Cooperative low-voltage ride-through control method based on network construction type and network following type three-terminal interconnection system and related device

Abstract

The invention provides a cooperative low-voltage ride-through control method and related device based on a network construction type and network following type three-terminal interconnection system, and the method comprises the steps: triggering a cooperative low-voltage ride-through control mechanism in response to a voltage drop fault generated at an AC side connected with a network construction type converter; performing cooperative control on each converter based on the mechanism; wherein the control network construction type converter only undertakes AC side voltage establishment and current safety survival functions, realizes current limiting control through adaptive virtual impedance, and does not participate in DC side energy adjustment of a common DC bus; the DC voltage control type grid-following converter is controlled to suppress DC voltage double-frequency fluctuation of the common DC bus, and actively absorb and compensate double-frequency power disturbance introduced by the grid-forming converter; and controlling the active power control type grid-following converter to avoid active power adjustment during the voltage drop fault period. A collaborative low-voltage ride through control mechanism is provided, the direct-current voltage fluctuation is reduced, and the overall low-voltage ride through capacity of the system is improved.

Description

Technical Field

[0001] This invention belongs to the field of low voltage ride-through control technology for three-terminal interconnected systems in power systems, specifically relating to a collaborative low voltage ride-through control method and related devices based on grid-connected and grid-following three-terminal interconnected systems. Background Technology

[0002] With the large-scale grid connection of new energy sources and the construction of new power systems, the structure of power systems is undergoing significant changes. It is gradually evolving from a centralized structure dominated by synchronous generators to a multi-terminal interconnected structure centered on power electronic converters. This structural change makes the operating characteristics and transient responses of power systems more complex.

[0003] In a three-terminal interconnected system, multiple AC-side converters achieve energy coupling via a common DC bus. Their operating status and transient response exhibit significant system-level correlation characteristics. If a fault such as a voltage dip occurs on the AC side, it is no longer limited to a single port but will affect the operating status of other ports through energy transfer on the DC side. Therefore, low-voltage ride-through capability is crucial for ensuring the safe and stable operation of a three-terminal interconnected system. In existing engineering applications, grid-connected converters are a common component of three-terminal interconnected systems, employing master-slave control to achieve power distribution during normal operation. Simultaneously, to improve system stability under weak grid conditions and fault conditions, grid-connected converters are increasingly being applied in system operation.

[0004] Currently, in the area of ​​low-voltage ride-through control for three-terminal interconnected systems, most existing technologies focus on a single converter as the control object, lacking a system-level collaborative control mechanism for mixed grid-connected and grid-linked operating structures. When a voltage drop occurs on the AC side connected to a grid-connected converter, the division of responsibilities among the ports in participating in DC voltage regulation and power regulation is unreasonable, easily leading to DC-side energy oscillations and control coupling. Furthermore, existing DC voltage second harmonic suppression methods are mostly local improvements and cannot guarantee the coordinated stability of DC voltage and AC current during faults at the system level, thus limiting the improvement of the overall low-voltage ride-through capability of the system. Summary of the Invention

[0005] In view of this, the present invention provides a collaborative low-voltage ride-through control method and related apparatus based on a network-type and a follow-network-type three-terminal interconnection system, aiming to solve the problem that the existing low-voltage ride-through control strategies of three-terminal interconnection systems lack a system-level collaborative control mechanism and have limited overall low-voltage ride-through capability.

[0006] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0007] In a first aspect, the present invention provides a cooperative low-voltage ride-through control method based on a three-terminal interconnection system of grid-type and grid-connected types, which is implemented based on a three-terminal interconnection system composed of an active power control grid-connected converter, a grid-type converter, and a DC voltage control grid-connected converter; the three-terminal interconnection system realizes energy coupling between converters through a common DC bus;

[0008] The method includes:

[0009] In response to a voltage dip fault occurring on the AC side of the grid-connected converter, a cooperative low-voltage ride-through control mechanism is triggered.

[0010] The converters are controlled in a coordinated manner based on the coordinated low voltage ride-through control mechanism.

[0011] Among them, the control grid-type converter only undertakes the functions of AC side voltage establishment and current safety survival, and realizes current limiting control through adaptive virtual impedance, and does not participate in the DC side energy regulation of the common DC bus.

[0012] The DC voltage control type grid converter suppresses the second harmonic fluctuation of the DC voltage on the common DC bus and actively absorbs and compensates for the second harmonic power disturbance introduced by the grid converter.

[0013] Active power control type grid converters yield active power regulation during voltage dip faults.

[0014] Furthermore, the control-grid converter achieves current-limiting control through adaptive virtual impedance, including:

[0015] The fault current is low-pass filtered, and the amplitude of the filtered fault current is compared with a preset current threshold.

[0016] When the fault current amplitude exceeds the preset current threshold, a virtual impedance is applied; the calculation expressions for virtual impedance and virtual reactance are as follows:

[0017]

[0018] In the formula, The equivalent virtual resistance of a grid-type converter. This is the virtual resistance adjustment coefficient. For low-pass filter gain, The fault current output by the grid-type converter. The preset current threshold is used; The equivalent virtual reactance of the grid-type converter, This is the virtual impedance ratio;

[0019] In the dq coordinate system, the voltage drop of the virtual impedance on the d-axis and q-axis is calculated. The voltage drop is used as a voltage correction quantity and superimposed with the virtual internal potential of the grid-type converter to form a voltage inner loop reference quantity, thereby indirectly limiting the fault current.

[0020] Furthermore, the expression for calculating the voltage drop is:

[0021]

[0022] In the formula, and These represent the equivalent voltage drops generated by the virtual impedance along the d-axis and q-axis of the dq coordinate system, respectively. and These are the measured current values ​​of the grid-type converter along the d-axis and q-axis in the dq coordinate system, respectively.

[0023] Furthermore, the DC voltage-controlled grid converter suppresses the second harmonic fluctuation of the DC voltage on the common DC bus through a proportional-integral-resonant controller. The transfer function of the proportional-integral-resonant controller is:

[0024]

[0025] In the formula, Let be the transfer function of the proportional-integral-resonant controller. For proportional control parameters, For integral control parameters, For resonant control gain, For complex frequency domain variables, It is the resonant angular frequency.

[0026] Furthermore, the active power regulation yielding mechanism of the active power control type and grid-type converter is as follows:

[0027] During a fault, the active power closed-loop regulation of the active power control type and grid type converter is suspended, and the active power command before the fault is maintained or the active power command is reduced to a preset safe value.

[0028] Secondly, the present invention provides a cooperative low-voltage ride-through control device based on a grid-type and grid-connected three-terminal interconnection system, which is based on a three-terminal interconnection system composed of an active power control grid-connected converter, a grid-type converter, and a DC voltage control grid-connected converter; the three-terminal interconnection system realizes energy coupling between converters through a common DC bus;

[0029] The device includes:

[0030] The fault monitoring module is used to trigger the cooperative low-voltage ride-through control mechanism in response to voltage dip faults occurring on the AC side of the grid-connected converter.

[0031] The collaborative control module is used to coordinate the control of each converter based on the collaborative low-voltage ride-through control mechanism;

[0032] Among them, the control grid-type converter only undertakes the functions of AC side voltage establishment and current safety survival, and realizes current limiting control through adaptive virtual impedance, and does not participate in the DC side energy regulation of the common DC bus.

[0033] The DC voltage control type grid converter suppresses the second harmonic fluctuation of the DC voltage on the common DC bus and actively absorbs and compensates for the second harmonic power disturbance introduced by the grid converter.

[0034] Active power control type grid converters yield active power regulation during voltage dip faults.

[0035] Furthermore, the control-grid converter achieves current-limiting control through adaptive virtual impedance, including:

[0036] The fault current is low-pass filtered, and the amplitude of the filtered fault current is compared with a preset current threshold.

[0037] When the fault current amplitude exceeds the preset current threshold, a virtual impedance is applied; the calculation expressions for virtual impedance and virtual reactance are as follows:

[0038]

[0039] In the formula, The equivalent virtual resistance of a grid-type converter. This is the virtual resistance adjustment coefficient. For low-pass filter gain, The fault current output by the grid-type converter. The preset current threshold is used; The equivalent virtual reactance of the grid-type converter, This is the virtual impedance ratio;

[0040] In the dq coordinate system, the voltage drop of the virtual impedance on the d-axis and q-axis is calculated. The voltage drop is used as a voltage correction quantity and superimposed with the virtual internal potential of the grid-type converter to form a voltage inner loop reference quantity, thereby indirectly limiting the fault current.

[0041] Furthermore, the expression for calculating the voltage drop is:

[0042]

[0043] In the formula, and These represent the equivalent voltage drops generated by the virtual impedance along the d-axis and q-axis of the dq coordinate system, respectively. and These are the measured current values ​​of the grid-type converter along the d-axis and q-axis in the dq coordinate system, respectively.

[0044] Thirdly, the present invention provides a computer device, the device including a processor and a memory:

[0045] The memory is used to store computer programs and send the instructions of the computer programs to the processor;

[0046] The processor executes a cooperative low-voltage ride-through control method based on a network-type and follow-network-type three-terminal interconnection system, as described in the first aspect, according to the instructions of the computer program.

[0047] Fourthly, the present invention provides a computer-readable storage medium storing a computer program, which, when executed by a processor, implements a cooperative low-voltage ride-through control method based on a network-type and follow-network-type three-terminal interconnection system as described in the first aspect.

[0048] In summary, this invention provides a cooperative low-voltage ride-through control method and related apparatus based on a grid-type and grid-connected three-terminal interconnected system. The method includes the following steps: triggering a cooperative low-voltage ride-through control mechanism in response to a voltage dip fault occurring on the AC side connected to the grid-type converter; performing cooperative control of each converter based on the cooperative low-voltage ride-through control mechanism; wherein, the grid-type converter is controlled to only undertake the AC side voltage establishment and current safety survival functions, and current limiting control is achieved through adaptive virtual impedance, without participating in the DC side energy regulation of the common DC bus; the DC voltage control type grid-connected converter is controlled to suppress the DC voltage second harmonic fluctuation of the common DC bus, and actively absorb and compensate for the second harmonic power disturbance introduced by the grid-type converter; the active power control type grid-connected converter is controlled to yield active power regulation during voltage dip faults. This invention proposes a collaborative low-voltage ride-through control mechanism. The grid-type converter is only responsible for AC side voltage building-up and current limiting, the DC voltage control grid-type converter is responsible for suppressing DC voltage second harmonic fluctuations and compensating for second harmonic power disturbances, and the active power control grid-type converter yields to active power regulation. This system-level solution addresses the problems of DC side energy oscillation and control coupling caused by the unreasonable division of responsibilities among ports during low-voltage ride-through in three-terminal interconnected systems. It ensures the coordinated stability of DC voltage and AC current during faults and improves the overall low-voltage ride-through capability of the system. Attached Figure Description

[0049] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0050] Figure 1 This is a block diagram of a three-terminal interconnected system collaborative fault traversal control provided in an embodiment of the present invention;

[0051] Figure 2 The topology and control block diagram of the network-type three-terminal interconnection system provided by this invention;

[0052] Figure 3 The present invention provides a block diagram for virtual impedance fault ride-through control under the voltage and current inner loop of a grid-type converter.

[0053] Figure 4 The block diagram of the DC voltage second harmonic fault ride-through control for grid-connected converters provided by this invention;

[0054] Figure 5 This is an adaptive virtual impedance generation diagram for the grid-type converter 2 provided in this embodiment of the invention;

[0055] Figure 6 A schematic diagram of the improved DC power loop of the grid-connected converter 3 provided in an embodiment of the present invention;

[0056] Figure 7 The simulated VSC 2 voltage waveform diagram provided for this invention;

[0057] Figure 8 The simulated VSC 2 current waveform diagram provided for this invention;

[0058] Figure 9 The simulated DC voltage waveform diagram provided for this invention;

[0059] Figure 10 The simulated VSC 3 active power waveform diagram provided for this invention;

[0060] Figure 11 This is a block diagram of a collaborative low-voltage ride-through control device based on a network-type and follow-network-type three-terminal interconnection system provided in an embodiment of the present invention.

[0061] Figure 12 This is a block diagram of a computer device provided in an embodiment of the present invention. Detailed Implementation

[0062] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0063] Please see Figure 1-4 The background technology of the present invention will be further introduced below.

[0064] Against the backdrop of large-scale grid connection of new energy sources and the construction of new power systems, power systems are gradually evolving from a centralized structure dominated by synchronous generators to a multi-terminal interconnected structure centered on power electronic converters. Especially in three-terminal interconnected systems, multiple AC-side converters achieve energy coupling through a common DC bus, and their operating status and transient response exhibit significant system-level correlation characteristics. In such systems, faults such as AC-side voltage dips are no longer limited to a single port but may affect the operating status of other ports through DC-side energy transfer. Therefore, low voltage ride-through (LVRT) capability has become a crucial technical indicator for ensuring the safe and stable operation of three-terminal interconnected systems.

[0065] Low-voltage ride-through (LVR) refers to the ability of a converter to maintain grid-connected operation within a specified voltage dip depth and duration during AC grid faults such as short circuits or severe voltage dips, while continuously participating in system operation while meeting its own current and device safety constraints. For three-terminal interconnected systems, LVR requires not only individual converters to survive voltage dips, but also that all ports maintain DC-side energy balance during faults to avoid drastic DC voltage fluctuations caused by power surges. If any port becomes unstable or disconnects from the grid during a voltage dip, its power disturbance will be transmitted to other ports through the common DC bus, thereby amplifying the overall system operational risk.

[0066] In current engineering applications, grid-following (GFL) converters remain a common component in three-terminal interconnected systems, such as... Figure 2 As shown in the diagram. VSC1 and VSC2 are directly connected, while VSC3 is connected to the other two terminals via a DC line. The system employs a master-slave control method, meaning the DC voltage is controlled by the d-axis under vector control in VSC3, while VSC1 and VSC2 control the transmitted active power through their respective d-axis controls. The q-axis of all three VSC terminals can be used to control the transmitted reactive power. For AC side variables, taking VSC1 as an example: the filter impedance Z consists of an inductor L and a resistor R; the ground capacitance filter is C. f Z s Connecting the point of common coupling (PCC) to the power grid, by inductor L s and resistance R sThe composition represents the equivalent inductance of the actual line and transformer. Variables located on the VSC (voltage source converter) machine side, grid connection point, and grid side are denoted by the subscripts "c", "t", and "s", respectively, while "m" represents the variable measured in the dq system through a low-pass filter (LPF). The DC-side capacitor is C, and the DC line consists of resistor R. ldc and inductor L ldc Composition. Under normal operating conditions, the above control method can achieve stable power distribution in a three-terminal interconnected system. However, when a voltage drop occurs on the AC side of a port in the system, especially under unbalanced fault conditions, the dynamic performance of the phase-locked loop (PLL) deteriorates significantly, resulting in limited current control capability and thus causing fluctuations in the port's output power. To address this issue, existing technologies typically employ current limiting, negative sequence current suppression, or power command reduction to improve the fault adaptability of grid-connected converters. However, these methods mainly focus on single-terminal protection and are insufficient to suppress energy oscillations in three-terminal interconnected systems at the system level.

[0067] To improve the stability of three-terminal interconnected systems under weak grid and fault conditions, grid-forming (GFM) converters are gradually being introduced into system operation. GFM converters actively establish voltage and frequency references, simulating the voltage source characteristics of synchronous generators, providing equivalent inertia and damping support for the system. However, when a voltage drop occurs on the AC side connected to the grid-forming converter, its control system is directly exposed to the fault environment. The AC side voltage imbalance leads to a significant second harmonic component in the converter's output power. This second harmonic power propagates through the common DC bus in the three-terminal interconnected system, causing significant second harmonic voltage fluctuations on the DC side, significantly increasing the difficulty of system energy management and stability control.

[0068] For AC side voltage dip conditions in grid-connected converters, existing technologies typically introduce virtual impedance (VI) into the grid-connected control. This is achieved by constructing an equivalent impedance characteristic at the control level to limit current, thereby reducing the risk of overcurrent during faults. For example... Figure 3 As shown, the grid-type converter uses a virtual internal potential as the control reference, where E ref θ represents the magnitude reference of the virtual internal potential. ref The phase angle reference represents the virtual internal potential; the virtual impedance module generates an equivalent voltage drop in a synchronously rotating coordinate system based on the output current feedback, and superimposes it with the virtual internal potential as the reference input for the voltage inner loop. Both the voltage inner loop and the current inner loop use a proportional-integral (PI) controller. The voltage inner loop achieves d-axis and q-axis decoupling by introducing a capacitor voltage feedforward term ωCf, while the current inner loop achieves current decoupling control by introducing an inductor voltage feedforward term ωL, which is related to v. dv q The feedforward compensations are added together to finally generate the modulation voltage command v. d *、v q * and drives the converter via the pulse width modulation module. When the system is not configured with a voltage inner loop or current inner loop, the output of the virtual impedance module can be directly used as the modulation voltage command input to the pulse width modulation link to control the converter equipment. In the above control structure, the virtual impedance current limiting method can be divided into two categories: fixed virtual impedance and adaptive virtual impedance. The impedance parameters of the fixed virtual impedance are preset according to the AC side voltage drop depth, while the adaptive virtual impedance is dynamically adjusted according to the real-time change of the output current of the grid-type converter, and its impedance coefficient is determined by equation (1). Where i d i q I th I k These represent the d-axis and q-axis current values, the set current threshold, and the effective value of the short-circuit current, respectively; n X / R The impedance ratio of the applied virtual impedance; m R Ri represents the virtual impedance coefficient, used to characterize the intensity of virtual impedance input. By adaptively adjusting the virtual impedance parameter, flexible current limiting can be achieved during AC side voltage dips, thereby maintaining the basic voltage support capability while ensuring the safe operation of the grid-connected converter. The corresponding virtual resistance and virtual reactance are Ri and Ri, respectively. VI With X VI .

[0069] (1)

[0070] When a severe voltage drop is detected, the grid-connected converter exits voltage source control mode and temporarily switches to current control operation, thereby ensuring the safe operation of the converter during faults. The above method can improve the fault survivability of grid-connected converters to some extent, but its main effect is concentrated on AC side current constraint, making it difficult to suppress DC side energy imbalance caused by AC side power fluctuations at the system level. In a three-terminal interconnected system, if DC side energy fluctuations are not effectively suppressed, their impact will simultaneously affect other grid-connected ports through the common DC bus, leading to a further increase in the DC voltage oscillation amplitude. On the other hand, regarding the DC voltage second-harmonic oscillation problem caused by AC side imbalance faults or power fluctuations at grid-connected ports in grid-connected converters, existing research has proposed introducing a notch filter and a proportional-resonant (PR) controller into the outer loop of the DC voltage of the grid-connected converter to selectively suppress specific frequency components in the DC bus voltage. The overall control structure is as follows: Figure 4As shown. This method improves the outer loop of the DC voltage based on the conventional DC voltage-current dual closed-loop control, enabling it to simultaneously regulate both the DC component and the second harmonic AC component.

[0071] like Figure 4 As shown on the left, the DC voltage control adopts a "PI+PR" parallel structure. Wherein, the DC bus voltage v dc Compared with reference value v dcref The deviation first enters the outer loop of the DC voltage. To extract the second harmonic component from the DC voltage, a notch filter is introduced into the control structure, and its transfer function G1(s) is expressed as follows:

[0072] (2)

[0073] In the formula, ζ is the notch factor, ω n Let ω be the center angular frequency of the notch filter. For a system with a grid fundamental frequency of 50Hz, the main oscillating component in the DC voltage is concentrated at the second harmonic, therefore ω is taken as... n =200π, and by setting ζω n =1000 to balance filtering depth and dynamic performance. After processing with a notch filter, the DC bus voltage with the second harmonic component removed can be obtained:

[0074] (3)

[0075] Among them, v dc0 This represents the bus voltage signal containing only the DC component. Based on this, the outer loop control quantity for the DC voltage consists of two parts: one part is used to adjust the average value of the DC voltage, and the other part is specifically used to compensate for the second harmonic fluctuations in the DC voltage. Specifically, the DC component v... dcref -v dc The signal enters the proportional-integral (PI) controller, generating a steady-state voltage regulation; while the second harmonic component v... dc -v dc0 The proportional-resonant (PR) controller is used to selectively amplify and suppress specific frequency components. The resulting inner current loop reference value i... dref It can be represented as:

[0076] (4)

[0077] In the formula, the first term is the DC component adjustment part, K p,PI and K i,PI These are the proportional and integral coefficients of the DC voltage outer-loop PI controller, respectively; the second term is the second harmonic component compensation part, K. p,PR and K R,PRHere, ω0 represents the proportional gain and resonant gain of the PR controller, respectively, and ω0 is the resonant angular frequency of the PR controller. Since this controller is designed for the second harmonic component of the DC voltage, ω0 is also taken as 200π.

[0078] Depend on Figure 4 As seen on the right, the generated current reference values ​​enter the d-axis and q-axis current inner loops, respectively. The current inner loops employ a proportional-integral-resonant (PIR) control structure and introduce an ωL inductor voltage feedforward term to decouple the d-axis and q-axis currents, ultimately generating the modulation voltage command v. d *、v q The grid-connected converter is driven by a modulation circuit. Simultaneously, the q-axis continues to be adjusted via reactive power external regulation to meet AC side voltage support or reactive power command requirements. However, this type of scheme is typically used as a local control improvement for a single converter, primarily focusing on the DC voltage stability of individual ports. It does not fully consider the overall structural characteristics of the three-terminal interconnected system and lacks a systematic design for the control responsibilities and interactions between the grid-connected and grid-connected converters during faults.

[0079] In a three-terminal interconnected system, the grid-type converter, the grid-connected converter responsible for DC voltage control, and the grid-connected converter responsible for power control have different functional roles and control objectives. When a voltage dip occurs on the AC side connected to the grid-type converter, if multiple ports simultaneously participate in DC voltage or power regulation during the fault, control coupling and mutual interference can easily occur, leading to amplification of DC voltage second-harmonic oscillations, an unsmooth fault recovery process, and reduced system stability margin. Current technology lacks a low-voltage ride-through method that can rationally divide and coordinate the control responsibilities of each port under voltage dip conditions, taking into account the structural characteristics of a three-terminal interconnected system.

[0080] Based on the above analysis, it is necessary to propose a collaborative low-voltage ride-through control method for grid-type and grid-connected three-terminal interconnected systems. When a voltage drop occurs on the AC side of the grid-type converter, by clarifying the control division of labor and collaborative mechanism of each port, the AC side fault survivability and DC side energy stability can be coordinated and unified, thereby improving the overall operational stability and reliability of the three-terminal interconnected system under fault conditions.

[0081] In the engineering applications and research of existing three-terminal interconnected systems and multi-terminal DC (MTDC) systems, grid-connected (GFM) converters, grid-following (GFL) converters that handle DC voltage control, and grid-following converters that handle power control typically operate in different control modes connected to the common DC bus, with significant differences in functional positioning and control objectives at each port. However, under fault conditions such as voltage dips on the AC side, most existing low-voltage ride-through control strategies still focus on a single converter as the control object, lacking a design concept that unifies and coordinates the control behavior of multiple converters at the system level.

[0082] Specifically, when a voltage drop occurs on the AC side of a grid-connected converter, a significant second harmonic component inevitably appears in its output power. This power fluctuation propagates to the remaining ports of the system through the common DC bus. In existing technologies, grid-connected converters typically achieve current limiting by introducing virtual impedance or switching control modes, while the grid-connected converter, which handles DC voltage or power control, continues to participate in DC voltage or power regulation according to its original control strategy. Since multiple ports simultaneously act on the DC-side energy regulation stage during a fault, control coupling and mutual interference are easily triggered, leading to amplification of the DC voltage second harmonic oscillation amplitude, a significant decrease in DC bus voltage stability, and even causing oscillations in the control system or discontinuous and uneven fault recovery processes. On the other hand, to address the DC voltage second harmonic fluctuation problem, existing technologies often employ methods such as introducing notch filters or proportional-resonant controllers in a single grid-connected converter to locally suppress specific frequency components in the DC voltage. While such methods can improve DC voltage quality to some extent in single-ended scenarios, their control objectives are mainly focused on local ports, failing to fully consider the system-level coupling effect caused by power fluctuations propagating through the common DC bus in three-terminal interconnected systems. When grid-connected converters operate in combination with multiple grid-connected converters, existing technologies struggle to effectively coordinate the control priorities and responsibilities of each port during faults, easily leading to redundant adjustments, control conflicts, or energy distribution imbalances, thus limiting further improvements in the overall low-voltage ride-through capability of the system.

[0083] In summary, existing technologies for low-voltage ride-through control in three-terminal interconnected systems have at least the following shortcomings: First, they lack a system-level collaborative control mechanism for hybrid grid-type and grid-following-type operating structures; second, during AC-side voltage dips, they fail to reasonably divide the responsibilities of each port in DC voltage regulation and power regulation, which can easily lead to DC-side energy oscillations and control coupling; and third, existing DC voltage second harmonic suppression methods are mostly local improvements, making it difficult to guarantee the coordinated stability of DC voltage and AC current during faults at the system level.

[0084] Based on the shortcomings of the existing technology, the purpose of this invention is to propose a cooperative low-voltage ride-through control method and related device for grid-type and follow-grid type three-terminal interconnected systems. The following describes various embodiments of this invention.

[0085] This embodiment provides a collaborative low-voltage ride-through control method based on a three-terminal interconnection system of grid-type and grid-connected types. It is implemented based on a three-terminal interconnection system composed of an active power control grid-connected converter, a grid-type converter, and a DC voltage control grid-connected converter. The three-terminal interconnection system realizes energy coupling between converters through a common DC bus.

[0086] It should be noted that a grid-connected converter is a power electronic converter with voltage source characteristics, capable of autonomously establishing AC side voltage and frequency to provide voltage support for the grid, possessing external characteristics similar to a synchronous generator. A grid-following converter is a power electronic converter with current source characteristics, lacking independent voltage establishment capability and needing to follow the grid's voltage and frequency signals for power regulation. It can be categorized into different types based on its control objective. An active power control grid-following converter is a grid-following converter whose control objective is AC side active power regulation, responsible for the distribution and transmission of active power in the system under normal operating conditions. A DC voltage control grid-following converter is a grid-following converter whose control objective is the stable regulation of the common DC bus voltage, undertaking the core responsibility of DC side energy balance and voltage stability under normal operating conditions. A three-terminal interconnected system is a power electronic interconnected system in which three converters achieve energy coupling through a common DC bus. In this embodiment, a hybrid topology of active power control grid-following converters, grid-connected converters, and DC voltage control grid-following converters is used. The common DC bus is the common channel for energy exchange and transmission between the DC sides of each converter in a three-terminal interconnection system.

[0087] The method includes:

[0088] S1: In response to a voltage dip fault occurring on the AC side of the grid-connected converter, the cooperative low-voltage ride-through control mechanism is triggered.

[0089] It should be noted that the collaborative low-voltage ride-through control mechanism is a system-level control strategy designed for voltage dip faults in three-terminal interconnected systems. It achieves the overall low-voltage ride-through control logic system by unified triggering and coordinated action of each converter according to preset responsibilities.

[0090] S2: Coordinated control of each converter is performed based on the cooperative low-voltage ride-through control mechanism;

[0091] Among them, the control grid-type converter only undertakes the functions of AC side voltage establishment and current safety survival, and realizes current limiting control through adaptive virtual impedance, and does not participate in the DC side energy regulation of the common DC bus.

[0092] The DC voltage control type grid converter suppresses the second harmonic fluctuation of the DC voltage on the common DC bus and actively absorbs and compensates for the second harmonic power disturbance introduced by the grid converter.

[0093] Active power control type grid converters yield active power regulation during voltage dip faults.

[0094] It should be noted that adaptive virtual impedance is a virtual impedance element whose parameters can be dynamically adjusted according to the system operating status (such as voltage dips and current amplitudes). When connected to the converter control loop, it can limit fault current. DC voltage second-harmonic fluctuation is a phenomenon in three-terminal interconnected systems where, during a fault, the frequency of the common DC bus voltage fluctuates at twice the fundamental frequency of the AC side due to factors such as AC side voltage imbalance and power disturbances. Second-harmonic power disturbance is a power fluctuation introduced into the common DC bus by the grid-type converter under AC side voltage dip faults, due to its own control characteristics and changes in AC side operating conditions, with a frequency twice the fundamental frequency of the AC side.

[0095] The overall implementation logic of the cooperative low-voltage ride-through control method in this embodiment is as follows: First, the voltage status of the AC side connected to the grid-type converter is monitored. When a voltage drop fault is detected on the side, the preset cooperative low-voltage ride-through control mechanism is immediately triggered, and the fault cooperative control mode is switched. In the cooperative control mode, targeted control commands are issued and functions are adjusted for the three converters. The grid-type converter abandons the original control objective and retains only the core function of AC side voltage establishment and the protection function of current safety survival. The fault current is limited by the dynamic adjustment of the adaptive virtual impedance link. At the same time, it completely withdraws from the DC side energy regulation of the common DC bus to avoid introducing additional disturbances to the DC side. While retaining the basic DC voltage stability function, the DC voltage-controlled grid-connected converter adds control objectives of second harmonic fluctuation suppression and power disturbance compensation. Through corresponding control algorithms, it actively absorbs and compensates for the second harmonic power disturbance introduced by the grid-connected converter, suppressing the second harmonic fluctuation of DC voltage on the common DC bus from the source. The active power-controlled grid-connected converter temporarily relinquishes its original active power regulation function and stops sending active power regulation commands to the system to avoid control coupling between its power regulation actions and other converters during faults. Ultimately, through the division of labor and functional complementarity of the three converters, coordinated stability control of the AC and DC sides of the three-terminal interconnected system is achieved during faults, completing the system's low-voltage ride-through.

[0096] The method provided in this embodiment, when a voltage drop occurs on the AC side of a grid-connected converter, clarifies the control division of labor and coordination among the grid-connected converter, the DC voltage-controlled grid-connected converter, and the power-controlled grid-connected converter during the fault period. This avoids control coupling caused by multiple ports simultaneously participating in DC side energy regulation, thereby effectively suppressing DC voltage second-harmonic oscillation, improving AC side fault survivability, achieving coordinated unity between DC side energy stability and AC side safe operation, and ultimately significantly improving the overall operational stability and reliability of the three-terminal interconnected system under low-voltage conditions.

[0097] Please refer to it again. Figure 1 , Figure 1 The following is a block diagram of the topology and cooperative fault ride-through control of the three-terminal VSC system composed of flexible interconnection equipment. Figure 1 The methods proposed in the above embodiments will be further described. For example... Figure 1 As shown, this system achieves energy coupling through a common DC bus. Converter 1 (VSC1) and Converter 2 (VSC2) are directly connected on the DC side, while Converter 3 (VSC3) is connected to the other two terminals via DC lines, thus forming a three-terminal DC interconnected system. The three converters are connected to the AC system, and their corresponding AC points of common coupling (PCC) are labeled PCC1, PCC2, and PCC3. The three-terminal interconnected system operates in a master-slave control mode, where VSC3 is responsible for DC voltage regulation, and the other two terminals achieve power distribution through active power control. Under this control architecture, VSC3 regulates the DC voltage through d-axis current in a vector control framework; VSC1 and VSC2 respectively achieve active power transmission through their respective d-axis control; and the q-axis of all three converters is used for reactive power control, thereby meeting the AC side voltage regulation requirements.

[0098] When a voltage drop occurs on the AC side of the grid-connected converter VSC2, a second harmonic component inevitably appears in the output power of VSC2 due to the decrease or imbalance of the AC voltage amplitude. This power fluctuation propagates to the other ports via the common DC bus, easily inducing significant second harmonic voltage oscillations on the DC side. Under traditional control methods, multiple ports simultaneously participate in DC voltage or power regulation, which can easily lead to enhanced control coupling, oscillation amplification, and an unsmooth fault recovery process.

[0099] To address the above issues, the following collaborative control principles are proposed:

[0100] (1) Grid-type converter (GFM) side: During voltage dips, it only undertakes the functions of AC side voltage establishment and current safety survival, and achieves current limiting control through adaptive virtual impedance. It does not participate in DC side energy regulation.

[0101] (2) Grid-connected converter (GFL) side with controlled DC voltage: As the core of DC energy regulation, it actively absorbs and compensates for the second harmonic power disturbance introduced by the grid-connected port, and suppresses the second harmonic fluctuation of DC voltage through proportional-integral-resonant (PIR) control;

[0102] (3) Power control grid-connected converter (GFL) side: During faults, active power regulation is deferred to avoid participating in the DC side fast energy balance process, thereby reducing the degree of system control coupling.

[0103] Through the above functional division, the three-terminal interconnected system achieves stable operation during AC-side faults. Specifically, VSC1 adopts a grid-following control mode with active / reactive power control, and its output active and reactive power are represented as P1 and Q1, respectively; VSC2 adopts a grid-building control mode, with its control core being a virtual synchronous machine structure, which supports the AC system by constructing voltage and frequency references, and its output power is represented as P2 and Q2, respectively; VSC3 adopts a grid-following control mode with DC voltage / reactive power control, where the d-axis is used for DC voltage (VDC) regulation and the q-axis is used for reactive power control, and its output reactive power is represented as Q3, achieving overall system energy balance through the DC side.

[0104] The following explanation uses VSC1 as an example to illustrate the AC side electrical structure and variable markings. The machine-side filter impedance Z1 of VSC1 consists of inductor L1 and resistor R1, used to suppress converter switching harmonics; the capacitance to ground connected in parallel to the grid connection point PCC1 is C. f1 Used to filter out high-frequency voltage components; the connection point to the AC grid is separated from the grid by the line impedance Z. s1 Connection, Z s1 From inductor L s1 and resistance R s1 This structure is used to equivalently represent the electrical characteristics of actual lines and transformers. AC side voltage and current variables are distinguished by different subscripts according to their physical location: variables located on the converter side are denoted by the subscript "c", variables located at the grid connection point are denoted by the subscript "t", and variables located on the grid side are denoted by the subscript "s".

[0105] In the control implementation, the three-phase voltage and current signals on the AC side are transformed from the abc coordinate system to the synchronously rotating dq coordinate system through a coordinate transformation module, with the synchronization angle provided by the phase-locked loop module. The voltage and current signals in the dq coordinate system are processed by a low-pass filter (LPF) to obtain the measured values, with the corresponding variables denoted by the subscript "m", which is used to reduce the impact of high-frequency noise on the control system.

[0106] The VSC1 control structure consists of an outer power control loop and an inner current control loop. The active power control loop is controlled by a proportional-integral controller. Generate d-axis current reference value, and the reactive power control loop is controlled by a proportional-integral controller. Generate q-axis current reference values; the inner current control loop also uses a proportional-integral (PI) controller. and Furthermore, an inductor voltage feedforward term (ωL1 term) is introduced to achieve decoupling control of d-axis and q-axis currents, thereby improving dynamic response performance.

[0107] As a grid-type converter, the VSC2 also includes a machine-side filter inductor L2, a damping resistor R2, and a grid-connection capacitor C on its AC side. f2 and the grid-side impedance L s2 and R s2 Its control employs a virtual synchronous machine structure, introducing virtual inertia J and damping D by simulating the rotation equation of a synchronous generator to achieve dynamic adjustment of frequency and active power; voltage amplitude control is achieved through a reactive power-voltage regulation loop. A virtual impedance VI module is introduced into its voltage and current dual closed-loop control structure (the voltage control loop uses a proportional-integral controller). and And introduce a capacitor voltage feedforward term (ωC) f (Item); The current control loop uses a proportional-integral controller. and And an inductor voltage feedforward term (ωL1 term) is introduced to construct an equivalent virtual resistance R. v and virtual reactance X v This feature enables current limiting during AC side voltage dips or unbalanced faults, thereby ensuring the safe operation of the grid-type converter. The grid-type converter uses a virtual internal potential as its control core, with a virtual internal potential amplitude of E. ref The phase angle of the virtual internal potential is θ ref The virtual internal potential, after passing through the virtual impedance module, generates a voltage command, which is then input as a reference quantity for the voltage inner loop to subsequent voltage and current control stages, such as... Figure 5 As shown.

[0108] In one embodiment of the present invention, the controlled grid converter achieves current limiting control through adaptive virtual impedance, including:

[0109] Step 1: Perform low-pass filtering on the fault current and compare the amplitude of the filtered fault current with the preset current threshold.

[0110] First, the fault current I... f Perform low-pass filtering G LPF To eliminate the effects of high-frequency noise.

[0111] Step 2: When the fault current amplitude exceeds the preset current threshold, activate the virtual impedance; the calculation expressions for virtual impedance and virtual reactance are:

[0112] (5)

[0113] In the formula, The equivalent virtual resistance of a grid-type converter. This is the virtual resistance adjustment coefficient. For low-pass filter gain, The fault current output by the grid-type converter. The preset current threshold is used; The equivalent virtual reactance of the grid-type converter, This represents the virtual impedance ratio; the preset virtual resistance adjustment coefficient for both is k. R .

[0114] The filtered current amplitude and the preset current threshold I th The comparison shows that when the current amplitude exceeds the threshold, the virtual impedance input mechanism is triggered, corresponding to the equivalent virtual resistance R. v and virtual reactance X v The expression is shown in equation (5).

[0115] Step 3: In the dq coordinate system, calculate the voltage drop of the virtual impedance on the d-axis and q-axis. Use the voltage drop as a voltage correction quantity and superimpose it with the virtual internal potential of the grid-type converter to form a voltage inner loop reference quantity, thereby indirectly limiting the fault current.

[0116] In a further embodiment of the present invention, a method for calculating voltage drop is proposed. In the dq coordinate system, the virtual impedance voltage drop acts on both the d-axis and the q-axis, and its expression is:

[0117] (6)

[0118] Among them, i tdm2 and i tqm2 These are the current measurements of the grid-type converter along the d-axis and q-axis in the dq coordinate system after passing through a low-pass filter. Finally, the voltage correction output by the virtual impedance module is related to the virtual internal potential E. ref The superposition forms the voltage inner loop reference quantity E. dref and E qref This allows for the indirect limitation of the inner current loop reference value by reducing the voltage reference value during AC-side voltage dips, thereby suppressing short-circuit current. This adaptive virtual impedance method can dynamically adjust the equivalent impedance according to the current magnitude, ensuring the safe survival of the grid-type converter during faults, but it does not regulate DC-side energy fluctuations.

[0119] VSC3 is responsible for DC voltage control. Its DC side includes a DC capacitor C3 for storing DC energy and smoothing the DC voltage; the DC circuit consists of a resistor R. ldc and inductor L ldc This structure is used to equivalently represent the electrical characteristics of a DC transmission line. The outer DC voltage control loop of VSC3 takes the difference between the DC voltage reference value and the actual measured value as input, and processes it through a proportional-integral-resonant controller. A d-axis current reference is generated, in which the resonant circuit selectively adjusts the second harmonic component of the DC voltage to suppress DC voltage second harmonic fluctuations; its q-axis control is still used for reactive power regulation, employing a proportional-integral controller. Implementation; the inner current control loop also uses a proportional-integral controller. and Furthermore, an inductor voltage feedforward term (ωL3 term) is introduced to achieve decoupling control of d-axis and q-axis currents, thereby improving dynamic response performance.

[0120] Figure 6 This diagram illustrates the improved DC voltage-power control structure of the grid-connected converter VSC3, which is responsible for DC voltage control. The outer control layer of VSC3 includes a DC voltage-power control loop and a reactive power control loop. The DC voltage control loop uses the DC voltage reference value v... dcref Compared with the actual measured value v dc The difference is used as the input signal. In order to simultaneously suppress the DC component deviation and the second harmonic AC component fluctuation of the DC voltage, a proportional-integral-resonant (PIR) controller is directly introduced into the outer loop of the DC voltage, compared with the use of notch filter and PR controller.

[0121] In one embodiment of the present invention, its transfer function is expressed as:

[0122] (7)

[0123] Among them, K p,PIR and K i,PIR These are the proportional-integral control parameters, K r,PIR The resonant control gain is given by ω0, which is the resonant angular frequency, taken as the angular frequency corresponding to the second harmonic of the DC voltage. The output of the PIR controller is used as the d-axis current reference value i. tdref3 Used to regulate DC-side energy balance. Simultaneously, the reactive power control loop generates a q-axis current reference value i via a proportional-integral (PI) controller. tqref3 This is used to meet the reactive power support requirements on the AC side. The inner current control loop adjusts the d-axis and q-axis currents separately, using a PI controller and introducing an inductor voltage feedforward term ωL3 to achieve decoupled control of the d-axis and q-axis currents, ultimately generating a voltage modulation signal v. tdm3 and v tqm3The signal is fed into the modulation loop to control the switching state of the converter. By introducing a resonant control loop into the outer loop of the DC voltage, VSC3 can actively absorb and compensate for the second harmonic power fluctuation on the DC side when a voltage drop occurs on the AC side of the grid-type converter VSC2, thereby effectively suppressing the second harmonic oscillation of the DC voltage.

[0124] In the aforementioned three-terminal interconnected system, when a voltage drop occurs on the AC side connected to the grid-connected converter VSC2, the AC side voltage imbalance will cause a significant second harmonic component in its output power. This power fluctuation propagates through the common DC bus in the three-terminal system, easily triggering DC voltage second harmonic oscillations. To address this problem, the cooperative low-voltage ride-through control method proposed in this invention prioritizes current limiting and safety survival on the grid-connected converter side through virtual impedance, avoiding participation in DC side energy regulation. DC side energy balance and second harmonic power fluctuation suppression are accomplished by the grid-connected converter VSC3, which undertakes DC voltage control. Its proportional-integral-resonant (PIR) controller in its DC voltage control loop actively suppresses the second harmonic component in the DC voltage, weakening the coupling propagation of power fluctuations between the three terminals at the system level.

[0125] Through the above control method, a clear division of responsibilities and coordinated cooperation among the control responsibilities of each converter in the three-terminal interconnection system are achieved when a voltage drop occurs on the AC side of the grid-type converter. While ensuring the safe operation of the AC side of the grid-type converter, the DC voltage second harmonic oscillation is effectively suppressed, and the overall low voltage ride-through capability and operational stability of the system are significantly improved.

[0126] To verify the cooperative low-voltage ride-through control method based on a three-terminal interconnected system of grid-type and follow-grid-type proposed in this invention, a simulation model of the three-terminal interconnected system is first built, and the corresponding system parameters are shown below.

[0127] a. System baseline value:

[0128] For ease of analysis, per-unit values ​​are used for calculation. First, the system's baseline values ​​are defined for per-unit scaling, as shown in Table 1, including the converter's rated power, rated frequency, grid voltage, and DC voltage.

[0129] Table 1 System Baseline Values

[0130]

[0131] b. Communication side

[0132] The AC parameters are shown in Table 2. The parameters for sides 1 and 3 are identical, while the filter parameters for side 2 differ due to the weak network characteristics of the line. The line impedance amplitude (per unit) is 1 / SCR, and the line inductance and resistance are based on SCR and θ. z Calculate: Ls =(1 / SCR)*sin(θ z ), R s =(1 / SCR)*cos(θ z ).

[0133] Table 2 AC side parameters

[0134]

[0135] c. DC side

[0136] The DC section includes the VSC DC-side capacitor and the DC line impedance, as shown in Table 3.

[0137] Table 3 DC side parameters

[0138]

[0139] d. Control loop

[0140] For the control loop, active power and reactive power control are used on VSC1 and VSC3 respectively. The typical grid-connected converter (GFL) control of DC voltage reactive power is used. The initial settings of the inner loop and reactive power loop parameters are the same. The grid-connected converter (GFM) control of VSG is used on VSC2. The corresponding control parameters are different. The specific parameters are shown in Table 4 below.

[0141] Table 4 Control Parameters

[0142]

[0143] When a single-phase voltage drop occurs on the AC side of VSC2 and falls to 20% of the rated voltage, a comparative simulation analysis was conducted between the three-terminal interconnection system cooperative fault ride-through control method proposed in this invention and the traditional three-terminal interconnection system fault ride-through control method without the introduction of adaptive virtual impedance and DC voltage control loop improvement. The VSC2 AC side voltage waveform is shown below. Figure 7 As shown, the corresponding output current waveform of VSC2 is as follows: Figure 8 As shown, where Figure 7 and Figure 8 Figure (a) corresponds to the traditional fault ride-through control method for three-terminal interconnected systems without adding adaptive virtual impedance and direct voltage loop improvement, while Figure (b) corresponds to the proposed cooperative fault ride-through control method for three-terminal interconnected systems. Simulation results show that under severe voltage drop conditions, when using the control strategy of this invention, the peak output current of VSC2 can be effectively limited to below 1 p.u., significantly improving the current limiting performance during faults and avoiding current over-limit phenomena, thereby enhancing the safe operation capability of the grid-type converter under AC side fault conditions.

[0144] Meanwhile, the corresponding DC bus voltage waveform and the active power output waveform on the VSC3 side are as follows: Figure 9 and Figure 10 As shown, where Figure 9 and Figure 10 Figure (a) corresponds to the traditional fault ride-through control method for a three-terminal interconnected system without adding adaptive virtual impedance and direct voltage loop improvement, while Figure (b) corresponds to the proposed cooperative fault ride-through control method for a three-terminal interconnected system. Comparative results show that, after adopting the cooperative fault ride-through control method proposed in this invention, the DC-side voltage fluctuation amplitude is effectively reduced from approximately 7% under the traditional control strategy to less than 3%, significantly improving DC voltage stability. Furthermore, the second harmonic oscillation amplitude in the active power output of VSC3 is also reduced from approximately 6% to less than 3%, effectively suppressing power oscillation and ensuring the overall operational stability of the three-terminal interconnected system during faults and the fault recovery process.

[0145] Based on the above embodiments, the advantages of the present invention are as follows:

[0146] (1) When a voltage drop occurs on the AC side of the grid-connected converter, the control responsibilities of the three-terminal converter are clearly defined, so that the grid-connected converter only undertakes the AC side voltage establishment and current safety survival functions, the grid-connected converter that controls DC voltage independently undertakes the DC side energy balance and second harmonic power fluctuation suppression tasks, and the grid-connected converter that controls power yields active power regulation during the fault period. From the system level, the control coupling and oscillation amplification problems caused by multiple ports participating in DC regulation at the same time are avoided, thereby significantly improving the overall stability and low voltage ride-through capability of the three-terminal interconnected system under AC side fault conditions.

[0147] (2) Under the aforementioned collaborative control framework, a collaborative control strategy is proposed to address the second harmonic power disturbance caused by the AC side voltage drop of the grid-type converter. This strategy is "adaptive virtual impedance current limiting of the grid-type converter + PIR control of the grid-type converter to suppress the second harmonic DC voltage fluctuation". The grid-type converter achieves safe current limiting and does not participate in DC energy regulation by dynamically adjusting the equivalent impedance based on the adaptive virtual impedance triggered by the current threshold. The grid-type converter for DC voltage introduces a proportional-integral-resonant controller in the outer loop of the DC voltage to synchronously adjust the DC component and the second harmonic component in the DC voltage. This effectively suppresses the second harmonic oscillation in the DC voltage and power without increasing the system complexity, thereby improving the smoothness and reliability of the system operation during faults and recovery processes.

[0148] Based on the same inventive concept, this application also provides a cooperative low-voltage ride-through control device for implementing the cooperative low-voltage ride-through control method based on grid-type and follow-grid three-terminal interconnection systems as described above. The solution provided by this device is similar to the solution described in the above method. Therefore, the specific limitations in the embodiments of the cooperative low-voltage ride-through control device based on grid-type and follow-grid three-terminal interconnection systems provided below can be found in the limitations of the cooperative low-voltage ride-through control method based on grid-type and follow-grid three-terminal interconnection systems described above, and will not be repeated here.

[0149] Please see Figure 11 The present invention also provides a cooperative low-voltage ride-through control device based on a grid-type and grid-connected three-terminal interconnection system, which is based on a three-terminal interconnection system composed of an active power control grid-connected converter, a grid-type converter, and a DC voltage control grid-connected converter; the three-terminal interconnection system realizes energy coupling between converters through a common DC bus;

[0150] The device includes:

[0151] The fault monitoring module is used to trigger the cooperative low-voltage ride-through control mechanism in response to voltage dip faults occurring on the AC side of the grid-connected converter.

[0152] The collaborative control module is used to coordinate the control of each converter based on the collaborative low-voltage ride-through control mechanism;

[0153] Among them, the control grid-type converter only undertakes the functions of AC side voltage establishment and current safety survival, and realizes current limiting control through adaptive virtual impedance, and does not participate in the DC side energy regulation of the common DC bus.

[0154] The DC voltage control type grid converter suppresses the second harmonic fluctuation of the DC voltage on the common DC bus and actively absorbs and compensates for the second harmonic power disturbance introduced by the grid converter.

[0155] Active power control type grid converters yield active power regulation during voltage dip faults.

[0156] Furthermore, the control-grid converter achieves current-limiting control through adaptive virtual impedance, including:

[0157] The fault current is low-pass filtered, and the amplitude of the filtered fault current is compared with a preset current threshold.

[0158] When the fault current amplitude exceeds the preset current threshold, a virtual impedance is applied; the calculation expressions for virtual impedance and virtual reactance are as follows:

[0159] (8)

[0160] In the formula, The equivalent virtual resistance of a grid-type converter. This is the virtual resistance adjustment coefficient. For low-pass filter gain, The fault current output by the grid-type converter. The preset current threshold is used; The equivalent virtual reactance of the grid-type converter, This is the virtual impedance ratio;

[0161] In the dq coordinate system, the voltage drop of the virtual impedance on the d-axis and q-axis is calculated. The voltage drop is used as a voltage correction quantity and superimposed with the virtual internal potential of the grid-type converter to form a voltage inner loop reference quantity, thereby indirectly limiting the fault current.

[0162] Furthermore, the expression for calculating the voltage drop is:

[0163] (9)

[0164] In the formula, and These represent the equivalent voltage drops generated by the virtual impedance along the d-axis and q-axis of the dq coordinate system, respectively. and These are the measured current values ​​of the grid-type converter along the d-axis and q-axis in the dq coordinate system, respectively.

[0165] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the above-described division of functional units and modules is merely an example. In practical applications, the above functions can be assigned to different functional units and modules as needed, that is, the internal structure of the system can be divided into different functional units or modules to complete all or part of the functions described above. The functional units and modules in the embodiments can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit. Furthermore, the specific names of the functional units and modules are only for easy differentiation and are not intended to limit the scope of protection of this application. The specific working process of the units and modules in the above system can be referred to the corresponding process in the foregoing method embodiments, and will not be repeated here.

[0166] Reference Figure 12 The present invention also provides a computer device, including: a memory and a processor, and a computer program stored in the memory. When the computer program is executed on the processor, it implements the cooperative low voltage ride-through control method based on a network-type and follow-network-type three-terminal interconnection system as described in any of the above methods.

[0167] The computer device may be a desktop computer, laptop, handheld computer, or cloud server, etc. This computer device may include, but is not limited to, a processor and memory. Those skilled in the art will understand that... Figure 12 The examples of computer devices are merely examples and do not constitute a limitation on computer devices. They may include more or fewer components than shown in the illustration, or combinations of certain components, or different components. For example, they may also include input / output devices, network access devices, etc.

[0168] The processor referred to can be a Central Processing Unit (CPU), but it can also be other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. A general-purpose processor can be a microprocessor or any conventional processor.

[0169] In some embodiments, the memory may be an internal storage unit of the computer device, such as a hard drive or RAM. In other embodiments, the memory may be an external storage device of the computer device, such as a plug-in hard drive, Smart Media Card (SMC), Secure Digital (SD) card, or Flash Card. Furthermore, the memory may include both internal and external storage units of the computer device. The memory is used to store the operating system, applications, boot loader, data, and other programs, such as the program code of the computer program. The memory can also be used to temporarily store data that has been output or will be output.

[0170] This invention also provides a computer-readable storage medium storing a computer program thereon. When the computer program is run by a processor, it implements the cooperative low-voltage ride-through control method based on a network-type and follow-network-type three-terminal interconnection system as described in any of the above methods.

[0171] In this embodiment, if the integrated unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, all or part of the processes in the methods of the above embodiments can be implemented by a computer program instructing related hardware. The computer program can be stored in a computer-readable storage medium, and when executed by a processor, it can implement the steps of the various method embodiments described above. The computer program includes computer program code, which can be in the form of source code, object code, executable files, or certain intermediate forms. The computer-readable medium can include at least: any entity or device capable of carrying computer program code to a photographing device / terminal device, a recording medium, a computer memory, a read-only memory (ROM), a random access memory (RAM), an electrical carrier signal, a telecommunication signal, and a software distribution medium. Examples include USB flash drives, portable hard drives, magnetic disks, or optical disks. In some jurisdictions, according to legislation and patent practice, computer-readable media cannot be electrical carrier signals or telecommunication signals.

[0172] This invention provides a computer program product, including a computer program that, when executed by a processor, implements the cooperative low-voltage ride-through control method based on a network-type and follow-network-type three-terminal interconnected system as described in any of the above methods.

[0173] In the above embodiments, the descriptions of each embodiment have different focuses. For parts that are not described in detail or recorded in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0174] Those skilled in the art will recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0175] In the embodiments disclosed in this application, it should be understood that the disclosed devices / terminal equipment and methods can be implemented in other ways. For example, the device / terminal equipment embodiments described above are merely illustrative. For instance, the division of modules or units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the displayed or discussed mutual coupling or direct coupling or communication connection may be through some interfaces; the indirect coupling or communication connection between devices or units may be electrical, mechanical, or other forms.

[0176] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A cooperative low-voltage ride-through control method based on a network-type and a follow-network-type three-terminal interconnected system, characterized in that, It is based on a three-terminal interconnected system consisting of an active power control grid-connected converter, a grid-connected converter, and a DC voltage control grid-connected converter; The three-terminal interconnection system achieves energy coupling between converters through a common DC bus. The method includes: In response to a voltage dip fault occurring on the AC side of the grid-connected converter, a cooperative low-voltage ride-through control mechanism is triggered. The converters are controlled in a coordinated manner based on the aforementioned coordinated low-voltage ride-through control mechanism. The grid-type converter is only responsible for AC side voltage establishment and current safety survival, and current limiting control is achieved through adaptive virtual impedance. It does not participate in the DC side energy regulation of the common DC bus. The DC voltage control type grid converter is controlled to suppress the second harmonic fluctuation of the DC voltage of the common DC bus and actively absorb and compensate for the second harmonic power disturbance introduced by the grid converter; The active power control type grid converter shall give way to active power regulation during voltage dip faults.

2. The cooperative low-voltage ride-through control method based on a network-type and follow-network-type three-terminal interconnected system according to claim 1, characterized in that, The grid-type converter is controlled to achieve current limiting control through adaptive virtual impedance, including: The fault current is low-pass filtered, and the amplitude of the filtered fault current is compared with a preset current threshold. When the fault current amplitude exceeds the preset current threshold, a virtual impedance is applied; the calculation expressions for the virtual impedance and virtual reactance are as follows: In the formula, The equivalent virtual resistance of a grid-type converter. This is the virtual resistance adjustment coefficient. For low-pass filter gain, The fault current output by the grid-type converter. The preset current threshold is used; The equivalent virtual reactance of the grid-type converter, This is the virtual impedance ratio; In the dq coordinate system, the voltage drop of the virtual impedance on the d-axis and q-axis is calculated. The voltage drop is used as a voltage correction quantity and superimposed with the virtual internal potential of the grid-type converter to form a voltage inner loop reference quantity, thereby indirectly limiting the fault current.

3. The cooperative low-voltage ride-through control method based on a network-type and follow-network-type three-terminal interconnected system according to claim 2, characterized in that, The formula for calculating the voltage drop is: In the formula, and These represent the equivalent voltage drops generated by the virtual impedance along the d-axis and q-axis of the dq coordinate system, respectively. and These are the measured current values ​​of the grid-type converter along the d-axis and q-axis in the dq coordinate system, respectively.

4. The cooperative low-voltage ride-through control method based on a network-type and follow-network-type three-terminal interconnected system according to claim 1, characterized in that, The DC voltage-controlled grid-connected converter suppresses the second harmonic fluctuation of the DC voltage on the common DC bus through a proportional-integral-resonant controller. The transfer function of the proportional-integral-resonant controller is: In the formula, Let be the transfer function of the proportional-integral-resonant controller. For proportional control parameters, For integral control parameters, For resonant control gain, For complex frequency domain variables, It is the resonant angular frequency.

5. The cooperative low-voltage ride-through control method based on a network-type and follow-network-type three-terminal interconnected system according to claim 1, characterized in that, The active power regulation yielding mechanism of the active power control type grid converter is as follows: During a fault, the active power closed-loop regulation of the active power control type grid converter is suspended, and the active power command before the fault is maintained or the active power command is reduced to a preset safe value.

6. A cooperative low-voltage ride-through control device based on a network-type and grid-connected three-terminal interconnection system, characterized in that, It is based on a three-terminal interconnected system consisting of an active power control grid-connected converter, a grid-connected converter, and a DC voltage control grid-connected converter; The three-terminal interconnection system achieves energy coupling between converters through a common DC bus. The device includes: The fault monitoring module is used to trigger the cooperative low voltage ride-through control mechanism in response to a voltage drop fault occurring on the AC side of the grid-connected converter. The collaborative control module is used to collaboratively control each converter based on the collaborative low-voltage ride-through control mechanism. The grid-type converter is only responsible for AC side voltage establishment and current safety survival, and current limiting control is achieved through adaptive virtual impedance. It does not participate in the DC side energy regulation of the common DC bus. The DC voltage control type grid converter is controlled to suppress the second harmonic fluctuation of the DC voltage of the common DC bus and actively absorb and compensate for the second harmonic power disturbance introduced by the grid converter; The active power control type grid converter shall give way to active power regulation during voltage dip faults.

7. The cooperative low-voltage ride-through control device based on a network-type and follow-network-type three-terminal interconnection system according to claim 6, characterized in that, The grid-type converter is controlled to achieve current limiting control through adaptive virtual impedance, including: The fault current is low-pass filtered, and the amplitude of the filtered fault current is compared with a preset current threshold. When the fault current amplitude exceeds the preset current threshold, a virtual impedance is applied; the calculation expressions for the virtual impedance and virtual reactance are as follows: In the formula, The equivalent virtual resistance of a grid-type converter. This is the virtual resistance adjustment coefficient. For low-pass filter gain, The fault current output by the grid-type converter. The preset current threshold is used; The equivalent virtual reactance of the grid-type converter, This is the virtual impedance ratio; In the dq coordinate system, the voltage drop of the virtual impedance on the d-axis and q-axis is calculated. The voltage drop is used as a voltage correction quantity and superimposed with the virtual internal potential of the grid-type converter to form a voltage inner loop reference quantity, thereby indirectly limiting the fault current.

8. The cooperative low-voltage ride-through control device based on a network-type and follow-network-type three-terminal interconnection system according to claim 7, characterized in that, The formula for calculating the voltage drop is: In the formula, and These represent the equivalent voltage drops generated by the virtual impedance along the d-axis and q-axis of the dq coordinate system, respectively. and These are the measured current values ​​of the grid-type converter along the d-axis and q-axis in the dq coordinate system, respectively.

9. A computer device, characterized in that, The device includes a processor and a memory: The memory is used to store computer programs and send the instructions of the computer programs to the processor; The processor executes, according to the instructions of the computer program, a cooperative low-voltage ride-through control method based on a three-terminal interconnected system of network and follow-network types as described in any one of claims 1-5.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program, which, when executed by a processor, implements a cooperative low-voltage ride-through control method based on a network-type and follow-network-type three-terminal interconnection system as described in any one of claims 1-5.

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