A grid-forming converter fault ride-through hierarchical control method

Abstract

The application discloses a kind of network configuration type converter fault ride-through hierarchical control methods, this method includes: current inner ring dynamic limiting control layer, after converter enters fault ride-through state, the virtual resistance that is introduced by fault current negative feedback composition is introduced to the voltage and current inner ring control of network configuration type converter based on VSG control, the size of virtual resistance is adjusted in real time with fault current amplitude, to carry out virtual resistance dynamic current limiting control;Power outer ring dynamic compensation control layer, after converter enters fault ride-through state, according to the active power instruction value and reactive power instruction value of network configuration type converter based on VSG control during fault ride-through are dynamically adjusted according to grid voltage drop degree, and active power instruction value and reactive power instruction value are based on active-frequency control and reactive-voltage control respectively.In addition, the method also includes state coupling and parameter adaptive module.The application can make GFM converter have better dynamic performance in fault recovery stage.

Description

Technical Field

[0001] This invention belongs to the field of grid-type converter control technology, specifically relating to a fault ride-through hierarchical control method for grid-type converters. Background Technology

[0002] Grid-forming (GFM) converters, with their voltage source control characteristics and self-supporting capabilities, exhibit stronger adaptability under weak grid conditions. However, when encountering large disturbances, the reactive power regulation capability of GFM converters differs from that of traditional synchronous generators due to the limitations of the overcurrent capacity of their power semiconductor devices. The latter can provide a larger short-circuit current during a fault, facilitating the correct operation of relay protection. In contrast, the short-circuit current of GFM converters is limited, potentially affecting the reliability of relay protection and further impacting grid stability. During a fault, GFM converters need to enter current-limiting mode to improve fault ride-through capability. However, if the control parameter setting accuracy is insufficient or the multi-stage coordination mechanism in the control strategy has defects, frequency and voltage oscillations and malfunctions of protection devices may occur during recovery. In severe cases, this can lead to a chain reaction in the power system, affecting a wider range of power supply.

[0003] Existing fault ride-through control methods for GFM converters focus solely on rapid current limiting control after a fault occurs to ensure that the fault current does not exceed the tolerance limits of power electronic devices. However, these control methods have the following limitations: First, they lack the ability to determine the existence of a stable operating point after a fault occurs, which may lead to the system entering an unstable state after the fault is cleared. Second, existing methods are insufficient in the dynamic control process after fault clearing, failing to effectively optimize dynamic response performance such as oscillation suppression and smooth recovery. These problems mean that while current fault ride-through control strategies for GFM converters can meet the basic objective of short-circuit current limiting, they fail to simultaneously address multiple control requirements such as maintaining a stable operating point and smooth recovery after fault clearing, thus hindering the establishment of a system-level coordination mechanism for GFM converters.

[0004] Therefore, there is an urgent need for a fault ride-through control method for GFM converters to improve the transient stability and fault ride-through capability of GFM converters and ensure the safe and efficient operation of new power systems with a high proportion of power electronic equipment. Summary of the Invention

[0005] The purpose of this invention is to provide a hierarchical control method for fault ride-through of a grid-type converter, which establishes a hierarchical control framework of dynamic limiting in the inner loop and dynamic regulation in the outer loop of the GFM converter. By integrating current limiting control during the fault period and dynamic recovery control after the fault, stable control of the GFM converter is achieved throughout the entire process from fault ride-through to steady-state recovery.

[0006] The first aspect of the present invention provides a fault ride-through hierarchical control method for a grid-type converter, the method comprising a current inner loop dynamic limiting control layer and a power outer loop dynamic compensation control layer.

[0007] The current inner loop dynamic limiting control layer introduces a virtual resistor composed of fault current negative feedback into the voltage and current inner loop control of the grid-type converter based on VSG control after the converter enters the fault ride-through state. The size of the virtual resistor is adjusted in real time according to the fault current amplitude to perform virtual resistor dynamic current limiting control.

[0008] The power outer loop dynamic compensation control layer dynamically adjusts the active power command value and reactive power command value of the grid-type converter based on VSG control during the fault ride-through period according to the degree of grid voltage drop after the converter enters the fault ride-through state. It also performs active-frequency control and reactive-voltage control based on the active power command value and reactive power command value, respectively.

[0009] Following the above scheme, this method also includes a fault detection layer;

[0010] The fault detection layer monitors the grid voltage sag and current overrun in real time to determine whether the converter should enter fault ride-through mode, as detailed below:

[0011] ;

[0012] In the formula, This refers to the amplitude of the fault current. This represents the maximum current amplitude. This is the per-unit value of the grid voltage, which is the ratio of the actual grid voltage to the rated grid voltage.

[0013] When the fault current amplitude exceeds the maximum allowable current amplitude of the grid-type converter based on VSG control, and the grid voltage drops to between 0.2 and 0.9 pu, the converter should enter the fault ride-through state.

[0014] Following the above scheme, the dynamic mathematical model of the virtual resistor is as follows:

[0015] ;

[0016] In the formula, For virtual resistance, This is the virtual resistance gain coefficient. Calculated value for virtual resistance;

[0017] The virtual resistance gain coefficient is adjusted based on the change in the magnitude of the fault current. Adaptive adjustment of virtual resistance gain coefficient The dynamic adjustment control strategy is as follows:

[0018] ;

[0019] In the formula, Virtual resistance gain coefficient The value, , ...the virtual resistance gain coefficient under different oscillation levels Numerical parameters, The fault current amplitude, , , ...the magnitude of the fault current The threshold parameter; the larger the fault current amplitude, the greater the virtual resistance gain coefficient. The larger the numerical parameter, the better.

[0020] Following the above scheme, the virtual resistance calculation value The calculation expression is:

[0021] ;

[0022] In the formula, This refers to the converter terminal voltage after the fault occurs. This refers to the grid voltage after the fault occurred; and The converter output current after the fault occurs. d-axis and q-axis components in the dq synchronous rotating coordinate system; The phase angle of the converter terminal voltage after the fault occurs; This refers to the resistance of the power grid line.

[0023] Following the above scheme, the active power command value and reactive power command value of the grid-type converter based on VSG control during fault ride-through are:

[0024] ;

[0025] ;

[0026] In the formula, and The active power command value and reactive power command value of the grid-type converter based on VSG control during fault ride-through; and These are the initial command values ​​for active power command value and reactive power command value; This is the per-unit value of the grid voltage, which is the ratio of the actual grid voltage to the rated grid voltage. and These are the terminal voltages of the grid-type converter. d-axis and q-axis components in the dq synchronous rotating coordinate system; For grid-side current of grid-type converter q-axis component in the dq synchronous rotating coordinate system; This is the rated output current of the converter; The expression for injecting reactive current into the converter is:

[0027] ;

[0028] in, The dynamic reactive current coefficient is given when the grid voltage drops to 0.2-0.9 pu. The dynamic reactive current coefficient is the factor for when the grid voltage drops below 0.2 pu.

[0029] Following the above scheme, this method also includes a state coupling and parameter adaptation module;

[0030] The state coupling and parameter adaptation module utilizes power angle deviation feedback to adjust the virtual resistance response speed, thereby optimizing the fault current limiting effect. Simultaneously, it adjusts the power command value to improve transient stability during fault ride-through, including:

[0031] Calculate the power angle offset :

[0032] ;

[0033] In the formula, For power grid line reactance; This refers to the converter terminal voltage after the fault occurs. This refers to the grid voltage after the fault occurred;

[0034] offset of power angle Introducing virtual resistance Dynamic response, virtual resistance The dynamic response is corrected to:

[0035] ;

[0036] Dynamic power compensation coefficient Introducing a power outer loop, the active power command value of the grid-type converter based on VSG control during fault ride-through. and reactive power command value Revised to:

[0037] ;

[0038] In the formula, This refers to the dynamic power compensation coefficient. This is an adaptive gain coefficient used to amplify or suppress the dynamic power compensation coefficient; This is an adaptive adjustment coefficient used to adjust the convergence speed; and These are the active power command value and reactive power command value after adjustment by the dynamic power compensation coefficient, respectively.

[0039] The final closed-loop control law of the state coupling and parameter adaptive module is as follows:

[0040] ;

[0041] Fault ride-through hierarchical coordinated control is performed on grid-type converters based on closed-loop control laws.

[0042] Furthermore, the active power droop coefficient in active power-frequency control The dynamic adjustment formula is:

[0043] ;

[0044] In the formula, This is the initial value of the active power droop coefficient; Adjust the gain to accommodate the active power droop factor; This is the per-unit value of the grid voltage, which is the ratio of the actual grid voltage to the rated grid voltage.

[0045] In the power outer loop dynamic compensation control layer, the active power-frequency control equation is:

[0046] ;

[0047] The reactive power-voltage control equation is:

[0048] ;

[0049] In the formula, The electrical angular frequency of the converter; This is a reference value for the converter voltage amplitude; and These are the initial command values ​​for active power command value and reactive power command value; and The active and reactive power outputs of the converter; This is a reference value for angular frequency; It is the moment of inertia; The Laplace transform factor; The damping coefficient; This represents the voltage amplitude at the converter terminals during no-load operation. The integral coefficient of the reactive power-voltage control loop; This refers to the voltage amplitude of the converter. This is the reactive power droop coefficient;

[0050] For the electrical angular frequency of the converter Integrating the phase reference value of the converter output voltage yields the value. The reference value of the converter voltage amplitude Together, the command values ​​of the d-axis and q-axis components of the terminal voltage are obtained through Park transformation. and ;

[0051] In the current inner loop dynamic limiting control layer, the voltage and current inner loop control of the grid-type converter based on VSG control includes a voltage outer loop control strategy and a current inner loop control strategy.

[0052] The voltage outer loop control strategy is as follows:

[0053] ;

[0054] The current inner loop control strategy is as follows:

[0055] ;

[0056] In the formula, and The d-axis and q-axis components of the converter output current are used as command values ​​for the inner current loop input. and These are the grid-side currents of the converter. d-axis and q-axis components in the dq synchronous rotating coordinate system; and These are the converter terminal voltages. d-axis and q-axis components in the dq synchronous rotating coordinate system; For converter filter capacitors; For virtual resistance; and For the d-axis and q-axis components of the converter output voltage; and These are the command values ​​for the d-axis and q-axis components of the output current, respectively. For converter filter inductance; , , , This is the transfer function for the controller.

[0057] According to a second aspect of the present invention, the present invention provides a grid-type converter based on VSG control, wherein the converter applies the fault ride-through hierarchical control method for grid-type converters described in any one of the first aspects for fault ride-through control.

[0058] According to a third aspect of the present invention, a computer device is provided, comprising: a processor and a memory, the memory storing a program or instructions executable on the processor, wherein the program or instructions, when executed by the processor, implement the steps of the grid-type converter fault ride-through hierarchical control method as described in any one of the first aspects.

[0059] According to a fourth aspect of the present invention, a readable storage medium is provided having a program or instructions stored thereon, which, when executed by a processor, implement the steps of the grid-type converter fault ride-through hierarchical control method as described in any one of the first aspects.

[0060] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects:

[0061] To address the multiple challenges posed by deep faults in GFM converters, such as current overruns, power oscillations, and power angle instability, this invention proposes a hierarchical fault ride-through control method for grid-type converters. After a fault occurs, this method rapidly suppresses the fault current through virtual resistance control in the voltage and current inner loop, maintains power angle stability through coordinated control of power commands and active power droop coefficients in the power outer loop, and adaptively adjusts parameters between the inner and outer loops via frequency deviation. This achieves dual control of current and power angle during fault ride-through, while simultaneously enhancing the dynamic performance of the GFM converter during the fault recovery phase. Attached Figure Description

[0062] Figure 1 A schematic diagram of the main circuit topology of a GFM converter provided in this application embodiment;

[0063] Figure 2(a) is a droop diagram of the converter terminal voltage frequency with respect to the output active power provided in an embodiment of this application;

[0064] Figure 2(b) is a droop characteristic diagram of the converter terminal voltage amplitude with respect to the output reactive power provided in an embodiment of this application;

[0065] Figure 3 A block diagram of droop control for a GFM converter provided in an embodiment of this application;

[0066] Figure 4 A control block diagram of a GFM converter based on VSG control is provided for embodiments of this application;

[0067] Figure 5 A block diagram of dual closed-loop voltage and current control for a GFM converter is provided in this application embodiment;

[0068] Figure 6 An equivalent circuit for VSG grid connection before and after a fault occurs, provided in an embodiment of this application;

[0069] Figure 7 An equivalent circuit diagram of VSG grid connection after introducing virtual impedance is provided for an embodiment of this application;

[0070] Figure 8 A block diagram of VSG current inner loop dynamic limiting control provided in this application embodiment;

[0071] Figure 9 A block diagram of VSG power outer loop dynamic compensation control provided in this application embodiment;

[0072] Figure 10 A VSG fault traversal hierarchical coordination control architecture diagram is provided for embodiments of this application;

[0073] Figure 11 A VSG fault traversal hierarchical coordination control block diagram provided in this application embodiment;

[0074] Figure 12 A block diagram of a fault ride-through hierarchical control method for a grid-type converter provided in an embodiment of this application. Detailed Implementation

[0075] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0076] This application provides a hierarchical control method for fault ride-through of a grid-type converter. It establishes a hierarchical control framework for the GFM converter, which consists of dynamic limiting in the inner loop and dynamic regulation in the outer loop. By integrating current limiting control during the fault period and dynamic recovery control after the fault, it achieves stable control of the GFM converter throughout the entire process from fault ride-through to steady-state recovery.

[0077] Figure 1 This is a topology diagram of the main circuit of a GFM converter. The GFM converter uses a typical three-phase bridge voltage source converter topology. The GFM converter is connected in parallel to the AC grid via a filter circuit. The filter circuit uses an LC circuit to filter out high-frequency harmonics and is connected to the grid through the point of common coupling (PCC). For the DC-side capacitor of the GFM converter; This indicates the DC-side capacitor voltage of the GFM converter; This indicates the output voltage of the GFM converter. , , These represent the output voltages of the GFM converter. The A, B, and C phase components; This indicates the output current of the GFM converter. , , These represent the output current of the GFM converter. The A, B, and C phase components; For GFM converter filter inductance; The equivalent resistance of the filter inductor in the GFM converter; For GFM converter filter capacitors; This indicates the terminal voltage of the GFM converter. , , These represent the terminal voltages of the GFM converter. The A, B, and C phase components; This indicates the grid-side current of the GFM converter. , , These represent the grid-side currents of the GFM converter. The A, B, and C phase components; The impedance of the power supply side line; Indicates AC power supply voltage. , , These represent the AC power supply voltages respectively. The A, B, and C phase components.

[0078] like Figure 1 As shown, the main circuit of the GFM converter consists of a voltage source converter, a DC power supply, and a filter circuit. The voltage source converter, as the main component of the GFM converter, has various topologies; this embodiment uses a typical three-phase bridge voltage source converter topology for study. The DC power supply of the GFM converter is typically provided by new energy systems such as photovoltaic power generation systems, energy storage systems, and wind power generation systems. Relying on the active power provided by the DC power supply, the GFM converter can effectively simulate the primary frequency regulation characteristics of a synchronous generator. Since this application focuses on studying the AC output characteristics of the GFM converter, the control process of its DC-side new energy power supply is ignored.

[0079] According to Kirchhoff's voltage law, the voltage loop equation of the GFM converter in the ABC three-phase stationary coordinate system can be obtained as follows:

[0080] .

[0081] According to Kirchhoff's current law, the current equation of the GFM converter in the ABC three-phase stationary coordinate system can be obtained as follows:

[0082] .

[0083] The variables in the above two equations are AC quantities, which are not convenient for analysis and control system design. Therefore, the Park transformation is used to convert the AC quantities into DC quantities in the dq synchronous rotating coordinate system. When performing the coordinate transformation, the converter terminal voltage is selected. Let the phase angle be denoted as the reference phasor. Therefore, the expression for the Park transformation is:

[0084] .

[0085] Multiplying both sides of the above two equations by the Park transformation matrix on the left, we can obtain the voltage loop equation and current equation of the GFM converter in the dq synchronous rotating coordinate system as follows:

[0086] ;

[0087] ;

[0088] In the formula, Indicates the electrical angular frequency of the GFM converter; and The output current of the GFM converter is respectively d-axis and q-axis components in the dq synchronous rotating coordinate system; and These are the grid-side currents of the GFM converter. d-axis and q-axis components in the dq synchronous rotating coordinate system; and The output voltages of the GFM converter are respectively d-axis and q-axis components in the dq synchronous rotating coordinate system; and These are the terminal voltages of the GFM converter. The d-axis and q-axis components in the dq synchronous rotating coordinate system.

[0089] Depend on Figure 1 The output active power of the GFM converter can be obtained. and reactive power The calculation expression in the dq synchronous rotating coordinate system is:

[0090] .

[0091] The above explains the Park transform and the power calculation module. The Park transform can calculate... , , and The power calculation module is used to calculate the output active power of the GFM converter. and reactive power .

[0092] The following section presents theoretical modeling of two power control methods for grid-type converters (droop control and VSG control), and demonstrates power decoupling through voltage and current dual closed-loop control, providing theoretical support and a control framework for subsequent research.

[0093] (1) Modeling of power control loop based on droop control

[0094] The frequency of the converter terminal voltage is related to the output active power ( ) and the amplitude of the terminal voltage relative to the output reactive power ( The drooping properties of the α and β are shown in Figures 2(a) and 2(b), respectively. In the figures, This refers to the rated active power output of the converter. This represents the maximum active power that the converter is allowed to output when the frequency drops. This refers to the no-load operating angular frequency of the converter. This is the minimum angular frequency corresponding to the maximum output active power of the converter; This is the maximum reactive power that the converter is allowed to output when the voltage amplitude decreases. This represents the voltage amplitude at the converter terminals during no-load operation. This represents the minimum voltage amplitude corresponding to the maximum reactive power output of the converter.

[0095] set up The droop coefficient is , The droop coefficient is From Figures 2(a) and 2(b), the mathematical expression for the power control loop of droop control can be obtained as follows:

[0096] ;

[0097] In the formula, Angular frequency output value ; This represents the voltage amplitude output value. The active power droop factor and reactive power droop factor can be obtained from the following formulas:

[0098] .

[0099] Assume the active power command value of the GFM converter output is The output reactive power command value is , and Given the reference values ​​for angular frequency and voltage, respectively, the frequency and voltage control equations for the GFM converter can be obtained as follows:

[0100] ;

[0101] In the formula, This is the output value of the power angle.

[0102] To avoid the impact of grid harmonics on the converter, in practical engineering applications, a low-pass filter is usually added at the droop control front end to filter out higher-order terms in the instantaneous power. Let... Let be the transfer function of the low-pass filter. Then, the expressions for the active and reactive power actually entering the feedback loop after passing through the low-pass filter are:

[0103] .

[0104] The resulting block diagram for the droop control of the GFM converter is as follows: Figure 3 As shown. The power calculation module obtains the terminal voltage of the GFM converter. and grid-side current The instantaneous value of the converter output power is calculated. and The instantaneous power value is passed through a low-pass filter to obtain the active power output of the GFM converter that enters the feedback loop. and reactive power ;pass control loop and The control loop obtains the power angle output value of the GFM converter. and voltage amplitude output value , the results and The reference voltage d-axis component is obtained from the input voltage and current inner loop control calculation. and q-axis components The required modulation voltage is calculated through inverse Park transform. Finally, the PWM modulation output trigger pulse controls the switching on and off of the converter's power devices, enabling the GFM converter to output the corresponding output voltage amplitude and phase angle to support the grid voltage and frequency. The voltage and current inner loop control in the control block diagram will be studied in detail below.

[0105] (2) Power control loop modeling based on virtual synchronous generator control

[0106] The electrical mathematical model of the GFM converter based on VSG control corresponds to the stator electrical equations of the synchronous generator. The mechanical motion equations of the synchronous generator can be expressed as:

[0107] ;

[0108] In the formula, This refers to the mechanical torque of the synchronous generator; For synchronous generator electromagnetic torque; This is the damping torque of the synchronous generator; The moment of inertia of the synchronous generator; is the mechanical angular frequency of the synchronous generator rotor. Wherein, the mechanical torque of the synchronous generator... Electromagnetic torque and damping torque It can be represented as:

[0109] ;

[0110] In the formula, This refers to the mechanical power of the synchronous generator; For synchronous generator electromagnetic power; Damping coefficient; This is the electrical angular frequency of the synchronous generator rotor.

[0111] Substituting the above equation into the mechanical motion equation of the synchronous generator, we get:

[0112] .

[0113] Choosing a salient-pole synchronous generator model with 1 pole pair, the rotor electric angular frequency is... Mechanical angular frequency of synchronous generator rotor If they are equal, then the equation of motion for the synchronous generator rotor can be expressed as:

[0114] .

[0115] From the above equation, we can see that the moment of inertia This reflects the rotor inertia of the synchronous generator, giving the converter inertia during power and frequency dynamic processes; the damping coefficient This reflects the damping characteristics of a synchronous generator, enabling the converter to dampen grid power oscillations. Moment of inertia and damping coefficient Two variables are key to the synchronous generator's ability to participate in grid frequency and voltage regulation. The VSG applies the synchronous generator's operating mode and working principle to the converter control, giving the VSG similar primary frequency and voltage regulation characteristics to the synchronous generator.

[0116] Figure 1 DC power supply in the main circuit of the GFM converter It can be equivalent to a prime mover; the actual value of the active power output of the GFM converter. Electromagnetic power of synchronous generator Correspondingly, the actual output active power of the GFM converter based on VSG control can be obtained from the above formula. With instruction value The relational expression is:

[0117] .

[0118] Combining the electrical and mechanical mathematical models of the GFM converter based on VSG control, it is evident that VSG can simulate the rotor motion equations and stator electrical equations of a synchronous generator. Since the VSG model and the synchronous generator model are equivalent, VSG can draw upon traditional synchronous generator control methods, incorporating damping and inertia into VSG control to simulate the active power-frequency regulation and reactive power-voltage regulation processes of a synchronous generator.

[0119] According to the active power-frequency droop characteristic, the droop characteristic equation between the active power and frequency of the synchronous generator governor is as follows:

[0120] ;

[0121] Taking the Laplace transform of the above equation, we get:

[0122] .

[0123] To incorporate virtual inertia and damping into the converter control loop, i.e., to simulate the rotor motion equation of a synchronous generator, the droop characteristic equation is substituted into the above equation, and then... ,get:

[0124] ;

[0125] Therefore, the active-frequency regulation equation for the GFM converter based on VSG control is obtained as follows:

[0126] .

[0127] As shown in the above equation, the active-frequency control of the VSG simulates the active-frequency droop control characteristics of a traditional synchronous generator. It regulates the frequency by controlling the output electromagnetic torque after detecting the error between the actual and reference values ​​of the active power. In the equation, the moment of inertia... The existence of VSG gives it virtual inertia during frequency dynamics, and the damping coefficient The presence of VSG dampes oscillations, thus enabling the converter to regulate power and frequency during grid fluctuations. Through the analysis of... By integrating, the phase reference value of the converter output voltage can be generated. The VSG active-frequency control loop block diagram is as follows: Figure 4 middle The control loop is shown.

[0128] Synchronous generators control their output voltage through excitation current to ensure the orderly distribution of reactive power. Based on the reactive-voltage droop characteristic, the droop characteristic equation between reactive power and voltage in a synchronous generator governor can be expressed as follows:

[0129] ;

[0130] In the formula, This provides the virtual reactive power output for the converter. This is the no-load potential of the converter.

[0131] By employing reactive power-voltage droop control, the system can adjust its output voltage based on changes in the reactive power of the load. The above equation represents open-loop control. To enable the VSG-based GFM converter to achieve stable AC voltage control in grid-connected mode, an integral controller is introduced to improve voltage control performance, namely:

[0132] ;

[0133] In the formula, The integral coefficient of the reactive power-voltage control loop; This is a reference value for the voltage amplitude of the converter.

[0134] Substituting the droop characteristic equation into the above formula, we obtain the mathematical model of the VSG reactive power-voltage control loop as follows:

[0135] .

[0136] This yields the reactive power-voltage control loop diagram of the GFM converter based on VSG control, as shown below. Figure 4 middle The control loop is shown. The electrical angular frequency of the converter is... Integrating the phase reference value of the converter output voltage yields the value. The reference value of the converter voltage amplitude Together, the command values ​​of the d-axis and q-axis components of the terminal voltage are obtained through Park transformation. and .

[0137] Based on the above, the control block diagram of the GFM converter based on VSG control is as follows: Figure 4 As shown. Since the VSG-controlled GFM converter can provide virtual inertia and damping compared to droop control, it has better dynamic characteristics. Therefore, this application uses the VSG-controlled GFM converter to carry out research on transient stability and fault ride-through problems. The VSG-controlled GFM converter will be referred to as VSG in the following text.

[0138] Next, we will model the voltage and current closed-loop control of the grid-type converter, namely... Figure 3 and Figure 4 The voltage and current inner loop control section.

[0139] neglect By performing a Laplace transform on the voltage loop equation of the GFM converter in the ABC three-phase stationary coordinate system, the d-axis component of the output voltage of the GFM converter can be obtained. q-axis components With the d-axis component of the output current q-axis components The transfer function expression is:

[0140] .

[0141] By performing a Laplace transform on the current equation of the GFM converter in the ABC three-phase stationary coordinate system, the d-axis component of the output current of the GFM converter can be obtained. q-axis components With the d-axis component of the terminal voltage q-axis components The transfer function expression is:

[0142] .

[0143] Analysis of the above formula shows that the d-axis component of the terminal voltage of the GFM converter... and q-axis components Since a coupling relationship exists, independent control cannot be achieved. Therefore, this application adopts a feedforward decoupling control method, letting:

[0144] ;

[0145] In the formula, , For the controller's transfer function, , These are the command values ​​for the d-axis and q-axis components of the terminal voltage, respectively.

[0146] By combining the above two equations, the d-axis component of the terminal voltage can be obtained. and q-axis components The expression for the transfer function is:

[0147] .

[0148] As can be seen from the above equation, the d-axis component of the terminal voltage is realized through the feedforward decoupling control method. and q-axis components Decoupling control. If a PI controller is used, the above equation can be written as:

[0149] ;

[0150] In the formula, , The proportional gain of the PI controller. , This represents the integral coefficient of the PI controller.

[0151] From the above formula, we can see that the d-axis component of the terminal voltage... and q-axis components The transfer functions of all PI controllers are second-order closed-loop expressions. Their dynamic response characteristics can be adjusted by changing the proportional and integral coefficients of the PI controller. Therefore, the outer-loop voltage control strategy for the GFM converter can be designed as follows:

[0152] .

[0153] The d-axis component of the output current of the GFM converter obtained in the above formula and q-axis components The command value is used as the input to the inner current loop. This is due to the d-axis component of the output current of the GFM converter. and q-axis components There is a coupling relationship; therefore, a feedforward decoupling control method is adopted, let:

[0154] ;

[0155] In the formula, , For the controller's transfer function, , These are the command values ​​for the d-axis and q-axis components of the output current, respectively.

[0156] This allows us to obtain the d-axis component of the output current. and q-axis components The expression for the transfer function is:

[0157] .

[0158] As can be seen from the above equation, the d-axis component of the output current is achieved through the feedforward decoupling control method. and q-axis components Decoupling control. If a PI controller is used, the above equation can be written as:

[0159] ;

[0160] In the formula, , The proportional gain of the PI controller. , This represents the integral coefficient of the PI controller.

[0161] From the above equation, we can see that the d-axis component of the output current... and q-axis components The transfer functions of all PI controllers are second-order closed-loop expressions. Their dynamic response characteristics can be adjusted by changing the proportional and integral coefficients of the PI controller. Therefore, the inner-loop current control strategy for the GFM converter can be designed as follows:

[0162] .

[0163] Based on the above, the voltage and current dual closed-loop control block diagram of the GFM converter can be obtained as follows: Figure 5 As shown, the voltage and current dual closed-loop control employs a hierarchical regulation mechanism and a PI controller to ensure the stability of the output voltage and current. Simultaneously, a decoupling element is introduced to reduce the coupling relationship between voltage and current, thereby improving the system's dynamic response performance.

[0164] The above systematic research focuses on the control principle and model of the GFM converter. First, based on Kirchhoff's laws, the voltage and current equations of the GFM converter in a three-phase stationary coordinate system were established. These equations were then transformed into a mathematical model in a dq rotating coordinate system using the Park transformation, deriving expressions for active and reactive power, thus laying the foundation for control strategy design. Second, mathematical models for droop control and VSG control were constructed: droop control achieves power distribution by simulating the power frequency and voltage characteristics of a synchronous generator, while VSG control introduces rotor motion equations and stator electrical equations, endowing the converter with virtual inertia and damping characteristics, thereby achieving primary frequency and voltage regulation functions similar to a synchronous generator. Finally, a dual-loop control strategy for voltage and current was designed by combining a feedforward decoupling strategy to suppress coupling between the dq axes and improve the system's dynamic response performance. The above theoretical derivation of the mathematical model and control strategy of the GFM converter and its control block diagram design present the implementation path of the GFM converter from main circuit modeling to power control loop and voltage and current inner loop control modeling, providing a mathematical model and control framework for subsequent transient stability analysis and fault ride-through strategy research of GFM converter.

[0165] When a system malfunctions, such as two typical instability modes: Type I and Type II, the grid voltage will drop, and the voltage difference between the grid voltage and the VSG terminal voltage will increase sharply, thereby triggering a transient inrush current far exceeding the rated value. Such overcurrents may not only trigger the protection device to malfunction, but also cause thermal stress accumulation or even permanent damage to the converter power devices, seriously affecting the safe and stable operation of the power system.

[0166] The equivalent circuit of VSG grid connection before and after the fault is as follows: Figure 6 As shown in the figure This is the VSG terminal voltage during normal operation. This is the grid voltage during normal operation. The VSG grid-side output current during normal operation, and the AC grid line impedance. , For the resistance of the power grid line, For power grid line reactance. This refers to the VSG output voltage after the fault occurs. This refers to the VSG output current after the fault occurs. This refers to the VSG terminal voltage after the fault occurred. This refers to the grid voltage after the fault occurred. This refers to the VSG network-side output current after a fault occurs.

[0167] When the grid voltage drops, the inverter faces the risk of both steady-state overcurrent and transient inrush current overcurrent. A deeper voltage drop and lower impedance both imply a greater risk of overcurrent. To reduce the fault inrush current generated by the VSG after a grid fault, the time constant can be shortened. This causes the transient component to decay to a smaller value before the steady-state component reaches its peak. The time constant... This is related to the equivalent impedance between the VSG and the power grid. Therefore, by introducing a virtual impedance element consisting of current negative feedback into the VSG control loop, the equivalent impedance of the system after a fault can be increased, thereby limiting overcurrent phenomena.

[0168] The equivalent circuit of VSG grid connection after introducing virtual impedance is as follows: Figure 7 As shown in the figure, For virtual impedance, For virtual resistance, This is a virtual reactance. After a fault occurs, the voltage of the mains grid will be used. For reference phasor Let the VSG terminal voltage be... , The phase angle of the VSG terminal voltage. A virtual impedance is introduced. After the fault, the steady-state output current of the VSG The expression is:

[0169] .

[0170] Steady-state output current after fault amplitude The expression is:

[0171] .

[0172] Thus, the virtual impedance is obtained. Caused voltage drop for:

[0173] .

[0174] Assume the steady-state output current of the VSG after the fault. The d-axis and q-axis components in the dq synchronous rotating coordinate system are respectively and Voltage drop caused by virtual impedance The d-axis and q-axis components in the dq synchronous rotating coordinate system are respectively and Therefore, the virtual impedance in the dq synchronous rotating coordinate system The voltage drop across is:

[0175] .

[0176] In VSG control, stator impedance can be simulated by introducing a virtual impedance, which is reflected in the d-axis component of the output voltage of the GFM converter. q-axis components With the d-axis component of the output current q-axis components Based on the transfer function expression, the d-axis component of the VSG internal potential and q-axis components Subtract virtual impedance The voltage drop is obtained by considering the virtual impedance in the dq synchronous rotating coordinate system to obtain the VSG modulated wave voltage. and The expression is:

[0177] ;

[0178] In the formula, the virtual inductance is represented as .

[0179] The steady-state output current of the VSG after the fault The relationship between the grid voltage, VSG terminal voltage, and fault current in the dq synchronous rotating coordinate system can be obtained as follows:

[0180] .

[0181] Thus, the virtual resistance is obtained. and virtual reactance The calculation expression and They are respectively:

[0182] ;

[0183] .

[0184] After introducing virtual impedance, the equivalent virtual impedance between the converter and the grid is... Represented as:

[0185] ;

[0186] In the formula, To introduce virtual impedance, the equivalent inductance between the converter and the grid is introduced. This is the equivalent resistance between the converter and the grid after introducing virtual impedance; This is to introduce virtual impedance to achieve the equivalent reactance between the converter and the power grid.

[0187] Attenuation component time constant The effect of virtual impedance becomes:

[0188] ;

[0189] In the formula, .

[0190] The above analysis shows that the larger the magnitude of the virtual impedance added after the fault, the larger the fault current. The smaller the virtual impedance, the better. This means the virtual impedance can limit the steady-state component of the current after a fault, but the transient component of the fault current may still exceed the current limit. Under the influence of the virtual impedance, the transient impact current and the time constant of the decay component... Related, by and The effect of the ratio is that, since the inherent impedance of the line remains constant, if it is necessary to accelerate the attenuation of the transient current component, the virtual resistance needs to be increased. Or reduce virtual reactance Increasing the equivalent reactance of the line will reduce the dynamic response performance of the converter, which is detrimental to the transient stability of the converter. Therefore, to prevent the steady-state and transient components of the current from exceeding the current limit value during a fault, a current-limiting control method that increases the virtual resistance can be adopted. After a short-circuit fault occurs, the virtual reactance remains unchanged. By increasing the virtual resistance, the magnitude of the VSG virtual impedance increases, thereby limiting the steady-state component of the short-circuit current. The increase in virtual resistance can also accelerate the decay rate of the transient inrush current, thus achieving dual limiting of the steady-state and transient components of the current after the fault.

[0191] While increasing the virtual resistance can reduce overcurrent during grid faults, existing methods for current limiting via virtual impedance typically employ a fixed current limiting value. When power system parameters or structures are disturbed, the equivalent impedance between the converter and the grid changes in real time, and a fixed virtual resistance value cannot achieve accurate current limiting during converter fault ride-through. Therefore, this application proposes a dynamic current limiting method using the virtual resistance within the VSG inner loop, with the control flow as follows:

[0192] (1) Fault detection. First, detect the difference between the actual value and the rated value of the grid voltage to determine if the grid voltage has dropped. According to the fault ride-through technology requirements of the GFM converter, when the actual value of the grid voltage is detected... When the voltage drops between 0.2 and 0.9 pu, the converter should enter fault ride-through mode. The VSG output current amplitude should be monitored in real time, and the maximum allowable current amplitude for the VSG should be set. (Generally taken as 1.1~1.5pu), when the actual value of the VSG output current exceeds Then the converter should enter fault ride-through mode. Therefore, the condition for activating the inner loop virtual resistor dynamic current limiting circuit is:

[0193] .

[0194] (2) Dynamic current limiting of the inner loop virtual resistance. After the VSG enters the fault ride-through state, the dynamic current limiting control of the virtual resistance is quickly started. The magnitude of the virtual resistance provided by the VSG to the power system is adjusted in real time according to the fault current amplitude. The dynamic mathematical model of the virtual resistance is:

[0195] ;

[0196] In the formula, This is the virtual resistance gain coefficient. This is the calculated value for the virtual resistance.

[0197] Since the magnitude of the virtual resistance that the VSG provides to the power system depends on the gain... Increase the gain of the control system This will increase the virtual resistance, but excessive gain... This will also prolong the time it takes for the VSG to reach stability. In order to balance fault current limiting and converter dynamic response speed, the gain of the VSG virtual resistance dynamic current limiting strategy can be adjusted according to the change in current amplitude. Adaptive adjustment, The dynamic adjustment control strategy is as follows:

[0198] ;

[0199] In the formula, For gain The value, , Gain under different oscillation levels Numerical parameters, , , ,……for Threshold parameters.

[0200] The above two equations constitute the proposed VSG virtual resistance dynamic current limiting strategy. Through this control strategy, the VSG can adjust the virtual resistance it provides to the power system according to the change in the fault current amplitude, thereby rapidly limiting the fault current and improving the frequency stability of the power system to a certain extent.

[0201] (3) Virtual resistor exits operation. When the short-circuit current is less than Furthermore, when the grid voltage recovers to above 0.9 pu, the virtual resistor exits operation. Thus, the dynamic limiting control strategy based on the dynamic virtual resistor effectively suppresses the steady-state value and transient peak value of the short-circuit current, achieving fault current limiting during VSG fault ride-through and improving the VSG's fault ride-through capability and transient stability performance.

[0202] Based on the above analysis, the block diagram of the VSG current inner loop dynamic limiting control based on virtual resistance is as follows: Figure 8 As shown, it suppresses VSG fault current through dynamic virtual impedance limiting, preventing converter devices from being impacted by fault current. During fault ride-through, to maintain the active support characteristics of the VSG, transient stability of the power angle and voltage support also need to be maintained. For the VSG under grid fault conditions, its active power command value... and actual value of output active power Significant deviations exist between the input and output of active power, resulting in substantial power imbalance and a continuous increase in the system's power angle. The fundamental cause of VSG transient power angle instability is the imbalance between active power input and output during the fault period. Next, from the perspective of the VSG transient power angle instability mechanism, we consider the active voltage support performance of the VSG based on reactive current injection during fault ride-through. By real-time adjustment of the active and reactive power command values ​​of the VSG during the fault process, we can respectively achieve the functions of maintaining power angle stability and maintaining voltage support.

[0203] According to the converter fault ride-through requirements, the converter should maintain continuous operation within a certain voltage range without disconnecting from the grid during fault ride-through. Therefore, the converter needs to inject reactive current into the grid connection point to achieve active voltage support. Combining the converter fault ride-through requirements, the degree of grid voltage drop and the reactive current injected by the converter are considered. The relationship expression between them is:

[0204] ;

[0205] In the formula, The dynamic reactive current coefficient is used when the grid voltage drops below 0.2 pu, and is generally taken as 1.05.

[0206] From the above, we can obtain the relationship between the grid voltage sag and the active current component of the VSG. The relationship expression between them is:

[0207] .

[0208] As can be seen from the above two equations, during fault ride-through, the VSG should switch from active power control priority mode to reactive power control priority mode to support the grid voltage. The reactive current injected by the converter is higher than the reactive current during normal operation, while the active current decreases accordingly. The magnitude of the reactive current is adjusted according to the degree of grid voltage drop, thereby supporting the grid voltage and improving the VSG's fault ride-through capability.

[0209] In practical applications, the output current is usually controlled by the converter's output power, rather than by direct current control. Therefore, the active power output by the VSG to the grid during fault ride-through can be obtained from the above formula. and reactive power for:

[0210] ;

[0211] Substituting the values ​​yields the VSG active power command value during fault ride-through. and reactive power command value for:

[0212] ;

[0213] ;

[0214] In the formula, This is the per-unit value of the grid-connected voltage; These are the grid-side currents of the GFM converter. q-axis component in the dq synchronous rotating coordinate system; These are the terminal voltages of the GFM converter. The q-axis component in the dq synchronous rotating coordinate system; rated output current of the grid-connected converter. , The dynamic reactive current coefficient is used when the grid voltage drops to 0.2-0.9 pu, and is generally taken as 1.5; The dynamic reactive current factor is typically taken as 1.05 when the grid voltage drops below 0.2 pu; the reactive current injected into the converter... .

[0215] The above two formulas can be used to calculate the VSG active power command value and reactive power command value, which are dynamically adjusted as the grid voltage drops during a fault. At that time, the VSG outputs active power and reactive power The corresponding initial instruction values ​​will be retained respectively. and When the grid voltage drops between 0.2 and 0.9 pu, the VSG outputs active power. This will be reduced to maintain system power angle stability, reactive power This will increase the reactive current injected into the system. When the grid voltage After that, active power This will be further reduced to increase the deceleration area and decrease the acceleration area, thereby improving the system's stability margin and reactive power. It will be further increased to maintain a certain voltage support.

[0216] By employing a dynamic power compensation control strategy, the active power command value is corrected, thereby reducing the power angle offset. :

[0217] .

[0218] Based on the above analysis, the VSG power outer loop dynamic compensation control block diagram is shown in Figure 9.

[0219] During the fault ride-through of GFM converters, grid voltage dips can trigger multiple challenges, including current overruns, power oscillations, and power angle instability. To address these issues simultaneously, this application proposes a hierarchical coordinated control strategy for VSG fault ride-through based on dynamic limiting of the inner current loop and dynamic compensation of the outer power loop. This strategy achieves dual optimized control of current and power angle during VSG fault ride-through through a synergistic mechanism of rapid fault current suppression in the inner loop and power angle stability maintenance in the outer loop, resulting in better dynamic performance of the VSG during fault ride-through.

[0220] VSG fault-crossing hierarchical coordination control architecture, such as Figure 10 As shown, the VSG fault ride-through hierarchical coordinated control architecture includes an inner current dynamic limiting control layer, an outer power dynamic compensation control layer, and a state coupling and parameter adaptation module. The inner current dynamic limiting control layer, as a fast response level, detects grid voltage dips and current overruns in real time. Based on the VSG inner loop virtual resistance dynamic current limiting method, it dynamically adjusts the virtual resistance to limit the steady-state component and transient peak value of the fault current, reducing the overcurrent risk of power devices and protecting the converter itself. The outer power dynamic compensation control layer, as a dynamic compensation level, dynamically adjusts the active power reference value and reactive power reference value according to the degree of grid voltage dip. Based on the power compensation model, it maintains system power angle stability during faults and reduces grid instability risk through voltage support. The state coupling and parameter adaptation module, as a coordinated feedback level, realizes dynamic interaction between the inner and outer loops. It uses power angle deviation feedback to adjust the virtual resistance response speed to optimize the fault current limiting effect and simultaneously adjusts the power command value to improve transient stability during fault ride-through.

[0221] To achieve the above feedback mechanism, the power angle deviation will be... As a common feedback signal for both the inner and outer loops, it is used both for adjusting the virtual impedance in the inner loop and for correcting the power command in the outer loop. For the current inner loop dynamic limiting control layer, this application will use the power deviation... Introducing virtual resistance Dynamic response, thus virtual resistance The dynamic response is corrected to:

[0222] .

[0223] From the above formula, it can be seen that when the work angle deviation... Larger or fault current instantaneous value The larger the virtual resistance, the greater the virtual resistance. The faster the adjustment speed, the faster the current overshoot can be suppressed.

[0224] Power angle deviation The change is driven by power imbalance. During a fault, the grid voltage drops sharply, causing a sudden drop in the active power output of the VSG and the virtual mechanical power. Greater than the active power output, The power compensation coefficient increases rapidly. Therefore, for the power outer loop dynamic compensation control layer, the power compensation coefficient will be... By introducing a power outer loop and adjusting the influence of the feedback signal through an adaptive coefficient, the VSG outputs active power based on the original value. and reactive power Further revised to:

[0225] ;

[0226] In the formula, This refers to the dynamic power compensation coefficient. This is an adaptive gain coefficient used to amplify or suppress the dynamic power compensation coefficient; This is an adaptive adjustment coefficient used to adjust the convergence speed; and These are the active power command value and reactive power command value after adjustment by the dynamic power compensation coefficient, respectively.

[0227] From the above formula, it can be seen that when At the same time, the outer power loop increases the power reference value, enhancing voltage and frequency support capabilities; At the same time, lowering the power reference value during the fault period allows the VSG to maintain a stable operating point, improving its transient stability performance during the fault. Regarding the power angle deviation... ,when When the angle deviation is small, smoothing adjustment is performed; when That is, when the angle of attack deviation is large, the convergence process is accelerated. And the square term... This allows the adaptive adjustment intensity to vary with the power angle deviation. The increase in nonlinearity enhances the convergence under large deviations, smooths the fast response under small deviations, and is effective regardless of the angle deviation. Is it positive or negative, the square term? All guaranteed The adjustment direction is consistent to avoid parameter oscillation. This adjustment strategy addresses the power angle deviation. When the voltage is high, the active power command is reduced to decrease the acceleration area and enhance system stability, while the reactive power is increased to improve voltage support capability.

[0228] Combining the above two equations, the closed-loop control law of the state-coupled and parameter-adaptive module is obtained as follows:

[0229] .

[0230] Based on the above analysis, the proposed VSG fault ride-through hierarchical coordinated control strategy achieves hierarchical coordinated control between the inner and outer loops by introducing power deviation feedback and adaptive parameter adjustment. When the fault current is large or the power angle deviation increases, the inner loop rapidly increases the virtual impedance to limit the fault current amplitude. Simultaneously, the outer loop, based on the increased power angle deviation, reduces the active power command and increases the reactive power command to compensate for voltage drops and suppress further power angle shifts. This balances the requirements of fault current limiting and power angle stability, improving the VSG fault ride-through capability and transient stability. Therefore, the VSG fault ride-through hierarchical coordinated control block diagram is as follows: Figure 11 As shown.

[0231] Based on this, this application proposes a hierarchical fault ride-through control method for GFM converters, such as... Figure 12 As shown, its control architecture includes a current inner loop dynamic limiting control layer, a power outer loop dynamic compensation control layer, and a state coupling and parameter adaptation module.

[0232] (1) The GFM converter's inner current loop dynamic limiting control layer detects grid voltage dips and current overruns in real time. After the GFM converter enters fault ride-through mode, the staged dynamic virtual resistance current limiting control is quickly activated. The magnitude of the virtual resistance provided by the GFM converter to the power system is adjusted in real time according to the fault current amplitude. The dynamic mathematical model of the virtual resistance is as follows:

[0233] ;

[0234] In the formula, This is the virtual resistance gain coefficient. This is the calculated value for the virtual resistance.

[0235] Since the magnitude of the virtual resistance provided by the GFM converter to the power system depends on the gain... Increase the gain of the control system This will increase the virtual resistance, but excessive gain... This will also prolong the time it takes for the GFM converter to reach stability. In order to balance fault current limiting and converter dynamic response speed, the gain of the GFM converter's virtual resistance dynamic current limiting strategy can be adjusted according to the change in current amplitude. Adaptive adjustment, The dynamic adjustment control strategy is the same as above.

[0236] This section is based on a dynamic mathematical model of virtual resistance and The control strategy is dynamically adjusted to regulate the virtual resistance of the GFM converter, limiting the steady-state component and transient peak value of the fault current, reducing the overcurrent risk of power devices, and protecting the safety of the converter itself.

[0237] (2) The GFM converter's outer power loop dynamic compensation control layer dynamically adjusts the power command value and active power droop coefficient according to the degree of grid voltage drop, so that the GFM converter meets the stability criteria and reduces the risk of grid instability. Active power command value of the GFM converter during fault ride-through. and reactive power command value The calculation method is the same as above.

[0238] During a fault, as the grid voltage drops, the GFM converter dynamically adjusts the active and reactive power command values ​​based on the above two formulas. When the grid voltage... hour, and These are the corresponding initial instruction values. =0.4MW and =0MVar; when the mains voltage drops between 0.2-0.9 pu, This will be reduced to maintain the stability of the system's power angle. This will increase the reactive current injected into the system. When the grid voltage back, This will be further reduced to increase the deceleration area and decrease the acceleration area, thereby improving the system's stability margin. It will be further increased to maintain a certain voltage support.

[0239] When the power grid experiences load changes or other disturbances, the active power droop factor determines the sensitivity of the inverter's output active power to frequency changes. Active power droop factor It can be dynamically adjusted based on the degree of grid voltage drop to optimize the dynamic recovery process after fault clearance. Active power droop factor. The dynamic adjustment formula is:

[0240] ;

[0241] In the formula, This is the initial value of the active power droop coefficient; The gain is adjusted by the active droop factor, which is typically set to 10.

[0242] The active power command value is corrected through the outer loop dynamic control layer of the GFM converter, thereby reducing the power angle offset. :

[0243] ;

[0244] In the formula, For line reactance; This refers to the converter terminal voltage after the fault occurs. This represents the grid voltage after the fault occurred.

[0245] (3) The state coupling and parameter adaptive module of the GFM converter utilizes the power angle deviation The feedback adjusts the response speed of the virtual resistor and simultaneously adjusts the power command value to achieve dynamic interaction between the inner and outer loops, thereby optimizing the limiting effect of the fault current and improving the transient stability during fault ride-through.

[0246] The parameter adaptive module will adjust the power deviation. Introducing virtual resistance Dynamic response, thus virtual resistance The dynamic response is corrected to:

[0247] .

[0248] From the above formula, it can be seen that when the work angle deviation... Larger or fault current instantaneous value The larger the virtual resistance, the greater the virtual resistance. The faster the adjustment speed, the faster the current overshoot can be suppressed.

[0249] Power angle deviation The change is driven by power imbalance. During the fault, the grid voltage drops sharply, causing a sudden drop in the active power output of the GFM converter and the virtual mechanical power. Greater than the active power output, The power compensation coefficient increases rapidly. Therefore, for the outer ring dynamic control layer, the power compensation coefficient will be... By introducing a power outer loop and adjusting the influence of the feedback signal through an adaptive coefficient, the GFM converter outputs active power. and reactive power Further modifications were made, using the same formula as above, ultimately yielding the closed-loop control law for the parameter adaptive module.

[0250] Based on the above analysis, the fault ride-through layered control strategy for GFM converters proposed in this application achieves layered control of the inner and outer loops by introducing power deviation feedback and adaptive parameter adjustment. When the fault current is large or the power angle deviation increases, the inner loop rapidly increases the virtual impedance to limit the fault current amplitude; simultaneously, the outer loop, based on the increased power angle deviation, reduces the active power command and increases the reactive power command to compensate for voltage drops and suppress further power angle shifts, thereby balancing the requirements of fault current limiting and power angle stability, and improving the fault ride-through capability and transient stability of the GFM converter. Thus, the block diagram of the proposed fault ride-through layered control strategy for GFM converters is as follows: Figure 12 As shown.

[0251] It should be noted that the steps shown in the above process or in the flowchart of the accompanying figures can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0252] This application also provides a grid-type converter based on VSG control, which uses the fault ride-through hierarchical control method for grid-type converters described in the above method embodiments for fault ride-through control.

[0253] In addition, combined Figure 12 The fault ride-through hierarchical control method for grid-type converters described in this application embodiment can be implemented by a computer device. The computer device includes a processor and a memory, the memory storing programs or instructions executable on the processor. When the program or instructions are executed by the processor, they implement the steps of the fault ride-through hierarchical control method for grid-type converters described in the above method embodiment.

[0254] Finally, in conjunction with the fault ride-through hierarchical control method for grid-type converters in the above embodiments, this application embodiment can provide a computer-readable storage medium for implementation. This computer-readable storage medium stores computer program instructions; when executed by a processor, these computer program instructions implement any of the fault ride-through hierarchical control methods for grid-type converters in the above embodiments.

[0255] It should be noted that the technical features of the above embodiments can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments have been described. However, as long as the combination of these technical features does not contradict each other, it should be considered within the scope of this specification. In addition, depending on the implementation needs, the various steps / components described in this application can be broken down into more steps / components, or two or more steps / components or parts of steps / components can be combined into new steps / components to achieve the purpose of this invention.

[0256] Those skilled in the art will readily understand that the above-described embodiments merely illustrate several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application.

Claims

1. A hierarchical control method for fault ride-through in a grid-type converter, characterized in that, The method includes a current inner loop dynamic limiting control layer, a power outer loop dynamic compensation control layer, and a state coupling and parameter adaptation module; The current inner loop dynamic limiting control layer introduces a virtual resistor composed of fault current negative feedback into the voltage and current inner loop control of the grid-type converter based on VSG control after the converter enters the fault ride-through state. The size of the virtual resistor is adjusted in real time according to the fault current amplitude to perform virtual resistor dynamic current limiting control. The power outer loop dynamic compensation control layer dynamically adjusts the active power command value and reactive power command value of the grid-type converter based on VSG control during the fault ride-through period according to the degree of grid voltage drop after the converter enters the fault ride-through state. It also performs active-frequency control and reactive-voltage control based on the active power command value and reactive power command value, respectively. The state coupling and parameter adaptation module utilizes power angle deviation feedback to adjust the virtual resistance response speed, thereby optimizing the fault current limiting effect. Simultaneously, it adjusts the power command value to improve transient stability during fault ride-through, including: Calculate the power angle offset : ； In the formula, For power grid line reactance; This refers to the converter terminal voltage after the fault occurs. This refers to the grid voltage after the fault occurred; This refers to the active power command value of the grid-type converter based on VSG control during fault ride-through. The initial command value for active power command; offset of power angle Introducing virtual resistance Dynamic response, then virtual resistance The dynamic response is corrected to: ； In the formula, This is the virtual resistance gain coefficient; This refers to the magnitude of the fault current. Dynamic power compensation coefficient Introducing a power outer loop, the active power command value of the grid-type converter based on VSG control during fault ride-through is... and reactive power command value Revised to: ； In the formula, This refers to the dynamic power compensation coefficient. This is an adaptive gain coefficient used to amplify or suppress the dynamic power compensation coefficient; This is an adaptive adjustment coefficient used to adjust the convergence speed; and These are the active power command value and reactive power command value after adjustment by the dynamic power compensation coefficient, respectively. The final closed-loop control law of the state coupling and parameter adaptive module is as follows: ； Fault ride-through hierarchical coordinated control is performed on grid-type converters based on closed-loop control laws.

2. The fault ride-through hierarchical control method for grid-type converters according to claim 1, characterized in that, The method also includes a fault detection layer; The fault detection layer monitors the grid voltage sag and current overrun in real time to determine whether the converter should enter fault ride-through mode, as detailed below: ； In the formula, This refers to the magnitude of the fault current. This represents the maximum current amplitude. This is the per-unit value of the grid voltage, which is the ratio of the actual grid voltage to the rated grid voltage. When the fault current amplitude exceeds the maximum allowable current amplitude of the grid-type converter based on VSG control, and the grid voltage drops to between 0.2 and 0.9 pu, the converter should enter the fault ride-through state.

3. The fault ride-through hierarchical control method for grid-type converters according to claim 1, characterized in that, The dynamic mathematical model of the virtual resistor is as follows: ； In the formula, For virtual resistance, This is the virtual resistance gain coefficient. Calculated value for virtual resistance; The virtual resistance gain coefficient is adjusted based on the change in the magnitude of the fault current. Adaptive adjustment of virtual resistance gain coefficient The dynamic adjustment control strategy is as follows: ； In the formula, Virtual resistance gain coefficient The value, , ...the virtual resistance gain coefficient under different oscillation levels Numerical parameters, The fault current amplitude, , , ...the magnitude of the fault current Threshold parameter; The larger the fault current amplitude, the greater the virtual resistance gain coefficient. The larger the numerical parameter, the better.

4. The fault ride-through hierarchical control method for grid-type converters according to claim 3, characterized in that, Virtual resistance calculation value The calculation expression is: ； In the formula, This refers to the converter terminal voltage after the fault occurs. This refers to the grid voltage after the fault occurred; and The converter output current after the fault occurs. d-axis and q-axis components in the dq synchronous rotating coordinate system; The phase angle of the converter terminal voltage after the fault occurs; This refers to the resistance of the power grid line.

5. The fault ride-through hierarchical control method for grid-type converters according to claim 1, characterized in that, During fault ride-through, the active power command and reactive power command values ​​for the VSG-controlled grid converter are: ； ； In the formula, and The active power command value and reactive power command value of the grid-type converter based on VSG control during fault ride-through; and These are the initial command values ​​for active power command value and reactive power command value; This is the per-unit value of the grid voltage, which is the ratio of the actual grid voltage to the rated grid voltage. and These are the terminal voltages of the grid-type converter. d-axis and q-axis components in the dq synchronous rotating coordinate system; For grid-side current of grid-type converter q-axis component in the dq synchronous rotating coordinate system; This refers to the rated output current of the converter. The expression for injecting reactive current into the converter is: ； in, The dynamic reactive current coefficient is given when the grid voltage drops to 0.2-0.9 pu. The dynamic reactive current coefficient is the factor for when the grid voltage drops below 0.2 pu.

6. The fault ride-through hierarchical control method for grid-type converters according to any one of claims 1 to 5, characterized in that, Active power droop coefficient in active power-frequency control The dynamic adjustment formula is: ； In the formula, This is the initial value of the active power droop coefficient; Adjust the gain to accommodate the active power droop factor; This is the per-unit value of the grid voltage, which is the ratio of the actual grid voltage to the rated grid voltage. In the power outer loop dynamic compensation control layer, the active power-frequency control equation is: ； The reactive power-voltage control equation is: ； In the formula, The electrical angular frequency of the converter; This is a reference value for the converter voltage amplitude; and These are the initial command values ​​for active power command value and reactive power command value; and The active and reactive power outputs of the converter; This is a reference value for angular frequency; It is the moment of inertia; This is the Laplace transform factor; The damping coefficient; This represents the voltage amplitude at the converter terminals during no-load operation. The integral coefficient of the reactive power-voltage control loop; This refers to the voltage amplitude of the converter. This is the reactive power droop coefficient; For the electrical angular frequency of the converter Integrating the phase reference value of the converter output voltage yields the value. The reference value of the converter voltage amplitude Together, the command values ​​of the d-axis and q-axis components of the terminal voltage are obtained through Park transformation. and ; In the current inner loop dynamic limiting control layer, the voltage and current inner loop control of the grid-type converter based on VSG control includes a voltage outer loop control strategy and a current inner loop control strategy. The voltage outer loop control strategy is as follows: ； The current inner loop control strategy is as follows: ； In the formula, and The d-axis and q-axis components of the converter output current are used as command values ​​for the inner current loop input. and These are the grid-side currents of the converter. d-axis and q-axis components in the dq synchronous rotating coordinate system; and These are the converter terminal voltages. d-axis and q-axis components in the dq synchronous rotating coordinate system; For converter filter capacitors; For virtual resistance; and These are the d-axis and q-axis components of the converter output voltage. and These are the command values ​​for the d-axis and q-axis components of the output current, respectively. For converter filter inductance; , , , This is the transfer function for the controller.

7. A grid-type converter based on VSG control, characterized in that, The converter uses the fault ride-through hierarchical control method for grid-type converters as described in any one of claims 1 to 6 for fault ride-through control.

8. A computer device, characterized in that, include: A processor and a memory, wherein the memory stores a program or instructions that can run on the processor, and when the program or instructions are executed by the processor, implement the steps of the fault ride-through hierarchical control method for grid-type converters as described in any one of claims 1 to 6.

9. A readable storage medium, characterized in that, It stores programs or instructions, which, when executed by a processor, implement the steps of the fault ride-through hierarchical control method for grid-type converters as described in any one of claims 1 to 6.

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