Net construction type converter stability analysis method based on inner and outer loop control coupling

Abstract

The invention discloses a stability analysis method for a network-constructed converter based on inner and outer loop control coupling, which comprises the following steps of: establishing a single-input single-output model of the network-constructed converter, clarifying a control coupling mechanism of the network-constructed converter, revealing a key coupling link causing instability of the network-constructed converter, and proposing a stability criterion based on a Bode criterion as shown in a formula (13) according to the key coupling link. And the critical stable short-circuit ratio and the oscillation frequency of the grid-forming converter can be simply, conveniently and accurately obtained. Different from an existing impedance analysis method and an existing characteristic value analysis method, the method solves the problem that the existing method can only perform qualitative analysis and cannot perform quantitative analysis.

Description

Technical Field

[0001] This invention relates to the field of stability analysis of grid-type converters, specifically a stability analysis method for grid-type converters based on the coupling of inner and outer loop control. Background Technology

[0002] With the large-scale replacement of synchronous generators by new energy power generation using converters as interfaces, the system's inertia and voltage support capabilities have been significantly weakened. This is because current converters generally employ grid-following control, which cannot provide the necessary inertia response and dynamic reactive power support for the system, seriously threatening system safety. Grid-following control, however, simulates the electromagnetic-mechanical dynamic characteristics of synchronous generators through algorithms, enabling converters to autonomously construct and maintain AC grid voltage and frequency. This control paradigm allows converters not only to provide virtual inertia damping for frequency fluctuations but also to achieve power angle stability and voltage support through autonomous adjustment, becoming a necessary stability foundation for new power systems and a core solution for ensuring safe system operation.

[0003] However, grid-connected converters suffer from severe stability issues under high-strength power grids. If the grid strength exceeds their critical stability value, the output of the grid-connected converter will generate divergent low-frequency oscillations, causing severe overvoltage and overcurrent, leading to the disconnection of new energy units from the grid, and seriously affecting the safety and stability of the new power system. Currently, the large-scale grid connection of new energy sources has significantly increased the operational difficulty of the power grid. The inherent intermittency and volatility of power generation mean that the addition or removal of any power source or load can trigger drastic changes in grid strength. Therefore, it is necessary to propose a stability analysis method for grid-connected converters to identify their critical stable operating grid strength, avoid operating grid-connected converters in grids exceeding this strength, and prevent low-frequency oscillations.

[0004] Current stability analyses of grid-connected converters mainly follow two directions. One is based on the study of port impedance external characteristics. This method treats the converter as a whole, analyzing its interaction with grid impedance by establishing its sequence impedance or an impedance model that considers frequency coupling. This type of research reveals that as grid strength increases, the impedance resonance peak of the grid-connected converter shifts to lower frequencies, even exhibiting negative RC characteristics in certain frequency bands, thus easily inducing low-frequency or subsynchronous oscillations. This has led to the development of techniques such as impedance reshaping to improve stability. However, this external characteristic-based analysis method has fundamental limitations because the converter's impedance characteristics are not a physical entity but merely an external manifestation of its control algorithm. Starting with external characteristics cannot reveal the intrinsic mechanism of instability, nor can it uncover the core factors strongly correlated with its intrinsic stability.

[0005] To reveal the essence of instability, another type of research has turned to the internal control coupling mechanism. This type of research attempts to delve into the internal control loop of the converter, analyzing its dynamic process by deriving its closed-loop transfer function or applying methods such as the damped torque method and the complex torque coefficient method. These methods have successfully shown that grid-connected converters exhibit negative damped torque characteristics proportional to the grid strength under strong grid conditions, and the fundamental source of this negative damping lies in the coupling between the power loop and the voltage loop in its internal control. Although this addresses the root of the problem, the models established by this type of analysis are usually extremely complex multi-input multi-output systems, resulting in highly coupled transfer functions of each component, and consequently, exceptionally complex stability criteria. This complexity makes it difficult for researchers to clearly and intuitively grasp the specific interaction mechanism and coupling form between the power loop and the voltage loop, thus hindering the progress of developing simple and efficient stability optimization schemes. Therefore, the current research field urgently needs to develop an analytical framework that can both deeply reflect the nature of instability and quickly and accurately derive the critical stability short-circuit ratio and oscillation frequency, so as to completely overcome the problem of stable operation of grid-type converters under strong power grids. Summary of the Invention

[0006] The purpose of this invention is to provide a stability analysis method for a grid-type converter based on inner and outer loop control coupling, comprising the following steps:

[0007] Step 1) Perform rotational speed control on the grid-connected system of the grid-type converter according to the dq rotating coordinate system.

[0008] The system is divided into two subsystems: the power grid and main circuit system, and the grid-type converter system.

[0009] Step 2) Obtain the small disturbance variable relationships between the power grid, main circuit system, and grid-type converter system;

[0010] Step 3) Establish small disturbance models of the power grid and main circuit system, and small disturbance models of the grid-type converter system in the corresponding rotating coordinate system;

[0011] Step 4) Based on the small disturbance variable relationship and the small disturbance model, establish a unified single-input single-output model for the grid-connected system of the grid-connected converter;

[0012] Step 5) Use a single-input single-output model to determine the open-loop transfer function of the key control coupling link that leads to system instability;

[0013] Step 6) Analyze the network structure variation using the open-loop transfer function of the key control coupling links.

[0014] Stability of grid-connected inverter systems.

[0015] Furthermore, in step 1), the rotational speed of the dq rotating coordinate system in which the grid-type converter system is located is the synchronous speed;

[0016] The rotational speeds of the dq rotating coordinate system in the grid-type converter system are not synchronized.

[0017] Furthermore, in step 2), the relationships between small disturbance variables among the power grid, main circuit system, and grid-type converter system are as follows:

[0018] (1)

[0019] in, This represents the dq rotating coordinate system in which vector x is located in the network converter control system. The dq component in The coordinate system is oriented with grid connection point voltage. The d-axis coincides with the d-axis vector of the grid connection point voltage, the q-axis is perpendicular to the d-axis, the positive direction is in the counterclockwise direction of the d-axis, and the origin is the intersection of the d and q axes. This represents the dq rotating coordinate system in which vector x is located within the power grid and main circuit system. The dq component in The coordinate system is oriented with grid voltage, the d-axis coincides with the grid voltage d-axis vector, the q-axis is perpendicular to the d-axis with its positive direction counterclockwise from the d-axis, and the origin is the intersection of the d and q axes; (symbol) This represents the small perturbation component of the corresponding variable; The output disturbance is used for the network-type control algorithm. , This represents the dq component of vector x under the reference state.

[0020] Furthermore, the output disturbance of the network-type control algorithm As shown below:

[0021] (2)

[0022] In the formula, This represents the small disturbance component of the active power output of the converter. Indicates synchronous angular velocity; D p J represents the virtual synchronous control damping coefficient and moment of inertia; s represents the complex frequency variable.

[0023] Furthermore, in step 3), the small disturbance model of the power grid and main circuit system is as follows:

[0024] (3)

[0025] (4)

[0026] (5)

[0027] In the formula, L g R g L represents the equivalent inductance and resistance of the power grid.f R f C f This represents the converter's filter inductance, parasitic resistance, and filter capacitor. , These are the d-axis and q-axis disturbance components of the converter inductor current; , These are the d-axis and q-axis disturbance components of the grid-connected current; , These are the d-axis and q-axis disturbance components of the grid connection point voltage; , d-axis and q-axis disturbance components of the grid equivalent voltage; , These are the d-axis and q-axis disturbance components of the converter output line voltage. The above variables are... In the coordinate system.

[0028] Furthermore, the small disturbance model of the grid-type converter system is shown below:

[0029] (6)

[0030] (7)

[0031] (8)

[0032] Where, m dq For the d-axis and q-axis components of the modulated signal; T d For equivalent delay; K pwm These are the PWM modulation coefficients; , These are the reference values ​​for the d-axis and q-axis disturbances of the converter inductor current. , These are the reference values ​​for the d-axis and q-axis disturbances of the converter output voltage. , G represents the d-axis and q-axis disturbance components of the converter output voltage. VL G CL Transfer functions for voltage loop and current loop PI controllers; , These are the d- and q-axis disturbance components of the converter inductor current. , These are the d-axis and q-axis perturbation components of the modulated signal; , These are the d- and q-axis disturbance components of the converter output line voltage. This represents the equivalent time delay of the control system. The above variables are in... In the coordinate system.

[0033] Furthermore, in step 4), the unified single-input single-output model of the grid-connected system for the grid-connected converter is shown below:

[0034] (9)

[0035] (10)

[0036] In the formula, G VL G CL This represents the transfer function of the voltage loop and current loop PI controller; G represents the closed-loop transfer function of the key control coupling link; q This represents the integral stage of the virtual exciter; Represents the initial values ​​of the d-axis components of the grid connection point voltage; G1, G2, G GFM Z v To simplify the expression of parameters.

[0037] Furthermore, the closed-loop transfer function of the key control coupling element is shown below:

[0038] (11)

[0039] Furthermore, in step 5), the open-loop transfer function G of the key control coupling link that leads to system instability... o As shown below:

[0040] (12)

[0041] Furthermore, in step 6), if the open-loop transfer function G... o If the stability criterion is met, then the system is stable;

[0042] The stability criterion is as follows:

[0043] (13)

[0044] In the formula, Phase margin; for The amplitude crossover frequency is the angular frequency corresponding to the amplitude of the amplitude-frequency characteristic being 1. This is the gain margin; for The phase crossover frequency is the angular frequency corresponding to a phase characteristic phase of -180°.

[0045] The technical effects of this invention are undeniable. This invention can simply and directly derive the critical stability short-circuit ratio and its oscillation frequency of a grid-type converter, solving the problem that existing methods can only perform qualitative analysis and cannot perform quantitative analysis. Attached Figure Description

[0046] Figure 1 Vector diagram for dividing subsystems

[0047] Figure 2 Amplitude and phase characteristics of Go under different short-circuit ratios

[0048] Figure 3 Simulation results of q-axis voltage components under different short-circuit ratios Detailed Implementation

[0049] The present invention will be further described below with reference to embodiments, but it should not be construed that the scope of the present invention is limited to the following embodiments. Various substitutions and modifications made based on ordinary technical knowledge and common practices in the art without departing from the above-described technical concept of the present invention should be included within the scope of protection of the present invention.

[0050] Example 1:

[0051] A stability analysis method for a grid-type converter based on inner and outer loop control coupling includes the following steps:

[0052] Step 1) Perform rotational speed control on the grid-connected system of the grid-type converter according to the dq rotating coordinate system.

[0053] The system is divided into two subsystems: the power grid and main circuit system, and the grid-type converter system.

[0054] Step 2) Obtain the small disturbance variable relationships between the power grid, main circuit system, and grid-type converter system;

[0055] Step 3) Establish small disturbance models of the power grid and main circuit system, and small disturbance models of the grid-type converter system in the corresponding rotating coordinate system;

[0056] Step 4) Based on the small disturbance variable relationship and the small disturbance model, establish a unified single-input single-output model for the grid-connected system of the grid-connected converter;

[0057] Step 5) Use a single-input single-output model to determine the open-loop transfer function of the key control coupling link that leads to system instability;

[0058] Step 6) Analyze the network structure variation using the open-loop transfer function of the key control coupling links.

[0059] Stability of grid-connected inverter systems.

[0060] Example 2:

[0061] A stability analysis method for a grid-type converter based on inner and outer loop control coupling, with the same technical content as in Example 1, further wherein, in step 1), the rotational speed of the dq rotating coordinate system of the grid-type converter system is the synchronous speed;

[0062] The rotational speeds of the dq rotating coordinate system in the grid-type converter system are not synchronized.

[0063] Example 3:

[0064] A stability analysis method for grid-type converters based on inner and outer loop control coupling, with the same technical content as any one of embodiments 1-2, further wherein, in step 2), the small disturbance variable relationships between the power grid, main circuit system, and grid-type converter system are as follows:

[0065] (1)

[0066] in, This represents the dq rotating coordinate system in which vector x is located in the network converter control system. The dq component in The coordinate system is oriented with grid connection point voltage. The d-axis coincides with the d-axis vector of the grid connection point voltage, the q-axis is perpendicular to the d-axis, the positive direction is in the counterclockwise direction of the d-axis, and the origin is the intersection of the d and q axes. This represents the dq rotating coordinate system in which vector x is located within the power grid and main circuit system. The dq component in The coordinate system is oriented with grid voltage, the d-axis coincides with the grid voltage d-axis vector, the q-axis is perpendicular to the d-axis with its positive direction counterclockwise from the d-axis, and the origin is the intersection of the d and q axes; (symbol) This represents the small perturbation component of the corresponding variable; The output disturbance is used for the network-type control algorithm. , This represents the dq component of vector x under the reference state.

[0067] Example 4:

[0068] A stability analysis method for a grid-type converter based on inner and outer loop control coupling is proposed, with the technical content being the same as any one of embodiments 1-3. Further, the output disturbance of the grid-type control algorithm is... As shown below:

[0069] (2)

[0070] In the formula, This represents the small disturbance component of the active power output of the converter. Indicates synchronous angular velocity; D p J represents the virtual synchronous control damping coefficient and moment of inertia; s represents the complex frequency variable.

[0071] Example 5:

[0072] A stability analysis method for grid-type converters based on inner and outer loop control coupling, with the same technical content as any one of embodiments 1-4, further wherein, in step 3), the small disturbance model of the power grid and main circuit system is as follows:

[0073] (3)

[0074] (4)

[0075] (5)

[0076] In the formula, L g R g L represents the equivalent inductance and resistance of the power grid. f R f C f This represents the converter's filter inductance, parasitic resistance, and filter capacitor. , These are the d-axis and q-axis disturbance components of the converter inductor current; , These are the d-axis and q-axis disturbance components of the grid-connected current; , These are the d-axis and q-axis disturbance components of the grid connection point voltage; , d-axis and q-axis disturbance components of the grid equivalent voltage; , These are the d-axis and q-axis disturbance components of the converter output line voltage. The above variables are... In the coordinate system.

[0077] Example 6:

[0078] A stability analysis method for a grid-type converter based on inner and outer loop control coupling is provided, with the technical content being the same as any one of embodiments 1-5. Furthermore, the small disturbance model of the grid-type converter system is shown below:

[0079] (6)

[0080] (7)

[0081] (8)

[0082] Where, m dq For the d-axis and q-axis components of the modulated signal; T d For equivalent delay; K pwm These are the PWM modulation coefficients; , These are the reference values ​​for the d-axis and q-axis disturbances of the converter inductor current. , These are the reference values ​​for the d-axis and q-axis disturbances of the converter output voltage. , G represents the d-axis and q-axis disturbance components of the converter output voltage. VL G CL Transfer functions for voltage loop and current loop PI controllers; , These are the d- and q-axis disturbance components of the converter inductor current. , These are the d-axis and q-axis perturbation components of the modulated signal; , These are the d- and q-axis disturbance components of the converter output line voltage. This represents the equivalent time delay of the control system. The above variables are in... In the coordinate system.

[0083] Example 7:

[0084] A stability analysis method for grid-connected converters based on inner and outer loop control coupling, with technical content identical to any one of embodiments 1-6, further comprising the following step 4): The unified single-input single-output model of the grid-connected converter system is as follows:

[0085] (9)

[0086] (10)

[0087] In the formula, G VL G CL This represents the transfer function of the voltage loop and current loop PI controller; G represents the closed-loop transfer function of the key control coupling link; q This represents the integral stage of the virtual exciter; Represents the initial values ​​of the d-axis components of the grid connection point voltage; G1, G2, G GFM Z v To simplify the expression of parameters.

[0088] Example 8:

[0089] A stability analysis method for a grid-type converter based on inner and outer loop control coupling is provided, with the technical content being the same as any one of embodiments 1-7. Furthermore, the closed-loop transfer function of the key control coupling link is shown below:

[0090] (11)

[0091] Example 9:

[0092] A stability analysis method for a grid-type converter based on inner and outer loop control coupling, with the same technical content as any one of embodiments 1-8, further comprising, in step 5), the open-loop transfer function G of the key control coupling link that leads to system instability. o As shown below:

[0093] (12)

[0094] Example 10:

[0095] A stability analysis method for a grid-type converter based on inner and outer loop control coupling, with the same technical content as any one of embodiments 1-9, further, in step 6), if the open-loop transfer function G o If the stability criterion is met, then the system is stable;

[0096] The stability criterion is as follows:

[0097] (13)

[0098] In the formula, Phase margin; for The amplitude crossover frequency is the angular frequency corresponding to the amplitude of the amplitude-frequency characteristic being 1. This is the gain margin; for The phase crossover frequency is the angular frequency corresponding to a phase characteristic phase of -180°.

[0099] Example 11:

[0100] A stability analysis method for a grid-type converter based on inner and outer loop control coupling, comprising the following steps:

[0101] Step 1: Adjust the rotational speed of the grid-connected system of the grid-type converter according to the rotational speed in the dq rotating coordinate system.

[0102] The rows are divided into two subsystems.

[0103] Step 2: Obtain the relationship between small perturbation variables between asynchronous subsystems in Step 1.

[0104] Step 3: Establish small perturbation models for the two subsystems in the corresponding rotating coordinate systems.

[0105] Step 4: Combining Step 2 and Step 3, establish a unified grid-connected system for grid-connected converters.

[0106] Single-input single-output model.

[0107] Step 5: Use the single-input single-output model obtained in Step 4 to identify the key control coupling links that cause system instability.

[0108] Step 6: Utilize key control coupling links and combine with the Byrd criterion to analyze network structure variations.

[0109] Stability of grid-connected inverter systems.

[0110] The meanings of the variables in the following content are shown in Table 1:

[0111]

[0112] In step 1, the grid-connected system of the grid-type converter is divided according to the rotational speed in the dq rotating coordinate system, forming two subsystems: the power grid and main circuit system (the rotational speed in the dq rotating coordinate system is always synchronous speed) and the grid-type converter system (the rotational speed in the dq rotating coordinate system is determined by the outer loop control and deviates from the synchronous speed when disturbed).

[0113] In step 2, the relationship between small perturbation variables between asynchronous subsystems obtained in step 1 is obtained through the vector mapping rule. This relationship is shown in the following equation:

[0114] (1)

[0115] in, Indicates that vector x is in The dq components in the coordinate system (the dq rotating coordinate system in which the grid-type converter system is located); Indicates that vector x is in The dq components in a coordinate system (the dq rotating coordinate system in which the power grid and main circuit system are located). Symbols This represents the small perturbation component of the corresponding variable. The output disturbance of the network-type control algorithm can be obtained from the virtual rotor control equations:

[0116] (2)

[0117] In step 3, the small disturbance model of the power grid and main circuit system is in a state of... In the coordinate system, its mathematical expression is shown in equation (3-5); the small disturbance model of the grid-type converter system is in In the coordinate system, its mathematical expression is shown in equation (6-8).

[0118] (3)

[0119] (4)

[0120] (5)

[0121] (6)

[0122] (7)

[0123] (8)

[0124] Where, m dq For the d-axis and q-axis components of the modulated signal; T d For equivalent delay; K pwm is the PWM modulation coefficient.

[0125] In step 4, the small disturbance model of the grid-type converter system is projected onto dq using equation (1). g In the coordinate system, combined with the small disturbance model of the power grid and main circuit system, the unified single-input single-output model of the grid-connected system of the grid-type converter is obtained, as shown in the following formula.

[0126] (9)

[0127] in:

[0128] (10)

[0129] In step 5, it can be seen from equation (9) that the grid-type converter has a typical feedback loop coupling, and the open-loop transfer function of this feedback loop determines the open-loop transfer function of the entire system. Therefore, the typical feedback loop G f This is a critical control coupling element that leads to system instability. The closed-loop transfer function expression for this element is as follows:

[0130] (11)

[0131] In step 6, the key control coupling link that leads to system instability is G. f G f The open-loop transfer function of G is the open-loop transfer function of the entire system. According to the Nyquist theorem, G f The open-loop transfer function can be used to analyze system stability. Its open-loop transfer function G... o As shown below:

[0132] (12)

[0133] Because of G o Since there are no poles in the right half-plane, the system stability can be analyzed using its amplitude and phase characteristics. That is, the system is stable when the following equation is satisfied:

[0134] (13)

[0135] In the formula, Phase margin; for The amplitude crossover frequency is the angular frequency corresponding to the amplitude of the amplitude-frequency characteristic being 1. This is the gain margin; for The phase crossover frequency is the angular frequency corresponding to a phase characteristic of -180°. Therefore, G under different operating conditions can be obtained. o The amplitude and phase characteristics can be used to analyze the system stability using equation (13).

[0136] Example 12:

[0137] A stability analysis method for a grid-type converter based on inner and outer loop control coupling is provided. The technical content is the same as any one of embodiments 1-11. Furthermore, by establishing a single-input single-output model of the grid-type converter, its control coupling mechanism is clarified, and the key coupling links leading to the instability of the grid-type converter are revealed. Based on this, a stability criterion based on the Bode criterion, as shown in equation (13), is proposed, which can easily and accurately obtain the critical stability short-circuit ratio and oscillation frequency of the grid-type converter. Unlike existing impedance analysis methods and eigenvalue analysis methods, this invention solves the problem that existing methods can only perform qualitative analysis and cannot perform quantitative analysis.

[0138] Example 13:

[0139] A stability analysis method for a grid-type converter based on inner and outer loop control coupling is presented below. The specific implementation steps of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0140] Step 1: Divide the grid-connected system of the grid-type converter into two subsystems according to the rotational speed in the dq rotating coordinate system.

[0141] Figure 1 The diagram below shows vector representations of two subsystems, where the grid voltage vector U is... g Grid connection point voltage vector U C Located at a constant synchronous speed of In the coordinate system, the converter output voltage vector E is located at the rotational speed of... of In the coordinate system, the rotational speed of this subsystem is determined by the virtual synchronization algorithm, and the rotational speed deviates from the synchronous speed after being disturbed.

[0142] Step 2: Obtain the relationship between small perturbation variables between asynchronous subsystems in Step 1.

[0143] The relationship between small perturbation variables in asynchronous subsystems is obtained using the vector mapping rule. This relationship is shown in the following equation:

[0144]

[0145] in, Indicates that vector x is in The dq components in the coordinate system (the dq rotating coordinate system in which the grid-type converter system is located); Indicates that vector x is in The dq components in a coordinate system (the dq rotating coordinate system in which the power grid and main circuit system are located). Symbols This represents the small perturbation component of the corresponding variable. The output disturbance of the network-type control algorithm can be obtained from the virtual rotor control equations:

[0146] (2)

[0147] Step 3: Establish small perturbation models for the two subsystems in the corresponding rotating coordinate systems.

[0148] The small disturbance model of the power grid and main circuit system is in In the coordinate system, its mathematical expression is shown in equation (3-5); the small disturbance model of the grid-type converter system is in In the coordinate system, its mathematical expression is shown in equation (6-8).

[0149] (3)

[0150] (4)

[0151] (5)

[0152] (6)

[0153] (7)

[0154] (8)

[0155] Where, m dq For the d-axis and q-axis components of the modulated signal; T d For equivalent delay; K pwm is the PWM modulation coefficient.

[0156] Step 4: Combining Step 2 and Step 3, establish a unified single-input single-output model for grid-connected grid-type converter systems.

[0157] Using equation (1), the small disturbance model of the grid-type converter system is projected onto... In the coordinate system, combined with the small disturbance model of the power grid and main circuit system, the unified single-input single-output model of the grid-connected system of the grid-type converter is obtained, as shown in the following formula.

[0158] (9)

[0159] in:

[0160] (10)

[0161] Step 5: Use the single-input single-output model obtained in Step 4 to identify the key control coupling links that cause system instability.

[0162] Equation (9) shows that the grid-type converter has a typical feedback loop coupling, and the open-loop transfer function of this feedback loop determines the open-loop transfer function of the entire system. Therefore, the typical feedback loop G fThis is a critical control coupling element that leads to system instability. The closed-loop transfer function expression for this element is as follows:

[0163] (11)

[0164] Step 6: Analyze the stability of the grid-connected system of the grid-connected converter by utilizing the key control coupling links and combining the Bode criterion.

[0165] The key control coupling link that leads to system instability is G in claim 6. f G f The open-loop transfer function of G is the open-loop transfer function of the entire system. According to the Nyquist theorem, G f The open-loop transfer function can be used to analyze system stability. Its open-loop transfer function G... o As shown below:

[0166] (12)

[0167] Because of G o Since there are no poles in the right half-plane, the system stability can be analyzed using its amplitude and phase characteristics. That is, the system is stable when the following equation is satisfied:

[0168] (13)

[0169] In the formula, Phase margin; for The amplitude crossover frequency is the angular frequency corresponding to the amplitude of the amplitude-frequency characteristic being 1. This is the gain margin; for The phase crossover frequency is the angular frequency corresponding to a phase characteristic of -180°. Therefore, G under different operating conditions can be obtained. o The amplitude and phase characteristics can be used to analyze the system stability using equation (13).

[0170] Plot G for different short-circuit ratios under a certain operating condition. o Amplitude and phase characteristics are as follows Figure 2 As shown in the figure, when SCR=7.2, the system stability margin is 0, indicating a critical stability state. The crossover frequency under this condition is 5.3Hz. Therefore, the system will operate stably when SCR is less than 7.2, exhibit constant-amplitude oscillations at 5.3Hz when SCR is equal to 7.2, and divergent oscillations when SCR is greater than 7.2. (See attached figure.) Figure 3 The simulation results shown are consistent.

Claims

1. A stability analysis method for a grid-type converter based on inner and outer loop control coupling, characterized in that, Includes the following steps: Step 1) Perform rotational speed control on the grid-connected system of the grid-type converter according to the dq rotating coordinate system. The system is divided into two subsystems: the power grid and main circuit system, and the grid-type converter system. Step 2) Obtain the small disturbance variable relationships between the power grid, main circuit system, and grid-type converter system; Step 3) Establish small disturbance models of the power grid and main circuit system, and small disturbance models of the grid-type converter system in the corresponding rotating coordinate system; Step 4) Based on the small disturbance variable relationship and the small disturbance model, establish a unified single-input single-output model for the grid-connected system of the grid-connected converter; Step 5) Use a single-input single-output model to determine the open-loop transfer function of the key control coupling link that leads to system instability; Step 6) Analyze the network structure variation using the open-loop transfer function of the key control coupling links. Stability of grid-connected inverter systems.

2. The stability analysis method for a grid-type converter based on inner and outer loop control coupling according to claim 1, characterized in that, In step 1), the rotational speed of the dq rotating coordinate system of the grid-type converter system is the synchronous speed; The rotational speeds of the grid-type converter system in the dq rotating coordinate system are not synchronized.

3. The stability analysis method for a grid-type converter based on inner and outer loop control coupling according to claim 1, characterized in that, In step 2), the small disturbance variable relationships between the power grid, main circuit system, and grid-type converter system are as follows: （1） in, This represents the dq rotating coordinate system in which vector x is located in the network converter control system. The dq component in The coordinate system is oriented with grid connection point voltage. The d-axis coincides with the d-axis vector of the grid connection point voltage, the q-axis is perpendicular to the d-axis, the positive direction is in the counterclockwise direction of the d-axis, and the origin is the intersection of the d and q axes. This represents the dq rotating coordinate system in which vector x is located within the power grid and main circuit system. The dq component in The coordinate system is oriented with grid voltage, the d-axis coincides with the grid voltage d-axis vector, the q-axis is perpendicular to the d-axis with its positive direction counterclockwise from the d-axis, and the origin is the intersection of the d and q axes; (symbol) This represents the small perturbation component of the corresponding variable; The output disturbance is used for the network-type control algorithm. , This represents the dq component of vector x under the reference state.

4. The stability analysis method for a grid-type converter based on inner and outer loop control coupling according to claim 3, characterized in that, Output disturbance of network control algorithm As shown below: （2） In the formula, This represents the small disturbance component of the active power output of the converter. Indicates synchronous angular velocity; D p J represents the virtual synchronous control damping coefficient and moment of inertia; s represents the complex frequency variable.

5. The stability analysis method for a grid-type converter based on inner and outer loop control coupling according to claim 1, characterized in that, In step 3), the small disturbance model of the power grid and main circuit system is shown below: （3） （4） （5） In the formula, L g R g L represents the equivalent inductance and resistance of the power grid. f R f C f This represents the converter's filter inductance, parasitic resistance, and filter capacitor. , These are the d-axis and q-axis disturbance components of the converter inductor current; , These are the d-axis and q-axis disturbance components of the grid-connected current; , These are the d-axis and q-axis disturbance components of the grid connection point voltage; , d-axis and q-axis disturbance components of the grid equivalent voltage; , These are the d-axis and q-axis disturbance components of the converter output line voltage.

6. The stability analysis method for a grid-type converter based on inner and outer loop control coupling according to claim 1, characterized in that, The small disturbance model of the grid converter system is shown below: （6） （7） （8） Where, m dq For the d-axis and q-axis components of the modulated signal; T d For equivalent delay; K pwm These are the PWM modulation coefficients; , These are the reference values ​​for the d-axis and q-axis disturbances of the converter inductor current. , These are the reference values ​​for the d-axis and q-axis disturbances of the converter output voltage. , G represents the d-axis and q-axis disturbance components of the converter output voltage. VL G CL Transfer functions for voltage loop and current loop PI controllers; , These are the d- and q-axis disturbance components of the converter inductor current. , These are the d-axis and q-axis perturbation components of the modulated signal; , These are the d- and q-axis disturbance components of the converter output line voltage. This is the equivalent time delay for the control system.

7. The stability analysis method for a grid-type converter based on inner and outer loop control coupling according to claim 1, characterized in that, In step 4), the unified single-input single-output model of the grid-connected system for the grid-connected converter is shown below: （9） （10） In the formula, G VL G CL This represents the transfer function of the voltage loop and current loop PI controller; G represents the closed-loop transfer function of the key control coupling link; q This represents the integral stage of the virtual exciter; Represents the initial values ​​of the d-axis components of the grid connection point voltage; G1, G2, G GFM Z v To simplify the expression of parameters.

8. The stability analysis method for a grid-type converter based on inner and outer loop control coupling according to claim 6, characterized in that, The closed-loop transfer function of the key control coupling element is shown below: （11） In the formula, This represents the closed-loop transfer function of the key control coupling link.

9. The stability analysis method for a grid-type converter based on inner and outer loop control coupling according to claim 1, characterized in that, In step 5), the open-loop transfer function G of the key control coupling link that leads to system instability o As shown below: （12） In the formula, This represents the initial value of the d-axis component of the grid connection point voltage.

10. The stability analysis method for a grid-type converter based on inner and outer loop control coupling according to claim 1, characterized in that, In step 6), if the open-loop transfer function G o If the stability criterion is met, then the system is stable; The stability criterion is as follows: （13） In the formula, Phase margin; for The amplitude crossover frequency is the angular frequency corresponding to the amplitude of the amplitude-frequency characteristic being 1. This is the gain margin; for The phase crossover frequency is the angular frequency corresponding to a phase characteristic phase of -180°.

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