A universal control method for LC and LCL grid-connected inverters

Abstract

The present invention relates to the field of power electronics technology, and specifically discloses a universal control method for LC and LCL grid-connected inverters, including: establishing a universal mathematical model for the AC side that is applicable to both the LC grid-connected inverter and the LCL grid-connected inverter; obtaining the current control block diagrams of the LC grid-connected inverter and the LCL grid-connected inverter respectively according to the universal mathematical model for the AC side; obtaining the corresponding closed-loop transfer functions according to the current control block diagram to achieve accurate current tracking of the LC grid-connected inverter and the LCL grid-connected inverter; converting the current control block diagram of the LCL grid-connected inverter into the current feedback and capacitor voltage feedback control block diagram of the LCL grid-connected inverter side, and then obtaining the open-loop transfer function of the LCL grid-connected inverter to achieve resonance suppression of the LCL grid-connected inverter. The present invention utilizes the inductor current and capacitor voltage on the inverter side to respectively achieve accurate current control and resonance suppression, does not require additional sensors when hardware changes, and has good algorithm versatility.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics, and in particular to a universal control method for LC type and LCL type grid-connected inverters. Background Art

[0002] Photovoltaic grid-connected inverters are energy conversion devices in photovoltaic power generation systems. Although the installation position of LCL filters is reserved in the grid-connected inverter equipment in actual projects, LC filters without grid-side inductance are generally used. The different control methods of LCL inverters and LC inverters result in the need to update software when adding inductance in actual projects. The control method of LC inverters is relatively simple. LCL grid-connected inverters have better high-order harmonic filtering capabilities than LC grid-connected inverters, but also bring resonance problems.

[0003] However, in actual projects, LCL grid-connected inverters usually only retain the inverter-side inductance and filter capacitors of the LCL filter. The grid-side inductance of the LCL filter is directly short-circuited to form an LC filter, but a position for the grid-side inductance is reserved to facilitate the subsequent addition of inductance to form an LCL filter. Regardless of whether there is a grid-side inductor, the inverter only provides inverter-side inductor current and capacitor voltage sensors. Even if the grid-side inductor installation position is reserved, no additional grid-side inductor current sensor will be added. LC-type inverters can achieve precise control of the inverter-side inductor current and capacitor voltage, but the commonly used control scheme for LCL filters is the dual-current closed-loop mode, which uses grid-connected current and capacitor current feedback to achieve current control and resonance suppression. When the actual inverter is modified from an LC filter to an LCL filter, the capacitor current cannot be calculated for resonance suppression in the absence of a grid-side inductor current sensor.

[0004] Therefore, the most commonly used resonance suppression method at present is capacitor current feedback, but the frequency of capacitor current is relatively high. On the one hand, high-frequency sensors have high accuracy requirements and high costs. On the other hand, when the inductance on the grid side is increased, its current sensor is not increased. Therefore, the capacitor current cannot be calculated through the inductor current values ​​on both sides of the LCL filter.

[0005] In summary, in actual grid-connected inverters, in order to reduce the complexity of the AC side filter changes and improve the versatility of the algorithm, a grid-connected inverter control method that can be applied to both LC and LCL filters is imperative. Summary of the invention

[0006] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a universal control method for LC type and LCL type grid-connected inverters, which utilizes the inductor current and capacitor voltage on the inverter side to respectively realize precise current control and resonance suppression, without the need for additional sensors when hardware changes, and has good algorithm versatility. The implementation process is simple without the need for coordinate transformation.

[0007] As a first aspect of the present invention, a general control method for LC type and LCL type grid-connected inverters is provided, and the general control method for LC type and LCL type grid-connected inverters comprises:

[0008] Step S1: establishing a universal mathematical model of the AC side applicable to both LC-type grid-connected inverter and LCL-type grid-connected inverter;

[0009] Step S2: According to the general mathematical model of the AC side, derive the grid-connected inverter in a switching cycle T s The total duty cycle within

[0010] Step S3: According to the grid-connected inverter in a switching cycle T s The total duty cycle in is obtained, and the current control block diagram of the LC type grid-connected inverter and the current control block diagram of the LCL type grid-connected inverter are obtained respectively;

[0011] Step S4: according to the current control block diagram of the LC type grid-connected inverter, a closed-loop transfer function of the LC type grid-connected inverter is obtained to achieve accurate current tracking of the LC type grid-connected inverter; and according to the current control block diagram of the LCL type grid-connected inverter, a closed-loop transfer function of the LCL type grid-connected inverter is obtained to achieve accurate current tracking of the LCL type grid-connected inverter;

[0012] Step S5: Based on the capacitor current feedback coefficient, the current control block diagram of the LCL type grid-connected inverter is converted into a current feedback and capacitor voltage feedback control block diagram of the LCL type grid-connected inverter side; and according to the current feedback and capacitor voltage feedback control block diagram of the LCL type grid-connected inverter side, an open-loop transfer function of the LCL type grid-connected inverter is obtained to achieve resonance suppression of the LCL type grid-connected inverter.

[0013] Furthermore, the establishment of a universal mathematical model for the AC side applicable to both the LC type grid-connected inverter and the LCL type grid-connected inverter also includes:

[0014] Step S11: The AC side filter of the LC type grid-connected inverter is LC type, the AC side filter of the LCL type grid-connected inverter is LCL type, U dc is the DC side voltage, C a1 and C a2 is the DC side capacitance, u Ca1 and u Ca2 Respectively represent C a1 The voltage value and C a2 Voltage value, L x and C x It forms an LC filter, L x , L xg and Cx The LCL filter is formed. x and i xg Respectively represent L x and L xg The x-phase current, e x represents the x-phase grid voltage, where x = a, b, c;

[0015] Step S12: According to Kirchhoff's voltage law, the universal mathematical model of the AC side applicable to both the LC type grid-connected inverter and the LCL type grid-connected inverter can be obtained as formula (1):

[0016]

[0017] In the formula, u xo is the output voltage of the x-phase inverter to point o; u no is the common mode voltage between the grid neutral point n and the DC side midpoint o; u Cx Represents C x Voltage value.

[0018] Furthermore, according to the general mathematical model of the AC side, it is derived that the grid-connected inverter has a switching cycle T s The total duty cycle also includes:

[0019] Under the premise of midpoint potential balance, when the x-phase modulation wave is in the positive half cycle, formula (1) is transformed into formula (2) in the [n, n+1]th switching cycle:

[0020]

[0021] Where n represents the sampling time, t Pon Indicates +U in one switching cycle dc / 2 duration, t(n+1)-t(n)=T s , T s is a switching cycle;

[0022] When the x-phase modulation wave is in the negative half cycle, formula (1) is converted into formula (3) in the [n, n+1]th switching cycle:

[0023]

[0024] In the formula, t Non Indicates one switching cycle -U dc / 2 duration;

[0025] Output voltage u of grid-connected inverter xo There are three level states: +U dc / 2, 0, and -U dc / 2, and set the duty ratios corresponding to these three levels to d xP d xO and d xN ;

[0026] (1) Positive half cycle of modulation wave

[0027] When the x-phase modulation wave is in the positive half cycle, the output voltage u of the grid-connected inverter xo Contains only +U dc / 2, 0 two levels, the corresponding duty cycle expression is set to formula (4):

[0028]

[0029] Where, d x represents the total duty cycle of phase x;

[0030] break down:

[0031] By decomposing the inductor current on the grid-connected inverter side, the following formula (5) can be obtained:

[0032]

[0033] In the formula, Δi xz+ Represents the current change of the x-phase inductor corresponding to the z working state in the positive half cycle of the modulation wave, z = P, O, N;

[0034] Sum:

[0035] The current changes corresponding to the three levels in formula (5) are summed to obtain the complete current change expression in the positive half cycle of the modulation wave as formula (6):

[0036]

[0037] Substituting the duty cycle set in formula (4) into formula (6), we can get formula (7):

[0038]

[0039] In the formula, Δi x+ Represents the current change corresponding to the x-phase inductance in the positive half cycle of the modulation wave;

[0040] (2) Negative half cycle of modulation wave

[0041] When the x-phase modulation wave is in the negative half cycle, the output voltage u of the grid-connected inverter xo Contains only -U dc / 2, 0 two levels, the corresponding duty cycle expression is set to formula (8):

[0042]

[0043] break down:

[0044] By decomposing the inductor current on the inverter side, the following formula (9) can be obtained:

[0045]

[0046] In the formula, Δi xz- Represents the current change of the x-phase inductor corresponding to the z working state in the negative half cycle of the modulation wave;

[0047] Sum:

[0048] The current changes corresponding to the three levels in formula (9) are summed to obtain the complete inverter side inductor current change expression in the negative half cycle of the modulation wave as formula (10):

[0049]

[0050] Substituting the duty cycle set in formula (8) into formula (10), we can obtain formula (11):

[0051]

[0052] In the formula, Δi x- Represents the current change of the inductor on the x-phase inverter side corresponding to the negative half cycle of the modulation wave;

[0053] By combining the changes in the LC filter inductor current in the positive and negative half-cycles of the modulation wave, that is, formula (7) and formula (11), we can get formula (12):

[0054] 2L x (Δi x+ +Δi x- )=2L x Δi x (12)

[0055] In the formula, Δi x Represents the current change corresponding to the x-phase inductance in one switching cycle;

[0056] In summary, a switching cycle T s The complete current change in is given by formula (13):

[0057] 2L x Δi x =U dc d x T s -2T s (u Cx +uno )(13) According to formula (13), a switching period T can be derived s The total duty cycle expression is as follows:

[0058]

[0059] One switching cycle T s The change in the inductor current can be expressed as formula (15):

[0060] Δi x =i x (n+1)-i x (n)(15)

[0061] In the formula, Δi x Indicates the actual inductor current change of phase x, i x (n+1) represents the value of the inductor current at the sampling time n+1, i x (n) represents the value of the inductor current at sampling time n;

[0062] In order to achieve the ideal inductor current tracking effect, the actual sampled inductor current value is replaced by the reference inductor current value at the sampling time n+1. The effect in control is equivalent to the actual inductor current value tracking the reference inductor current value at the next moment. Therefore, formula (15) is converted into formula (16):

[0063] Δi x =I xref (n+1)-i x (n)(16)

[0064] In the formula, I xref (n+1) represents the reference value of the inductor current at sampling time n+1;

[0065] Applying the above conversion form of inductor current change to the inductor current change on the inverter side, we can get formula (17):

[0066]

[0067] Let K1 = 2L x / (U dc T s ) and K2=2 / U dc , the duty cycle expression can be obtained as formula (18):

[0068] d x =K1[I xref (n+1)-i x (n)]+K2(u Cx +u no )(18).

[0069] Further, the step of obtaining a closed-loop transfer function of the LC-type grid-connected inverter according to the current control block diagram of the LC-type grid-connected inverter to achieve accurate current tracking of the LC-type grid-connected inverter further includes:

[0070] The closed-loop transfer function G1(s) of the LC-type grid-connected inverter is expressed as formula (19):

[0071]

[0072] Further, the step of obtaining a closed-loop transfer function of the LCL type grid-connected inverter according to the current control block diagram of the LCL type grid-connected inverter to achieve accurate current tracking of the LCL type grid-connected inverter further includes:

[0073] The closed-loop transfer function G of the LCL type grid-connected inverter close (s) is formula (20):

[0074]

[0075] Furthermore, the step S4 further includes:

[0076] According to the current control block diagram of the LCL type grid-connected inverter, the open-loop transfer function G of the LCL type grid-connected inverter is obtained: open (s);

[0077] Among them, the open-loop transfer function G of the LCL type grid-connected inverter is open (s) is formula (21);

[0078]

[0079] Furthermore, based on the capacitor current feedback coefficient, the current control block diagram of the LCL type grid-connected inverter is converted into a current feedback and capacitor voltage feedback control block diagram of the LCL type grid-connected inverter side, and further includes:

[0080] Converting the current control block diagram of the LCL type grid-connected inverter into a current feedback and capacitor current feedback control block diagram of the LCL type grid-connected inverter side;

[0081] Based on the capacitor current feedback coefficient, the LCL type grid-connected inverter side current feedback and capacitor current feedback control block diagram are converted into the LCL type grid-connected inverter side current feedback and capacitor voltage feedback control block diagram.

[0082] Further, the open-loop transfer function of the LCL type grid-connected inverter is obtained according to the current feedback and capacitor voltage feedback control block diagram of the LCL type grid-connected inverter to achieve resonance suppression of the LCL type grid-connected inverter, and further includes:

[0083] According to the LCL type grid-connected inverter side current feedback and capacitor voltage feedback control block diagram, the open-loop transfer function G of the LCL type grid-connected inverter is calculated. open (s) is corrected to obtain the corrected open-loop transfer function G of the LCL grid-connected inverter open1 (s) to achieve resonance suppression of LCL type grid-connected inverter;

[0084] Among them, the modified open-loop transfer function G of the LCL type grid-connected inverter is open1 (s) is formula (22):

[0085]

[0086] The universal control method for LC and LCL grid-connected inverters provided by the present invention has the following advantages: on the one hand, when the filter of the inverter is switched, there is no need to modify the software algorithm, and it can be applied to the precise current control under the two filter conditions at the same time, avoiding the software update when the filter of the inverter changes, and on the other hand, it also reduces the demand for additional sensors. In order to make the control method applicable to three-phase unbalanced working conditions, the present invention adopts a D-Σ algorithm without coordinate transformation and capacitor voltage feedforward to respectively realize the precise control of current and resonance suppression. BRIEF DESCRIPTION OF THE DRAWINGS

[0087] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. Together with the following specific embodiments, they are used to explain the present invention, but do not constitute a limitation of the present invention.

[0088] Figure 1 The present invention provides a flowchart of a general control method for LC-type and LCL-type grid-connected inverters.

[0089] Figure 2 A topological diagram of a T-type three-level grid-connected inverter with an LC filter provided by the present invention.

[0090] Figure 3 A topological diagram of a T-type three-level grid-connected inverter with an LCL filter provided by the present invention.

[0091] Figure 4 A current control block diagram of a T-type three-level grid-connected inverter with an LC filter provided by the present invention.

[0092] Figure 5A current control block diagram of a T-type three-level grid-connected inverter with an LCL filter provided by the present invention.

[0093] Figure 6 A Bode diagram of a current closed-loop transfer function of a T-type three-level grid-connected inverter with an LC filter provided by the present invention.

[0094] Figure 7 The present invention provides a Bode diagram of the current open-loop transfer function of a T-type three-level grid-connected inverter with an LCL filter.

[0095] Figure 8 The invention provides a Bode diagram of a current closed-loop transfer function of a T-type three-level grid-connected inverter with an LCL filter.

[0096] Fig. 9 This is a control block diagram of the inverter side current feedback and capacitor current feedback provided by the present invention.

[0097] Fig.10 This is a control block diagram of inverter side current feedback and capacitor voltage feedback provided by the present invention.

[0098] Fig.11 The Bode diagram of the open-loop transfer function before and after correction provided by the present invention.

[0099] Fig.12 The three-phase grid-connected current waveform diagram of the LCL type grid-connected inverter under the method proposed by the present invention.

[0100] Figure 13(a) shows Fig.12 THD analysis diagram of grid-connected current before capacitor voltage correction.

[0101] Figure 13(b) shows Fig.12 THD analysis diagram of grid-connected current after capacitor voltage correction.

[0102] Fig.14 The three-phase grid-connected current waveform diagram of the LC type grid-connected inverter under the method proposed in the present invention. DETAILED DESCRIPTION

[0103] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0104] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiment of the present invention. Obviously, the described embodiment is only a part of the embodiment 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 making creative work should fall within the scope of protection of the present invention.

[0105] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so as to describe the embodiments of the present invention described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0106] In this embodiment, a general control method for LC type and LCL type grid-connected inverter is provided, such as Figure 1 As shown, the general control method of the LC type and LCL type grid-connected inverter includes:

[0107] Step S1: establishing a universal mathematical model of the AC side applicable to both LC-type grid-connected inverter and LCL-type grid-connected inverter;

[0108] Preferably, Figure 2-3 As shown, the establishment of the AC side universal mathematical model applicable to both the LC type grid-connected inverter and the LCL type grid-connected inverter also includes:

[0109] Step S11: The present invention is described by taking a T-type three-level grid-connected inverter as an example. Figure 2 The AC side filter of the LC type grid-connected inverter shown is LC type. Figure 3 The LCL type grid-connected inverter shown in the figure has an AC side filter of LCL type, U dc is the DC side voltage, C a1 and C a2 is the DC side capacitance, u Ca1 and u Ca2 Respectively represent C a1 The voltage value and C a2 Voltage value, L x and C x It forms an LC filter, L x , L xg and Cx The LCL filter is formed. x and i xg Respectively represent L x and L xg The x-phase current, e x represents the x-phase grid voltage, where x = a, b, c;

[0110] Step S12: According to Kirchhoff's voltage law, the universal mathematical model of the AC side applicable to both the LC type grid-connected inverter and the LCL type grid-connected inverter can be obtained as formula (1):

[0111]

[0112] In the formula, u xo is the output voltage of the x-phase inverter to point o; u no is the common mode voltage between the grid neutral point n and the DC side midpoint o; u Cx Represents C x Voltage value;

[0113] It can be seen from formula (1) that, whether it is an LC filter or an LCL filter, if only the inductor current and filter capacitor voltage on the inverter side are sampled, the same AC side mathematical model can be obtained.

[0114] Step S2: According to the general mathematical model of the AC side, derive the grid-connected inverter in a switching cycle T s The total duty cycle within

[0115] Preferably, according to the universal mathematical model of the AC side, it is derived that the grid-connected inverter has a switching cycle T s The total duty cycle also includes:

[0116] Under the premise of midpoint potential balance, when the x-phase modulation wave is in the positive half cycle, formula (1) is transformed into formula (2) in the [n, n+1]th switching cycle:

[0117]

[0118] Where n represents the sampling time, t Pon Indicates +U in one switching cycle dc / 2 duration, t(n+1)-t(n)=T s , T s is a switching cycle;

[0119] When the x-phase modulation wave is in the negative half cycle, formula (1) is converted into formula (3) in the [n, n+1]th switching cycle:

[0120]

[0121] In the formula, t Non Indicates one switching cycle -U dc / 2 duration;

[0122] Output voltage u of grid-connected inverter xo There are three level states: +U dc / 2, 0, and -U dc / 2, and set the duty ratios corresponding to these three levels to d xP ,d xO and d xN ;

[0123] (1) Positive half cycle of modulation wave

[0124] When the x-phase modulation wave is in the positive half cycle, the output voltage u of the grid-connected inverter xo Contains only +U dc / 2, 0 two levels, the corresponding duty cycle expression is set to formula (4):

[0125]

[0126] Where, d x represents the total duty cycle of phase x;

[0127] break down:

[0128] By decomposing the inductor current on the grid-connected inverter side, the following formula (5) can be obtained:

[0129]

[0130] In the formula, Δi xz+ Represents the current change of the x-phase inductor corresponding to the z working state in the positive half cycle of the modulation wave, z = P, O, N;

[0131] Sum:

[0132] The current changes corresponding to the three levels in formula (5) are summed to obtain the complete current change expression in the positive half cycle of the modulation wave as formula (6):

[0133]

[0134] Substituting the duty cycle set in formula (4) into formula (6), we can get formula (7):

[0135]

[0136] In the formula, Δi x+ Represents the current change corresponding to the x-phase inductance in the positive half cycle of the modulation wave;

[0137] (2) Negative half cycle of modulation wave

[0138] When the x-phase modulation wave is in the negative half cycle, the output voltage u of the grid-connected inverter xo Contains only -U dc / 2, 0 two levels, the corresponding duty cycle expression is set to formula (8):

[0139]

[0140] break down:

[0141] By decomposing the inductor current on the inverter side, the following formula (9) can be obtained:

[0142]

[0143] In the formula, Δi xz- Represents the current change of the x-phase inductor corresponding to the z working state in the negative half cycle of the modulation wave;

[0144] Sum:

[0145] The current changes corresponding to the three levels in formula (9) are summed to obtain the complete inverter side inductor current change expression in the negative half cycle of the modulation wave as formula (10):

[0146]

[0147] Substituting the duty cycle set in formula (8) into formula (10), we can obtain formula (11):

[0148]

[0149] In the formula, Δi x- Represents the current change of the inductor on the x-phase inverter side corresponding to the negative half cycle of the modulation wave;

[0150] By combining the changes in the LC filter inductor current in the positive and negative half-cycles of the modulation wave, that is, formula (7) and formula (11), we can get formula (12):

[0151] 2L x (Δi x+ +Δi x- )=2L x Δi x (12)

[0152] In the formula, Δi x Represents the current change corresponding to the x-phase inductance in one switching cycle;

[0153] In summary, a switching cycle T s The complete current change in is given by formula (13):

[0154] 2L x Δi x =U dc d x T s -2T s (u Cx +u no ) (13) According to formula (13), a switching period T can be derived s The total duty cycle expression is as follows:

[0155]

[0156] One switching cycle T s The change in the inductor current can be expressed as formula (15):

[0157] Δi x =i x (n+1)-i x (n) (15)

[0158] In the formula, Δi x Indicates the actual inductor current change of phase x, i x (n+1) represents the value of the inductor current at the sampling time n+1, i x (n) represents the value of the inductor current at sampling time n;

[0159] In order to achieve the ideal inductor current tracking effect, the actual sampled inductor current value is replaced by the reference inductor current value at the sampling time n+1. The effect in control is equivalent to the actual inductor current value tracking the reference inductor current value at the next moment. Therefore, formula (15) is converted into formula (16):

[0160] Δi x =I xref (n+1)-i x (n) (16)

[0161] In the formula, I xref (n+1) represents the reference value of the inductor current at sampling time n+1;

[0162] Applying the above conversion form of inductor current change to the inductor current change on the inverter side, we can get formula (17):

[0163]

[0164] Let K1 = 2L x / (U dcT s ) and K2=2 / U dc , the duty cycle expression can be obtained as formula (18):

[0165] d x =K1[I xref (n+1)-i x (n)]+K2(u Cx +u no ) (18).

[0166] Step S3: According to the grid-connected inverter in a switching cycle T s The total duty cycle within is obtained as follows: Figure 4 The current control block diagram of the LC type grid-connected inverter shown in Figure 5 The current control block diagram of the LCL type grid-connected inverter shown;

[0167] It should be noted that after adopting the above-mentioned general control method based on D-Σ and capacitor voltage feedforward, the following can be obtained: Figure 4 The current control block diagram of the LC type grid-connected inverter shown in Figure 5 The current control block diagram of the LCL type grid-connected inverter is shown.

[0168] Step S4: according to the current control block diagram of the LC type grid-connected inverter, a closed-loop transfer function of the LC type grid-connected inverter is obtained to achieve accurate current tracking of the LC type grid-connected inverter; and according to the current control block diagram of the LCL type grid-connected inverter, a closed-loop transfer function of the LCL type grid-connected inverter is obtained to achieve accurate current tracking of the LCL type grid-connected inverter;

[0169] Preferably, the step of obtaining a closed-loop transfer function of the LC-type grid-connected inverter according to the current control block diagram of the LC-type grid-connected inverter to achieve accurate current tracking of the LC-type grid-connected inverter further includes:

[0170] The closed-loop transfer function G1(s) of the LC-type grid-connected inverter is expressed as formula (19):

[0171]

[0172] Preferably, the step of obtaining the closed-loop transfer function of the LCL type grid-connected inverter according to the current control block diagram of the LCL type grid-connected inverter to achieve accurate current tracking of the LCL type grid-connected inverter further includes:

[0173] The closed-loop transfer function G of the LCL type grid-connected inverter close (s) is formula (20):

[0174]

[0175] Using the hardware parameters shown in Table 1, we can draw Figure 6-Figure 8 The transfer function Bode diagram is shown. Figure 6 and Figure 8 It can be seen from the figure that the method proposed in the present invention can enable the inverter to obtain a current tracking effect with higher accuracy at 50Hz. Figure 7 It can be seen that the resonance is not suppressed, and a resonance suppression method needs to be further superimposed, which is specifically described in step S5.

[0176] Table 1 Hardware parameters

[0177]

[0178] Step S5: Based on the capacitor current feedback coefficient, the current control block diagram of the LCL type grid-connected inverter is converted into a current feedback and capacitor voltage feedback control block diagram of the LCL type grid-connected inverter side; and according to the current feedback and capacitor voltage feedback control block diagram of the LCL type grid-connected inverter side, an open-loop transfer function of the LCL type grid-connected inverter is obtained to achieve resonance suppression of the LCL type grid-connected inverter.

[0179] Preferably, the step S4 further includes:

[0180] According to the current control block diagram of the LCL type grid-connected inverter, the open-loop transfer function G of the LCL type grid-connected inverter is obtained: open (s);

[0181] Among them, the open-loop transfer function G of the LCL type grid-connected inverter is open (s) is formula (21);

[0182]

[0183] Preferably, the current control block diagram of the LCL type grid-connected inverter is converted into a current feedback and capacitor voltage feedback control block diagram of the LCL type grid-connected inverter side based on the capacitor current feedback coefficient, and further includes:

[0184] Will be like Figure 5 The current control block diagram of the LCL type grid-connected inverter shown in FIG. Fig. 9 The LCL type grid-connected inverter side current feedback and capacitor current feedback control block diagram shown;

[0185] Based on the capacitor current feedback coefficient, Fig. 9 The LCL type grid-connected inverter side current feedback and capacitor current feedback control block diagram shown in FIG. Fig.10 The LCL type grid-connected inverter side current feedback and capacitor voltage feedback control block diagram is shown; wherein H1 is the capacitor current feedback coefficient.

[0186] It should be noted that the present invention only samples the inverter side current and capacitor voltage for control. In order to achieve a good resonance suppression effect, the correction coefficient of the capacitor voltage feedforward is derived according to the traditional capacitor current feedback resonance suppression method.

[0187] Preferably, the method of obtaining an open-loop transfer function of the LCL type grid-connected inverter according to the current feedback and capacitor voltage feedback control block diagram of the LCL type grid-connected inverter to achieve resonance suppression of the LCL type grid-connected inverter further includes:

[0188] According to the LCL type grid-connected inverter side current feedback and capacitor voltage feedback control block diagram, the open-loop transfer function G of the LCL type grid-connected inverter is calculated. open (s) is corrected to obtain the corrected open-loop transfer function G of the LCL grid-connected inverter open1 (s) to achieve resonance suppression of LCL type grid-connected inverter;

[0189] Among them, the modified open-loop transfer function G of the LCL type grid-connected inverter is open1 (s) is formula (22):

[0190]

[0191] from Fig.11 It can be seen that after converting the capacitor current feedback into capacitor voltage feedback, the peak of the amplitude-frequency characteristic curve after the correction of the capacitor voltage feedforward coefficient is suppressed below 0 dB at the resonant frequency of 1218 Hz, and the resonance is suppressed.

[0192] Fig.12 This is a simulation waveform of the method proposed in the present invention in the LCL type grid-connected inverter, such as Fig.12 As shown, before 0.225s is the current waveform before the capacitor voltage feedforward coefficient is corrected, and Figure 13(a) is its corresponding current THD analysis diagram. After 0.225s, it is the current waveform after the capacitor current feedforward coefficient is corrected, and Figure 13(b) is its corresponding current THD analysis diagram. It can be seen from the waveform and its THD analysis diagram that under the D-Σ algorithm and capacitor voltage feedforward method proposed in the present invention, regardless of whether there is resonance, the grid-connected current can be stabilized at 10A, but the resonance is not suppressed before the capacitor voltage feedforward coefficient is corrected. It can be seen from its THD diagram that the harmonics at the resonant frequency of 1218Hz are relatively large. After the capacitor voltage feedforward coefficient is corrected, the resonance is completely suppressed, and the grid-connected current THD is less than 5%. In the LC type grid-connected inverter, such as Fig.14 As shown, the method proposed by the present invention can also stabilize the grid-connected current at 10 A. The present invention can simultaneously meet the control requirements of the grid-connected inverter under two filters.

[0193] It should be noted that in order to solve the problem of non-universal control algorithms when the filter type of the grid-connected inverter is converted in actual projects, according to the sensor settings of the actual grid-connected inverter, the present invention uses the D-Σ algorithm and capacitor voltage feedforward to realize universal control of LC type and LCL type grid-connected inverters, the inverter side inductor current feedback to realize accurate tracking of the grid-connected current, and the capacitor voltage feedforward to realize resonance suppression.

[0194] In an embodiment of the present invention, the inverter-side inductor current feedback and capacitor voltage feedback are used to implement a control algorithm that satisfies the switching of two types of filters. The D-Σ algorithm is derived to achieve precise tracking of the grid-connected current. Resonance suppression can be achieved by correcting the capacitor voltage feedforward coefficient, thereby achieving the universality of a control algorithm when the filters are different.

[0195] Therefore, the present invention proposes a universal control method for LC and LCL grid-connected inverters. On the one hand, when the filter of the inverter is switched, there is no need to modify the software algorithm, and it can be applied to the precise current control under the two filter conditions at the same time, avoiding the software update when the filter of the inverter changes, and on the other hand, it also reduces the demand for additional sensors. In order to make the control method applicable to three-phase unbalanced working conditions, the present invention adopts a D-Σ algorithm without coordinate transformation and capacitor voltage feedforward to respectively realize the precise control of current and resonance suppression.

[0196] It is to be understood that the above embodiments are merely exemplary embodiments used to illustrate the principles of the present invention, but the present invention is not limited thereto. For those of ordinary skill in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also considered to be within the scope of protection of the present invention.

Claims

1. A general control method for LC and LCL grid-connected inverters, characterized in that: The general control method of the LC type and LCL type grid-connected inverter includes: Step S1: establishing a universal mathematical model of the AC side applicable to both LC-type grid-connected inverter and LCL-type grid-connected inverter; Step S2: According to the general mathematical model of the AC side, derive the grid-connected inverter in a switching cycle T s The total duty cycle within Step S3: According to the grid-connected inverter in a switching cycle T s The total duty cycle in is obtained, and the current control block diagram of the LC type grid-connected inverter and the current control block diagram of the LCL type grid-connected inverter are obtained respectively; Step S4: according to the current control block diagram of the LC type grid-connected inverter, a closed-loop transfer function of the LC type grid-connected inverter is obtained to achieve accurate current tracking of the LC type grid-connected inverter; and according to the current control block diagram of the LCL type grid-connected inverter, a closed-loop transfer function of the LCL type grid-connected inverter is obtained to achieve accurate current tracking of the LCL type grid-connected inverter; Step S5: based on the capacitor current feedback coefficient, converting the current control block diagram of the LCL type grid-connected inverter into a current feedback and capacitor voltage feedback control block diagram of the LCL type grid-connected inverter side; and obtaining an open-loop transfer function of the LCL type grid-connected inverter according to the current feedback and capacitor voltage feedback control block diagram of the LCL type grid-connected inverter side, so as to realize resonance suppression of the LCL type grid-connected inverter; The establishment of a universal mathematical model for the AC side applicable to both the LC type grid-connected inverter and the LCL type grid-connected inverter also includes: Step S11: The AC side filter of the LC type grid-connected inverter is LC type, the AC side filter of the LCL type grid-connected inverter is LCL type, U dc is the DC side voltage, C a1 and C a2 is the DC side capacitance, u Ca1 and u Ca2 Respectively represent C a1 The voltage value and C a2 Voltage value, L x and C x It forms an LC filter, L x , L xg and C x The LCL filter is formed. x and i xg Respectively represent L x and L xg The x-phase current, e x represents the x-phase grid voltage, where x = a, b, c; Step S12: According to Kirchhoff's voltage law, the universal mathematical model of the AC side applicable to both the LC type grid-connected inverter and the LCL type grid-connected inverter can be obtained as formula (1): In the formula, u xo is the output voltage of the x-phase inverter to point o; u no is the common mode voltage between the grid neutral point n and the DC side midpoint o; u Cx Represents C x Voltage value.

2. The universal control method for LC type and LCL type grid-connected inverter according to claim 1, characterized in that: According to the general mathematical model of the AC side, it is deduced that the grid-connected inverter has a switching cycle T s The total duty cycle also includes: Under the premise of midpoint potential balance, when the x-phase modulation wave is in the positive half cycle, formula (1) is transformed into formula (2) in the [n, n+1]th switching cycle: Where n represents the sampling time, t Pon Indicates +U in one switching cycle dc / 2 duration, t(n+1)-t(n)=T s , T s is a switching cycle; When the x-phase modulation wave is in the negative half cycle, formula (1) is converted into formula (3) in the [n, n+1]th switching cycle: Where, t Non Indicates one switching cycle -U dc / 2 duration; Output voltage u of grid-connected inverter xo There are three level states: +U dc / 2, 0, and -U dc / 2, and set the duty ratios corresponding to these three levels to d xP ,d xO and d xN ; (1) Positive half cycle of modulation wave When the x-phase modulation wave is in the positive half cycle, the output voltage u of the grid-connected inverter xo Contains only +U dc / 2, 0 two levels, the corresponding duty cycle expression is set to formula (4): Where, d x represents the total duty cycle of phase x; break down: By decomposing the inductor current on the grid-connected inverter side, the following formula (5) can be obtained: In the formula, Δi xz+ Represents the current change of the x-phase inductor corresponding to the z working state in the positive half cycle of the modulation wave, z = P, O, N; Sum: The current changes corresponding to the three levels in formula (5) are summed to obtain the complete current change expression in the positive half cycle of the modulation wave as formula (6): Substituting the duty cycle set in formula (4) into formula (6), we can get formula (7): In the formula, Δi x+ Represents the current change corresponding to the x-phase inductance in the positive half cycle of the modulation wave; (2) Negative half cycle of modulation wave When the x-phase modulation wave is in the negative half cycle, the output voltage u of the grid-connected inverter xo Contains only -U dc / 2, 0 two levels, the corresponding duty cycle expression is set to formula (8): break down: By decomposing the inductor current on the inverter side, the following formula (9) can be obtained: In the formula, Δi xz- Represents the current change of the x-phase inductor corresponding to the z working state in the negative half cycle of the modulation wave; Sum: The current changes corresponding to the three levels in formula (9) are summed to obtain the complete inverter side inductor current change expression in the negative half cycle of the modulation wave as formula (10): Substituting the duty cycle set in formula (8) into formula (10), we can obtain formula (11): In the formula, Δi x- Represents the current change of the inductor on the x-phase inverter side corresponding to the negative half cycle of the modulation wave; By combining the changes in the LC filter inductor current in the positive and negative half-cycles of the modulation wave, that is, formula (7) and formula (11), we can get formula (12): 2L x (Δi x+ +Δi x- )=2L x Δi x (12) In the formula, Δi x Represents the current change corresponding to the x-phase inductance in one switching cycle; In summary, a switching cycle T s The complete current change in is given by formula (13): 2L x Δi x =U dc d x T s -2T s (u Cx +u no ) (13) According to formula (13), a switching period T can be derived s The total duty cycle is expressed as follows: One switching cycle T s The change in the inductor current can be expressed as formula (15): Δi x =i x (n+1)-i x (n) (15) In the formula, Δi x Indicates the actual inductor current change of phase x, i x (n+1) represents the value of the inductor current at the sampling time n+1, i x (n) represents the value of the inductor current at sampling time n; In order to achieve the ideal inductor current tracking effect, the actual sampled inductor current value is replaced by the reference inductor current value at the sampling time n+1. The effect in control is equivalent to the actual inductor current value tracking the reference inductor current value at the next moment. Therefore, formula (15) is converted into formula (16): Δi x =I xref (n+1)-i x (n) (16) In the formula, I xref (n+1) represents the reference value of the inductor current at sampling time n+1; Applying the above conversion form of inductor current change to the inductor current change on the inverter side, we can get formula (17): Let K1 = 2L x / (U dc T s ) and K2=2 / U dc , the duty cycle expression can be obtained as formula (18): d x =K1[I xref (n+1)-i x (n)]+K2(u Cx +u no ) (18)。 3. The universal control method for LC type and LCL type grid-connected inverter according to claim 2, characterized in that: The method of obtaining a closed-loop transfer function of the LC-type grid-connected inverter according to the current control block diagram of the LC-type grid-connected inverter to achieve accurate current tracking of the LC-type grid-connected inverter further includes: The closed-loop transfer function G1(s) of the LC-type grid-connected inverter is expressed as formula (19):

4. The universal control method for LC type and LCL type grid-connected inverter according to claim 2, characterized in that: The method of obtaining a closed-loop transfer function of the LCL type grid-connected inverter according to the current control block diagram of the LCL type grid-connected inverter to achieve accurate current tracking of the LCL type grid-connected inverter further includes: The closed-loop transfer function G of the LCL type grid-connected inverter close (s) is formula (20):

5. The universal control method for LC type and LCL type grid-connected inverter according to claim 2, characterized in that: The step S4 further includes: According to the current control block diagram of the LCL type grid-connected inverter, the open-loop transfer function G of the LCL type grid-connected inverter is obtained: open (s); Among them, the open-loop transfer function G of the LCL type grid-connected inverter is open (s) is formula (21); 6. The universal control method for LC type and LCL type grid-connected inverter according to claim 5, characterized in that: The method of converting the current control block diagram of the LCL type grid-connected inverter into a current feedback and capacitor voltage feedback control block diagram of the LCL type grid-connected inverter based on the capacitor current feedback coefficient also includes: Converting the current control block diagram of the LCL type grid-connected inverter into a current feedback and capacitor current feedback control block diagram of the LCL type grid-connected inverter side; Based on the capacitor current feedback coefficient, the LCL type grid-connected inverter side current feedback and capacitor current feedback control block diagram are converted into the LCL type grid-connected inverter side current feedback and capacitor voltage feedback control block diagram.

7. A universal control method for LC type and LCL type grid-connected inverters according to claim 6, characterized in that: The method further comprises obtaining an open-loop transfer function of the LCL type grid-connected inverter according to the current feedback and capacitor voltage feedback control block diagram of the LCL type grid-connected inverter to achieve resonance suppression of the LCL type grid-connected inverter: According to the LCL type grid-connected inverter side current feedback and capacitor voltage feedback control block diagram, the open-loop transfer function G of the LCL type grid-connected inverter is calculated. open (s) is corrected to obtain the corrected open-loop transfer function G of the LCL grid-connected inverter open1 (s) to achieve resonance suppression of LCL type grid-connected inverter; Among them, the modified open-loop transfer function G of the LCL type grid-connected inverter is open1 (s) is formula (22):

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