Three-level photovoltaic grid-connected inverter control method based on I-D-sigma algorithm

Abstract

The invention discloses a three-level photovoltaic grid-connected inverter control method based on an I-D-sigma algorithm. The method comprises the following steps: (1) constructing an LCL type three-level grid-connected inverter mathematical model according to a T type three-level photovoltaic grid-connected inverter; (2) according to the LCL type three-level grid-connected inverter mathematical model, calculating a duty ratio expression giving consideration to grid-connected current tracking, neutral-point potential balance, resonance suppression and third-order common-mode voltage suppression at the same time; (3) calculating according to the duty ratio expression to obtain a third-order common-mode voltage; and (4) substituting the third-order common-mode voltage in the step (3) into the duty ratio expression in the step (2) again, and further controlling the three-level photovoltaic grid-connected inverter through grid-connected current tracking, neutral-point potential balance, resonance suppression and third-order common-mode voltage suppression. According to the method, grid-connected current tracking, neutral-point potential balance and resonance suppression are considered, the influence of the triple frequency component in the common-mode voltage is considered, and integrated control over the LCL type three-level photovoltaic grid-connected inverter can be achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics, and in particular to a three-level photovoltaic grid-connected inverter control method based on an ID-Σ algorithm. Background Art

[0002] As an important energy conversion device in photovoltaic power generation system, the performance improvement of photovoltaic grid-connected inverter is crucial to the power generation efficiency and power quality of the entire photovoltaic power generation system.

[0003] At present, the three-level inverter is one of the most common topologies of photovoltaic grid-connected inverters due to its higher grid-connected power quality and efficiency. The LCL filter has higher harmonic filtering capability and lower volume, and is one of the first filters for photovoltaic grid-connected inverters. The LCL type three-level photovoltaic grid-connected inverter has many control targets, such as grid-connected current control, midpoint potential balance, resonance suppression, and common-mode voltage suppression. In the existing control technology, each control target is usually controlled separately, such as PI or PR control of grid-connected current, zero-sequence injection or optimized modulation control of midpoint potential, common capacitor current feedback method for resonance suppression, and optimized modulation strategy method for common-mode voltage. Although there are effective solutions for each control target in the existing control technology, there are relatively few comprehensive control methods for multiple targets. In the existing control technology, the impact of the triple frequency component in the common-mode voltage on the control technology has not been solved.

[0004] The existing control technologies for photovoltaic grid-connected inverters have effective control methods for their control targets, but there are few technologies that integrate multiple control targets into one control algorithm. Model predictive control can achieve integrated control of multiple targets, but for the midpoint clamped three-level grid-connected inverter, its common mode voltage lacks closed-loop control technology due to the lack of sensors.

[0005] Generally speaking, it can be suppressed by improved modulation strategies and hardware isolation methods. Therefore, model predictive control is often used with improved modulation strategies to suppress the amplitude of common-mode voltage. The principle is to select a vector with a smaller common-mode voltage during the switching cycle. However, the third-order common-mode voltage caused by the triple frequency component in the modulation wave of the mid-point clamped three-level grid-connected inverter is often ignored. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a three-level photovoltaic grid-connected inverter control method based on the ID-Σ algorithm, which takes into account grid-connected current tracking, mid-point potential balance, and resonance suppression, while considering the influence of the triple frequency component in the common-mode voltage, and can realize the integrated control of the LCL type three-level photovoltaic grid-connected inverter.

[0007] In order to solve the above technical problems, the present invention provides a three-level photovoltaic grid-connected inverter control method based on ID-Σ algorithm, comprising the following steps:

[0008] Step 1: construct a mathematical model of an LCL type three-level grid-connected inverter based on a T-type three-level photovoltaic grid-connected inverter;

[0009] Step 2, according to the mathematical model of the LCL type three-level grid-connected inverter, calculate the duty cycle expression that takes into account grid-connected current tracking, midpoint potential balance, resonance suppression, and third-order common mode voltage suppression;

[0010] Step 3, calculating the third-order common-mode voltage according to the duty cycle expression obtained in step 2;

[0011] Step 4: Substitute the third-order common-mode voltage in step 3 back into the duty cycle expression in step 2, and then control the three-level photovoltaic grid-connected inverter through grid-connected current tracking, midpoint potential balance, resonance suppression, and third-order common-mode voltage suppression.

[0012] Furthermore, the specific steps of constructing the mathematical model of the LCL type three-level grid-connected inverter in step 1 are as follows:

[0013] Step 1.1: The AC side filter in the T-type three-level photovoltaic 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 their voltage values; L xi , L xg and C x It is a component of the LCL filter; xi and i xg Respectively represent L xi and L xg The x-phase current; U PCCx Represents the x-phase grid voltage; where x = a, b, c;

[0014] Step 1.2: According to Kirchhoff's voltage law, the mathematical model of the AC side is obtained as Formula 1:

[0015]

[0016] Among them, u xo is the output voltage of the x-phase inverter at point o, u no is the common mode voltage between point o and point n;

[0017] Step 1.3, assuming that a virtual zero potential point o1 is at the midpoint of the DC side, the above formula can be converted into Formula 2:

[0018]

[0019] in, is the output voltage of the x-phase inverter to point o1, which refers to the output voltage when the midpoint potential is balanced; is the voltage between point o1 and point o;

[0020] Based on the above assumptions, the voltage expression between point o1 and point o is obtained as formula 3:

[0021]

[0022] Furthermore, the specific steps of calculating the duty cycle expression taking into account grid current tracking, midpoint potential balance, resonance suppression, and third-order common mode voltage suppression in step 2 are as follows:

[0023] Step 2.1: Decompose and sum the inductor current in the positive and negative half cycles of a complete switching cycle, and substitute Formula 3 in Step 1.3 into Formula 2 to obtain Formula 4:

[0024]

[0025] Where n is the sampling time, t Pon Refers to +U dc / 2 action time; t Non Refers to -U dc / 2 action time;

[0026] Step 2.2: The output voltage of the T-type three-level inverter contains 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 ;

[0027] When the modulation wave is in the positive half cycle, the output voltage of the T-type three-level inverter contains only +U dc / 2, 0 two levels, the corresponding duty cycle expression is set to formula 5:

[0028]

[0029] Step 2.3: The inductance on both sides of the LCL filter is decomposed at the same time to obtain the following formula 6:

[0030]

[0031] Among them, Δi xyz+Represents the current change of the inductor on the y side of the x phase corresponding to the z working state in the positive half cycle of the modulation wave; y = i (inverter, inverter), g (grid, grid); z = P, O, N;

[0032] The current changes corresponding to the three levels in formula 6 are summed to obtain the complete current change expression in the positive half cycle of the modulation wave, which is formula 7:

[0033]

[0034] Among them, Δi xy+ Represents the current change corresponding to the inductance on the y side of the x-phase in the positive half cycle of the modulation wave;

[0035] Substituting the duty cycle set in Formula 5 into Formula 7, we get Formula 8:

[0036]

[0037] When the modulation wave is in the negative half cycle, the output voltage of the T-type three-level inverter contains only -U dc / 2, 0 two levels, the corresponding duty cycle expression is set to formula nine:

[0038]

[0039] Step 2.4: Apply the decomposition process to the inductors on both sides of the LCL filter at the same time to obtain Formula 10:

[0040]

[0041] Among them, Δi xyz- Represents the current change of the inductor on the y side of the x phase corresponding to the z working state in the negative half cycle of the modulation wave; y = i (inverter, inverter), g (grid, grid); z = P, O, N;

[0042] Step 2.5, sum the current changes corresponding to the three levels in Formula 10, and obtain the complete current change expression in the positive half cycle of the modulation wave as Formula 11:

[0043]

[0044] Among them, Δi xy- Represents the current change corresponding to the inductance on the y side of the x phase in the negative half cycle of the modulation wave;

[0045] Substituting the duty cycle set in Formula 9 into Formula 11, we get Formula 12:

[0046]

[0047] By synthesizing the changes in the inductor current on both sides of the LCL filter where the modulation wave is located in the positive and negative half-cycles, that is, Formula 8 and Formula 12, we get Formula 13:

[0048] 2L xg Δi xg +2L xi Δi xi =U dc d x T s +2[-(u C1 -u C2 )+u on ]T s -2U PCCx T s

[0049] According to formula 13, a switching period T is derived s The internal total duty cycle expression, that is, Formula 14, is as follows:

[0050]

[0051] Apply the conversion form of the inductor current change to the inductor current change on both sides of the LCL filter, and get Formula 15:

[0052]

[0053] Among them, I xgref (n+1) and I xiref (n+1) are the reference values ​​of the grid-connected current and the inverter-side inductor current at the sampling time n+1; i xg (n) and i xi (n) are the actual sampling values ​​of the grid-connected current and the inverter-side inductor current at sampling time n.

[0054] Furthermore, the specific steps of calculating the third-order common-mode voltage in step 3 are as follows:

[0055] According to formula 15, the expression of common mode voltage can be obtained as formula 16:

[0056]

[0057] Therefore, the common mode voltage expression at the first two sampling moments can be obtained as Formula 17:

[0058]

[0059] The calculations according to Formula 16 and Formula 17 are based on the basic signals of the state variables, so the estimated common-mode voltage is only three times the fundamental frequency component; since the switching frequency is much higher than the fundamental frequency and three times the fundamental frequency, that is, the common-mode voltage changes linearly within the switching cycle; the final estimated common-mode voltage expression is Formula 18:

[0060] u on1 (n) = 2u on (n-1)-u on (n-2).

[0061] Furthermore, the specific steps of controlling the three-level photovoltaic grid-connected inverter through grid-connected current tracking, midpoint potential balance, resonance suppression, and third-order common-mode voltage suppression in step 4 are as follows:

[0062] Substituting the common-mode voltage estimated in step 3 into equation 15, we get the final duty cycle expression; It is the inverter side current feedback control, which is related to resonance suppression; It is grid-connected current feedback control, which is related to grid-connected current tracking; It is grid voltage feedforward to reduce grid impact; It is the midpoint potential difference feedback control, which is related to the midpoint potential balance control; is the common-mode voltage feedback and is related to low-frequency common-mode voltage rejection.

[0063] The beneficial effects of the present invention are:

[0064] 1. The present invention realizes the integrated control of grid-connected current tracking, midpoint potential balance, resonance suppression, and triple frequency common mode voltage in a three-level photovoltaic grid-connected inverter through the duty cycle expression in step 2, thereby solving the problem of separate control of multiple control targets in the existing control method.

[0065] 2. The present invention uses the duty cycle expression in step 3 and the linear change relationship of the common-mode voltage at the previous two moments to obtain the value of the common-mode voltage at the current moment, and then feeds it back to the duty cycle expression to achieve the suppression of low-frequency common-mode voltage, solving the problem of sensorless common-mode voltage estimation. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] Figure 1 is a flow chart of the present invention;

[0067] Figure 2 It is a topological diagram of an LCL type T-type three-level grid-connected inverter in a specific embodiment;

[0068] Figure 3 A grid-connected current waveform diagram under balanced grid conditions in a specific embodiment;

[0069] Figure 4It is a waveform diagram of the startup performance of the grid-connected current control based on the DD-Σ method in a specific embodiment;

[0070] Figure 5 It is a diagram showing the effect of NPP balance control under different control methods in a specific embodiment;

[0071] Figure 6 It is a comparative waveform diagram before and after using the ID-Σ method considering the third-order CMV in a specific embodiment. DETAILED DESCRIPTION

[0072] like Figure 1 As shown, the present invention provides a three-level photovoltaic grid-connected inverter control method based on ID-Σ algorithm, comprising the following steps:

[0073] Step 1: construct a mathematical model of an LCL type three-level grid-connected inverter based on a T-type three-level photovoltaic grid-connected inverter;

[0074] Step 2, according to the mathematical model of the LCL type three-level grid-connected inverter, calculate the duty cycle expression that takes into account grid-connected current tracking, midpoint potential balance, resonance suppression, and third-order common mode voltage suppression;

[0075] Step 3, calculating the third-order common-mode voltage according to the duty cycle expression obtained in step 2;

[0076] Step 4: Substitute the third-order common-mode voltage in step 3 back into the duty cycle expression in step 2, and then control the three-level photovoltaic grid-connected inverter through grid-connected current tracking, midpoint potential balance, resonance suppression, and third-order common-mode voltage suppression.

[0077] Furthermore, the specific steps of constructing the mathematical model of the LCL type three-level grid-connected inverter in step 1 are as follows:

[0078] Step 1.1: Figure 2 The AC side filter in the T-type three-level photovoltaic grid-connected inverter shown is an 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 their voltage values; L xi , L xg and C x It is a component of the LCL filter; xi and i xg Respectively represent L xi and L xg The x-phase current; U PCCx Represents the x-phase grid voltage; where x = a, b, c;

[0079] Step 1.2: According to Kirchhoff's voltage law, the mathematical model of the AC side is obtained as Formula 1:

[0080]

[0081] Among them, u xo is the output voltage of the x-phase inverter at point o, u no is the common mode voltage between point o and point n;

[0082] Step 1.3, assuming that a virtual zero potential point o1 is at the midpoint of the DC side, the above formula can be converted into Formula 2:

[0083]

[0084] in, is the output voltage of the x-phase inverter to point o1, which refers to the output voltage when the midpoint potential is balanced; is the voltage between point o1 and point o;

[0085] Based on the above assumptions, the voltage expression between point o1 and point o is obtained as formula 3:

[0086]

[0087] Furthermore, the specific steps of calculating the duty cycle expression taking into account grid current tracking, midpoint potential balance, resonance suppression, and third-order common mode voltage suppression in step 2 are as follows:

[0088] Step 2.1: Decompose and sum the inductor current in the positive and negative half cycles of a complete switching cycle, and substitute Formula 3 in Step 1.3 into Formula 2 to obtain Formula 4:

[0089]

[0090] Where n is the sampling time, t Pon Refers to +U dc / 2 action time; t Non Refers to -U dc / 2 action time;

[0091] Step 2.2: The output voltage of the T-type three-level inverter contains 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 ;

[0092] When the modulation wave is in the positive half cycle, the output voltage of the T-type three-level inverter contains only +Udc / 2, 0 two levels, the corresponding duty cycle expression is set to formula 5:

[0093]

[0094] Step 2.3: The inductance on both sides of the LCL filter is decomposed at the same time to obtain the following formula 6:

[0095]

[0096] Among them, Δi xyz+ Represents the current change of the inductor on the y side of the x phase corresponding to the z working state in the positive half cycle of the modulation wave; y = i (inverter, inverter), g (grid, grid); z = P, O, N;

[0097] The current changes corresponding to the three levels in formula 6 are summed to obtain the complete current change expression in the positive half cycle of the modulation wave, which is formula 7:

[0098]

[0099] Among them, Δi xy+ Represents the current change corresponding to the inductance on the y side of the x-phase in the positive half cycle of the modulation wave;

[0100] Substituting the duty cycle set in Formula 5 into Formula 7, we get Formula 8:

[0101]

[0102] When the modulation wave is in the negative half cycle, the output voltage of the T-type three-level inverter contains only -U dc / 2, 0 two levels, the corresponding duty cycle expression is set to formula nine:

[0103]

[0104] Step 2.4: Apply the decomposition process to the inductors on both sides of the LCL filter at the same time to obtain Formula 10:

[0105]

[0106] Among them, Δi xyz- Represents the current change of the inductor on the y side of the x phase corresponding to the z working state in the negative half cycle of the modulation wave; y = i (inverter, inverter), g (grid, grid); z = P, O, N;

[0107] Step 2.5, sum the current changes corresponding to the three levels in Formula 10, and obtain the complete current change expression in the positive half cycle of the modulation wave as Formula 11:

[0108]

[0109] Among them, Δi xy- Represents the current change corresponding to the inductance on the y side of the x phase in the negative half cycle of the modulation wave;

[0110] Substituting the duty cycle set in Formula 9 into Formula 11, we get Formula 12:

[0111]

[0112] By synthesizing the changes in the inductor current on both sides of the LCL filter where the modulation wave is located in the positive and negative half-cycles, that is, Formula 8 and Formula 12, we get Formula 13:

[0113] 2L xg Δi xg +2L xi Δi xi =U dc d x T s +2[-(u C1 -u C2 )+u on ]T s -2U PCCx T s

[0114] According to formula 13, a switching period T is derived s The internal total duty cycle expression, that is, Formula 14, is as follows:

[0115]

[0116] Apply the conversion form of the inductor current change to the inductor current change on both sides of the LCL filter, and get Formula 15:

[0117]

[0118] Among them, I xgref (n+1) and I xiref (n+1) are the reference values ​​of the grid-connected current and the inverter-side inductor current at the sampling time n+1; i xg (n) and i xi (n) are the actual sampling values ​​of the grid-connected current and the inverter-side inductor current at sampling time n.

[0119] Furthermore, the specific steps of calculating the third-order common-mode voltage in step 3 are as follows:

[0120] According to formula 15, the expression of common mode voltage can be obtained as formula 16:

[0121]

[0122] Therefore, the common mode voltage expression at the first two sampling moments can be obtained as Formula 17:

[0123]

[0124] The calculations according to Formula 16 and Formula 17 are based on the basic signals of the state variables, so the estimated common-mode voltage is only three times the fundamental frequency component; since the switching frequency is much higher than the fundamental frequency and three times the fundamental frequency, that is, the common-mode voltage changes linearly within the switching cycle; the final estimated common-mode voltage expression is Formula 18:

[0125] u on1 (n) = 2u on (n-1)-u on (n-2).

[0126] Furthermore, the specific steps of controlling the three-level photovoltaic grid-connected inverter through grid-connected current tracking, midpoint potential balance, resonance suppression, and third-order common-mode voltage suppression in step 4 are as follows:

[0127] Substituting the common-mode voltage estimated in step 3 into equation 15, we get the final duty cycle expression; It is the inverter side current feedback control, which is related to resonance suppression; It is grid-connected current feedback control, which is related to grid-connected current tracking; It is grid voltage feedforward to reduce grid impact; It is the midpoint potential difference feedback control, which is related to the midpoint potential balance control; is the common-mode voltage feedback and is related to low-frequency common-mode voltage rejection.

[0128] like Figure 3 The figure shows the control effect of DD-Σ digital control under balanced power grid. (a) is the DC side capacitor voltage u C1 The three-phase voltage of the PCC and the DC side capacitor voltage are 100V, then the neutral point potential is balanced. (b) in the figure is the three-phase grid-connected current. It can be seen from the figure that the grid-connected current is a sinusoidal symmetrical current, which verifies the feasibility of the DD-Σ digital control inverter designed by the present invention. (c) in the figure is the total harmonic distortion (THD) of the a-phase current, in which the total harmonic distortion (THD) is less than 3%, meeting the grid-connected requirements. At the same time, the grid-connected current and the corresponding grid voltage are in phase.

[0129] like Figure 4The figure shows the starting performance of the grid-connected current control based on the DD-Σ method, where (a) is soft start, (b) is current mutation, and (c) is the local diagram of current mutation. The starting performance uses the soft start mode to slowly change the current from 0A to 5A. At the same time, the current change process from 5A to 10A uses the current mutation mode, as shown in (b) and (c). The current mutation process time is very short, only 1.5ms.

[0130] like Figure 5 Shown is the NPP balance control effect under different control methods. Figure 3 (a) adopts the ID-Σ method proposed in this paper, while Figure 3 (b) uses the DD-Σ method with zero-sequence injection. Figure 3 The experimental process in did not consider the influence of the third-order CMV. During the test, by changing the reference value of the NPP difference, it can be seen that the two methods have the same control effect. However, when the NPP fluctuates, it will cause a spike in the CMV. It has also been proven that the third-order CMV is related to the components of the NPP balance control. NPP imbalance will increase the amplitude of the third-order CMV.

[0131] like Figure 6 The figure shows the comparison waveforms before and after using the ID-Σ method considering the third-order CMV, where (a) does not consider the third-order CMV, and (b) considers the third-order CMV. It can be seen from the figure that the proposed third-order CMV estimation method can accurately estimate the CMV, which is achieved through the DA conversion function of the controller. The comparative experiment proves that the ID-Σ method proposed in this paper can effectively reduce the third-order CMV caused by the presence of the third-order component in the modulated wave. Figure 5 and Figure 6 It can be proved that the ID-Σ method has stronger comprehensive characteristics than the DD-Σ method. Under the NPP equilibrium equivalent control condition, the third-order CMV is also suppressed.

Claims

1. A three-level photovoltaic grid-connected inverter control method based on ID-Σ algorithm, characterized in that: The steps include: Step 1: construct a mathematical model of an LCL type three-level grid-connected inverter based on a T-type three-level photovoltaic grid-connected inverter; Step 2, according to the mathematical model of the LCL type three-level grid-connected inverter, calculate the duty cycle expression that takes into account grid-connected current tracking, midpoint potential balance, resonance suppression, and third-order common mode voltage suppression; Step 3, calculating the third-order common-mode voltage according to the duty cycle expression obtained in step 2; Step 4: Substitute the third-order common-mode voltage in step 3 back into the duty cycle expression in step 2, and then control the three-level photovoltaic grid-connected inverter through grid-connected current tracking, midpoint potential balance, resonance suppression, and third-order common-mode voltage suppression.

2. A three-level photovoltaic grid-connected inverter control method based on ID-Σ algorithm according to claim 1, characterized in that: The specific steps of constructing the mathematical model of the LCL type three-level grid-connected inverter in step 1 are as follows: Step 1.1: The AC side filter in the T-type three-level photovoltaic 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 their voltage values; L xi , L xg and C x It is a component of the LCL filter; xi and i xg Respectively represent L xi and L xg The x-phase current; U PCCx Represents the x-phase grid voltage; where x = a, b, c; Step 1.2: According to Kirchhoff's voltage law, the mathematical model of the AC side is obtained as Formula 1: Among them, u xo is the output voltage of the x-phase inverter at point o, u no is the common mode voltage between point o and point n; Step 1.3, assuming that a virtual zero potential point o1 is at the midpoint of the DC side, the above formula can be converted into Formula 2: in, is the output voltage of the x-phase inverter to point o1, which refers to the output voltage when the midpoint potential is balanced; is the voltage between point o1 and point o; Based on the above assumptions, the voltage expression between point o1 and point o is obtained as formula 3:

3. A three-level photovoltaic grid-connected inverter control method based on ID-Σ algorithm according to claim 2, characterized in that: The specific steps of calculating the duty cycle expression taking into account grid-connected current tracking, midpoint potential balance, resonance suppression, and third-order common-mode voltage suppression in step 2 are as follows: Step 2.1: Decompose and sum the inductor current in the positive and negative half cycles of a complete switching cycle, and substitute Formula 3 in Step 1.3 into Formula 2 to obtain Formula 4: Where n is the sampling time, t Pon Refers to +U dc / 2 action time; t Non Refers to -U dc / 2 action time; Step 2.2: The output voltage of the T-type three-level inverter contains 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 ; When the modulation wave is in the positive half cycle, the output voltage of the T-type three-level inverter contains only +U dc / 2, 0 two levels, the corresponding duty cycle expression is set to formula 5: Step 2.3: The inductance on both sides of the LCL filter is decomposed at the same time to obtain the following formula 6: Among them, Δi xyz+ Represents the current change of the inductor on the y side of the x phase corresponding to the z working state in the positive half cycle of the modulation wave; y = i (inverter, inverter), g (grid, grid); z = P, O, N; The current changes corresponding to the three levels in Formula 6 are summed to obtain the complete current change expression in the positive half cycle of the modulation wave as Formula 7: Among them, Δi xy+ Represents the current change corresponding to the inductance on the y side of the x-phase in the positive half cycle of the modulation wave; Substituting the duty cycle set in Formula 5 into Formula 7, we get Formula 8: When the modulation wave is in the negative half cycle, the output voltage of the T-type three-level inverter contains only -U dc / 2, 0 two levels, the corresponding duty cycle expression is set to formula nine: Step 2.4: Apply the decomposition process to the inductors on both sides of the LCL filter at the same time to obtain Formula 10: Among them, Δi xyz- Represents the current change of the inductor on the y side of the x phase corresponding to the z working state in the negative half cycle of the modulation wave; y = i (inverter, inverter), g (grid, grid); z = P, O, N; Step 2.5, sum the current changes corresponding to the three levels in Formula 10, and obtain the complete current change expression in the positive half cycle of the modulation wave as Formula 11: Among them, Δi xy- Represents the current change corresponding to the inductance on the y side of the x phase in the negative half cycle of the modulation wave; Substituting the duty cycle set in Formula 9 into Formula 11, we get Formula 12: By synthesizing the changes in the inductor current on both sides of the LCL filter where the modulation wave is located in the positive and negative half-cycles, that is, Formula 8 and Formula 12, we get Formula 13: 2L xg Δi xg +2L xi Δi xi =U dc d x T s +2[-(u C1 -u C2 )+u on ]T s -2U PCCx T s According to formula 13, a switching period T is derived s The internal total duty cycle expression, that is, Formula 14, is as follows: Apply the conversion form of the inductor current change to the inductor current change on both sides of the LCL filter, and get Formula 15: Among them, I xgref (n+1) and I xiref (n+1) are the reference values ​​of the grid-connected current and the inverter-side inductor current at the sampling time n+1; i xg (n) and i xi (n) are the actual sampling values ​​of the grid-connected current and the inverter-side inductor current at sampling time n.

4. A three-level photovoltaic grid-connected inverter control method based on ID-Σ algorithm according to claim 3, characterized in that: The specific steps of calculating the third-order common-mode voltage in step 3 are as follows: According to formula 15, the expression of common mode voltage can be obtained as formula 16: Therefore, the common mode voltage expression at the first two sampling moments can be obtained as Formula 17: The calculations according to Formula 16 and Formula 17 are based on the basic signals of the state variables, so the estimated common-mode voltage is only three times the fundamental frequency component; since the switching frequency is much higher than the fundamental frequency and three times the fundamental frequency, that is, the common-mode voltage changes linearly within the switching cycle; the final estimated common-mode voltage expression is Formula 18: u on1 (n)=2u on (n-1)-u on (n-2)。 5. The three-level photovoltaic grid-connected inverter control method based on ID-Σ algorithm according to claim 3, characterized in that: The specific steps of controlling the three-level photovoltaic grid-connected inverter by grid-connected current tracking, midpoint potential balance, resonance suppression, and third-order common mode voltage suppression in step 4 are as follows: Substituting the common-mode voltage estimated in step 3 into equation 15, we get the final duty cycle expression; It is the inverter side current feedback control, which is related to resonance suppression; It is grid-connected current feedback control, which is related to grid-connected current tracking; It is grid voltage feedforward to reduce grid impact; It is the midpoint potential difference feedback control, which is related to the midpoint potential balance control; is the common-mode voltage feedback and is related to low-frequency common-mode voltage rejection.

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