A simplified vector constant frequency predictive current control method for grid-connected inverter

Abstract

The present invention relates to a simplified vector fixed-frequency predictive current control method for a grid-connected inverter, comprising the following steps: step S1: calculating a current prediction value according to a current state parameter of a system, and calculating a predicted current difference; step S2: determining two groups of voltage vector combinations that need to be traversed by judging the sector where the predicted current difference is located; step S3: calculating the action time of each voltage vector in step S2, substituting it into a cost function for traversal optimization, and obtaining an optimal voltage vector combination; step S4: inputting the optimal voltage vector combination and action time into a DPWM1 modulator to obtain a pulse signal to control the inverter, and obtaining a pulse signal. The present invention effectively reduces harmonics. By calculating and compensating for the predicted current error, three voltage vectors are used to act on the grid-connected inverter in each control cycle, so that the current ripple is significantly reduced and the current control accuracy is improved.

Description

Technical Field

[0001] The present invention relates to the field of new energy grid connection, and in particular to a simplified vector constant-frequency predictive current control method for a grid-connected inverter based on discontinuous modulation. Background Art

[0002] As a connection device between the photovoltaic power generation system and the power grid, the quality of the control method of the three-phase grid-connected inverter has a great influence on the grid connection effect, so its control method has received more and more research attention. Model Predictive Control (MPC), as an emerging control strategy in recent years, is widely used in power electronics and motor control because of its strong robustness, good dynamic response, and no need for complex parameter design. Traditional model predictive control methods are all single vector control, with only one voltage vector involved in each control cycle, unable to accurately synthesize the control vector, and poor steady-state performance. In addition, the voltage vector acts on the entire cycle, making the current tracking inaccurate and the current harmonic content high. The dual-vector model predictive current control method, on the basis of single vector control, adds a zero vector to each control cycle and allocates zero vector control time, introduces the concept of duty cycle control, and can accurately synthesize the amplitude of the voltage vector. However, since the second vector is always a zero vector, the direction of the inverter output voltage vector is still fixed, and the duty cycle is determined after the first vector is selected, which cannot guarantee the global optimality. The basic voltage vectors are combined nearby, and a total of 12 groups are substituted into the traversal calculation, which brings a huge burden to the controller and is not conducive to practical application. A large number of studies have shown that the control effect of dual-vector is better than that of single-vector, but dual-vector control lacks a modulation link, and the state switching of the switch in each control cycle is still irregular, resulting in the dispersion of current harmonics, which brings great inconvenience to subsequent filtering, and the harmonic content is still high. Summary of the invention

[0003] In view of this, the purpose of the present invention is to provide a simplified vector constant frequency predictive current control method for a grid-connected inverter based on discontinuous modulation, which solves the harmonic dispersion and high harmonic content of the output current of the grid-connected inverter and simplifies the control calculation of the grid-connected inverter, thereby effectively improving the control efficiency.

[0004] To achieve the above object, the present invention adopts the following technical solutions:

[0005] A simplified vector constant frequency predictive current control method for a grid-connected inverter comprises the following steps:

[0006] Step S1: Calculate the current prediction value according to the current state parameters of the system, and calculate the predicted current difference;

[0007] Step S2: determining the sector where the predicted current difference is located, and determining two groups of voltage vector combinations that need to be traversed;

[0008] Step S3: Calculate the action time of each voltage vector in step S2, substitute it into the cost function to perform traversal optimization, and obtain the optimal voltage vector combination;

[0009] Step S4: Input the optimal voltage vector combination and action time into the DPWM1 modulator to obtain a pulse signal to control the inverter and obtain a pulse signal.

[0010] Furthermore, the step S1 is specifically as follows:

[0011] Step S11: Obtain the current state parameters of the system, including the actual three-phase current (in the αβ coordinate system) I m , refer to the three-phase current (in αβ coordinate system) I ref ,angular velocityω;

[0012] Step S12: Calculate the predicted current value at the next moment using the vector compensation method;

[0013] Step S13: Calculate the predicted current difference based on the obtained current prediction value.

[0014] Further, the step S12 is specifically as follows:

[0015] The vector representation of the reference current in steady state is

[0016] i ref (k)=|i ref |e jθ(k) (2)

[0017] Where θ(k) is the phase angle of the reference current at the current moment. Assuming that the vector rotates at an angular velocity ω and the amplitude remains unchanged, the phase angle value of the reference current at the next moment is expressed as

[0018] θ(k+1)=θ(k)+ωT s (3)

[0019] Combining equations (2) and (3) to get the predicted current value at the next moment

[0020]

[0021] Furthermore, the predicted current difference is calculated as follows:

[0022]

[0023] Furthermore, the two groups of voltage vector combinations are shown in Table 1.

[0024] Furthermore, the step S3 is specifically as follows:

[0025] Through the voltage law and Clarke transformation, the dynamic current equation of the three-phase grid-connected inverter in the αβ coordinate system is obtained as follows:

[0026]

[0027] u α 、u β are the inverter output voltage vectors of the α-axis and β-axis, respectively, e α 、e β are the α-axis and β-axis voltage vectors respectively.

[0028] Rewriting equation (5) to obtain the instantaneous current change rate in the αβ coordinate system is

[0029]

[0030] According to formula (6), ignoring the influence of resistance R, the instantaneous rate of change of current is related to the inverter output voltage vector and the grid voltage vector;

[0031] In each cycle, two vectors adjacent to the sector where the predicted current error is located are selected as valid vectors, and the zero vector is selected according to the rules of discontinuous modulation DPWM1.

[0032] When the zero vector acts, the current change rate is

[0033]

[0034] Where S α0 is the rate of change of the α-axis current under the action of zero vector, S β0 is the rate of change of β-axis current under the action of zero vector.

[0035] Combining equations (6) and (7), the current change rate when two non-zero vectors act is

[0036]

[0037] Where u opt1α 、u opt1β 、u opt2α 、u opt2β are the components of two non-zero vectors in the αβ coordinate system;

[0038] Where i represents the i-th voltage vector output by the inverter.

[0039] According to the control target of compensating the predicted current difference, when the current error is zero, we have

[0040]

[0041] T s =t0+t1+t2 (10)

[0042] Combining the above equations (7)-(10), the action time of each vector is solved as follows:

[0043]

[0044] Substitute the calculated action time of each vector into formula (12) to perform traversal optimization and obtain the optimal voltage vector combination;

[0045]

[0046] in The predicted current of α-axis and β-axis at time k, s ai 、s βi are the current change rates corresponding to the i-th voltage vector acting on the α-axis and β-axis at time k, t i is the action time of the i-th voltage vector at time k.

[0047] Compared with the prior art, the present invention has the following beneficial effects:

[0048] 1. The present invention combines DPWM1 modulation to keep one phase switch state fixed in each control cycle, thereby reducing the number of switching actions, playing the role of fixing the switching frequency and effectively reducing harmonics;

[0049] 2. The present invention calculates and compensates for the predicted current error, and uses three voltage vectors to act on the grid-connected inverter in each control cycle, so that the current ripple is significantly reduced and the current control accuracy is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 is a control flow chart of an embodiment of the present invention;

[0051] Figure 2 A three-phase grid-connected inverter in one embodiment of the present invention;

[0052] Figure 3 It is a sector diagram re-divided in one embodiment of the present invention;

[0053] Figure 4 It is the DPWM1 modulation timing of the first sector in one embodiment of the present invention;

[0054] Figure 5 It is a structural block diagram of a simplified vector selection fixed-frequency predictive current control of a grid-connected inverter in one embodiment of the present invention. DETAILED DESCRIPTION

[0055] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0056] In this embodiment, the three-phase grid-connected inverter circuit topology is as follows: Figure 2 As shown in the figure, U dc is the DC side voltage, R is the line resistance, L is the filter inductance, e is the grid voltage, S i (i=1,2,…,6) are six switch tubes of the inverter;

[0057] In this embodiment, a simplified vector constant frequency predictive current control method for a grid-connected inverter is provided, comprising the following steps:

[0058] Step S1: Calculate the current prediction value according to the current state parameters of the system, and calculate the predicted current difference;

[0059] Step S2: determining the sector where the predicted current difference is located, and determining two groups of voltage vector combinations that need to be traversed;

[0060] Step S3: Calculate the action time of each voltage vector in step S2, substitute it into the cost function to perform traversal optimization, and obtain the optimal voltage vector combination;

[0061] Step S4: Input the optimal voltage vector combination and action time into the DPWM1 modulator to obtain a pulse signal to control the inverter and obtain a pulse signal.

[0062] In this embodiment, the current error component in the αβ axis is calculated as follows:

[0063]

[0064] The vector compensation method is used to calculate the current reference value at the next moment. The vector representation of the reference current in steady state is:

[0065] i ref (k)=|i ref |e jθ(k) (3)

[0066] Where θ(k) is the phase angle of the reference current at the current moment. Assuming that the vector rotates at an angular velocity ω and the amplitude remains unchanged, the phase angle value of the reference current at the next moment can be expressed as

[0067] θ(k+1)=θ(k)+ωT s (4)

[0068] Combining equations (2) and (3), we can get

[0069]

[0070] In practical applications, the voltage vector with the smallest current error angle is selected for control. Therefore, the sectors are re-divided as Figure 3As shown. Each re-divided sector contains a voltage vector. According to the idea of ​​SVPWM sector judgment, the sector where the predicted current error is located can be obtained. Assuming that the predicted voltage error falls in sector I, due to the influence of factors such as current ripple and control error, the actual predicted current error may be above or below the voltage vector v1, so the voltage vector combination has two cases: (v1, v2, v0) and (v1, v6, v7). Other sectors are similar, and the voltage vector combination determined by the simplified vector selection is shown in Table 1 below.

[0071] Table 1

[0072]

[0073]

[0074] In this embodiment, the action time of each vector is calculated as follows:

[0075] right Figure 2 Through the voltage law and Clarke transformation, the dynamic current equation of the three-phase grid-connected inverter in the αβ coordinate system is obtained:

[0076]

[0077] Rewriting equation (5) to obtain the instantaneous current change rate in the αβ coordinate system is

[0078]

[0079] According to formula (6), ignoring the influence of resistance R, the instantaneous rate of change of current is related to the inverter output voltage vector and the grid voltage vector. In each cycle, two vectors adjacent to the sector where the predicted current error is located are selected as valid vectors, and the zero vector is selected according to the rules of discontinuous modulation DPWM1.

[0080] When the zero vector acts, the current change rate is

[0081]

[0082] Combining equations (6) and (7), we can get the current change rate when two non-zero vectors act as follows:

[0083]

[0084] Where u opt1α 、u opt1β 、u opt2α 、u opt2β are the components of two non-zero vectors in the αβ coordinate system respectively.

[0085] According to the control target of compensating the predicted current difference, when the current error is zero, we have

[0086]

[0087] T s =t0+t1+t2 (10)

[0088] Combining the above equations (7)-(10), the action time of each vector is solved as follows:

[0089]

[0090] In this embodiment, discontinuous modulation DPWM1 is performed to output a PWM wave, specifically:

[0091] When implementing SVPWM, when only one of the two zero vectors is used, discontinuous DPWM is generated. During the entire switching cycle, the state of one of the inverter bridge arms remains unchanged and remains connected to the positive or negative DC bus. In discontinuous modulation DPWM1, the rule for using zero vectors is that v0 (0,0,0) is eliminated in the even sector and v7 (1,1,1) is eliminated in the odd sector. Based on this rule, in I, the action timing of one of the vector groups is as follows Figure 4 Similarly, the action order of other vectors is shown in Table 2.

[0092] Table 2

[0093]

[0094] Substitute the calculated action time of each vector into formula (12) to perform traversal optimization and obtain the optimal voltage vector combination;

[0095]

[0096] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention should fall within the scope of the present invention.

Claims

1. A simplified vector constant frequency predictive current control method for a grid-connected inverter, characterized in that: The following steps are involved: Step S1: Calculate the current prediction value according to the current state parameters of the system, and calculate the predicted current difference; Step S2: determining the sector where the predicted current difference is located, and determining two groups of voltage vector combinations that need to be traversed; Step S3: Calculate the action time of each voltage vector in step S2, substitute it into the cost function to perform traversal optimization, and obtain the optimal voltage vector combination; Step S4: input the optimal voltage vector combination and action time into the DPWM1 modulator to obtain a pulse signal to control the inverter and obtain a pulse signal; The step S1 is specifically as follows: Step S11: Obtain the current state parameters of the system, including the actual three-phase current I m , reference three-phase current I ref ,angular velocityω; Step S12: Calculate the predicted current value at the next moment using the vector compensation method; Step S13: Calculate the predicted current difference according to the obtained current predicted value; The predicted current difference calculation is specifically as follows: Δ α , Δ β are the predicted current differences between the α-axis and the β-axis, i refα (k+1), i refβ (k+1) are the current reference values ​​of the α-axis and β-axis at time k+1, i α (k+1), i β (k+1) are the predicted current values ​​of the α-axis and β-axis at time k+1 respectively; The step S3 is specifically as follows: Through the voltage law and Clarke transformation, the dynamic current equation of the three-phase grid-connected inverter in the αβ coordinate system is obtained as follows: u α 、u β are the inverter output voltage vectors of the α-axis and β-axis, respectively, and e α 、e β are the α-axis and β-axis voltage vectors respectively; Rewriting equation (5) to obtain the instantaneous current change rate in the αβ coordinate system is According to formula (6), ignoring the influence of resistance R, the instantaneous rate of change of current is related to the inverter output voltage vector and the grid voltage vector; In each cycle, two vectors adjacent to the sector where the predicted current error is located are selected as valid vectors, and the zero vector is selected according to the rules of discontinuous modulation DPWM1. When the zero vector acts, the current change rate is Where S α0 is the rate of change of α-axis current under zero vector action, S β0 is the rate of change of β-axis current under the action of zero vector; Combining equations (6) and (7), the current change rate when two non-zero vectors act is Where u opt1α 、u opt1β 、u opt2α 、u opt2β are the components of two non-zero vectors in the αβ coordinate system; Where i represents the i-th voltage vector output by the inverter; According to the control target of compensating the predicted current difference, when the current error is zero, we have T s =t0+t1+t2 (10) Combining the above equations (7)-(10), the action time of each vector is solved as follows: Substitute the calculated action time of each vector into formula (12) to perform traversal optimization and obtain the optimal voltage vector combination; in The predicted current of α-axis and β-axis at time k, s ai 、s βi are the current change rates corresponding to the i-th voltage vector acting on the α-axis and the β-axis at time k, t i is the action time of the i-th voltage vector at time k.

2. The simplified vector constant frequency predictive current control method for grid-connected inverter according to claim 1, characterized in that: The step S12 is specifically: The vector representation of the reference current in steady state is and ref (k)=|i ref |e jθ(k) (2) Where θ(k) is the phase angle of the reference current at the current moment. Assuming that the vector rotates at an angular velocity ω and the amplitude remains unchanged, the phase angle value of the reference current at the next moment is expressed as θ(k+1)=θ(k)+ωT s (3) Combining equations (2) and (3) to get the predicted current value at the next moment

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