Control method and system for improving transient stability of virtual resistance VSG

Abstract

This application belongs to the field of new energy grid connection technology, specifically a control method and system for improving the transient synchronization stability of a virtual resistance VSG, comprising: a virtual common coupling point reconstruction module, which converts the virtual resistance into a series voltage drop term located before the real common coupling point to construct a virtual common coupling point; a power characteristic mapping module, which generates an active power-power angle characteristic curve; a stability boundary determination module, which determines, based on the active power-power angle characteristic curve, the critical virtual power angle corresponding to the unstable equilibrium point of the system under voltage sag conditions, and the maximum allowable power angle overshoot margin corresponding to the stable equilibrium point; and a transient stability control module, which adaptively lowers the active power reference value to constrain the power angle trajectory from exceeding the critical virtual power angle. This application adaptively lowers the active power reference value when a voltage sag or synchronization risk is detected, thereby achieving active dynamic constraint of the power angle, suppressing excessive power angle overshoot, and significantly improving transient synchronization stability.

Description

Technical Field

[0001] This application belongs to the field of new energy grid connection technology, specifically a control method and system for improving the transient synchronization stability of virtual resistance VSG. Background Technology

[0002] In high-proportion renewable energy grid-connected systems, a large number of power sources are connected to the grid through voltage source converters (VSCs), resulting in a significant decrease in system inertia and increasingly prominent stability issues. Virtual synchronous generator (VSG) control, as a typical grid-connected control strategy, introduces the oscillation equation of a synchronous machine into the control, enabling the converter to possess inertial and damping characteristics similar to a synchronous generator, thus providing voltage and frequency support in weak grid environments. However, the dynamic characteristics of the power outer loop of the VSG may induce small-disturbance instability problems such as synchronous resonance (SR). To suppress synchronous resonance, virtual resistors (VRs) are typically introduced into the control structure in engineering to enhance system damping performance. However, research has found that while virtual resistors can improve small-disturbance stability, they reduce the peak active power transmission and compress the power angle stability domain under large-disturbance conditions such as grid voltage sags, leading to a decrease in system transient synchronous stability. Existing improvement methods, such as adaptive inertia, mode switching, and additional damping strategies, can improve system response to some extent, but they generally suffer from computational complexity, reliance on differential detection, and insufficient robustness. Summary of the Invention

[0003] This application provides a control method and system for improving the transient synchronization stability of virtual resistors (VSGs), solving the problems of dependence on differential detection and insufficient robustness in existing methods.

[0004] The first aspect of this application provides a system for improving the transient synchronization stability of a virtual resistance VSG, comprising:

[0005] The virtual common coupling point reconfiguration module is used to construct a virtual common coupling point by equating the virtual resistance to a series voltage drop term located before the real common coupling point according to the control structure of the virtual synchronous generator, and to define the virtual power angle as the difference between the reference phase of the virtual synchronous generator output voltage and the grid phase.

[0006] The power characteristic mapping module is used to establish a mapping relationship between the active power injected into the power grid from the real common coupling point and the virtual power angle based on the virtual common coupling point and the virtual power angle, and generate an active power-power angle characteristic curve.

[0007] The stability boundary determination module is used to determine the critical virtual power angle corresponding to the unstable equilibrium point of the system under voltage sag condition, and the maximum allowable power angle overshoot margin corresponding to the stable equilibrium point, based on the active power-power angle characteristic curve.

[0008] The transient stability control module is used to adaptively reduce the active power reference value based on the critical virtual power angle and the maximum allowable power angle overshoot margin when a grid voltage sag is detected, so as to constrain the power angle trajectory to not exceed the critical virtual power angle.

[0009] Furthermore, the transient stability control module includes an additional adjustment channel, which is connected between the active power reference value input terminal and the active power feedback terminal, and is used to adaptively correct the active power reference value according to the power mismatch between the steady-state active power reference value and the actual output active power.

[0010] Furthermore, the additional adjustment channel includes: a power mismatch calculation unit for calculating the power mismatch between the steady-state active power reference value and the actual output active power; a proportional adjustment unit with control gain for proportionally amplifying the power mismatch to generate an additional adjustment amount; and an addition unit for subtracting the additional adjustment amount from the steady-state active power reference value to generate a transient active power reference value.

[0011] Furthermore, the transient active power reference value is calculated as follows:

[0012] ,

[0013] in, This is a reference value for transient active power. This is the steady-state active power reference value. The value can be 0 or 1. When the value is 1, the additional adjustment channel is activated. This refers to the actual output active power. To control the gain.

[0014] Furthermore, the additional adjustment channel also includes: a switching unit connected between the power mismatch calculation unit and the proportional adjustment unit, used to close when a transient stability control command is received and to open during normal operation; and a smooth cut-in unit used to gradually increase the additional adjustment amount from zero to the target value through a ramp function or low-pass filtering when the switching unit is closed.

[0015] Furthermore, the transient stability control module also includes: a disturbance detection unit, used to detect whether the voltage amplitude drop depth at the virtual common coupling point exceeds a preset threshold, and output a trigger signal when the threshold is exceeded; a control decision unit, used to generate a transient stability control command based on the trigger signal; and the additional adjustment channel is controlled by the control decision unit and is put into operation when the transient stability control command is received.

[0016] Furthermore, the transient stability control module also includes:

[0017] The critical virtual power angle determination unit is used to determine the critical virtual power angle corresponding to the unstable equilibrium point of the system under voltage sag conditions based on the active power-power angle characteristic curve.

[0018] The margin calculation unit is used to calculate the maximum allowable power angle overshoot margin δosm=δce-δe, where δe is the virtual power angle corresponding to the stable equilibrium point and δce is the critical virtual power angle corresponding to the unstable equilibrium point.

[0019] The additional adjustment channel also limits the downward adjustment range of the transient active power reference value based on the maximum permissible power angle overshoot margin.

[0020] Furthermore, the transient stability control module also includes:

[0021] The synchronous risk detection module is used to calculate the virtual power angle and the rate of change of the virtual power angle in real time; based on the degree of proximity between the virtual power angle and the critical virtual power angle, and the magnitude of the rate of change of the virtual power angle, a synchronous risk coefficient is generated.

[0022] The control decision unit generates transient stability control commands based on the trigger signal and the synchronization risk coefficient output by the synchronization risk detection module.

[0023] A second aspect of this application provides a method for improving the transient synchronization stability of a virtual resistance VSG, comprising:

[0024] Based on the control structure of the virtual synchronous generator, the virtual resistance is equivalent to a series voltage drop term located before the real common coupling point, the virtual common coupling point is constructed, and the virtual power angle is defined as the difference between the reference phase of the virtual synchronous generator output voltage and the phase of the grid.

[0025] Based on the virtual common coupling point and the virtual power angle, a mapping relationship between the active power injected into the power grid from the real common coupling point and the virtual power angle is established, and an active power-power angle characteristic curve is generated.

[0026] Based on the active power-power angle characteristic curve, determine the critical virtual power angle corresponding to the unstable equilibrium point of the system under voltage sag condition, and the maximum allowable power angle overshoot margin corresponding to the stable equilibrium point.

[0027] When a voltage dip in the grid is detected, the active power reference value is adaptively lowered based on the critical virtual power angle and the maximum allowable power angle overshoot margin, constraining the power angle trajectory not to exceed the critical virtual power angle.

[0028] Furthermore, the active power reference value is adaptively lowered, including:

[0029] Calculate the power mismatch between the steady-state active power reference value and the actual output active power;

[0030] The power mismatch is proportionally amplified to generate an additional adjustment amount;

[0031] The additional adjustment is subtracted from the steady-state active power reference value to generate the transient active power reference value.

[0032] Compared with the prior art, the advantages of this application are as follows:

[0033] When a voltage sag or synchronization risk is detected, this application adaptively lowers the active power reference value to achieve active constraint on the dynamics of the power angle, thereby suppressing excessive power angle overshoot and significantly improving transient synchronization stability. Attached Figure Description

[0034] Figure 1 A system block diagram for improving the transient synchronization stability of the virtual resistance VSG provided in the embodiments of this application;

[0035] Figure 2 A block diagram of the additional adjustment channel provided in the embodiments of this application;

[0036] Figure 3 A block diagram of the transient stability control module provided in the embodiments of this application. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0038] To address the issue of instability in virtual synchronous generators (VSGs) with virtual resistance under large disturbances, a system and method for improving the transient synchronization stability of VSGs with virtual resistance are proposed. When a voltage sag or synchronization risk is detected, the active power reference value is adaptively lowered through an additional adjustment channel to actively constrain the transient power angle dynamics, thereby suppressing excessive power angle overshoot and significantly improving transient synchronization stability.

[0039] See Figure 1 Combination Figure 2 and Figure 3 As shown, a system for improving the transient synchronization stability of a virtual resistance VSG includes:

[0040] The virtual common coupling point reconfiguration module is used to construct a virtual common coupling point by equating the virtual resistance to a series voltage drop term located before the real common coupling point according to the control structure of the virtual synchronous generator, and to define the virtual power angle as the difference between the reference phase of the virtual synchronous generator output voltage and the grid phase.

[0041] Based on the virtual synchronous generator control structure, the virtual common coupling point can be equivalent to a virtual node located before the real common coupling point, and the virtual power angle is defined as... , The reference phase for the output voltage of the virtual synchronous generator. For grid phase. Virtual resistance. Voltage reference reconstruction is achieved by modifying the virtual common coupling point voltage reference. .in The voltage reference space vector generated for the external power loop. The space vector of the current injected into the power grid. This serves as the reconstructed voltage reference. Because the inner voltage / current loop bandwidth is higher than the outer power loop, therefore... It can be regarded as an equivalent series voltage drop term acting on the voltage reference side, so that the virtual synchronous generator is equivalent to being in front of the real synchronous generator in a control sense.

[0042] The power characteristic mapping module is used to establish a mapping relationship between the active power injected into the power grid from the real common coupling point and the virtual power angle based on the virtual common coupling point and the virtual power angle, and generate an active power-power angle characteristic curve.

[0043] Active power injected into the grid by a real synchronous generator from the real point of common coupling. With reactive power The parsing expression:

[0044] ,

[0045] ,

[0046] in, This is the grid voltage. For virtual power angle, For line reactance, This represents the grid resistance. Virtual resistance is also considered. Grid resistance and line reactance The combined impact.

[0047] Based on this, the voltage reference space vector generated by the external power loop is further derived. With virtual power angle The relationship between them:

[0048] + ,

[0049] in This represents the reactive power droop coefficient. This is a reference value for reactive power, combined with active power. The expression is used to obtain the mapping relationship between the active power injected into the grid at the real common coupling point and the virtual power angle, thus establishing a complete active power-power angle characteristic curve. This active power-power angle characteristic curve reveals the influence of the virtual resistance on the power transmission capacity and the peak value of the curve.

[0050] The stability boundary determination module is used to determine the critical virtual power angle corresponding to the unstable equilibrium point of the system under voltage sag condition, and the maximum allowable power angle overshoot margin corresponding to the stable equilibrium point, based on the active power-power angle characteristic curve.

[0051] A transient sag shifts the active power-power angle characteristic curve downwards and reduces peak power, thereby weakening the equivalent active power transmission capacity during a fault. Consequently, two scenarios may occur during a fault: a balance point may still exist, or the balance point may disappear, inevitably leading to instability. When a balance point exists during a fault, the system typically has both a stable equilibrium point (SEP) and an unstable equilibrium point (UEP), corresponding to virtual power angles δe and δce, respectively. During the transient process, once the trajectory crosses the unstable equilibrium point, the system will lose synchronization and enter a power angle divergence process. Therefore, the maximum permissible power angle overshoot margin δosm = δce - δe is defined, reflecting the "safety margin" of the virtual power angle that the system can tolerate under large disturbances. Thus, transient synchronization stability can be equivalently expressed as: whether the power angle trajectory after the disturbance crosses the critical virtual power angle δce corresponding to the unstable equilibrium point.

[0052] The transient stability control module is used to adaptively reduce the active power reference value based on the critical virtual power angle and the maximum allowable power angle overshoot margin when a grid voltage sag is detected, so as to constrain the power angle trajectory to not exceed the critical virtual power angle.

[0053] In one embodiment, the fault process under voltage sag disturbance is analyzed based on the energy function and the equal area criterion. During the disturbance, when the steady-state active power reference value is greater than the actual output active power, the system is in an accelerating state, forming an accelerating area; when the steady-state active power reference value is less than the actual output active power, the system is in a decelerating state, forming a decelerating area.

[0054] The acceleration area and deceleration area are calculated using the following formulas:

[0055] ,

[0056] ,

[0057] in, To accelerate the area, For deceleration area, This is the steady-state active power reference value. It is related to the virtual power angle. The initial virtual power angle is the angle at which the disturbance occurs.

[0058] Transient synchronization stability is essentially dominated by the equilibrium point distribution and peak transmission capability determined by the active power-power angle characteristics. Specifically, virtual resistance reduces peak power and shifts the active power-power angle characteristic curve downwards overall. Under voltage sag conditions, the equivalent transmission capability is further weakened, leading to a smaller deceleration area. Simultaneously, the critical virtual power angle moves closer to the stable equilibrium point, resulting in a significant reduction in the maximum permissible power angle overshoot margin. Therefore, although virtual resistance helps improve system damping performance, it essentially weakens transient synchronization stability by reducing the equivalent active power transmission capability and compressing the permissible power angle margin.

[0059] Based on the above analysis, this embodiment performs transient reconstruction of the active power reference value during voltage sag: when a grid fault occurs, the system enters transient mode, and the active power reference value is promptly adjusted from the steady-state active power reference value to the transient active power reference value. The new maximum allowable power angle overshoot margin is significantly increased compared to the original maximum allowable power angle overshoot margin, thereby effectively improving the transient synchronization stability margin during the fault.

[0060] Although lowering the active power reference during voltage dips helps to increase the power margin, how to rationally design the transient active power reference value still needs further discussion.

[0061] In one embodiment, based on suppressing virtual power angle overshoot by lowering the active power reference value, a transient power reference reconfiguration structure based on power mismatch and synchronization risk is introduced to ensure stability while taking into account dynamic response performance.

[0062] The system also includes a synchronization risk detection module, which is used to detect the virtual power angle and its rate of change in real time and assess the synchronization operation risk of the virtual synchronous generator. A transient stability control module, connected to the active power control loop of the virtual synchronous generator, is used to generate a transient active power reference value based on the synchronization risk assessment result output by the synchronization risk detection module when a grid voltage sag is detected.

[0063] The synchronous risk detection module is used to calculate the virtual power angle and the rate of change of the virtual power angle in real time; based on the degree of proximity between the virtual power angle and the critical virtual power angle, and the magnitude of the rate of change of the virtual power angle, a synchronous risk coefficient is generated; the critical virtual power angle is determined according to the active power-power angle characteristic curve, corresponding to the unstable equilibrium point of the system.

[0064] At this time, the transient stability control module employs: a disturbance detection unit, used to detect whether the voltage amplitude drop depth at the virtual common coupling point exceeds a preset threshold, and outputs a trigger signal when the threshold is exceeded; and a control decision unit, used to generate a transient stability control command based on the trigger signal and the synchronization risk coefficient output by the synchronization risk detection module. For example, a transient stability control command is generated when a trigger signal is present and the synchronization risk coefficient is greater than a set threshold.

[0065] The transient active power reference value is calculated as follows:

[0066] ,

[0067] in, This is a reference value for transient active power. This is the steady-state active power reference value. The value can be 0 or 1. When the value is 1, the additional adjustment channel is activated. This refers to the actual output active power. To control the gain.

[0068] Let the state variable , The system can be written in the following state equation form:

[0069] ,

[0070] It is the rate of change of the virtual work angle. It is the rate of change of angular velocity deviation. Angular velocity, For virtual rotational inertia, The damping coefficient is... The angular frequency of the power grid. The rated angular frequency, Represents virtual power angle The actual output active power, when the additional regulation term is introduced, effectively suppresses the acceleration power when positive power mismatch occurs, thereby reducing the acceleration area in the initial stage of disturbance and increasing the system stability margin. As the control gain increases, this suppression effect is further enhanced, the power angle swing amplitude is effectively controlled, the unfavorable power angle overshoot is weakened, and the transient synchronization stability of the system is improved accordingly.

[0071] In one embodiment, the transient stability control module includes an additional adjustment channel connected between the active power reference value input terminal and the active power feedback terminal, which is used to adaptively correct the active power reference value based on the power mismatch between the steady-state active power reference value and the actual output active power.

[0072] Additional adjustment channels include:

[0073] The power mismatch calculation unit is used to calculate the power mismatch between the steady-state active power reference value and the actual output active power.

[0074] A proportional adjustment unit, having a control gain, is used to proportionally amplify the power mismatch and generate an additional adjustment amount.

[0075] An addition unit is used to subtract the additional adjustment amount from the steady-state active power reference value to generate a transient active power reference value;

[0076] The switching unit is connected between the power mismatch calculation unit and the proportional regulation unit. It is used to close when a transient stability control command is received and to open under normal operating conditions.

[0077] The smooth cut-in unit is used to gradually increase the additional adjustment amount from zero to the target value through a ramp function or low-pass filtering when the switching unit is closed.

[0078] The introduction of the smooth cut-in unit effectively avoids the sudden change in the additional regulation amount at the moment of triggering the transient stability control command, preventing the converter output current or voltage from being impacted by the step change in the control amount. The slope of the ramp function can be tuned according to the system inertia level and the severity of the fault. The cutoff frequency of the low-pass filter needs to take into account both response speed and smoothness requirements, and is usually set in the range of 0.5Hz to 2Hz to filter out high-frequency noise while ensuring the dynamic performance of transient regulation.

[0079] The power mismatch calculation unit uses a first-order inertial element to filter the actual output active power, eliminating high-frequency pulsation components in power measurement and ensuring the stability of the power mismatch calculation. The control gain of the proportional control unit is adaptively tuned based on the converter capacity and grid strength. Under weak grid conditions, the gain is appropriately reduced to avoid interactive oscillations with grid impedance, while under strong grid conditions, the gain can be increased to accelerate transient response speed.

[0080] The control logic of the switching unit is linked with the grid fault detection signal. When the voltage amplitude drops below the set threshold or the frequency change rate exceeds the allowable range, and combined with the synchronization risk coefficient output by the synchronization risk detection module, the transient stability control command is set, and the switching unit closes to start the additional regulation channel. After the grid voltage recovers to the normal range and remains stable for more than the preset delay, the transient stability control command is reset, and the switching unit is opened. At this time, the smooth cut-out unit gradually reduces the additional regulation amount to zero in a ramp manner symmetrical to the cut-in process, ensuring a smooth transition of the control mode switching.

[0081] The transient active power reference value output by the adder unit is processed by a limiting circuit and then sent to the active current inner loop controller. The limiting value is dynamically determined based on the converter's thermal limit current and the voltage level during grid faults, preventing excessive correction of the power reference value from triggering the converter's overcurrent protection. The decoupling design of this additional regulation channel from the main control loop allows the transient stability control function to be independently enabled and disabled, facilitating functional expansion and parameter debugging on existing converter control platforms.

[0082] In one embodiment, when the transient stability control module does not include a synchronization risk detection module, the transient stability control module includes: a disturbance detection unit, used to detect whether the voltage amplitude drop depth at the virtual common coupling point exceeds a preset threshold, and output a trigger signal when the threshold is exceeded;

[0083] A control decision unit is used to generate transient stability control commands based on the trigger signal;

[0084] The additional adjustment channel is controlled by the control decision unit and is put into operation when a transient stability control command is received.

[0085] In one embodiment, when the transient stability control module includes a synchronization risk detection module, the disturbance detection unit is used to detect whether the voltage amplitude drop depth at the virtual common coupling point exceeds a preset threshold, and outputs a trigger signal when the threshold is exceeded. The synchronization risk detection module is used to calculate the virtual power angle and the rate of change of the virtual power angle in real time; and generate a synchronization risk coefficient based on the proximity of the virtual power angle to the critical virtual power angle and the amplitude of the rate of change of the virtual power angle. At this time, the transient stability control module generates a transient stability control command based on the trigger signal and the synchronization risk coefficient output by the synchronization risk detection module.

[0086] The control decision unit can use a variety of strategies to generate transient stability control commands, which are described below.

[0087] Strategy 1: Binary Control Strategy

[0088] The binary control strategy is the simplest implementation, with control commands being Boolean values. When the trigger signal is valid and the synchronization risk coefficient exceeds a preset risk threshold, an input command is output; otherwise, an output command is output.

[0089] Strategy Two: Proportion Control Strategy

[0090] The proportional control strategy outputs continuous control commands, reflecting the intensity of input to the additional regulation channel. When the trigger signal is valid, the control command is proportional to the synchronization risk coefficient; when the synchronization risk coefficient exceeds the risk threshold, the control command saturates to 1.

[0091] Strategy 3: Hysteresis Control Strategy

[0092] The hysteresis control strategy introduces hysteresis characteristics to avoid frequent switching of control commands near thresholds. This strategy sets two thresholds: an activation threshold and an deactivation threshold, with the activation threshold being greater than the deactivation threshold. When the trigger signal is valid and the synchronization risk coefficient is greater than or equal to the activation threshold, an activation command is output; when the trigger signal is invalid or the synchronization risk coefficient is less than the deactivation threshold, a deactivation command is output; otherwise, the control command remains unchanged.

[0093] The control commands generated by the control decision unit are applied to the additional regulation channels in the following ways.

[0094] When the control command is an "engage" command, the switch unit closes, and the auxiliary control channel is put into operation. When the control command is a "disengage" command, the switch unit opens, and the auxiliary control channel is deactivated.

[0095] For continuous control commands, the control gain of the proportional control unit can also be adjusted. Specifically, the actual control gain is obtained by multiplying the base gain by the adjustable gain range and then adding the control command value. When the control command value is large, the control gain increases, and the power mismatch suppression effect is enhanced; when the control command value is small, the control gain decreases, and the power mismatch suppression effect is weakened.

[0096] Furthermore, control commands can also be used to adjust the cut-in rate of the smooth cut-in unit. When the control command value is large, the smooth cut-in time constant is small, and the additional adjustment amount increases rapidly; when the control command value is small, the smooth cut-in time constant is large, and the additional adjustment amount increases slowly. This adjustment method matches the control response speed with the level of risk.

[0097] The additional regulation channel is the execution unit of the transient stability control module, controlled by the control decision unit, and is activated upon receiving a transient stability control command. The switching unit connects the power mismatch calculation unit and the proportional regulation unit, and is also controlled by the control decision unit. When an activation command is received, the switching unit closes, connecting the additional regulation channel between the active power reference value input terminal and the active power feedback terminal; when an deactivation command is received, the switching unit opens.

[0098] The power mismatch calculation unit is used to calculate the power mismatch between the steady-state active power reference value and the actual output active power. The calculation formula is: the power mismatch equals the steady-state active power reference value minus the actual output active power. This power mismatch reflects the current degree of active power imbalance in the system: a positive value indicates that the reference power is greater than the actual power, and the system is in an accelerating state; a negative value indicates that the reference power is less than the actual power, and the system is in a decelerating state.

[0099] The proportional control unit has a control gain, which is used to proportionally amplify the power mismatch and generate an additional adjustment. The formula for calculating the additional adjustment is: the additional adjustment equals the control gain multiplied by the power mismatch. The value of the control gain is determined based on the stability boundary relationships stored in the parameter configuration module, and the typical value range is 0 to 2.0.

[0100] The physical meaning of control gain is as follows: when the control gain is greater than 0, the additional adjustment amount suppresses the power mismatch; when the control gain is greater than 1, the suppression effect exceeds the original power mismatch, and the equivalent acceleration power direction reversal is achieved; the larger the control gain, the stronger the suppression effect, and the more obvious the power angle overshoot suppression effect.

[0101] As a preferred embodiment, the transient stability control module further includes:

[0102] The critical virtual power angle determination unit is used to determine the critical virtual power angle corresponding to the unstable equilibrium point of the system under voltage sag conditions based on the active power-power angle characteristic curve.

[0103] The margin calculation unit is used to calculate the maximum allowable power angle overshoot margin δosm=δce-δe, where δe is the virtual power angle corresponding to the stable equilibrium point and δce is the critical virtual power angle corresponding to the unstable equilibrium point.

[0104] The additional adjustment channel also limits the downward adjustment of the transient active power reference value based on the maximum permissible power angle overshoot margin. The maximum permissible power angle overshoot margin reflects the allowable power angle fluctuation space between the system's stable equilibrium point and the loss-of-step boundary. The larger this margin, the more sufficient the system's transient stability reserve, and the more it can withstand greater disturbances without losing synchronism; the smaller the margin, the weaker the system's transient stability reserve, and the worse its tolerance to disturbances.

[0105] When generating transient active power reference values, the additional adjustment channel needs to limit the downward adjustment range according to the maximum allowable power angle overshoot margin to avoid over-adjustment leading to abnormal system response or wasted stability margin.

[0106] For example, the additional adjustment channel can limit the downward adjustment of the transient active power reference value according to the maximum permissible power angle overshoot margin in the following way: The downward adjustment of the transient active power reference value is limited to be proportional to the maximum permissible power angle overshoot margin. When the maximum permissible power angle overshoot margin is large, a larger downward adjustment is allowed; when the maximum permissible power angle overshoot margin is small, the downward adjustment is limited.

[0107] In other words, when a system has a large maximum permissible power angle overshoot margin, it means that the system can withstand relatively large transient disturbances without losing synchronization. In this case, a faster cut-in rate can be used to quickly bring the additional regulation to the target value, thereby suppressing the power angle overshoot as soon as possible. When the system's maximum permissible power angle overshoot margin is small, the system's tolerance to disturbances is weak. In this case, a slower cut-in rate should be used to allow the additional regulation to slowly reach the target value, avoiding transient shocks during the cut-in process that could cause the system to cross the stability boundary.

[0108] In a preferred embodiment, the additional adjustment channel also includes a smooth cut-in unit connected between the switching unit and the proportional adjustment unit. The smooth cut-in unit is used to gradually increase the additional adjustment amount from zero to the target value using a ramp function or low-pass filtering method when the switching unit is closed, thus avoiding sudden changes in the transient power reference value from impacting the system.

[0109] The smoothing cut-in unit can be implemented using a first-order low-pass filter. Its recursive formula is: the smoothed additional adjustment is equal to the filter coefficient multiplied by the current additional adjustment plus one minus the filter coefficient multiplied by the smoothed additional adjustment at the previous time step. The filter coefficient ranges from 0 to 1. The smaller the filter coefficient, the more obvious the smoothing effect and the slower the cut-in process; the larger the filter coefficient, the faster the response, but it may introduce abrupt changes.

[0110] The adder unit subtracts the additional regulation from the steady-state active power reference value to generate the transient active power reference value. The formula for calculating the transient active power reference value is: Transient active power reference value equals steady-state active power reference value minus the additional regulation. Substituting the expression for power mismatch, we obtain that the transient active power reference value equals steady-state active power reference value minus the control gain multiplied by the power mismatch.

[0111] In one embodiment, the transient stability control module may further include: a parameter configuration module for storing the stable boundary relationship between the virtual resistance and the control gain determined by numerical scanning, and providing parameter constraints for the proportional adjustment unit of the additional adjustment channel.

[0112] The specific implementation of numerical scanning is as follows: Multiple discrete virtual resistance values ​​are selected within a preset range. For each virtual resistance value, a critical control gain can be determined through time-domain simulation or frequency-domain analysis, ensuring that the system maintains transient synchronization and stability when the control gain is greater than or equal to this critical value. The critical control gain corresponding to each virtual resistance value is recorded, generating a stability boundary relationship. This stability boundary relationship can be stored in tabular form or fitted as a functional expression.

[0113] In actual operation, the parameter configuration module queries the corresponding minimum control gain from the stability boundary relationship based on the current virtual resistance value, and configures the control gain of the proportional control unit of the additional adjustment channel to be no less than this minimum control gain. When the virtual resistance changes, the parameter configuration module automatically adjusts the control gain to ensure that the system always maintains transient synchronization and stability.

[0114] On the other hand, see Figure 2 As shown, based on the above-described system for improving the transient synchronization stability of a virtual resistance VSG, a method for improving the transient synchronization stability of a virtual resistance VSG is provided, comprising:

[0115] Based on the control structure of the virtual synchronous generator, the virtual resistance is equivalent to a series voltage drop term located before the real common coupling point, the virtual common coupling point is constructed, and the virtual power angle is defined as the difference between the reference phase of the virtual synchronous generator output voltage and the phase of the grid.

[0116] Based on the virtual common coupling point and the virtual power angle, a mapping relationship between the active power injected into the power grid from the real common coupling point and the virtual power angle is established, and an active power-power angle characteristic curve is generated.

[0117] Based on the active power-power angle characteristic curve, determine the critical virtual power angle corresponding to the unstable equilibrium point of the system under voltage sag condition, and the maximum allowable power angle overshoot margin corresponding to the stable equilibrium point.

[0118] When a voltage dip in the grid is detected, the active power reference value is adaptively lowered based on the critical virtual power angle and the maximum allowable power angle overshoot margin, constraining the power angle trajectory not to exceed the critical virtual power angle.

[0119] Adaptive adjustment of active power reference value, including:

[0120] Calculate the power mismatch between the steady-state active power reference value and the actual output active power;

[0121] The power mismatch is proportionally amplified to generate an additional adjustment amount;

[0122] The additional adjustment is subtracted from the steady-state active power reference value to generate the transient active power reference value.

[0123] In one embodiment, when a voltage dip in the grid is detected, the depth of the voltage drop at the virtual point of common coupling is detected to see if it exceeds a preset threshold, and a trigger signal is output when the threshold is exceeded; the virtual power angle and the rate of change of the virtual power angle are calculated in real time; and a synchronization risk coefficient is generated based on the degree of proximity between the virtual power angle and the critical virtual power angle, as well as the magnitude of the rate of change of the virtual power angle.

[0124] The transient stability control module generates transient stability control commands based on the trigger signal and the synchronization risk coefficient output by the synchronization risk detection module. This adaptively lowers the active power reference value.

[0125] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A system for improving the transient synchronization stability of a virtual resistance VSG, characterized in that, include: The virtual common coupling point reconfiguration module is used to construct a virtual common coupling point by equating the virtual resistance to a series voltage drop term located before the real common coupling point according to the control structure of the virtual synchronous generator, and to define the virtual power angle as the difference between the reference phase of the virtual synchronous generator output voltage and the grid phase. The power characteristic mapping module is used to establish a mapping relationship between the active power injected into the power grid from the real common coupling point and the virtual power angle based on the virtual common coupling point and the virtual power angle, and generate an active power-power angle characteristic curve. The stability boundary determination module is used to determine the critical virtual power angle corresponding to the unstable equilibrium point of the system under voltage sag condition, and the maximum allowable power angle overshoot margin corresponding to the stable equilibrium point, based on the active power-power angle characteristic curve. The transient stability control module is used to adaptively reduce the active power reference value based on the critical virtual power angle and the maximum allowable power angle overshoot margin when a grid voltage sag is detected, so as to constrain the power angle trajectory to not exceed the critical virtual power angle.

2. The system for improving the transient synchronization stability of a virtual resistance VSG according to claim 1, characterized in that, The transient stability control module includes an additional adjustment channel, which is connected between the active power reference value input terminal and the active power feedback terminal. The additional adjustment channel is used to adaptively correct the active power reference value based on the power mismatch between the steady-state active power reference value and the actual output active power.

3. The system for improving the transient synchronization stability of a virtual resistance VSG according to claim 2, characterized in that, The additional adjustment channel includes: a power mismatch calculation unit for calculating the power mismatch between the steady-state active power reference value and the actual output active power; a proportional adjustment unit with control gain for proportionally amplifying the power mismatch to generate an additional adjustment amount; and an addition unit for subtracting the additional adjustment amount from the steady-state active power reference value to generate a transient active power reference value.

4. The system for improving the transient synchronization stability of virtual resistance VSG according to claim 3, characterized in that, The transient active power reference value is calculated in the following manner: ， in, This is a reference value for transient active power. This is the steady-state active power reference value. The value can be 0 or 1. When the value is 1, the additional adjustment channel is activated. This refers to the actual output active power. To control the gain.

5. The system for improving the transient synchronization stability of a virtual resistance VSG according to claim 3, characterized in that, The additional adjustment channel further includes: a switching unit, connected between the power mismatch calculation unit and the proportional adjustment unit, used to close when a transient stability control command is received and open during normal operation; and a smooth cut-in unit, used to gradually increase the additional adjustment amount from zero to the target value through a ramp function or low-pass filtering when the switching unit is closed.

6. The system for improving the transient synchronization stability of a virtual resistance VSG according to claim 2, characterized in that, The transient stability control module further includes: a disturbance detection unit, used to detect whether the voltage amplitude drop depth at the virtual common coupling point exceeds a preset threshold, and output a trigger signal when the threshold is exceeded; a control decision unit, used to generate a transient stability control command based on the trigger signal; and an additional adjustment channel controlled by the control decision unit, which is put into operation when a transient stability control command is received.

7. The system for improving the transient synchronization stability of a virtual resistance VSG according to claim 2, characterized in that, The transient stability control module further includes: The critical virtual power angle determination unit is used to determine the critical virtual power angle corresponding to the unstable equilibrium point of the system under voltage sag conditions based on the active power-power angle characteristic curve. The margin calculation unit is used to calculate the maximum allowable power angle overshoot margin δosm=δce-δe, where δe is the virtual power angle corresponding to the stable equilibrium point and δce is the critical virtual power angle corresponding to the unstable equilibrium point. The additional adjustment channel also limits the downward adjustment range of the transient active power reference value based on the maximum permissible power angle overshoot margin.

8. A system for improving the transient synchronization stability of a virtual resistance VSG according to claim 6, characterized in that, The transient stability control module further includes: The synchronous risk detection module is used to calculate the virtual power angle and the rate of change of the virtual power angle in real time; based on the degree of proximity between the virtual power angle and the critical virtual power angle, and the magnitude of the rate of change of the virtual power angle, a synchronous risk coefficient is generated. The control decision unit generates transient stability control commands based on the trigger signal and the synchronization risk coefficient output by the synchronization risk detection module.

9. A method for improving the transient synchronization stability of a virtual resistance VSG, characterized in that, include: Based on the control structure of the virtual synchronous generator, the virtual resistance is equivalent to a series voltage drop term located before the real common coupling point, the virtual common coupling point is constructed, and the virtual power angle is defined as the difference between the reference phase of the virtual synchronous generator output voltage and the phase of the grid. Based on the virtual common coupling point and the virtual power angle, a mapping relationship between the active power injected into the power grid from the real common coupling point and the virtual power angle is established, and an active power-power angle characteristic curve is generated. Based on the active power-power angle characteristic curve, determine the critical virtual power angle corresponding to the unstable equilibrium point of the system under voltage sag condition, and the maximum allowable power angle overshoot margin corresponding to the stable equilibrium point. When a voltage dip in the grid is detected, the active power reference value is adaptively adjusted down based on the critical virtual power angle and the maximum allowable power angle overshoot margin, constraining the power angle trajectory not to exceed the critical virtual power angle.

10. A method for improving the transient synchronization stability of a virtual resistance VSG according to claim 9, characterized in that, Adaptive adjustment of active power reference value, including: Calculate the power mismatch between the steady-state active power reference value and the actual output active power; The power mismatch is proportionally amplified to generate an additional adjustment amount; The additional adjustment is subtracted from the steady-state active power reference value to generate the transient active power reference value.

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