LCL single-phase grid-connected inverter control method based on circuit energy state

Abstract

The present invention proposes a control method for an LCL type single-phase grid-connected inverter based on circuit energy state, comprising: step 1, obtaining an energy state model of the circuit according to the energy storage law of an energy storage element and selected circuit parameters; step 2, establishing a finite state machine controller for the LCL type single-phase grid-connected inverter, setting an effective state and an initial state, and obtaining a convergence condition and a switching condition of the finite state machine controller based on sliding mode control; step 3, establishing a finite state machine controller according to the finite state machine controller based on sliding mode control, and under a finite frequency condition, making the control variable converge to an equilibrium point within a finite time, that is, the output current of the single-phase grid-connected inverter tracks a given value without deviation.

Description

[0001] Control method of LCL type single-phase grid-connected inverter based on circuit energy state Technical Field

[0002] The present invention relates to the field of automatic control, and in particular to a control method of a single-phase grid-connected inverter based on sliding mode control. Background Art

[0003] The core module of the grid-connected inverter is the core module of the new energy power generation system. LCL filters have been widely used in grid-connected inverters due to their better high-frequency harmonic attenuation and filtering effect, as well as smaller size under the same filtering effect. However, since the LCL filter is a third-order system, there are problems such as high-frequency resonance and difficulty in stable control.

[0004] Some linear control strategies have been applied to LCL type single-phase grid-connected inverters. Linear control strategies are simple and easy to implement. However, they also have some disadvantages.

[0005] 1) The required effect cannot be achieved in terms of dynamic response and steady-state error;

[0006] 2) Sensitive to changes in circuit parameters. Circuit parameter drift may lead to system performance degradation and controlled parameter divergence.

[0007] Sliding mode control is a nonlinear control method that is robust to parameter uncertainty and external disturbances. It is a variable structure control. The difference from other controls is that the structure of the system is not fixed, but changes purposefully and continuously according to the current state of the system, so that the controlled object moves along the reference trajectory. Since the sliding mode can be designed and is independent of the object parameters and disturbances, the sliding mode control has the characteristics of fast response, insensitivity to parameter changes, and overcomes the uncertainty of the system. Summary of the invention

[0008] The present invention aims to at least solve the technical problems existing in the prior art, and in particular innovatively proposes a control method for an LCL type single-phase grid-connected inverter based on circuit energy state.

[0009] The present invention proposes a control method for an LCL type single-phase grid-connected inverter based on circuit energy state. By utilizing the stability characteristics of the LCL type filter, the idea of ​​the circuit energy state is proposed, and an energy model of the LCL filter is established to quantify the current circuit energy state. The method adopts a controller based on a state machine structure, and the circuit switching control quantity is calculated by calculating the current circuit energy state and the reference energy state, so that the LCL type grid-connected inverter has a good dynamic response and has good robustness to parameter uncertainty and load disturbance. The idea of ​​the circuit energy state proposed by the present invention is combined with a controller with a finite state machine structure, and a zero error is achieved in the steady state of the system by adding a delay link to limit the switching frequency.

[0010] In order to achieve the above-mentioned object of the present invention, the present invention provides a control method of an LCL type single-phase grid-connected inverter based on circuit energy state, the key of which is to include:

[0011] Step 1: According to the energy storage law of the energy storage element and the selected circuit parameters, the energy state equation of the circuit is obtained; the three selected circuit variables are the converter side inductor current i1, capacitor voltage U c and the grid current i2, and establish the energy state model of the circuit according to the energy storage characteristics of the inductor and capacitor;

[0012] Step 2, establish a finite state machine controller for the LCL type single-phase grid-connected inverter, set the effective state and the initial state, and obtain the convergence condition of the finite state machine controller based on sliding mode control;

[0013] Step 3, according to the finite state machine controller based on sliding mode control, a finite state machine controller with an increased time lag value is established, so that the control variable converges to the equilibrium point under the finite frequency condition, that is, the output current of the single-phase grid-connected inverter tracks the given value without deviation;

[0014] The control method of the LCL type single-phase grid-connected inverter based on energy state, preferably, the step 1 comprises:

[0015] Step 1-1, the three circuit variables selected are the converter side inductor current i1, capacitor voltage U c And the grid current i2, according to the energy storage characteristics of inductance and capacitance, establish the energy state model of the circuit.

[0016]

[0017] Here, E is introduced to represent the energy state of the circuit. According to the reference value of the grid-side current, the circuit reference energy state can be obtained:

[0018]

[0019] 3. The LCL type single-phase grid-connected inverter control method based on circuit energy state according to claim 1, characterized in that the step 2 comprises:

[0020] Step 2-1, set the valid state and initial state of the state machine so that the valid state corresponds to the output of the state machine; the state machine has four valid states A + , A - , A delay+ , A delay- and an initial state; when the circuit energy state E<E ref When the system works in the left half plane, the state machine is composed of A + and A del Drive, when the circuit energy state E>E ref When the system works in the right half plane, the state machine is composed of A - and A - Drive. A delay- and A delay Indicates that the state machine enters the delay phase at this time, the symbol "-" represents the right half plane, at this time the control amount d = -1, the symbol "+" represents the left half plane, at this time the control amount d = 1;

[0021] Step 2-1, based on the system energy state model obtained in step 1, by comparing with the reference value, the switching conditions for jumping between states are obtained.

[0022] Select the sliding surface:

[0023] s=EE ref (3)

[0024] V in =dV dc (4)

[0025] d=-sgn(s) (5)

[0026] The four valid states of the state machine have corresponding control quantities A + , A - , A delay+ , A delay- , where state A delay- and state A - The corresponding control quantity d = -1, at this time s is greater than zero, at this time switch tube 1 and switch tube 4 are turned off, switch tube 2 and switch tube 3 are turned on, and the input voltage V in =V dc .

[0027] V in =U c +L1di1 / dt (6)

[0028] In this state, according to the established circuit energy model, it can be inferred that the energy state E in the circuit will increase;

[0029] Status A + and A delay+ The control quantity corresponding to the state is d=1. At this time, when s is less than zero, the input voltage is V dc At this time, switch tube 2 and switch tube 3 are turned off, switch tube 1 and switch tube 4 are turned on, and the input voltage V in =-V dc .

[0030] -V in =U c +L1di1 / dt (7)

[0031] In this state, according to the established circuit energy model, it can be inferred that the energy state E in the circuit will decrease.

[0032] By switching between the above states, the circuit energy state will track to the reference energy state within a limited time.

[0033] 4. The LCL type single-phase grid-connected inverter control method based on circuit energy state according to claim 1, characterized in that the step 3 comprises:

[0034] in E=E ref Based on the assumption that

[0035]

[0036] Case 1

[0037] i1>i ref >i2 (9)

[0038] According to Kirchhoff's current law, we can get

[0039] i1=i2+i c (10)

[0040] At this time c >0, during this period, the voltage across the capacitor will rise.

[0041] U c >v g +L2 di2 / dt (11)

[0042] The grid current will increase and this process will continue until

[0043] i1=i2=i ref , U c =U ref (12)

[0044] Case 2

[0045] i2>i ref >i1 (13)

[0046] According to Kirchhoff's current law, we can get

[0047] i1=i2+i c (14)

[0048] At this time c <0, during this period, the voltage across the capacitor will drop.

[0049] U c <v g +L2di2 / dt (15)

[0050] The grid current will decrease and this process will continue until

[0051] i1=i2=i ref , U c =U ref (16)

[0052] Hypothesis 2

[0053]

[0054] Case 1

[0055] i2>i ref , U c <U ref (18)

[0056] According to Kirchhoff's current law, we can get

[0057] i1=i2+i c (19)

[0058] At this time c <0, during this period, the voltage across the capacitor will drop.

[0059] U c <v g +L2di2 / dt (20)

[0060] The grid current will decrease and this process will continue until

[0061] i1=i2=i ref , U c =U ref (twenty one)

[0062] Case 2

[0063] i2<iref , U c >U ref (twenty two)

[0064] because

[0065] U c >v g +L2di2 / dt (23)

[0066] The grid current will increase and this process will continue until

[0067] i1=i2=i ref , U c =U ref (twenty four)

[0068] Assumption 3

[0069]

[0070] Case 1

[0071] i1>i ref , U c <U ref (26)

[0072] According to Kirchhoff's current law, we can get

[0073] i1=i2+i c (27)

[0074] At this time c >0, during this period, the voltage across the capacitor will rise, and the converter side inductor current i1 will gradually decrease. This process will continue until

[0075] i1=i2=i ref , U c =U ref (28)

[0076] Case 2

[0077] i1<i ref , U c >U ref (29)

[0078] because

[0079] U c >v g +L2di2 / dt (30)

[0080] As the capacitor voltage drops, the grid-side current increases, and this process will continue until

[0081] U c =vg +L2di2 / dt (31)

[0082] because

[0083] E=E ref (32)

[0084] If at this time i1=i ref , then the circuit energy state reaches the reference circuit energy state, if at this time i1<i ref ,because

[0085] i1<i2 (33)

[0086] i1=i2+i c (34)

[0087] As the capacitor voltage continues to decrease, the current on the converter side continues to increase, and the current on the grid side continues to decrease. This process will continue until

[0088] i1=i2=i ref , U c =U ref (35)

[0089] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0090] 1. Compared with traditional linear control, the dynamic and steady-state characteristics of the output current of the LCL type single-phase grid-connected inverter are improved;

[0091] 2. The energy storage characteristics of the LCL filter are utilized to simplify the design process of the LCL type single-phase grid-connected inverter controller;

[0092] 3. Good robustness to the drift of circuit parameters;

[0093] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0094] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0095] Figure 1 It is a circuit diagram of an LCL type single-phase grid-connected inverter of a control method of an LCL type single-phase grid-connected inverter based on circuit energy state of the present invention;

[0096] Figure 2 It is a state machine controller structure diagram of a control method of an LCL type single-phase grid-connected inverter based on circuit energy state of the present invention;

[0097] Figure 3 It is a method schematic diagram of a control method of an LCL type single-phase grid-connected inverter based on circuit energy state of the present invention;

[0098] Figure 4 It is an example circuit diagram of a control method of an LCL type single-phase grid-connected inverter based on circuit energy state of the present invention; DETAILED DESCRIPTION

[0099] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be understood as limiting the present invention.

[0100] In the description of the present invention, it is necessary to understand that the terms "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0101] In the description of the present invention, unless otherwise specified and limited, it should be noted that the terms "installation", "connection" and "connection" should be understood in a broad sense. For example, it can be a mechanical connection or an electrical connection, or it can be the internal connection of two elements. It can be a direct connection or an indirect connection through an intermediate medium. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to the specific circumstances.

[0102] Figure 1 The circuit structure of the LCL type single-phase grid-connected inverter is shown, and the model of the circuit energy state is established.

[0103]

[0104] Similarly, the circuit energy state reference model is obtained

[0105]

[0106] Different states of switches S1, S2, S3, and S4 determine different states of the circuit.

[0107] When the control quantity d=1, the switch tubes S1 and S4 are turned on, and the switch tubes S2 and S3 are turned off. The following differential equation group can be obtained from the input and output differential equations of the controller:

[0108] V dc =U c +L1di1 / dt (3)

[0109] When the control quantity d = -1, the switch tubes S1 and S4 are turned off, and the switch tubes S2 and S3 are turned on. The following differential equation group can be obtained from the input and output differential equations of the controller:

[0110] -V dc =U c +L1di1 / dt (4)

[0111] Select the sliding surface:

[0112] s=EE ref (5)

[0113] V in =dV dc (6)

[0114] d=-sgn(s) (7)

[0115] Figure 2 The diagram shows the structure of the state machine. The state machine consists of four valid states and one initial state. The switching conditions between the states have been explained in detail above. The four valid states of the state machine have corresponding control quantities A. + , A - , A delay , A dela , where state A delay and state A - The corresponding control quantity d = -1, at this time s is greater than zero, at this time switch tube 1 and switch tube 4 are turned off, switch tube 2 and switch tube 3 are turned on, and the input voltage V in =V dc .

[0116] V in =U c +L1di1 / dt (8)

[0117] In this state, according to the established circuit energy model, it can be inferred that the energy state E in the circuit will increase;

[0118] Status A + and A delay+ The control quantity corresponding to the state is d=1. At this time, when s is less than zero, the input voltage is V dc At this time, switch tube 2 and switch tube 3 are turned off, switch tube 1 and switch tube 4 are turned on, and the input voltage V in =-V dc .

[0119] -V in =U c+L1di1 / dt (9)

[0120] In this state, according to the established circuit energy model, it can be inferred that the energy state E in the circuit will decrease.

[0121] By switching between the above states, the circuit energy state will track to the reference energy state within a limited time.

[0122] in E=E ref On the basis of

[0123] Assumption 1

[0124]

[0125] Case 1

[0126] i1>i ref >i2 (11)

[0127] According to Kirchhoff's current law, we can get

[0128] i1=i2+i c (12)

[0129] At this time c >0, during this period, the voltage across the capacitor will rise.

[0130] U c >v g +L2 di2 / dt (13)

[0131] The grid current will increase and this process will continue until

[0132] i1=i2=i ref , U c =U ref (14)

[0133] Case 2

[0134] i2>i ref >i1 (15)

[0135] According to Kirchhoff's current law, we can get

[0136] i1=i2+i c (16)

[0137] At this time c <0, during this period, the voltage across the capacitor will drop.

[0138] U c <v g +L2 di2 / dt (17)

[0139] The grid current will decrease and this process will continue until

[0140] i1=i2=i ref , U c =U ref (18)

[0141] Hypothesis 2

[0142]

[0143] Case 1

[0144] i2>i ref , U c <U ref (20)

[0145] According to Kirchhoff's current law, we can get

[0146] i1=i2+i c (twenty one)

[0147] At this time c <0, during this period, the voltage across the capacitor will drop.

[0148] U c <v g +L2di2 / dt (22)

[0149] The grid current will decrease and this process will continue until

[0150] i1=i2=i ref , U c =U ref (twenty three)

[0151] Case 2

[0152] i2<i ref , U c >U ref (twenty four)

[0153] because

[0154] U c >v g +L2di2 / dt (25)

[0155] The grid current will increase and this process will continue until

[0156] i1=i2=i ref , U c =U ref (26)

[0157] Assumption 3

[0158]

[0159] Case 1

[0160] i1>i ref , U c <U ref (28)

[0161] According to Kirchhoff's current law, we can get

[0162] i1=i2+i c (29)

[0163] At this time c >0, during this period, the voltage across the capacitor will rise, and the converter side inductor current i1 will gradually decrease. This process will continue until

[0164] i1=i2=i ref , U c =U ref (30)

[0165] Case 2

[0166] i1<i ref , U c >U ref (31)

[0167] because

[0168] U c >v g +L2di2 / dt (32)

[0169] As the capacitor voltage drops, the grid-side current increases, and this process will continue until

[0170] U c =v g +L2di2 / dt (33)

[0171] because

[0172] E=E ref (34)

[0173] If at this time i1=i ref , then the circuit energy state reaches the reference circuit energy state, if at this time i1<i ref ,because

[0174] i1<i2 (35)

[0175] i1=i2+i c (36)

[0176] As the capacitor voltage continues to decrease, the current on the converter side continues to increase, and the current on the grid side continues to decrease. This process will continue until

[0177] i1=i2=i ref , U c =U ref (37)

[0178] Figure 3 The method diagram of the control method of LCL type single-phase grid-connected inverter based on circuit energy is shown. The control method is improved based on the switching function idea of ​​the second-order sliding mode. The energy state model of the circuit is obtained according to the energy storage law of the energy storage element and the selected circuit parameters. Then, a finite state machine controller is established for the LCL type single-phase grid-connected inverter, and the effective state and initial state are set. The convergence condition and switching condition of the finite state machine controller based on sliding mode control are obtained, and then the conditions for the convergence of the circuit energy state are obtained. The three steps fully describe the design process and workflow of the state machine controller.

[0179] Figure 4 The example circuit diagram is shown. The inductor current and capacitor voltage are returned to the state machine controller as feedback signals, and the state machine controller outputs control signals to the four switch MOSFETs. The controller processes the collected signals to obtain the energy state of the circuit, and then generates a PWM signal to control the operation of the switch tube, thereby achieving control of the circuit working state.

[0180] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0181] Although embodiments of the present invention have been shown and described, those skilled in the art will appreciate that various changes, modifications, substitutions and variations can be made to the embodiments without departing from the principles and spirit of the invention, and the scope of the invention is defined by the claims and their equivalents.

Claims

1. A LCL type single-phase grid-connected inverter control method based on circuit energy state, characterized in that: The LCL type single-phase grid-connected inverter includes: four switch tubes, the drain of the first switch tube S1 is connected to the drain of the third switch tube S3 and the converter power supply V dc The positive electrode of the first switch tube S1 is connected to the drain of the second switch tube S2 and one end of the converter side inductor L1, and the other end of the converter side inductor L1 is connected to one end of the grid side inductor L2 and the capacitor U c One end is connected to the grid side inductor L2, and the other end is connected to the grid output terminal v g The positive electrode of the second switch tube S2 is connected to the positive electrode of the converter power supply V dc The cathode is connected to the source of the fourth switch tube S4, and the drain of the fourth switch tube S4 is connected to the source of the third switch tube S3 and the capacitor U c The other end and the grid output end v g The negative pole is connected; The control method comprises: Step 1, obtaining the energy state equation of the circuit according to the energy storage law of the energy storage element and the selected circuit parameters; Step 1-1, the three circuit variables selected are the converter side inductor current i1, capacitor voltage U c and the grid current i2, and establish the energy state model of the circuit according to the energy storage characteristics of the inductor and capacitor; Step 2, establish a finite state machine controller for the LCL type single-phase grid-connected inverter, set the effective state and the initial state, and obtain the convergence condition and switching condition of the finite state machine controller based on sliding mode control; Step 3: According to the finite state machine controller based on sliding mode control, under finite frequency conditions, the control variable is made to converge to an equilibrium point within a finite time, that is, the output current of the single-phase grid-connected inverter tracks the given value without deviation.

2. The LCL type single-phase grid-connected inverter control method based on circuit energy state according to claim 1, characterized in that: The step 1 comprises: Modeling the energy states of a circuit Here, E is introduced to represent the energy state of the circuit, where i1 is the inductor current on the converter side, i2 is the grid current, and U c is the capacitor voltage, C is the capacitance; According to the reference value of the grid-side current, the circuit reference energy state can be obtained Among them, E ref It is the energy state reference value of the circuit.

3. The LCL type single-phase grid-connected inverter control method based on circuit energy state according to claim 1, characterized in that: The step 2 comprises: Step 2-1, set the valid state and initial state of the state machine so that the valid state corresponds to the output of the state machine; the state machine has four valid states A + , A _ , A delay+ , A delay- and an initial state; when the circuit energy state E <E ref When the system works in the left half plane, the state machine is composed of A + and A delay+ Drive, when the circuit energy state E>E ref When the system works in the right half plane, the state machine is composed of A - and A delay- Drive; A delay- and A delay+ Indicates that the state machine enters the delay phase at this time, the symbol "-" represents the right half plane, at this time the control amount d = -1, the symbol "+" represents the left half plane, at this time the control amount d = 1; Step 2-2, according to the system energy state model obtained in step 1, by comparing with the reference value, the switching conditions for jumping between states are obtained; Select sliding surface s=EE ref (3) V in =dV dc (4) d=-sgn(s) (5) Where E is the energy state of the circuit, E ref is the energy state reference value of the circuit, V in is the grid-side input voltage; The four valid states of the state machine have corresponding control quantities A + , A - , A delay+ , A delay- , where state A delay- and state A - The corresponding control quantity d = -1, at this time s is greater than zero, at this time the switch tube S1 and the switch tube S4 are turned off, the switch tube S2 and the switch tube S3 are turned on, and the input voltage V in =V dc ; V in =U c +L1di1 / dt (6) In this state, according to the established circuit energy model, it can be inferred that the energy state E in the circuit will increase; Status A + and A delay+ The control quantity corresponding to the state is d=1. At this time, when s is less than zero, the input voltage is V dc At this time, switch tubes S2 and S3 are turned off, switch tubes S1 and S4 are turned on, and the input voltage V in =-V dc ; -V in =U c +L1di1 / dt (7) In this state, according to the established circuit energy model, it can be inferred that the energy state E in the circuit will decrease; By switching between the above states, the circuit energy state will track to the reference energy state within a limited time.

4. The LCL type single-phase grid-connected inverter control method based on circuit energy state according to claim 1, characterized in that: The step 3 comprises: in E=E ref On the basis of like Case 1 i1>i ref >i2 (9) According to Kirchhoff's current law, we can get i1=i2+i c (10) At this time c >0, during this period, the voltage across the capacitor will rise. U c >v g +L2di2 / dt (11) The grid current will increase and this process will continue until i1=i2=i ref ,IN c =U ref (12) Case 2 i2>i ref >i1 (13) According to Kirchhoff's current law, we can get i1=i2+i c (14) At this time c <0, during this period, the voltage across the capacitor will drop. U c <v g +L2di2 / dt (15) The grid current will decrease and this process will continue until i1=i2=i ref ,IN c =U ref (16) like Case 1 i2>i ref ,U c <U ref (18) According to Kirchhoff's current law, we can get i1=i2+i c (19) At this time c <0, during this period, the voltage across the capacitor will drop. U c <v g +L2di2 / dt (20) The grid current will decrease and this process will continue until i1=i2=i ref ,IN c =U ref (21) Case 2 i2 ref ,YOU c >You ref (22)​ because U c >v g +L2di2 / dt (23) The grid current will increase and this process will continue until i1=i2=i ref ,IN c =U ref (24) like Case 1 i1>i ref ,IN c ref (26)​ According to Kirchhoff's current law, we can get i1=i2+i c (27) At this time c >0, during this period, the voltage across the capacitor will rise, and the converter side inductor current i1 will gradually decrease. This process will continue until i1=i2=i ref ,IN c =U ref (28) Case 2 and1 ref ,IN c >In ref (29)​ because U c >v g +L2di2 / dt (30) As the capacitor voltage drops, the grid-side current increases, and this process will continue until U c =v g +L2di2 / dt (31) because E=E ref (32) If at this time i1=i ref , then the circuit energy state reaches the reference circuit energy state. If at this time i1 ref ,​ because i1 <i2 (33) i1=i2+i c (34) As the capacitor voltage continues to decrease, the current on the converter side continues to increase, and the current on the grid side continues to decrease. This process will continue until i1=i2=i ref ,IN c =U ref (35).

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