A sliding mode and SVG-based power grid control system and control method for multi-level inverters

Abstract

A power grid control system and control method based on sliding mode and SVG of a multi-level inverter, and relates to the field of power grid control. The present invention aims to solve the problem of poor stability of the multi-level inverter disconnection and connection process in the prior art. The pre-synchronization unit of the present invention generates a voltage change and a phase angle change through a sliding mode control method; the SVG control unit obtains a reference voltage amplitude according to a reactive power reference value, the current reactive power of the inverter and the voltage amplitude, and obtains a phase angle reference value according to an active power reference value, the current active power P of the inverter and the phase angle; the reference voltage generation module generates a reference voltage according to the reference voltage amplitude and the phase angle reference value; the SMC control unit makes the inverter output voltage track the reference voltage U ref The present invention can quickly track the voltage, frequency and phase of the power grid and realize the grid connection process more quickly and smoothly.

Description

Technical Field

[0001] The invention relates to the field of power grid control, and in particular to a power grid control system based on sliding mode and SVG of a multi-level inverter. Background Art

[0002] Compared with other cascade methods, the cascaded three-phase bridge multilevel inverter can achieve natural self-balancing of three-phase power; however, when the three-phase high-voltage and high-power grid is connected, due to the increase in cascades, the system will lack inertia and damping and cannot autonomously participate in frequency and voltage regulation; at the same time, the microgrid has large system power in the isolated and grid-connected operation state, resulting in voltage, frequency and power angle stability problems, especially in the grid connection of renewable energy power generation. PI control, PR control and repetitive control are relatively widely used in the current grid-connected DC / AC. PI control cannot track static errors in the two-phase αβ coordinate system and needs to be transformed into a DC component in the dq coordinate system. PI control should be used The coordinates are changed multiple times, and the inverter mathematical model has a coupling in the dq axis that is difficult to remove; the gain of the PR controller at the industrial frequency is infinite, so in the two-phase αβ coordinate system, it can follow the fundamental sinusoidal current without difference, but its gain will be smaller at places that are not the fundamental frequency, so when the frequency of the power grid fluctuates, its gain will be relatively significantly reduced; the shortcoming of repetitive control is that it has a delay that makes the system dynamic response slow, its output lags behind a whole fundamental cycle, and there is no way to remove the interference signal that appears in this fundamental cycle, so it needs to be used in conjunction with other types of control to improve the control effect.

[0003] Therefore, those skilled in the art are committed to developing a sliding mode and SVG-based power grid control system for realizing a fast and stable grid connection process. Summary of the invention

[0004] In view of the above-mentioned defects of the prior art, the technical problem to be solved by the present invention is to provide a sliding mode power grid control system based on a multi-level inverter and SVG, which effectively solves the impact on the power grid caused by high-power grid connection and ensures the stability of system voltage and frequency.

[0005] To achieve the above object, the present invention provides a sliding mode and SVG-based power grid control system of a multi-level inverter, comprising:

[0006] A pre-synchronization unit that tracks the grid voltage and phase angle using a sliding mode control method;

[0007] The SVG control unit includes a reactive power control module, an active power control module and a reference voltage generation module. The reactive power control module generates a reference voltage according to a reactive power reference value Q ref, the inverter's current reactive power Q and voltage amplitude are used to obtain the reference voltage amplitude U, and the active power control module is based on the active power reference value P ref , the current active power P and phase angle of the inverter are used to obtain a phase angle reference value w, and the reference voltage generation module generates a reference voltage U according to the reference voltage amplitude U and the phase angle reference value w ref ;

[0008] SMC control unit, used to make the inverter output voltage track the reference voltage U ref .

[0009] Another aspect of the present invention provides a sliding mode and SVG-based power grid control method of a multi-level inverter, comprising:

[0010] S1, pre-synchronization control, tracking the grid voltage and phase angle through the sliding mode control method;

[0011] S2, SVG control, according to the reactive power reference value Q ref , the inverter's current reactive power Q and voltage amplitude are used to obtain the reference voltage amplitude U, and the active power reference value P ref , the inverter's current active power P and phase angle obtain the phase angle reference value w, and generate the reference voltage U according to the reference voltage amplitude U and the phase angle reference value w ref ;

[0012] S3, SMC control, used to make the inverter output voltage track the reference voltage U ref .

[0013] Furthermore, the control equation of the reactive power control module is:

[0014]

[0015] The control equation of the active power control module is:

[0016]

[0017] Further, step S1 includes:

[0018] S11, through sliding mode control, the reference voltage amplitude U output by VSG tracks the grid voltage amplitude E, the tracking error is set to e=UE, and the sliding mode surface is established as:

[0019]

[0020] Among them, K 21 Represents the sliding surface coefficient, which is always a positive number;

[0021] The sliding mode control law is obtained as:

[0022]

[0023] Step S12: Use sliding mode control to make the phase angle w output by the VSG track the grid phase angle w g , set the tracking error to e = ww g , the sliding surface is defined as:

[0024]

[0025] Among them, K 22 Represents the sliding surface coefficient, which is always a positive number;

[0026] The sliding mode control law is obtained as:

[0027]

[0028] Further, step S3 includes:

[0029] S31. Establish a mathematical model of the system;

[0030] S32. Define the state equation as:

[0031] where x = e α , u=u k , W(t) is the unknown interference;

[0032] The function of constructing the sliding surface is:

[0033] e=u k -u ref is the tracking error, m and n represent the sliding surface coefficients, both are positive numbers;

[0034] Then the sliding mode control law is obtained as:

[0035]

[0036] Wherein, K(t) is the switching gain, K(t)=max(W(t))+η, and η=0;

[0037] Furthermore, the multi-level inverter adopts a cascaded three-phase inverter.

[0038] Furthermore, the control system includes a carrier phase shift unit for generating a PWM waveform.

[0039] Compared with the prior art, this invention has the following technical effects:

[0040] 1. The cascaded three-phase bridge inverter topology adopted in this invention can effectively improve the AC voltage level with fewer components; and ensure that the average active power output by each module is equal. From the perspective of topological structure, it can achieve three-phase power self-balancing and eliminate the problem of power imbalance between three phases.

[0041] 2. The present invention realizes the control of pre-synchronous grid-connected cascaded three-phase bridge inverters. When new energy three-phase high-voltage and high-power grid-connected, it effectively solves the impact of high-power grid-connected on the power grid, ensures the stability of system voltage and frequency, simplifies the algorithm and speeds up the calculation speed; it provides a more effective method for the control of pre-synchronous grid-connected cascaded three-phase bridge inverters.

[0042] 3. The present invention adopts the SMC pre-synchronization grid-connected control algorithm based on VSG. After comparing with various traditional control methods and comprehensive evaluation, it is found that this method greatly improves the microgrid's tracking of grid voltage, frequency, and phase, and improves the grid connection speed.

[0043] 4. This method is applicable to various high-power cascade three-phase bridge topologies. The hardware circuit is easy to implement and has many application occasions.

[0044] The concept, specific structure and technical effects of the present invention will be further described below in conjunction with the accompanying drawings to fully understand the purpose, characteristics and effects of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 It is a topological structure diagram of the cascaded three-phase bridge inverter of the present invention;

[0046] Figure 2 The SMC pre-synchronization grid-connected control block diagram of the VSG of the present invention;

[0047] Figure 3 A schematic diagram of the carrier phase shift allocation PWM modulation strategy of the present invention;

[0048] Figure 4 This is the pre-synchronization phase angle and voltage curve diagram of the present invention;

[0049] Figure 5 It is the conventional pre-synchronization phase angle and voltage curve diagram; DETAILED DESCRIPTION

[0050] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict.

[0051] It should be noted that the illustrations provided in the following embodiments are only used to illustrate the basic concept of the present invention in a schematic manner, and thus the illustrations only show components related to the present invention rather than being drawn according to the number, shape and size of components in actual implementation. In actual implementation, the type, quantity and proportion of each component may be changed arbitrarily, and the component layout may also be more complicated.

[0052] While some exemplary embodiments of the present invention have been described for the purpose of illustration, it should be understood that the present invention may be implemented in other ways not specifically shown in the drawings.

[0053] In the operation of high-power cascade inverters, due to the presence of a large number of power devices, real-time and rapid control is required to simplify the corresponding control algorithm; in addition, especially in the process from islanding to grid connection, the microgrid's tracking of grid voltage, frequency, phase, and the impact current on the grid when connected to the grid have always been an important part of our attention; in order to make the power supply and load similar to the synchronous motor in terms of operating mechanism and external characteristics, the impact on the grid when connected to the grid is reduced.

[0054] Therefore, in a specific embodiment, a sliding mode and SVG-based power grid control system is provided, such as Figure 1 As shown, it includes: a pre-synchronization unit, an SVG control unit, an SMC control unit and a carrier phase shift unit.

[0055] The multi-level inverter in this embodiment is a cascaded three-phase bridge inverter, such as Figure 2 As shown, the current relationship of the two-stage cascade three-phase bridge inverter is:

[0056]

[0057] in,

[0058] I is the effective value of current, i is the three-phase current of the bridge arm of the corresponding module;

[0059] The voltage relationship of the two-stage cascade three-phase bridge inverter is:

[0060]

[0061]

[0062] Where V is the effective value of voltage, N 1 is a reference point;

[0063] Then the equivalent mathematical model of the two-stage cascade three-phase bridge inverter is obtained as follows:

[0064]

[0065] in, is the average value of the DC side bus voltage of a single power module. The above equivalent mathematical model shows that the two-stage cascaded three-phase bridge inverter can be equivalent to a traditional three-phase bridge inverter, where the equivalent DC side bus voltage is:

[0066] V dc-eq =2V dc-av

[0067] Furthermore, the state space mathematical model of the cascaded three-phase bridge grid-connected inverter with LCL filter structure is:

[0068]

[0069]

[0070] Where k represents a, b, c; u kN represents the voltage difference between the node k on the AC side of the three-phase bridge and the DC bus reference point N; u N′N Represents the voltage difference between the filter common node N′ and the DC bus reference point N; u N′O represents the voltage difference between the filter common node N′ and the grid reference point O; e a ,e b ,e c Indicates the grid voltage;

[0071] A pre-synchronization unit that tracks the grid voltage and phase angle using a sliding mode control method;

[0072] The SVG control unit includes a reactive power control module, an active power control module and a reference voltage generation module. The reactive power control module generates a reference voltage according to a reactive power reference value Q ref , the current reactive power Q of the inverter and the voltage amplitude obtained by the pre-synchronization control unit are used to obtain the reference voltage amplitude U, and the active power control module is based on the active power reference value P ref The current active power P of the inverter and the phase angle output by the pre-synchronization unit are used to obtain a phase angle reference value w, and the reference voltage generation module generates a reference voltage U according to the reference voltage amplitude U and the phase angle reference value θ. ref ;

[0073] SMC control unit, used to make the inverter output voltage track the reference voltage U ref ;

[0074] The carrier phase shift unit is used to generate a PWM waveform.

[0075] In a specific embodiment, a sliding mode and SVG-based power grid control method of a multilevel inverter is provided, comprising the following steps:

[0076] S1, pre-synchronous control, through the sliding mode control method to make the voltage and phase angle of the VSG output track the grid voltage and phase angle; through the synchronous control, the voltage amplitude, phase and frequency of the microgrid are consistent with the grid to solve the on / off grid operation mode.

[0077] The issues of conversion include:

[0078] Step S11, setting the tracking target: making the reference voltage amplitude U output by the VSG track the grid voltage amplitude E, and according to the VSG algorithm, taking the derivative of the reactive power control equation, the state equation is obtained as follows:

[0079]

[0080] in, W(t) is the unknown interference;

[0081] Assume the tracking error is e = UE, and establish the sliding surface as:

[0082]

[0083] Among them, K 21 Represents the sliding surface coefficient, which is a positive number. Taking the derivative of the above formula, we can get:

[0084]

[0085] When W(t) is not included, let By combining the state equations, the equivalent control is obtained as follows:

[0086]

[0087] Therefore, the sliding mode control law is obtained as:

[0088]

[0089] Among them U eq Realize the control of the determined part to ensure that the system moves on s; U sw It is used to achieve the approaching motion of the system to s to suppress uncertain disturbances.

[0090] Step S12: Set the tracking target to make the phase angle w output by the VSG track the grid phase angle w g , According to the VSG algorithm, the active power control equation, the state equation is:

[0091]

[0092] in, W(t) is the unknown interference;

[0093] Assume the tracking error is e = wwg , the sliding surface is defined as:

[0094]

[0095] Among them, K 22 Represents the sliding surface coefficient, which is a positive number. Taking the derivative of the above formula, we can get:

[0096]

[0097] When W(t) is not included, let By combining the state equation formula of step S12, the equivalent control is obtained as follows:

[0098]

[0099] Therefore, the sliding mode control law is obtained as:

[0100]

[0101] where w eq Realize the control of the determined part to ensure that the system moves on s; w sw It is used to achieve the approaching motion of the system to s to suppress uncertain disturbances.

[0102] S2, according to the reactive power reference value Q ref , the current reactive power Q of the inverter and the voltage amplitude are used to obtain the reference voltage amplitude U, and the active power reference value P is used to obtain the reference voltage amplitude U. ref , the inverter's current active power P and phase angle obtain the phase angle reference value w, and generate the reference voltage U according to the reference voltage amplitude U and the phase angle reference value θ ref ;

[0103] In this embodiment, the second-order mathematical model of the synchronous generator is selected as:

[0104]

[0105]

[0106] in, is the electromagnetic electromotive force; is the stator terminal voltage; D is the constant damping coefficient; θ is the phase angle of the excitation electromotive force; J is the moment of inertia; ω is the electrical angular velocity; ω m is the mechanical angular velocity; p is the number of pole pairs; r a is the electronic armature resistance; X d is the synchronous reactance; P m is the mechanical power; P e is the electromagnetic power.

[0107] Then the control equation of the reactive power control module is obtained as follows:

[0108]

[0109] The control equation of the active power control module is obtained as follows:

[0110]

[0111] Among them, D q is the reactive voltage droop coefficient, U cn is the rated value of the terminal voltage, U c The actual output value of the terminal voltage;

[0112] S3, SMC control, using the sliding mode control method to make the inverter output voltage track the reference voltage U ref , including:

[0113] Step S31, establishing a state mathematical model of the system:

[0114] Since the mathematical model in the synchronous rotating coordinate system will aggravate the nonlinearity of the mathematical model and increase the recursive calculation time, this embodiment selects the mathematical model of the two-stage cascade three-phase bridge inverter in the αβ stationary coordinate system to avoid the above problems, and the three-phase inverter model does not have coupling under the αβ coordinate axes, and the αβ axes can be controlled separately; compared with the synchronous rotating coordinate system, it can save calculation time, shorten the sampling period, and improve the estimation accuracy.

[0115] The cascaded three-phase bridge inverter topology is equivalent to a single inverter topology, and the LCL filter is used for grid connection. At the same time, the current suppression inductor L between the two-stage cascade topology m Converted to the three-phase AC side, we get L eq =L+L m / 3, where L is the inductance on the inverter side; when considering the three-phase balance of the system, because L g L eq Small, in order to simplify the calculation, L is not considered g The voltage drop on the system, combined with Kirchhoff's law, the state mathematical model of the system can be expressed as:

[0116]

[0117] Step S32: transform the system state mathematical model to the αβ stationary coordinate system:

[0118]

[0119] Among them, L, C and R f They are filter inductance, filter capacitance, inductor equivalent resistance, U k is the inverter output voltage; e kis the grid connection point voltage, i is the three-phase current, and k=a,b,c are the three-phase subscripts.

[0120] Step S33, establish a sliding mode controller; sliding mode control is essentially a special type of nonlinear control, and the nonlinearity is manifested as discontinuity of control; this control strategy is different from other controls in that the "structure" of the system is not fixed, but can be purposefully changed according to the current state of the system during the dynamic process, forcing the system to move according to the state trajectory of the predetermined "sliding mode"; since the sliding mode can be designed and is independent of the object parameters and disturbances, this makes the sliding mode control have fast response, insensitivity to corresponding parameter changes and disturbances, and strong robustness, and is an effective method for solving nonlinear problems;

[0121] Define the state equation (taking the α axis as an example):

[0122]

[0123] where x = e α , u=u k , W(t) is the unknown interference;

[0124] The control target is set to the output voltage u of the cascaded three-phase bridge inverter k Track the reference voltage u output by the upper VSG algorithm ref , let the tracking error be e=u k -u ref , the sliding surface function is defined as:

[0125]

[0126] Among them, m and n represent the sliding surface coefficients, both of which are positive numbers. Taking the derivative of the above formula, we can get:

[0127]

[0128] When W(t) is not included, let Combining the above formulas, the equivalent control is obtained as:

[0129]

[0130] Then the sliding mode control law is obtained as:

[0131]

[0132] Among them, u eq Realize the control of the determined part to ensure that the system moves on s; u sw It is used to realize the approaching motion of the system to s to suppress the uncertain disturbance. K(t) is the switching gain, and K(t)=max(W(t))+η, and η=0;

[0133] Prove the reachability of the control law:

[0134] Define the Lyapunov function as:

[0135]

[0136] Combining the two Lyapunov function equations, we can get:

[0137]

[0138] Because K(t) is greater than the upper bound of the disturbance, V(s)≤0; this proves the accessibility of the sliding mode control law.

[0139] S4. Generate PWM waveform by carrier phase shift. The carrier phase shift PWM modulation strategy is one of the carrier PWM strategies and is one of the modulation strategies commonly used by multi-level converters. It uses multiple triangular carriers with the same amplitude and phases that lag by a certain angle to compare with the modulation wave. The generated PWM pulses control the switching devices of each module of the multi-level converter respectively. Through this carrier phase shift method, the harmonic content of the output voltage waveform of the multi-level inverter can be greatly reduced, and the waveform quality is improved. The phase voltage and phase current analysis data of the carrier phase shift of this embodiment are shown in Table 1.

[0140] Table 1: Carrier phase shift PWM modulation phase voltage and phase current analysis

[0141]

[0142] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A power grid control method based on sliding mode and SVG of a multilevel inverter, characterized in that: include: S1, pre-synchronization control, tracking the grid voltage and phase angle through the sliding mode control method, including: S11, through sliding mode control, the reference voltage amplitude of VSG output Tracking grid voltage amplitude , setting the tracking error to , the sliding surface is established as: ； The sliding mode control law is obtained as: ； in, , D q is the reactive voltage droop coefficient; Represents the sliding surface coefficient, which is a positive number; Achieve control over the determined part to ensure that the system moves on s; It is used to realize the approaching motion of the system to s to suppress the uncertain disturbance; Step S12: Use sliding mode control to adjust the phase angle of the VSG output Tracking grid phase angle , setting the tracking error to , the sliding surface is defined as: ； The sliding mode control law is obtained as: ； in, , D is the constant damping coefficient, J is the moment of inertia; Represents the sliding surface coefficient, which is a positive number; Achieve control over the determined part to ensure that the system moves on s; It is used to realize the approaching motion of the system to s to suppress the uncertain disturbance; S2, SVG control, reactive power control module according to the reactive power reference value , the current reactive power of the inverter The voltage amplitude is the reference voltage amplitude. , the active power control module according to the active power reference value , The current active power of the inverter The phase angle reference value is obtained by , the reference voltage generation module is based on the reference voltage amplitude and phase angle reference value Generate reference voltage ; S3, SMC control, used to make the inverter output voltage track the reference voltage ,include: S31. Establish a mathematical model of the system; S32. Define the state equation as: ; in, , , , , , is the inverter output voltage, is the inductor equivalent resistance, For unknown interference; The function of constructing the sliding surface is: ; in, is the tracking error, m and n represent the sliding surface coefficients, both are positive numbers; Then the sliding mode control law is obtained as: ； in, For the switching gain, take ,and .

2. The power grid control method based on sliding mode and SVG of a multilevel inverter according to claim 1, characterized in that: The control equation of the reactive power control module is: ； The control equation of the active power control module is: ； in, is the reactive voltage droop coefficient, is the rated value of the terminal voltage, The actual output value of the terminal voltage, J is the moment of inertia, is the mechanical power; is the electromagnetic power, is the electrical angular velocity; is the mechanical angular velocity, and D is the steady damping coefficient.

3. The power grid control method based on sliding mode and SVG of a multilevel inverter according to claim 1, characterized in that: The multi-level inverter adopts a cascade three-phase inverter.

4. The power grid control method based on sliding mode and SVG of a multilevel inverter according to claim 1, characterized in that: The control system includes a carrier phase shift unit for generating a PWM waveform.

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