A symmetrical control method based on grid-connected inverter system

Abstract

The application discloses a symmetrical control method based on a grid-connected inverter system, in which a phase-locked loop simultaneously subtracts steady values of d-axis and q-axis PCC voltages after coordinate transformation, then sends errors into a phase-locked loop regulator and carries out integration to obtain d-axis and q-axis outputs k and theta respectively, and the d-axis and q-axis outputs k and theta respectively act on stretching transformation and rotation transformation in coordinate transformation to realize symmetry of the d-axis and q-axis in the control process coordinate transformation; a power control loop is used for outputting the grid-connected current reference, a stretching transformation function is added into a power reference of the power control loop, the power reference is subjected to stretching transformation, meanwhile, power feedforward is set in a power calculation module to reshape output power, and a symmetrical power control loop is obtained. The application realizes simplification of a mathematical model of a three-phase grid-connected inverter system, makes a stability analysis process more simple, and has great significance for stability analysis of the inverter system.

Description

TECHNICAL FIELD

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

[0002] Distributed generation is one of the effective ways to develop and utilize renewable energy such as solar and wind energy on a large scale. As the energy conversion device between the distributed generation unit and the grid, the grid-connected inverter is crucial to the stable operation of the distributed generation system. With the grid becoming weaker, the grid impedance will not be negligible and will change in a wide range, making the coupling between the grid-connected inverter and the grid more obvious, and the interaction between them leading to the instability risk of the grid-connected inverter system. The state-space-based and impedance-based stability analysis methods are the main stability analysis methods at present. The latter is widely used to analyze the stability of the system because it does not need to know 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 in the control structure of the inverter, which converts the power frequency quantity into the direct current quantity, has a nonlinear characteristic, it is necessary to establish a small-signal impedance model of the grid-connected inverter and the grid in the dq coordinate system or the positive and negative sequence. In the above model, the inverter output admittance and the grid impedance are both 2x2 matrices, so it is necessary to analyze whether the eigenvalues of the return ratio matrix composed of the product of the two meet the generalized Nyquist stability criterion (GNC) to determine the stability of the system.

[0003] Recent researches have pointed out that the phase-locked loop used to obtain the grid phase information in the grid-connected inverter and the active power loop and the reactive power loop used to control the output power will all affect the stability of the system. Therefore, when analyzing the stability of the grid-connected inverter system, the influence of the phase-locked loop and the power control loop should 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 a non-symmetrical matrix, making the radical in the expressions of the two eigenvalues of the system return ratio matrix unable to be simplified, which makes it difficult to be used for intuitive analysis of the influence of the phase-locked loop and the power loop parameters on the stability of the grid-connected inverter and to guide the parameter design. SUMMARY

[0005] The present application provides a symmetrical control method based on a grid-connected inverter system to solve the problems in the prior art.

[0006] In order to achieve the above purpose, the technical scheme adopted by the present application 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 is sent to the current regulator. The output of the current regulator is added to the proportional feedback of the capacitor current to suppress the resonance peak of the LCL filter. The output modulation voltage is output to the SPWM module through inverse coordinate transformation. The SPWM module outputs the driving signal of the inverter bridge arm switch tube.

[0009] The phase-locked loop is used to synchronize the grid-connected current with the PCC voltage. The d-axis and q-axis PCC voltages after coordinate transformation are subtracted by the steady-state value, and then the error is sent to the phase-locked loop regulator and integrated 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 realize the symmetry of the d-axis and q-axis in the control process coordinate transformation.

[0010] The power control loop is used to output the grid-connected current reference. The scaling transformation function is added to the power reference in the power control loop, and the power feedforward is set in the power calculation module to reshape the output power, thereby obtaining the symmetrical power control loop.

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

[0012] In the phase-locked loop control, the d-axis and q-axis PCC voltages after coordinate transformation are subtracted by the steady-state values V ref and 0, respectively, to obtain the deviation of the d-axis and q-axis PCC voltages after coordinate transformation from the reference, and then the result is sent to the regulator G c (s) of the phase-locked loop. The d-axis and q-axis outputs obtained after operation are denoted as k and θ, respectively, wherein θ is the phase angle of the PCC voltage, and k is the scaling ratio. The k is used to realize the scaling transformation in the coordinate transformation, and the scaling transformation function is f(k)=Ce -k , wherein C is a non-zero constant (C=1 is taken as an example for illustration). The θ is used to realize the rotation transformation in the coordinate transformation, and the rotation transformation function is g(θ)=e -jθ .

[0013] Further, the symmetrical power control loop adopts the scaling transformation of the power reference multiplied by the function e -2k to obtain the corrected power reference. In the active power calculation in the power calculation module, the power feedforward of the active power is subtracted to obtain the reshaped active power. In the reactive power calculation, the power feedforward of the reactive power is added to obtain the reshaped reactive power.

[0014] The power feedforward of the active power is composed of the product of the q-axis PCC voltage after coordinate transformation and the q-axis grid-connected current, and the function G vff (s).

[0015] The power feedforward of the reactive power is the product of the q-axis PCC voltage after the coordinate transformation and the d-axis grid-connected current, plus a function G vff (s) is constituted.

[0016] Further, the phase-locked loop outputs k and θ respectively on the d-axis and q-axis, and the small perturbation signals and are shown in equation (1),

[0017]

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

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

[0020]

[0021] wherein, x' dq = [x d x q ] T is the matrix form of the state variable in the d'q' coordinate system, wherein x' d and x' q are its d' axis and q' axis components respectively, X d , X q are the steady state values of the state variables x d and x q respectively; x abc = [x a x b x c ] T is the matrix form of the state variable in the abc coordinate system, wherein x a , x b , x c are its a-axis, b-axis and c-axis components respectively; θ o and k0 are the steady state values of θ and k respectively; is the small perturbation signal of x i ; G PLL (s) is the closed loop transfer function of the phase-locked loop, P θo is the Park transformation matrix with the rotation angle being θ o , and its expression is shown in equation (4) and equation (5),

[0022]

[0023] Further, in the control structure, the grid-connected current feedback H ig i gabc , the capacitor current feedback H iC i Cabc , the PCC voltage v PCCabc , the small signal model of H ig i′ gdq , H iC i′ Cdq and v′ PCCdq are obtained through the abc / d′q′ transformation; and the small signal model of the modulating signal v′ Mdq is obtained through the d′q′ / abc transformation, and the expression is shown in equation (6):

[0024]

[0025] where I gi , I Ci , V PCCi , V Mi are the steady-state values of i gi , i Ci , v PCCi , v Mi at i=d, q respectively.

[0026] Further, the power reference correction module is added, and the expression of the output power and its small signal model are shown in equation (7) and equation (8):

[0027]

[0028] The power reference is multiplied by the scaling function e -2k to obtain the corrected power reference and its small signal model, and the expression is shown in equation (9) and equation (10):

[0029]

[0030] where P o ′ is the active power, Q o ′ is the reactive power, P ref is the power reference of the active power, Q ref is the power reference of the reactive power; P r ′ ef is the corrected power reference of the active power, Q r ′ ef is the corrected power reference of the 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 modifying the power reference, that is, by introducing a PCC voltage to the power summing point via a feedforward G. PQref_PLL (s) to offset G v_Sym (s).

[0032] Furthermore, in the control structure, the power error at the power summation point after adding power feedforward and its small-signal model expression are shown in equations (11) and (12).

[0033]

[0034] Among them, G vff (s) is the feedforward function, P error and Q error These represent the active power error and reactive power error at the power summation point, respectively. This invention addresses the G... vff Add a high-pass filter G to (s) HPF (s) is used to avoid the influence of feedforward on the power calculation results at 0Hz in the dq coordinate system and the transfer function G. v (s) is used to counteract the effects of coordinate transformation, G vff (s) and G v The expression for (s) is shown in formulas (13) and (14).

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

[0036]

[0037] As shown in equation (6), the transfer function matrix Y of the feedforward branch introduced by the symmetrical phase-locked loop from the PCC voltage to the coordinate transformation point 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 Equation (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 matrix outside the bandwidth is symmetric, and the transfer function matrix G introduced is the feedback branch from the grid-connected current to the power reference. i_PQ (s) is a symmetric matrix.

[0038] Further, the grid-connected inverter system comprises a three-phase LCL grid-connected inverter and a control system thereof, and the three-phase LCL grid-connected inverter and the control system thereof comprise a three-phase LCL grid-connected inverter, a current control loop, a phase-locked loop and a power control loop.

[0039] Further, the main circuit of the three-phase LCL grid-connected inverter comprises a DC power supply, three-phase three-bridge arms, a three-phase LCL filter and three-phase grid-connected switches, which are sequentially connected, wherein the LCL filter of each phase is composed of two inductors L1 and L2 and a capacitor C, the inductor L1, the inductor L2 and the grid-connected switch are connected in series, and one end of the capacitor C is connected to a node between the inductors L1 and L2, and the other end is connected to the remaining nodes of the capacitors of the other two phases.

[0040] Compared with the prior art, the grid-connected inverter system has the following beneficial effects:

[0041] The symmetric control method based on the grid-connected inverter system solves the problems of complex stability analysis of the three-phase grid-connected inverter system and tedious parameter design process 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, the stability analysis process is relatively simple, and the foundation is laid for the parameter optimization design of the three-phase grid-connected inverter system. The stability analysis of the inverter system has great significance and great engineering application value.

[0042] In the method, the symmetric phase-locked loop is adopted, the d-axis and q-axis PCC voltages after coordinate transformation are simultaneously subtracted from the steady-state value in the phase-locked loop structure, then the error is sent to the phase-locked loop regulator and integrated to obtain the k and θ of the d-axis and q-axis outputs, which are used for controlling the dq symmetry, k is used for realizing the scaling transformation in the coordinate transformation, and θ is used for realizing the rotation transformation in the coordinate transformation.

[0043] In the method, the symmetric power control loop is adopted, the scaling transformation function is added to the power reference in the power loop to modify the power reference, and the power feedforward is introduced in the power calculation to realize the symmetric design of the power control loop. The transfer function matrix in the small signal model of the control link is a symmetric matrix, the symmetric design of the phase-locked loop and the power control loop is realized, and the problem of the asymmetric matrix in the small signal model caused by the asymmetric original control method is overcome. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 The main circuit and the control structure block diagram of the grid-connected inverter are provided.

[0045] Figure 2 The control structure block diagram of the symmetric phase-locked loop is provided.

[0046] Figure 3 Control structure block diagram of symmetrical power loop of the present application;

[0047] Figure 4 For Figure 3 Structure block diagram of medium power calculation module;

[0048] Figure 5 For system back ratio matrix four element frequency characteristic curve of symmetrical control method based on grid-connected inverter system of the present application;

[0049] Figure 6 Waveform diagram of PCC voltage and grid-connected current when grid-connected inverter system oscillates by using prior art method;

[0050] Figure 7 For Figure 6 Spectrum diagram of grid-connected current corresponding to

[0051] Figure 8 Waveform diagram of PCC voltage and grid-connected current when grid-connected inverter system oscillates by using symmetrical control method of the present application;

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

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

[0054] Example one:

[0055] As Figure 1 shown, the symmetrical control method based on grid-connected inverter system of the present application comprises a grid-connected inverter system, and the grid-connected inverter system comprises 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 the control system thereof comprise 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 for converting direct current into high-quality alternating current and feeding into a power grid; the phase-locked loop is used for realizing synchronization of grid-connected current i gabc and common coupling point voltage v PCCabc ; the power control loop is used for controlling output power, and its output serves as grid-connected current reference i′ refdq of the current control loop.

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

[0057] In current loop control, sample grid-connected current i gabc , after coordinate transformation, the grid-connected current i refdqThe error signal is outputted after comparison, and the error signal is sent to the current regulator G i (s) whose output is the feedback of the capacitor current H iC i′ Cdq The modulated voltage v′ Mdq is obtained after comparison Mdq The output signal v is obtained through coordinate transformation Mabc , and then the driving signals of the inverter bridge arm switching tubes Q1-Q6 are outputted through the SPWM module.

[0058] The phase-locked loop is used to realize the grid-connected current i gabc and the synchronization with the PCC voltage v PCCabc .

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

[0060] As shown in Figure 2 , the control structure of the symmetrical phase-locked loop is to subtract the steady-state value V ref and 0 from the d-axis and q-axis PCC voltages after coordinate transformation to obtain the deviation of the d-axis and q-axis PCC voltages after coordinate transformation from the reference, and then send the result to the regulator G c (s) of the phase-locked loop, and then send the result to the integral link 1 / s, and the obtained d-axis and q-axis outputs are denoted as k and θ respectively; 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 taken as an example for illustration); θ is used to realize the rotation transformation in coordinate transformation, and the rotation transformation function is g(θ)=e -jθ . The symmetrical control of the phase-locked loop means that the phase-locked loop has symmetrical influence on the coordinate transformation in the control structure.

[0061] As shown in Figure 3 , the symmetrical power control loop is used to control the output power, and the output thereof is taken as the grid-connected current reference of the current control loop. The control structure of the symmetrical power loop is that: first, the active power and the reactive power are calculated by performing power calculation on the PCC voltage after coordinate transformation and the grid-connected current, then the errors are sent to the power regulator G PQ (s) after comparison with the power reference respectively to obtain the grid-connected current reference i′ refdq . The control structure of the power loop adds the scaling transformation function e -2k for correction in the power reference.

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

[0063] Embodiment Two:

[0064] In this example, based on Embodiment One, further in the symmetrical phase-locked loop control, the d-axis and q-axis PCC voltages after coordinate transformation are simultaneously subtracted by the steady-state value V ref and 0 to obtain the deviation from the reference, and then the result is sent to the regulator G c (s) of the phase-locked loop. The regulator output is sent to the integral element 1 / s, and the obtained d-axis and q-axis outputs are denoted as k and θ, respectively. Wherein, and The expression is shown in formula (1),

[0065]

[0066] Wherein, 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 taken as an example for illustration); θ is used to realize the rotation transformation in coordinate transformation, and the rotation transformation function is g(θ)=e -jθ ; when the scaling transformation function f(k)=e -k , the small-signal models of the abc / d′q′ transformation and the d′q′ / abc transformation are shown in formula (2) and formula (3),

[0068]

[0069] Wherein, x′ dq =[x d x q ] T is the matrix form of the state variable in the d′q′ coordinate system, wherein x′ d and x′ qThese are its d′ and q′ axis components, respectively, X d X q State variables x d and x q steady-state value; x abc =[x a x b x c ] T Let x be the matrix form of the state variables in the abc coordinate system, where x a x b x c These are its a-axis, b-axis, and c-axis components, respectively; θ o Let θ and k0 be the steady-state values ​​of θ and k, respectively; For x i Small perturbation 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 equations (4) and (5).

[0070]

[0071] The phase-locked loop (PLL) has a symmetrical structure, as can be seen from the small-signal model of coordinate transformation after using a symmetrical PLL, as shown in formulas (2) and (3). The influence of the PLL in formulas (2) and (3) is dq symmetry.

[0072] Furthermore, a small-signal model of coordinate transformation in the control structure is obtained; at this point, H ig i gabc H iC i Cabc and v PCCabc H is obtained through the abc / d′q′ transformation. ig i′ gdq H iC i′ Cdq and v′ PCCdq , v′ Mdq v is obtained through the d′q′ / abc transformation 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] Example 3:

[0076] This example, based on Example 2, further derives the small-signal model for power calculation in the symmetrical power control loop by using the expression of the power calculation stage; then, a scaling transformation function f(k) = e is added to the power reference of the power loop. -2k The small-signal model of the corrected power reference is obtained; then, the small-signal model of the power loop is obtained based on the two small-signal models. The expressions for the power calculation loop and its small-signal model before the power reference correction are shown in formulas (7) and (8).

[0077]

[0078] Multiply the power reference by the scaling function e -2k The modified power reference and its small-signal model are expressed as shown in equations (9) and (10).

[0079]

[0080] Among them, P ref Q serves as the power reference for active power. ref P serves as the power reference for reactive power. r ′ ef Q is the power reference for the corrected active power. r ′ ef This 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 modifying the power reference, that is, by introducing a PCC voltage to the power summing point via a feedforward G. PQref_PLL (s) to offset G v_Sym (s).

[0082] Furthermore, subtracting the power feedforward consisting of the product of the q-axis PCC voltage and the q-axis grid-connected current after coordinate transformation from the control block diagram for active power calculation yields the reshaped active power. Adding the power feedforward consisting of the product of the q-axis PCC voltage and the d-axis grid-connected current after coordinate transformation to the control block diagram for reactive power calculation yields the reshaped reactive power. A function G is then 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 power error at the power summation point and its small-signal model expression after adding the power feedforward are shown in equations (11) and (12).

[0083]

[0084] Among them, Gvff (s) is a feed-forward function, P error and Q error are active power error and reactive power error at power sum point respectively. The power control loop is symmetrical structure by power loop small signal model after symmetrical power loop by formula (12), because the model of power control loop in formula (12) is dq symmetrical. The present application adds high-pass filter G vff (s) in G HPF (s) to avoid the influence of feed-forward quantity on power calculation result at 0Hz in dq coordinate system and transfer function G v (s) to offset the influence of coordinate transformation, G vff (s) and G v (s) are shown in formula (13) and formula (14),

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

[0086]

[0087] The transfer function matrix Y g ′ _PLL (s) of feed-forward branch from PCC voltage to coordinate transformation introduced by symmetrical phase-locked loop is shown in formula (6), i ′ C_PLL (s), G v ′ _PLL (s) and G′ M_PLL (s) are all symmetrical matrix; the transfer function matrix G′ v_PQ (s) of feed-forward branch from PCC voltage to power reference introduced by symmetrical power loop is shown in formula (12), G′ HPF (s) is symmetrical matrix outside the bandwidth of high-pass filter G i_PQ (s) is symmetrical 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 ). The implementation is shown in formula (15) and formula (16), Figure 4The middle pink block diagram shows.

[0089] In the symmetrical power loop of the present 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 ; and 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, power feedforward is added in the original calculation of active power and reactive power, as shown in the middle red block diagram. Figure 4

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

[0091]

[0092] Example Four

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

[0094] In the current loop control process, the grid-connected current i gabc is first sampled, and i' gdq is obtained through coordinate transformation, and then compared with the current reference i' refdq , and the error is sent to the current regulator G i (s). Here, the capacitor current i' Cdq proportional feedback is adopted to realize the suppression of LCL resonance peak. The modulation voltage v' Mdq is obtained through coordinate inverse transformation to obtain v Mabc , and the driving signals of the inverter bridge arm switching tubes Q1-Q6 are obtained through SPWM.

[0095] Subsequently, in the phase-locked loop control, the d-axis and q-axis PCC voltages after coordinate transformation are simultaneously subtracted by the steady-state value V ref and 0 to obtain the deviation of the d-axis and q-axis PCC voltages after coordinate transformation from the reference, and then the result is sent to the regulator G c of the phase-locked loop.In (s), the result is fed into the integration stage 1 / s, and the d-axis and q-axis outputs obtained after the calculation are denoted as k and θ, respectively; k is used to implement the scaling transformation in the coordinate transformation, and the scaling transformation function is f(k) = Ce -k (C is an arbitrary constant; C = 1 is used as an example for explanation); θ is used to implement the rotation transformation in 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 for power calculation is obtained based on the expression in the power calculation stage; then, the scaling transform function e is added to the power reference in the power loop. -2k We obtain a small-signal model of the power reference scaling transformation; then, based on the two small-signal models, we obtain a small-signal model of the power loop.

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

[0098] This invention effectively achieves dq symmetry in the control of the phase-locked loop and power control loop in the grid-connected inverter system, and the control method is relatively simple. It simplifies the mathematical model of the three-phase grid-connected inverter system, making the stability analysis process more concise. It lays the foundation for the parameter optimization design of the three-phase grid-connected inverter system, and has great significance for the stability analysis of the inverter system, with significant engineering application value.

[0099] Application Example 1:

[0100] To better illustrate the effects of this invention, specific embodiments of the symmetrical control method based on a grid-connected inverter system based on this invention are described below.

[0101] This example uses a symmetrical phase-locked loop to obtain the PCC voltage phase θ and scaling ratio k. A symmetrical power loop is used to obtain the grid-connected current reference i′. refdq Grid-connected current i gabc i′ is obtained through coordinate transformation gdq Then, with its current reference i′ refdq The comparison is performed, and the error is fed into the current regulator G. i (s). Here, the capacitor current i′ is used. Cdq Proportional feedback suppresses the LCL resonance peak. Modulation voltage v′ Mdqv is obtained through inverse coordinate transformation Mabc The drive signals for the switching transistors Q1 to Q6 are obtained through SPWM.

[0102] Table 1

[0103]

[0104] like Figure 5 The figure shows the frequency response curves of the four elements of the system return ratio matrix after symmetrical control of the grid-connected inverter system according to the present invention, wherein the main diagonal element of the return ratio matrix is ​​T. dd (s), T qq (s), with T as the secondary diagonal element. 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 main circuit, current loop, phase-locked loop, and power loop parameters of the grid-connected inverter are shown in Table 1. The high-pass filter G... HPF The bandwidth of (s) is taken as 5Hz. From Figure 5 The curves showing the phase angle (Deg) versus frequency (f) and the gain versus frequency (f) show that, outside the bandwidth of the high-pass filter, T... dd (s)=T qq (s) and T dq (s)=-T qd (s), meaning the back-ratio matrix is ​​a symmetric matrix. Figure 5 The control method of this application shows that the transfer function matrix of each link in the system is a symmetric matrix. This means that the small-signal models of the phase-locked loop and the power loop are symmetric matrices. That is, the phase-locked loop and the power loop have achieved symmetric design. At this time, the system meets the simplified conditions for stability analysis.

[0105] Application Example 2:

[0106] This example, based on application example one, uses the proportional coefficient k of the phase-locked loop regulator in Table 1. p_PLL Integral coefficient k of phase-locked loop regulator i_PLL Power loop regulator proportional coefficient k p_PQ Integral coefficient k of power loop regulator i_PQThe parameters are modified to 0.196, 5.88, 0.0000127, 0.025 respectively to make the system oscillate, at this time the waveform of PCC voltage and grid-connected current of the grid-connected inverter system with existing asymmetric control is as shown in Figure 6 The spectrum of corresponding grid-connected current is as shown in Figure 7 .

[0107] The proportional coefficient k p_PLL of phase-locked loop regulator, the integral coefficient k i_PLL of phase-locked loop regulator, the proportional coefficient k p_PQ of power loop regulator and the integral coefficient k i_PQ of power loop regulator are modified to 0.49, 36.75, 0.0000127, 0.025 respectively on the basis of the parameters in Table 1 to make the system oscillate, at this time the waveform of PCC voltage and grid-connected current of the grid-connected inverter system with symmetric control according to the application is as shown in Figure 8 The spectrum of corresponding grid-connected current is as shown in Figure 9 .

[0108] It can be seen from Figure 6 , Figure 7 that when the system oscillates, there is oscillation frequency coupling effect, that is, there are oscillation components with 2 times of power frequency in the system. Figure 8 , Figure 9 It can be seen from that when the system oscillates, there is no oscillation frequency coupling effect, that is, the system is symmetric, and the symmetric control strategy is correct and effective.

[0109] The basic principle, main features and advantages of the application are shown and described above. It should be understood by the person skilled in the art that the application is not limited by the above examples, the above examples and descriptions in the specification are only to illustrate the principle of the application, and the application can be variously changed and improved without departing from the spirit and scope of the application, and these changes and improvements all fall within the scope of the application. The scope of protection of the application is defined by the appended 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 a grid-connected current reference after coordinate transformation. The resulting error is fed into a current regulator, added to the capacitor current proportional feedback, and the output modulated voltage is output to the SPWM module through inverse coordinate transformation. The SPWM module then outputs the drive signal for the inverter bridge arm switching transistors. Its characteristic is that... The phase-locked loop is used to synchronize the grid-connected current with the common coupling point voltage. The phase-locked loop uses the steady-state value to simultaneously subtract the PCC voltage of the d-axis and q-axis after coordinate transformation, and then sends the error to the phase-locked loop regulator for integration. The outputs of the d-axis and q-axis are used for scaling and rotation transformations in the coordinate transformation, respectively, to achieve 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 transformation function is added to the power reference of the power control loop to perform scaling transformation on the power reference. Power feedforward is set in the power calculation module to reshape the output power, resulting in a symmetrical power control loop. The grid-connected inverter system includes a three-phase LCL grid-connected inverter and its control system. The main circuit of the three-phase LCL grid-connected inverter and its control system include a three-phase LCL grid-connected inverter, a current control loop, a phase-locked loop, and a power control loop. The main circuit of the three-phase LCL grid-connected inverter includes: a DC power supply, three-phase three-bridge arms, a three-phase LCL filter, and a three-phase grid-connected switch. The DC power supply, three-phase three-bridge arms, three-phase LCL filter, and three-phase grid-connected switch are connected in sequence. Each phase's LCL filter consists of two inductors L1 and L2 and a capacitor C. Inductors L1 and L2 and the grid-connected switch are connected in series. One end of the capacitor C is connected to the node between inductors L1 and L2, and the other end is connected to the remaining nodes of the capacitors of the other two phases.

2. The symmetrical control method based on a grid-connected inverter system according to claim 1, characterized in that, The specific method for symmetrical coordinate transformation of the d-axis and q-axis in the phase-locked loop control process is as follows: In phase-locked loop control, the steady-state value V is subtracted from the PCC voltages on the d-axis and q-axis after coordinate transformation. ref The deviations of the PCC voltages on the d-axis and q-axis after coordinate transformation from the reference are obtained by adding 0, and then the results are sent to the regulator G of the phase-locked loop. c (s), the result is fed into the integration stage 1 / s, and the d-axis and q-axis outputs obtained after the calculation are denoted as k and θ, respectively, where θ is the PCC voltage phase angle and k is the scaling ratio; k is used to implement the scaling transformation in the coordinate transformation, and the scaling transformation function is f(k) = Ce -k C is a non-zero constant; θ is used to implement the rotation transformation in coordinate transformation, and the rotation transformation function is g(θ) = e -jθ .

3. The symmetrical control method based on a grid-connected inverter system according to claim 2, characterized in that, The symmetrical power control loop uses a power reference multiplied by a function e. -2k A scaling transformation is performed to obtain the corrected power reference; when calculating active power in the power calculation module, the power feedforward of active power is subtracted to obtain the reshaped active power; when calculating reactive power, the power feedforward of reactive power is added to obtain the reshaped reactive power. The active power feedforward is calculated by multiplying the q-axis PCC voltage (after coordinate transformation) and the q-axis grid-connected current, and adding the function G. vff (s) constitute; The reactive power feedforward is the product of the q-axis PCC voltage and the d-axis grid-connected current after the coordinate transformation, plus the function G. vff (s) constitutes.

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

5. The symmetrical control method based on a grid-connected inverter system according to claim 4, characterized in that, In coordinate transformation, for the case of C=1, the small-signal models of the abc / d′q′ transformation and the d′q′ / abc transformation are shown in equation (2) and equation (3), respectively. Where, x′ dq =[x d x q ] T Let x′ be the matrix form of the state variables in the d′q′ coordinate system, where x′ d and x′ q These are its d′ and q′ axis components, respectively, and Xd and Xq are the state variables x and q, respectively. d and x q steady-state value; x abc =[x a x b x c ] T Let x be the matrix form of the state variables in the abc coordinate system, where x a x b x c These are its a-axis, b-axis, and c-axis components, respectively; θ o k0 and X i θ, k and x respectively i The steady-state values, where i = a, b, c, d, q; For x i Small perturbation signals, where i = a, b, c, d, q; 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 equations (4) and (5).

6. The symmetrical control method based on a grid-connected inverter system according to claim 5, characterized in that, In the control structure, the grid-connected current feedback H ig i gabc Capacitor current feedback H iC i Cabc PCC voltage v PCCabc H is obtained through the abc / d′q′ transformation ig i′ gdq H iC i′ Cdq and v′ PCCdq Small signal model; And the modulated signal is transformed by d′q′ / abc to obtain v Mabc The expression for the small-signal model is shown in formula (6): Among them, I gi I Ci V PCCi V Mi i gi i Ci v PCCi v Mi The steady-state value of H when i = d, q ig H is the sampling coefficient for grid-connected current; iC The sampling coefficient for capacitor current.

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

8. The symmetrical control method based on a grid-connected inverter system according to claim 7, characterized in that, In the control structure, the power error at the power summation point after adding power feedforward and its small-signal model expression are shown in equations (11) and (12). Among them, G vff (s) is the feedforward function, P error and Q error These represent the active power error and reactive power error at the power summation point, respectively; G vff (s) and G v The expression for (s) is shown in formulas (13) and (14). G vff (s)=3G HPF (s)G v (s) (13) Among them, G HPF (s) represents the transfer function of the high-pass filter.

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