Symmetric control method based on grid-connected inverter system

Abstract

The invention discloses a symmetric control method based on a grid-connected inverter system, and the method comprises the steps: enabling a phase-locked loop to simultaneously subtract a steady-state value from the PCC voltages of a d axis and a q axis after coordinate transformation, then transmitting an error into a phase-locked loop regulator, carrying out the integration, and obtaining the output k and theta of the d axis and the q axis, respectively acting on telescopic transformation and rotation transformation in coordinate transformation to realize symmetry of a d axis and a q axis in coordinate transformation in a control process; the power control loop is used for outputting the grid-connected current reference, a telescopic transformation function is added into the power reference of the power control loop, telescopic transformation is performed on the power reference, and meanwhile, power feedforward is set in a power calculation module to remodel output power, so that the symmetrical power control loop is obtained. According to the method, the mathematical model of the three-phase grid-connected inverter system is simplified, the stability analysis process is more concise, and the method has great significance in stability analysis of the inverter system.

Description

Technical Field

[0001] The present invention relates to the field of power system control, and in particular to a symmetrical control method based on a grid-connected inverter system. Background Art

[0002] Distributed generation is one of the effective ways to develop and utilize renewable energy such as solar energy and wind energy on a large scale. As an energy conversion device between distributed generation units and the power grid, the grid-connected inverter is crucial to the stable operation of the distributed generation system. As the power grid becomes weaker and weaker, the grid impedance will not be negligible and will vary in a wide range, making the coupling between the grid-connected inverter and the power grid more obvious. Their interaction leads to the risk of instability in the grid-connected inverter system. The state space-based and impedance-based stability analysis methods are currently the main stability analysis methods. The latter is widely used to analyze the stability of the system because it does not require knowing the internal parameters of the system and has a clear physical meaning. When using the impedance-based stability analysis method, considering that the Park transformation that converts the power frequency quantity into the DC quantity in the control structure of the inverter has nonlinear characteristics, it is necessary to establish a small signal impedance model of the grid-connected inverter and the power grid in the dq coordinate system or positive and negative sequence. In the above model, the inverter output admittance and the grid impedance are both 2×2 matrices, so it is necessary to analyze whether the eigenvalue of the ratio matrix formed by the product of the two satisfies the generalized Nyquist stability criterion (Generalized Nyquist Stability Criterion, GNC) to judge the system stability.

[0003] Recent studies have pointed out that the phase-locked loop used to obtain grid phase information in the grid-connected inverter, as well as the active power loop and reactive power loop used to control the output power, will affect the system stability. Therefore, when analyzing the stability of the grid-connected inverter system, the impact of the phase-locked loop and the power control loop must be fully considered.

[0004] However, the asymmetry of the control structure of the phase-locked loop and the power control loop in the dq coordinate system will make the inverter output admittance an asymmetric matrix, making it impossible to simplify the radicals in the two eigenvalue expressions of the system back-propagation matrix, making it difficult to intuitively analyze the impact of the phase-locked loop and power loop parameters on the stability of the grid-connected inverter, and difficult to guide parameter design. Summary of the invention

[0005] The present invention aims at solving the problems in the prior art and provides a symmetrical control method based on a grid-connected inverter system.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] A symmetrical control method based on a grid-connected inverter system, the method comprising:

[0008] The sampled grid-connected current is compared with the grid-connected current reference after coordinate transformation, and the error obtained is sent to the current regulator, and is added with the capacitor current proportional feedback to suppress the resonance peak of the LCL filter. The output modulation voltage is output to the SPWM module through coordinate inverse transformation, and the SPWM module outputs the drive signal of the inverter bridge arm switch tube; this process is the current loop control process;

[0009] The phase-locked loop is used to synchronize the grid-connected current with the voltage at the common coupling point. The phase-locked loop simultaneously subtracts the steady-state value from the d-axis and q-axis PCC voltages after coordinate transformation, and then sends the error to the phase-locked loop regulator and integrates it to obtain the d-axis and q-axis outputs, which are used for the scaling transformation and rotation transformation in the coordinate transformation, respectively, to achieve the symmetry of the d-axis and q-axis in the coordinate transformation of the control process.

[0010] The power control loop is used to output the grid-connected current reference. A scaling function is added to the power reference of the power control loop to perform scaling transformation on the power reference. A power feedforward is set in the power calculation module to reshape the output power to obtain a symmetrical power control loop.

[0011] Furthermore, the symmetry method of the d-axis and the q-axis in the coordinate transformation of the phase-locked loop control process is as follows:

[0012] In the phase-locked loop control, the steady-state value V is subtracted from the d-axis and q-axis PCC voltages after coordinate transformation. ref and 0, and obtain the deviation between the d-axis and q-axis PCC voltages and the reference after coordinate transformation, and then send the result to the regulator G of the phase-locked loop c (s), and the result is sent to the integral link 1 / s. The d-axis and q-axis outputs obtained after calculation are recorded as k and θ respectively, where θ is the PCC voltage phase angle and k is the telescopic ratio; k is used to realize the telescopic transformation in the coordinate transformation, and the telescopic transformation function is f(k)=Ce -k , C is a non-zero constant (take C = 1 as an example for explanation); θ is used to realize the rotation transformation in the coordinate transformation, and the rotation transformation function is g(θ) = e -jθ .

[0013] Furthermore, the symmetrical power control loop adopts the power reference multiplication function e -2k Perform a scaling transformation to obtain a corrected power reference; in the power calculation module, subtract the power feedforward of the active power when calculating the active power to obtain the reshaped active power; add the power feedforward of the reactive power when calculating the reactive power to obtain the reshaped reactive power;

[0014] The power feed-forward of the active power is obtained by multiplying the q-axis PCC voltage and the q-axis grid-connected current after coordinate transformation, and adding the function G vff (s) composition;

[0015] The reactive power feedforward is obtained by multiplying the q-axis PCC voltage and the d-axis grid-connected current after the coordinate transformation, and adding the function G vff (s) composition.

[0016] Furthermore, the d-axis and q-axis in the phase-locked loop output k and θ respectively, and their small disturbance signals and The expression of is shown in formula (1),

[0017]

[0018] in, is the q-axis component of the PCC voltage after coordinate transformation, is the d-axis component of the PCC voltage after coordinate transformation, s is the Laplace operator, G c (s) is a phase-locked loop regulator.

[0019] Furthermore, in the coordinate transformation, for the case of C=1, the small signal models of abc / d′q′ transformation and d′q′ / abc transformation are shown in formula (2) and formula (3), respectively.

[0020]

[0021] Among them, x′ dq =[x d x q ] T is the matrix form of the state variables in the d′q′ coordinate system, where x′ d and x′ q are their d′ and q′ axis components respectively, X d , X q They are state variables x d and x q Steady-state value of x abc =[x a x b x c ] T is the matrix form of the state variables in the abc coordinate system, where x a 、x b 、x c are the a-axis, b-axis and c-axis components respectively; θ o and k0 are the steady-state values ​​of θ and k, respectively; For x i Small disturbance signal G PLL (s) is the closed-loop transfer function of the phase-locked loop, P θo The rotation angle is θ o The Park transformation matrix is ​​expressed as shown in formula (4) and formula (5).

[0022]

[0023] Furthermore, in the control structure, the grid current feedback H ig i gabc , capacitor current feedback H iC i Cabc 、PCC voltage v PCCabc After transformation through abc / d′q′, H is obtained ig i′ gdq , H iC i′ Cdq and v′ PCCdq The small signal model of the modulated signal is obtained by d′q′ / abc transformation to obtain v′ Mdq The expression of the small signal model is as follows:

[0024]

[0025] Among them, I gi ,I Ci 、V PCCi 、V Mi i gi 、i Ci 、v PCCi 、v Mi Steady-state value when i=d,q.

[0026] Furthermore, the expressions of the output power of the power calculation module and its small signal model before adding the power reference correction are shown in formula (7) and formula (8),

[0027]

[0028] Multiply the power reference by the scaling function e -2k The expressions of the corrected power reference and its small signal model are shown in formula (9) and formula (10).

[0029]

[0030] Among them, P o ′ is active power, Q o ′ is reactive power, P ref is the power reference for active power, Q ref is the power reference of reactive power; P r ' ef is the power reference of the corrected active power, Q r ' ef is the power reference for the corrected reactive power.

[0031] Comparing equations (8) and (10), it can be seen that the influence of the phase-locked loop on the power loop can be eliminated by correcting the power reference, that is, introducing a feedforward G from the PCC voltage to the power addition point. PQref_PLL (s) to offset G v_Sym (s).

[0032] Furthermore, in the control structure, the expressions of the power error at the power addition point and its small signal model after adding power feedforward are shown in formula (11) and formula (12),

[0033]

[0034] Among them, G vff (s) is the feedforward function, P error and Q error are respectively the active power error and reactive power error at the power addition point. vff (s) adds a high-pass filter G HPF (s) is used to avoid the influence of the feedforward on the power calculation result at 0Hz in the dq coordinate system and the transfer function G v (s) is used to offset the influence of coordinate transformation, G vff (s) and G v The expression of (s) is shown in formula (13) and formula (14).

[0035] G vff (s)=3G HPF (s)G v (s)(13)

[0036]

[0037] As shown in formula (6), the transfer function matrix Y of the feedforward branch from the PCC voltage to the coordinate transformation introduced by the symmetrical phase-locked loop is g ' _PLL (s), G i ' C_PLL (s), G v ' _PLL (s) and G′ M_PLL (s) are all symmetric matrices; as shown in formula (12), the transfer function matrix G′ of the feedforward branch from the PCC voltage to the power reference introduced by the symmetric power loop is v_PQ (s) in the high-pass filter G HPF (s) The bandwidth is a symmetric matrix, and the transfer function matrix G introduced from the grid current to the power reference feedback branch is i_PQ (s) is a symmetric matrix.

[0038] Furthermore, the grid-connected inverter system includes a three-phase LCL type grid-connected inverter and a control system thereof, and the three-phase LCL type grid-connected inverter main circuit and its control system include a three-phase LCL type grid-connected inverter, a current control loop, a phase-locked loop and a power control loop.

[0039] Furthermore, the main circuit of the three-phase LCL type grid-connected inverter includes: a DC side power supply, a three-phase three-bridge arm, a three-phase LCL filter and a three-phase grid-connected switch, the DC side power supply, the three-phase three-bridge arm, the three-phase LCL filter and the three-phase grid-connected switch are connected in sequence, wherein the LCL filter of each phase is composed of two inductors L1, L2 and a capacitor C, the inductor L1, the inductor L2 and the grid-connected switch are connected in series, one end of the capacitor C is connected to the node between the inductors L1 and L2, and the other end is connected to the remaining nodes of the other two-phase capacitors.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] The symmetrical control method based on the grid-connected inverter system of the present invention solves the problems of complex stability analysis and cumbersome parameter design process of the three-phase grid-connected inverter system caused by the asymmetric phase-locked loop and the asymmetric power control loop in the control process. The mathematical model of the three-phase grid-connected inverter system is simplified, and the stability analysis process is relatively simple, which lays a foundation for the parameter optimization design of the three-phase grid-connected inverter system, has great significance for the stability analysis of the inverter system, and has great engineering application value.

[0042] The method of the present invention adopts a symmetrical phase-locked loop, in which the steady-state values ​​are simultaneously subtracted from the d-axis and q-axis PCC voltages after coordinate transformation in the phase-locked loop structure, and then the errors are sent to the phase-locked loop regulator and integrated, and the obtained d-axis and q-axis output k and θ are used to control dq symmetry, k is used to realize the telescopic transformation in the coordinate transformation, and θ is used to realize the rotation transformation in the coordinate transformation.

[0043] The method of the present invention adopts a symmetrical power control loop, adds a telescoping function to the power reference of the power loop to correct the power reference, and introduces power feedforward in the power calculation to achieve a symmetrical design of the power control loop. The transfer function matrices in the small signal model of the control link are all symmetrical matrices, which realizes the symmetrical design of the phase-locked loop and the power control loop, and overcomes the asymmetry of the original control method, resulting in the existence of an asymmetric matrix in the small signal model. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 The main circuit and control structure block diagram of the grid-connected inverter of the present invention;

[0045] Figure 2 is a control structure block diagram of a symmetrical phase-locked loop of the present invention;

[0046] Figure 3 is a control structure block diagram of a symmetrical power loop of the present invention;

[0047] Figure 4 for Figure 3 The structural block diagram of the medium power calculation module;

[0048] Figure 5 The frequency characteristic curves of four elements of the system back-ratio matrix of the symmetrical control method based on the grid-connected inverter system of the present invention;

[0049] Figure 6 It is a waveform diagram of PCC voltage and grid-connected current when the grid-connected inverter system using the existing method oscillates;

[0050] Figure 7 For the corresponding Figure 6 The spectrum diagram of grid-connected current;

[0051] Figure 8 It is a waveform diagram of PCC voltage and grid-connected current when the grid-connected inverter system using the symmetrical control method of the present invention oscillates;

[0052] Fig. 9 For the corresponding Figure 8 Spectrum diagram of the grid-connected current. DETAILED DESCRIPTION

[0053] The present invention will be further described below in conjunction with the accompanying drawings.

[0054] Embodiment 1:

[0055] like Figure 1 As shown, the symmetrical control method based on the grid-connected inverter system of the present invention includes a grid-connected inverter system, and the grid-connected inverter system includes a three-phase LCL type grid-connected inverter main circuit and a control system thereof; wherein the three-phase LCL type grid-connected inverter main circuit and its control system include a three-phase LCL type grid-connected inverter, a current control loop, a phase-locked loop and a power control loop; the three-phase LCL type grid-connected inverter is used to convert direct current into high-quality alternating current and feed it into the power grid; the phase-locked loop is used to realize the grid-connected current i gabc The voltage at the common coupling point v PCCabc Synchronous; the power control loop is used to control the output power, and its output is used as the grid-connected current reference i′ of the current control loop refdq .

[0056] The symmetrical control method based on the grid-connected inverter system of the present invention comprises:

[0057] In the current loop control, the grid-connected current i is sampled gabc , after coordinate transformation, the current reference i′ output by the power control loop refdqAfter comparison, the error signal is output and sent to the current regulator G i (s), its output is related to the capacitor current feedback H iC i′ Cdq Compare and get the modulation voltage v′ Mdq , modulation voltage v′ Mdq The output signal v is obtained by coordinate transformation Mabc , and then the drive signal of the inverter bridge arm switch tubes Q1~Q6 is output through the SPWM module;

[0058] Phase-locked loop control, the phase-locked loop is used to achieve grid-connected current i gabc The voltage at the common coupling point v PCCabc synchronous;

[0059] In the power control loop, the grid-connected current i is sampled gabc and PCC voltage v PCCabc , respectively, after coordinate transformation, they are input into the power calculation link. The output power of the power calculation link is compared with the power reference and then passes through the power regulator G PQ (s) Output current base current i′ refdq .

[0060] like Figure 2 As shown in the figure, the control structure of the symmetrical phase-locked loop is to subtract the steady-state value V from the d-axis and q-axis PCC voltages after coordinate transformation. ref and 0, and obtain the deviation between the d-axis and q-axis PCC voltages and the reference after coordinate transformation, and then send the result to the regulator G of the phase-locked loop c (s), and then send the result to the integral link 1 / s. The d-axis and q-axis outputs obtained after calculation are recorded as k and θ respectively; k is used to realize the scaling transformation in the coordinate transformation, and the scaling transformation function is f(k)=Ce -k (C is an arbitrary constant, and C=1 is used as an example for explanation); θ is used to realize the rotation transformation in the coordinate transformation, and the rotation transformation function is g(θ)=e -jθ Symmetrical control of a phase-locked loop means that the effect of the phase-locked loop on the coordinate transformation in the control structure is symmetrical.

[0061] like Figure 3 As shown in the figure, the symmetrical power control loop is used to control the output power, and its output is used as the grid-connected current reference of the current control loop. The control structure of the symmetrical power loop is as follows: first, the PCC voltage and grid-connected current after coordinate transformation are calculated to obtain active power and reactive power, and then the error is compared with the power reference and sent to the power regulator G. PQ (s), and obtain the grid-connected current reference i′ refdq The control structure of the power loop adds a scaling function e to the power reference. -2k Make corrections.

[0062] like Figure 4 As shown, the power feedforward consisting of the product of the q-axis PCC voltage and the q-axis grid-connected current after coordinate transformation is subtracted from the control block diagram of the active power calculation of the symmetrical power control loop to obtain the reshaped active power. The power feedforward consisting of the product of the q-axis PCC voltage and the d-axis grid-connected current after coordinate transformation is added to the control block diagram of the reactive power calculation to obtain the reshaped reactive power. And the function G is added to the above power feedforward. vff (s) Correct the feedforward. Figure 4 The red part in the medium power calculation module is the increased power feedforward.

[0063] Embodiment 2:

[0064] This example further adopts the symmetrical phase-locked loop control based on the first embodiment, by subtracting the steady-state value V from the d-axis and q-axis PCC voltages after coordinate transformation. ref and 0, and obtain its deviation from the reference, and then send the result to the regulator G of the phase-locked loop c (s), the regulator output is sent to the integral link 1 / s, and the d-axis and q-axis outputs obtained after calculation are recorded as k and θ respectively. and The expression of is shown in formula (1),

[0065]

[0066] in, is the q-axis component of the PCC voltage after coordinate transformation, is the d-axis component of the PCC voltage after coordinate transformation, and s is the Laplace operator.

[0067] k is used to realize the scaling transformation in coordinate transformation, and the scaling transformation function is f(k)=Ce -k (C is an arbitrary constant, and C=1 is used as an example for explanation); θ is used to realize the rotation transformation in the coordinate transformation, and the rotation transformation function is g(θ)=e -jθ ; Telescopic transformation function f(k) = e -k When abc / d′q′ transformation and d′q′ / abc transformation, the small signal model is shown in formula (2) and formula (3).

[0068]

[0069] Among them, x′ dq =[x d x q ] T is the matrix form of the state variables in the d′q′ coordinate system, where x′ d and x′ qare their d′ and q′ axis components respectively, X d , X q They are state variables x d and x q Steady-state value of x abc =[x a x b x c ] T is the matrix form of the state variables in the abc coordinate system, where x a 、x b 、x c are the a-axis, b-axis and c-axis components respectively; θ o and k0 are the steady-state values ​​of θ and k, respectively; For x i Small disturbance signal G PLL (s) is the closed-loop transfer function of the phase-locked loop, P θo The rotation angle is θ o The Park transformation matrix is ​​expressed as shown in formula (4) and formula (5).

[0070]

[0071] The symmetrical structure of the phase-locked loop can be seen from the small signal model of the coordinate transformation after the symmetrical phase-locked loop shown in formula (2) and formula (3). The effect brought by the phase-locked loop in formula (2) and formula (3) is dq symmetry.

[0072] Furthermore, the small signal model of coordinate transformation in the control structure is obtained; at this time, H ig i gabc , H iC i Cabc and v PCCabc By transforming abc / d′q′, we can get H ig i′ gdq , H iC i′ Cdq and v′ PCCdq , v′ Mdq By d′q′ / abc transformation, we can get v Mabc , its small signal model is shown in formula (6),

[0073]

[0074] Among them, I gi ,I Ci 、V PCCi 、V Mi i gi 、i Ci 、v PCCi 、v Mi The steady-state value of (i=d,q).

[0075] Embodiment three:

[0076] Based on the second embodiment, this example further obtains a small signal model of power calculation according to the expression of the power calculation link in the symmetrical power control loop control; then adds the scaling transformation function f(k)=e to the power reference of the power loop. -2k , and obtain the small signal model of the corrected power reference; then, the small signal model of the power loop is obtained based on the small signal models of the two. The expressions of the power calculation link and its small signal model before the power reference correction are shown in formula (7) and formula (8).

[0077]

[0078] Multiply the power reference by the scaling function e -2k The expressions of the corrected power reference and its small signal model are shown in formula (9) and formula (10).

[0079]

[0080] Among them, P ref is the power reference for active power, Q ref is the power reference of reactive power; P r ' ef is the power reference of the corrected active power, Q r ' ef is the power reference for the corrected reactive power.

[0081] Comparing equations (8) and (10), it can be seen that the influence of the phase-locked loop on the power loop can be eliminated by correcting the power reference, that is, introducing a feedforward G from the PCC voltage to the power addition point. PQref_PLL (s) to offset G v_Sym (s).

[0082] Furthermore, the power feedforward consisting of the product of the q-axis PCC voltage and the q-axis grid-connected current after coordinate transformation is subtracted from the control block diagram of active power calculation to obtain the reshaped active power. The power feedforward consisting of the product of the q-axis PCC voltage and the d-axis grid-connected current after coordinate transformation is added to the control block diagram of reactive power calculation to obtain the reshaped reactive power. Function G is added to the above power feedforward. vff (s) The feedforward is corrected to obtain the small signal model of the power loop after adding the feedforward. The expressions of the power error at the power addition point and its small signal model after adding the power feedforward are shown in formula (11) and formula (12).

[0083]

[0084] Among them, Gvff (s) is the feedforward function, P error and Q error are the active power error and reactive power error at the power addition point, respectively. By using the power loop small signal model after the symmetrical power loop in formula (12), it can be obtained that the power control loop is a symmetrical structure, because the model of the power control loop in formula (12) is dq symmetrical. vff (s) adds a high-pass filter G HPF (s) is used to avoid the influence of the feedforward on the power calculation result at 0Hz in the dq coordinate system and the transfer function G v (s) is used to offset the influence of coordinate transformation, G vff (s) and G v The expression of (s) is shown in formula (13) and formula (14).

[0085] G vff (s)=3G HPF (s)G v (s) (13)

[0086]

[0087] As shown in formula (6), the transfer function matrix Y of the feedforward branch from the PCC voltage to the coordinate transformation introduced by the symmetrical phase-locked loop is g ' _PLL (s), G i ' C_PLL (s), G v ' _PLL (s) and G′ M_PLL (s) are all symmetric matrices; as shown in formula (12), the transfer function matrix G′ of the feedforward branch from the PCC voltage to the power reference introduced by the symmetric power loop is v_PQ (s) in the high-pass filter G HPF (s) The bandwidth is a symmetric matrix, and the transfer function matrix G introduced from the grid current to the power reference feedback branch is i_PQ (s) is a symmetric matrix.

[0088] In the original power loop, the calculation method of active power is: P′ o =1.5(v′ PCCd i′ gd +v′ PCCq i′ gq ); The calculation method of reactive power is: Q′ o =1.5(-v′ PCCq i′ gd +v′ PCCd i′ gq ). Its implementation method is as follows Figure 4As shown in the pink box.

[0089] In the symmetrical power loop of this application, the calculation method of active power is: P′ o_recon =1.5(v′ PCCd i′ gd +v′ PCCq i′ gq )-G vff (s)v′ PCCq i′ gq ; The calculation method of reactive power is: Q′ o_recon =1.5(v′ PCCd i′ gq -v′ PCCq i′ gd )+G vff (s)v′ PCCq i′ gd That is, add power feedforward to the original active power calculation and reactive power calculation, such as Figure 4 As shown in the red box.

[0090] The above formula is reflected in formula (7) and formula (11) in the specification.

[0091]

[0092] Embodiment 4:

[0093] The symmetrical control method based on the grid-connected inverter system of the present invention comprises the following steps:

[0094] During the current loop control process, the grid-connected current i is first sampled gabc By coordinate transformation, we can get i′ gdq , and then with its current reference i′ refdq The error is then sent to the current regulator G i (s). Here, the capacitor current i′ is used Cdq Proportional feedback is used to suppress the LCL resonance peak. Mdq By inverse coordinate transformation, we can get v Mabc , the driving signals of the inverter bridge arm switch tubes Q1~Q6 are obtained through SPWM.

[0095] Subsequently, in the phase-locked loop control, the steady-state value V is subtracted from the d-axis and q-axis PCC voltages after coordinate transformation. ref and 0, and obtain the deviation between the d-axis and q-axis PCC voltages and the reference after coordinate transformation, and then send the result to the regulator G of the phase-locked loop c(s), the result is sent to the integral link 1 / s, and the d-axis and q-axis outputs obtained after calculation are recorded as k and θ respectively; k is used to realize the scaling transformation in the coordinate transformation, and the scaling transformation function is f(k) = Ce -k (C is an arbitrary constant, and C=1 is used as an example for explanation); θ is used to realize the rotation transformation in the coordinate transformation, and the rotation transformation function is g(θ)=e -jθ ; Then the small signal model of coordinate transformation in the control structure is obtained;

[0096] Next, the small signal model of power calculation is obtained according to the expression of the power calculation link; then the telescoping function e is added to the power reference of the power loop. -2k , and obtain the small signal model of the power reference scaling transformation; and then obtain the small signal model of the power loop based on the small signal models of the two;

[0097] Finally, in the control block diagram of active power calculation, the power feedforward consisting of the product of the q-axis PCC voltage and the q-axis grid-connected current after coordinate transformation is subtracted to obtain the reshaped active power; in the control block diagram of reactive power calculation, the power feedforward consisting of the product of the q-axis PCC voltage and the d-axis grid-connected current after coordinate transformation is added to obtain the reshaped reactive power; the function G is added to the above power feedforward. vff (s) The feedforward is modified to obtain the small signal model of the power loop after adding the feedforward.

[0098] The present invention effectively realizes the dq symmetry of the control of the phase-locked loop and the power control loop in the grid-connected inverter system, and the control method is relatively simple, which realizes the simplification of the mathematical model of the three-phase grid-connected inverter system, makes the stability analysis process more concise, and lays a foundation for the parameter optimization design of the three-phase grid-connected inverter system. It is of great significance for the stability analysis of the inverter system and has great engineering application value.

[0099] Application example 1:

[0100] In order to better illustrate the use effect of the present invention, a specific embodiment of the symmetrical control method based on the grid-connected inverter system using the present invention is introduced below.

[0101] This example uses a symmetrical phase-locked loop to obtain the PCC voltage phase θ and the expansion ratio k. A symmetrical power loop is used to obtain the grid-connected current reference i′ refdq Grid-connected current i gabc By coordinate transformation, we can get i′ gdq , and then with its current reference i′ refdq The error is then sent to the current regulator G i (s). Here, the capacitor current i′ is used Cdq Proportional feedback is used to suppress the LCL resonance peak. MdqBy inverse coordinate transformation, we can get v Mabc , the driving signals of the switch tubes Q1~Q6 are obtained through SPWM.

[0102] Table 1

[0103]

[0104] like Figure 5 As shown, the frequency characteristic curves of the four elements of the system back-ratio matrix after symmetrical control of the grid-connected inverter system of the present invention are shown, wherein the main diagonal elements of the back-ratio matrix are T dd (s), T qq (s), the secondary diagonal elements are T dq (s), T qd (s). Among them, the current loop, phase-locked loop and power loop regulators all use PI regulators, and their expressions are G i (s) = k p_i +k i_i / s, G PLL (s) = k p_PLL +k i_PLL / s and G PQ (s) = k p_PQ +k i_PQ / s. The parameters of the main circuit, current loop, phase-locked loop and power loop of the grid-connected inverter are shown in Table 1. The high-pass filter G HPF The bandwidth of (s) is taken as 5Hz. Figure 5 From the curves of phase angle (Deg) and frequency (f), as well as the curves of gain (Gain) and frequency (f), we can see that in the frequency band outside the bandwidth of the high-pass filter, T dd (s) = T qq (s) and T dq (s)=-T qd (s), that is, the back-bias matrix is ​​a symmetric matrix. Figure 5 It shows that after adopting the control method of the present application, the transfer function matrix of each link in the system is a symmetric matrix, which means that the small signal models of the phase-locked loop and the power loop are both symmetric matrices, that is, the phase-locked loop and the power loop are both symmetrically designed. At this time, the system meets the simplified stability analysis conditions.

[0105] Application Example 2:

[0106] In this example, based on the application example 1, the proportional coefficient k of the phase-locked loop regulator of the parameters in Table 1 is p_PLL , Phase-locked loop regulator integral coefficient k i_PLL , power loop regulator proportional coefficient k p_PQ , power loop regulator integral coefficient k i_PQThey are modified to 0.196, 5.88, 0.0000127, and 0.025 respectively to make the system oscillate. At this time, the waveforms of the PCC voltage and grid-connected current of the existing asymmetric controlled grid-connected inverter system are as follows: Figure 6 As shown, the spectrum of the grid-connected current in the corresponding figure is as follows Figure 7 shown.

[0107] Modify the phase-locked loop regulator proportional coefficient k based on the parameters in Table 1 p_PLL , Phase-locked loop regulator integral coefficient k i_PLL , power loop regulator proportional coefficient k p_PQ , power loop regulator integral coefficient k i_PQ The PCC voltage and grid-connected current waveforms of the grid-connected inverter system symmetrically controlled by the present invention are as follows: Figure 8 As shown, the corresponding frequency spectrum of the grid-connected current is as follows Fig. 9 shown.

[0108] from Figure 6 , Figure 7 It can be seen that when there is an asymmetric control link in the system, when the system oscillates, there will be an oscillation frequency coupling effect, that is, oscillation components with a difference of 2 times the power frequency will appear in the system at the same time. Figure 8 , Fig. 9 It can be seen that after adopting the symmetrical control strategy of the present invention, when the system oscillates, there will be no oscillation frequency coupling effect, which means that the system is a symmetrical system and the proposed symmetrical control strategy is correct and effective.

[0109] The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A symmetrical control method based on a grid-connected inverter system, the method comprising: The sampled grid-connected current is compared with the grid-connected current reference after coordinate transformation, and the error obtained is sent to the current regulator, and is added with the capacitor current proportional feedback. The output modulation voltage is output to the SPWM module through coordinate inverse transformation, and the SPWM module outputs the drive signal of the inverter bridge arm switch tube; it is characterized in that: The phase-locked loop is used to synchronize the grid-connected current with the voltage at the common coupling point. The phase-locked loop simultaneously subtracts the steady-state value from the d-axis and q-axis PCC voltages after coordinate transformation, and then sends the error to the phase-locked loop regulator and integrates it to obtain the d-axis and q-axis outputs, which are used for the scaling transformation and rotation transformation in the coordinate transformation, respectively, to achieve the symmetry of the d-axis and q-axis in the coordinate transformation of the control process. The power control loop is used to output the grid-connected current reference. A scaling function is added to the power reference of the power control loop to perform scaling transformation on the power reference. A power feedforward is set in the power calculation module to reshape the output power to obtain a symmetrical power control loop.

2. The symmetrical control method based on the grid-connected inverter system according to claim 1, characterized in that: The symmetry method of the d-axis and q-axis in the coordinate transformation of the phase-locked loop control process is as follows: In the phase-locked loop control, the steady-state value V is subtracted from the d-axis and q-axis PCC voltages after coordinate transformation. ref and 0, and obtain the deviation between the d-axis and q-axis PCC voltages and the reference after coordinate transformation, and then send the result to the regulator G of the phase-locked loop c (s), and the result is sent to the integral link 1 / s. The d-axis and q-axis outputs obtained after calculation are recorded as k and θ respectively, where θ is the PCC voltage phase angle and k is the telescopic ratio; k is used to realize the telescopic transformation in the coordinate transformation, and the telescopic transformation function is f(k)=Ce -k , C is a non-zero constant; θ is used to realize the rotation transformation in coordinate transformation, and the rotation transformation function is g(θ)=e -jθ .

3. The symmetrical control method based on the grid-connected inverter system according to claim 2, characterized in that: The symmetrical power control loop uses the power reference multiplication function e -2k Perform a scaling transformation to obtain a corrected power reference; in the power calculation module, subtract the power feedforward of the active power when calculating the active power to obtain the reshaped active power; add the power feedforward of the reactive power when calculating the reactive power to obtain the reshaped reactive power; The power feed-forward of the active power is obtained by multiplying the q-axis PCC voltage and the q-axis grid-connected current after coordinate transformation, and adding the function G vff (s) composition; The reactive power feedforward is obtained by multiplying the q-axis PCC voltage and the d-axis grid-connected current after the coordinate transformation, and adding the function G vff (s) composition.

4. The symmetrical control method based on the grid-connected inverter system according to claim 3, characterized in that: The d-axis and q-axis in the phase-locked loop output k and θ respectively, and their small disturbance signals and The expression of is shown in formula (1), in, is the q-axis component of the PCC voltage after coordinate transformation, is the d-axis component of the PCC voltage after coordinate transformation, s is the Laplace operator, G c (s) is a phase-locked loop regulator.

5. The symmetrical control method based on the grid-connected inverter system according to claim 4, characterized in that: In the coordinate transformation, for the case of C = 1, the small signal models of abc / d′q′ transformation and d′q′ / abc transformation are shown in formula (2) and formula (3), respectively. Among them, x′ dq =[x d x q ] T is the matrix form of the state variables in the d′q′ coordinate system, where x′ d and x′ q are their d′ axis and q′ axis components respectively, and Xd and Xq are the state variables x d and x q Steady-state value of x abc =[x a x b x c ] T is the matrix form of the state variables in the abc coordinate system, where x a 、x b 、x c are the a-axis, b-axis and c-axis components respectively; θ o , k0 and X i (i=a,b,c,d,q) are θ, k and x respectively i The steady-state value of For x i Small disturbance signal G PLL (s) is the closed-loop transfer function of the phase-locked loop, P θo The rotation angle is θ o The Park transformation matrix is ​​expressed as shown in formula (4) and formula (5).

6. The symmetrical control method based on the grid-connected inverter system according to claim 5, characterized in that: In the control structure, the grid current feedback H ig i gabc , capacitor current feedback H iC i Cabc 、PCC voltage v PCCabc After transformation through abc / d′q′, H is obtained ig i′ gdq , H iC i′ Cdq and v′ PCCdq Small signal model of And the modulated signal is converted into v′ by d′q′ / abc Mdq The expression of the small signal model is as follows: Among them, I gi ,I Ci 、V PCCi 、V Mi i gi 、i Ci 、v PCCi 、v Mi Steady-state value when i=d,q.

7. The symmetrical control method based on the grid-connected inverter system according to claim 6, characterized in that: The expressions of the output power of the power calculation module and its small signal model before adding the power reference correction are shown in formula (7) and formula (8). Multiply the power reference by the scaling function e -2k The expressions of the corrected power reference and its small signal model are shown in formula (9) and formula (10). Among them, P′ o is the active power, Q′ o is the reactive power, P ref is the power reference for active power, Q ref is the power reference of reactive power; P′ ref is the power reference of the corrected active power, Q′ ref is the power reference for the corrected reactive power.

8. The symmetrical control method based on the grid-connected inverter system according to claim 7, characterized in that: In the control structure, the expressions of the power error at the power addition point and its small signal model after adding power feedforward are shown in formula (11) and formula (12). Among them, G vff (s) is the feedforward function, P error and Q error are the active power error and reactive power error at the power addition point respectively; G vff (s) and G v The expression of (s) is shown in formula (13) and formula (14). G vff (s)=3G HPF (s)G v (s) (13) 9. The symmetrical control method based on the grid-connected inverter system according to claim 1, characterized in that: The grid-connected inverter system includes a three-phase LCL type grid-connected inverter and a control system thereof. The three-phase LCL type grid-connected inverter main circuit and a control system thereof include a three-phase LCL type grid-connected inverter, a current control loop, a phase-locked loop and a power control loop.

10. The symmetrical control method based on the grid-connected inverter system according to claim 9, characterized in that: The main circuit of the three-phase LCL type grid-connected inverter includes: a DC side power supply, a three-phase three-bridge arm, a three-phase LCL filter and a three-phase grid-connected switch. The DC side power supply, the three-phase three-bridge arm, the three-phase LCL filter and the three-phase grid-connected switch are connected in sequence, wherein the LCL filter of each phase is composed of two inductors L1, L2 and a capacitor C, the inductor L1, the inductor L2 and the grid-connected switch are connected in series, one end of the capacitor C is connected to the node between the inductors L1 and L2, and the other end is connected to the remaining nodes of the other two-phase capacitors.

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