Transformer nonlinear control method based on MMC-UPFC structure

Abstract

The invention discloses a transformer nonlinear control method based on an MMC-UPFC structure, and the method comprises the steps: building a mathematical model according to the topological structure of the MMC-UPFC, and analyzing the internal characteristics of the mathematical model; secondly, designing a positive sequence Lyapunov controller and a negative sequence Lyapunov controller under the unbalanced condition of the MMC-UPFC, and carrying out stability proving; then, the signals are transmitted to the MMC through carrier phase-shifting modulation by means of sub-module voltage-sharing control; and finally, an MMC-UPFC simulation system is established to carry out simulation verification. Aiming at the problems caused by the unbalanced condition of a power grid, an inner-loop control method and an outer-loop control method of the MMC-UPFC transformer are respectively designed, and the problems of incomplete decoupling, many control parameters, difficulty in selection and the like in the prior art are solved, so that the problems of low control performance and the like are solved.

Description

Technical Field

[0001] The invention belongs to the technical field of MMC-UPFC power electronic transformer control, and in particular relates to a transformer nonlinear control method based on an MMC-UPFC structure. Background Art

[0002] In actual systems, the three-phase voltage of the power grid is often in an unbalanced state. The unbalanced negative sequence component of the power grid will cause a double frequency ripple voltage on the DC side of the MMC. These harmonic components will generate voltage harmonics after the AC reactor is connected, which will aggravate the imbalance of the power grid. When the negative sequence component in the MMC AC current is too large, it will cause overcurrent in the converter, endangering the safe operation of the MMC-UPFC system. Therefore, compensation measures need to be taken to offset the negative sequence component of the power grid, so as to ensure the safe and stable operation and reliable power flow regulation capability of the MMC-UPFC system under unbalanced conditions.

[0003] At present, the research focus of MMC-UPFC is converter topology, mathematical modeling, control method, optimal configuration and practical application. The most widely used linear control method in UPFC is PI control, in which the voltage injection is divided into active and reactive parts. In summary, at present, the control strategy of MMC-UPFC is mostly based on PI control, and PI control is simple and mature. However, if the system parameters change or there are errors or uncertainties in the system model, the control strategy has the disadvantages of slow response speed, long debugging cycle, and general stability. The control effect needs to be further improved, and it may not meet the requirements for some rapidly changing systems. At the same time, PI control is suitable for simple linear systems, while MMC-UPFC is a complex nonlinear system. Therefore, if PI control is used on MMC-UPFC, the nonlinear MMC-UPFC needs to be linearized in a small range near the stability, and the multi-variable MMC-UPFC needs to be decoupled into multiple single-variable objects. These treatments have problems such as incomplete decoupling, many control parameters and difficult selection, which leads to low control performance.

[0004] Although some nonlinear control strategies such as feedback linearization, passive, sliding mode, passive sliding mode, and inversion have been proposed to address the problems of PI control, feedback control relies on the precise model of the object and is limited to affine nonlinear systems. In addition, the mathematical tools are relatively abstract, the algorithms are relatively complex, and implementation is difficult. Passive control relies on the precise model of the object, and the construction of the Lyapunov function is irregular. When subjected to internal and external changes, the steady-state error is large, and the system response is slow. The switching frequency of the sliding mode control is not fixed, the output has high-frequency jitter, and it is difficult to select the ideal sliding mode switching surface. Passive sliding mode control is a combination of passive and sliding mode methods, with a complex system structure and a large amount of calculation. Inversion control relies on the mathematical model of the object and is only suitable for nonlinear systems with strict parameter feedback, and the amount of calculation of the regression function is large. Summary of the invention

[0005] The purpose of the present invention is to provide a transformer nonlinear control method based on MMC-UPFC structure in order to overcome the defects of the above-mentioned prior art.

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

[0007] On the one hand, the present invention provides a transformer nonlinear control method based on an MMC-UPFC structure, comprising the following steps:

[0008] Step S1: establishing mathematical models of the parallel side and the series side according to the topological structure of the MMC-UPFC transformer, wherein the MMC-UPFC transformer comprises a parallel side MMC and a series side MMC, and the parallel side MMC and the series side MMC are connected to the same DC bus in a back-to-back manner;

[0009] Step S2: performing abc-dq coordinate transformation on the mathematical models of the parallel side and the series side to obtain the mathematical models of the parallel side and the series side in the d and q two-phase coordinate systems;

[0010] Step S3: constructing a mathematical model of the DC side of the MMC-UPFC;

[0011] Step S4: constructing the AC and DC side relationship of the MMC-UPFC transformer in the positive sequence system according to the mathematical models of the parallel side and the series side, the mathematical models of the parallel side and the series side in the d, q two-phase coordinate system, and the mathematical model of the MMC-UPFC DC side;

[0012] Step S5: Based on the AC and DC side relationship of the MMC-UPFC transformer, combined with the mathematical models of the parallel side and the series side and the mathematical models of the parallel side and the series side in the d, q two-phase coordinate system, the inner loop control method and the outer loop control method of the MMC-UPFC transformer are constructed.

[0013] Furthermore, the parallel side mathematical model in step S1 is:

[0014]

[0015] R eq =R 1

[0016] L eq =L 1 +L m / 2

[0017] Among them, u shj is the output voltage of the parallel side MMC, u sj is the voltage at the sending end of the AC system, i shj is the output current of the parallel side MMC, R eq , L eq is the equivalent resistance and inductance of the parallel side MMC, R 1 , L 1 is the AC connection resistance and inductance of the parallel side MMC, L m is the bridge arm inductance.

[0018] Furthermore, the series side mathematical model in step S1 is:

[0019]

[0020] Among them, u 12j is the voltage of the series-side MMC connected to the system, u rj is the voltage at the receiving end of the AC system, u sej 、i sej is the output voltage and current of the series side, R r , L r is the line impedance, i rj is the line current, C 1 is the series side capacitor. When the voltage ratio between the primary and secondary sides of transformer T is k, u sej =ku sej .

[0021] Furthermore, the parallel side mathematical model in the d and q two-phase coordinate system is:

[0022]

[0023] Where ω is the grid angular frequency, i shd 、i shq is the d-axis and q-axis components of the output current of the parallel-side MMC in the two-phase rotating coordinate system, u shd 、u shq is the d-axis and q-axis components of the output voltage of the parallel-side MMC in the two-phase rotating coordinate system, usd 、u sq are the d-axis and q-axis components of the voltage at the sending end of the AC system in the two-phase rotating coordinate system.

[0024] Furthermore, the mathematical model of the series side in the d and q two-phase coordinate system is:

[0025]

[0026] Among them, u sed 、u seq is the d-axis and q-axis components of the output voltage on the series side in the two-phase rotating coordinate system, u 12d 、u 12q is the d-axis and q-axis components of the voltage of the series-side MMC connected to the system in the two-phase rotating coordinate system, i rd 、i rq are the d-axis and q-axis components of the line current in the two-phase rotating coordinate system, i sed 、i seq are the d and q axis components of the output current on the series side in the two-phase rotating coordinate system.

[0027] Furthermore, the mathematical model of the DC side of the MMC-UPFC is:

[0028]

[0029] Among them, u dc is the intermediate DC bus voltage, u jp 、u jn 、i jp 、i jn are the voltage and current of the upper and lower bridge arms respectively.

[0030] Furthermore, the step S4 specifically includes the following steps:

[0031] Step S41: Ignore the fluctuation of capacitor voltage and rewrite the mathematical model of the series side to obtain the mathematical model of the voltage of the series side MMC connected in series to the system;

[0032] Step S42: obtaining the positive sequence component in the dq rotating coordinate system according to the mathematical model of the voltage of the series-side MMC connected in series to the system;

[0033] Step S43: obtaining the positive sequence component in dq coordinates according to the mathematical model of the DC side of the MMC-UPFC;

[0034] Step S44: constructing the AC and DC side relationship of the MMC-UPFC transformer in the positive sequence system according to the positive sequence component in the dq rotating coordinate system and the positive sequence component in the dq coordinate system.

[0035] Furthermore, the mathematical model of the voltage of the series-side MMC connected in series to the system is:

[0036]

[0037] Among them, S jni , S jpi is the switching signal of the i-th submodule SM of the j-th phase lower bridge arm n and upper bridge arm p; S j is the switching signal of the j-th phase bridge arm submodule, and N is the number of submodules in the bridge arm;

[0038] The positive sequence component in the dq rotating coordinate system is:

[0039]

[0040] in, are the d-axis and q-axis components of the voltage in the two-phase rotating coordinate system of the series-side MMC connected in series to the system under the positive sequence system, are the d-axis and q-axis components of the output current on the series side in the positive sequence system in the two-phase rotating coordinate system, is the switching function under the positive sequence system;

[0041] The positive sequence component in the dq coordinate is:

[0042]

[0043] Where C is the capacitance value in the submodule, N is the number of submodules in the bridge arm, i dc is the intermediate DC bus current.

[0044] Furthermore, the AC and DC side relationship of the MMC-UPFC transformer in the positive sequence system is:

[0045]

[0046] in, is the converted switch function; is the steady-state value of the switching function; Δd and Δq are the fluctuations of the switching function.

[0047] Furthermore, the step S5 specifically includes:

[0048] Define system status variables:

[0049]

[0050] Set DC side resistance X dc , then i d =u k / X d, according to the AC-DC side relationship of the MMC-UPFC transformer, the Lyapunov model of the MMC-UPFC under the positive sequence system is obtained;

[0051] According to the Lyapunov asymptotic stability theorem, combined with the Lyapunov model of MMC-UPFC under the positive sequence system, the mathematical model of the parallel side and the series side, and the mathematical model of the parallel side and the series side under the d, q two-phase coordinate system, the switching function of MMC-UPFC based on the Lyapunov positive sequence system is obtained, which is the inner loop control method and outer loop control method of the MMC-UPFC transformer:

[0052]

[0053] in, are the d-axis switching function and q-axis switching function of the MMC power electronic transformer under the positive sequence system, α and β are Lyapunov control gains, α<0, β<0, is the steady-state value of the switching function, is the DC side voltage reference value, It is the output current reference value of the series side in the positive sequence system.

[0054] Compared with the prior art, the present invention has the following advantages:

[0055] (1) In order to solve the above problems existing in MMC-UPFC control, the present invention introduces nonlinear Lyapunov control into nonlinear MMC-UPFC object control. In view of the problems caused by the unbalanced power grid, the inner loop control method and the outer loop control method of the MMC-UPFC transformer are designed respectively, which solves the problems of incomplete decoupling, multiple control parameters and difficulty in selecting in the prior art, resulting in low control performance, etc., and compared with the traditional PI control strategy, the present invention has the advantages of small overshoot, fast adjustment speed and good anti-disturbance stability.

[0056] (2) This paper proposes a Lyapunov control strategy for MMC-UPFC under unbalanced power grid. The corresponding Lyapunov control model, controller, and control gain range are derived, and the three control objectives of power control are used to achieve current balance, active power pulsation, and reactive power pulsation. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 This is a system structure diagram of the MMC-UPFC of the present invention;

[0058] Figure 2 The λ of the present invention 1min Follow γ 1 The change curve diagram of

[0059] Figure 3 It is the overall control structure block diagram of MMC-UPFC of the present invention;

[0060] Figure 4 It is a power supply voltage sag simulation curve at the receiving end of the AC system of the present invention;

[0061] Figure 5 It is the line current and negative sequence component diagram of the present invention;

[0062] Figure 6 The waveform diagrams of power flow control under two control methods in the embodiments of the present invention are as follows;

[0063] Figure 7 This is a waveform diagram of the power supply voltage at the receiving end in an embodiment of the present invention;

[0064] Figure 8 The waveform diagrams of line current and negative sequence component under two control methods in the embodiments of the present invention;

[0065] Fig. 9 2 are power flow control waveform diagrams under two control methods in the embodiments of the present invention. DETAILED DESCRIPTION

[0066] 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.

[0067] Embodiment 1:

[0068] The structure of MMC-UPFC system is as follows: Figure 1 As shown in the figure, the MMC-UPFC consists of two converters: a series converter on the right side of the figure connected in series with the grid, and a parallel converter on the left side of the figure connected in parallel with the grid. Both converters use the MMC structure and are connected back-to-back through the DC bus. The series-side MMC converter provides a series voltage u to the system through a series connection. 12 , thereby regulating the active and reactive power flows of the system; the parallel-side MMC converter maintains the DC voltage stability between the two MMCs by exchanging active power with the system, and also inputs reactive power to the system. Each of the three phases of the MMC converter has an upper bridge arm and a lower bridge arm, and each bridge arm consists of N half-bridge sub-modules (SM) and a series-connected bridge arm inductor.

[0069] Figure 1 In the above example, subscript j = a, b, c, u sj 、u rjis the voltage at the sending and receiving ends of the AC system, R r , L r is the line impedance, u 1 、u 2 is the AC bus voltage of the parallel and series converter nodes, u 12j is the voltage of the series-side MMC connected to the system, u shj 、u sej 、i shj 、i sej are the output voltage and current of the parallel and series sides of the MMC, u sej1 is the primary side voltage of transformer T, i fj is the series capacitor current, i rj is the line current, u dc is the intermediate DC bus voltage, R 1 , L 1 With R 2 , L 2 L is the AC connection resistance and inductance of the MMC series and parallel sides. m is the bridge arm inductance, C 1 is the series side capacitance, C 2 is the submodule capacitance, P r , Q r It is the active and reactive power connected in series to the receiving end of the AC system.

[0070] According to Kirchhoff circuit law, the mathematical model of the parallel-side MMC converter is:

[0071]

[0072] Where: R eq , L eq is the equivalent resistance and inductance of the parallel side MMC, R eq =R 1 , L eq =L 1 +L m / 2.

[0073] Transforming the above formula through the abc-dq coordinate system yields:

[0074]

[0075] Where: ω is the grid angular frequency, ω = 2πf, f = 50Hz.

[0076] According to Kirchhoff circuit law, the mathematical model of the series side MMC converter can be obtained as follows:

[0077]

[0078] When the primary and secondary voltage ratio of transformer T is k, u sej1 =ku sej .

[0079] By transforming equations (3) to (5) from the abc-dq coordinate system, we can obtain:

[0080]

[0081] According to the mathematical model of the parallel-side MMC converter in equation (2) and the mathematical model of the series-side MMC converter in equations (6) to (8) established above, the Lyapunov controller of the MMC-UPFC can be further designed.

[0082] In order to solve the existing problems of PI control, this embodiment proposes a nonlinear Lyapunov control method for MMC-UPFC, and also introduces the Lyapunov control law of the positive and negative sequence of MMC-UPFC under unbalanced conditions, and performs stability analysis.

[0083] The idea of ​​Lyapunov control of MMC-UPFC is: when the voltage of the three-phase grid is unbalanced, the series and parallel converters are used to manage the imbalance. By establishing the Lyapunov controller of the positive and negative sequence inner loop control system, the negative sequence control system on the parallel side of UPFC offsets the negative sequence current caused by the three-phase imbalance on the line by the negative sequence current output by the parallel converter; the series side compensates for the unbalanced voltage by injecting negative sequence voltage into the grid. The control signal is generated by Lyapunov control, and the trigger pulse is generated by carrier phase shift modulation.

[0084] Taking the mathematical model of the series side as an example, when the grid voltage is unbalanced, the system needs to be converted into a positive and negative sequence system in the d and q coordinate systems for control respectively. Because the analysis of the positive and negative sequence systems is similar, only the Lyapunov control method of the positive sequence system is discussed below.

[0085] Ignoring the fluctuation of capacitor voltage, the mathematical model of the AC side of the series side UPFC converter in equation (4) is written as:

[0086]

[0087] in,

[0088]

[0089] Where: S jni , S jpi is the switching signal of the i-th submodule (SM) of the j-th phase lower bridge arm (n) and upper bridge arm (p); S j is the switching signal of the j-th phase bridge arm submodule.

[0090] According to formula (9), the positive sequence component in the dq rotating coordinate system is:

[0091]

[0092] The mathematical model of the DC side of MMC-UPFC is:

[0093]

[0094] Where: u jp 、u jn and i jp 、i jn are the voltage and current of the upper and lower bridge arms respectively.

[0095] Rewrite equation (10) as the relationship in the abc stationary coordinate system:

[0096]

[0097] Where: C is the equivalent capacitance of the bridge arm; i dc is the DC bus current.

[0098] Taking the derivative of the above equation and ignoring the DC current differential derivative, we can get:

[0099]

[0100] Then convert the above formula into the positive sequence component in dq coordinates:

[0101]

[0102] Therefore, the AC and DC side relationship of the MMC under the positive sequence system is:

[0103]

[0104] Where: is the converted switch function; is the steady-state value of the switching function; Δd and Δq are the fluctuations of the switching function.

[0105] The main control target of MMC-UPFC under unbalanced power grid is to inject voltage into the series side line to compensate for the three-phase voltage difference of the unbalanced power grid. The current of the series side controller adopts the set current value. If the series side current is stable at the set value, the relationship of the positive sequence system is:

[0106]

[0107] In the formula: superscript * indicates the reference value of the variable.

[0108] From formula (16), we can deduce:

[0109]

[0110] Define system status variables:

[0111]

[0112] Set DC side resistance X dc , then i dc =u dc / X dc According to formula (15), the Lyapunov model of MMC-UPFC under positive sequence system can be obtained:

[0113]

[0114] According to Lyapunov's asymptotic stability theorem, when: ①V(x)=0; ②For any x≠0, V(x)>0; ③For any x≠0, ④When ||x||→∞, V(x)→∞. When ||x||→∞, the system is asymptotically stable.

[0115] Let the Lyapunov function of the MMC-UPFC positive sequence be:

[0116]

[0117] Derivative of formula (20):

[0118]

[0119] Substitute equation (19) into the above equation and simplify it to:

[0120]

[0121] make:

[0122]

[0123] Where: α, β are Lyapunov control gains, α<0, β<0.

[0124] Therefore, the switching function of MMC-UPFC based on Lyapunov positive sequence system is:

[0125]

[0126] Through the above analysis, it can be concluded that the Lyapunov controller used in this embodiment meets the conditions of global asymptotic stability. All parameters of the Lyapunov controller are given, but only the control gain has not been determined. Therefore, as long as a suitable adjustment gain is selected, a better adjustment effect can be achieved. Compared with the traditional PI controller, this controller saves many parameters selection and adjustment links, greatly saves the resources and time required for the experiment, and also improves the operation efficiency of the system.

[0127] In the actual operating system, due to changes in internal model parameters and external signals, the system may operate under non-ideal conditions. At this time, if the optimal controller parameters of the system can be found, the system's anti-interference and accuracy can be improved, so that the system can maintain stable and accurate operation under non-ideal conditions.

[0128] Let the expected value and the actual expected value at a certain time t be and but:

[0129]

[0130] Therefore, formula (23) can be equivalent to:

[0131]

[0132] like but Negative definite. Assumption Then (25) can be transformed into:

[0133]

[0134] Let r 1 =-αu^dcu*dc>0,r 2 =-βu^dcu*dc>0, then:

[0135]

[0136] in,

[0137]

[0138] If f 1 (m 1 ,m 3 )>0 and f 2 (m 2 ,m 3 )>0, then Negative determination.

[0139] Let m 3 =h 1m 1 , m 3 =h 2 m 2 ,but:

[0140]

[0141] Where: 1 (r 1 ,γ 1 ,h 1 ) is about h 1 If h is a quadratic function, then 1 (0) = (1 + γ 1 ) / (2γ 1 ), the function is at its minimum. At this time:

[0142] λ 1min =R eq +r 1 [1-(1+γ 1 ) 2 / (4γ 1 )] (29)

[0143] If 1min >0, then no matter h 1 Why value, f 1 (m 1 ,m 3 )>0, in order to ensure asymptotic stability, let γ a <γ 1 <γ b ,at the same time:

[0144]

[0145] Among them, γ 0 =1+2R eq / r 1 . 1min Follow γ 1 The changing trend is Figure 2 shown.

[0146] γ 1 The smaller the value, the more stable the system parameters will be when they change. If the uncertainty interval of the parameter is 1-ε<γ 1 <1+ε, according to formula (31):

[0147]

[0148] Similarly, when f 2 (m 2 ,m 3 )>0, the maximum value of β is:

[0149]

[0150] Through the above analysis, the intervals of α and β can be determined under imprecise control, thus improving work efficiency.

[0151] Figure 3 The overall control block diagram of MMC-UPFC is shown in Figure 1. The series-parallel converter is controlled by Lyapunov and combined with DC capacitor voltage control, circulating current and voltage sharing control. The output control signal is modulated by carrier phase-shift SPWM to obtain the switch trigger signal, thereby controlling the on and off of the converter switch.

[0152] Embodiment 2:

[0153] The parts not mentioned in this embodiment are the same as those in Embodiment 1.

[0154] This embodiment is constructed on the MATALB / Simulink software platform Figure 7 The MMC-UPFC system simulation model is constructed, and the performance advantages of the Lyapunov control system of the MMC-UPFC of this embodiment are illustrated by simulation analysis under two unbalanced working conditions, thereby verifying the effectiveness of the method of this embodiment. The MMC-UPFC system parameters are shown in Table 1.

[0155] Table 1 Parameters of MMC-UPFC system

[0156]

[0157] In order to verify the effectiveness of MMC-UPFC in an unbalanced state, the single-phase voltage of phase A at the receiving end is temporarily reduced by 10% at 0.2s to simulate the unbalanced working condition, and the negative sequence unbalanced control system is put into use at 0.4s, and the active power is set to be constant at 200kW and the reactive power is 0Var. The single-phase power supply voltage at the receiving end of the AC system temporarily drops by 10%, and the simulation curve is shown in Figure 4 .

[0158] Figure 5 is the line current and negative sequence component. Among them: Figure 5 (a) is the line current effect diagram under unbalanced control. Figure 5 (b) and (c) are the negative sequence components of line current under Lyapunov and PI control respectively.

[0159] Depend on Figure 5 It can be seen that before 0.2s, the system was in a balanced state, the line current was sinusoidal, and the negative sequence component was 0; after 0.2s, the A phase of the receiving end power supply voltage temporarily dropped by 10%. At this time, negative sequence components began to appear in the line current, resulting in current imbalance. At 0.4s, unbalanced control was put into operation. Due to the input of the negative sequence compensation current of the parallel converter, the negative sequence component of the line current was controlled to 0, and the line current returned to a balanced state.

[0160] Depend on Figure 5 (b) and (c) show that when the system is unbalanced, negative sequence components appear in both control strategies, but the stability performance under the PI control strategy is the worst. After the unbalanced control is put into use, the PI control takes the slowest time to reach stability, has poor dynamic performance, and the control effect is still insufficient. However, under Lyapunov control, the system can control the negative sequence current well, has the fastest control speed, and has the best waveform quality.

[0161] Figure 6 The waveforms of the power flow control under Lyapunov and PI control methods are shown in Figure 1. At 0.2s, the active and reactive power of the line both have a 2-fold frequency fluctuation due to the imbalance of the system. After 0.4s, the unbalanced control is put into use. The MMC-UPFC compensates for the negative sequence current and voltage respectively, effectively suppressing the generation of the second harmonics, so that the power flow of the system can quickly return to a stable state.

[0162] Depend on Figure 6 It can be seen that both control methods can achieve flow control under unbalanced conditions, which proves the effectiveness of the two control strategies. The Lyapunov control proposed in this embodiment has a faster response speed and lower overshoot than the PI control. The results show the effectiveness and superiority of the Lyapunov method proposed in this embodiment.

[0163] Asymmetric three-phase loads lead to system imbalance, affecting the normal operation of the system. Asymmetric three-phase loads are usually caused by asymmetric load impedance. In this embodiment, a negative sequence voltage source is connected in series to the symmetrical voltage source at the receiving end at 0.2s. Asymmetric loads will inevitably lead to asymmetric voltage at the receiving end. The simulation curve is as follows: Figure 7 shown.

[0164] Figure 8 The waveforms of line current and negative sequence component under Lyapunov and PI control methods are shown. At 0.2s, the system is unbalanced due to load asymmetry, the line current is unbalanced, and negative sequence component appears. At 0.4s, unbalanced control is put into use, the line current is balanced, and the negative sequence current is 0.

[0165] It can be seen from the figure that when the system is unbalanced, both Lyapunov and PI control methods can stabilize the system. Their stabilization times are 0.03s and 0.08s, and their overshoots are 1A and 4A, respectively. Therefore, Lyapunov control has a shorter stabilization time and smaller overshoot than PI control.

[0166] Fig. 9Figure 1 is the flow control waveform under Lyapunov and PI control methods. As can be seen from the figure, both Lyapunov and PI control methods can achieve stable flow control under unbalanced conditions. Their stabilization times are 0.02s and 0.04s, respectively, and their overshoots are 0.1W and 0.3W, respectively. Compared with PI control, Lyapunov control has a shorter stabilization time and lower overshoot. Therefore, the Lyapunov control proposed in this embodiment is more effective and has better control performance.

[0167] In summary, compared with PI control, the Lyapunov control strategy proposed in this embodiment has the advantages of shorter adjustment time, faster control speed, and better system stability when an unbalanced working condition occurs in the system.

[0168] If the above functions are implemented in the form of software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, or the part of the technical solution, can be embodied in the form of a software product. The computer software product is stored in a storage medium, including several instructions for a computer device (which can be a personal computer, a server, or a network device, etc.) to perform all or part of the steps of the method described in each embodiment of the present invention. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), disk or optical disk, etc., which can store program code.

[0169] 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 transformer nonlinear control method based on MMC-UPFC structure, characterized in that: The following steps are involved: Step S1: establishing mathematical models of the parallel side and the series side according to the topological structure of the MMC-UPFC transformer, wherein the MMC-UPFC transformer comprises a parallel side MMC and a series side MMC, and the parallel side MMC and the series side MMC are connected to the same DC bus in a back-to-back manner; Step S2: performing abc-dq coordinate transformation on the mathematical models of the parallel side and the series side to obtain the mathematical models of the parallel side and the series side in the d and q two-phase coordinate systems; Step S3: constructing a mathematical model of the DC side of the MMC-UPFC; Step S4: constructing the AC and DC side relationship of the MMC-UPFC transformer in the positive sequence system according to the mathematical models of the parallel side and the series side, the mathematical models of the parallel side and the series side in the d, q two-phase coordinate system, and the mathematical model of the MMC-UPFC DC side; Step S5: Based on the AC and DC side relationship of the MMC-UPFC transformer, combined with the mathematical models of the parallel side and the series side and the mathematical models of the parallel side and the series side in the d, q two-phase coordinate system, the inner loop control method and the outer loop control method of the MMC-UPFC transformer are constructed.

2. The transformer nonlinear control method based on MMC-UPFC structure according to claim 1 is characterized in that: The parallel side mathematical model in step S1 is: R eq =R1 L eq =L1+L m / 2 Among them, u shj is the output voltage of the parallel side MMC, u sj is the voltage at the sending end of the AC system, i shj is the output current of the parallel side MMC, R eq , L eq is the equivalent resistance and inductance of the parallel side MMC, R1 and L1 are the AC connection resistance and inductance of the parallel side MMC, L m is the bridge arm inductance.

3. The transformer nonlinear control method based on MMC-UPFC structure according to claim 1 is characterized in that: The mathematical model of the series connection in step S1 is: Among them, u 12j is the voltage of the series-side MMC connected to the system, u rj is the voltage at the receiving end of the AC system, u sej 、i sej is the output voltage and current of the series side, R r , L r is the line impedance, i rj is the line current, C1 is the series capacitor, when the voltage ratio between the primary and secondary sides of transformer T is k, u sej =ku sej .

4. The transformer nonlinear control method based on MMC-UPFC structure according to claim 1 is characterized in that: The mathematical model of the parallel side in the d and q two-phase coordinate system is: Where ω is the grid angular frequency, i shd 、i shq is the d-axis and q-axis components of the output current of the parallel-side MMC in the two-phase rotating coordinate system, u shd 、u shq is the d-axis and q-axis components of the output voltage of the parallel-side MMC in the two-phase rotating coordinate system, u sd 、u sq are the d-axis and q-axis components of the voltage at the sending end of the AC system in the two-phase rotating coordinate system.

5. The transformer nonlinear control method based on MMC-UPFC structure according to claim 1, characterized in that: The mathematical model of the series side in the d and q two-phase coordinate system is: Among them, u sed 、u seq is the d-axis and q-axis components of the output voltage on the series side in the two-phase rotating coordinate system, u 12d 、u 12q is the d-axis and q-axis components of the voltage of the series-side MMC connected to the system in the two-phase rotating coordinate system, i rd 、i rq are the d-axis and q-axis components of the line current in the two-phase rotating coordinate system, i sed 、i seq are the d and q axis components of the output current on the series side in the two-phase rotating coordinate system.

6. The transformer nonlinear control method based on MMC-UPFC structure according to claim 1, characterized in that: The mathematical model of the DC side of the MMC-UPFC is: Among them, u dc is the intermediate DC bus voltage, u jp 、u jn 、i jp 、i jn are the voltage and current of the upper and lower bridge arms respectively.

7. The transformer nonlinear control method based on MMC-UPFC structure according to claim 1, characterized in that: The step S4 specifically comprises the following steps: Step S41: Ignore the fluctuation of capacitor voltage and rewrite the mathematical model of the series side to obtain the mathematical model of the voltage of the series side MMC connected in series to the system; Step S42: obtaining the positive sequence component in the dq rotating coordinate system according to the mathematical model of the voltage of the series-side MMC connected in series to the system; Step S43: obtaining the positive sequence component in dq coordinates according to the mathematical model of the DC side of the MMC-UPFC; Step S44: constructing the AC and DC side relationship of the MMC-UPFC transformer in the positive sequence system according to the positive sequence component in the dq rotating coordinate system and the positive sequence component in the dq coordinate system.

8. The transformer nonlinear control method based on MMC-UPFC structure according to claim 7, characterized in that: The mathematical model of the voltage of the series-side MMC connected in series to the system is: Among them, S jni , S jpi is the switching signal of the i-th submodule SM of the j-th phase lower bridge arm n and upper bridge arm p; S j is the switching signal of the j-th phase bridge arm submodule, and N is the number of submodules in the bridge arm; The positive sequence component in the dq rotating coordinate system is: in, are the d-axis and q-axis components of the voltage in the two-phase rotating coordinate system of the series-side MMC connected in series to the system under the positive sequence system, are the d-axis and q-axis components of the output current on the series side in the positive sequence system in the two-phase rotating coordinate system, is the switching function under the positive sequence system; The positive sequence component in the dq coordinate is: Where C is the capacitance value in the submodule, N is the number of submodules in the bridge arm, i dc is the intermediate DC bus current.

9. A transformer nonlinear control method based on MMC-UPFC structure according to claim 1 or 8, characterized in that: The AC and DC side relationship of the MMC-UPFC transformer under the positive sequence system is: in, is the converted switch function; is the steady-state value of the switching function; Δd and Δq are the fluctuations of the switching function.

10. The transformer nonlinear control method based on MMC-UPFC structure according to claim 1, characterized in that: The step S5 specifically includes: Define system status variables: Set DC side resistance X dc , then i d =u k / X d , according to the AC-DC side relationship of the MMC-UPFC transformer, the Lyapunov model of the MMC-UPFC under the positive sequence system is obtained; According to the Lyapunov asymptotic stability theorem, combined with the Lyapunov model of MMC-UPFC under the positive sequence system, the mathematical model of the parallel side and the series side, and the mathematical model of the parallel side and the series side under the d, q two-phase coordinate system, the switching function of MMC-UPFC based on the Lyapunov positive sequence system is obtained, which is the inner loop control method and outer loop control method of the MMC-UPFC transformer: in, are the d-axis switching function and q-axis switching function of the MMC power electronic transformer under the positive sequence system, α and β are Lyapunov control gains, α<0, β<0, is the steady-state value of the switching function, is the DC side voltage reference value, It is the output current reference value of the series side in the positive sequence system.

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