Transient synchronous stability control method and system for modular multilevel converter under asymmetric bridge arm condition

Abstract

The invention relates to a transient synchronous stability control method and system for a modular multilevel converter under the condition of asymmetric bridge arms. The method comprises the following steps: firstly, establishing an MMC mathematical model under the condition that bridge arms are asymmetric; in the MMC mathematical model, defining an MMC state variable expected trajectory under the condition that the bridge arms are asymmetric; designing a system expectation energy function, and combining the system expectation energy function with the MMC mathematical model to generate an MMC transient synchronous stability control closed-loop system expectation state equation under the condition of asymmetric bridge arms; a passive control law is established according to the expected state equation of the control closed-loop system; compared with the prior art, the method has the advantages that transient synchronous stability of the MMC under the asymmetric bridge arm fault can be realized, and safe and stable operation of the system is facilitated.

Description

Technical Field

[0001] The present invention relates to the technical field of modular multi-level converter control, and in particular to a method and system for transient synchronous stability control of a modular multi-level converter under the condition of asymmetric bridge arms. Background Art

[0002] At present, modular multilevel converters are widely used in the field of large-scale renewable energy grid connection due to their advantages such as low harmonic content, low switching loss, strong fault ride-through capability, and convenient modular expansion and industrial production. However, MMC uses multiple sub-modules to cascade to build bridge arms. In actual operation, the failure of a single / multiple sub-modules or the perturbation of the bridge arm resistance and inductance parameters may cause the MMC bridge arm to operate asymmetrically and the total energy of the MMC to change. The reduction of the total energy of the MMC leads to a reduction in the output power on the AC side, which leads to transient synchronization stability problems, affecting the safe and stable operation of the system.

[0003] Under the existing asymmetric bridge arm situation, the transient synchronous stability control of MMC mostly adopts the traditional vector control method, which does not start from the energy perspective and designs the controller based on the nonlinear nature of the MMC system under the asymmetric bridge arm situation. Therefore, the stability domain is narrow. When the system is actually running in the presence of uncertain disturbances, the interference resistance and robustness of the vector controller face challenges.

[0004] Compared with the traditional vector control method, the nonlinear control method starts from the energy perspective and designs a controller that can reflect the nonlinear nature of the MMC system under the condition of asymmetric bridge arms. The control performance is improved in terms of stability and robustness of the closed-loop control system, but the calculation is more complicated and there are still deficiencies in the optimization of dynamic response performance, which is not conducive to solving practical engineering problems. Summary of the invention

[0005] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and to provide a method and system for transient synchronous stability control of a modular multi-level converter under the condition of asymmetric bridge arms.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

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

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

[0009] S2. In the MMC mathematical model, define the expected trajectory of the MMC state variables under the asymmetric bridge arm condition;

[0010] S3. Design the system expected energy function and combine it 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 asymmetric bridge arms;

[0011] S4, establishing a passive control law based on the desired state equation of the control closed-loop system;

[0012] S5. Adjust the working 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: first, build the x-phase state space equation when the MMC is in asymmetric operation of the bridge arm, then determine the state variables and control variables, and use the x-phase state space equation to establish the MMC mathematical model based on PCHD under the condition of asymmetric bridge arm.

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

[0015]

[0016] Where, L xc , L xh are the sum and difference of the upper and lower bridge arm inductances of phase x respectively; i xs is the x-phase grid-connected current, i xo is the x-phase circulation; v xs is the x-phase output voltage; v xg is the grid-side voltage of phase x; v xo is the voltage in the x-phase bridge arm; C is the rated capacitance of the submodule; n u 、n l are the number of submodules put into use in the upper and lower bridge arms respectively; is the differential operator, and t is the current time.

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

[0018]

[0019] In the formula, 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; u is the MMC model system voltage.

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

[0021]

[0022]

[0023] In the formula, are the expected trajectories of the capacitor voltage of the upper and lower bridge arm submodules, L xc , L xh are the sum and difference of the upper and lower bridge arm inductances of phase x respectively; i xs is the x-phase grid-connected current, i xo is the x-phase circulation; ω, are the angular frequency and phase angle of the fundamental frequency component of the x-phase output current respectively; n u 、n l are the number of submodules put into use in the upper and lower bridge arms respectively; C is the rated capacitance of the submodule; is the expected trajectory of the x-phase circulation; is the expected trajectory of the x-phase output current; S MMC MMC apparent power; m = V xg / (U dc / 2) is the modulation ratio, U dc is the DC side voltage,

[0024] As a preferred technical solution, the specific process of generating the desired state equation of the MMC transient synchronous stability control closed-loop system under the condition of asymmetric bridge arms in S3 is:

[0025] S31, calculate the output frequency f ac and the expected trajectory of frequency f acN Deviation between

[0026] S32, by the output frequency f ac The expected trajectory with frequency f acN The deviation between them is solved to obtain the expected trajectory of the capacitor voltage of the upper and lower bridge arm submodules;

[0027] S33, obtaining the system expected trajectory from the capacitor voltage expected trajectory as the control target of the MMC transient synchronization problem, and then designing the system expected energy function under the asymmetric bridge arm situation;

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

[0029] As a preferred technical solution, the specific formula of the system expected energy function in S33 is:

[0030]

[0031] In the formula, H d is the expected energy function of the system; L xc , L xh are the sum and difference of the upper and lower bridge arm inductances of phase x respectively; C is the rated capacitance of the submodule; n u 、nl are the number of submodules put into use in the upper and lower bridge arms, respectively, n is the system state variable, is the expected state variable of the system.

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

[0033]

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

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

[0036]

[0037] Where g(x) is the port matrix; J d (x) is the expected interconnection matrix of the system; R d (x) is the system expected damping matrix; H d (x) is the expected energy function of the system; J(x) is the interconnection matrix; R(x) is the damping matrix; H(x) is the energy function.

[0038] According to another aspect of the present invention, a modular multilevel converter transient synchronous stability control system under a bridge arm asymmetry condition is provided, the system is operated by a modular multilevel converter transient synchronous stability control method under a bridge arm asymmetry condition as described above, the system comprises 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 the MMC mathematical model under the condition of asymmetric bridge arms;

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

[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 working state of the multi-level converter according to the passive control law.

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

[0044] 1. In the present invention, firstly, an MMC mathematical model is established under the condition of asymmetric bridge arms; then, in the MMC mathematical model, the expected trajectory of the MMC state variables under the condition of asymmetric bridge arms is defined; then, the system expected energy function 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 asymmetric bridge arms; and a passive control law is established based on the expected state equation of the control closed-loop system; finally, the working state of the multi-level converter is adjusted according to the passive control law. Through this method, the MMC transient synchronous stability can be achieved under the condition of asymmetric bridge arm fault, which is beneficial to the safe and stable operation of the system.

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

[0046] 3. In the present invention, the x-phase state space equation of the MMC is first constructed when the bridge arm is in asymmetric operation, and then the state variables and control variables are determined, and the PCHD-based MMC mathematical model under the asymmetric bridge arm is established in conjunction with the x-phase state space equation. In this method, the PCHD characteristics and passivity theory are used to shape the energy function so that it can obtain the minimum value at the expected equilibrium point, thereby improving the output energy efficiency of the MMC, realizing the rapid tracking of the grid-side frequency by the MMC, and ensuring the global asymmetric stability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 A schematic diagram of the steps of a method for transient synchronous stability control of a modular multi-level converter under the condition of asymmetric bridge arms in the present invention;

[0048] Figure 2 The circuit structure and submodule topology diagram of the three-phase MMC in the case of asymmetric bridge arms in the embodiment;

[0049] Figure 3 It is a network-side frequency diagram of the MMC transient synchronous control in the embodiment. DETAILED DESCRIPTION

[0050] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0051] At present, modular multilevel converters are widely used in the field of large-scale renewable energy grid connection due to their advantages such as low harmonic content, low switching loss, strong fault ride-through capability, and convenient modular expansion and industrial production. However, MMC uses multiple sub-modules to cascade to build bridge arms. In actual operation, the failure of a single / multiple sub-modules or the perturbation of the bridge arm resistance and inductance parameters may cause the MMC bridge arm to operate asymmetrically and the total energy of the MMC to change. The reduction of the total energy of the MMC leads to a reduction in the output power on the AC side, which leads to transient synchronization stability problems, affecting the safe and stable operation of the system.

[0052] Under the existing asymmetric bridge arm situation, the transient synchronous stability control of MMC mostly adopts the traditional vector control method, which does not start from the energy perspective and designs the controller based on the nonlinear nature of the MMC system under the asymmetric bridge arm situation. Therefore, the stability domain is narrow. When the system is actually running in the presence of uncertain disturbances, the interference resistance and robustness of the vector controller face challenges.

[0053] Compared with the traditional vector control method, the nonlinear control method starts from the energy perspective and designs a controller that can reflect the nonlinear nature of the MMC system under the condition of asymmetric bridge arms. The control performance is improved in terms of stability and robustness of the closed-loop control system, but the calculation is more complicated and there are still deficiencies in the optimization of dynamic response performance, which is not conducive to solving practical engineering problems.

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

[0055] Example 1

[0056] In this embodiment, a method for transient synchronous stability control of a modular multilevel converter under the condition of asymmetric bridge arms is applied. The method steps are as follows: Figure 1 As shown, the following steps are included:

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

[0058] S2. In the MMC mathematical model, define the expected trajectory of the MMC state variables under the asymmetric bridge arm condition;

[0059] S3. Design the system expected energy function and combine it 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 asymmetric bridge arms;

[0060] S4, establishing a passive control law based on the desired state equation of the control closed-loop system;

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

[0062] In this embodiment, the three-phase MMC circuit structure and submodule topology diagram under the condition of asymmetric bridge arms are as follows: Figure 2 As shown. Figure 2 It can be obtained that when a submodule failure occurs in the x (x∈{u,v,w}) phase or the bridge arm parameter perturbation occurs, the MMC is in asymmetric operation of the bridge arm. The x-phase state space equation of the MMC in asymmetric operation of the bridge arm 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] Where, L xc , L xh are the sum and difference of the upper and lower bridge arm inductances of phase x respectively; R xc , R xh are the sum and difference of the upper and lower bridge arm resistances of phase x, respectively, i xs is the x-phase grid-connected current, i xo is the x-phase circulation; v xs is the x-phase output voltage; v xg is the grid-side voltage of phase x; v xo is the voltage in the x-phase bridge arm, v xu 、v xl are the upper and lower bridge arm capacitor voltages of phase x, respectively, C is the rated capacitance of the submodule; u cu 、u cl are the capacitor voltages of the upper and lower bridge arm submodules of phase x, respectively, u 、n l are the number of submodules put into use in the upper and lower bridge arms respectively; U dc is the DC side voltage, is the differential operator, and t is the current time.

[0068] In this embodiment, state variables and control variables are defined, and the 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] Combined with the x-phase state space equation of the MMC in the asymmetric operation of the bridge arm, the PCHD-based MMC mathematical model under the asymmetric bridge arm can be established as follows:

[0072]

[0073] In the formula, 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; u is the MMC model system voltage.

[0074] Among them, the interconnection matrix is:

[0075]

[0076] The damping matrix is:

[0077]

[0078] The port matrix is:

[0079]

[0080] The energy function is:

[0081]

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

[0083]

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

[0085]

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

[0087]

[0088] In the formula, are the expected trajectories of the capacitor voltage of the upper and lower bridge arm submodules, L xc , L xhare the sum and difference of the upper and lower bridge arm inductances of phase x respectively; i xs is the x-phase grid-connected current, i xo is the x-phase circulation; ω, are the angular frequency and phase angle of the fundamental frequency component of the x-phase output current respectively; n u 、n l are the number of submodules put into use in the upper and lower bridge arms respectively; C is the rated capacitance of the submodule; is the expected trajectory of the x-phase circulation; is the expected trajectory of the x-phase output current; S MMC MMC apparent power; m = V xg / (U dc / 2) is the modulation ratio, U dc is the DC side voltage,

[0089] In this embodiment, the specific process of generating the desired state equation of the MMC transient synchronous stability control closed-loop system under the condition of asymmetric bridge arms in S3 is:

[0090] S31, calculate the output frequency f ac and the expected trajectory of frequency f acN Deviation between

[0091] S32, by the output frequency f ac and the expected trajectory of frequency f acN The deviation between them is solved to obtain the expected trajectory of the capacitor voltage of the upper and lower bridge arm submodules;

[0092] S33, obtaining the system expected trajectory from the capacitor voltage expected trajectory as the control target of the MMC transient synchronization problem, and then designing the system expected energy function under the asymmetric bridge arm situation;

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

[0094] First, calculate the deviation, the MMC output frequency f when the bridge arm is asymmetric ac and the expected trajectory of frequency f acN There is a deviation between them, and the relationship between them satisfies:

[0095]

[0096] Where: P dc , P ac are the power of MMC DC side and AC side respectively, H MMC is the equivalent inertia of the internal capacitor of MMC, K D is the equivalent damping coefficient of MMC, N is the number of MMC single bridge arm submodules, N fis the number of MMC single bridge arm submodule failure exits, is the submodule capacitor voltage reference value.

[0097] In this embodiment, the MMC output frequency f ac and the expected trajectory of frequency f acN The expected value of the submodule capacitor voltage is obtained by the deviation between Then the expected trajectory of the capacitor voltage of the upper and lower bridge arm submodules is obtained as:

[0098]

[0099] The specific formula of the system expected energy function in S33 is:

[0100]

[0101] In the formula, H d is the expected energy function of the system; L xc , L xh are the sum and difference of the upper and lower bridge arm inductances of phase x respectively; C is the rated capacitance of the submodule; n u 、n l are the number of submodules put into use in the upper and lower bridge arms, respectively, n is the system state variable, is the expected state variable of the system.

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

[0103]

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

[0105] The system expects the interconnection matrix to be:

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

[0107] The expected damping matrix of the system is:

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

[0109] In the formula, the system injects the interconnection matrix J a (x) = 0, the system expected damping matrix Ra =diag[R1,R2,R3,R4], R1, R2, R3, R4 are the injected positive damping parameters, among which R1=R2, R3=R4.

[0110] From the expected state equation of the MMC transient synchronous stability control closed-loop system, the specific formula of the passive control law based on PCHD under the condition of asymmetric bridge arms is:

[0111]

[0112] Where g(x) is the port matrix; J d (x) is the expected interconnection matrix of the system; R d (x) is the system expected damping matrix; H d (x) is the expected energy function of the system; J(x) is the interconnection matrix; R(x) is the damping matrix; H(x) is the energy function.

[0113] In this embodiment, the passive control law can ensure that the closed-loop control system can achieve rapid tracking of the desired trajectory of the state variables under the premise of global asymmetric stability, and achieve the MMC transient synchronous stability control goal under the condition of asymmetric bridge arms.

[0114] In this embodiment, a simulation model of the MMC transient synchronous stability control system under the condition of asymmetric bridge arms is built in MATLAB / Simulink, and simulation tests are performed using the traditional vector control method and the proposed PCHD-based passivity control method. The simulation results are shown in Figure 2. Figure 3 As shown. Figure 3 Analysis shows that at t=50s, the MMC submodule fails and exits, resulting in asymmetric operation of the bridge arm. Compared with the traditional vector control method, the proposed method has small overshoot, short adjustment time, and excellent dynamic response performance. It can achieve transient synchronous stability of the MMC under asymmetric fault of the bridge arm, which is conducive to the safe and stable operation of the system.

[0115] Example 2

[0116] In this embodiment, a modular multi-level converter transient synchronous stability control system under the condition of asymmetric bridge arms 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 asymmetric bridge arms; the expected trajectory determination module is used to define the expected trajectory of the MMC state variables under the condition of asymmetric bridge arms in the MMC mathematical model; and the system expected energy function 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 asymmetric bridge arms; the control law generation module is used to construct the passive control law using the expected state equation of the control closed-loop system; the control module adjusts the working state of the multi-level converter according to the passive control law.

[0118] In this embodiment, the three-phase MMC circuit structure and submodule topology diagram under the condition of asymmetric bridge arms are as follows: Figure 2 As shown. Figure 2 It can be obtained that when a submodule failure occurs in the x (x∈{u,v,w}) phase or the bridge arm parameter perturbation occurs, the MMC is in asymmetric operation of the bridge arm. The x-phase state space equation of the MMC in asymmetric operation of the bridge arm 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] Where, L xc , L xh are the sum and difference of the upper and lower bridge arm inductances of phase x respectively; R xc , R xh are the sum and difference of the upper and lower bridge arm resistances of phase x, respectively, i xs is the x-phase grid-connected current, i xo is the x-phase circulation; v xs is the x-phase output voltage; v xg is the grid-side voltage of phase x; v xo is the voltage in the x-phase bridge arm, v xu 、v xl are the upper and lower bridge arm capacitor voltages of phase x, respectively, C is the rated capacitance of the submodule; u cu 、u clare the capacitor voltages of the upper and lower bridge arm submodules of phase x, respectively, u 、n l are the number of submodules put into use in the upper and lower bridge arms respectively; U dc is the DC side voltage, is the differential operator, and t is the current time.

[0124] In this embodiment, state variables and control variables are defined, and the 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] Combined with the x-phase state space equation of the MMC in the asymmetric operation of the bridge arm, the PCHD-based MMC mathematical model under the asymmetric bridge arm can be established as follows:

[0128]

[0129] In the formula, 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; u is the MMC model system voltage.

[0130] Among them, the interconnection matrix is:

[0131]

[0132] The damping matrix is:

[0133]

[0134] The port matrix is:

[0135]

[0136] The energy function is:

[0137]

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

[0139]

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

[0141]

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

[0143]

[0144] In the formula, are the expected trajectories of the capacitor voltage of the upper and lower bridge arm submodules, L xc , L xh are the sum and difference of the upper and lower bridge arm inductances of phase x respectively; i xs is the x-phase grid-connected current, i xo is the x-phase circulation; ω, are the angular frequency and phase angle of the fundamental frequency component of the x-phase output current respectively; n u 、n l are the number of submodules put into use in the upper and lower bridge arms respectively; C is the rated capacitance of the submodule; is the expected trajectory of the x-phase circulation; is the expected trajectory of the x-phase output current; S MMC MMC apparent power; m = V xg / (U dc / 2) is the modulation ratio, U dc is the DC side voltage,

[0145] In this embodiment, the specific process of generating the desired state equation of the MMC transient synchronous stability control closed-loop system under the condition of asymmetric bridge arms in S3 is:

[0146] S31, calculate the output frequency f ac The expected trajectory with frequency f acN Deviation between

[0147] S32, by the output frequency f ac The expected trajectory with frequency f acN The deviation between them is solved to obtain the expected trajectory of the capacitor voltage of the upper and lower bridge arm submodules;

[0148] S33, obtaining the system expected trajectory from the expected trajectory of the capacitor voltage as the control target of the MMC transient synchronization problem, and then designing the system expected energy function under the asymmetric bridge arm situation;

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

[0150] First, calculate the deviation, the MMC output frequency f when the bridge arm is asymmetric ac The expected trajectory with frequency f acN There is a deviation between them, and the relationship between them satisfies:

[0151]

[0152] Where: P dc , P ac are the power of MMC DC side and AC side respectively, H MMC is the equivalent inertia of the internal capacitor of MMC, K D is the equivalent damping coefficient of MMC, N is the number of MMC single bridge arm submodules, N f is the number of MMC single bridge arm submodule failure exits, is the submodule capacitor voltage reference value.

[0153] In this embodiment, the MMC output frequency f ac and the expected trajectory of frequency f acN The expected value of the submodule capacitor voltage is obtained by the deviation between Then the expected trajectory of the capacitor voltage of the upper and lower bridge arm submodules is obtained as:

[0154]

[0155] The specific formula of the system expected energy function in S33 is:

[0156]

[0157] In the formula, H d is the expected energy function of the system; L xc , L xh are the sum and difference of the upper and lower bridge arm inductances of phase x respectively; C is the rated capacitance of the submodule; n u 、n l are the number of submodules put into use in the upper and lower bridge arms, respectively, n is the system state variable, is the expected state variable of the system.

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

[0159]

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

[0161] The system expects the interconnection matrix to be:

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

[0163] The expected damping matrix of the system is:

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

[0165] In the formula, the system injects the interconnection matrix J a (x) = 0, the system expected damping matrix R a =diag[R1,R2,R3,R4], R1, R2, R3, R4 are the injected positive damping parameters, among which R1=R2, R3=R4.

[0166] From the expected state equation of the MMC transient synchronous stability control closed-loop system, the specific formula of the passive control law based on PCHD under the condition of asymmetric bridge arms is:

[0167]

[0168] Where g(x) is the port matrix; J d (x) is the expected interconnection matrix of the system; R d (x) is the system expected damping matrix; H d (x) is the expected energy function of the system; J(x) is the interconnection matrix; R(x) is the damping matrix; H(x) is the energy function.

[0169] In this embodiment, the passive control law can ensure that the closed-loop control system can achieve rapid tracking of the desired trajectory of the state variables under the premise of global asymmetric stability, and achieve the MMC transient synchronous stability control goal under the condition of asymmetric bridge arms.

[0170] In this embodiment, a simulation model of the MMC transient synchronous stability control system under the condition of asymmetric bridge arms is built in MATLAB / Simulink, and simulation tests are performed using the traditional vector control system and the proposed PCHD-based passivity control system. The simulation results are shown in Figure 2. Figure 3 As shown. Figure 3 Analysis shows that at t=50s, the MMC submodule fails and exits, resulting in asymmetric operation of the bridge arm. Compared with the traditional vector control system, the proposed system has small overshoot, short adjustment time, and excellent dynamic response performance. It can achieve MMC transient synchronous stability under asymmetric fault of the bridge arm, which is conducive to the safe and stable operation of the system.

[0171] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can easily think of various equivalent modifications or replacements within the technical scope disclosed by the present invention, and these modifications or replacements should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A method for transient synchronous stability control of a modular multilevel converter under the condition of asymmetric bridge arms, characterized in that: The following steps are involved: S1. Establish the MMC mathematical model under the condition of asymmetric bridge arms; S2. In the MMC mathematical model, define the expected trajectory of the MMC state variables under the asymmetric bridge arm condition; S3. Design the system expected energy function and combine it 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 asymmetric bridge arms; S4, establishing a passive control law based on 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.

2. The method for transient synchronous stability control of a modular multilevel converter under the condition of asymmetric bridge arms according to claim 1, characterized in that: The specific process of establishing the MMC mathematical model in S1 is: first, build the x-phase state space equation of the MMC when the bridge arm is in asymmetric operation, then determine the state variables and control variables, and use the x-phase state space equation to establish the MMC mathematical model based on PCHD under the condition of asymmetric bridge arm.

3. The method for transient synchronous stability control of a modular multilevel converter under asymmetric bridge arm conditions according to claim 2, characterized in that: The x-phase state space equation of the MMC when the bridge arm is in asymmetric operation is: Where, L xc , L xh are the sum and difference of the upper and lower bridge arm inductances of phase x respectively; i xs is the x-phase grid-connected current, i xo is the x-phase circulation; v xs is the x-phase output voltage; v xg is the grid-side voltage of phase x; v xo is the voltage in the x-phase bridge arm; C is the rated capacitance of the submodule; n u 、n l are the number of submodules put into use in the upper and lower bridge arms respectively; is the differential operator, and t is the current time.

4. The method for transient synchronous stability control of a modular multilevel converter under asymmetric bridge arm conditions according to claim 2, characterized in that: The MMC mathematical model is: In the formula, 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; u is the MMC model system voltage.

5. The method for transient synchronous stability control of a modular multilevel converter under asymmetric bridge arm conditions according to claim 1, characterized in that: The expected trajectory of the MMC state variables in S2 is: In the formula, are the expected trajectories of the capacitor voltage of the upper and lower bridge arm submodules, L xc , L xh are the sum and difference of the upper and lower bridge arm inductances of phase x respectively; i xs is the x-phase grid-connected current, i xo is the x-phase circulation; ω, are the angular frequency and phase angle of the fundamental frequency component of the x-phase output current respectively; n u 、n l are the number of submodules put into use in the upper and lower bridge arms respectively; C is the rated capacitance of the submodule; is the expected trajectory of the x-phase circulation; is the expected trajectory of the x-phase output current; S MMC MMC apparent power; m = V xg / (U dc / 2) is the modulation ratio, U dc is the DC side voltage.

6. The method for transient synchronous stability control of a modular multilevel converter under asymmetric bridge arm conditions according to claim 1, characterized in that: The specific process of generating the desired state equation of the MMC transient synchronous stability control closed-loop system under the condition of asymmetric bridge arms in S3 is: S31, calculate the output frequency f ac The expected trajectory with frequency f acN Deviation between S32, by the output frequency f ac The expected trajectory with frequency f acN The deviation between them is solved to obtain the expected trajectory of the capacitor voltage of the upper and lower bridge arm submodules; S33, obtaining the system expected trajectory from the capacitor voltage expected trajectory as the control target of the MMC transient synchronization problem, and then designing the system expected energy function under the asymmetric bridge arm situation; S34. By analyzing the bridge arm of the MMC mathematical model and the system expected energy function under asymmetric conditions, the expected state equation of the MMC transient synchronous stability control closed-loop system is obtained.

7. The method for transient synchronous stability control of a modular multilevel converter under the condition of asymmetric bridge arms according to claim 6, characterized in that: The specific formula of the system expected energy function in S33 is: In the formula, H d is the expected energy function of the system; L xc , L xh are the sum and difference of the upper and lower bridge arm inductances of phase x respectively; C is the rated capacitance of the submodule; n u 、n l are the number of submodules put into use in the upper and lower bridge arms, respectively, n is the system state variable, is the expected state variable of the system.

8. The method for transient synchronous stability control of a modular multilevel converter under the condition of asymmetric bridge arms according to claim 6, characterized in that: The specific formula of the desired state equation of the MMC transient synchronous stability control closed-loop system in S34 is: In the formula, is the expected state of the system; J d (x) is the system expected interconnection matrix; R d (x) is the system expected damping matrix; H d (x) is the expected energy function of the system.

9. The method for transient synchronous stability control of a modular multilevel converter under asymmetric bridge arm conditions according to claim 1, characterized in that: The specific formula of the passive control law in S4 is: Where g(x) is the port matrix; J d (x) is the expected interconnection matrix of the system; R d (x) is the system expected damping matrix; H d (x) is the expected energy function of the system; J(x) is the interconnection matrix; R(x) is the damping matrix; H(x) is the energy function.

10. A modular multilevel converter transient synchronous stability control system under the condition of asymmetric bridge arms, characterized in that: The system is operated by a modular multi-level converter transient synchronization stability control method under a bridge arm asymmetry condition as described in any one of claims 1 to 9, and the system comprises 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 the MMC mathematical model under the condition of asymmetric bridge arms; The expected trajectory determination module is used to define the expected trajectory of the MMC state variables in the case of asymmetric bridge arms in the MMC mathematical model; and to design the system expected energy function, and combine it with the MMC mathematical model to generate the expected state equation of the MMC transient synchronous stability control closed-loop system in the case of asymmetric bridge arms; 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 multi-level converter according to the passive control law.

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