A method and system for transient synchronization stability control of modular multilevel converter under asymmetric bridge arm situation

Abstract

The application relates to a kind of bridge arm asymmetric situation modular multilevel converter transient synchronization stability control method and system.The method first establishes MMC mathematical model under bridge arm asymmetric situation;Then define MMC state variable desired trajectory under bridge arm asymmetric situation in MMC mathematical model;Then design system desired energy function, and combine it with MMC mathematical model, generate MMC transient synchronization stability control closed-loop system desired state equation under bridge arm asymmetric situation;And establish passive control law from control closed-loop system desired state equation;Finally, according to passive control law, adjust the working state of multilevel converter.Compared with prior art, the application can realize MMC transient synchronization stability under bridge arm asymmetric fault, which is beneficial to the safe and stable operation of system.

Description

Technical Field

[0001] This invention relates to the field of modular multilevel converter control technology, and in particular to a transient synchronous stability control method and system for modular multilevel converters under bridge arm asymmetry. Background Technology

[0002] Currently, modular multilevel converters (MMCs) are widely used in large-scale renewable energy grid connection applications due to their advantages such as low harmonic content, low switching losses, strong fault ride-through capability, ease of modular expansion, and industrial production. However, MMCs use a multi-submodule cascaded bridge arm configuration. In actual operation, the failure of one or more submodules or perturbations in the bridge arm resistance and inductance parameters may lead to asymmetrical operation of the MMC bridge arms and changes in the total energy of the MMC. The reduction in the total energy of the MMC results in a decrease in the AC side output power, which in turn leads to transient synchronization and stability issues, affecting the safe and stable operation of the system.

[0003] In existing bridge arm asymmetry scenarios, most MMC transient synchronous stability control methods adopt traditional vector control methods without considering the energy aspect and designing controllers to address the nonlinear nature of MMC systems under bridge arm asymmetry. As a result, the stability domain is narrow, and the disturbance rejection and robustness of the vector controller face challenges in actual operation when the system is subjected to uncertain disturbances.

[0004] Compared to traditional vector control methods, nonlinear control methods, starting from an energy perspective, design controllers that can reflect the nonlinear nature of MMC systems under asymmetrical bridge arm conditions. This improves control performance in terms of stability and robustness of closed-loop control systems. However, the calculations are more complex, and there are still shortcomings in optimizing dynamic response performance, which is not conducive to solving practical engineering problems. Summary of the Invention

[0005] The purpose of this invention is to overcome the defects of the prior art and provide a transient synchronous stability control method and system for modular multilevel converters under bridge arm asymmetry.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] According to one aspect of the present invention, a transient synchronous stability control method for a modular multilevel converter under bridge arm asymmetry is provided, comprising the following steps:

[0008] S1. Establish the MMC mathematical model under the condition of asymmetrical bridge arms;

[0009] S2. In the MMC mathematical model, define the expected trajectory of the MMC state variables under the case of asymmetrical bridge arms;

[0010] S3. Design the desired energy function of the system and combine it with the MMC mathematical model to generate the desired state equation of the MMC transient synchronous stable control closed-loop system under the case of bridge arm asymmetry.

[0011] S4. Establish a passive control law from the desired state equation of the control closed-loop system;

[0012] S5. Adjust the operating state of the multilevel converter according to the passive control law.

[0013] As a preferred technical solution, the specific process of establishing the MMC mathematical model in S1 is as follows: First, the x-phase state space equation of the MMC when the bridge arm is running asymmetrically is constructed. Then, the state variables and control variables are determined, and the PCHD-based MMC mathematical model is established in conjunction with the x-phase state space equation.

[0014] As a preferred technical solution, the x-phase state-space equation of the MMC during asymmetrical operation of the bridge arm is:

[0015]

[0016] In the formula, L xc L xh The sum and difference of the inductances of the upper and lower bridge arms of phase x are respectively; i xs Let i be the grid-connected current of phase x. xo For phase x circulation; v xs V is the output voltage of phase x; xg V is the voltage on the grid side of phase x; xo The voltage within phase x of the bridge arm; C is the rated capacitance of the submodule; n u n l These represent the number of sub-modules deployed in the upper and lower bridge arms, respectively. Here, t is the differential operator, and t is the current time.

[0017] As a preferred technical solution, the MMC mathematical model is as follows:

[0018]

[0019] In the formula, denoted as , where is the expectation of the MMC mathematical model system; J(x) is the interconnection matrix; R(x) is the damping matrix; H(x) is the energy function; g(x) is the port matrix; and u is the voltage of the MMC model system.

[0020] As a preferred technical solution, the expected trajectory of the MMC state variables in S2 is:

[0021]

[0022]

[0023] In the formula, These are the expected trajectories of the capacitor voltages of the upper and lower bridge arm submodules, respectively. xc L xh The sum and difference of the inductances of the upper and lower bridge arms of phase x are respectively; i xs Let i be the grid-connected current of phase x. xo For x-phase circulation; ω, These represent the angular frequency and phase angle of the fundamental frequency component of the x-phase output current, respectively; n u n l These represent the number of sub-modules deployed in the upper and lower bridge arms, respectively; C represents the rated capacitance of the sub-module. The desired trajectory of the x-phase circulation; S represents the desired trajectory of the output current in phase x; MMC For the apparent power of MMC; m = V xg / (U dc / 2) is the modulation ratio, U dc DC side voltage

[0024] As a preferred technical solution, the specific process for generating the desired state equation of the MMC transient synchronous stable control closed-loop system under the bridge arm asymmetry case in S3 is as follows:

[0025] S31. Calculate the output frequency f ac With frequency expected trajectory f acN Deviation between;

[0026] S32, from the output frequency f ac With frequency expected trajectory f acN The deviation between the two sides is used to solve for the expected trajectories of the capacitor voltages of the upper and lower bridge arm submodules.

[0027] S33. Obtain the system's expected trajectory from the expected trajectory of the capacitor voltage, and use it as the control objective for the MMC transient synchronization problem. Then, design the system's expected energy function under the case of bridge arm asymmetry.

[0028] S34. By jointly analyzing the bridge arm of the MMC mathematical model and the expected energy function of the system under asymmetric conditions, the expected state equation of the MMC transient synchronous stable control closed-loop system is obtained.

[0029] As a preferred technical solution, the specific formula for the system's expected energy function in S33 is as follows:

[0030]

[0031] In the formula, H d Let L be the system's desired energy function; xc L xh These represent the sum and difference of the inductances of the upper and lower bridge arms of phase x, respectively; C is the rated capacitance of the submodule; n u nl These represent the number of sub-modules deployed in the upper and lower bridge arms, respectively, x n For system state variables, These are the expected state variables of the system.

[0032] As a preferred technical solution, the specific formula for the desired state equation of the MMC transient synchronous stable control closed-loop system in S34 is as follows:

[0033]

[0034] In the formula, J represents the desired state of the system. d (x) is the system's desired interconnection matrix; R d (x) is the system's desired damping matrix; H d (x) is the system's expected energy function.

[0035] As a preferred technical solution, the specific formula of the passive control law in S4 is as follows:

[0036]

[0037] In the formula, g(x) is the port matrix; J d (x) is the system's desired interconnection matrix; R d (x) is the system's desired damping matrix; H d J(x) is the system's desired energy function; J(x) is the interconnection matrix; R(x) is the damping matrix; and H(x) is the energy function.

[0038] According to another aspect of the present invention, a transient synchronous stability control system for a modular multilevel converter under asymmetric bridge arm conditions is provided. The system operates using a transient synchronous stability control method for a modular multilevel converter under asymmetric bridge arm conditions as described above. The system includes a model building module, a desired trajectory determination module, a control law generation module, and a control module.

[0039] The model building module is used to establish an MMC mathematical model in the case of asymmetrical bridge arms;

[0040] The desired trajectory determination module is used to define the desired trajectory of the MMC state variables in the case of bridge arm asymmetry in the MMC mathematical model; and to design the system desired energy function, and combine it with the MMC mathematical model to generate the desired state equation of the MMC transient synchronous stable control closed-loop system in the case of bridge arm asymmetry.

[0041] The control law generation module is used to construct a passive control law using the desired state equation of the control closed-loop system.

[0042] The control module adjusts the operating state of the multilevel converter according to the passive control law.

[0043] Compared with the prior art, the present invention has the following beneficial effects;

[0044] 1. In this invention, a mathematical model of the MMC is first established under the condition of bridge arm asymmetry. Then, the expected trajectory of the MMC state variables under the condition of bridge arm asymmetry is defined in the MMC mathematical model. Next, the expected energy function of the system is designed and combined with the MMC mathematical model to generate the expected state equation of the MMC transient synchronous stability control closed loop system under the condition of bridge arm asymmetry. A passive control law is then established from the expected state equation of the control closed loop system. Finally, the working state of the multilevel converter is adjusted according to the passive control law. Through this method, the transient synchronous stability of the MMC can be achieved under the condition of bridge arm asymmetry fault, which is beneficial to the safe and stable operation of the system.

[0045] 2. The passive control law established in this invention is mainly composed of an interconnection matrix, a damping matrix, an energy function and its corresponding expectation. Compared with the traditional vector control method, this control law has a simple form, no singularities, and small overshoot, resulting in short adjustment time and excellent dynamic response performance.

[0046] 3. In this invention, the x-phase state-space equations for the MMC under asymmetrical bridge arm operation are first constructed. Then, the state variables and control variables are determined, and a PCHD-based mathematical model of the MMC under asymmetrical bridge arm operation is established by combining the x-phase state-space equations. This method utilizes PCHD characteristics and passive theory, and through energy function shaping, minimizes the energy at the desired equilibrium point, improving the MMC output energy efficiency, enabling the MMC to quickly track the grid-side frequency, and ensuring the global asymptotic stability of the system. Attached Figure Description

[0047] Figure 1 This is a schematic diagram illustrating the steps of a transient synchronous stability control method for a modular multilevel converter under bridge arm asymmetry in this invention.

[0048] Figure 2 This is a diagram showing the three-phase MMC circuit structure and sub-module topology under the condition of asymmetrical bridge arms in the embodiment;

[0049] Figure 3 This is the frequency diagram of the MMC transient synchronization control network side in the embodiment. Detailed Implementation

[0050] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0051] Currently, modular multilevel converters (MMCs) are widely used in large-scale renewable energy grid connection applications due to their advantages such as low harmonic content, low switching losses, strong fault ride-through capability, ease of modular expansion, and industrial production. However, MMCs use a multi-submodule cascaded bridge arm configuration. In actual operation, the failure of one or more submodules or perturbations in the bridge arm resistance and inductance parameters may lead to asymmetrical operation of the MMC bridge arms and changes in the total energy of the MMC. The reduction in the total energy of the MMC results in a decrease in the AC side output power, which in turn leads to transient synchronization and stability issues, affecting the safe and stable operation of the system.

[0052] In existing bridge arm asymmetry scenarios, most MMC transient synchronous stability control methods adopt traditional vector control methods without considering the energy aspect and designing controllers to address the nonlinear nature of MMC systems under bridge arm asymmetry. As a result, the stability domain is narrow, and the disturbance rejection and robustness of the vector controller face challenges in actual operation when the system is subjected to uncertain disturbances.

[0053] Compared to traditional vector control methods, nonlinear control methods, starting from an energy perspective, design controllers that can reflect the nonlinear nature of MMC systems under asymmetrical bridge arm conditions. This improves control performance in terms of stability and robustness of closed-loop control systems. However, the calculations are more complex, and there are still shortcomings in optimizing dynamic response performance, which is not conducive to solving practical engineering problems.

[0054] How to improve dynamic and static response performance while ensuring further improvement in the global asymmetric stability and robustness of the system, while keeping the controller design as simple as possible, is a key problem that MMC transient synchronous stability control must solve under bridge arm asymmetry.

[0055] Example 1

[0056] In this embodiment, a transient synchronous stability control method for a modular multilevel converter under bridge arm asymmetry is applied. The method steps are as follows: Figure 1 As shown, it includes the following steps:

[0057] S1. Establish the MMC mathematical model under the condition of asymmetrical bridge arms;

[0058] S2. In the MMC mathematical model, define the expected trajectory of the MMC state variables under the case of asymmetrical bridge arms;

[0059] S3. Design the desired energy function of the system and combine it with the MMC mathematical model to generate the desired state equation of the MMC transient synchronous stable control closed-loop system under the case of bridge arm asymmetry.

[0060] S4. Establish a passive control law from the desired state equation of the control closed-loop system;

[0061] S5. Adjust the operating state of the multilevel converter according to the passive control law.

[0062] In this embodiment, the three-phase MMC circuit structure and sub-module topology under the bridge arm asymmetry case are as follows: Figure 2 As shown. By Figure 2 It can be seen that when a submodule fault or bridge arm parameter perturbation occurs in phase x (x∈{u,v,w}), the MMC is in bridge arm asymmetric operation. The state space equation of phase x when the MMC is in bridge arm asymmetric operation is:

[0063]

[0064] in,

[0065] v xu =n u u cu v xl =n l u cl ,

[0066] a = L xc R xc -L xh R xh b = L xh R xc -L xc R xh

[0067] In the formula, L xc L xh These are the sum and difference of the inductances of the upper and lower bridge arms of phase x, respectively; R xc R xh The sum and difference of the resistances of the upper and lower bridge arms of phase x are respectively, i xs Let i be the grid-connected current of phase x. xo For phase x circulation; v xs V is the output voltage of phase x; xg V is the voltage on the grid side of phase x; xo V is the voltage within phase x of the bridge arm. xu v xl These represent the capacitor voltages of the upper and lower bridge arms of phase x, respectively, and C is the rated capacitance of the submodule; u cu u cl These represent the capacitor voltages of the upper and lower bridge arm submodules for phase x, respectively, and n u n l These represent the number of sub-modules deployed in the upper and lower bridge arms, respectively; U dc DC side voltage Here, t is the differential operator, and t is the current time.

[0068] In this embodiment, state variables and control variables are defined, and their specific formulas are as follows:

[0069]

[0070] u = [u1, u2, u3, u4] T =[v xo ,v xg -v xs i xs i xo ] T

[0071] By combining the x-phase state-space equations of the combined MMC under asymmetric arm operation, a PCHD-based mathematical model of the MMC under asymmetric arm operation can be established as follows:

[0072]

[0073] In the formula, denoted as , where is the expectation of the MMC mathematical model system; J(x) is the interconnection matrix; R(x) is the damping matrix; H(x) is the energy function; g(x) is the port matrix; and u is the voltage of the MMC model system.

[0074] The interconnection matrix is ​​as follows:

[0075]

[0076] The damping matrix is:

[0077]

[0078] The port matrix is ​​as follows:

[0079]

[0080] The energy function is:

[0081]

[0082] In this embodiment, the expected trajectory of the MMC state variable in S2 is:

[0083]

[0084] The expected trajectory of the x-phase circulation is:

[0085]

[0086] The desired trajectory of the x-phase output current is:

[0087]

[0088] In the formula, These are the expected trajectories of the capacitor voltages of the upper and lower bridge arm submodules, respectively. xc L xhThe sum and difference of the inductances of the upper and lower bridge arms of phase x are respectively; i xs Let i be the grid-connected current of phase x. xo For x-phase circulation; ω, These represent the angular frequency and phase angle of the fundamental frequency component of the x-phase output current, respectively; n u n l These represent the number of sub-modules deployed in the upper and lower bridge arms, respectively; C represents the rated capacitance of the sub-module. The desired trajectory of the x-phase circulation; S represents the desired trajectory of the output current in phase x; MMC For the apparent power of MMC; m = V xg / (U dc / 2) is the modulation ratio, U dc DC side voltage

[0089] In this embodiment, the specific process of generating the desired state equation of the MMC transient synchronous stable control closed-loop system under the bridge arm asymmetry case in S3 is as follows:

[0090] S31. Calculate the output frequency f ac With frequency expected trajectory f acN Deviation between;

[0091] S32, from the output frequency f ac With frequency expected trajectory f acN The deviation between the two sides is used to solve for the expected trajectories of the capacitor voltages of the upper and lower bridge arm submodules.

[0092] S33. Obtain the system's expected trajectory from the expected trajectory of the capacitor voltage, and use it as the control objective for the MMC transient synchronization problem. Then, design the system's expected energy function under the case of bridge arm asymmetry.

[0093] S34. By jointly analyzing the bridge arm of the MMC mathematical model and the expected energy function of the system under asymmetric conditions, the expected state equation of the MMC transient synchronous stable control closed-loop system is obtained.

[0094] First, calculate the deviation, specifically the MMC output frequency f under the condition of bridge arm asymmetry. ac With frequency expected trajectory f acN There is a discrepancy between them, and the relationship between them satisfies:

[0095]

[0096] In the formula: P dc P ac These represent the DC and AC power of the MMC, respectively, H MMC K represents the equivalent inertia of the internal capacitance of the MMC. D N is the equivalent damping coefficient of MMC, and N is the number of MMC single-bridge arm submodules. fThis represents the number of MMC single-bridge arm submodules that failed to exit. This is the reference value for the capacitor voltage of the submodule.

[0097] In this embodiment, the MMC output frequency f ac With frequency expected trajectory f acN The expected value of the submodule capacitor voltage is obtained by calculating the inter-deviation. The expected trajectories of the capacitor voltages of the upper and lower bridge arm submodules can then be obtained as follows:

[0098]

[0099] The specific formula for the system's expected energy function in S33 is as follows:

[0100]

[0101] In the formula, H d Let L be the system's desired energy function; xc L xh These represent the sum and difference of the inductances of the upper and lower bridge arms of phase x, respectively; C is the rated capacitance of the submodule; n u n l These represent the number of sub-modules deployed in the upper and lower bridge arms, respectively, x n For system state variables, These are the expected state variables of the system.

[0102] In this embodiment, the specific formula for the desired state equation of the MMC transient synchronous stable control closed-loop system in S34 is as follows:

[0103]

[0104] In the formula, J represents the desired state of the system. d (x) is the system's desired interconnection matrix; R d (x) is the system's desired damping matrix; H d (x) is the system's expected energy function.

[0105] The system's expected interconnection matrix is:

[0106] J d (x)=J(x)+J a (x)

[0107] The system's desired damping matrix is:

[0108] R d (x)=R(x)+R a (x)

[0109] In the formula, the system injection interconnection matrix J a (x) = 0, the system's expected damping matrix Ra =diag[R1,R2,R3,R4], where R1, R2, R3, and R4 are the injected positive damping parameters, and R1 = R2 and R3 = R4.

[0110] From the desired state equation of the MMC transient synchronous stable control closed-loop system, the specific formula for the passive control law based on PCHD in the case of bridge arm asymmetry is as follows:

[0111]

[0112] In the formula, g(x) is the port matrix; J d (x) is the system's desired interconnection matrix; R d (x) is the system's desired damping matrix; H d J(x) is the system's desired energy function; J(x) is the interconnection matrix; R(x) is the damping matrix; and H(x) is the energy function.

[0113] In this embodiment, the passive control law can ensure that the closed-loop control system can quickly track the desired trajectory of the state variable under the premise of global asymptotic stability, and achieve the goal of MMC transient synchronous stable control under the condition of bridge arm asymmetry.

[0114] In this embodiment, a simulation model of the MMC transient synchronous stable control system under the condition of bridge arm asymmetry is built in MATLAB / Simulink. Simulation tests are conducted using both traditional vector control methods and the proposed passive control method based on PCHD. The simulation results are as follows: Figure 3 As shown. By Figure 3 Analysis shows that at t=50s, the MMC submodule fails and exits, resulting in asymmetrical operation of the bridge arm. Compared with the traditional vector control method, the proposed method has a smaller overshoot, shorter adjustment time, and better dynamic response performance. It can achieve transient synchronous stability of MMC under asymmetrical bridge arm faults, which is beneficial to the safe and stable operation of the system.

[0115] Example 2

[0116] In this embodiment, a modular multilevel converter transient synchronous stability control system for bridge arm asymmetry is applied. The system includes a model building module, a desired trajectory determination module, a control law generation module, and a control module.

[0117] The model building module is used to establish the MMC mathematical model under the condition of bridge arm asymmetry; the desired trajectory determination module is used to define the desired trajectory of the MMC state variables under the condition of bridge arm asymmetry in the MMC mathematical model; and to design the desired energy function of the system, and combine it with the MMC mathematical model to generate the desired state equation of the MMC transient synchronous stable control closed loop system under the condition of bridge arm asymmetry; the control law generation module is used to construct a passive control law using the desired state equation of the control closed loop system; the control module adjusts the working state of the multilevel converter according to the passive control law.

[0118] In this embodiment, the three-phase MMC circuit structure and sub-module topology under the bridge arm asymmetry case are as follows: Figure 2 As shown. By Figure 2 It can be seen that when a submodule fault or bridge arm parameter perturbation occurs in phase x (x∈{u,v,w}), the MMC is in bridge arm asymmetric operation. The state space equation of phase x when the MMC is in bridge arm asymmetric operation is:

[0119]

[0120] in,

[0121] v xu =n u u cu v xl =n l u cl ,

[0122] a = L xc R xc -L xh R xh b = L xh R xc -L xc R xh

[0123] In the formula, L xc L xh These are the sum and difference of the inductances of the upper and lower bridge arms of phase x, respectively; R xc R xh The sum and difference of the resistances of the upper and lower bridge arms of phase x are respectively, i xs Let i be the grid-connected current of phase x. xo For phase x circulation; v xs V is the output voltage of phase x; xg V is the voltage on the grid side of phase x; xo V is the voltage within phase x of the bridge arm. xu v xl These represent the capacitor voltages of the upper and lower bridge arms of phase x, respectively, and C is the rated capacitance of the submodule; u cu u clThese represent the capacitor voltages of the upper and lower bridge arm submodules for phase x, respectively, and n u n l These represent the number of sub-modules deployed in the upper and lower bridge arms, respectively; U dc DC side voltage Here, t is the differential operator, and t is the current time.

[0124] In this embodiment, state variables and control variables are defined, and their specific formulas are as follows:

[0125]

[0126] u = [u1, u2, u3, u4] T =[v xo ,v xg -v xs i xs i xo ] T

[0127] By combining the x-phase state-space equations of the combined MMC under asymmetric arm operation, a PCHD-based mathematical model of the MMC under asymmetric arm operation can be established as follows:

[0128]

[0129] In the formula, denoted as , where is the expectation of the MMC mathematical model system; J(x) is the interconnection matrix; R(x) is the damping matrix; H(x) is the energy function; g(x) is the port matrix; and u is the voltage of the MMC model system.

[0130] The interconnection matrix is ​​as follows:

[0131]

[0132] The damping matrix is:

[0133]

[0134] The port matrix is ​​as follows:

[0135]

[0136] The energy function is:

[0137]

[0138] In this embodiment, the expected trajectory of the MMC state variable in S2 is:

[0139]

[0140] The expected trajectory of the x-phase circulation is:

[0141]

[0142] The desired trajectory of the x-phase output current is:

[0143]

[0144] In the formula, These are the expected trajectories of the capacitor voltages of the upper and lower bridge arm submodules, respectively. xc L xh The sum and difference of the inductances of the upper and lower bridge arms of phase x are respectively; i xs Let i be the grid-connected current of phase x. xo For x-phase circulation; ω, These represent the angular frequency and phase angle of the fundamental frequency component of the x-phase output current, respectively; n u n l These represent the number of sub-modules deployed in the upper and lower bridge arms, respectively; C represents the rated capacitance of the sub-module. The desired trajectory of the x-phase circulation; S represents the desired trajectory of the output current in phase x; MMC For the apparent power of MMC; m = V xg / (U dc / 2) is the modulation ratio, U dc DC side voltage

[0145] In this embodiment, the specific process of generating the desired state equation of the MMC transient synchronous stable control closed-loop system under the bridge arm asymmetry case in S3 is as follows:

[0146] S31. Calculate the output frequency f ac With frequency expected trajectory f acN Deviation between;

[0147] S32, from the output frequency f ac With frequency expected trajectory f acN The deviation between the two sides is used to solve for the expected trajectories of the capacitor voltages of the upper and lower bridge arm submodules.

[0148] S33. The expected trajectory of the system is obtained from the expected trajectory of the capacitor voltage, which serves as the control objective for the transient synchronization problem of MMC, and then the expected energy function of the system under the case of bridge arm asymmetry is designed.

[0149] S34. By jointly analyzing the bridge arm of the MMC mathematical model and the expected energy function of the system under asymmetric conditions, the expected state equation of the MMC transient synchronous stable control closed-loop system is obtained.

[0150] First, calculate the deviation, specifically the MMC output frequency f under the condition of bridge arm asymmetry. ac With frequency expected trajectory f acN There is a discrepancy between them, and the relationship between them satisfies:

[0151]

[0152] In the formula: P dc P ac These represent the DC and AC power of the MMC, respectively, H MMC K represents the equivalent inertia of the internal capacitance of the MMC. D N is the equivalent damping coefficient of MMC, and N is the number of MMC single-bridge arm submodules. f This represents the number of MMC single-bridge arm submodules that failed to exit. This is the reference value for the capacitor voltage of the submodule.

[0153] In this embodiment, the MMC output frequency f ac With frequency expected trajectory f acN The expected value of the submodule capacitor voltage is obtained by calculating the inter-deviation. The expected trajectories of the capacitor voltages of the upper and lower bridge arm submodules can then be obtained as follows:

[0154]

[0155] The specific formula for the system's expected energy function in S33 is as follows:

[0156]

[0157] In the formula, H d Let L be the system's desired energy function; xc L xh These represent the sum and difference of the inductances of the upper and lower bridge arms of phase x, respectively; C is the rated capacitance of the submodule; n u n l These represent the number of sub-modules deployed in the upper and lower bridge arms, respectively, x n For system state variables, These are the expected state variables of the system.

[0158] In this embodiment, the specific formula for the desired state equation of the MMC transient synchronous stable control closed-loop system in S34 is as follows:

[0159]

[0160] In the formula, J represents the desired state of the system. d (x) is the system's desired interconnection matrix; R d (x) is the system's desired damping matrix; H d (x) is the system's expected energy function.

[0161] The system's expected interconnection matrix is:

[0162] J d (x)=J(x)+Ja (x)

[0163] The system's desired damping matrix is:

[0164] R d (x)=R(x)+R a (x)

[0165] In the formula, the system injection interconnection matrix J a (x) = 0, the system's expected damping matrix R a =diag[R1,R2,R3,R4], where R1, R2, R3, and R4 are the injected positive damping parameters, and R1 = R2 and R3 = R4.

[0166] From the desired state equation of the MMC transient synchronous stable control closed-loop system, the specific formula for the passive control law based on PCHD in the case of bridge arm asymmetry is as follows:

[0167]

[0168] In the formula, g(x) is the port matrix; J d (x) is the system's desired interconnection matrix; R d (x) is the system's desired damping matrix; H d J(x) is the system's desired energy function; J(x) is the interconnection matrix; R(x) is the damping matrix; and H(x) is the energy function.

[0169] In this embodiment, the passive control law can ensure that the closed-loop control system can quickly track the desired trajectory of the state variable under the premise of global asymptotic stability, and achieve the goal of MMC transient synchronous stable control under the condition of bridge arm asymmetry.

[0170] In this embodiment, a simulation model of the MMC transient synchronous stable control system under the condition of bridge arm asymmetry is built in MATLAB / Simulink. Simulation tests are conducted using a traditional vector control system and the proposed passive control system based on PCHD. The simulation results are as follows: Figure 3 As shown. By Figure 3 Analysis shows that at t=50s, the MMC submodule fails and exits, causing the bridge arm to operate asymmetrically. Compared with the traditional vector control system, the proposed system has a smaller overshoot, shorter adjustment time, and better dynamic response performance. It can achieve transient synchronization and stability of the MMC under bridge arm asymmetrical fault, which is conducive to the safe and stable operation of the system.

[0171] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A transient synchronous stability control method for a modular multilevel converter under bridge arm asymmetry, characterized in that, Includes the following steps: S1. Establish the MMC mathematical model under the case of asymmetrical bridge arms; S2. In the MMC mathematical model, define the expected trajectory of the MMC state variables under the case of asymmetrical bridge arms; S3. Design the desired energy function of the system and combine it with the MMC mathematical model to generate the desired state equation of the MMC transient synchronous stable control closed-loop system under the case of bridge arm asymmetry. S4. Establish a passive control law from the desired state equation of the control closed-loop system; S5. Adjust the working state of the multilevel converter according to the passive control law; The specific process for generating the desired state equation of the MMC transient synchronous stable control closed-loop system under the asymmetric bridge arm condition in S3 is as follows: S31. Calculate the output frequency With frequency expected trajectory Deviation between; S32, based on the output frequency With frequency expected trajectory The deviation between the two sides is used to solve for the expected trajectories of the capacitor voltages of the upper and lower bridge arm submodules. S33. Obtain the system's expected trajectory from the capacitor voltage expected trajectory, and use it as the control objective for the MMC transient synchronization problem. Then, design the system's expected energy function under the bridge arm asymmetry case. S34. By analyzing the bridge arm of the MMC mathematical model and the expected energy function of the system under asymmetric conditions, the expected state equation of the MMC transient synchronous stable control closed-loop system is obtained. The specific formula for the system's expected energy function in S33 is as follows: In the formula, Let be the system's desired energy function; , These are the sum and difference of the inductances of the upper and lower bridge arms of phase x, respectively. The rated capacitor for the submodule; , These represent the number of sub-modules deployed in the upper and lower bridge arms, respectively. For system state variables, The expected state variables of the system are represented by the index n = 1, 2, 3 or 4, which is the state variable number. The specific formula for the desired state equation of the MMC transient synchronous stable control closed-loop system in S34 is as follows: In the formula, This represents the desired state of the system. The desired interconnect matrix of the system; Here is the system's desired damping matrix; Let be the system's desired energy function.

2. The transient synchronous stability control method for a modular multilevel converter under bridge arm asymmetry as described in claim 1, characterized in that, The specific process of establishing the MMC mathematical model in S1 is as follows: First, the model is constructed when the MMC is operating in asymmetrical bridge arm mode. x Phase state-space equations, then determine state variables and control variables, and combine them. x A PCHD-based mathematical model of MMC is established using phase state space equations.

3. The transient synchronous stability control method for a modular multilevel converter under bridge arm asymmetry as described in claim 2, characterized in that, The MMC is in the process of asymmetrical operation of the bridge arm. x The phase state space equation is: In the formula, , They are respectively x The sum of the inductances of the upper and lower bridge arms and the difference thereof; for x Phase-connected grid current, for x Phase circulation; for x Phase output voltage; for x Phase grid side voltage; for x Phase bridge arm voltage; The rated capacitor for the submodule; , These represent the number of sub-modules deployed in the upper and lower bridge arms, respectively. For differential operators, The current time; , These are the capacitor voltages of the upper and lower bridge arm submodules, respectively.

4. The transient synchronous stability control method for a modular multilevel converter under bridge arm asymmetry as described in claim 2, characterized in that, The MMC mathematical model is as follows: In the formula, The expected value of the MMC mathematical model system; For interconnection matrix; Here is the damping matrix; It is an energy function; Port matrix; This represents the system voltage in the MMC model.

5. The transient synchronous stability control method for a modular multilevel converter under bridge arm asymmetry as described in claim 1, characterized in that, The expected trajectory of the MMC state variables in S2 is: In the formula, , These are the expected trajectories of the capacitor voltages of the upper and lower bridge arm submodules, respectively. , They are respectively x The sum of the inductances of the upper and lower bridge arms is equal to the difference between them; for x Phase-connected grid current, for x Phase circulation; , They are respectively x The fundamental frequency component of the phase output current has an angular frequency and a phase angle; , These represent the number of sub-modules deployed in the upper and lower bridge arms, respectively. The rated capacitor for the submodule; The desired trajectory of the x-phase circulation; The desired trajectory of the output current of phase x; For MMC apparent power; The modulation ratio, for x Phase grid voltage, This is the DC side voltage.

6. The transient synchronous stability control method for a modular multilevel converter under bridge arm asymmetry as described in claim 1, characterized in that, The specific formula for the passive control law in S4 is as follows: In the formula, Port matrix; The system's desired interconnect matrix; Here is the system's desired damping matrix; Let be the system's desired energy function; For interconnection matrix; Here is the damping matrix; It is the energy function.

7. A transient synchronous stability control system for a modular multilevel converter under asymmetrical bridge arm conditions, characterized in that, The system operates using a transient synchronous stability control method for a modular multilevel converter under bridge arm asymmetry as described in any one of claims 1-6. The system includes a model building module, a desired trajectory determination module, a control law generation module, and a control module. The model building module is used to establish an MMC mathematical model in the case of asymmetrical bridge arms; The desired trajectory determination module is used to define the desired trajectory of the MMC state variables in the case of bridge arm asymmetry in the MMC mathematical model; and to design the system desired energy function, and combine it with the MMC mathematical model to generate the desired state equation of the MMC transient synchronous stable control closed-loop system in the case of bridge arm asymmetry. The control law generation module is used to construct a passive control law using the desired state equation of the control closed-loop system. The control module adjusts the operating state of the multilevel converter according to the passive control law.

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