Method for improving transient stability of grid-forming converter based on voltage amplitude adjustment

Abstract

The invention relates to a method for improving transient stability of a network-building type converter based on voltage amplitude regulation, which comprises the following steps of: simplifying a network-building type converter system model; obtaining three-phase active power from the voltage of the network-constructing converter to the voltage of the power grid and a droop relation between the active power and the frequency; based on the three-phase active power, drawing a relation curve between the active power and the phase angle difference between the grid-forming converter and the power grid, obtaining the active power conversion condition of the grid-forming converter, and determining the condition and balance point of the grid-forming converter for maintaining the transient stability according to the active power conversion condition; and when transient instability of the network-forming converter occurs, according to the conditions, adjusting the voltage and the line resistance of the network-forming converter to reach a new balance point, improving the transient stability of the network-forming converter, and keeping the voltage of the network-forming converter to be greater than the voltage amplitude of the network-forming converter.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy and new power systems, and in particular to a method for improving transient stability of a grid-connected converter based on voltage amplitude regulation. Background Art

[0002] With the transformation of the global energy structure, the development of distributed energy, the construction of smart grids and the advancement of power electronics technology, issues such as the grid connection of new energy, the stability and reliability of power systems, the need for energy conservation and emission reduction, and the rise of energy Internet are receiving increasing attention. In this context, grid-connected converters, as an important tool to solve these problems, have received extensive attention and research.

[0003] The grid-type converter adopts energy conversion technology based on power electronic devices, and realizes the conversion and transmission of electric energy by controlling the switch state. It has the advantages of fast response, high control accuracy, strong adaptability, high reliability, energy saving and environmental protection. However, the grid-type converter also has some limitations. For example, in the application of power system, the stability of the grid-type converter is a key consideration. The stability problem mainly involves synchronization stability, damping stability, power balance stability, dynamic stability and voltage stability. Transient stability refers to the synchronization stability of the grid-type converter when it is subject to large disturbances and can track the phase and frequency of the grid voltage and maintain synchronous operation with the grid. This is the basic requirement for the converter when it is connected to the grid. If the converter cannot keep pace with the grid, it will lead to a decrease in power quality and even cause system oscillation or collapse.

[0004] However, in some large disturbances such as grid voltage drop, transmission line fault and serious load nonlinearity, the transient stability problem of grid-connected converter may be caused. In order to improve the transient stability of power system, we can start from the following aspects, such as fast fault removal, configuration of appropriate control system and change the damping factor, active power and reactive power and other indicators affecting transient stability.

[0005] According to the above methods, when a fault occurs in the power system, rapid fault removal can effectively reduce the impact of the fault on the system, avoid fault expansion and system collapse, and is an important measure to improve transient stability. However, in order to achieve rapid fault removal, a series of measures need to be taken, including the use of fast relay protection devices, improving the reliability of relay protection, and achieving rapid fault removal of equipment such as lines and transformers. This process is too complicated and cumbersome; by reasonably configuring various control devices of the power system, such as using control systems such as AGC (automatic generation control), the automatic adjustment and control of the power system can be achieved, which can improve the transient stability of the system. However, this method depends on the strength of the signal and may cause a certain degree of delay, which is difficult to meet the application occasions with high real-time requirements; system damping refers to the mechanism of suppressing oscillation in the system. By increasing the system damping, the oscillation amplitude of the system can be reduced, and the transient stability of the system can be improved, but the overshoot of the power angle will be increased. When the grid voltage drops suddenly, transient stability can be improved by reducing the active power, but it will lead to a reduction in active power and affect frequency regulation. If reactive power is injected, transient voltage will be generated to reduce transient stability. Therefore, there are certain disadvantages in improving transient stability by changing indicators that affect transient stability, such as damping factor, active power and reactive power.

[0006] Maintaining the transient stability of grid-type converters is the key to ensuring the stable operation of power systems, and a series of effective measures and methods need to be taken to solve the problem, requiring comprehensive consideration from multiple aspects such as safety, stability and economy. However, the difficulty lies in finding a method that is easy to implement while improving transient stability and can keep the active power transmission unchanged in view of the asymmetric active power injection and absorption of grid-type converters. Summary of the invention

[0007] In order to solve the problems existing in the above-mentioned prior art, the purpose of the present invention is to propose a method for improving the transient stability of a grid-type converter based on voltage amplitude regulation, which takes into account the injection and absorption of active power, and can enable the grid-type converter to maintain stable operation under large disturbance conditions. It does not require additional hardware and can be easily implemented. It can also be adjusted according to real time to keep the active power transmission unchanged, thereby significantly improving the transient stability of the grid-type converter.

[0008] To achieve the above object, the present invention provides the following solutions:

[0009] A method for improving transient stability of a grid-connected converter based on voltage amplitude regulation, comprising:

[0010] Simplify the grid-type converter system model, and obtain the three-phase active power from the voltage of the grid-type converter to the grid voltage and the droop relationship between the active power and the frequency according to the simplified grid-type converter system model;

[0011] Based on the three-phase active power, a relationship curve between the active power and the phase angle difference between the grid-type converter and the power grid is drawn to obtain the active power conversion status of the grid-type converter, and through the active power conversion status, the conditions and balance point for the grid-type converter to maintain transient stability are determined;

[0012] When the grid-type converter is transiently unstable, the voltage and line resistance of the grid-type converter are adjusted according to the conditions to reach a new balance point, improve the transient stability of the grid-type converter, and maintain the voltage of the grid-type converter greater than the voltage amplitude of the grid-type converter.

[0013] Optionally, the method for obtaining the three-phase active power from the voltage of the grid-connecting converter to the grid voltage is:

[0014]

[0015] Among them, p g Represents the three-phase active power from the voltage of the grid-connected converter to the grid voltage, V g Represents the grid-type converter voltage, V s represents the grid voltage, δ g Represents the phase angle difference between the grid-connected converter and the grid, R gs and X gs They represent the resistance and reactance between the grid-connected converter and the grid respectively.

[0016] Optionally, the method for obtaining the droop relationship between the active power and the frequency is:

[0017] dδ g / dt=ω0K pf (p g_ref / S0-p g / s0)

[0018] Among them, δ g represents the phase angle difference between the grid-connected converter and the grid, s0 and ω0 represent the nominal power and angular frequency of the grid-connected converter, respectively, and K pf represents the droop gain of active power control, p g Represents active power, p g_ref Indicates the active power reference.

[0019] Optionally, obtaining the active power conversion status of the grid-type converter includes: the grid-type converter outputs active power to the grid and the grid-type converter absorbs active power from the grid.

[0020] Optionally, determining the conditions and balance points for the grid-connected converter to maintain transient stability includes:

[0021] The conditions for determining whether the grid-connected converter maintains transient stability include:

[0022] p g_max ≥p g_ref or p' g_max ≥p g_ref

[0023] Determining the balance point includes:

[0024] dδ g / dt=0

[0025] Among them, p g_ref and p' g_ref They represent the active power base of the active power output of the grid-forming converter to the grid and the active power absorbed by the grid-forming converter from the grid, respectively. g_max and p' g_max They represent the active power transmission limit of the active power output from the grid-forming converter to the grid and the active power absorbed from the grid by the grid-forming converter, δ g Represents the phase angle difference between the grid-connected converter and the grid.

[0026] Optionally, adjusting the line resistance to improve the transient stability of the grid-connected converter includes:

[0027] The active power curve output by the grid-type converter to the grid and the active power curve absorbed by the grid-type converter from the grid are plotted, and the first derivatives of the active power curve output by the grid-type converter to the grid and the active power curve absorbed by the grid-type converter from the grid are determined:

[0028]

[0029]

[0030] Among them, p g Represents the three-phase active power from the voltage of the grid-connected converter to the grid voltage, V g Represents the grid-type converter voltage, V s Represents the grid voltage, R gs and X gs They represent the resistance and reactance between the grid-connected converter and the grid, respectively, g_max and p' g_max They represent the active power transmission limits of the active power output from the grid-forming converter to the grid and the active power absorbed from the grid by the grid-forming converter respectively;

[0031] The power angle δ of the extreme point g_pmax ∈[π / 2,π) and δ g_pmax∈[-π / 2, 0) into the first derivative to obtain the second derivative:

[0032]

[0033] When the grid-type converter outputs active power to the power grid, it is determined based on the first derivative and the second derivative to increase the resistance of the grid-type converter to improve the voltage amplitude control. When the grid-type converter absorbs active power from the power grid, it is determined based on the first derivative and the second derivative to reduce the resistance of the grid-type converter to improve the voltage amplitude control.

[0034] Optionally, obtaining the voltage amplitude of the grid-connected converter includes:

[0035] According to the active power transmission limit and the balance point characteristics under the critical stable state, the power angle of the extreme point is obtained, and the power angle of the extreme point is substituted into the active power transmission limit to obtain the voltage amplitude of the grid-forming converter.

[0036] Optionally, a method for obtaining the power angle of the extreme point is:

[0037]

[0038]

[0039] Among them, R gs and X gs They represent the resistance and reactance between the grid-connected converter and the grid respectively.

[0040] Optionally, the method for obtaining the active power transmission limit is:

[0041]

[0042]

[0043] Among them, p g_ref and p' g_ref They represent the active power base of the active power output of the grid-forming converter to the grid and the active power absorbed by the grid-forming converter from the grid, respectively. g_max and p' g_max They represent the active power transmission limit of the active power output from the grid-forming converter to the grid and the active power absorbed from the grid by the grid-forming converter, δ g represents the phase angle difference between the grid-connected converter and the grid, δ g_pmax and δ' g_pmax Represents the power angle of the extreme point, R gs and X gs They represent the resistance and reactance between the grid-connected converter and the grid respectively.

[0044] Optionally, the method for obtaining the voltage amplitude of the grid-connected converter is:

[0045]

[0046] or

[0047]

[0048] Among them, p g_ref and p' g_ref They represent the active power base of the active power output of the grid-forming converter to the grid and the active power absorbed by the grid-forming converter from the grid, respectively. g_max and p' g_max They represent the active power transmission limit of the active power output from the grid-forming converter to the grid and the active power absorbed from the grid by the grid-forming converter, δ g represents the phase angle difference between the grid-connected converter and the grid, δ g_pmax and δ' g_pmax Represents the power angle of the extreme point, R gs and X gs Respectively represent the resistance and reactance between the grid-connected converter and the grid, V s Indicates the grid voltage.

[0049] The beneficial effects of the present invention are:

[0050] When facing large disturbances, grid-type converters often face the challenge of transient stability imbalance. In response to this key issue, the present invention innovatively proposes a method for improving the transient stability of grid-type converters based on voltage amplitude regulation. This method cleverly balances the injection and absorption of active power, ensuring that the grid-type converter can maintain its stable operating state even when subjected to strong disturbances.

[0051] Further quantitative analysis shows that compared with other strategies for improving the transient stability of grid-connected converters, the present invention has the following beneficial effects:

[0052] (1) Significantly improve stability: Greatly improve the stability performance of grid-connected converters during transient processes.

[0053] (2) Easy to implement and adjustable in real time: The method is easy to implement and can be adjusted in real time to quickly respond to system requirements.

[0054] (3) Constant power transmission: During the regulation process, ensure that the transmission of active power is not affected and maintain the power balance of the system.

[0055] (4) No additional hardware required: This method does not require the introduction of any additional hardware equipment, reducing implementation costs.

[0056] (5) Smooth voltage regulation: It provides a smooth voltage adjustment mechanism to effectively avoid system shocks caused by drastic adjustments. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0058] Figure 1 A schematic diagram of a system configuration of a grid-connected converter according to an embodiment of the present invention;

[0059] Figure 2 A schematic diagram of a simplified model of a grid-connected converter and a grid according to an embodiment of the present invention;

[0060] Figure 3 PF droop control block diagram of an embodiment of the present invention;

[0061] Figure 4 The embodiment of the present invention has a balance point and different R gs p g -δ g Curve diagram; Figure 4 (a) is p with equilibrium point g -δ g Curve diagram, Figure 4 (b) for different R gs p g -δ g Schematic diagram of the curve;

[0062] Figure 5 is the p before and after the failure of the embodiment of the present invention g -δ g Schematic diagram of the curve;

[0063] Figure 6 The embodiment of the present invention has a voltage amplitude adjustment p g -δ g Schematic diagram of the curve;

[0064] Figure 7 is a phase diagram with voltage amplitude adjustment according to an embodiment of the present invention; wherein, Figure 7 (a) is the phase diagram in case 1, Figure 7 (b) is the phase diagram in case 2;

[0065] Figure 8is a critical stable state p in an embodiment of the present invention g -δ g Schematic diagram of the curve;

[0066] Fig. 9 The embodiment of the present invention is g_max -V g Curves and different R gs p g_max -V g Curve diagram; Fig. 9 (a) is p in case 1 g_max -V g Curve diagram, Fig. 9 (b) is p' in case 2 g_max -V g Schematic diagram of the curve;

[0067] Fig.10 It is a schematic diagram of transient response simulation of a grid-connected converter according to an embodiment of the present invention with or without voltage amplitude regulation; Fig.10 (a) is a schematic diagram of no voltage amplitude adjustment. Fig.10 (b) is a schematic diagram of voltage amplitude adjustment. DETAILED DESCRIPTION

[0068] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0069] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0070] like Figure 1 As shown, the power grid is modeled as a resistor R gs 、Inductor L s and voltage source v sabc The grid-type converter is connected in series by L gi , C gf and L gg The LCL filter formed is connected to the power grid. Among them, v gfabc and i giabc Represent the capacitor voltage and converter side current, v gabc and i ggabc Represent the voltage and grid injection current at the point of common coupling (PCC), respectively. Figure 1The control block diagram of the grid-connected converter system is given. The internal current controller is designed to damp the LCL resonance and limit the converter current for protection purposes, and the intermediate voltage controller is designed to strictly regulate the capacitor voltage v gfabc The amplitude and phase angle of the active power and reactive power. Usually, the external power controller targets active power and reactive power and tracks their references. The reactive power controller can be replaced by a voltage controller, which provides accurate and fast voltage control, that is, the grid-type converter can be controlled by voltage.

[0071] In addition, the phase angle of the grid-connected converter is determined by the active power controller. Figure 1 In, θ g_ref is the reference phase angle used for frame conversion, θ g The actual phase angle of the grid-connected converter, θ s is the phase angle provided by the PLL, which tracks the phase of the grid voltage, δ g is the phase angle difference between the grid-connected converter and the grid. Before grid connection, the relay S gr Disconnect, then θ g_ref =θ s , the grid phase angle is locked by PLL. Then the converter adjusts the capacitor voltage v gfabc Following the grid voltage, when v gfabc With v gabc At the same time, by closing S gr Connect the converter to the grid. In the initial stage of grid connection, θ g_ref =δ g +θ s , the phase angle is given by the power angle δ g and PLL phase angle θ s After the converter is successfully connected to the grid, as long as the active power is well regulated, the PLL is no longer needed. g_ref =θ g The phase angle is independently provided by the power controller. This process is the grid synchronization process.

[0072] The present embodiment discloses a method for improving the transient stability of a grid-type converter based on voltage amplitude regulation, comprising: simplifying a grid-type converter system model, and obtaining the three-phase active power from the voltage of the grid-type converter to the voltage of the grid and the droop relationship between the active power and the frequency according to the simplified grid-type converter system model; drawing a relationship curve between the active power and the phase angle difference between the grid-type converter and the grid based on the three-phase active power, obtaining the active power conversion of the grid-type converter, and determining the conditions and balance point for the grid-type converter to maintain transient stability through the active power conversion; when the grid-type converter is transiently unstable, adjusting the voltage and line resistance of the grid-type converter according to the determined conditions to reach a new balance point, thereby improving the transient stability of the grid-type converter, and in order to prevent the grid-type converter from transient instability, the voltage of the grid-type converter needs to be greater than the voltage amplitude of the grid-type converter.

[0073] The specific steps are as follows:

[0074] Step 1, simplify the grid-connected converter system model, and use the droop control method to analyze the droop relationship between active power and frequency (pf).

[0075] Step 2: Analyze the active power and power angle (p g -δ g ) curve and transient instability mechanism.

[0076] Step 3: Propose a method to improve transient stability by voltage amplitude regulation and analyze its principle, conduct a quantitative analysis of the voltage amplitude regulation rate, and discuss the impact of line resistance on voltage amplitude control.

[0077] Step 4: Figure 1 The grid-type converter system block diagram is constructed, and a MATLAB / Simulink (R2022 a) model is built to perform simulations to verify the feasibility of the proposed method.

[0078] Step 1: Simplify the system model and analyze the power control method:

[0079] (1) Simplified model of grid-connected converter and power grid:

[0080] The grid-connected converter system model is simplified as follows: Figure 2 As shown. g ∠0 represents the grid-type converter voltage, where V g is the phase voltage amplitude, specified as a reference value of zero radians. V s ∠-δ g and I gg ∠ψ represent the grid voltage and grid current respectively. Based on this simplified model, from V g Transmit to V s The three-phase active power pg It can be deduced as:

[0081]

[0082] Among them, V g Represents the grid-type converter voltage, V s represents the grid voltage, δ g Represents the phase angle difference between the grid-connected converter and the grid, R gs and X gs They represent the resistance and reactance between the grid-connected converter and the grid respectively.

[0083] (2) Power control:

[0084] This embodiment uses a droop control method. Droop control mainly refers to the droop relationship between active power and frequency (pf) and reactive power and voltage (qv). This embodiment only introduces the relationship between active power and frequency (pf). Figure 3 Shown is the pf droop control block diagram. Figure 3 The symbol Δ indicates parameter change, and the subscript pu indicates unit value. S0 and ω0 are the nominal power and angular frequency of the grid-connected converter, respectively. pf Represents the droop gain of active power control. Figure 3 PF droop control block diagram, PF control equation can be written as:

[0085] dδ g / dt=ω0K pf (p g_ref / S0-p g / S0) (2)

[0086] Among them, δ g represents the phase angle difference between the grid-connected converter and the grid, S0 and ω0 are the nominal power and angular frequency of the grid-connected converter, respectively, and K pf represents the droop gain of active power control, p g Represents active power, p g_ref Indicates the active power reference.

[0087] Step 2: According to p g -δ g Curve analysis of transient instability mechanism

[0088] According to formula (1), this embodiment can obtain p g and δ g The relationship curve between Figure 4 (a)-(b) As shown in the figure, since the grid-connected converter with integrated energy storage can absorb active power, p g -δ gThe curve contains two cases:

[0089] Case 1: p g >0, indicating that the grid-connected converter outputs active power to the grid.

[0090] Case 2: p g <0, indicating that the grid-connected converter absorbs active power from the grid.

[0091] exist Figure 4 In (a), p g_ref and p' g_ref Represent the active power reference in case 1 and case 2 respectively. g_max and p' g_max They represent the active power transfer limits in case 1 and case 2 respectively. g_pmax and δ' g_pmax represents the boundary power angle in case 1 and case 2. a, a', b and b' are equilibrium points, at which p g =p g_ref The shaded area represents the stable operating region.

[0092] exist Figure 4 (b) shows the gs =0 and R gs >0 in both cases g -δ g Curve. When the line resistance R gs >0, the active power is consumed by the line resistance, p g -δ g The curve is offset. g >0, the grid-connected converter needs to provide R gs Output additional active power, which increases the active power transmission limit, the curve shifts upward to easily reach the equilibrium point. g <0, R gs Part of the active power is absorbed, thereby reducing the active power transmission, the curve shifts upward, and is prone to deviate from the balance point.

[0093] (2) Transient instability mechanism:

[0094] like Figure 5 As shown in Figure 2, when a fault occurs, the active power transfer limit is reduced, resulting in two types of transient instability. g_max ≥p g_ref or When , there is transient instability with an equilibrium point. According to formula (2), there exists dδ g / dt=0 equilibrium point, δ g It is possible to enter the equilibrium point and stay at the equilibrium point, at which time the grid-connected converter can be synchronized with the grid, as shown in curve I. or When , there is transient instability without equilibrium point. If there is no equilibrium point, according to formula (2), this will lead to dδ g / dt>0 or dδ g / dt<0,δ g It will increase or decrease monotonically towards +∞ or -∞. At this time, the grid-connected converter is transiently unstable and cannot be synchronized with the grid, as shown in curve II.

[0095] In summary, when a fault occurs, the necessary condition for the grid-connected converter to maintain transient stability and keep synchronization with the grid is p g_max ≥p g_ref That is, there exists dδ g / dt=0 equilibrium point.

[0096] Step 3: Voltage amplitude regulation to improve transient stability:

[0097] (1) Voltage amplitude regulation:

[0098] By analyzing the transient instability mechanism, this embodiment can know that if there is a balance point, the system can still operate stably, but if a serious fault occurs, the active power transmission limit will be significantly reduced, resulting in loss of the balance point. At this time, the balance point can be re-established by increasing the active power transmission limit to ensure stable operation of the system. According to formula (1), the active power is directly affected by the grid-type converter voltage V g In this embodiment, the grid-type converter voltage V g Increase the active power transfer limit to reach the balance point and improve the transient stability of the grid-connected converter.

[0099] like Figure 6 As shown in the figure, when the grid-connected converter works in the voltage control mode, the grid voltage V s When V drops from 110V to 15V, g Response V s Then it drops to 15V, as shown in curve I. At this time, there is no equilibrium point. Gradually increase V g , we get curves II, III and IV. Obviously, the active power transfer limit increases with V g increases with the increase of V g When it increases to 20V, the active power transmission limit is greater than the active power reference, reaching the balance point, improving the transient stability of the grid-connected converter, and at this time it can keep synchronized with the power grid.

[0100] Figure 7 (a)-(b) show the graphical solutions of formula (2), from which we can more intuitively see that at each point, δg and its derivative dδ g / dt. As V g As the curve gets closer to the dotted line, when it reaches the dotted line, dδ g / dt=0, the active power transfer limit is equal to the active power reference, and the balance point is reached. Therefore, voltage regulation is an effective method to improve the transient stability of grid-connected converters.

[0101] Furthermore, obtaining the voltage amplitude of the grid-type converter includes: obtaining the power angle of the extreme point according to the active power transmission limit and the balance point characteristics under the critical stable state, substituting the power angle of the extreme point into the active power transmission limit, and obtaining the voltage amplitude of the grid-type converter.

[0102] (2) Quantitative analysis of voltage amplitude regulation rate:

[0103] like Figure 8 As shown, there is a critical stable state between the stable and unstable states, at which p g With p g_ref There is a unique intersection point, which is the unique equilibrium point. At this time, the active power transfer limit p g_max or p' g_max are equal to their reference p g_ref or p' g_ref In this embodiment, the voltage amplitude of the grid-type converter in the critical stable state is expressed as V gc . V gc represents the voltage characteristic of the converter at the edge of stability and is the threshold value for distinguishing between stable and unstable states. g By properly adjusting V g The value of can gradually enhance the stability of the converter, thereby achieving a smooth transition from an unstable state to a stable state. In order to prevent the grid-type converter from transient instability and ensure that the grid-type converter maintains a stable working state under any circumstances, the actual regulated voltage V g Set above the critical steady-state voltage V gc At the level of V g >V gc Therefore, V gc It is very important.

[0104] Since the active power transfer limit p g_max or p' g_max Yes g -δ g The extreme value of the curve, then according to the critical stable state, dδ g / dt=0, the power angle δ of the extreme point is derived from formula (2) g_pmaxor δ' g_pmax for:

[0105]

[0106]

[0107] Then according to formula (1), the active power transfer limit can be derived as:

[0108]

[0109]

[0110] By solving (5) and (6), V gc To export as:

[0111]

[0112] or

[0113]

[0114] According to formulas (7) and (8), V in case 1 and case 2 can be calculated. gc They are 19.98V and 19.88V respectively. Figure 6 and Figure 7 As shown, in case 1 and case 2, V g <V gc Curves I and II are unstable, while V g >V gc The curves III and IV are stable.

[0115] (3) Influence of line resistance on voltage amplitude control:

[0116] Since line resistance consumes active power, the line resistance will have a certain impact on the voltage amplitude regulation effect of the grid-type converter transient stability. In order to explore the impact of line resistance, this embodiment plots the p g_max -V g Curve and p' of Case 2 g_max -V g Curves, such as Fig. 9 As shown in (a)-(b).

[0117] according to Fig. 9 , p g_max -V g and p' g_max -V g The derivative of the curve is derived as:

[0118]

[0119]

[0120] According to formulas (3) and (4), we can deduce δ g_pmax ∈[π / 2,π) and δ g_pmax ∈[-π / 2, 0), substituting into formulas (9) and (10), we get:

[0121]

[0122] When R gs =0, dp g_max / dV g and dp' g_max / dV g are all constants, such as Fig. 9 As shown by the two straight lines in gs >0, according to formulas (9) and (11), dp g_max / dV g Always positive and increases with V g Increase and increase, such as Fig. 9 As shown in (a), p g_max With V g The faster the increase of V g For R gs The increase will lead to p g_max Increase, so that the system is easier to reach a stable state. Therefore, increase R gs The voltage amplitude regulation in case 1 will enhance the effect of improving transient stability. However, according to formulas (10) and (11), dp' g_max / dV g It is negative at the beginning and then increases as V g increases and becomes positive, such as Fig. 9 As shown in (b), R gs Increasing will lead to p' g_max The absolute value of R decreases, which means that it becomes more difficult for the system to reach a stable state. Therefore, increasing R gs This will weaken the effect of improving transient stability by adjusting the voltage amplitude in case 2.

[0123] In summary, line resistance will have a certain impact on the effect of regulating the transient stability of the grid-type converter by voltage amplitude. In case 1, R should be increased. gs To improve the voltage control effect, R should be reduced in case 2. gs to mitigate its adverse effects.

[0124] Step 4: Simulation to verify the feasibility of the proposed method:

[0125] according to Figure 1The grid-type converter system configuration and control block diagram, in this embodiment, a MATLAB / Simulink (R2022 a) model is constructed to simulate and verify the feasibility of the proposed method.

[0126] When the grid-connected converter operates in voltage control mode, the grid voltage V s From 110V to 15V, the converter voltage V g Following the grid voltage V s Also drops to 15V. Fig.10 (a) shows that at p g , δ g and i gg Low-frequency oscillations are observed in all waveforms, indicating that the sudden drop in grid voltage produces a large disturbance, which triggers the transient instability of the grid-connected converter. To improve transient stability, V g Adjust to 20V, the simulation results are as follows Fig.10 As shown in (b), no p was observed during the transient process. g , δ g and i gg Oscillation indicates that the transient stability of the grid-type converter is improved. In summary, the simulation results confirm the feasibility of improving the transient stability of the grid-type converter by voltage amplitude regulation.

[0127] The embodiments described above are only descriptions of the preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for improving transient stability of a grid-connected converter based on voltage amplitude regulation, characterized in that: include: Simplify the grid-type converter system model, and obtain the three-phase active power from the voltage of the grid-type converter to the grid voltage and the droop relationship between the active power and the frequency according to the simplified grid-type converter system model; Based on the three-phase active power, a relationship curve between the active power and the phase angle difference between the grid-type converter and the power grid is drawn to obtain the active power conversion status of the grid-type converter, and through the active power conversion status, the conditions and balance point for the grid-type converter to maintain transient stability are determined; When the grid-type converter is transiently unstable, the voltage and line resistance of the grid-type converter are adjusted according to the conditions to reach a new balance point, improve the transient stability of the grid-type converter, and maintain the voltage of the grid-type converter greater than the voltage amplitude of the grid-type converter.

2. The method for improving transient stability of grid-connected converter based on voltage amplitude regulation according to claim 1, characterized in that: The method for obtaining the three-phase active power from the voltage of the grid-connected converter to the grid voltage is: Among them, p g Represents the three-phase active power from the voltage of the grid-connected converter to the grid voltage, V g Represents the grid-type converter voltage, V s represents the grid voltage, δ g Represents the phase angle difference between the grid-connected converter and the grid, R gs and X gs They represent the resistance and reactance between the grid-connected converter and the grid respectively.

3. The method for improving transient stability of grid-connected converter based on voltage amplitude regulation according to claim 1, characterized in that: The method for obtaining the droop relationship between the active power and the frequency is: dδ g / dt=ω0K pf (p g_ref / S0-p g / S0) Among them, δ g represents the phase angle difference between the grid-type converter and the grid, S0 and ω0 represent the nominal power and angular frequency of the grid-type converter, respectively, and K pf represents the droop gain of active power control, p g Represents active power, p g_ref Indicates the active power reference.

4. The method for improving transient stability of grid-connected converter based on voltage amplitude regulation according to claim 1, characterized in that: Acquiring the active power conversion status of the grid-type converter includes: the grid-type converter outputting active power to the grid and the grid-type converter absorbing active power from the grid.

5. The method for improving transient stability of grid-connected converter based on voltage amplitude regulation according to claim 4, characterized in that: The conditions and balance points for determining the grid-type converter to maintain transient stability include: The conditions for determining whether the grid-connected converter maintains transient stability include: p g_max ≥p g_ref or p′ g_max ≥p g_ref Determining the balance point includes: dδ g / dt=0 Among them, p g_ref and p′ g_ref They represent the active power base of the active power output of the grid-forming converter to the grid and the active power absorbed by the grid-forming converter from the grid, respectively. g_max and p′ g_max They represent the active power transmission limit of the active power output from the grid-forming converter to the grid and the active power absorbed from the grid by the grid-forming converter, δ g Represents the phase angle difference between the grid-connected converter and the grid.

6. The method for improving transient stability of grid-connected converter based on voltage amplitude regulation according to claim 4, characterized in that: Adjusting the line resistance to improve the transient stability of the grid-connected converter includes: The active power curve output by the grid-type converter to the grid and the active power curve absorbed by the grid-type converter from the grid are plotted, and the first derivatives of the active power curve output by the grid-type converter to the grid and the active power curve absorbed by the grid-type converter from the grid are determined: Among them, p g Represents the three-phase active power from the voltage of the grid-connected converter to the grid voltage, V g Represents the grid-type converter voltage, V s Represents the grid voltage, R gs and X gs They represent the resistance and reactance between the grid-connected converter and the grid, respectively, g_max and p′ g_max They represent the active power transmission limits of the active power output from the grid-forming converter to the grid and the active power absorbed from the grid by the grid-forming converter respectively; The power angle δ of the extreme point g_pmax ∈[π / 2,π) and δ g_pmax ∈[-π / 2, 0) into the first derivative to obtain the second derivative: When the grid-type converter outputs active power to the power grid, it is determined based on the first derivative and the second derivative to increase the resistance of the grid-type converter to improve the voltage amplitude control. When the grid-type converter absorbs active power from the power grid, it is determined based on the first derivative and the second derivative to reduce the resistance of the grid-type converter to improve the voltage amplitude control.

7. The method for improving transient stability of grid-connected converter based on voltage amplitude regulation according to claim 1, characterized in that: Obtaining the voltage amplitude of the grid-connected converter includes: According to the active power transmission limit and the balance point characteristics under the critical stable state, the power angle of the extreme point is obtained, and the power angle of the extreme point is substituted into the active power transmission limit to obtain the voltage amplitude of the grid-forming converter.

8. The method for improving transient stability of grid-connected converter based on voltage amplitude regulation according to claim 7, characterized in that: The method for obtaining the power angle of the extreme point is: Among them, R gs and X gs They represent the resistance and reactance between the grid-connected converter and the grid respectively.

9. The method for improving transient stability of grid-connected converter based on voltage amplitude regulation according to claim 7, characterized in that: The method for obtaining the active power transmission limit is: Among them, p g_ref and p′ g_ref They represent the active power base of the active power output of the grid-forming converter to the grid and the active power absorbed by the grid-forming converter from the grid, respectively. g_max and p′ g_max They represent the active power transmission limit of the active power output from the grid-forming converter to the grid and the active power absorbed from the grid by the grid-forming converter, δ g represents the phase angle difference between the grid-connected converter and the grid, δ g_pmax and δ g_pmax Represents the power angle of the extreme point, R gs and X gs They represent the resistance and reactance between the grid-connected converter and the grid respectively.

10. The method for improving transient stability of grid-connected converter based on voltage amplitude regulation according to claim 7, characterized in that: The method for obtaining the voltage amplitude of the grid-connected converter is: or Among them, p g_ref and p′ g_ref They represent the active power base of the active power output of the grid-forming converter to the grid and the active power absorbed by the grid-forming converter from the grid, respectively. g_max and p′ g_max They represent the active power transmission limit of the active power output from the grid-forming converter to the grid and the active power absorbed from the grid by the grid-forming converter, δ g represents the phase angle difference between the grid-connected converter and the grid, δ g_pmax and δ g_pmax Represents the power angle of the extreme point, R gs and X gs Respectively represent the resistance and reactance between the grid-connected converter and the grid, V s Indicates the grid voltage.

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