Fault current limiting method and system for voltage source converter based on control mode switching

Abstract

The application discloses a control mode switching-based fault current limiting method and system for a flexible direct-current converter, and belongs to the technical field of flexible direct-current transmission control, which comprises the following steps: in response to detecting that the flexible direct-current converter has a fault, switching the control mode of the flexible direct-current converter from a grid-constructing control mode to a grid-following control mode based on preset initial values of grid-following integrators of grid-following controllers; in the grid-following control mode, limiting the grid-following controllers according to a reactive current-first current limiting strategy, determining active currents and reactive currents, and then determining initial values of grid-constructing integrators of the grid-constructing controllers; and in response to detecting that the flexible direct-current converter fault is eliminated, switching the control mode of the flexible direct-current converter from the grid-following control mode back to the grid-constructing control mode based on the initial values of the grid-constructing integrators. The scheme can ensure system transient stability, does not need to change original controller structures or add new control links, greatly simplifies the complexity of engineering implementation, and can realize bidirectional smooth switching of control modes.

Description

Technical Field

[0001] This application relates to the field of flexible DC transmission control technology, specifically to a fault current limiting method and system for flexible DC converters based on control mode switching. Background Technology

[0002] Flexible DC transmission offers advantages such as smaller footprint, flexible control, and compatibility with active or passive networks, making it a friendly solution for large-scale renewable energy transmission. Traditional flexible DC converters employ a grid-following control strategy, using a phase-locked loop (PLL) to measure the grid voltage phase for synchronization, which limits their ability to actively support the grid. Grid-based flexible DC converters, on the other hand, automatically synchronize with the grid based on DC voltage or active power, providing proactive grid support. However, due to the voltage source characteristics of grid-based converters, to maintain a constant internal potential, their output current increases sharply when the grid voltage drops, easily exceeding the tolerance limits of their power semiconductor devices. Therefore, fault current limiting is one of the key issues that urgently needs to be addressed in grid-based flexible DC technology.

[0003] Currently, the main fault current limiting method for grid-type flexible DC transmission is the additional virtual impedance method. This method reduces the converter output voltage by simulating impedance voltage division inside the converter to achieve current limiting. However, the parameter design of the virtual impedance needs to take into account the fault current limiting effect, the system stability boundary, and the reactive power support requirements at the same time. The calculation is complex, and the engineering implementation is complicated and difficult. Summary of the Invention

[0004] This paper presents a fault current limiting method and system for flexible DC converters based on control mode switching, aiming to simplify the complexity of engineering implementation.

[0005] Firstly, a fault current limiting method for flexible DC converters based on control mode switching is provided, comprising the following steps: In response to the detection of a fault in the flexible DC converter, the control mode of the flexible DC converter is switched from the grid construction control mode to the grid connection control mode based on the preset initial value of the grid connection integrator of the grid connection controller. In the grid-connected control mode, the grid-connected controller is current-limited according to the reactive current priority current-limiting strategy to determine the active current and reactive current. Based on the active current and the reactive current, determine the initial value of the grid integrator of the grid controller; In response to the detection that the fault of the flexible DC converter has been eliminated, the control mode of the flexible DC converter is switched back from the grid-following control mode to the grid-building control mode based on the initial value of the grid integrator.

[0006] In some embodiments, prior to detecting a fault in the flexible DC converter, the method includes: In response to detecting that the flexible DC converter is operating in the grid control mode, the current operating condition of the flexible DC converter is obtained; Based on the current operating conditions, determine the initial value of the grid-connected integrator of the current inner loop integrator of the grid-connected controller.

[0007] In some embodiments, determining the initial value of the grid-connected integrator of the current inner loop integrator of the grid-connected controller based on the current operating conditions includes: Based on the voltage phase output of the phase-locked loop in the grid controller, the reference voltage output by the grid controller, the voltage and current output by the flexible DC converter are determined respectively. Based on the reference voltage output by the grid controller, the voltage and current output by the flexible DC converter, the initial value of the grid integrator of the current inner loop integrator of the grid controller is determined.

[0008] In some embodiments, the fault detection step of the flexible DC converter includes: In response to detecting that the output current of the flexible DC converter exceeds a preset current threshold, the positive sequence voltage component of the flexible DC converter is abnormal, and / or the negative sequence voltage component of the flexible DC converter is abnormal, it is determined that the flexible DC converter has malfunctioned.

[0009] In some embodiments, in the grid-connected control mode, current limiting of the grid-connected controller according to a reactive current-priority current limiting strategy, and determination of active current and reactive current, includes: In the grid-connected control mode, based on the preset transient low voltage and reactive current curves, the reactive component of the output current corresponding to the voltage of the flexible DC converter is determined. Based on the reactive component and the total current withstand limit of the flexible DC converter, the active and reactive components of the inner loop current of the grid controller are limited respectively to determine the active and reactive currents.

[0010] In some embodiments, determining the initial value of the grid integrator of the grid controller based on the active current and the reactive current includes: Based on the active current and the reactive current, the reference voltage output by the grid controller is phase-locked to determine the phase and angular frequency corresponding to the reference voltage output by the grid controller. Based on the phase and angular frequency corresponding to the reference voltage output by the network controller, the initial value of the network integrator of the network controller is determined.

[0011] In some embodiments, determining the initial value of the network integrator of the network controller based on the phase and angular frequency corresponding to the reference voltage output by the network controller includes: Based on the phase and angular frequency corresponding to the reference voltage output by the grid controller, the amplitude of the reference voltage output by the grid controller is determined; Based on the amplitude of the reference voltage output by the grid controller and the active and reactive power control equations of the grid controller, the initial value of the grid integrator of the grid controller is determined.

[0012] Secondly, a fault current limiting system for flexible DC converters based on control mode switching is also provided, including: The grid-connected control mode switching module is used to switch the control mode of the flexible DC converter from the grid-connected control mode to the grid-connected control mode in response to the detection of a fault in the flexible DC converter, based on the preset initial value of the grid-connected integrator of the grid-connected controller. The fault current limiting module is used to limit the current of the grid controller according to the reactive current priority current limiting strategy in the grid control mode, and to determine the active current and reactive current. The initial value setting module for grid control is used to determine the initial value of the grid integrator of the grid controller based on the active current and the reactive current. The grid control mode switching module is used to switch the control mode of the flexible DC converter back from the grid-following control mode to the grid control mode after detecting a preset time for the fault elimination of the flexible DC converter, based on the initial value of the grid integrator.

[0013] Thirdly, an electronic device is also provided, including a memory and a processor, wherein the memory stores a computer program, which, when executed by the processor, implements the steps of the flow control method described in any of the above methods.

[0014] Fourthly, a computer-readable storage medium is also provided, on which a computer program is stored, the computer program being loaded by a processor to perform the steps of the flow control method described in any of the above methods.

[0015] Beneficial effects: By pre-calculating and setting the ideal initial value of the integrator of the target controller before switching, voltage and current surges caused by mismatch in the initial state of the integrator can be effectively avoided, ensuring the transient stability of the system. At the same time, this method does not require modification of the original controller structure or addition of new control links, greatly simplifying the complexity of engineering implementation. During fault handling, a reactive current priority current limiting strategy is adopted, which can dynamically and accurately provide reactive power support based on the AC voltage drop depth at the grid connection point of the flexible DC converter, while strictly limiting the total current within the safe tolerance range of the converter. This achieves synergistic optimization of fault current limiting and grid reactive power support, thus providing comprehensive and effective protection for the grid-connected flexible DC converter to safely pass through grid faults. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a simplified flowchart of the fault current limiting method for flexible DC converter based on control mode switching provided by an exemplary embodiment of this disclosure; Figure 2 This is a schematic diagram of the overall process of the fault current limiting method for flexible DC converter based on control mode switching provided by the exemplary embodiments of this disclosure; Figure 3 This is a flowchart of the grid connection control process of a direct converter provided in an exemplary embodiment of this disclosure; Figure 4 This is a flowchart of a network-type flexible vertical control system provided in an exemplary embodiment of this disclosure; Figure 5 This is a schematic diagram of the electrical quantity waveform of the converter output during the instantaneous switch from grid-based control to grid-following control, provided by an exemplary embodiment of this disclosure. Figure 6 This is a schematic diagram of the electrical quantity waveform output of the converter during the instantaneous switch from grid control to grid control provided by an exemplary embodiment of this disclosure; Figure 7 This is a schematic diagram of the electrical quantity waveforms output by the converter before and after the control mode switching when a single-phase ground fault occurs in the power grid, according to an exemplary embodiment of this disclosure. Figure 8 This is a schematic diagram of a flexible DC-DC converter fault current limiting system based on control mode switching provided in an exemplary embodiment of this disclosure; Figure 9 This is a schematic diagram of the modules of an electronic device provided in an exemplary embodiment of this disclosure. Detailed Implementation

[0018] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0019] In the description of this application, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are used only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of the stated features. In the description of this application, "a plurality of" means two or more, unless otherwise explicitly specified.

[0020] "A and / or B" includes the following three combinations: A only, B only, and a combination of A and B.

[0021] The use of "applies to" or "configured to" in this application implies open and inclusive language, which does not exclude the applicability to or configuration to devices performing additional tasks or steps. Additionally, the use of "based on" implies openness and inclusivity, because processes, steps, calculations, or other actions "based on" one or more of the stated conditions or values ​​may in practice be based on additional conditions or values ​​beyond those stated.

[0022] In this application, the term "exemplary" is used to mean "used as an example, illustration, or description." Any embodiment described as "exemplary" in this application is not necessarily to be construed as being more preferred or advantageous than other embodiments. The following description is provided to enable any person skilled in the art to make and use this application. Details are set forth in the following description for purposes of explanation. It should be understood that those skilled in the art will recognize that this application can be made without using these specific details. In other instances, well-known structures and processes are not described in detail to avoid obscuring the description of this application with unnecessary detail. Therefore, this application is not intended to be limited to the embodiments shown, but is consistent with the broadest scope of the principles and features disclosed in this application.

[0023] On the one hand, this embodiment provides a fault current limiting method for flexible DC converters based on control mode switching, applicable to scenarios requiring fault current limiting of flexible DC converters through switching between grid-based control and grid-following control, such as grid-type flexible DC transmission scenarios. This embodiment does not impose specific limitations on this, such as... Figure 1 and Figure 2 As shown, it includes the following steps: Step S100: In response to the detection of a fault in the flexible DC converter, the control mode of the flexible DC converter is switched from the grid construction control mode to the grid connection control mode based on the preset initial value of the grid connection integrator of the grid connection controller.

[0024] Specifically, during the normal operation of the flexible DC converter in grid-connected control mode, an integral pre-synchronization strategy is adopted to set the initial value of the grid-connected integrator of the grid-connected controller, and the system monitors in real time whether a fault occurs in the flexible DC converter. That is, during grid-connected control mode operation, the system calculates in advance the ideal output value of the inner-loop current integrator of the grid-connected controller under the current operating conditions based on the real-time value of the integrator of the grid-connected controller used to control the flexible DC converter. The integral pre-synchronization strategy refers to a pre-synchronization strategy used to dynamically set the initial value of the integrator of the controller to be switched to based on the real-time value of the integrator of the controller in the current control mode of the flexible DC converter. This ensures a smooth transition during mode switching and avoids overcurrent during mode switching. In this embodiment, the current control mode and the control mode to be switched refer to the grid-connected control mode and the grid-connected control mode, respectively.

[0025] In response to the detection of a fault in the flexible DC converter, the control mode of the flexible DC converter is switched from grid-connected control mode to grid-connected control mode according to the preset initial value of the grid-connected integrator of the grid-connected controller. At the instant of control mode switching, the preset initial value of the grid-connected integrator of the grid-connected controller is directly set to the initial value of the integrator of the grid-connected controller, thereby minimizing voltage and current surges caused by mismatch in the initial state of the integrator and achieving a smooth, fast, and reliable switch from grid-connected control to grid-connected control.

[0026] Step S200: In grid-connected control mode, current limiting is applied to the grid-connected controller according to the reactive current priority current limiting strategy to determine the active current and reactive current.

[0027] Specifically, after the flexible DC converter's control mode switches to grid-connected control mode, the system achieves fault current limiting by limiting the active and reactive components of the inner loop current of the grid-connected controller. This process employs a reactive current-priority current limiting strategy. First, based on a preset transient low voltage and reactive current relationship curve, the system queries the maximum reactive current reference value corresponding to the current AC voltage dip depth (i.e., grid voltage) at the flexible DC converter's grid connection point to ensure maximum reactive power support for the flexible DC converter. The transient low voltage and reactive current relationship curve refers to the functional relationship curve that dynamically sets the reactive current reference value based on the degree of AC voltage dip at the flexible DC converter's grid connection point during a fault. This curve reflects the magnitude of reactive current that the converter should output to match different AC voltage dip levels at the flexible DC converter's grid connection point. Then, ensuring that the total current does not exceed the withstand limit of the flexible DC converter's power electronic devices, the remaining current capacity is allocated to the active current. This strategy ensures that reactive power is actively and precisely injected into the flexible DC converter while strictly limiting the fault current. This is beneficial for the timely recovery of AC voltage at the grid connection point of the flexible DC converter, and achieves synergistic optimization of fault current limiting and reactive power support.

[0028] Step S300: Determine the initial value of the grid integrator of the grid controller based on the active current and reactive current.

[0029] Specifically, during the operation of the flexible DC converter in grid-connected control mode, the system performs phase-locked loop (PLL) on the reference voltage output by the grid-connected controller in real time. The amplitude of the reference voltage is calculated by acquiring its phase and angular frequency, and the active and reactive power in the active and reactive power loops of the grid controller are monitored. Then, using an integral pre-synchronization strategy, the acquired amplitude, phase, and angular frequency of the reference voltage, along with the measured values ​​of active and reactive power, are substituted into the control equations of the active and reactive power loops of the grid controller. This process reverses the calculation of the ideal output values ​​of the active and reactive power loop integrators in the grid controller at this time, serving as the initial values ​​for the grid integrators. This prepares the system for switching back to grid-connected control mode after a fault is cleared, ensuring that the initial state of the grid controller matches the controller output value under grid-connected control at the instant the control mode is switched after a fault is cleared, thus enabling a smooth transition in the control mode of the flexible DC converter.

[0030] Step S400: In response to the detection that the fault of the flexible DC converter has been eliminated, the control mode of the flexible DC converter is switched back from the grid-following control mode to the grid-building control mode based on the initial value of the grid integrator.

[0031] Specifically, in response to the detection that the fault in the flexible DC converter has been cleared, the system switches from grid-following control mode to grid-building control mode. At the moment of switching, the system sets the pre-calculated ideal output values ​​of the active and reactive power loop integrators of the grid-building controller as the initial values ​​of the grid-building controller's integrators. Therefore, this embodiment, through an integral pre-synchronization strategy, ensures that the reference voltage output by the grid-building controller seamlessly connects to the previous output of the grid-following controller at the moment of control mode switchback, thereby avoiding transient power oscillations or voltage / current surges introduced by the control mode switch and achieving a smooth reverse switching.

[0032] Furthermore, to improve the stability of the flexible DC converter during control mode switching, this embodiment does not immediately switch the control mode upon detecting the elimination of a fault in the flexible DC converter. Instead, it waits for a preset delay time before switching from grid-following control mode to grid-connecting control mode once the system has stabilized. The delay time is determined based on the time required for the flexible DC converter to stabilize, typically several hundred milliseconds. By setting this delay time, this embodiment ensures that the switch from grid-following control mode to grid-connecting control mode is performed only after the grid fault has been completely cleared and the system operating point has stabilized. This effectively prevents false tripping and failure to trip, thereby improving the reliability of fault identification and system recovery processes.

[0033] In this embodiment, by pre-calculating and setting the ideal initial value of the integrator of the target controller before switching, voltage and current surges caused by mismatch in the initial state of the integrator can be effectively avoided, ensuring the transient stability of the system. At the same time, this method does not require modification of the original controller structure or addition of new control links, greatly simplifying the complexity of engineering implementation. During fault handling, a reactive current priority current limiting strategy is adopted, which can dynamically and accurately provide reactive power support based on the AC voltage drop depth at the grid connection point of the flexible DC converter, under the premise of strictly limiting the total current within the safe tolerance range of the converter. This achieves synergistic optimization of fault current limiting and grid reactive power support, thereby realizing bidirectional smooth switching of control modes, and providing comprehensive and effective protection for the grid-connected flexible DC converter to safely pass through grid faults.

[0034] In a preferred embodiment, step S100, prior to detecting a fault in the flexible DC converter, includes step S500, specifically comprising: Step S510: In response to detecting that the flexible DC converter is operating in grid control mode, obtain the current operating condition of the flexible DC converter.

[0035] Specifically, when the system detects that the flexible DC converter is operating stably in the grid-connected control mode, it acquires and updates key electrical quantities that characterize the current operating conditions of the system in real time. These include, but are not limited to, control command signals generated by the grid controller itself, i.e., the reference voltage output by the grid controller, as well as real-time operating parameter values ​​such as the AC voltage measured at the grid connection point of the flexible DC converter and the current output by the flexible DC converter. This is to accurately capture the dynamic data of the system under the current stable operating state, so as to provide real and reliable data support for the subsequent accurate calculation of the initial value of the ideal integrator of the grid controller.

[0036] Step S520: Based on the current operating conditions, determine the initial value of the grid integrator of the current inner loop integrator of the grid controller.

[0037] Specifically, the system calculates the initial value of the grid-connected integrator of the current inner loop integrator of the grid-connected controller based on the key electrical quantities under the current operating conditions. That is, when the grid-connected control is actually running, the system simulates and calculates what output state the current inner loop integrator of the grid-connected controller should be in if the system switches to grid-connected control at this moment, so that the reference voltage generated by the grid-connected controller can be seamlessly connected with the reference voltage generated by the current grid-connected controller.

[0038] Specifically, the system utilizes the phase-locked loop (PLL) in the grid controller to provide the AC voltage phase at the grid connection point of the flexible DC converter. This allows the system to convert the reference voltage output by the grid controller, the measured AC voltage at the grid connection point, and the converter output current into the grid voltage synchronous coordinate system (dq coordinate system) for the flexible DC converter output current, obtaining the d-axis and q-axis components. Then, based on these components and inherent parameters such as the converter's equivalent inductance, the ideal output values ​​of the grid controller's d-axis and q-axis current inner-loop integrators under the current operating conditions are calculated. These values ​​serve as the initial values ​​for the grid integrators. In the event of a fault requiring switching, these initial values ​​can be instantly applied to the grid controller's integrators, achieving a smooth switching and effectively avoiding severe transient shocks caused by integrator initial value mismatch.

[0039] In this embodiment, when the system is stably operating in grid-connected control mode, the current operating conditions, such as AC voltage measurement at the grid connection point of the flexible DC converter and converter output current, are continuously and in real time acquired. Based on these real data, the ideal output value of the grid-connected controller's inner loop integrator under the current state is accurately calculated. This allows for preparation in advance before a sudden grid fault occurs, so that when a sudden grid fault occurs and the system needs to switch to grid-connected control, the grid-connected controller's integrator can start working from an ideal initial state that perfectly matches the current operating point of the system. This fundamentally avoids integral saturation or output jumps that are inevitably caused by improper integrator initial value settings (such as starting from zero or having a large deviation). This effectively suppresses voltage and current surges that may occur during control mode switching, greatly improves the smoothness and reliability of the switching process, and facilitates rapid and safe fault current limiting.

[0040] In a preferred embodiment, step S520, based on the current operating conditions, determines the initial value of the grid-connected integrator of the current inner loop integrator of the grid-connected controller, including: Step S521: Based on the voltage phase output of the phase-locked loop in the grid controller, determine the reference voltage output by the grid controller, the voltage and current output by the flexible DC converter, respectively.

[0041] Specifically, a unified coordinate reference is established using the phase-locked loop (PLL) of the grid controller. This means the output current of the flexible DC converter is in the grid voltage synchronous coordinate system (dq coordinate system). The PLL determines a stable reference angle by tracking and locking the phase of the AC voltage measured at the grid connection point of the flexible DC converter in real time. Based on this reference angle, the system performs coordinate transformation on key electrical quantities such as the reference voltage generated by the grid controller under the current operating conditions, the actual measured AC voltage at the grid connection point of the flexible DC converter, and the output current of the flexible DC converter. This transforms these key electrical quantities from the original three-phase stationary coordinate system (abc coordinate system) to the synchronous coordinate system (dq coordinate system) guided by the AC voltage measurement vector at the grid connection point of the flexible DC converter. This yields the direct axis (d-axis) and quadrature axis (q-axis) components of these key electrical quantities in the synchronous coordinate system, ensuring that all electrical quantities are represented under the same dynamic reference system.

[0042] Step S522: Based on the reference voltage output by the grid controller, the voltage and current output by the flexible DC converter, determine the initial value of the grid integrator of the current inner loop integrator of the grid controller.

[0043] Specifically, based on the d-axis and q-axis components of each key electrical quantity in the synchronous coordinate system, the ideal output value of the grid controller's current inner loop integrator, i.e., the initial value of the grid integrator, is calculated using the dynamic equation of the grid controller's current inner loop.

[0044] As a preferred example, Figure 3 The diagram shows the grid-connected control flow of a DC converter. This control flow is centered around a phase-locked loop (PLL) and operates by measuring the grid voltage in real time. phase A synchronous coordinate system is established, guided by the AC voltage measurement vector at the grid connection point of the flexible DC converter, to ensure that the converter output is synchronized with the grid. The control loop starts from the outer loop controller, with the DC voltage command value... Compared with actual value The deviation is processed by a proportional-integral (PI) controller to generate an active power reference value. Meanwhile, reactive power reference value The reference currents for the d-axis and q-axis are given externally. Then, the reference values ​​for active power and reactive power are calculated using formula (1). and ,Right now: (1) Wherein, the d-axis reference current Used to control active power (or DC voltage), q-axis reference current. Used to control reactive power. Based on the withstand limit of the flexible DC converter and the "reactive power priority" strategy... and These two current reference values ​​are limited, and after limiting, the following results are obtained: and This is to ensure the safe operation of the flexible DC converter during a fault.

[0045] Subsequently, the current command, after being limited, is sent to the inner current loop controller. Both the d-axis and q-axis inner current loops use independent PI controllers to obtain control parameters as follows: , and , These regulators compare a reference current with the actual measured converter output current. and The deviation, plus the feedforward compensation term of the grid voltage. and and cross-coupling terms and Output the reference voltages for the d-axis and q-axis respectively. and ,Right now: (2) in, The voltage is the grid voltage, and the subscript 's' indicates the location of the flexible DC converter's grid connection point. This represents the converter output voltage. The subscript GFL indicates the grid voltage coordinate system component, the superscript * represents the reference value, and the subscript lim represents the limiting value. As the reference angular frequency, L The equivalent inductance of the flexible DC converter, , and , These are the inner loop control parameters for the d-axis and q-axis currents, respectively, with s in the denominator being the Laplace variable.

[0046] Finally, these two reference voltages are transformed from the rotating dq coordinate system back to the three-phase stationary coordinate system through the dq / abc coordinate transformation module in the figure, obtaining the reference voltages in the three-phase stationary coordinate system, which ultimately drive the converter power devices to generate the required output voltage. This enables precise control of grid power, including output voltage. The expression is as follows: (3) Before switching from grid-based control to grid-following control, grid-based control controls active power to the active power command value (or DC voltage command value) and reactive power to the reactive power command value. Assuming the power command value remains unchanged after the switch, then... , Therefore, according to equation (1), the current command value output by the grid controller should be equal to the actual value, that is... , To combine with , Substituting into the dynamic equation of the grid controller's inner current loop as expressed in equation (2), we obtain the ideal output value of the grid controller's inner current integral element, namely: (4) in, , These are the ideal output values ​​of the d-axis and q-axis current inner loop integrators, respectively.

[0047] During grid-based control, the output of the grid-connected controller's inner-loop current integrator is limited in real time based on the ideal output value calculated using equation (4). In other words, the calculated ideal output value of the grid-connected controller's inner-loop current integrator is used as the initial value of the grid-connected integrator. This limitation is lifted after switching to grid-connected control.

[0048] As can be seen, this example derives the initial state that the integrator section of the current inner loop of the grid-connected controller should possess, i.e., the initial value of the integrator. This allows the grid-connected controller to immediately output a voltage command that matches the grid-connected controller under the current operating condition. This ensures that the output of the grid-connected controller is continuous and smooth at the moment of switching from grid-connected control to grid-connected control, without generating large transient shocks due to deviations in the initial value of the integrator, thus achieving seamless switching between the two control modes. The method in this embodiment can effectively avoid voltage and current surges or controller saturation problems caused by mismatch in the initial value of the integrator, significantly improving the smoothness and reliability of the mode switching process. At the same time, this calculation process is entirely based on existing controller parameters and real-time operating data, without introducing additional control links or complex parameter tuning, combining ease of engineering implementation with high reliability, laying a solid foundation for the rapid and safe response of grid-connected flexible DC systems in fault conditions.

[0049] In a preferred embodiment, the fault determination step of the flexible DC converter in step S100 includes: In response to the detection that the output current of the flexible DC converter exceeds a preset current threshold, the positive sequence voltage component of the flexible DC converter is abnormal, and / or the negative sequence voltage component of the flexible DC converter is abnormal, it is determined that the flexible DC converter has failed.

[0050] Specifically, the system simultaneously monitors whether the converter output current exceeds a preset current threshold, and whether the positive-sequence component of the AC voltage measured at the grid connection point of the flexible DC converter is lower than the lower set value of the positive-sequence voltage, and whether the negative-sequence component is higher than the higher set value of the negative-sequence voltage. Comparing the positive-sequence component with the lower set value reflects the AC voltage drop at the grid connection point of the flexible DC converter, a typical characteristic of short-circuit faults. Comparing the negative-sequence component with the higher set value reflects the rise in AC voltage at the grid connection point of the flexible DC converter, a typical characteristic of asymmetrical faults (such as single-phase grounding). When both conditions of increased current and abnormal voltage are met simultaneously, the system determines that a grid fault has occurred.

[0051] As a preferred example, the system detects whether the positive-sequence voltage component is lower than the lower setpoint for positive-sequence voltage and whether the negative-sequence voltage component is higher than the higher setpoint for negative-sequence voltage. A fault is determined to have occurred when both the current increase criterion and the voltage anomaly criterion are simultaneously met. The specific logic is as follows: (5) in, This is the threshold value for the converter output current, for example, 1.1 times the rated current; , These are the positive and negative sequence components of the grid connection point voltage, respectively. and These are the low setpoint for positive sequence voltage and the high setpoint for negative sequence voltage, for example... and Take 0.9 pu and 0.2 pu respectively.

[0052] In this embodiment, a dual fault identification method combining overcurrent and voltage anomaly criteria is adopted, which effectively avoids malfunctions caused by a single signal anomaly (such as instantaneous sampling error, normal power fluctuation or excitation inrush current, etc.), significantly improves the accuracy and reliability of fault identification, and provides a solid guarantee for the correct and rapid switching of subsequent control modes.

[0053] In a preferred embodiment, step S200, in grid-connected control mode, involves current limiting the grid-connected controller according to a reactive current priority current limiting strategy, determining the active current and reactive current, including: Step S210: In grid-connected control mode, based on the preset transient low voltage and reactive current curves, determine the reactive component of the output current corresponding to the voltage of the flexible DC converter.

[0054] Specifically, after switching to grid-connected control mode, the system monitors the AC voltage level at the grid connection point of the flexible DC converter after a fault in real time. Then, it queries the preset relationship curve between transient low voltage and reactive current to quickly determine the maximum value of the reactive component of the output current required to support the AC voltage at the grid connection point of the flexible DC converter. This refers to the reference value of the reactive component of the output current, which ensures that the flexible DC converter can actively and accurately inject reactive current according to the severity of the fault, thereby providing effective support for fault recovery.

[0055] Step S220: Based on the reactive component and the total current withstand limit of the flexible DC converter, limit the active and reactive components of the inner loop current of the grid controller to determine the active and reactive currents.

[0056] Specifically, after determining the reference value for the reactive current component, and under the premise of limited total current capacity, the system will set the total current withstand limit. The maximum value of the reactive component of the required current The difference is taken as the maximum value of the active component. This ensures that the total output current of the converter does not exceed the safety limits that its power electronic devices can withstand. Finally, the system will determine the maximum value of the reactive current component. and the maximum value of active current component The limit values ​​are set as reference values ​​for the q-axis (reactive) and d-axis (active) currents of the inner loop current loop of the grid controller, respectively. In this way, under strict fault current constraints, the optimal allocation of reactive power support and active power transmission to the power grid is achieved to the maximum extent.

[0057] In this embodiment, the reactive current priority current limiting strategy prioritizes meeting the output demand of the reactive component of the output current corresponding to the voltage of the flexible DC converter, and then allocates the remaining current capacity to the active current. This reliably limits the fault current to a safe range during a fault, protects the power electronic devices such as the flexible DC converter, maximizes the reactive power support capability of the flexible DC converter for the faulty power grid, and improves the fault ride-through performance of the system.

[0058] In a preferred embodiment, step S300, determining the initial value of the grid integrator of the grid controller based on active and reactive current, includes: Step S310: Based on the active current and reactive current, perform phase-locking on the reference voltage output by the grid controller to determine the phase and angular frequency corresponding to the reference voltage output by the grid controller.

[0059] Specifically, in grid-following control mode, the reference voltage output by the grid-following controller is phase-locked, and the waveform of the reference voltage is tracked and analyzed in real time to accurately obtain its current phase angle. and angular frequency This ensures that the system can accurately capture the system synchronization reference established by the grid controller during and after a fault, so that the grid controller to be switched can achieve phase and frequency pre-synchronization with the current operating state of the flexible DC converter.

[0060] Step S320: Determine the initial value of the network integrator of the network controller based on the phase and angular frequency corresponding to the reference voltage output by the network controller.

[0061] Specifically, the phase and angular frequency of the grid controller's output reference voltage are set so that the phase and angular frequency of the grid-connected controller's output reference voltage at the instant of switching are completely consistent with the current grid controller's output reference voltage. Simultaneously, the amplitude of the current grid controller's output reference voltage is calculated. These voltage commands (amplitude, phase, and frequency) along with the actual measured values ​​of DC voltage, active power, and reactive power corresponding to the current stable operating point of the system are substituted into the active and reactive power loop control equations of the grid controller. The ideal output values ​​that the grid controller's active and reactive power loop integrators should possess are then solved, serving as the initial values ​​for the grid integrators. This ensures that at the instant of switching back to grid control, the reference voltage output by the grid controller perfectly matches the current system operating conditions, thereby achieving a smooth transition, effectively avoiding power oscillations and electrical shocks caused by mismatches in the controller's internal state, and significantly improving the system's stability and reliability during the fault recovery phase.

[0062] In a preferred embodiment, determining the initial value of the network integrator of the network controller based on the phase and angular frequency corresponding to the reference voltage output by the network controller in step S320 includes: Step S321: Based on the phase and angular frequency corresponding to the reference voltage output by the grid controller, determine the amplitude of the reference voltage output by the grid controller.

[0063] Specifically, the system uses the d-axis and q-axis reference voltage components output by the inner loop of the grid controller to calculate the amplitude of the grid controller output reference voltage through equation (3), and sets this amplitude as the amplitude of the grid controller output reference voltage, thereby achieving pre-synchronization of the reference voltage amplitude before and after the control mode switch, so that the voltage amplitude output of the grid controller will not jump when it is put into operation.

[0064] Step S322: Based on the amplitude of the reference voltage output by the grid controller and the active and reactive power control equations of the grid controller, determine the initial value of the grid integrator of the grid controller.

[0065] Specifically, since the switching operation is delayed until the fault is cleared and the system stabilizes, the actual operating parameters of the system, such as DC voltage, active power, and reactive power, are already very close to their command values. Therefore, in this embodiment, these stable actual measured values, along with the determined reference voltage commands such as amplitude, phase, and frequency, are substituted into the control equations of the active and reactive power loops of the grid controller. This allows for the derivation of the ideal output values ​​that the active and reactive power loop integrators of the grid controller should possess, which are then used as the initial values ​​for the grid integrators. This ensures that the output of the grid controller is smooth and continuous during the switching process, achieving a smooth transition from grid-following control to grid-building control.

[0066] As a preferred example, based on Figure 4 The flowchart shown is for a grid-type flexible DC control system. This control process mainly consists of two parts: a phase generation stage and a voltage amplitude generation stage. In the phase generation stage, the actual value of the DC voltage... With instruction value The deviation is first fed into a proportional-integral (PI) controller, which outputs a reference value for the active power. This active power reference value differs from the actual active power output of the converter. The deviation values ​​were compared. The program then proceeds to the core algorithm module of the Virtual Synchronous Generator (VSG), which contains virtual rotational inertia. The transfer function, constructed using the damping coefficient D, simulates the inertial damping characteristics of a real synchronous generator rotor, outputting the angular frequency deviation. This angular frequency deviation is different from the system's reference angular frequency. The summation yields the internal potential angular frequency of the converter, which is then integrated to finally generate the phase angle of the internal potential. This allows for the independent establishment of a synchronization benchmark for the power grid.

[0067] This process can be represented as: (6) Wherein, the subscript GFM indicates the DC voltage synchronous coordinate system component. , These are the PI control parameters for the DC voltage power loop. Here, D is the virtual moment of inertia, and D is the damping coefficient. and These are the reference angular frequency and its reference value, respectively. This represents the phase of the output voltage.

[0068] In the voltage amplitude generation stage, the actual value of reactive power Its instruction value The deviation is also processed by a PI controller, which outputs a voltage deviation component. Voltage deviation component With the set no-load electromotive force (The default value is 1.0 pu) are added together to obtain the magnitude of the internal potential. E To simulate the equivalent impedance of a synchronous generator, the amplitude of the internal potential is used. E Subtracting the voltage drop across the virtual impedance yields the d-axis and q-axis reference voltages at the converter output. and These reference voltages are then subjected to inverse coordinate transformation to generate a three-phase modulated wave, which drives the output of the flexible DC converter, thereby actively providing voltage and frequency support to the power grid. The entire control process does not rely on grid voltage phase-locking, realizing true grid construction function.

[0069] During the operation of the grid controller, the reference voltage output by the grid controller is phase-locked to obtain its phase. and angular frequency The amplitude of the reference voltage is calculated according to equation (3). The phase, angular frequency, and amplitude of the forced grid control output reference voltage are equal to those of the grid.

[0070] Since the switch back to grid-connected control mode was delayed until the steady-state operating point, the actual active and reactive power outputs of the converter under previous grid-connected control were already very close to their commanded values. , , Substituting this value along with the phase, angular frequency, and amplitude of the already determined reference voltage into equation (6), the ideal output value of the integrator in the DC voltage loop and reactive power loop of the grid control system can be calculated in reverse: (7) During grid-following control, the ideal output values ​​of the active and reactive power loop integrators of the grid controller are calculated according to equation (7), and the output of the corresponding integrators of the grid controller is limited in real time. This limitation is lifted after switching to grid control.

[0071] In this embodiment, the amplitude of the reference voltage output by the grid controller is calculated and directly set as the target voltage amplitude of the grid-building controller, achieving pre-synchronization of the output voltage amplitude. Furthermore, the ideal initial output value required by the internal integrator is precisely calculated using the active and reactive power loop control equations in reverse. This ensures that the internal state perfectly matches the current operating point of the system at the instant of switching back to grid-building mode, thus achieving a smooth transition of the output voltage and effectively avoiding power oscillations that may be caused by control mode switching. This process is entirely based on existing control logic and real-time data, requiring no additional hardware or complex tuning. It combines ease of engineering implementation with high reliability, effectively ensuring that the system safely and stably recovers to the normal grid-building control mode after fault clearance.

[0072] In summary, the fault current limiting method for flexible DC converters based on control mode switching proposed in this application employs grid-connected control during normal operation of the flexible DC converter, and smoothly switches to grid-following control during grid faults. The fault current limiting is naturally achieved using the current loop of grid-following control. After the fault is cleared, it switches back to grid-connected control after a fixed delay. Without altering the original controller structure and control parameters, an integral pre-synchronization strategy is adopted to reduce transient impacts caused by integral deviation or saturation during switching, achieving bidirectional smooth switching of control modes. Fault identification uses a comprehensive voltage and current criterion to improve reliability. During fault conditions, a reactive current reference value is set based on a preset transient low voltage and a preset reactive current curve, which can quickly and reliably limit the converter output current, providing reactive power support to the grid.

[0073] The method proposed in this application is used for experimental verification. The names and values ​​of the key parameters involved in the actual calculation process are shown in Table 1. Based on these parameters, experiments were conducted to monitor and collect electrical quantity data output by the flexible DC converter at the instant of switching between grid-connected control mode and grid-following control mode, and to monitor and collect electrical quantity data output by the flexible DC converter before and after the control mode switching when a single-phase ground fault occurs in the power grid. The experimental results are as follows: Figures 5-7 As shown.

[0074] Table 1:

[0075] In Table 1, Y represents the winding configuration.

[0076] like Figure 5The diagram shows the electrical quantity waveforms output by the converter at the instant of switching from grid-based control to grid-following control. It can be seen that the active power, reactive power, system DC voltage, and d-axis and q-axis components of the current output by the converter all remain stable. This is because, during the grid-based control phase (… t <2s), based on equation (4), the output value of the grid controller's inner loop current integrator is calculated and preset in real time, thereby ensuring the output of the grid controller at the moment of switching. d, q The shaft current reference value closely matches the actual current, effectively avoiding saturation or abnormal accumulation caused by deviations in the integrator's initial value.

[0077] like Figure 6 The diagram shows the electrical waveform of the converter output at the instant of switching from grid-following control back to grid-connecting control. It can be seen that the converter output also shows no significant disturbance. This is because during the grid-following control phase (2s < ... t <4.3s), by constraining the amplitude, frequency and phase of the reference voltage output by the grid controller, it can track and match the current reference voltage state of the grid controller in real time; at the same time, according to equation (7), the output of the active and reactive loop integrators in the grid controller is calculated and limited in real time to ensure that its active and reactive reference values ​​are consistent with the actual operating values, thereby achieving smooth switching.

[0078] like Figure 7 The diagram shows the electrical waveforms of the converter output before and after the control mode switch when a single-phase ground fault occurs in the power grid. It can be seen that within 3ms after the single-phase ground fault occurs, the proposed fault identification based on a comprehensive voltage and current criterion completes rapid and accurate fault identification. The control mode is immediately switched from grid-based control to grid-following control based on the fault signal. After switching to grid-following control, its inner current controller quickly intervenes and successfully limits the increase of the fault current, limiting the maximum current of the converter arm to 2.866kA, successfully achieving fault ride-through. After the single-phase ground fault occurs, the grid voltage drops to 0.67pu. According to the preset transient low voltage and reactive current curves, the reactive current output by the converter should be 0.52pu, corresponding to a theoretical reactive power value of 0.67*0.52*2000=697MVar. This value highly matches the simulation results, indicating that the proposed method can provide precise and controllable reactive power support to the power grid during the fault period according to preset requirements. After 500ms of fault recovery, the grid-following control smoothly switches back to grid-based control.

[0079] Based on the above analysis, it can be concluded that Figures 5-7 The experimental results fully verify the effectiveness and feasibility of the fault current limiting method for flexible DC converters proposed in this application.

[0080] On the other hand, such as Figure 8 As shown, this embodiment provides a fault current limiting system for a flexible DC converter based on control mode switching, including: The grid-connected control mode switching module 801 is used to switch the control mode of the flexible DC converter from the grid-connected control mode to the grid-connected control mode in response to the detection of a fault in the flexible DC converter, based on the preset initial value of the grid-connected integrator of the grid-connected controller. The fault current limiting module 802 is used to limit the current of the grid controller in grid-connected control mode according to the reactive current priority current limiting strategy, and to determine the active current and reactive current. The initial value setting module 803 for network control is used to determine the initial value of the network integrator of the network controller based on active current and reactive current. The grid control mode switching module 804 is used to switch the control mode of the flexible DC converter from the grid-following control mode back to the grid-building control mode after a preset time for detecting the elimination of the fault in the flexible DC converter, based on the initial value of the grid integrator.

[0081] This embodiment also provides an electronic device, such as... Figure 9 As shown, it includes a memory and a processor. In a specific example, the memory stores a computer program, which, when executed by the processor, implements the method of any of the above embodiments.

[0082] This embodiment also provides a computer-readable storage medium having a computer program stored thereon, the computer program being loaded by a processor to execute the arrangement in any of the methods described above.

[0083] In the embodiments of this application, the storage medium may be a magnetic disk, an optical disk, a read-only memory (ROM), or a random access memory (RAM), etc.

[0084] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions in other embodiments.

[0085] The foregoing has provided a detailed description of a fault current limiting method, system, electronic device, and computer-readable storage medium for a flexible DC converter based on control mode switching, as provided in the embodiments of this application. Specific examples have been used to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A fault current limiting method for flexible DC converters based on control mode switching, characterized in that, Includes the following steps: In response to the detection of a fault in the flexible DC converter, the control mode of the flexible DC converter is switched from the grid construction control mode to the grid connection control mode based on the preset initial value of the grid connection integrator of the grid connection controller. In the grid-connected control mode, based on the preset transient low voltage and reactive current curves, the reactive component of the output current corresponding to the voltage of the flexible DC converter is determined. Based on the reactive component and the total current withstand limit of the flexible DC converter, the active and reactive components of the inner loop current of the grid controller are limited respectively to determine the active and reactive currents. Based on the active current and the reactive current, the reference voltage output by the grid controller is phase-locked to determine the phase and angular frequency corresponding to the reference voltage output by the grid controller. Based on the phase and angular frequency corresponding to the reference voltage output by the network controller, the initial value of the network integrator of the network controller is determined; In response to the detection that the fault of the flexible DC converter has been eliminated, the control mode of the flexible DC converter is switched back from the grid-following control mode to the grid-building control mode based on the initial value of the grid integrator.

2. The fault current limiting method for flexible DC converter based on control mode switching according to claim 1, characterized in that, Prior to detecting a fault in the flexible DC converter, including: In response to detecting that the flexible DC converter is operating in the grid control mode, the current operating condition of the flexible DC converter is obtained; Based on the current operating conditions, determine the initial value of the grid-connected integrator of the current inner loop integrator of the grid-connected controller.

3. The fault current limiting method for flexible DC converter based on control mode switching according to claim 2, characterized in that, The step of determining the initial value of the grid-connected integrator of the current inner loop integrator of the grid-connected controller based on the current operating conditions includes: Based on the voltage phase output of the phase-locked loop in the grid controller, the reference voltage output by the grid controller, the voltage and current output by the flexible DC converter are determined respectively. Based on the reference voltage output by the grid controller, the voltage and current output by the flexible DC converter, the initial value of the grid integrator of the current inner loop integrator of the grid controller is determined.

4. The fault current limiting method for flexible DC converter based on control mode switching according to claim 1, characterized in that, The fault detection steps for the flexible DC converter include: In response to detecting that the output current of the flexible DC converter exceeds a preset current threshold, the positive sequence voltage component of the flexible DC converter is abnormal, and / or the negative sequence voltage component of the flexible DC converter is abnormal, it is determined that the flexible DC converter has malfunctioned.

5. The fault current limiting method for flexible DC converter based on control mode switching according to claim 1, characterized in that, The process of determining the initial value of the network integrator of the network controller based on the phase and angular frequency corresponding to the reference voltage output by the network controller includes: Based on the phase and angular frequency corresponding to the reference voltage output by the grid controller, the amplitude of the reference voltage output by the grid controller is determined; Based on the amplitude of the reference voltage output by the grid controller and the active and reactive power control equations of the grid controller, the initial value of the grid integrator of the grid controller is determined.

6. A fault current limiting system for a flexible DC converter based on control mode switching, characterized in that, include: The grid-connected control mode switching module is used to switch the control mode of the flexible DC converter from the grid-connected control mode to the grid-connected control mode in response to the detection of a fault in the flexible DC converter, based on the preset initial value of the grid-connected integrator of the grid-connected controller. The fault current limiting module is used to determine the reactive component of the output current corresponding to the voltage of the flexible DC converter based on the preset transient low voltage and reactive current curves in the grid-connected control mode. Based on the reactive component and the total current withstand limit of the flexible DC converter, the active and reactive components of the inner loop current of the grid controller are limited respectively to determine the active and reactive currents. The grid control initial value setting module is used to perform phase-locking on the reference voltage output by the grid controller based on the active current and the reactive current, and to determine the phase and angular frequency corresponding to the reference voltage output by the grid controller. Based on the phase and angular frequency corresponding to the reference voltage output by the network controller, the initial value of the network integrator of the network controller is determined; The grid control mode switching module is used to switch the control mode of the flexible DC converter back from the grid-following control mode to the grid control mode after detecting a preset time for the fault elimination of the flexible DC converter, based on the initial value of the grid integrator.

7. An electronic device, characterized in that, It includes a memory and a processor, wherein the memory stores a computer program that, when executed by the processor, implements the method as described in any one of claims 1-5.

8. A computer-readable storage medium, characterized in that, It stores a computer program, which is loaded by a processor to perform the steps of the method according to any one of claims 1-5.

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