Low-voltage ride-through control method based on reactive power synchronous networking type grid-connected converter

Abstract

The invention provides a low-voltage ride-through control method for a grid-forming type grid-connected converter based on reactive power synchronization, and belongs to the technical field of converter low-voltage ride-through control. When the voltage of a power grid drops, a current amplitude limiting module in the power grid sends a control switching signal to trigger low-voltage ride-through control, and the low-voltage ride-through control is switched to a reactive power synchronization mode; a reactive power synchronous control gain KQs is calculated according to an inner loop gain coefficient Kcp, a current amplitude limiting value, a power grid voltage Vg and a grid side inductance Lgi, a virtual power angle reference value is generated by combining a set reactive power direct current component reference value QN, so that a power angle delta is stabilized in a range close to a normal working point, and a control effect is verified by comparing with a traditional method. According to the method, the system can quickly recover to a normal working state regardless of the duration of the fault and the severity of the fault, meanwhile, the system can effectively work under different power grid fault levels, and the robustness of the system is further enhanced.

Description

Technical Field

[0001] This invention belongs to the field of converter low voltage ride-through control technology, specifically a low voltage ride-through control method for grid-connected converters based on reactive power synchronous grid construction. Background Technology

[0002] In power systems, with the large-scale integration of new energy sources into the grid, grid-connected converters, as key equipment connecting new energy sources to the grid, play a crucial role in the operation of the power system through their performance and control strategies. Traditional grid-connected converters rely on phase-locked loops (PLLs) to synchronize with the grid phase. While this can meet grid connection requirements, it is difficult to effectively support grid voltage and frequency during grid faults, failing to meet the high reliability and stability requirements of modern power systems for grid-connected equipment, thus exhibiting numerous limitations. In recent years, grid-connected converters have received increasing attention as an emerging control method. Grid-connected converters employ self-synchronizing control units, simulating the external characteristics of traditional synchronous generators. Phase self-synchronization is achieved through a power outer loop, automatically adjusting the active or reactive power output of the grid-connected converter to provide frequency and voltage support to the grid, demonstrating good adaptability and flexibility. Among these, droop control is the most widely used control method in grid-connected converters.

[0003] However, traditional low-voltage ride-through control strategies based on active power synchronization droop have significant shortcomings under fault conditions such as grid voltage dips. On the one hand, successful ride-through is highly dependent on the fault duration. When the fault duration exceeds the critical fault clearing time, the system cannot successfully recover to a stable equilibrium point, causing the grid-connected converter to inject unwanted reactive power into the grid, outputting excessive current and resulting in additional transmission losses, severely impacting the stable operation of the power system and the efficient transmission of energy. On the other hand, even if the fault duration is within the critical fault clearing time, the recovery time after the fault is relatively long, which is unacceptable for improving the reliability and power quality of the power system. Furthermore, the robustness of existing control strategies under severe voltage dips is also challenged, making it difficult to guarantee stable system operation under different fault conditions.

[0004] Existing low-voltage ride-through control strategies for grid-connected converters have many performance issues under grid fault conditions, making it difficult to meet the high reliability and stability requirements of modern power systems for grid-connected equipment. A new control strategy is urgently needed to solve these problems, improve the performance of grid-connected converters under grid voltage dip faults, ensure the stable operation of the power system, and facilitate the efficient grid connection of distributed power sources such as new energy sources. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention proposes a low-voltage ride-through control method for grid-connected converters based on reactive power synchronous grid configuration. The core of this method lies in changing the direct correlation between the power angle and active power in traditional active power synchronous control. Instead, it utilizes reactive power to generate a power angle reference, thereby providing a stable equilibrium point for the system during faults, ensuring the power angle remains stable within a range close to the normal operating point. Based on this control method, regardless of the duration or severity of the fault, the system can quickly recover to normal operating conditions, significantly improving the stability and reliability of the grid-connected converter under grid voltage dip faults. Furthermore, this invention includes detailed parameter design for this control method, ensuring the stability and rapid response capability of the control system, enabling it to operate effectively under various grid fault conditions, further enhancing the system's robustness.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0007] A low-voltage ride-through control method for grid-connected converters with reactive power synchronous grid configuration includes the following steps:

[0008] Step 1: When the grid voltage drops, trigger the low voltage ride-through control to switch to reactive power synchronization mode. The trigger signal for the control switch comes from the current limiting module in the grid.

[0009] Step 2: Based on the inner loop gain coefficient K cp Current limit value, grid voltage V g and grid-side inductor L gi The control gain K for reactive power synchronization is calculated. Qs ;

[0010] Step 3: Combine with the set reference value Q for the DC component of reactive power N Generate virtual power angle reference value To keep the power angle δ stable within a range close to the normal operating point;

[0011] Step 4: Set the SCR of the grid-connected converter system under weak grid conditions to 1.5 and 3, and conduct comparative verification using the traditional low voltage ride-through control method and the low voltage ride-through control method based on reactive power synchronization, respectively.

[0012] Furthermore, the formula for calculating the reactive power synchronization control bandwidth is as follows:

[0013]

[0014] Among them, V n The voltage rating is s, the apparent power is K. Qs This is the reactive power synchronous proportional gain.

[0015] Furthermore, the reactive power synchronization control bandwidth ω Q(s) Designed for current-controlled bandwidth ω c of Therefore, the control gain K for reactive power synchronization Qs for:

[0016]

[0017] Among them, L gi For the grid-side inductor, K cp This is the inner loop gain coefficient.

[0018] Furthermore, the reactive power expression in the low voltage ride-through control mode is:

[0019]

[0020] Among them, V g X is the grid voltage vector. g This is the equivalent impedance of the power grid.

[0021] Furthermore, the reference value Q for the DC component of reactive power N The calculation formula is:

[0022]

[0023] Among them, X gm =ω n L gm It is the measured power grid impedance, L gm For the measured filter inductance, ω n I serves as the reference value for the grid converter and the grid angular frequency. max It is the maximum current allowed to pass through a grid-connected converter.

[0024] Furthermore, the virtual power angle for low-voltage ride-through control based on reactive power synchronization is:

[0025]

[0026] Among them, K Qs For reactive power synchronization proportional gain, P and Q are the active power and reactive power output of the grid-connected converter, respectively. N , Q N These are the rated active power and reactive power of the grid-connected converter, respectively, K p This is the active power droop factor.

[0027] Furthermore, during a fault, the DC component Q of the reactive power output Q... N Independent of the fault voltage amplitude, determined solely by the grid impedance X gm and maximum current limit value I maxDecide.

[0028] Furthermore, after the fault is cleared, the system automatically exits low-voltage ride-through control and resumes normal active power droop control, with the recovery time being only the power outer loop response time t. ps .

[0029] Furthermore, a d-axis priority current limiter is used to limit the fault current.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] 1) Eliminates the dependence of traditional active power synchronization control on critical fault clearing time, and can maintain stable operation regardless of the duration of the fault;

[0032] 2) The recovery time is only the power outer loop response time, which is more than 5.5 times shorter than the traditional method;

[0033] 3) Change the direct correlation between power angle and active power in traditional active power synchronization control during low voltage ride-through, and instead use reactive power to generate power angle reference. Establish a stable equilibrium point at δ=0 through reactive power synchronization, and avoid the risk of system instability caused by power angle divergence in traditional control.

[0034] 4) During a fault, the system automatically matches the grid impedance characteristics and outputs the set reactive power value to avoid overcompensation;

[0035] 5) It can still stabilize quickly even under deep voltage drops, and its robustness is better than traditional solutions;

[0036] 6) By using a bandwidth-tiered design, control loop decoupling is ensured, reducing debugging complexity;

[0037] 7) Applicable to existing grid-type converter architectures, requiring only modification of the control algorithm without the need for additional hardware;

[0038] 8) Provide continuous active power support during faults to enhance power supply reliability during grid fault ride-through;

[0039] 9) Reduce transmission losses caused by faults and improve energy efficiency. Attached Figure Description

[0040] Figure 1 This is a flowchart of the low-voltage ride-through control method for a grid-connected converter based on reactive power synchronous grid configuration, as described in an embodiment of the present invention.

[0041] Figure 2 This is a topology diagram of a grid-connected three-phase two-level grid-connected converter system according to an embodiment of the present invention;

[0042] Figure 3 This is a block diagram illustrating the control principle of droop control in an embodiment of the present invention.

[0043] Figure 4 Block diagram of the droop control voltage and current dual closed-loop control principle;

[0044] Figure 5 This is a block diagram illustrating the topology and control principle of a grid-connected converter system under voltage sag according to an embodiment of the present invention.

[0045] Figure 6 This describes the principle of the d-axis priority current limiter in an embodiment of the present invention.

[0046] Figure 7 This is a simplified form of the grid-connected converter system based on droop control in this embodiment of the invention;

[0047] Figure 8 This is a vector diagram of a grid-connected converter system based on droop control according to an embodiment of the present invention.

[0048] Figure 9 This is a simplified form of a grid-connected converter system considering the influence of the current limiter in an embodiment of the present invention;

[0049] Figure 10 Vector diagram of a grid-connected converter system considering the influence of the current limiter in an embodiment of the present invention;

[0050] Figure 11 This invention relates to the stable equilibrium point working mechanism of the grid-connected converter under voltage drop conditions in an embodiment of the present invention.

[0051] Figure 12 This invention relates to the active power synchronization successful LVRT working mechanism of the grid-connected converter under voltage drop conditions in an embodiment of the present invention.

[0052] Figure 13 This invention relates to the active power synchronization failure LVRT working mechanism of the grid-connected converter under voltage drop in an embodiment of the present invention.

[0053] Figure 14 This is the active power synchronization LVRT working mechanism of the grid-connected converter under voltage drop conditions in an embodiment of the present invention;

[0054] Figure 15 This is a normal droop control and reactive power synchronization LVRT control model for embodiments of the present invention;

[0055] Figure 16 This is a diagram illustrating the working mechanism of low voltage ride-through stability control under reactive power synchronous control according to an embodiment of the present invention.

[0056] Figure 17 Simulation of a grid-connected converter with active power synchronization and reactive power synchronization droop control according to an embodiment of the present invention;

[0057] Figure 18This is a simulation diagram of the power grid voltage in an embodiment of the present invention (Group A);

[0058] Figure 19 This is a simulation diagram of the grid connection point voltage (Group A) according to an embodiment of the present invention;

[0059] Figure 20 This is a simulation diagram of the grid connection point current (Group A) according to an embodiment of the present invention;

[0060] Figure 21 This is a simulation diagram of the active power response of an embodiment of the present invention (Group A);

[0061] Figure 22 This is a simulation diagram of the reactive power response of an embodiment of the present invention (Group A);

[0062] Figure 23 This is a simulation diagram of the power grid voltage in an embodiment of the present invention (Group B);

[0063] Figure 24 This is a simulation diagram of the grid connection point voltage (Group B) according to an embodiment of the present invention;

[0064] Figure 25 This is a simulation diagram of the grid connection point current (Group B) according to an embodiment of the present invention;

[0065] Figure 26 This is a simulation diagram of the active power response of an embodiment of the present invention (Group B);

[0066] Figure 27 This is a simulation diagram of the reactive power response of an embodiment of the present invention (Group B);

[0067] Figure 28 This is a voltage simulation diagram of low-voltage ride-through of the power grid based on reactive power synchronization, according to an embodiment of the present invention.

[0068] Figure 29 This is a simulation diagram of the low voltage ride-through grid connection point voltage based on reactive power synchronization, according to an embodiment of the present invention.

[0069] Figure 30 This is a simulation diagram of the low-voltage ride-through grid connection point current based on reactive power synchronization in an embodiment of the present invention;

[0070] Figure 31 This is a simulation diagram of the low-voltage ride-through active power response based on reactive power synchronization when the grid-connected converter provides 5kW of active power to the power grid according to an embodiment of the present invention.

[0071] Figure 32 This is a simulation diagram of the low-voltage ride-through active power response based on reactive power synchronization when the grid-connected converter provides 3.1 kvar of reactive power to the power grid according to an embodiment of the present invention.

[0072] Figure 33 This is an amplified waveform diagram of the low-voltage ride-through grid connection point current based on reactive power synchronization in an embodiment of the present invention.

[0073] Figure 34 This is a simulation diagram of low voltage ride-through of the power grid based on reactive power synchronization, where the voltage drops by 0.8 pu according to an embodiment of the present invention.

[0074] Figure 35 This is a simulation diagram of the low voltage ride-through grid connection point voltage based on reactive power synchronization with a drop of 0.8 pu in an embodiment of the present invention.

[0075] Figure 36 This is a simulation diagram of the low-voltage ride-through grid connection point current based on reactive power synchronization with a drop of 0.8 pu in an embodiment of the present invention.

[0076] Figure 37 This is a simulation diagram of the low-voltage ride-through active power response based on reactive power synchronization with a voltage drop of 0.8 pu according to an embodiment of the present invention.

[0077] Figure 38 This is a simulation diagram of the low-voltage ride-through reactive power response based on reactive power synchronization under a voltage drop of 0.8 pu according to an embodiment of the present invention. Detailed Implementation

[0078] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. 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.

[0079] Example

[0080] See Figures 1-38 The low-voltage ride-through control method for grid-connected converters with synchronous reactive power grid connection is as follows:

[0081] 1. Topology and control strategy of grid-connected converter under normal operating conditions

[0082] Grid-connected converters employ self-synchronizing control units, using a power outer loop for phase and voltage self-synchronization to regulate active and reactive power. Droop control is the most common control method in grid-connected converters. This control strategy simulates the primary frequency and voltage regulation characteristics of a synchronous generator, automatically adjusting the active or reactive power output of the grid-connected converter to provide frequency and voltage support to the grid when grid voltage or frequency fluctuates. A typical droop-controlled grid-connected converter structure is shown below. Figure 2 As shown. Where, v d 、v q 、i d and i qLet V be the output voltage and output current of the grid-connected converter in the dq synchronous rotating coordinate system, respectively; let P and Q be the active power and reactive power output of the grid-connected converter, respectively; and let v be the output voltage and output current of the grid-connected converter. ref ω is the reference value for the output voltage. ref The frequency reference value is used. Droop control obtains the actual voltage and current at the grid connection point through sampling, calculates the active and reactive power of the grid-connected converter through the power calculation module, and obtains the setpoint values ​​for the output voltage and phase (i.e., frequency integral) of the grid-connected converter through the droop control module. The voltage loop calculates the difference between the output voltage and the reference voltage, and obtains the reference current through the PI controller; the current loop calculates the difference between the inductor current and the reference current, and obtains the voltage through the PI controller, which is then input to the PWM generator to control the switching of the power switching transistors.

[0083] In the power calculation module, the three-phase instantaneous active power is first calculated using the voltage and current of the grid-connected converter in the dq coordinate system. and instantaneous reactive power for:

[0084]

[0085] Among them, v od and v oq Let i represent the d-axis and q-axis components of the voltage in the dq coordinate system, respectively. od and i oq These represent the d-axis and q-axis components of the current in the dq coordinate system, respectively.

[0086] Instantaneous power and After passing through a low-pass filter, the average power P and Q can be obtained, satisfying:

[0087]

[0088] in ω is the low-pass filter time constant. 1ow This is the cutoff frequency of the low-pass filter.

[0089] Figure 3 This is a block diagram of the droop control module, P N , Q N These are the rated active power and reactive power of the power grid, ω N and v N K represents the rated angular frequency and the rated voltage of the converter. p and K q These are the active power droop factor and the reactive power droop factor, respectively; the reference values ​​for voltage and frequency calculated using the droop equation are as follows:

[0090]

[0091] According to equation (3), when the power grid fluctuates, due to the droop control effect, the output active power P and reactive power Q of the grid-connected converter change accordingly, and they differ from their respective given values ​​P. N and Q N The power deviation, after being adjusted by the droop coefficient, produces frequency and voltage changes. These changes are related to the rated frequency ω of the grid-connected converter. N and rated voltage v N The summation yields the actual output voltage reference value v and frequency reference value ω of the grid-connected converter. The frequency reference value, after integration, provides the actual phase reference θ of the output voltage. Combining the phase reference θ and the amplitude reference v yields the output voltage vector v* of the grid-connected converter. To better limit current, reduce voltage ripple, and achieve optimized control, the grid-connected converter employs a dual closed-loop control structure for voltage and current, such as... Figure 4 As shown.

[0092] Figure 5 The topology and control principle of a grid-connected converter system based on droop control under voltage dip conditions are presented. The control section consists of a power calculation loop, an outer voltage control loop, current limiting, and an inner current control loop. The power calculation loop is divided into active and reactive power components, each employing droop control to generate the virtual power angle δ and voltage reference v* of the grid-connected converter. The droop coefficients for the active and reactive power components are k and k, respectively. p and k q The d-axis reference value of the outer voltage loop is given by the output v* of the reactive power component, and the q-axis voltage reference value is 0. and These are the d-axis and q-axis current reference components of the voltage outer loop output, respectively; i dref and i qref These are the d-axis and q-axis components after passing through the current limiter, representing the actual reference values ​​for the inner current loop; v d and v q These represent the d-axis and q-axis voltage components output from the current loop, respectively. The limiting module employs the widely used and simple d-axis priority current limiter method, and its current limiting algorithm expression is:

[0093]

[0094] Among them, I max This is the maximum current amplitude allowed to pass through a grid-connected converter. For example... Figure 6 As shown, when When the reference value of the current loop is equal to the output value of the outer voltage loop; when At that time, the d-axis component of the current loop reference value is I. max The q-axis component is 0. To prevent voltage outer loop saturation, the integral parameter k of the PI controller in the voltage outer loop is...i Set to 0 in rate limiting mode.

[0095] 2. Transient stability analysis of grid-connected converters under voltage dips

[0096] 1) Transient model of grid-connected converter under voltage sag

[0097] Grid-connected converters construct their own voltage and frequency by simulating generator characteristics, thus they can be considered equivalent to a controlled voltage source. Figure 7 A simplified circuit for a grid-connected converter based on droop control is presented. Where Z... g =R g +jX g X g =ω n L g All are equivalent impedances of the power grid. The power grid voltage vector V... g For reference, the voltage vector V output of the grid-connected converter is defined. f With grid voltage vector V g The phase difference between them, i.e., the virtual power angle δ, is:

[0098] δ=θ ref -θ=∫(ω0-ω g )dt (5)

[0099] In the formula, ω0 and ω g These are the grid converter and the grid angular frequency, respectively, with a reference value of ω. n Based on this, we can obtain Figure 8 The vector diagram shown.

[0100] Due to the voltage source characteristics of grid-connected converters, the subscript "fault" represents the physical quantity at which a fault occurs. At the instant a fault occurs in the power grid, the voltage V at the grid connection point... f The voltage amplitude V remains unchanged, while the grid voltage amplitude V g The sudden drop caused the grid connection point current I to... g A sudden surge occurs. Therefore, under grid voltage dip faults, grid-connected converters will inject extremely large current values ​​to maintain their output voltage. Although this voltage source characteristic is beneficial for maintaining voltage stability in the grid-connected system, current limiting measures must be taken for grid-connected converters because the overcurrent capability of the power semiconductor devices in the grid-connected converter is very limited.

[0101] When a fault occurs, the grid-connected converter enters low-voltage ride-through control to limit the fault current. In low-voltage ride-through control, the introduction of a current limiter can equate the grid-connected converter to a current-controlled voltage source. Therefore, a simplified circuit for the grid-connected converter considering the current limiting effect is as follows: Figure 9 As shown in the figure. When When the current is d, the limiter can be called a d-axis current-priority limiter; when At this time, the limiter is also called a q-axis current-priority limiter. This patent uses a d-axis current-priority limiter, therefore... When the grid voltage drops and enters the low voltage ride-through control triggering current limiting, the output current I of the grid-connected converter is... max Controlled to phase reference θ ref Or d-axis alignment, such as Figure 10 As shown.

[0102] According to the definition of three-phase complex power, the active and reactive power outputs of a grid-connected converter based on droop control under normal operating conditions can be expressed as:

[0103]

[0104] Further analysis reveals:

[0105]

[0106] in, Let be the power grid impedance angle, and The expression is:

[0107]

[0108] Further simplification of equation (7) yields the following expressions for the output active and reactive power of a grid-connected converter under normal operating conditions:

[0109]

[0110] Similarly, when a grid-connected converter is under low-voltage ride-through control, according to Figure 9 and Figure 10 Its apparent power, active power, and reactive power output can be calculated as follows:

[0111]

[0112] 2) Transient stability analysis of low voltage ride-through in traditional active power synchronization

[0113] Based on the control principle of electromechanical coupling characteristics of synchronous generator sets, the stability analysis of grid-connected converter systems can follow the traditional power grid phase angle stability criterion. By monitoring the transient process of the power angle δ during equipment operation, the transient stability of the grid-connected converter under severe power grid fault conditions can be effectively evaluated. When a sudden system fault occurs, if δ can return to the initial equilibrium point after a period of dynamic adjustment, or establish a new stable operating point within the periodic phase range, specifically manifested as follows: Within the specified range, this indicates that the grid-connected converter system exhibits good transient stability. Conversely, when the power angle δ shows a continuous divergence trend, it will trigger continuous fluctuations in the electrical quantities at the grid-connected converter ports, ultimately leading to system instability. This instability manifests specifically as periodic oscillations in power transmission, increased voltage waveform distortion, and abnormal fluctuations in current amplitude, causing the entire grid-connected system to lose controllability. This analysis method based on the dynamic characteristics of the power angle provides a theoretical basis for the equivalence study of grid-connected converters and traditional rotating units in terms of stability. This patent study investigates the transient behavior of a grid-connected converter with basic active power synchronous droop under low-voltage ride-through control conditions during voltage dips.

[0114] For ease of analysis, we assume that the grid impedance is predominantly inductive, i.e., X g >>R g The power grid frequency is fixed. Therefore, there exists and Sag control under normal operating conditions, V g and V f Assumed to be the nominal value V n When a power grid fault causes a voltage drop, the magnitude of the grid voltage decreases from V. fault This means that the following conditions are met:

[0115]

[0116] in, The derivative of the virtual power angle δ is determined by the droop equation. The power angle of a traditional synchronous generator is defined as the internal electromotive force. With grid voltage The angle between them reflects the magnitude of the synchronous generator's output power. Under normal operating conditions (steady-state conditions), such as... Figure 8 As shown, the output voltage of the grid-connected converter Along the d-axis, the virtual power angle δ is equal to the actual power angle of the grid-connected converter. However, due to the dynamic process of the voltage outer loop, they do not remain equal during transient processes. Especially after entering the low-voltage ride-through control trigger current limiting stage, the voltage outer loop saturates and loses its function, such as... Figure 10 As shown, the virtual power angle δ of the grid-connected converter is much smaller than the actual power angle. If the fault has been cleared at this time, the system will not exit the current limiting control and will continue to operate under low voltage ride-through control.

[0117] According to equations (9) and (10), the active power output P of the grid-connected converter in different modes is calculated as follows:

[0118]

[0119] In Equation (12), Condition ① represents the normal operating condition, i.e., the operating condition before a fault occurs and after the fault is cleared and the low-voltage ride-through control is exited; Condition ② represents the condition after the fault has been cleared but before the low-voltage ride-through control is exited; Condition ③ represents the condition during the low-voltage ride-through control when a fault occurs.

[0120] As can be seen from Equation (11), when the reference power P N > P, will increase, and when P N < P, will decrease. The point where P N = P is the point, corresponding to the equilibrium operating point of the system. Among the equilibrium points, the point with negative feedback is the stable equilibrium point. If after a fault, the system trajectory can converge to the stable equilibrium operating point, it indicates that the grid-forming grid-connected converter can ensure transient stability. Using this relationship, the operating mechanism under voltage dip is as Figure 12 shown, and the virtual power angle curve obtained according to Equation (12) is plotted. Under the droop control in normal conditions, the P and virtual power angle curves follow the pre-fault curve, i.e., follow the Figure 11 curve of Condition ① in Figure 11 . When the grid voltage drops and enters the low-voltage ride-through control, the relationship between P and the virtual power angle curve will follow the fault curve, i.e., follow the curve of Condition ③. When the grid voltage recovers, but the grid-forming grid-connected converter is still operating in the low-voltage ride-through control mode, the relationship between P and the virtual power angle curve will follow the post-fault Condition ② curve. When the grid-forming grid-connected converter exits the low-voltage ride-through control after the fault is cleared, the curve follows the pre-fault relationship between P and the virtual power angle, i.e., follows the curve of Condition ①. In when the system operates at point a before a fault occurs, if the system encounters an instantaneous small disturbance at this time, resulting in a small increment Δδ in the power angle δ, the operating point moves from the original point a to point a1. As can be seen from Equation (11) N the actual active power P output by the grid-connected converter is greater than the reference power P the actual active power P output by the grid-connected converter is less than the reference power P N, δ will increase, and the operating point will return to point a again. It can be analyzed that when the grid-forming grid-connected converter operates at point a, it has the ability to self-recover to the original equilibrium state after encountering an instantaneous small disturbance. Therefore, point a is the stable equilibrium point of the pre-fault system. According to the above analysis, the operating situation of point f is opposite. No matter whether the small disturbance causes a slight increase or decrease of Δδ in δ, the operating point will not return to point f again. When δ increases by Δδ, the operating point moves from the original f to f1, At this time, δ will continue to increase; when δ decreases by Δδ, the operating point moves from the original f to f2, The operating point will move along operating condition ① towards point a and finally reach point a and remain at a new equilibrium point. Therefore, point f is an unstable equilibrium operating point. It can be seen from the analysis that before the grid voltage drops, that is, under normal operating conditions, the grid-forming grid-connected converter operates stably at point a, and the power angle is δ0.

[0121] First, according to Figure 11 Analyze the low-voltage ride-through process that can recover to the stable equilibrium point, as Figure 12 shown. When the grid voltage V g suddenly drops from V n to V fault , the grid-forming grid-connected converter enters the low-voltage ride-through control, and the current limit module is immediately triggered, but δ will not change suddenly. Therefore, the operating point moves from Figure 12 point a in to point b, that is, from the pre-fault operating condition ① curve to the fault operating condition ③ curve, as Figure 12 shown by the dashed line in. Because at the operating point b, P N >P, so the power angle δ continuously increases along the fault operating condition ③ curve. At point c, the fault is cleared, and when the voltage V g recovers from V fault to V n , the operating point also jumps from point c to point d and enters the post-fault operating condition ② curve. At point d, since P N <P, therefore δ decreases and moves along the post-fault operating condition ② curve until it reaches point e, which is the intersection of the operating condition ① curve under normal control after the fault and the operating condition ② curve still working under low-voltage ride-through control after the fault. After point e, the low-voltage ride-through control switches to the control under normal operating conditions, the current limit module is deactivated, and the operating point moves along the operating condition ① curve under normal control after the fault is cleared. Because at point e, P N <P, δ decreases from point e until point a, where P N =P, and point a is the stable equilibrium point under normal control. The above analyzes a successful low-voltage ride-through process.

[0122] An unsuccessful low-voltage ride-through process is as Figure 13 shown. When the grid voltage Vg Falling to V fault When it does, the operating point will move from point a to point b. At the operating point b, P N >P, so the power angle δ will continuously increase along the curve of operating condition ③ during the fault until V g returns to the state c' point, the fault is cleared, and the voltage V g from V fault returns to V n , the operating point jumps from c' to d'. However, at the d' point, P N >P, so δ will continue to increase and move along the curve of operating condition ② after the fault until the e' point. For the balanced operating point e', it operates on the virtual power angle curve that is still under low-voltage ride-through control after the fault, rather than on the virtual power angle curve under normal control. From Equation (12), it can be seen that at this time, the grid-forming grid-connected converter operates with the maximum limit value of the current. If the grid-connected converter continuously operates with the maximum limit current under normal grid conditions, such as at the e' point, it will generate undesirable transmission losses and reactive power injection. Therefore, the e' point is not the desired balanced operating point of the grid-connected converter.

[0123] At the moment when the grid voltage V g returns to V n , δ is expressed as δ vr . As Figure 14 shown, when δ vr <δ cc , after the grid fault is cleared, the operating point jumps from the curve of operating condition ③ during the fault to the curve of operating condition ② after the fault. At this time, due to P N <P, δ will decrease and move along the curve of operating condition ② after the fault, and the operating point can finally return to the balanced stable operating point a. When δ vr >δ cc , after the fault is cleared, the operating point jumps from the curve of operating condition ③ during the fault to the curve of operating condition ② after the fault. Due to P N >P, δ continues to increase and cannot return to the original stable balanced operating point. Based on the above analysis, to ensure the success of the low-voltage ride-through process of the grid-forming grid-connected converter, it is necessary to satisfy δ vr <δ cc . δ cc is defined as the critical fault clearing angle (CCA), that is, the angle at the intersection of P N and the curve of operating condition ② after the fault. δ cc can be calculated as:

[0124]

[0125] Define the grid fault voltage V fault from δ0 to δ ccTime t cc The critical fault clearing time (CCT) is t. cc It can be calculated by solving a differential equation, as follows:

[0126]

[0127] The final solution is:

[0128]

[0129] The parameters satisfy:

[0130]

[0131] exist Figure 14 In the middle, when δ vr <δ cc When a fault is cleared and the system can stably recover, there are two types of situations. The δ value at the intersection of the curve for condition ① under normal control after the fault and the curve for condition ③ under LVRT control after the fault is denoted as δ. vci When δ vr <δ vci At this time, the grid-connected converter can directly return to the operating condition ① curve under normal control after the fault. Where (0, δ vci The range of () can be considered to be within the linearized range of the operating condition ① curve under normal control after a fault. Therefore, the recovery time after a fault is the same as the response time of the active power control loop of the grid-connected converter, denoted as t. ps However, if δ vci <δ vr <δ cc Firstly, the grid-connected converter continues to operate under LVRT control under condition ③ curve after a fault, until δ recovers to the range under normal control. The control mode recovery time, i.e. Figure 14 The time taken for d→e is represented by t. mr After the fault, if the system remains under LVRT control condition ③, the power angle equation is:

[0132]

[0133] from Figure 14 From this, we can know that δ vr The closer to δ cc , t mr The longer the duration, the longer the recovery process will take (t). ps +t mr In summary, the success of low-voltage ride-through control based on traditional active power synchronization requires that δ < δ during grid fault recovery.cc To ensure the critical fault clearing angle, the fault duration should be less than the critical fault clearing time t. cc . t cc It also depends on the fault level; the more severe the grid fault, the higher the requirement for t cc The shorter the duration, the longer the fault duration and the more severe the voltage drop, which may lead to a longer recovery time after the fault, or even cause the low-voltage ride-through based on active power synchronization control to fail.

[0134] 3. Low-voltage ride-through stability control strategy based on reactive power synchronization

[0135] 1) Low voltage ride-through stability control based on reactive power synchronization

[0136] The above analysis shows that the success of low-voltage ride-through control based on traditional active power synchronization is highly dependent on the duration of the fault, because there may be no stable equilibrium point during the fault period. Therefore, δ will deviate from the value of the normal steady-state operating point. Even if the fault duration is within the critical fault clearing time t... cc Internally, recovery after a fault also takes a long time. Although droop control based on active power synchronization can change the active power reference, it is difficult to determine the fault level and active power reference value from local information under fault conditions. In addition, according to equation (12), under normal operating conditions, the active power P and the virtual power angle δ are in... Within a certain range, P and δ are positively correlated. However, for grid voltage dip faults, under low-voltage ride-through control based on active power synchronization, P and δ are positively correlated within the range of -π < δ < 0. Under normal operating conditions and under low-voltage ride-through control based on active power synchronization, the ranges in which active power P and virtual power angle δ have positive feedback do not overlap. This means that even if an equilibrium point exists under low-voltage ride-through control based on active power synchronization, δ may still be far from the normal operating point, making post-fault recovery longer and more difficult. To address these issues, a synchronization method needs to be found that ensures a stable equilibrium point around δ during normal operation during faults.

[0137] As shown in equation (10), under low voltage ride-through control, the reactive power Q becomes the sum of the DC component and a trigonometric function about δ. The reactive power in... The reactive power output decreases monotonically within the range of δ, overlapping with the monotonic interval of active power synchronous control under normal operating conditions. Furthermore, when δ = 0, the reactive power output equals the DC component. δ = 0 is within the monotonic region. The intermediate position, close to the normal operating point, is an operating point that facilitates rapid fault recovery. Therefore, in order to make the grid-connected converter operate near δ=0, this embodiment selects the reactive power generated from the reactive power under low voltage ride-through control during a fault to generate the power angle reference, and sets the Q reference to the DC component:

[0138]

[0139] Among them, X gm = ω n L gm is the measured grid impedance. If it is assumed that X gm = X g . This DC component is only related to the grid impedance value and the maximum current limit, and is independent of the amplitude of the fault grid voltage. Therefore, the fault level does not need to be considered based on the synchronous reference value of reactive power.

[0140] The low voltage ride-through controller based on reactive power synchronization is as Figure 15 shown. Under normal condition control, the power angle controller adopts normal active power synchronous droop control. When the grid voltage suddenly drops, the low voltage ride-through control is triggered, and the reactive power synchronous control will also be triggered, where K Qs is the proportional gain of reactive power synchronization, and the trigger signal for control switching comes from the current limiting module.

[0141] When the low voltage ride-through control and the reactive power synchronous control are triggered, it satisfies:

[0142]

[0143] According to Equation (19), if Q N > Q, then δ will decrease; when Q N < Q, then δ will increase. The point where Q N = Q, that is, δ = 0 is the equilibrium point of δ under low voltage ride-through control. To sum up, the virtual power angle of the low voltage ride-through control based on reactive power synchronization can be determined as:

[0144]

[0145] Among them, condition ① is the normal condition droop control, and condition ③ is after triggering the low voltage ride-through control when the voltage drops. The expressions of P and Q under different condition modes are described as:

[0146]

[0147] 2) Analysis of the low voltage ride-through stable control mechanism

[0148] Because the virtual power angle δ synchronization mechanism is respectively related to the control of the normal condition and the P and Q of the low voltage ride-through control after the fault, therefore, according to Equations (21) and (22), the curves of the active power synchronization and the reactive power synchronization control are plotted, as Figure 16 shown. According to at V g = V nThe operating conditions under normal operating conditions and the operating conditions after a fault are analyzed. The dynamic trend of the virtual power angle is determined using the active power synchronous control curve. When V occurs... g =V fault In the case of voltage drop fault conditions, the virtual power angle is determined by referring to the reactive power angle curve based on the low voltage ride-through of reactive power synchronous control.

[0149] The low-voltage ride-through process using a reactive power synchronous control strategy follows Figure 16 The sequence shown is a→b→c→d→a. First, at the equilibrium operating point a, P... N =P, as can be seen from equation (20) Grid-connected converters operate at their steady-state equilibrium point under normal operating conditions with synchronized active power. When a voltage dip occurs in the grid and low-voltage ride-through control is triggered... Figure 16 The reactive power synchronization control section of the virtual power angle control is triggered. Since the virtual power angle δ does not change suddenly, the operating point state will jump from point a to point b, that is, from the pre-fault operating condition ① curve to the fault operating condition ③ curve. At the balanced operating point b, when Q... N When Q >, it can be seen from equation (20) that δ will decrease along the reactive power angle curve ③ during the fault until it reaches the equilibrium point c, satisfying Q. N =Q, as can be seen from equation (20) Therefore, point c is the stable equilibrium point based on the reactive power synchronous low voltage ride-through control condition ③ curve. In this case, regardless of the duration of the fault, the grid-connected converter will always operate at the steady-state equilibrium point c. When the grid voltage recovers, the low voltage ride-through control automatically deactivates, and the grid-connected converter immediately returns to normal droop control. At this time, the operating state will jump from point c to point d, that is, from the state of the fault low voltage ride-through control condition ③ curve to the state of the normal droop control condition ① curve. Then, the virtual power angle δ is determined by the active power P. According to equation (20), the operating point will move along the active power angle normal control condition ① curve until P... N =P, reaching the original stable equilibrium operating point a. Furthermore, since (0, δ0) is within the linearization range of active power control, the time taken for d→a is only t. ps .

[0150] In summary, the above analysis shows that during a grid voltage dip fault, the low-voltage ride-through stability control strategy based on reactive power synchronization always maintains a steady-state equilibrium point. Therefore, the success of low-voltage ride-through is unaffected by the fault level or duration. Furthermore, fault recovery based on the reactive power synchronization low-voltage ride-through control strategy only requires t... ps .

[0151] 3) Stable controller design

[0152] The parameter design for reactive power synchronous control will be introduced below. According to equation (10), the reactive power expression in the low voltage ride-through control mode can be obtained as follows:

[0153]

[0154] Assume the steady-state operating point is δ0, and the corresponding reactive power is Q0. When there is a small disturbance Δδ, the new δ is δ0 + δ, and the corresponding reactive power Q is Q0 + ΔQ. After linearization, ΔQ can be approximated as:

[0155]

[0156] According to equation (23), taking the partial derivative with respect to δ, we can obtain:

[0157]

[0158] Therefore, the small-signal model of reactive power Q is:

[0159]

[0160] Based on the above analysis, the steady-state operating point under low-voltage ride-through control and reactive power synchronization control modes is δ0 = 0, therefore cosδ = 1. After simplification, the small-signal model of reactive power Q can be obtained as follows:

[0161]

[0162] Where ^ represents a small disturbance. According to Figure 15 The reactive power synchronous control model shown below, when the limiting signal is 1, the open-loop transfer function becomes:

[0163]

[0164] Therefore, the reactive power synchronous control bandwidth can be calculated as follows:

[0165]

[0166] Under normal control and fault conditions, the desired virtual power angle for a grid-connected converter is δ = 0. The goal of the inner current loop control is to respond quickly to changes in the current reference value and reduce current errors; therefore, a larger bandwidth is required. The outer power loop control generally uses a lower bandwidth to ensure smooth regulation and avoid over-response to high-frequency changes. Therefore, to ensure the normal operation of each control element and reduce mutual interference, the power angle control bandwidth ω... Q(s) Designed for current-controlled bandwidth ω c of

[0167] In power electronic converters, the current control loop typically uses a proportional-integral (PI) controller to regulate the output current for fast response. The grid-side inductor L... gi It is an important parameter in the circuit, affecting the rate of change of current. Inner loop gain coefficient K cp This is the proportional gain component of the controller, which determines the controller's response speed to current errors. Therefore, the current control bandwidth ω... c It can be approximated as:

[0168]

[0169] According to equation (29), the bandwidth ω of reactive power synchronization is... Q(s) At different fault voltages V n The voltage fluctuates, and higher grid voltages result in higher voltage control bandwidth. To ensure that the control bandwidth ω remains constant under different grid fault levels... c The control bandwidth ω is always greater than that of reactive power synchronization. Q(s) More than ten times that of reactive power synchronization. Therefore, the control gain K of reactive power synchronization... Qs Designed as follows:

[0170]

[0171] 4. Simulation Testing

[0172] To verify the low-voltage ride-through control method for grid-connected converters based on reactive power synchronization proposed in this patent, and to compare it with the traditional low-voltage ride-through control method based on active power synchronization, according to... Figure 2 The topology shown was constructed using Matlab-simulink. Figure 17 The simulation model of the grid-connected converter is shown. The main simulation parameters are shown in Table 1.

[0173] Table 1 Simulation parameters of the grid-connected converter model

[0174]

[0175]

[0176] 1) Stability analysis and simulation verification of low voltage ride-through control based on active power synchronization

[0177] To verify that the success of traditional active synchronous low voltage ride-through control is affected by the fault clearance time, the simulation parameters are first substituted into equation (15) to obtain the fault clearance time t. cc =212ms, therefore, two sets of comparative simulation experiments, A and B, were set up to verify the impact of fault limit clearing time on traditional active synchronous low voltage ride-through control. The parameters of the two sets are shown in Table 2.

[0178] Table 2. Two sets of simulation parameters based on the active synchronous low voltage ride-through system.

[0179]

[0180] When parameters from groups A and B are applied respectively, a voltage drop occurs on the grid side, causing a voltage drop of 0.5 pu. Group A sets a symmetrical voltage drop at t=0.2s and clears the drop fault at t=0.4s; Group B sets a symmetrical voltage drop at t=0.2s and clears the drop fault at t=0.6s.

[0181] Figures 18 to 22 The grid voltage v obtained from the simulation of parameters A is given. g Grid-connected converter output voltage V abc Grid-connected converter output current i abc And the response curves of active power P and reactive power Q.

[0182] Simulation results from Group A parameters show that when the grid voltage suddenly drops from 311V to 155V, the output current of the grid-connected converter is limited to 34A. When the grid voltage recovers to 311V after 200ms, the grid-connected converter still operates under low-voltage ride-through control, i.e., post-fault condition ②. Since the fault duration of 200ms is less than the critical fault clearing time t... cc After the grid voltage recovers, the actual active power is still greater than the active power reference value. Therefore, the virtual power angle δ will decrease, and both active and reactive power will continue to increase. This process is... Figure 12 During the d→e phase, this phase lasts until 1.2 seconds after which the grid-connected converter exits low-voltage ride-through control. Afterwards, the operating point follows... Figure 21 In condition ① of the operation, the active power continues to decrease until it recovers to the reference value at 1.5s. The corresponding stage is... Figure 12 During the e→a phase, the operating point returns to the original operating point a and reaches stability. In this case, the recovery time after the fault is approximately 1.1 seconds, which is 5.5 times the fault duration of 0.2 seconds.

[0183] Figures 23 to 27 The grid voltage v obtained from the simulation of parameters B is given. g Grid-connected converter output voltage V abc Grid-connected converter output current i abc And the active power P and reactive power Q responses.

[0184] In the simulation using parameters B, the fault duration of 400ms was set to be greater than the fault clearing time t. cc .like Figures 23 to 27 As shown, when the grid voltage recovers to 311V, the active power is lower than the active power reference value, therefore the operating point will move along... Figure 26 The fault condition ② curve shows that the active power continues to decrease. This process is as follows: Figure 13 The analysis focuses on the d'→e' stage. Approximately 3.4 seconds after the grid voltage recovers, the active power P converges back to its pre-fault value, but the reactive power recovers to a different value. Therefore, it can be concluded that in grid-connected converters based on active power synchronization and low-voltage ride-through control, the prolonged fault duration causes the converter to converge to an incorrect equilibrium point after the grid voltage dip. Figure 13 Point e'. (By) Figure 25 and Figure 27 Simulation results show that at point e', the grid-connected converter injects unwanted reactive power into the grid, and the excessive output current leads to additional transmission losses.

[0185] In summary, the simulation results from groups A and B show that the success of low voltage ride-through control based on active power synchronization is highly dependent on the duration of the fault. An excessively long fault duration will cause the grid-connected converter to inject unwanted reactive power into the grid, resulting in overcurrent in the grid-connected converter and causing additional transmission losses. Furthermore, the recovery time after the fault is too long.

[0186] 2) Simulation verification of low voltage ride-through stability control strategy based on reactive power synchronization

[0187] The low-voltage ride-through stability control strategy based on reactive power synchronization proposed in this patent was simulated by applying a grid voltage dip, causing the grid-side voltage to drop to 0.5 pu. The simulation time was set to t = 0.2 s for a symmetrical voltage dip, and the dip fault was cleared at t = 0.6 s.

[0188] Figures 28 to 33 The grid voltage v obtained from low voltage ride-through simulation based on reactive power synchronization control is presented. g Grid-connected converter output voltage V abc Grid-connected converter output current i abc And the response curves of active power P and reactive power Q.

[0189] Depend on Figures 28 to 33 It can be seen that when the grid voltage drops from 311V to 155V, the output current of the grid-connected converter is limited to within 30A. Simultaneously, the output voltage, active power, and reactive power of the grid-connected converter converge to a stable equilibrium operating point within 30ms. Figure 16 Point c in the equation. A stable equilibrium point c means that even if the fault duration exceeds the fault clearing time limit, stability can be quickly restored. During the fault, the grid-connected converter provides 5kW of active power and 3.1kvar of reactive power to the grid. After the fault is cleared, the grid-connected converter successfully returns to its initial state before the fault within 130ms. Figure 16The d→a stage. Therefore, simulation results show that the low voltage ride-through control based on reactive power synchronization is not affected by the fault limit clearing time and can quickly recover to steady state.

[0190] To verify the robustness of low-voltage ride-through control based on reactive power synchronization under severe voltage dip conditions, a simulation experiment with a voltage dip of 0.8 pu was set. A symmetrical voltage dip was set to occur at t=0.2s, and the dip fault was cleared at t=0.6s. The simulation results are as follows: Figures 34 to 38 As shown.

[0191] The simulation results show that when the voltage drops by 0.8 pu, the low voltage ride-through stability control strategy based on reactive power synchronization can reach a steady state in a relatively short time.

[0192] Simulation results show that the low voltage ride-through control based on reactive power synchronization proposed in this patent has strong robustness against severe low voltage drops.

[0193] Finally, it should be noted that the above embodiments are merely preferred embodiments of the present invention and are only used to illustrate the technical solutions of the present invention, not to limit them. Any modifications, improvements, or other modifications or equivalent substitutions made by those skilled in the art to the technical solutions of the present invention, as long as they do not depart from the spirit and scope of the technical solutions of the present invention, should be included within the protection scope of the present invention.

Claims

1. A low-voltage ride-through control method for a grid-connected converter based on reactive power synchronous grid configuration, characterized in that, The following steps are involved: Step 1: When the grid voltage drops, trigger the low voltage ride-through control to switch to reactive power synchronization mode. The trigger signal for the control switch comes from the current limiting module in the grid. Step 2: Based on the inner loop gain coefficient K cp Current limit value, grid voltage V g and grid-side inductor L gi The control gain K for reactive power synchronization is calculated. Qs ; Step 3: Combine with the set reference value Q for the DC component of reactive power N Generate virtual power angle reference value To keep the power angle δ stable within a range close to the normal operating point; Step 4: Set the SCR of the grid-connected converter system under weak grid conditions to 1.5 and 3, and conduct comparative verification using the traditional low voltage ride-through control method and the low voltage ride-through control method based on reactive power synchronization, respectively.

2. The low-voltage ride-through control method for grid-connected converters based on reactive power synchronous grid configuration according to claim 1, characterized in that, The formula for calculating the reactive power synchronous control bandwidth is: Among them, V n The voltage rating is s, the apparent power is K. Qs This is the reactive power synchronous proportional gain.

3. The low-voltage ride-through control method for grid-connected converters based on reactive power synchronous grid configuration according to claim 1, characterized in that: The reactive power synchronous control bandwidth ω Q(s) Designed for current-controlled bandwidth ω c of Therefore, the control gain K for reactive power synchronization Qs for: Among them, L gi For the grid-side inductor, K cp This is the inner loop gain coefficient.

4. The low-voltage ride-through control method for grid-connected converters based on reactive power synchronous grid configuration according to claim 1, characterized in that, The reactive power expression in low voltage ride-through control mode is: Among them, V g X is the grid voltage vector. g This is the equivalent impedance of the power grid.

5. The low-voltage ride-through control method for a grid-connected converter based on reactive power synchronous grid configuration according to claim 1, characterized in that, The reference value Q for the DC component of reactive power N The calculation formula is: Among them, X gm =ω n L gm It is the measured power grid impedance, L gm For the measured filter inductance, ω n I serves as the reference value for the grid converter and the grid angular frequency. max It is the maximum current allowed to pass through a grid-connected converter.

6. The low-voltage ride-through control method for a grid-connected converter based on reactive power synchronous grid configuration according to claim 1, characterized in that, The virtual power angle for low-voltage ride-through control based on reactive power synchronization is: Among them, K Qs For reactive power synchronization proportional gain, P and Q are the active power and reactive power output of the grid-connected converter, respectively. N , Q N These are the rated active power and reactive power of the grid-connected converter, respectively, K p This is the active power droop factor.

7. The low-voltage ride-through control method for grid-connected converters based on reactive power synchronous grid configuration according to claim 5, characterized in that: During a fault, the DC component Q of the reactive power output Q N Independent of the fault voltage amplitude, determined solely by the grid impedance X gm and maximum current limit value I max Decide.

8. The low-voltage ride-through control method for grid-connected converters based on reactive power synchronous grid configuration according to claim 1, characterized in that: After the fault is cleared, the system automatically exits low-voltage ride-through control and resumes normal active power droop control. The recovery time is only the power outer loop response time t. ps .

9. The low-voltage ride-through control method for a grid-connected converter based on reactive power synchronous grid configuration according to claim 1, characterized in that, A d-axis priority current limiter is used to limit the fault current.

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