A low voltage ride through control method of a virtual synchronous generator considering power angle stability and overcurrent suppression

Abstract

The application relates to a low-voltage ride-through control method of a virtual synchronous generator considering power angle stability and overcurrent suppression, which comprises the following steps: obtaining a virtual synchronous generator control model, monitoring transient stability and voltage drop of the virtual synchronous generator control model, and obtaining a monitoring result; when transient synchronous instability appears in the monitoring result, adjusting an active power reference instruction during a transient period according to actual output active power by using an adaptive active power instruction strategy; and when voltage drop or voltage drop recovery appears in the monitoring result, suppressing transient impact overcurrent based on an adaptive virtual synchronous impedance control strategy. The application not only realizes non-divergence and non-fluctuation of a power angle during any voltage drop depth, but also realizes fast and smooth recovery to a normal influence mode without overcurrent.

Description

Technical Field

[0001] This invention relates to the field of virtual synchronous generator control technology, and in particular to a low-voltage ride-through control method for a virtual synchronous generator that balances power angle stability and overcurrent suppression. Background Technology

[0002] With the rapid development of new energy grid connection technology, new energy grid-connected inverters based on VSG control are increasingly becoming an important technical means to improve system stability in high-proportion new energy power systems because they can provide synchronization characteristics such as inertia and damping to the power grid.

[0003] However, existing VSG control strategies still face significant challenges in engineering practice, especially when grid faults cause voltage dips. VSG control systems are highly susceptible to both power angle instability and output overcurrent, severely threatening the transient stability and equipment safety of the grid-connected system. Currently, most existing research focuses on addressing individual issues in power angle stability or overcurrent suppression, failing to comprehensively address the combined effects of both from a system-wide perspective. Furthermore, existing methods typically neglect the inrush currents caused by voltage surges during the voltage dip transient process and subsequent recovery phase. These currents can cause irreversible damage to power devices, thus affecting converter lifespan and operational reliability.

[0004] Therefore, under complex and ever-changing transient operating conditions of the power grid, traditional control strategies are unable to effectively balance maintaining power angle stability, suppressing steady-state overcurrent, and limiting transient inrush current, which restricts the reliable operation of VSG under harsh conditions such as low voltage ride-through. Summary of the Invention

[0005] To address the problems existing in the prior art, the purpose of this invention is to propose a low-voltage ride-through control method for a virtual synchronous generator that balances power angle stability and overcurrent suppression. This method not only ensures that the power angle does not diverge or fluctuate during any voltage drop depth, but also enables a rapid and smooth recovery to the normal operating mode without overcurrent.

[0006] To achieve the above objectives, the present invention provides the following solution: A low-voltage ride-through control method for a virtual synchronous generator that balances power angle stability and overcurrent suppression includes: A virtual synchronous generator control model is obtained, and transient stability and voltage sag monitoring are performed on the virtual synchronous generator control model to obtain the monitoring results. When transient synchronization instability occurs in the monitoring results, the adaptive active power instruction strategy is used to adjust the active power reference instruction during the transient period according to the actual output active power. When the voltage drops or recovers in the monitoring results, the transient surge overcurrent is suppressed based on the adaptive virtual synchronous impedance control strategy.

[0007] Optionally, setting the adaptive active power command strategy includes: The rotor motion model of the virtual synchronous generator is transformed into a state space, and a sliding surface is defined. The rotor motion model after state space transformation is constrained on the sliding surface. When the sliding surface reaches the first target value, it proves that the rotor motion model has reached stability. Then, the derivative of the sliding surface is obtained. When the derivative of the sliding surface equals the approaching law, the control law is obtained.

[0008] Optionally, obtaining the control law includes: ; in, The inverter's reference active power. For the actual output electromagnetic power, This is the adjustment coefficient. The damping coefficient is... , These are the actual angular velocity and the rated angular velocity of the rotor, respectively. For rotational inertia, This is the gain coefficient. Scaling factor This represents the boundary layer thickness.

[0009] Optionally, the adaptive virtual synchronous impedance control strategy includes: a virtual impedance control strategy after voltage drop and a virtual impedance control strategy after voltage recovery.

[0010] Optionally, the virtual impedance control strategy after the voltage drop includes: Obtain the loop impedance magnitude after voltage drop in steady state, and when the phase angle between voltage and current in steady state is the second target value, obtain the virtual resistance and virtual inductance in steady state. Fault current analysis is performed on the virtual synchronous generator control model to obtain the inrush current value under the worst condition. When the inrush current value is the third target value, the current value that the inverter can withstand is determined. When the impedance angle is the fourth target value in steady state, the initial values ​​of the transient virtual resistance and virtual inductance are obtained based on the virtual resistance and virtual inductance in steady state combined with the current value that the inverter can withstand, so as to set the virtual impedance control strategy after the voltage drop.

[0011] Optionally, the virtual impedance control strategy after the voltage drop includes: ; in, The duration following a voltage drop fault. The virtual resistance in steady state. The virtual inductance in steady state. For transient virtual resistance, The initial value of the transient virtual resistance. The loop time constant during transients. For transient virtual inductance, This is the initial value of the transient virtual inductance.

[0012] Optionally, obtaining the surge current value under the worst-case scenario includes: ; in, This represents the surge current value under the worst-case scenario. Let the steady-state solution amplitude of the differential equation for phase a loop be given. Let be the time constant of phase a circuit.

[0013] Optionally, the virtual impedance control strategy after voltage recovery includes: The voltage of the virtual synchronous generator control model is obtained in steady state after the voltage drop. When the phase angle between the voltage and current in steady state is the fifth target value, the initial value of the transient virtual impedance at voltage recovery is obtained to set the virtual impedance control strategy after voltage recovery.

[0014] Optionally, the virtual impedance control strategy after voltage recovery includes: ; in, This is the transient virtual resistance during voltage recovery. This is the initial value of the transient virtual resistance during voltage recovery. The duration after the grid voltage is restored. The transient equivalent loop time constant after the grid voltage recovers. The virtual inductance after the grid voltage is restored. This is the initial value of the virtual inductance after the grid voltage is restored.

[0015] The beneficial effects of this invention are as follows: This invention addresses two core challenges faced by virtual synchronous generators (VSGs) during low-voltage ride-through in power grids: transient power angle instability and output overcurrent. It proposes an innovative collaborative control strategy. For transient power angle stabilization, real-time active power command tracking technology is employed. When a voltage dip occurs, the active power reference value is tracked in real-time to the actual output value, effectively eliminating the acceleration energy caused by active power imbalance, thereby maintaining the synchronous stability between the VSG rotor and the grid and preventing grid disconnection due to power angle instability. For fault current suppression, dynamic virtual impedance technology is introduced. Its impedance value adaptively adjusts according to the real-time detected voltage dip depth, quickly limiting the peak value of the inrush current at the moment of fault occurrence and maintaining the steady-state fault current within a safe operating range during the fault duration. During voltage recovery, a virtual impedance smooth exit mechanism is designed, allowing the virtual impedance to smoothly and continuously decay to zero during voltage recovery, avoiding potential power and current surges during control mode switching and ensuring a smooth transition from fault ride-through to normal operation.

[0016] This invention can simultaneously solve the problems of transient power angle stability and overcurrent suppression that are difficult to balance in traditional control schemes within a single control architecture, enhance the virtual synchronous generator's ride-through capability and grid connection support reliability during grid faults, and help improve the safe and stable operation level of power systems with a high proportion of new energy. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a structural diagram of the VSG control system according to an embodiment of the present invention; Figure 2 This is a power angle curve for different voltage drop depths according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the three-phase voltage drop of the VSG according to an embodiment of the present invention; Figure 4 This is a fault current phasor diagram according to an embodiment of the present invention; Figure 5 This is a waveform diagram of the maximum fault current in an embodiment of the present invention; Figure 6 This is a conventional active power command adaptive power angle curve diagram according to an embodiment of the present invention; Figure 7 This is a block diagram of the improved active power adaptive control according to an embodiment of the present invention; Figure 8This is a schematic diagram of a synchronous generator according to an embodiment of the present invention; Figure 9 This is a schematic diagram illustrating the sequence of virtual impedance input during LVRT according to an embodiment of the present invention; Figure 10 This is a schematic diagram of the equivalent circuit of the system after voltage drop according to an embodiment of the present invention; (a) is the transient equivalent circuit, and (b) is the steady-state equivalent circuit; Figure 11 This is a schematic diagram of the equivalent circuit of the voltage recovery instantaneous system according to an embodiment of the present invention; Figure 12 The experimental waveform diagram shows the voltage drop to 0.6 pu without LVRT strategy in an embodiment of the present invention. Figure 13 A waveform diagram showing the voltage drop to 0.6 pu in an embodiment of the present invention with the addition of a virtual impedance; Figure 14 The waveform diagram of the LVRT strategy of this invention is shown in the experimental diagram when the voltage drops to 0.6 pu according to an embodiment of the invention. Figure 15 The experimental waveform diagram shows the voltage drop to 0.2 pu without LVRT strategy in an embodiment of the present invention. Figure 16 The waveform diagram is an experimental waveform of a conventional active power adaptive strategy when the voltage drops to 0.2 pu according to an embodiment of the present invention. Figure 17 The waveform diagram of the LVRT strategy of this invention is shown in the embodiment of the invention when the voltage drops to 0.2 pu. Figure 18 The following are waveforms of virtual resistors and virtual inductors under the LVRT strategy in this embodiment of the invention. Figure 19 This is a flowchart of a low-voltage ride-through control method for a virtual synchronous generator that balances power angle stability and overcurrent suppression, according to an embodiment of the present invention. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0021] This embodiment discloses a low-voltage ride-through control method for a virtual synchronous generator that balances power angle stability and overcurrent suppression. The method includes: acquiring a virtual synchronous generator control model; monitoring the transient stability and voltage dip of the virtual synchronous generator control model and obtaining the monitoring results; when transient synchronous instability occurs in the monitoring results, adjusting the active power reference command during the transient period according to the actual output active power using an adaptive active power command strategy; and when voltage dips or voltage dip recovery occur in the monitoring results, suppressing transient inrush overcurrent based on an adaptive virtual synchronous impedance control strategy.

[0022] Specifically, this embodiment discloses a low-voltage ride-through (LVRT) control method for a virtual synchronous generator (VSG) that balances power angle stability and overcurrent suppression. Considering the most severe impact on the system during a three-phase symmetrical voltage dip in the grid, this invention uses this operating condition as its research background, with the core control objective being to maintain the transient power angle stability of the VSG and effectively suppress overcurrent. First, the dynamic characteristics of the VSG's transient power angle and the output current characteristics during a three-phase symmetrical voltage dip in the grid are theoretically analyzed. The reason for power angle instability is the imbalance between the VSG's active power command and the output active power. To address this problem, a strategy is designed where the active power reference command follows the actual output active power during the transient period. At this time, there is no imbalance between the power power command and the output active power, thus preventing power angle instability. Second, drawing on the short-circuit analysis theory of synchronous generators, transient virtual impedance and steady-state virtual impedance are introduced at the moment of grid-side voltage dip, during grid recovery, and during the voltage dip, respectively, and relevant design criteria are given. The introduction of this strategy ensures that the inverter will not experience transient overcurrent surges throughout the entire LVRT. Finally, the correctness and effectiveness of the proposed strategy are verified using the MATLAB / Simulink experimental platform.

[0023] VSG transient characteristics analysis: VSG Basic Principle: The virtual synchronous control grid connection structure adopted in this invention is as follows: Figure 1 As shown in the diagram. The DC portion of the main circuit is typically the DC voltage output from the energy storage unit. The inverter port voltage is filtered to obtain the three-phase voltage uabc. In the diagram, Lf, Cf, and Rf represent the filter inductance, capacitor, and equivalent resistance, respectively. i The voltage and current represented by VSG output, PCC is the common coupling point, Lg and Rg are the line inductance and resistance respectively, and Ug represents the grid voltage.

[0024] The hardware topology of the VSG is the same as that of a traditional GCI. By drawing upon the mechanical and electromagnetic equations of a traditional synchronous generator, the GCI can be simulated as a traditional synchronous generator, giving it the operating characteristics of a traditional synchronous generator. Therefore, its rotor motion equations are obtained as follows: (1); in: (2); In the formula, J is the moment of inertia; D is the damping coefficient; ω, These are the actual angular velocity and the rated angular velocity of the rotor, respectively. , These represent the mechanical power and the actual output electromagnetic power, respectively, with δ being the rotor's power angle. The inverter's reference active power. This is the adjustment coefficient. For time.

[0025] The power of the VSG after grid connection can be expressed as: (3); In the formula, E is the VSG output voltage, Ug is the grid voltage, and Xg is the line impedance. Output reactive power to VSG.

[0026] The reactive power control section of the virtual synchronous generator regulates the output reactive power to ensure system voltage stability by simulating the voltage-reactive power droop relationship in the excitation voltage regulator of a synchronous generator. (4); In the formula, Rated voltage, For reference reactive power, This represents the actual reactive power output of the VSG. This is the reactive power droop coefficient.

[0027] VSG transient power angle characteristic analysis: When the power grid is subjected to large disturbances, the VSG will face transient power angle instability problems similar to those of traditional generators. As can be seen from equation (3), when the power grid experiences a symmetrical voltage drop, the output active power of the VSG decreases accordingly, the active power input and output of the VSG become unbalanced, the rotor accelerates, and the power angle increases. The transient stability of the VSG will be analyzed below.

[0028] VSG power angle characteristic curves under different voltage drops are as follows Figure 2 As shown. Curve I is the power angle characteristic curve when the grid voltage is normal. Curve I and Pref intersect at points s and u, respectively. Point s is the stable equilibrium point (SEP), and point u is the unstable equilibrium point (UEP). Stable equilibrium occurs when the system operates stably at point s. Curves II and III are the power angle curves for different voltage drop depths. Curve II intersects with Pref, at which point the VSG may recover stability, but the following conditions must be met: .in and These are the acceleration area and deceleration area, respectively. The following analysis focuses on curve II. When the grid voltage drops instantaneously, because the power angle δ cannot change abruptly, the operating point of the VSG jumps from point s to point a. At this time, because... The rotor begins to accelerate, RoCoF > 0, and the power angle δ begins to increase; after the operating point passes point b, , The rotor begins to decelerate, the power angle δ begins to decrease, and the operating point moves towards point a. After several oscillation cycles, the operating point finally stabilizes at point b. For curve III, the grid voltage drops significantly, and the power angle curve does not intersect with Pref. At this time, Pref is always greater than Pe, the rotor continues to accelerate, the power angle diverges, and the system has no deceleration area. In this case, the faulty system needs to be disconnected in time to restore stability. From the above analysis, it can be concluded that the root cause of the VSG power angle instability is the imbalance between the output active power and the active power reference value.

[0029] VSG fault current analysis: Figure 3 The diagram shows a three-phase voltage drop in a VSG circuit. Before the voltage drop, the circuit is in a stable state. When a symmetrical voltage drop fault occurs in the power grid, the system remains three-phase symmetrical. Only one phase needs to be analyzed during the study.

[0030] The following analysis focuses on phase a. When the VSG is in steady-state operation before the fault occurs, its phase a current can be expressed as: (5); in: (6); In the formula, Z is the grid-side impedance. The modulus, The line impedance angle has a value of , This refers to the phase shift that occurs when the difference between the VSG output voltage and the mains voltage is synthesized into a new sinusoidal quantity during a fault. This is the output voltage of the VSG. For short circuit angle, For VSG steady-state power angle, This is the steady-state current amplitude. For grid-side line resistance, This refers to the inductance of the grid-side line.

[0031] When a fault occurs, assuming the voltage drop fault is in When this occurs, the instantaneous value of the current must satisfy the differential equation: (7); in, For grid-side line inductance, Let a be the phase current. This refers to the line resistance on the grid side.

[0032] Equation (5) is clearly a first-order linear non-homogeneous differential equation with constant coefficients. Its solution is the analytical expression of the total current during a fault. The solution of this differential equation includes a general solution and a particular solution. Its steady-state solution is: (8); in: (9); In the formula, This refers to the phase shift that occurs when the difference between the VSG output voltage and the mains voltage is synthesized into a new sinusoidal quantity during a fault. This refers to the grid fault voltage.

[0033] Therefore, the expression for the total current after the fault can be obtained: (10); In the formula, Ta is the decay time constant, and its value is Lg / Rg.

[0034] The above analysis shows that when a voltage dip fault occurs in the power grid, the phase fault current includes AC periodic and aperiodic components. The periodic component is related to the amplitude and phase difference of the VSG output voltage and the grid voltage, as well as the loop impedance; the aperiodic component is related not only to the amplitude and phase difference but also to time, and eventually decays to zero over time.

[0035] Figure 4 The diagram shows the phasor diagram of phase a voltage and current at t=0, where... and These represent the voltage difference between the VSG output voltage and the mains voltage before and after the fault; and They are respectively and The difference between the projections onto the time axis and the initial values ​​of the non-periodic components is the initial value of the projection. It is obvious from the diagram that when Initial values ​​of aperiodic components when parallel to the time axis maximum.

[0036] Considering that the reactance on the grid side is much greater than the resistance, the impedance angle is approximated. Combining equation (10), we can derive that when and When the conditions are most severe and the initial value of the AC component is the largest, the maximum instantaneous value of the fault current at this time is called the impact current. Substituting this condition into equation (10) yields: (11); Figure 5The graph shows the fault current waveform of phase a when the initial value of the non-periodic component is at its maximum. It can be seen from the graph that the maximum instantaneous value of the fault current occurs in the first half-wave period after the fault occurs. For a system with a power frequency of 50Hz, its maximum value occurs at... Place.

[0037] Based on the above analysis, the inrush current value of the VSG under the worst-case scenario can be obtained as follows: (12); After a VSG fault, the initial values ​​of the aperiodic components of the three-phase current cannot be maximized simultaneously, but at any initial phase, the initial value of one phase current will always be maximized.

[0038] LVRT control strategy: Adaptive Active Power Command Strategy: As the above analysis shows, when the grid voltage drops significantly, the active power curve and the active power reference command do not intersect, the power angle continues to increase and eventually diverges, and the VSG will experience transient synchronization instability. Equation (3) shows that the active power output of the VSG after the fault is... (13); Where k is the voltage sag coefficient. , This is the voltage of the power grid fault. The rated voltage of the power grid. For grid-side line reactance, For VSG power angle.

[0039] like Figure 6 The traditional adaptive active power regulation command shown typically sets the active power reference command to kPN after a voltage drop. Essentially, it generates a new command by scaling the rated power proportionally based on the voltage value after the fault. Although this open-loop method based on steady-state quantity calculation can ensure that the VSG will not become unstable after a voltage drop, it does not form a closed-loop feedback with transient state quantities such as rotor angular velocity deviation and power angle change rate. Therefore, the control effect is limited. At the moment of voltage drop and recovery, power angle fluctuations will occur due to the difference between Pref and Pe.

[0040] To address this issue, the present invention employs sliding mode control to enable Pref to dynamically track the output power Pe during the LVRT process, i.e., Pref-Pe=0. Combining equation (1) and the power angle curve, it can be seen that there is no power difference at this time, and the rotor will not accelerate but will remain unchanged. dω / dt=0, so the power angle remains stable throughout the entire LVRT period.

[0041] Rewrite the rotor motion equation (1) in state-space form and define the state variables. Control input Disturbance Then the system equation is: (14); in, For the first derivative of the state variable, For rotational inertia, is the damping coefficient.

[0042] When the grid voltage drops, in order to maintain the system state Fast convergence value (rated value) The angular frequency tracking error is defined as: (15); In order for the system to reach and stabilize at the rated angular frequency within a finite time, the sliding surface is defined as the error itself: (16); When the system state is constrained to s=0 on the sliding surface, the system reaches steady state. At this time, the active power reference command Pref is equal to the VSG output power Pe, thus achieving power balance.

[0043] Differentiating with respect to the sliding surface, we get: (17); To balance convergence speed and chattering, an exponential reaching law is adopted, and the sgn function is replaced with the smoother hyperbolic tangent function tanh. The derivative of the sliding surface is set to equal the reaching law expression: (18); The final control law can be obtained as follows: (19); in, The inverter's reference active power. For the actual output electromagnetic power, This is the adjustment coefficient. The damping coefficient is... , These are the actual angular velocity and the rated angular velocity of the rotor, respectively. For rotational inertia, This is the gain coefficient. Scaling factor This represents the boundary layer thickness.

[0044] The stability proof is given below. Assuming that the control strategy can quickly and accurately track Pe during grid voltage dips and subsequent transient processes, we can obtain the following: (20); in, To track error, ideally , This is the grid voltage.

[0045] Substituting equation (20) into equation (1), we get: (twenty one); Equation (21) is clearly a nonlinear system, and its proof is based on the energy-based Lyapunov direct method. Let , Therefore, Lyapunov functions can be constructed: (twenty two); in, For the angle of work deviation, For angular frequency deviation, For the angle of attack, For steady-state work angle, For reference active power, It outputs electromagnetic power to the VSG.

[0046] Differentiating equation (22) and substituting equations (20) and (21) into it, we get: (twenty three); Obviously under ideal tracking conditions at this time According to the Lasalle invariant set principle, the system eventually converges to... Therefore, the system will stabilize at the equilibrium point. s, The improved active power loop control block diagram is as follows: Figure 7 As shown, when the voltage drops, the adaptive active power command will take effect throughout the LVRT period.

[0047] Adaptive virtual synchronous impedance control strategy: To better guide the design of adaptive synchronous virtual impedance and achieve better dynamic performance and inrush current suppression under different voltage dip fault depths, the following analysis draws on traditional synchronous generators.

[0048] like Figure 8 As shown, after a three-phase short circuit in a synchronous generator, the rotor damping winding D and the excitation winding f will first induce a free DC current to resist the entry of the stator direct-axis armature reaction flux, forcing the flux to close along the leakage flux path of the D and f windings. At this time, the equivalent reactance on the stator side is the subtransient synchronous reactance. Corresponding to subtransient current This is the subtransient process; as the free DC component of the damping winding decays rapidly, the armature reaction flux can partially penetrate into the D winding, increasing the magnetic permeability, and the stator equivalent reactance becomes the transient synchronous reactance. Corresponding transient current When the free DC component of the excitation winding also decays to 0, the armature reaction flux completely penetrates the direct shaft, the magnetic permeability reaches its maximum, and the stator equivalent reactance becomes the synchronous reactance. Corresponding to the steady-state short-circuit current I, the synchronous generator enters a steady-state short-circuit state. After a fault, the synchronous generator experiences three stages, and the synchronous reactance relationship for each stage is as follows: Therefore, the fault current is the largest during the subtransient state, and can even reach more than ten times the rated current.

[0049] Due to the inherent structure of synchronous generators, the secondary transient current is very large in the initial stage of a fault. Since VSGs do not have winding and armature reactions, they do not exhibit the secondary and transient reactances of synchronous machines. Based on the above analysis, VSGs still face the problem of inrush current exceeding limits after a voltage drop fault on the grid side. However, VSG control is flexible and can use control algorithms to simulate transient synchronous reactances to suppress the inrush current. The process is as follows: Figure 9 As shown.

[0050] The virtual impedance strategy after voltage drop includes transient virtual impedance and steady-state virtual impedance, when 0 < When <4τ, the impedance strategy is a transient virtual impedance. When the impedance is greater than or equal to 4τ, the impedance strategy is steady-state virtual impedance.

[0051] Virtual impedance design after voltage dip: The power electronic devices in the inverter are highly sensitive to current, and the overcurrent is most severe during a three-phase fault, easily damaging the devices. Therefore, the current value that the inverter can withstand for a long time after a voltage drop is 1.2IN. Taking the grid voltage as the reference phase, substituting equation (9) into equation (8) and transforming it, we can obtain the circuit impedance magnitude in steady state after the voltage drop: (twenty four); in, The virtual resistance in steady state. This is the virtual inductance in steady state.

[0052] Obviously, equation (24) only yields the impedance magnitude and cannot separate the virtual resistance and virtual inductance. The impedance angle is needed. In steady state, a large resistance will lead to severe active power loss in the VSG output. Therefore, a smaller resistance is chosen. Therefore, the values ​​of the virtual resistance and virtual inductance in steady state can be obtained as follows: (25); Next, the value of the transient virtual impedance is calculated. As analyzed above, the VSG will face the threat of inrush current at the moment of fault occurrence. To avoid this problem, the inrush current calculation formula is shown in equation (12). Let Then we have: (26); Where R is the sum of the virtual resistance and the grid-side resistance, and L is the sum of the virtual inductance and the grid-side inductance.

[0053] The decay rate of the inrush current is related to the impedance ratio of the circuit; the larger the impedance ratio, the faster the decay rate. Therefore, a smaller impedance angle is chosen during transients. The initial values ​​of the transient virtual resistance and virtual inductance can then be calculated as follows: (27); Based on the exponential decay characteristic of transient current, its decay rate is related to the time constant. The transient component decays to almost zero after 4τ. Therefore, the transient virtual impedance transitions to the steady-state virtual impedance at 4τ. Because the transient and steady-state virtual impedances are different in magnitude, a simple step switching would inevitably cause a secondary impact due to the voltage difference. To make... and It decays to 4τ after τ and To avoid secondary impacts, the transient virtual impedance is designed as follows: (28); in, t F The duration following a voltage dip fault, when the fault occurs. t F = 0, τ' is the time constant of the loop during transients.

[0054] Therefore, the total impedance between the VSG internal potential and the power grid after the voltage drop is the sum of the line impedance and the virtual impedance. Thus, the simplified circuit diagram of the system after the voltage drop is obtained as follows: Figure 10 As shown in (a)-(b).

[0055] Virtual Impedance Design After Voltage Recovery: During a voltage drop, a traditional VSG increases the power angle δ to maintain active power output. When the grid voltage recovers, the power angle δ cannot change abruptly, causing the output power Pe to jump rapidly, far exceeding the rated active power, and resulting in inrush current. Although this invention uses an active power command-following output power control strategy to ensure the power angle remains unchanged throughout the LVRT period, during the steady-state period after a voltage drop, the PCC voltage experiences a voltage drop due to the virtual impedance caused by the VSG's internal potential E passing through the virtual impedance. Therefore, at the instant the grid voltage recovers, a potential difference exists between the PCC and the grid voltage, still resulting in inrush current.

[0056] From the calculated steady-state virtual impedance, the voltage across PCC in steady state after the voltage drop is: (29); Similar to voltage sags, the larger the resistance-to-inductance ratio, the faster the inrush current decays. The initial value of the transient virtual impedance during voltage recovery is: (30); Similarly, the inrush current decays to 0 after 4τ. Since the time constant τ of the transient circuit is very small during recovery, if the virtual impedance smoothly exits after 4τ, the active power curve of the VSG output will not have recovered to stability and there will still be an inrush. Therefore, the duration of the virtual impedance must be greater than the time for the active power to recover to stability. Thus, the control strategy for the recovery transient virtual impedance is as follows: (31); in, t R is the duration after voltage recovery, and the instantaneous voltage recovery... t R=0.

[0057] The system equivalent circuit at the moment of voltage recovery is as follows Figure 11 As shown.

[0058] Experimental Verification: To verify the correctness and effectiveness of the theoretical analysis and control strategy of this invention, a simulation platform was built on Matlab / Simulink. Figure 1 The VSG grid-connected model is shown below. Simulation parameters are shown in Table 1.

[0059] Table 1 System Parameters The VSG initially operates in steady state. At 2 seconds, a three-phase symmetrical voltage sag fault occurs in the power grid, lasting for 1 second. LVRT is verified for different voltage sag depths and different control strategies.

[0060] Figure 12 The experimental waveforms for traditional VSG control show that when the grid voltage drops to 0.6 pu, the VSG output active power can reach the set command system, allowing the system to recover stability after the voltage drop. However, the inrush current after the fault reaches 2.1 pu, the steady-state current reaches 1.61 pu, and the inrush current after voltage recovery reaches 2.15 pu. Clearly, the traditional VSG can recover stability when the voltage drops to 0.6 pu, but the excessive inrush current and steady-state overcurrent may damage the power devices in the inverter.

[0061] Figure 13The figure shows the experimental waveforms after a voltage dip with a virtual impedance applied. The virtual resistance is 0.48Ω and the virtual inductance is 0.194mH. The graph shows that after adding the virtual impedance, the inrush current during a voltage dip is 1.47 pu, the steady-state fault current is 1.53 pu, and the inrush current at the moment of voltage recovery is 2 p.u. While introducing a fixed-value virtual impedance during a voltage dip can reduce the inrush current and steady-state fault current to some extent, it still carries risks, as the steady-state fault current exceeds the inverter's long-term withstand current value.

[0062] Figure 14 The experimental waveforms using the LVRT control strategy proposed in this invention are shown in the figure. As can be seen from the power angle curve, when using the adaptive active power control strategy of this invention, the power angle of the VSG remains stable at the rated power angle value throughout the entire LVRT period without fluctuation. The inrush current at the moment of voltage drop is only 1.26 pu, the inrush current at the moment of voltage recovery is only 1.2 pu, and the steady-state fault current is 1.2 pu. This demonstrates that the strategy of this invention, by drawing on the transient process of synchronous generators and dividing the virtual impedance after a VSG fault into transient virtual impedance and steady-state virtual impedance, can effectively suppress the inrush current and ensure that there is no overcurrent problem in the VSG during LVRT.

[0063] Figure 15 The experimental waveforms for a traditional VSG with a voltage drop to 0.2 pu are shown. Due to the excessive voltage drop, the active power output of the VSG cannot reach the active power reference value, causing power angle instability and slippage during LVRT. At the moment of voltage drop, the inrush current is 3.6 pu; at the moment of voltage recovery, the inrush current is 5.6 pu; and the steady-state fault current is 2.9 pu. Therefore, when the grid voltage drops too deeply, the traditional VSG will experience power angle instability, and the inrush current will far exceed the inverter's tolerance.

[0064] Figure 16 The experimental waveforms for conventional active power adaptation are shown. When a voltage dip in the grid is detected, the active power command is set to Pref=kPn, and a virtual resistor of 0.856Ω and a virtual inductance of 0.3954mH are applied. The figure shows that when the conventional active power adaptation strategy is introduced, the VSG can stabilize under deep voltage dips, but its power angle fluctuates during LVRT. After adding a fixed virtual impedance value, although the steady-state fault current can be suppressed, the inrush current during voltage dips and the inrush current during voltage recovery cannot be effectively suppressed, with values ​​of 1.7pu and 1.54pu respectively. Furthermore, the power angle fluctuates during LVRT.

[0065] Figure 17The experimental waveforms using the LVRT strategy proposed in this invention are shown. As can be seen from the figures, the inrush current during the instantaneous voltage drop is only 1.23 pu, the steady-state fault current stabilizes at 1.2 pu, and the inrush current after voltage recovery is also only 1.2 pu. Furthermore, the power angle of the VSG remains at its rated power angle throughout the entire LVRT process. This demonstrates that the strategy proposed in this invention not only ensures power angle stability during LVRT but also effectively suppresses transient inrush currents, thanks to the design of the adaptive virtual synchronization impedance.

[0066] Figure 18 The waveforms of the virtual resistance and virtual inductance under the LVRT strategy of this invention are shown. This invention analyzes the causes of inrush current and draws on the transient process of synchronous power generation to divide the virtual impedance into transient virtual impedance and steady-state virtual impedance. As can be seen from the figure, at the moment of voltage drop fault, the transient virtual impedance that should be put into operation is calculated based on the current voltage drop depth. After 4τ, the non-periodic component of the fault current decays to 0, and the transient virtual impedance also decays to the steady-state virtual impedance. Similarly, at the moment of voltage recovery, the transient virtual impedance that dynamically decays to 0 is calculated and put into operation based on the voltage difference between PCC and the grid.

[0067] This invention proposes a VSG low-voltage ride-through control strategy considering transient power angle stability and overcurrent suppression. Firstly, by improving active power command adaptation, this strategy ensures that the power angle remains unchanged and does not fluctuate during any voltage dip depth. Secondly, the causes of post-fault surges are analyzed, and an adaptive virtual impedance is designed with calculation rules provided. The introduction of this virtual impedance effectively suppresses transient surge currents and ensures that the steady-state fault current does not exceed the inverter's long-term withstand value. After voltage recovery, this strategy allows the VSG to quickly and smoothly return to normal operating mode without overcurrent. Finally, the effectiveness of the proposed strategy is verified through experiments. Future research will focus on fault ride-through in multi-VSG parallel systems.

[0068] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A low-voltage ride-through control method for a virtual synchronous generator that balances power angle stability and overcurrent suppression, characterized in that, include: A virtual synchronous generator control model is obtained, and transient stability and voltage sag monitoring are performed on the virtual synchronous generator control model to obtain the monitoring results. When transient synchronization instability occurs in the monitoring results, the adaptive active power instruction strategy is used to adjust the active power reference instruction during the transient period according to the actual output active power. When the voltage drops or recovers in the monitoring results, the transient surge overcurrent is suppressed based on the adaptive virtual synchronous impedance control strategy.

2. The low-voltage ride-through control method for a virtual synchronous generator that balances power angle stability and overcurrent suppression according to claim 1, characterized in that, Setting the adaptive active power instruction strategy includes: The rotor motion model of the virtual synchronous generator is transformed into a state space, and a sliding surface is defined. The rotor motion model after state space transformation is constrained on the sliding surface. When the sliding surface reaches the first target value, it proves that the rotor motion model has reached stability. Then, the derivative of the sliding surface is obtained. When the derivative of the sliding surface equals the approaching law, the control law is obtained.

3. The low-voltage ride-through control method for a virtual synchronous generator that balances power angle stability and overcurrent suppression according to claim 2, characterized in that, The control law is obtained by: ； in, The inverter's reference active power. For the actual output electromagnetic power, This is the adjustment coefficient. The damping coefficient is... , These are the actual angular velocity and the rated angular velocity of the rotor, respectively. For rotational inertia, This is the gain coefficient. Scaling factor This represents the boundary layer thickness.

4. The low-voltage ride-through control method for a virtual synchronous generator that balances power angle stability and overcurrent suppression according to claim 1, characterized in that, The adaptive virtual synchronous impedance control strategy includes: a virtual impedance control strategy after voltage drop and a virtual impedance control strategy after voltage recovery.

5. The low-voltage ride-through control method for a virtual synchronous generator that balances power angle stability and overcurrent suppression according to claim 4, characterized in that, The virtual impedance control strategy after the voltage drop includes: Obtain the loop impedance magnitude after voltage drop in steady state, and when the phase angle between voltage and current in steady state is the second target value, obtain the virtual resistance and virtual inductance in steady state. Fault current analysis is performed on the virtual synchronous generator control model to obtain the inrush current value under the worst condition. When the inrush current value is the third target value, the current value that the inverter can withstand is determined. When the impedance angle is the fourth target value in steady state, the initial values ​​of the transient virtual resistance and virtual inductance are obtained based on the virtual resistance and virtual inductance in steady state combined with the current value that the inverter can withstand, so as to set the virtual impedance control strategy after the voltage drop.

6. The low-voltage ride-through control method for a virtual synchronous generator that balances power angle stability and overcurrent suppression according to claim 5, characterized in that, The virtual impedance control strategy after voltage drop includes: ； in, This refers to the duration following a voltage drop fault. The virtual resistance in steady state. The virtual inductance in steady state. For transient virtual resistance, The initial value of the transient virtual resistance. The loop time constant during transients. For transient virtual inductance, This is the initial value of the transient virtual inductance.

7. The low-voltage ride-through control method for a virtual synchronous generator that balances power angle stability and overcurrent suppression according to claim 5, characterized in that, Obtaining the surge current value under the worst-case scenario includes: ； in, This represents the surge current value under the worst-case scenario. Let the steady-state solution amplitude of the differential equation for phase a be denoted as . Let be the time constant of phase a circuit.

8. The low-voltage ride-through control method for a virtual synchronous generator that balances power angle stability and overcurrent suppression according to claim 7, characterized in that, The virtual impedance control strategy after voltage recovery includes: The voltage of the virtual synchronous generator control model is obtained in steady state after the voltage drop. When the phase angle between the voltage and current in steady state is the fifth target value, the initial value of the transient virtual impedance at voltage recovery is obtained to set the virtual impedance control strategy after voltage recovery.

9. The low-voltage ride-through control method for a virtual synchronous generator that balances power angle stability and overcurrent suppression according to claim 8, characterized in that, The virtual impedance control strategy after voltage recovery includes: ； in, This is the transient virtual resistance during voltage recovery. This is the initial value of the transient virtual resistance during voltage recovery. The duration after the grid voltage is restored. The transient equivalent loop time constant after the grid voltage recovers. The virtual inductance after the grid voltage is restored. This is the initial value of the virtual inductance after the grid voltage is restored.

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