion relationship between the power difference and the current command, a proportional control model is established with the active power difference at the receiving end as the input variable and the current command correction amount as the output variable. This model is compiled into machine language code and loaded into the reserved logic space of the second controller, forming a power fast recovery controller with real-time computing capabilities.

[0054] During dynamic execution, the power fast recovery controller receives the active power difference at the receiving end calculated in the aforementioned steps in real time. The proportional control unit inside the controller amplifies or converts the active power difference at the receiving end according to a preset proportional gain, thereby generating a target signal representing the required output current value of the DC-connected overcapacity valve, i.e., the power compensation current command. This command, after sign processing (such as inversion), is precisely superimposed onto the original DC current command reference value of the DC-connected overcapacity valve. This process injects a compensation component for the AC side power deficit into the current command, which was originally controlled solely by DC voltage. Thus, without changing the main control topology, the fast response capability of the DC-connected overcapacity valve provides power support to the receiving-end converter station. By embedding the power fast recovery controller within the second controller, a high degree of integration of control functions is achieved, external wiring delays are reduced, and the real-time generation and accuracy of the power compensation current command are ensured, providing a core control method for improving the power recovery speed after a fault.

[0055] like Figure 4 The figure shows a comparison of the receiving-end power waveforms before and after the application of the power generation method proposed in this patent. The horizontal axis represents time t (s), and the vertical axis represents the receiving-end active power (MW). From... Figure 4 It is clearly visible that the AC fault at the receiving end occurred at t=7s, causing the active power at the receiving end to drop rapidly from 1900MW to 1200MW; the fault was cleared at t=8s. Before this strategy was implemented, the receiving-end power could only recover slowly following the wind farm at the sending end, reaching approximately 1800MW by t=9s. At this point, there was still a difference of about 100MW from the rated value of 1900MW, limiting the power recovery speed. However, after implementing this strategy, the fast power recovery controller immediately activated, actively supplementing the power difference at the receiving end using a DC-connected overcapacity valve. The receiving-end power recovered more rapidly within the same timeframe, essentially restoring to the rated value of 1900MW by t=9s.

[0056] A direct comparison of the two methods reveals that, within the same recovery window after fault clearance, the power recovery curve is significantly steeper when using the method of this application. The time required for the receiving-end power to recover from 1200MW to the rated value of 1900MW is reduced from approximately 2 seconds without the strategy to approximately 1 second after the strategy is implemented. The power recovery speed after fault clearance is significantly improved. This result fully verifies the effectiveness of the control strategy proposed in this patent in improving the power recovery speed after AC faults at the receiving end, and also confirms the technical effect of the technical solution proposed in this patent.

[0057] In addition, combined Figure 4 The magnified local features also reveal that the method described in this application results in a smooth power recovery process without overshoot or oscillation, indicating that the control strategy not only improves the recovery speed but also exhibits excellent dynamic response quality. This demonstrates that this patent not only effectively solves the problem of slow power recovery at the receiving end in the prior art but also ensures the stability and reliability of the power recovery process.

[0058] Through the above steps, utilizing the rapid response characteristics of the DC-connected overcapacity valve, its output current is dynamically adjusted based on the active power difference at the receiving end, thereby achieving real-time power support for the receiving-end converter station. After adopting the control method provided in this application embodiment, the recovery speed of the receiving-end power after fault clearance is significantly accelerated, and the waveform slope is significantly increased, verifying the effectiveness of this control strategy in improving the transient stability of the system.

[0059] The receiving-end AC fault power recovery control method provided in the embodiments of this application includes: firstly, obtaining the receiving-end active power difference on the AC side of the receiving-end converter station in the active-supported DC transmission system; then, calculating the current command correction amount of the DC direct-connected overcapacity valve based on the receiving-end active power difference, the first rated power of the receiving-end converter station, and the second rated power of the DC direct-connected overcapacity valve in the active-supported DC transmission system, for correcting the DC current command of the DC direct-connected overcapacity valve; and then controlling the DC direct-connected overcapacity valve to perform power compensation to the receiving-end converter station based on the corrected DC current command. This method directly uses the difference in active power at the receiving end as the control target and utilizes the DC direct-connected overcapacity valve to actively generate power for precise compensation. Therefore, the power recovery of the receiving-end converter station is no longer limited by the slow power recovery rate of the sending-end wind farm. Instead, the overcapacity valve actively replenishes the power difference in a timely manner after the fault is cleared, which can effectively improve the power recovery speed after an AC fault at the receiving end and solve the technical problem in the prior art that the receiving end can only passively follow the slow power recovery of the sending end.

[0060] As mentioned above, the receiving-end converter station is equipped with a first controller, and the DC direct-connected overcapacity valve is equipped with a second controller. The first controller can calculate the instantaneous active power value on the AC side of the receiving-end converter station and transmit it to the second controller via communication. For example, the first controller can be configured in the control system of the receiving-end converter station to collect and process electrical quantity data from the AC side in real time. Based on the instantaneous AC voltage and current values ​​collected by the voltage and current transformers, the first controller performs multiplication operations or coordinate transformations to calculate the instantaneous active power value on the AC side of the receiving-end converter station. Subsequently, the first controller encapsulates the calculated instantaneous active power value into a data message conforming to a preset communication protocol and sends it in real time to the second controller located at the DC direct-connected overcapacity valve via the station's fiber optic communication network or industrial Ethernet communication link.

[0061] The second controller in the DC-connected overcapacity valve control system acts as the receiving end, unpacking and verifying the received communication messages to extract the instantaneous active power value. This instantaneous active power value is used as the input variable for calculating the current command correction. This control architecture enables state information sharing between the receiving-end converter station and the DC-connected overcapacity valve, providing an accurate data foundation for subsequent power difference calculation and command correction. Through communication interaction between the first and second controllers, key state quantities can be obtained using existing communication infrastructure without changing the main control strategy of the receiving-end converter station, thereby achieving precise coordinated control of the DC-connected overcapacity valve. The first controller is responsible for the accurate calculation and transmission of source data, while the second controller is responsible for data reception and application. The two form a closed-loop data interaction through a reliable communication transmission link. This hierarchical control structure ensures that the instantaneous active power value can be transmitted to the execution end in a timely and accurate manner, so that the current command correction calculated based on this instantaneous value can truly reflect the power gap of the receiving-end AC system, thus ensuring the effective implementation of the power compensation control strategy.

[0062] It is important to note that this application also provides limitations on the applicable operating conditions for the above control strategy, namely: the active power difference at the receiving end is non-negative; in the active-supported DC transmission system, the positive power direction is defined as the flow from the DC side to the AC side; and there is no power flow reversal. Under these limitations, the non-negative characteristic of the active power difference at the receiving end directly stems from the system's preset power flow direction definition and specific operating state constraints. In the operating logic of the active-supported DC transmission system, the direction from the DC side to the AC side is defined as the positive power direction, that is, the direction in which the DC line injects energy into the receiving-end grid. Combined with the constraint of no power flow reversal, the system maintains unidirectional energy transmission throughout the entire control process, and there is no situation where energy flows backward from the receiving-end AC side to the DC side. Based on the above unidirectional transmission characteristic, the instantaneous value of the actual active power on the AC side of the receiving-end converter station is always less than or equal to the rated active power of the system. Therefore, the difference calculated based on the instantaneous value of the rated active power and the actual active power is always non-negative. This non-negative property means that when calculating the current command correction for the DC direct-connected overcapacity valve, the control logic only needs to handle unidirectional compensation requirements, effectively avoiding the complexity of control logic or controller output oscillations that may be caused by the switching of the positive and negative signs of the power difference. By clearly defining the power flow direction and operating condition boundaries, the power compensation determination process is simplified, ensuring that during the fault recovery phase, the DC direct-connected overcapacity valve can always provide stable power support to the receiving-end converter station according to the predetermined strategy, thereby improving the determinism and reliability of the control strategy in specific unidirectional power transmission scenarios.

[0063] 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 recovery control of AC fault power at the receiving end through the method provided in the embodiments of this application. Since this method can solve the problems in the prior art, the active support DC transmission system can also solve the problems in the prior art, which will not be elaborated here.

[0064] 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 power recovery control during AC faults at the receiving end of an active-supported DC transmission system, characterized in that, The method includes: To obtain the difference in active power at the AC side of the receiving-end converter station in an active-supported DC transmission system; Based on the difference in active power at the receiving end, the first rated power of the receiving end converter station, and the second rated power of the DC direct-connected overcapacity valve in the active support type DC transmission system, the current command correction amount of the DC direct-connected overcapacity valve is calculated and used to correct the DC current command of the DC direct-connected overcapacity valve. The DC direct-connected overcapacity valve is controlled to perform power compensation to the receiving-end converter station according to the corrected DC current command.

2. The method according to claim 1, characterized in that, Obtaining the active power difference at the AC side of the receiving-end converter station in an active-supported DC transmission system includes: Obtain the instantaneous value of active power on the AC side of the receiving-end converter station; The difference in active power at the receiving end is calculated based on the rated active power and the instantaneous value of the active power, wherein the rated active power is the reference value of the first rated power in the per-unit system.

3. The method according to claim 1, characterized in that, Based on the active power difference at the receiving end, the first rated power of the receiving-end converter station, and the second rated power of the DC direct-connected overcapacity valve in the active-supported DC transmission system, the current command correction amount of the DC direct-connected overcapacity valve is calculated, including: The current command correction amount is calculated by a proportional controller based on the difference in active power at the receiving end, the first rated power, and the second rated power.

4. The method according to claim 3, characterized in that, Based on the difference in active power at the receiving end, the first rated power, and the second rated power, the current command correction is calculated by a proportional controller, including: The proportional gain of the proportional controller is determined based on the ratio of the second rated power to the first rated power. The active power difference at the receiving end is used as the input of the proportional controller, and after being amplified by the proportional gain, the current command correction is output. The current command correction is expressed in per-unit value to represent the current value required to be output by the DC direct-connected overcapacity valve. The product of the current command correction and the rated power of the receiving-end converter station is equal to the product of the active power difference at the receiving end and the rated power of the DC direct-connected overcapacity valve.

5. The method according to claim 4, characterized in that, The proportional gain of the proportional controller is determined according to the following formula: K=P superC / P MMC ; Among them, P superC This is the second rated power; P MMC K represents the first rated power; K represents the proportional gain.

6. The method according to claim 3, characterized in that, The method further includes: Check whether the AC side voltage of the receiving-end converter station has recovered to the rated value; The proportional controller is enabled when the AC side voltage of the receiving-end converter station returns to its rated value.

7. The method according to claim 1, characterized in that, The DC direct-connected overcapacity valve is equipped with an enable switch, which is connected to the output terminal of the proportional controller. When the AC side voltage of the receiving-end converter station recovers to the rated value, the enable switch closes, allowing the current command correction quantity to pass and participate in subsequent control. When the AC side voltage of the receiving-end converter station fails to recover to the rated value, the enable switch opens, blocking the current command correction quantity.

8. The method according to claim 1, characterized in that, The method further includes: inverting the current command correction amount and superimposing it onto the DC current command to obtain the corrected DC current command.

9. The method according to claim 8, characterized in that, The DC direct-connected overcapacity valve is further equipped with a DC current controller, and the method further includes: The corrected DC current command is input into the DC current controller; Obtain the actual current of the DC direct-connected overcapacity valve, compare the corrected DC current command with the actual current, and obtain the current deviation signal; The voltage reference value is generated by proportional-integral adjustment of the current deviation signal through the DC current controller. A switching pulse is generated based on the voltage reference value to control the output current of the DC-connected overcapacity valve to track the corrected DC current command.

10. An active-supported DC transmission system, characterized in that, The active support type DC transmission system performs recovery control of AC fault power at the receiving end by means of the method described in any one of claims 1 to 9.

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