Transient synchronous stability control method and system for grid-connected converter

Abstract

The invention provides a transient synchronous stability control method and system for a grid-connected converter, and the method comprises the steps: carrying out the closed-loop control of a current inner loop of the grid-connected converter, enabling the grid-connected converter to be equivalent to a current source, and providing an equivalent power grid; providing a phase-locked loop state equation for describing the dynamic state of the phase-locked loop according to the current source and the equivalent power grid; parameters meeting the control stability requirement of the phase-locked loop are obtained according to the set bandwidth and phase margin range; based on the phase-locked loop state equation, calculating an attraction domain of the phase-locked loop to obtain an output amplitude limiting value of a phase-locked loop integrator; analyzing amplitude limiting values output by a phase-locked loop integrator under different phase-locked loop parameters and different fault clearing times to obtain a strategy for maintaining transient synchronous stability of the phase-locked loop; based on the strategy, a phase-locked loop working point under various fault conditions can return to a balance point through simple constraint after disturbance is eliminated, and transient synchronous stable control of the grid-connected converter is realized.

Description

Technical Field

[0001] The present invention relates to the field of renewable energy power generation and power electronics technology, and in particular to a method and system for transient synchronous stability control of a grid-connected converter based on different parameter attraction domains and phase-locked loop limiting technology, and also provides a corresponding controller and a controller-readable storage medium. Background Art

[0002] In recent years, the penetration level of renewable energy resources (e.g., wind and solar) in power systems has been increasing. As an important interface for the integration of these resources with the grid, parallel voltage source converters (VSCs) have been widely used. Since the dynamic characteristics of VSCs are mainly determined by their control methods, the stability of VSCs is becoming a new issue that needs to be addressed to ensure the safe and stable operation of VSCs and power systems dominated by renewable energy. In this context, one of the main aspects of VSC stability research is its transient stability problem, especially the synchronization capability when subjected to large disturbances (e.g., grid faults). This has led to an active class of research topics on VSC stability, commonly known as transient synchronous stability (TSS) problems, which are mainly caused by the nonlinear behavior of the VSC phase locked loop (PLL), which manifests as a loss of synchronization after large grid disturbances (e.g., grid faults).

[0003] At present, many scholars have proposed control strategies for the problem of PLL transient synchronization. Most of these control mechanisms are too complicated and important links are ignored in the analysis, resulting in an inability to accurately describe the dynamics of the phase-locked loop during a fault, and the strategies made are too conservative. Summary of the invention

[0004] In view of the above-mentioned deficiencies in the prior art, the present invention provides a method and system for transient synchronous stability control of a grid-connected converter based on different parameter attraction domains and phase-locked loop limiting technology, and also provides a corresponding controller and a controller-readable storage medium.

[0005] According to one aspect of the present invention, a method for transient synchronous stability control of a grid-connected converter is provided, comprising:

[0006] Performing closed-loop control on the current inner loop of the grid-connected converter to make the grid-connected converter equivalent to a current source and provide an equivalent power grid;

[0007] According to the current source and the equivalent power grid, a phase-locked loop state equation is provided for describing the dynamics of the phase-locked loop;

[0008] Given different PLL bandwidths and phase margins, obtain PLL parameters that can meet the PLL control stability requirements;

[0009] Based on the phase-locked loop state equation and according to the acquired phase-locked loop parameters, the region of attraction (ROA) of the phase-locked loop is calculated to obtain the output limit value of the phase-locked loop integrator;

[0010] By analyzing the output limit value of the phase-locked loop integrator under different phase-locked loop parameters and different fault removal times, a strategy for maintaining the transient synchronization stability of the phase-locked loop is obtained;

[0011] Based on the strategy, transient synchronous stability control of the grid-connected converter is achieved.

[0012] Preferably, the closed-loop control of the inner loop of the current of the grid-connected converter is performed to make the grid-connected converter equivalent to a current source and provide an equivalent power grid, including:

[0013] The DC power supply is controlled by the inverter to output AC current, which is then filtered by the filter and fed into the grid. t After sampling, the input is input into the phase-locked loop to obtain the angle input current control loop. The current control loop outputs the modulation wave for SPWM modulation, and finally, after PWM modulation, it is fed back to the converter for DC-AC conversion to form a closed-loop control of the converter current inner loop, thereby making the grid-connected converter equivalent to a current source.

[0014] The Thevenin circuit is used as the equivalent power grid; where the power grid impedance Z g =R g +jω pll L g / ω s , R g and L g are the equivalent resistance and equivalent inductance of the AC power grid, respectively. j is an imaginary unit representing the reactance component of the power grid impedance. pll is the output frequency of the phase-locked loop, ω s is the synchronization angular frequency.

[0015] Preferably, providing a phase-locked loop state equation for describing the dynamics of the phase-locked loop includes:

[0016] The phase-locked loop PI controller integrator output x int and the phase-locked loop output angle θ and the grid voltage angle θ g The difference δ = θ-θ g As the state variable, the phase-locked loop state equation that can describe the dynamics of the phase-locked loop is constructed as:

[0017]

[0018] In formula (1), is the derivative of the power angle δ, is the derivative of the integrator output xint, k p is the proportional gain of the phase-locked loop, k i is the phase-locked loop integral gain, U g is the grid-side voltage amplitude; ω s is the synchronous angular frequency, t is the integration time, is the initial phase angle of the power grid; I sd is the d-axis current component, I sq is the q-axis current component.

[0019] Preferably, given different phase-locked loop bandwidths and phase margins, obtaining phase-locked loop parameters that can meet the phase-locked loop control stability requirements includes:

[0020] According to different bandwidth ω b and phase margin γ, to obtain the corresponding phase-locked loop parameters that meet the phase-locked loop stability requirements; where:

[0021] Construct the open-loop transfer function G of the phase-locked loop pll (s) is:

[0022]

[0023] In formula (2), k p is the proportional gain of the phase-locked loop, k i is the phase-locked loop integral gain, s=jω is the Laplace variable, ω is the selected frequency;

[0024] By using formula (3), we can calculate the bandwidth ω b The open-loop transfer function G of the phase-locked loop is pll (s) The amplitude margin is equal to 1, and the phase margin γ satisfies the open-loop transfer function required for phase-locked loop control stability |G pll (jω b )|:

[0025]

[0026] According to the open-loop transfer function |G pll (jω b )|, and obtain the phase-locked loop proportional gain k that can meet the phase-locked loop control stability requirements p and the phase-locked loop integral gain k i , which is the phase-locked loop parameter.

[0027] Preferably, the acquired phase-locked loop parameters include the phase-locked loop proportional gain k p and the phase-locked loop integral gain k iBased on the phase-locked loop state equation, according to the acquired phase-locked loop parameters, the attraction domain of the phase-locked loop is calculated to obtain the output limit value of the phase-locked loop integrator, including:

[0028] The phase-locked loop proportional gain k p and the phase-locked loop integral gain k i Substituting into the phase-locked loop state equation, we get the current phase-locked loop integrator output x int and the angle difference δ;

[0029] According to the current phase-locked loop integrator output x int and the angle difference δ, set the derivative of the phase-locked loop state equation to 0, solve the stable equilibrium point of the phase-locked loop, and identify the real attraction domain of the phase-locked loop by calculating the reverse time integral of the two unstable equilibrium points adjacent to the stable equilibrium point in the direction of the stable manifold;

[0030] According to the boundary data of the attraction domain, the phase angle range is selected to be the phase-locked loop integrator output x within two unstable equilibrium points. int , the phase-locked loop integrator output x between these two unstable equilibrium points int The maximum and minimum output values ​​of the phase-locked loop integrator are reflected, the maximum output value of the phase-locked loop integrator is used as a reference value for the upper limit, and the minimum output value of the phase-locked loop integrator is used as a reference value for the lower limit, so as to obtain the output limit value of the phase-locked loop integrator.

[0031] Preferably, the selected phase angle range is the phase-locked loop integrator output x within two unstable equilibrium points. int , including any of the following methods:

[0032] Using the fixed value limiting method, the integrator output x int The minimum range [-80,30] is used as the integrator limit design value;

[0033] The parameter attraction domain limit design method is used to design the limit value under specific parameters, where the maximum value of the output limit value x int_max and the minimum value x of the output limit value int_min They are all multiplied by the set ratio to ensure that the working point is constrained within a certain range where the attraction domain exists, and the working point returns to the equilibrium point in the attraction domain after the fault is cleared.

[0034] Preferably, the strategy for maintaining the transient synchronization stability of the phase-locked loop is obtained by analyzing the output limit value of the phase-locked loop integrator under different phase-locked loop parameters and different fault removal times, including:

[0035] During fault ride-through, the converter outputs reactive current according to the grid-connected guidelines to raise the grid-connected point voltage; after the fault is removed, the converter resumes active power output; the continuity of the attraction domain of the phase-locked loop is determined by whether the manifold curve starting from the unstable equilibrium point crosses the horizontal axis. If it crosses the horizontal axis, the attraction domain of the phase-locked loop is discontinuous; if it does not cross the horizontal axis, the attraction domain of the phase-locked loop is continuous;

[0036] If the attraction domain of the phase-locked loop is continuous, the LVRT control strategy is activated when a fault occurs; due to the resistive impedance of the power grid, the angle difference δ and the integrator output x int The derivative of is negative, and the working point moves in the negative direction. In addition, the attraction domain of the phase-locked loop is continuous and the integrator output x is set. int The working point will always be in the attraction domain of the phase-locked loop. If the fault is removed in the attraction domain of the original equilibrium point, the working point will directly return to the original equilibrium point. If the fault is removed when the working point moves to the attraction domain of other adjacent equilibrium points, the working point will stabilize at other equilibrium points. When the working point moves to the lower limit of the fault removal time, the working point moves along the limit value and returns to the equilibrium point after the fault is removed.

[0037] If the attraction domain of the phase-locked loop is discontinuous, the LVRT control strategy is activated when the fault occurs; the operating point moves in the negative direction; if the fault is removed in the gap of the attraction domain of the phase-locked loop, the operating point moves in the positive direction outside the attraction domain of the phase-locked loop until the integrator outputs x int The upper limit is reached and the working point moves along the upper limit until it enters the attraction domain and is attracted to the equilibrium point again in the attraction domain. If the working point is cut off in the attraction domain of the phase-locked loop, the working point is directly attracted back to the equilibrium point.

[0038] Regardless of whether the attraction domain of the phase-locked loop is continuous, the LVRT strategy is not activated during a fault; the AC side current is still output according to the original output plan, and the angle difference δ and the integrator output x int The derivative of is positive, and the working point moves in the positive direction; if the fault is removed within the attraction domain of the phase-locked loop, the working point is attracted back to the equilibrium point; if the fault is removed outside the attraction domain of the phase-locked loop, the integrator output x int If the limit value is not reached, the integrator output x int will continue to increase until the integrator output x int The upper limit is limited, and it moves along the limit value in the direction of increasing the angle difference δ. The working point moves along the limit upper limit until the fault is cleared. Then the working point moves along the limit value into the attraction domain of the phase-locked loop and is finally attracted to the equilibrium point.

[0039] According to another aspect of the present invention, there is provided a grid-connected converter transient synchronous stability control system, comprising:

[0040] A converter current inner loop control module, which is used to perform closed-loop control on the grid-connected converter current inner loop, so as to make the grid-connected converter equivalent to a current source and provide an equivalent power grid;

[0041] A phase-locked loop state equation building module, which provides a phase-locked loop state equation for describing the dynamics of the phase-locked loop according to the current source and the equivalent power grid;

[0042] A phase-locked loop parameter design module, which obtains phase-locked loop parameters that can meet the phase-locked loop control stability requirements according to the set phase-locked loop bandwidth and phase margin;

[0043] An attraction domain calculation module, which calculates the attraction domain of the phase-locked loop based on the phase-locked loop state equation and the acquired phase-locked loop parameters to obtain the output limit value of the phase-locked loop integrator;

[0044] A strategy analysis module, which obtains a strategy for maintaining the transient synchronization stability of the phase-locked loop by analyzing the output limit value of the phase-locked loop integrator under different phase-locked loop parameters and different fault removal times;

[0045] A converter control module realizes transient synchronous stability control of the grid-connected converter based on the strategy.

[0046] According to a third aspect of the present invention, there is provided a transient synchronous stability controller for a grid-connected converter, comprising: a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein when the processor executes the program, it can be used for the method described in any one of the above-mentioned methods of the present invention, or, to run the system described in any one of the above-mentioned systems of the present invention.

[0047] According to a fourth aspect of the present invention, there is provided a controller-readable storage medium having a computer program stored thereon, which, when executed by a processor, can be used to execute any of the above-described methods of the present invention, or to run any of the above-described systems of the present invention.

[0048] Due to the adoption of the above technical solution, the present invention has at least one of the following beneficial effects compared with the prior art:

[0049] The transient synchronous stability control method and system of the grid-connected converter provided by the present invention, the quantitative output limiting technology of the integrator of the phase-locked loop PI controller, the working point begins to deviate from the equilibrium point after a large disturbance, and the limiting value is calculated by a numerical method to maximize the dynamic response of the phase-locked loop, and the limiting can ensure that the working point is limited to a certain range, and ensure that the phase-locked loop working point returns to the attraction domain after the fault is cleared, thereby ensuring the transient synchronous stability of the converter.

[0050] The present invention provides a transient synchronous stability control method and system for a grid-connected converter, and a phase-locked loop integrator limiting design method under different phase-locked loop parameters. Transient synchronous stability is strongly correlated with the phase-locked loop parameters, so it is difficult to satisfy the stability under all conditions by designing a single parameter. Therefore, the limiting value can be designed by the numerical method proposed in this article, providing a reference for the integrator limiting under different parameters.

[0051] The transient synchronization stability control method and system of the grid-connected converter provided by the present invention aim at the problem of transient synchronization instability that may be triggered when a large disturbance fault occurs in the grid-connected converter under different phase-locked loop parameters, and propose a PLL integrator output limit calculation method. According to the stability domain under different phase-locked loop parameters, the corresponding limit extreme values ​​are calculated respectively, and the working point during the fault is constrained within a certain range so that it does not deviate too far from the equilibrium point during the fault period. After the fault is cleared, it can be guaranteed to return to the ROA as soon as possible and finally return to the equilibrium point. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0053] Figure 1 The present invention is a flowchart of a transient synchronous stability control method for a grid-connected converter in a preferred embodiment of the present invention.

[0054] Figure 2 It is a classic grid-connected converter topology and control structure during fault.

[0055] Figure 3 This is a classic phase-locked loop control block diagram.

[0056] Figure 4 is the ROA calculated by numerical method.

[0057] Figure 5 are the phase-locked loop parameters (part) under different bandwidths and phase margins; where (a) is the proportional gain k calculated under different phase-locked loop bandwidths and phase margins p , (b) is the integral gain k calculated under different PLL bandwidth and phase margin i .

[0058] Figure 6 is the attraction domain x under different bandwidth and phase margin int Extreme value (part); where (a) is the upper limit of the attraction domain boundary x calculated under different PLL bandwidths and phase margins int_max , (b) is the lower limit of the attraction domain boundary x calculated under different PLL bandwidths and phase margins int_min .

[0059] Figure 7Design flow chart for parameter attraction domain limiting.

[0060] Figure 8 The operating conditions of the working point when a transient fault occurs under different PLL parameters and different fault clearing times, where (a) the LVRT control strategy is started, the ROA is continuous, and the fault is removed within the original ROA; (b) the LVRT control strategy is started, the ROA is continuous, and the fault is removed within other ROAs; (c) the LVRT control strategy is started, the ROA is discontinuous, and the fault is removed at the gap; (d) the LVRT control strategy is not started, and the fault is removed outside the ROA.

[0061] Fig. 9 This is a flow chart of the working point's destination after a fault occurs.

[0062] Fig.10 The figure is a schematic diagram of the component modules of the transient synchronous stability control system of the grid-connected converter in a preferred embodiment of the present invention. DETAILED DESCRIPTION

[0063] The following is a detailed description of the embodiments of the present invention: This embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation method and a specific operation process are given. It should be pointed out that for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

[0064] According to existing technical research, such as Zhang Yu, Zhang Chen, Cai Xu, etc. Transient synchronization stability analysis of grid-connected converters: Stability region estimation and stabilization control [J]. Proceedings of the CSEE, 2022, 42(21): 7871-7884. DOI: 10.13334 / j.0258-8013.pcsee.211148., an integral limiter is used to effectively stabilize the phase-locked loop control loop that has already experienced synchronization instability. int The integral limit is set within the upper and lower boundaries of the stable domain, so that it passes through the stable domain. Then the integral limit will have the ability to guide the system state into the stable domain, so that the converter that has already experienced synchronous instability can be stabilized. The Lyapunov function is used to obtain the ROA boundary, and the x at the equilibrium point is taken. intValue is used as the limit value design standard. However, the prior art still has the following technical problems: 1. The Lyapunov function method is used to estimate the system ROA, and the limit value is obtained based on this ROA. This method is conservative to a certain extent, and the influence of the LVRT control strategy on the motion trajectory of the working point is not considered. The phase-locked loop parameters will change the continuity of the ROA, and different limits will be triggered due to the influence of the fault clearing time. The motion trajectory of the working point during the fault will also be different. Therefore, the corresponding limiting method should be selected for specific phase-locked loop parameters. In summary, the control strategies proposed for the problem of PLL transient synchronization are mostly too complicated in their control mechanism, and important links are ignored in the analysis, which makes it impossible to accurately describe the dynamics of the phase-locked loop during the fault, and the strategy made is too conservative.

[0065] In view of the above problems, an embodiment of the present invention proposes a transient synchronization stability control method for a grid-connected converter. The method analyzes the attraction domain (Region of area, ROA) of the phase-locked loop under different parameters, gives the corresponding quantitative limiting of the phase-locked loop integrator output, and realizes a simple transient synchronization stability strategy. This strategy can be commonly used in the currently widely used phase-locked loop-based grid-following new energy grid-connected converters. Through simple constraints, the phase-locked loop operating point in various fault conditions can return to the equilibrium point after the disturbance is cleared, and the dynamic response of the phase-locked loop is guaranteed to the maximum extent.

[0066] like Figure 1 As shown, the grid-connected converter transient synchronous stability control method provided by this embodiment may include:

[0067] S1, performing closed-loop control on the inner current loop of the grid-connected converter to make the grid-connected converter equivalent to a current source and provide an equivalent power grid;

[0068] S2, based on the current source and the equivalent power grid, providing a phase-locked loop state equation that can accurately describe the dynamics of the phase-locked loop, for calculating the region of attraction (ROA) of the phase-locked loop;

[0069] S3, according to the set phase-locked loop bandwidth and phase margin range, the phase-locked loop parameters that can meet the phase-locked loop control stability requirements are calculated through the phase-locked loop open-loop transfer function;

[0070] S4, based on the state equation of the phase-locked loop and according to the acquired phase-locked loop parameters, the region of attraction (ROA) of the phase-locked loop is calculated to obtain the output limit value of the phase-locked loop integrator;

[0071] S5, by analyzing the output limit value of the phase-locked loop integrator under different phase-locked loop parameters and different fault removal times, a strategy for maintaining the transient synchronization stability of the phase-locked loop is obtained;

[0072] S6, based on this strategy, the transient synchronous stability control of the grid-connected converter is realized.

[0073] In a preferred embodiment of S1, a closed-loop control is performed on the inner loop of the current of the grid-connected converter to make the grid-connected converter equivalent to a current source and provide an equivalent power grid, including:

[0074] The DC power supply is controlled by the inverter to output AC current, which is then filtered by the filter and fed into the grid. t After sampling, the input is input into the phase-locked loop to obtain the angle input current control loop. The current control loop outputs the modulation wave for SPWM modulation, and finally, after PWM modulation, it is fed back to the converter for DC-AC conversion to form a closed-loop control of the converter current inner loop, so that the grid-connected converter is equivalent to a current source;

[0075] The Thevenin circuit is used as the equivalent power grid; where the power grid impedance Z g =R g +jω pll L g / ω s , R g and L g are the equivalent resistance and equivalent inductance of the AC power grid, respectively. j is an imaginary unit representing the reactance component of the power grid impedance. pll is the output frequency of the phase-locked loop, ω s is the synchronization angular frequency.

[0076] In a preferred embodiment of S2, a phase-locked loop state equation capable of accurately describing the dynamics of the phase-locked loop is provided, including:

[0077] The phase-locked loop PI controller integrator output x int and the phase-locked loop output angle θ and the grid voltage angle θ g The difference δ = θ-θ g As the state variable, the phase-locked loop state equation that can accurately describe the dynamics of the phase-locked loop is constructed as:

[0078]

[0079] In formula (1), is the derivative of the power angle δ, is the derivative of the integrator output xint, k p is the proportional gain of the phase-locked loop, k i is the phase-locked loop integral gain, U g is the grid-side voltage amplitude; ω s is the synchronous angular frequency, t is the integration time, is the initial phase angle of the power grid; I sd is the d-axis current component, I sqis the q-axis current component.

[0080] In a preferred embodiment of S3, according to the set phase-locked loop bandwidth and phase margin range, the phase-locked loop parameters that can meet the phase-locked loop control stability requirements are obtained, including:

[0081] According to different bandwidth ω b The corresponding phase-locked loop parameters that meet the phase margin stability range of the phase-locked loop are obtained by summing the phase margin γ; where:

[0082] Construct the open-loop transfer function G of the phase-locked loop pll (s) is:

[0083]

[0084] In formula (2), k p is the proportional gain of the phase-locked loop, k i is the phase-locked loop integral gain, s=jω is the Laplace variable, ω is the selected frequency;

[0085] By using formula (3), we can calculate the bandwidth ω b The open-loop transfer function G of the phase-locked loop is pll (s) The amplitude margin is equal to 1, and the phase margin γ satisfies the open-loop transfer function required for phase-locked loop control stability |G pll (jω b )|:

[0086]

[0087] In formula (3), k p is the proportional gain of the phase-locked loop, k i is the phase-locked loop integral gain, ω b is the phase-locked loop bandwidth, γ is the phase margin; according to the open-loop transfer function |G pll (jω b )|, and obtain the phase-locked loop proportional gain k that can meet the phase-locked loop control stability requirements p and the phase-locked loop integral gain k i , which is the phase-locked loop parameter.

[0088] In a preferred embodiment, the phase margin γ generally ranges from 30° to 70°.

[0089] Furthermore, in a preferred embodiment, the bandwidth ω b The frequency range is 5 Hz to 20 Hz, and the phase margin γ is preferably 45° to 70°.

[0090] In a preferred embodiment of S4, the acquired phase-locked loop parameters include the phase-locked loop proportional gain k p and the phase-locked loop integral gain k iBased on the phase-locked loop state equation, according to the acquired phase-locked loop parameters, the attraction domain of the phase-locked loop is calculated to obtain the output limit value of the phase-locked loop integrator, including:

[0091] The phase-locked loop proportional gain k p and the phase-locked loop integral gain k i Substituting into the phase-locked loop state equation, we get the current phase-locked loop integrator output x int and the angle difference δ;

[0092] According to the current phase-locked loop integrator output x int and the angle difference δ, set the derivative of the phase-locked loop state equation to 0, solve the stable equilibrium point of the phase-locked loop, and identify the real attraction domain of the phase-locked loop by calculating the reverse time integral of the two unstable equilibrium points adjacent to the stable equilibrium point in the direction of the stable manifold;

[0093] According to the boundary data of the attraction domain, the phase angle range is selected to be the phase-locked loop integrator output x within two unstable equilibrium points. int , the phase-locked loop integrator output x between these two unstable equilibrium points int It can reflect the maximum and minimum output values ​​of the phase-locked loop integrator, take the maximum output value of the phase-locked loop integrator as the reference value of the upper limit, and take the minimum output value of the phase-locked loop integrator as the reference value of the lower limit, so as to obtain the output limit value of the phase-locked loop integrator.

[0094] Further, in a preferred embodiment of S4, the phase angle range is selected to be within two unstable equilibrium points, and the output x of the phase-locked loop integrator is selected to be within two unstable equilibrium points. int , including any of the following methods:

[0095] -Use the fixed value limiting method to set the integrator output x int The minimum range [-80,30] is used as the integrator limit design value;

[0096] -Use the parameter attraction domain limit design method to design the limit value under specific parameters, where the maximum value x of the output limit value is int_max The minimum value x of the output limit value int_min They are all multiplied by a set ratio (eg, 90%) to ensure that the operating point is constrained within a certain range of the attraction domain, and that the operating point returns to the equilibrium point within the attraction domain after the fault is cleared.

[0097] In a preferred embodiment of S5, by analyzing the output limit value of the phase loop integrator under different phase-locked loop parameters and different fault removal times, a strategy for maintaining the transient synchronization stability of the phase-locked loop is obtained, including:

[0098] During fault ride-through, the converter outputs reactive current according to the grid-connected guidelines to raise the grid-connected point voltage; after the fault is removed, the converter resumes active power output; the continuity of the attraction domain of the phase-locked loop is determined by whether the manifold curve starting from the unstable equilibrium point crosses the horizontal axis. If it crosses the horizontal axis, the attraction domain of the phase-locked loop is discontinuous; if it does not cross the horizontal axis, the attraction domain of the phase-locked loop is continuous;

[0099] If the attraction domain of the phase-locked loop is continuous, the LVRT control strategy is activated when a fault occurs; due to the resistive impedance of the power grid, the angle difference δ and the integrator output x int The derivative of is negative, and the working point moves in the negative direction. In addition, the attraction domain of the phase-locked loop is continuous and the integrator output x is set. int The working point will always be in the attraction domain of the phase-locked loop. If the fault is removed in the attraction domain of the original equilibrium point, the working point will directly return to the original equilibrium point. If the fault is removed when the working point moves to the attraction domain of other adjacent equilibrium points, the working point will stabilize at other equilibrium points. When the working point moves to the lower limit of the fault removal time, the working point moves along the limit value and returns to the equilibrium point after the fault is removed.

[0100] If the attraction domain of the phase-locked loop is discontinuous, the LVRT control strategy is activated when a fault occurs; the operating point will move in the negative direction; if the fault is removed in the gap of the attraction domain of the phase-locked loop, the operating point will move in the positive direction outside the attraction domain of the phase-locked loop until the integrator outputs x int The upper limit is reached and the working point moves along the upper limit until it enters the attraction domain and is attracted to the equilibrium point again in the attraction domain. If the working point is cut off in the attraction domain of the phase-locked loop, the working point is directly attracted back to the equilibrium point.

[0101] Regardless of whether the attraction domain of the phase-locked loop is continuous, the LVRT strategy is not activated during a fault; the AC side current is still output according to the original output plan, and the angle difference δ and the integrator output x int The derivative of is positive, and the working point moves in the positive direction; if the fault is removed within the attraction domain of the phase-locked loop, the working point is attracted back to the equilibrium point; if the fault is removed outside the attraction domain of the phase-locked loop, the integrator output x int If the limit value is not reached, the integrator output x int will continue to increase until the integrator outputs x int The upper limit is limited, and it moves along the limit value in the direction of increasing the angle difference δ. The working point moves along the limit upper limit until the fault is cleared. Then the working point moves along the limit value into the attraction domain of the phase-locked loop and is finally attracted to the equilibrium point.

[0102] The transient synchronization stability control method for the grid-connected converter provided by the above-mentioned embodiment of the present invention fully considers the wide range variation of the phase-locked loop parameters and the phase-locked loop limiting, and obtains parameters that meet the stability requirements of the phase-locked loop in a wide range through the control parameter design principle, so that the design method can be applied to most phase-locked loop systems; the attraction domain of the phase-locked loop is drawn through the numerical integration method, and the extreme values ​​of the limiting design are obtained from the upper and lower boundaries of the attraction domain, and the designed limiting value is obtained by selecting a suitable ratio and multiplying the extreme value; through the analysis and testing of the limiting method under different phase-locked loop parameters (corresponding to different ROA conditions) and different fault removal times, the versatility of the integrator limiting method for maintaining the transient synchronization stability of the PLL under various conditions is verified, so that the design method can be simply and effectively applied to the phase-locked loop.

[0103] The transient synchronous stability control method for the grid-connected converter provided by the above embodiment of the present invention is further described in detail below in conjunction with a specific application example and the accompanying drawings, so as to fully understand the quantitative output constraint technology of the phase-locked loop PI controller integrator proposed by the present invention.

[0104] In this specific application example, the following description is given through the specific application scenario of the grid-side converter.

[0105] like Figure 2 As shown in the figure, the classic current-type grid-connected converter topology and its low voltage ride-through control structure are shown. The DC side power supply outputs AC current after being controlled by the inverter, and then fed into the grid after being filtered by the filter. t ) is sampled and then input into the phase-locked loop (PLL) to obtain the angle input current control loop, and finally fed back to the converter through PWM modulation for DC-AC conversion to form a closed-loop control. Except for frequency and angle, all electrical quantities are expressed in per unit value. Due to the closed-loop control of the converter current inner loop, the grid-connected converter can be equivalent to a current source, and the Thevenin circuit equivalent grid is used, where Z g =R g +jω pll L g / ω s is the grid impedance, R g and L g are the equivalent resistance and equivalent inductance of the AC power grid respectively.

[0106] like Figure 3 As shown in the figure, the classic phase-locked loop control block diagram is shown, where ω pll is the output frequency of the PLL, ω s is the synchronous angular frequency, where U g is the grid-side voltage amplitude, is the grid voltage angle, t is the integration time; I=(I sd +jIsq ) jθ is the converter equivalent output current, I sd is the d-axis current component, I sq is the q-axis current component, and θ is the PLL output angle. int and the phase-locked loop (PLL) output angle θ and the grid voltage angle θ g The difference δ = θ-θ g As state variables, the state equation that accurately describes the dynamics of the phase-locked loop is obtained:

[0107]

[0108] in, is the derivative of the power angle δ, is the derivative of the integrator output xint, k p is the proportional gain of the phase-locked loop, k i is the phase-locked loop integral gain.

[0109] like Figure 4 As shown in the figure, the region of area (ROA) of the phase-locked loop is shown. The line impedance parameter selected by the present invention is Z g =0.5, inductance-to-resistance ratio X / R=4, where X is the reactance in the grid impedance and R is the resistance in the grid impedance. The attraction domain in the figure is obtained by numerical calculation. The specific calculation process involves the reverse time integral of two unstable equilibrium points adjacent to the stable equilibrium point in the direction of the stable manifold. This method can identify the real attraction domain range of the system, provide important information about the system behavior, and prepare for the selection of the integrator output limit value. Select the phase angle range as x within the two unstable equilibrium points. int (between the two dotted lines in the figure), the x between these two equilibrium points int It can reflect its maximum and minimum values ​​( Figure 4 The dashed line in the middle shows the maximum value x int_max The dotted line indicates the minimum value x int_min ), and select them separately through extreme value functions.

[0110] like Figure 5 As shown in (a) and (b), the figure shows the phase-locked loop parameters designed according to different bandwidths and phase margins. The following formula is the open-loop transfer function of the phase-locked loop:

[0111]

[0112] By calculating the formula (3), let ω bThe open-loop transfer function amplitude margin of the phase-locked loop is equal to 1, and the phase margin γ meets the stability requirement. The present invention selects a bandwidth of 5Hz-20Hz and a phase margin of 45°-70°, thereby obtaining PLL parameters with a wider coverage. Some data are shown in Tables 1 and 2.

[0113]

[0114] like Figure 6 As shown in (a) and (b), the figure shows the attraction domain x calculated according to different PLL parameters. int The lower the bandwidth, the lower the phase margin, and the corresponding x int The data in the figure show that under the phase-locked loop parameters given by the design method in this paper, x int The upper limit is not less than 30, and the lower limit is not higher than -80. It can be considered that under normal and stable phase-locked loop parameters, the ROA of the state equation has this characteristic. Therefore, this paper gives two limiting design methods:

[0115] (1) Fixed value limiting method. Use the x int The minimum range [-80,30] is used as the integrator limit design value. Under normal working conditions, the x of the phase-locked loop int The fluctuation is very small, and this fixed interval can meet its dynamic response.

[0116] (2) Parameter attraction domain limiting design method. Figure 7 The flow chart designs the limit value under specific parameters, where x int_max and x int_min They are all multiplied by 90% to ensure that the working point is constrained within a certain range where the attraction domain exists, and can return to the equilibrium point within the attraction domain after the fault is cleared.

[0117] like Figure 8 As shown in (a) to (d), the figure shows the operation of the operating point when a transient fault occurs under different PLL parameters (U g =1→U g =0.1), where the black solid point represents the stable equilibrium point, the star point represents the fault removal point, the black solid line trajectory is the ROA boundary curve, the black dotted line trajectory is the working point motion trajectory, and the names of the other lines are marked in the figure. When a serious fault is passed through, the converter generates full reactive power to raise the grid connection point voltage (I sd =0,I sq =1), after the fault is removed, the converter resumes active power output (I sd =1,I sq =0). The continuity of ROA is determined by whether the manifold curve starting from the unstable equilibrium point crosses the horizontal axis. If it crosses the horizontal axis, the ROA is discontinuous; if it does not cross the horizontal axis, the ROA is continuous.

[0118] There are three specific situations:

[0119] (1) When ROA is continuous (ω b =8Hz,γ=70°,k p =47.2, k i =864,x int_max =51.26,x int_min =-129.54), the LVRT control strategy starts when the fault occurs. Due to the resistive impedance of the power grid, δ and x int The derivative of is negative, and the working point moves in the negative direction. In addition, ROA is continuous and x is set. int The operating point will always be within the ROA. Figure 8 As shown in (a), if the fault is removed within the original ROA, the operating point will directly return to the original equilibrium point; Figure 8 As shown in (b), the working point moves to other adjacent ROA 0.625s after the fault occurs. At this time, the fault is removed and the system stabilizes at the equilibrium point of other ROA. Since the longitudinal movement speed of the working point is slow, it usually moves to the lower limit value 1-2s after the transient fault occurs. If it moves to the lower limit value, it moves along the limit value and returns to the equilibrium point after the fault is cleared.

[0120] (2) When ROA is discontinuous (ω b =10Hz,γ=50°,k p =48, k i =2537,x int_max =71.9,x int_min =-174.5), the LVRT control strategy starts when the fault occurs. As in the first case, the working point will move in the negative direction. Figure 8 As shown in (c), when the fault is removed in the gap of ROA, the working point will move in the positive direction outside ROA until x int The upper limit is reached and the object moves along the upper limit until it is within the ROA, where it is attracted back to the equilibrium point. Similarly, if the object is cut off within the ROA, it is directly attracted back to the equilibrium point.

[0121] (3) Regardless of whether the ROA is continuous or not, the LVRT strategy is not activated during the fault. The AC side current is still output according to the original output plan, and δ and x int The derivative of is positive, and the operating point will move in the positive direction. If the fault is removed within the ROA, the operating point will be attracted back to the equilibrium point, similar to the above situation of removing the fault within the ROA. If the fault is removed outside the ROA, x int If the limit value is not reached, it will continue to increase until x int The upper limit is limited, and moves along the limit value in the direction of increasing δ, such as Figure 8(d) b =8Hz,γ=70°,k p =47.2, k i =864,x int_max =51.26,x int_min =-129.54), the operating point moves along the upper limit of the limit until the fault is cleared, then the operating point moves along the limit value into the ROA and is finally attracted to the equilibrium point.

[0122] like Fig. 9 As shown in the figure, the flow chart of the working point after the fault occurs is shown. Depending on whether the LVRT control strategy is adopted, whether the ROA is continuous, and different fault clearing points, the working point will have different movement trajectories and the corresponding limit will also be different.

[0123] Table 1 Phase-locked loop k p (Only part of the display)

[0124]

[0125] Table 2 Phase-locked loop k i (Only part of the display)

[0126]

[0127]

[0128] An embodiment of the present invention further provides a grid-connected converter transient synchronous stability control system, such as Fig.10 As shown, the system includes:

[0129] A converter current inner loop control module, which is used to perform closed-loop control on the grid-connected converter current inner loop, so as to make the grid-connected converter equivalent to a current source and provide an equivalent power grid;

[0130] A phase-locked loop state equation building module, which provides a phase-locked loop state equation for describing the dynamics of the phase-locked loop based on a current source and an equivalent power grid;

[0131] A phase-locked loop parameter design module, which obtains phase-locked loop parameters that can meet the phase-locked loop control stability requirements according to the set phase-locked loop bandwidth and phase margin;

[0132] An attraction domain calculation module, which is based on the state equation of the phase-locked loop and according to the acquired phase-locked loop parameters, calculates the attraction domain of the phase-locked loop and obtains the output limit value of the phase-locked loop integrator;

[0133] A strategy analysis module, which obtains a strategy for maintaining the transient synchronization stability of the phase-locked loop by analyzing the output limit value of the phase-locked loop integrator under different phase-locked loop parameters and different fault removal times;

[0134] The converter control module realizes transient synchronous stability control of the grid-connected converter based on the strategy.

[0135] In a preferred embodiment of the converter current inner loop control module, a closed-loop control is performed on the grid-connected converter current inner loop to make the grid-connected converter equivalent to a current source and provide an equivalent power grid, including:

[0136] The current source equivalent unit is used to output AC current after the DC side power supply is controlled by the inverter, and then fed into the grid after filtering, which has an impact on the grid connection point voltage U t After sampling, the input is input into the phase-locked loop to obtain the angle input current control loop. The current control loop outputs the modulation wave for SPWM modulation, and finally, after PWM modulation, it is fed back to the converter for DC-AC conversion to form a closed-loop control of the converter current inner loop, so that the grid-connected converter is equivalent to a current source;

[0137] The grid equivalent unit uses the Thevenin circuit as the equivalent grid; the grid impedance Z g =R g +jω pll L g / ω s , R g and L g are the equivalent resistance and equivalent inductance of the AC power grid, respectively. j is an imaginary unit representing the reactance component of the power grid impedance. pll is the output frequency of the phase-locked loop, ω s is the synchronization angular frequency.

[0138] In a preferred embodiment of the phase-locked loop state equation building module, a phase-locked loop state equation capable of accurately describing the dynamics of the phase-locked loop is provided, including:

[0139] The phase-locked loop PI controller integrator output x int and the phase-locked loop output angle θ and the grid voltage angle θ g The difference δ = θ-θ g As the state variable, the phase-locked loop state equation that can accurately describe the dynamics of the phase-locked loop is constructed as:

[0140]

[0141] In formula (1), k p is the proportional gain of the phase-locked loop, k i is the phase-locked loop integral gain, U g is the grid-side voltage amplitude; ω s is the synchronous angular frequency, t is the integration time, is the initial phase angle of the power grid; I sdis the d-axis current component, I sq is the q-axis current component.

[0142] In a preferred embodiment of the phase-locked loop parameter design module, the phase-locked loop parameters that can meet the phase-locked loop control stability requirements are obtained according to the set phase-locked loop bandwidth and phase margin range, including:

[0143] According to different bandwidth ω b The corresponding phase-locked loop parameters that meet the phase margin stability range of the phase-locked loop are obtained by using the phase margin γ; where:

[0144] An open-loop transfer function building unit, which is used to build an open-loop transfer function G of a phase-locked loop pll (s) is:

[0145]

[0146] In formula (2), k p is the proportional gain of the phase-locked loop, k i is the phase-locked loop integral gain, s=jω is the Laplace variable, ω is the selected frequency;

[0147] Phase-locked loop parameter calculation unit, which is used to calculate the bandwidth ω through equation (3) b The open-loop transfer function G of the phase-locked loop is pll (s) The amplitude margin is equal to 1, and the phase margin γ satisfies the open-loop transfer function required for phase-locked loop control stability |G pll (jω b )|:

[0148]

[0149] According to the open-loop transfer function |G pll (jω b )|, and obtain the phase-locked loop proportional gain k that can meet the phase-locked loop control stability requirements p and the phase-locked loop integral gain k i , which is the phase-locked loop parameter. In a preferred embodiment, the phase margin γ generally ranges from 30° to 70°.

[0150] Furthermore, in a preferred embodiment, the bandwidth ω b The frequency range is 5 Hz to 20 Hz, and the phase margin γ is preferably 45° to 70°.

[0151] In a preferred embodiment of the attraction domain calculation module, based on the phase-locked loop state equation and according to the acquired phase-locked loop parameters, the attraction domain of the phase-locked loop is calculated to obtain the output limit value of the phase-locked loop integrator, including:

[0152] A parameter acquisition unit, which is used to acquire phase-locked loop parameters including the phase-locked loop proportional gain k p and the phase-locked loop integral gain k i ;

[0153] Parameter calculation unit, which is used to calculate the phase-locked loop proportional gain k p and the phase-locked loop integral gain k i Substituting into the phase-locked loop state equation, we get the current phase-locked loop integrator output x int and the angle difference δ;

[0154] The attraction domain range identification unit obtains the current phase-locked loop integrator output x int and the angle difference δ, set the derivative of the phase-locked loop state equation to 0, solve the stable equilibrium point of the phase-locked loop, and identify the real attraction domain of the phase-locked loop by calculating the reverse time integral of the two unstable equilibrium points adjacent to the stable equilibrium point in the direction of the stable manifold;

[0155] A limit value acquisition unit, which selects the phase-locked loop integrator output x within the phase angle range of two unstable equilibrium points according to the boundary data of the attraction domain range obtained. int , the phase-locked loop integrator output x between these two unstable equilibrium points int It can reflect the maximum and minimum output values ​​of the phase-locked loop integrator, take the maximum output value of the phase-locked loop integrator as the reference value of the upper limit, and take the minimum output value of the phase-locked loop integrator as the reference value of the lower limit, so as to obtain the output limit value of the phase-locked loop integrator.

[0156] Furthermore, in a preferred embodiment of the limit value acquisition unit, the phase angle range is selected to be within two unstable equilibrium points of the phase-locked loop integrator output x int , including any of the following methods:

[0157] Using the fixed value limiting method, the integrator output x int The minimum range [-80,30] is used as the integrator limit design value;

[0158] The parameter attraction domain limit design method is used to design the limit value under specific parameters, where the maximum value x of the output limit value is int_max and the minimum value x of the output limit value int_min They are all multiplied by a set ratio (eg, 90%) to ensure that the operating point is constrained within a certain range of the attraction domain, and that the operating point returns to the equilibrium point within the attraction domain after the fault is cleared.

[0159] In a preferred embodiment of the strategy analysis module, by analyzing the output limit value of the phase loop integrator under different phase-locked loop parameters and different fault removal times, a strategy for maintaining the transient synchronization stability of the phase-locked loop is obtained, including:

[0160] An attraction domain continuity judgment unit, which judges the continuity of the attraction domain of the phase-locked loop by whether the manifold curve starting from the unstable equilibrium point crosses the horizontal axis. If it crosses the horizontal axis, the attraction domain of the phase-locked loop is discontinuous; if it does not cross the horizontal axis, the attraction domain of the phase-locked loop is continuous;

[0161] The LVRT control strategy starts the control unit, which includes:

[0162] If the attraction domain of the phase-locked loop is continuous, the LVRT control strategy is activated when a fault occurs; the working point will always be in the attraction domain of the phase-locked loop; if the fault is removed in the attraction domain of the original equilibrium point, the working point will directly return to the original equilibrium point; if the fault is removed when the working point moves to the attraction domain of other adjacent equilibrium points, the working point will stabilize at other equilibrium points; when the working point moves to the lower limit of the fault removal time, the working point moves along the limit value and returns to the equilibrium point after the fault is cleared;

[0163] If the attraction domain of the phase-locked loop is discontinuous, the LVRT control strategy is activated when a fault occurs; the operating point will move in the negative direction; if the fault is removed in the gap of the attraction domain of the phase-locked loop, the operating point will move in the positive direction outside the attraction domain of the phase-locked loop until the integrator outputs x int The upper limit is reached and the working point moves along the upper limit until it enters the attraction domain and is attracted to the equilibrium point again in the attraction domain. If the working point is cut off in the attraction domain of the phase-locked loop, the working point is directly attracted back to the equilibrium point.

[0164] Regardless of whether the attraction domain of the phase-locked loop is continuous, the LVRT strategy is not activated when a fault occurs; at this time, the working point moves in the positive direction; if the fault is removed within the attraction domain of the phase-locked loop, the working point is attracted back to the equilibrium point; if the fault is removed outside the attraction domain of the phase-locked loop, the integrator output x int If the limit value is not reached, the integrator output x int will continue to increase until the integrator outputs x int The upper limit is limited, and it moves along the limit value in the direction of increasing the angle difference δ. The working point moves along the limit upper limit until the fault is cleared. Then the working point moves along the limit value into the attraction domain of the phase-locked loop and is finally attracted to the equilibrium point.

[0165] It should be noted that the steps in the method provided by the present invention can be implemented by using corresponding modules, devices, units, etc. in the system. Those skilled in the art can refer to the technical solution of the method to realize the composition of the system, that is, the embodiments in the method can be understood as preferred examples for constructing the system, which will not be elaborated here.

[0166] An embodiment of the present invention further provides a transient synchronous stability controller for a grid-connected converter, comprising: a memory, a processor, and a computer program stored in the memory and executable on the processor; when the processor executes the program, the processor can be used to execute any of the methods in the above embodiments, or to run any of the systems in the above embodiments of the present invention.

[0167] An embodiment of the present invention further provides a controller-readable storage medium on which a computer program is stored. When the program is executed by a processor, it can be used to execute any method of the above embodiments of the present invention, or to run any system of the above embodiments of the present invention.

[0168] The transient synchronous stability control method and system of the grid-connected converter provided by the above-mentioned embodiment of the present invention, the quantitative output limiting technology of the integrator of the phase-locked loop PI controller, the working point begins to deviate from the equilibrium point after a large disturbance, and the limiting value is calculated by a numerical method to maximize the dynamic response of the phase-locked loop, and the limiting can ensure that the working point is limited to a certain range, and the working point of the phase-locked loop returns to the attraction domain after the fault is cleared, so as to ensure the transient synchronous stability of the converter; the phase-locked loop integrator limiting design method under different phase-locked loop parameters, the transient synchronous stability is strongly related to the phase-locked loop parameters, so it is difficult to fully achieve the desired effect by designing a single parameter. The stability under all conditions is sufficient, so the limiting value can be designed by the numerical method proposed in this paper, providing a reference for the limiting of the integrator under different parameters; Aiming at the transient synchronization instability problem that may be triggered when a large disturbance fault occurs in the grid-type converter under different phase-locked loop parameters, a PLL integrator output limiting calculation method is proposed. According to the stability domain under different phase-locked loop parameters, the corresponding limiting extreme values ​​are calculated respectively, and the working point during the fault period is constrained within a certain range, so that it does not deviate too far from the equilibrium point during the fault period, and it can be guaranteed to return to the ROA as soon as possible and finally return to the equilibrium point after the fault is cleared.

[0169] Those skilled in the art know that, in addition to implementing the system and its various devices provided by the present invention in a purely computer-readable program code, the system and its various devices provided by the present invention can be made to implement the same functions in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers by logically programming the method steps. Therefore, the system and its various devices provided by the present invention can be considered as a hardware component, and the devices for implementing various functions included therein can also be considered as structures within the hardware component; the devices for implementing various functions can also be considered as both software modules for implementing the method and structures within the hardware component.

[0170] All matters not covered in the above embodiments of the present invention are well known in the art.

[0171] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for transient synchronous stability control of a grid-connected converter, characterized in that: include: Performing closed-loop control on the current inner loop of the grid-connected converter to make the grid-connected converter equivalent to a current source and provide an equivalent power grid; According to the current source and the equivalent power grid, a phase-locked loop state equation is provided for describing the dynamics of the phase-locked loop; Given different PLL bandwidths and phase margins, obtain PLL parameters that can meet the PLL control stability requirements; Based on the phase-locked loop state equation and according to the acquired phase-locked loop parameters, the attraction domain of the phase-locked loop is calculated to obtain the output limit value of the phase-locked loop integrator; By analyzing the output limit value of the phase-locked loop integrator under different phase-locked loop parameters and different fault removal times, a strategy for maintaining the transient synchronization stability of the phase-locked loop is obtained; Based on the strategy, transient synchronous stability control of the grid-connected converter is achieved.

2. The method for transient synchronous stability control of a grid-connected converter according to claim 1, characterized in that: The closed-loop control of the inner loop of the current of the grid-connected converter is performed to make the grid-connected converter equivalent to a current source and provide an equivalent power grid, including: The DC power supply is controlled by the inverter to output AC current, which is then filtered by the filter and fed into the grid. t After sampling, the input is input into the phase-locked loop to obtain the angle input current control loop. The current control loop outputs the modulation wave for SPWM modulation, and finally, after PWM modulation, it is fed back to the converter for DC-AC conversion to form a closed-loop control of the converter current inner loop, thereby making the grid-connected converter equivalent to a current source. The Thevenin circuit is used as the equivalent power grid; where the power grid impedance Z g =R g +jω pll L g / ω s , R g and L g are the equivalent resistance and equivalent inductance of the AC power grid, j is an imaginary unit representing the reactance component of the power grid impedance, ω pll is the output frequency of the phase-locked loop, ω s is the synchronization angular frequency.

3. The method for transient synchronous stability control of a grid-connected converter according to claim 1, characterized in that: The present invention provides a phase-locked loop state equation for describing the dynamics of the phase-locked loop, including: The phase-locked loop PI controller integrator output x int and the phase-locked loop output angle θ and the grid voltage angle θ g The difference δ = θ-θ g As the state variable, the phase-locked loop state equation that can describe the dynamics of the phase-locked loop is constructed as: in, is the derivative of the power angle δ, is the derivative of the integrator output xint, k p is the proportional gain of the phase-locked loop, k i is the phase-locked loop integral gain, U g is the grid-side voltage amplitude; ω s is the synchronous angular frequency, t is the integration time, is the initial phase angle of the power grid; I sd is the d-axis current component, I sq is the q-axis current component, R g and L g are the equivalent resistance and equivalent inductance of the AC power grid respectively.

4. The method for transient synchronous stability control of a grid-connected converter according to claim 1, characterized in that: The step of obtaining phase-locked loop parameters that can meet the control stability requirements of the phase-locked loop by giving different phase-locked loop bandwidths and phase margins includes: According to different bandwidth b and phase margin γ, to obtain the corresponding phase-locked loop parameters that meet the phase margin stability range of the phase-locked loop; where: Construct the open-loop transfer function G of the phase-locked loop pll (s) is: In formula (2), k p is the proportional gain of the phase-locked loop, k i is the phase-locked loop integral gain, s=jω is the Laplace variable, ω is the selected frequency, and j is an imaginary unit representing the reactive component of the grid impedance; By using formula (3), we can calculate the bandwidth ω b The open-loop transfer function G of the phase-locked loop is pll (s) The amplitude margin is equal to 1, and the phase margin γ satisfies the open-loop transfer function required for phase-locked loop control stability |G pll (jω b )|: Among them, k p is the proportional gain of the phase-locked loop, k i is the phase-locked loop integral gain; According to the open-loop transfer function |G pll (jω b )|, and obtain the phase-locked loop proportional gain k that can meet the phase-locked loop control stability requirements p and the phase-locked loop integral gain k i , which is the phase-locked loop parameter.

5. The method for transient synchronous stability control of a grid-connected converter according to claim 1, characterized in that: The acquired phase-locked loop parameters include the phase-locked loop proportional gain k p and the phase-locked loop integral gain k i ; Based on the phase-locked loop state equation and according to the acquired phase-locked loop parameters, the attraction domain of the phase-locked loop is calculated to obtain the output limit value of the phase-locked loop integrator, including: The phase-locked loop proportional gain k p and the phase-locked loop integral gain k i Substituting into the phase-locked loop state equation, we get the current phase-locked loop integrator output x int and the angle difference δ; According to the current phase-locked loop integrator output x int and the angle difference δ, set the derivative of the phase-locked loop state equation to 0, solve the stable equilibrium point of the phase-locked loop, and identify the real attraction domain of the phase-locked loop by calculating the reverse time integral of the two unstable equilibrium points adjacent to the stable equilibrium point in the direction of the stable manifold; According to the boundary data of the attraction domain, the phase angle range is selected to be the phase-locked loop integrator output x within two unstable equilibrium points. int , the phase-locked loop integrator output x between these two unstable equilibrium points int The maximum and minimum output values ​​of the phase-locked loop integrator are reflected, the maximum output value of the phase-locked loop integrator is used as a reference value for the upper limit, and the minimum output value of the phase-locked loop integrator is used as a reference value for the lower limit, so as to obtain the output limit value of the phase-locked loop integrator.

6. The method for transient synchronous stability control of a grid-connected converter according to claim 5, characterized in that: The selected phase angle range is the phase-locked loop integrator output x within two unstable equilibrium points. int , including any of the following methods: Using the fixed value limiting method, the integrator output x int The minimum range [-80,30] is used as the integrator limit design value; The parameter attraction domain limit design method is used to design the limit value under specific parameters, where the maximum value of the output limit value x int_max and the minimum value x of the output limit value int_min They are all multiplied by the set ratio to ensure that the working point is constrained within a certain range of the attraction domain, and the working point returns to the equilibrium point in the attraction domain after the fault is cleared.

7. The method for transient synchronous stability control of a grid-connected converter according to claim 1, characterized in that: The strategy for maintaining the transient synchronization stability of the phase-locked loop is obtained by analyzing the output limit value of the phase-locked loop integrator under different phase-locked loop parameters and different fault removal times, including: The continuity of the attraction domain of the phase-locked loop is determined by whether the manifold curve starting from the unstable equilibrium point crosses the horizontal axis. If it crosses the horizontal axis, the attraction domain of the phase-locked loop is discontinuous; if it does not cross the horizontal axis, the attraction domain of the phase-locked loop is continuous. If the attraction domain of the phase-locked loop is continuous, the LVRT control strategy is activated when a fault occurs; the working point will always be in the attraction domain of the phase-locked loop; if the fault is removed in the attraction domain of the original equilibrium point, the working point will directly return to the original equilibrium point; if the fault is removed when the working point moves to the attraction domain of other adjacent equilibrium points, the working point will stabilize at other equilibrium points; when the working point moves to the lower limit of the fault removal time, the working point moves along the limit value and returns to the equilibrium point after the fault is cleared; If the attraction domain of the phase-locked loop is discontinuous, the LVRT control strategy is activated when a fault occurs, and the operating point moves in the negative direction; if the fault is removed in the gap of the attraction domain of the phase-locked loop, the operating point moves in the positive direction outside the attraction domain of the phase-locked loop until the integrator outputs x int The upper limit is reached and the working point moves along the upper limit until it enters the attraction domain and is attracted to the equilibrium point again in the attraction domain. If the working point is cut off in the attraction domain of the phase-locked loop, the working point is directly attracted back to the equilibrium point. Regardless of whether the attraction domain of the phase-locked loop is continuous, the LVRT strategy is not activated when a fault occurs; the operating point moves in the positive direction; if the fault is removed within the attraction domain of the phase-locked loop, the operating point is attracted back to the equilibrium point; if the fault is removed outside the attraction domain of the phase-locked loop, the integrator outputs x int If the limit value is not reached, the integrator output x int Keep increasing until the integrator output x int The upper limit is limited, and it moves along the limit value in the direction of increasing the angle difference δ. The working point moves along the limit upper limit until the fault is cleared. Then the working point moves along the limit value into the attraction domain of the phase-locked loop and is finally attracted to the equilibrium point.

8. A grid-connected converter transient synchronous stability control system, characterized in that: include: A converter current inner loop control module, which is used to perform closed-loop control on the grid-connected converter current inner loop, so as to make the grid-connected converter equivalent to a current source and provide an equivalent power grid; A phase-locked loop state equation building module, which provides a phase-locked loop state equation for describing the dynamics of the phase-locked loop according to the current source and the equivalent power grid; A phase-locked loop parameter design module, which obtains phase-locked loop parameters that can meet the phase-locked loop control stability requirements according to the set phase-locked loop bandwidth and phase margin; An attraction domain calculation module, which calculates the attraction domain of the phase-locked loop based on the phase-locked loop state equation and the acquired phase-locked loop parameters to obtain the output limit value of the phase-locked loop integrator; A strategy analysis module, which obtains a strategy for maintaining the transient synchronization stability of the phase-locked loop by analyzing the output limit value of the phase-locked loop integrator under different phase-locked loop parameters and different fault removal times; A converter control module realizes transient synchronous stability control of the grid-connected converter based on the strategy.

9. A transient synchronous stability controller for a grid-connected converter, comprising: The invention comprises a memory, a processor and a computer program stored in the memory and executable on the processor, wherein the processor can be used to execute the method described in any one of claims 1 to 7, or to run the system described in claim 8 when executing the program.

10. A controller-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, it can be used to execute the method described in any one of claims 1 to 7, or to run the system described in claim 8.

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