Fluctuation suppression improved control method for distributed power generation grid-connected inverter

Abstract

The invention discloses a fluctuation suppression improved control method for a distributed power generation grid-connected inverter. The method comprises the following steps: firstly, describing a basic structure of a DG grid-connected converter, and establishing a mathematical model under a two-phase dq rotating coordinate system; on the basis, a self-adaptive sliding mode control strategy is provided to ensure that the grid-connected converter can track and suppress external interference of the system in real time, and the capability of the system to cope with power fluctuation is enhanced. According to the method, better error tracking can be realized when the system is interfered, the error waveform is more stable, and the recovery time is shorter. Simulation verification proves that the self-adaptive backstepping sliding mode control has better error tracking performance and dynamic response performance compared with traditional PI control, the sensitivity to system parameter changes is low, and the effectiveness of the control method is fully explained.

Description

Technical Field

[0001] The present invention belongs to the research field of distributed generation (DG), and more specifically, relates to a fluctuation suppression control method for a distributed generation grid-connected inverter based on adaptive backstepping sliding mode control. Background Art

[0002] With the continuous development of social economy and energy technology, the status of energy in society is becoming increasingly important. To this end, it is necessary to accelerate the construction of a new power system centered on renewable energy and enhance the access and absorption capacity of new energy. However, although distributed renewable energy, such as photovoltaic power generation and wind power generation, has the advantages of being environmentally friendly, clean and inexhaustible, there are still many problems in practical applications due to its volatility and intermittent characteristics. As a bridge between distributed generation (DG) and the power grid, the grid-connected inverter of distributed generation is the main carrier for achieving efficient energy conversion and flow. The control mechanism of the grid-connected inverter of distributed generation should ensure excellent performance in the transition process, ensure that the system smoothly passes through the transient stage, and show excellent dynamic response capabilities, including reducing overshoot, reducing the number of oscillations, shortening the adjustment time and other key indicators, to ensure the stability and operation efficiency of the system.

[0003] At present, direct methods are mostly used in the study of the stability of AC / DC hybrid systems, and they are extended in the field of AC system stability in order to find a method for constructing energy functions and obtaining critical energy suitable for AC / DC hybrid systems. However, the modeling difficulties caused by the strong nonlinear characteristics of DC systems have not been fundamentally solved. In addition, the influence of DC control methods and key control links on the transient stability of AC / DC hybrid systems mostly stays at the level of qualitative relationship disclosure, lacks quantitative relationship expression, and is difficult to describe the influence of the switching process of different control methods on system stability. There are few studies on DC control mode switching strategies aimed at improving system stability, and most of them focus on the optimization of control link parameters, but there are problems such as complex controller structure and lack of theoretical support for parameter selection.

[0004] At present, the widely used proportional integral (PI) control adopts the idea of ​​linearization at the equilibrium point, but can only guarantee the stability of the system to a certain extent, and the optimal design of the PI linear controller parameters usually sacrifices certain steady-state performance under the premise of ensuring the robustness of the system. However, how to improve the robust autonomous performance of the DG grid-connected converter control system is still a problem that needs to be solved in the development of new energy technologies. Since backstepping control has strong robustness to uncertain factors, backstepping control has been applied to fields such as motor control and has achieved good results. However, this backstepping control technology is often limited by its dependence on the precise model of the system, and its robustness is often insufficient. In order to overcome these shortcomings, some articles have adopted a bidirectional DC converter control method that combines backstepping sliding mode control with precise feedback linearization. However, the processing effect of sliding mode control theory is still conservative because it handles the interference terms of the system through the method of constrained approximation. Sliding mode control has the characteristics of rapid response, robustness to parameter changes and disturbances, and relatively simple physical implementation. Studies have shown that the double integral sliding mode control method used can improve the switching efficiency of the converter between different working modes and has good anti-interference ability, but this method cannot adjust the parameters of the sliding mode controller according to the changes in system parameters. Summary of the invention

[0005] The purpose of the present invention is to provide an improved control method for fluctuation suppression of distributed generation grid-connected inverters. First, the basic structure of the DG grid-connected converter is described, and a mathematical model is established in a two-phase dq rotating coordinate system. On this basis, an adaptive sliding mode control strategy is proposed to ensure that the grid-connected converter can track and suppress the external interference of the system in real time and enhance the system's ability to cope with power fluctuations. Finally, the effectiveness of the proposed control system is proved through simulation verification.

[0006] The technical solution of the present invention is:

[0007] A method for improving control of fluctuation suppression of a distributed generation grid-connected inverter comprises the following steps:

[0008] 1) The mathematical model of the grid-connected converter is established as follows:

[0009] According to the KVL law, we can write the voltage circuit equation and get

[0010]

[0011] Where R, L, and C are equivalent resistance, inductance, and capacitance respectively. a 、i b 、i c is the three-phase alternating current, E a 、E b 、E c is the three-phase voltage on the AC side, UaN , U bN , U cN are the three-phase voltages abc output by the converter, U N0 is the neutral point voltage. The mathematical model in the dq two-phase rotating coordinate system is as follows:

[0012]

[0013] Among them, U d and U q are the d-axis and q-axis components of the converter AC output voltage in the rotating coordinate system; E d and E q are the d-axis and q-axis components of the AC bus voltage in the rotating coordinate system; i d and i q are the d-axis and q-axis components of the converter output current in the rotating coordinate system respectively; ω is the fundamental frequency;

[0014] 2) The improved fluctuation suppression control method is as follows:

[0015] For the mathematical model established by formula (1), the integral term of the control target is introduced and the state variable is selected as

[0016]

[0017] Among them, i dref and i qref are the components of the converter current reference value on the d-axis and q-axis respectively, and x1, x2, x3, and x4 are the selected state variables respectively; when considering the disturbance term, the derivative of the system state variable is written as:

[0018]

[0019] In the formula, w d and w q is the disturbance term; U d and U q As control input, define virtual control v1 and v2, which are related to the actual control input U d and U q The relationship is:

[0020]

[0021] Therefore, the following formula is obtained:

[0022]

[0023] Take the following coordinate transformation

[0024]

[0025] Where c1 and c2 are smooth functions of x1 and x3 respectively, z1 and z3 are new coordinate systems; to enhance the robustness of the system, sliding mode control is introduced, and the integral sliding surface S of the d-axis and q-axis is d and S q Defined as:

[0026] S d =z2+k1z1,S q =z4+k2z3 (8)

[0027] Where k1 and k2 are the parameters of the positive integral sliding surface. Differentiating the sliding surface in equation (8), we can obtain:

[0028]

[0029] to w d and w q The value is estimated, which introduces estimation bias. and Defined as:

[0030]

[0031] In the formula and is the actual value of the disturbance;

[0032] To prove the stability of the control system, the following Lyapunov function is defined:

[0033]

[0034] The derivation of equation (11) is:

[0035]

[0036] According to the following inequality:

[0037] z n z n+1 ≤z n 2 / 2+z n+1 2 / 2 (13)

[0038] Formula (12) is written as:

[0039]

[0040] Set the intermediate control variables a1 and a2 to:

[0041]

[0042] When the derivative of the Lyapunov function satisfies the critical stability condition and is equal to 0, the following two equations should be satisfied:

[0043]

[0044] Solved

[0045]

[0046] In order to ensure that the derivative of the Lyapunov function is always less than 0, a negative constant coefficient should be introduced in the above formula; therefore, according to the Lyapunov asymptotic stability principle, it should be taken as:

[0047]

[0048] where α1, α2, α3, α4, λ d ,λ q is the positive feedback coefficient.

[0049] In the aforementioned improved control method for fluctuation suppression of distributed generation grid-connected inverter, parameters α1 and α2 are both greater than 1.

[0050] The above-mentioned improved control method for fluctuation suppression of distributed generation grid-connected inverter, when interference terms are involved, parameters α3, α4, λ d , q Greater than zero.

[0051] Compared with the prior art, the present invention has the following beneficial effects:

[0052] 1. Different from the improved control method for fluctuation suppression of conventional distributed generation grid-connected inverters, the adaptive sliding mode control strategy proposed in the present invention ensures that the grid-connected inverter can track and suppress external interference of the system in real time, and enhances the system's ability to cope with power fluctuations.

[0053] 2. The method of the present invention can achieve better tracking error when the system is disturbed, the error waveform is more stable, and the recovery time is faster. Through simulation verification, it is proved that the adaptive backstepping sliding mode control has better error tracking performance and dynamic response performance than the traditional PI control, and is less sensitive to changes in system parameters, which fully demonstrates the effectiveness of the control method of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 is a structural schematic diagram of a DG grid-connected converter according to an embodiment of the present invention;

[0055] Figure 2 It is a schematic diagram of a simulation model of a PSCAD / EMTDC platform according to an embodiment of the present invention;

[0056] Figure 3is a schematic diagram of an AC frequency simulation result test of an embodiment of the present invention; Figure 3 (a) is the system frequency under the control of the method of the present invention, Figure 3 (b) is the system frequency under normal control;

[0057] Figure 4 is a schematic diagram of an AC voltage simulation result test of an embodiment of the present invention; Figure 4 (a) is the DC voltage under control of the method of the present invention, Figure 4 (b) is the DC voltage under conventional control. DETAILED DESCRIPTION

[0058] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0059] The method for suppressing fluctuations of a distributed generation grid-connected inverter based on adaptive backstepping sliding mode control of the present invention comprises the following steps:

[0060] 1) Establish a mathematical model of the grid-connected converter;

[0061] like Figure 1 As shown, according to the KVL law, the voltage circuit equation can be obtained.

[0062]

[0063] where i ac , U ac is the AC side current and voltage, R, L, C are equivalent resistance, inductance and capacitance, i a 、i b 、i c is the three-phase alternating current, E a 、E b 、E c is the AC side voltage, U dc is the DC side capacitance, U aN , U bN , U cN are the three-phase voltages abc output by the converter, U N0 is the neutral point voltage. It can be seen that the voltage and current have components in the three coordinates of a, b, and c, and they are all AC variables, which is not conducive to controller design. The mathematical model in the dq two-phase rotating coordinate system is as follows:

[0064]

[0065] Among them, U d and U q are the d-axis and q-axis components of the converter AC output voltage in the rotating coordinate system; E d and E q are the d-axis and q-axis components of the AC bus voltage in the rotating coordinate system; i d and i q are the d-axis and q-axis components of the converter output current in the rotating coordinate system; ω is the fundamental frequency.

[0066] 2) Improved fluctuation suppression control method

[0067] For the mathematical model established in (1), the integral term of the control objective is introduced and the state variable is selected as

[0068]

[0069] Among them, i dref and i qref are the components of the converter current reference value on the d-axis and q-axis respectively, and x1, x2, x3, and x4 are the selected state variables respectively. For the actual system, modeling errors such as system parameter errors and switching losses need to be considered. When considering the disturbance term, the derivative of the system state variable should be written as

[0070]

[0071] In the formula, w d and w q is the disturbance term; U d and U q As control input, define virtual control v1 and v2, which are related to the actual control input U d and U q The relationship is

[0072]

[0073] Therefore, it can be written as

[0074]

[0075] Take the following coordinate transformation

[0076]

[0077] Where c1 and c2 are smooth functions of x1 and x3 respectively, and z1 and z3 are new coordinate systems. In order to enhance the robustness of the system, sliding mode control is introduced here. The integral sliding mode surface S of the d-axis and q-axis is d and S q Defined as

[0078] Sd =z2+k1z1,S q =z4+k2z3 (8)

[0079] In the formula, k1 and k2 are the parameters of the positive integral sliding surface. Differentiating the sliding surface in formula (8), we can get

[0080]

[0081] Need to w d and w q The value is estimated, and there will be deviations in the estimation process, so it is necessary to introduce estimation deviations. and Defined as

[0082]

[0083] In the formula and is the actual value of the disturbance.

[0084] In order to prove the stability of the control system, the following Lyapunov function is defined

[0085]

[0086] The derivation of equation (11) is

[0087]

[0088] According to the inequality

[0089] z n z n+1 ≤z n 2 / 2+z n+1 2 / 2 (13)

[0090] It can be written as

[0091]

[0092] Set the intermediate control variables a1 and a2 to

[0093]

[0094] When the derivative of the Lyapunov function satisfies the critical stability condition and is equal to 0, the following two equations should be satisfied:

[0095]

[0096] Solved

[0097]

[0098] In order to ensure that the derivative of the Lyapunov function is always less than 0, a negative constant coefficient should be introduced in the above formula. Therefore, according to the Lyapunov asymptotic stability principle, it should be taken as

[0099]

[0100] where α1, α2, α3, α4, λ d ,λ q is the positive feedback coefficient to be determined.

[0101] 3) Improved stability proof of fluctuation suppression control

[0102] According to Young's inequality

[0103]

[0104] This means

[0105] V'(x)≤-βV(x)+H (23)

[0106] Where β and H should satisfy

[0107]

[0108] Furthermore, when t>t0, the Lyapunov function of the system satisfies

[0109]

[0110] Therefore, the Lyapunov function V(x) can satisfy the bounded condition, thus ensuring the asymptotic stability of the system. If the system parameters α1 and α2 are both greater than 1, it can be considered that the system's Lyapunov function satisfies the asymptotic stability condition in a large range. When it comes to interference terms, if the tracking control can already achieve fast and accurate tracking, then for the parameters α3, α4, λ d ,λ q , as long as they are greater than zero, the system stability requirement can be met. In addition, it can be seen that the larger the value of the feedback coefficient α1~α4, the faster the system tends to asymptotic stability. After determining the feedback control law v(t) of the converter, the actual control input U can be calculated according to formula (5) d and U q This ensures the rapid response of the system, maintains the stability of the system, and avoids the problems that may be caused by excessive feedback coefficients.

[0111] In order to verify the effectiveness of the proposed control strategy, a microgrid model containing DG was built using the PSCAD simulation platform. Figure 2The disturbance of AC side power change is set in the model to test the effectiveness of various control methods. The simulation results of the proposed control method and the traditional control method are shown in Figure 3 , Figure 4 As shown. The simulation results show that when a disturbance occurs, the system will transition to a new equilibrium point under both the traditional PI control and the proposed adaptive back-pushing sliding mode control strategy, but there are different dynamic response performances. Under the traditional PI control strategy, the highest frequency value is 50.3Hz, the lowest value is 48.9Hz, the highest AC voltage value is 383V, the lowest value is 305V, and the system does not return to stability until about 0.75s. It can be seen that under the traditional PI control, the system has a more violent transient fluctuation process, which seriously affects the power quality of the system. Under the proposed adaptive back-pushing sliding mode control strategy, the highest frequency value is 50.1Hz, the lowest value is 49.2Hz, the highest AC voltage value is 368V, the lowest value is 330V, and the system has reached a stable state after about 0.5s, showing a faster response speed and more accurate tracking ability, while reducing overshoot and oscillation phenomena, ensuring a more stable and reliable operation performance, and the transient fluctuation of the system has also been greatly alleviated.

[0112] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for improving control of fluctuation suppression of distributed generation grid-connected inverters, characterized in that: The steps include: 1) The mathematical model of the grid-connected converter is established as follows: According to the KVL law, we can write the voltage circuit equation and get Where R, L, and C are equivalent resistance, inductance, and capacitance respectively. a 、i b 、i c is the three-phase alternating current, E a 、E b 、E c is the three-phase voltage on the AC side, U aN , U bN , U cN are the three-phase voltages abc output by the converter, U N0 is the neutral point voltage; the mathematical model in the dq two-phase rotating coordinate system is as follows: Among them, U d and U q are the d-axis and q-axis components of the converter AC output voltage in the rotating coordinate system; E d and E q are the d-axis and q-axis components of the AC bus voltage in the rotating coordinate system; i d and i q are the d-axis and q-axis components of the converter output current in the rotating coordinate system respectively; ω is the fundamental frequency; 2) The improved fluctuation suppression control method is as follows: For the mathematical model established by formula (1), the integral term of the control target is introduced and the state variable is selected as Among them, i dref and i qref are the components of the converter current reference value on the d-axis and q-axis respectively, and x1, x2, x3, and x4 are the selected state variables respectively; when considering the disturbance term, the derivative of the system state variable is written as: In the formula, w d and w q is the disturbance term; U d and U q As control input, define virtual control v1 and v2, which are related to the actual control input U d and U q The relationship is: Therefore, the following formula is obtained: Take the following coordinate transformation Where c1 and c2 are smooth functions of x1 and x3 respectively, and z1 and z3 are new coordinate systems. To enhance the robustness of the system, sliding mode control is introduced, and the integral sliding mode surfaces S of the d-axis and q-axis are d and S q Defined as: S d =z2+k1z1,S q =z4+k2z3 (8) Where k1 and k2 are the parameters of the positive integral sliding surface. Differentiating the sliding surface in equation (8), we can obtain: For w d and w q The value is estimated, which introduces estimation bias. and Defined as: In the formula and is the actual value of the disturbance; To prove the stability of the control system, the following Lyapunov function is defined: The derivation of equation (11) is: According to the following inequality: With n With n+1 ≤of n 2 / 2+z n+1 2 / 2 (13) Formula (12) is written as: Set the intermediate control variables a1 and a2 to: When the derivative of the Lyapunov function satisfies the critical stability condition and is equal to 0, the following two equations should be satisfied: Solved In order to ensure that the derivative of the Lyapunov function is always less than 0, a negative constant coefficient should be introduced in the above formula; therefore, according to the Lyapunov asymptotic stability principle, it should be taken as: where α1, α2, α3, α4, λ d ,λ q is the positive feedback coefficient.

2. The method for improving control of fluctuation suppression of a distributed generation grid-connected inverter according to claim 1, characterized in that: Parameters α1 and α2 are both greater than 1.

3. The method for improving control of fluctuation suppression of a distributed generation grid-connected inverter according to claim 1, characterized in that: When interference is involved, the parameters α3, α4, λ d , q Greater than zero.

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