A grid-connected converter control method based on hybrid control of following the grid and forming the grid

Abstract

This application relates to the field of new energy grid-connected converter control technology, and particularly to a grid-connected converter control method based on hybrid control of grid-following and grid-forming. By constructing an adaptive hybrid synchronization control model, the ratio of grid-following and grid-forming characteristics is flexibly adjusted, ensuring a smooth and efficient grid connection process. A voltage-current double-loop control model is constructed to precisely regulate the output voltage and current, meeting the high-standard power quality requirements of the power grid for new energy power generation. At the same time, grid impedance estimation is introduced for virtual impedance control, further enhancing the anti-interference ability and response speed of the system. In summary, the grid-connected converter control method using hybrid control of grid-following and grid-forming in this application significantly improves the adaptability and stability of the system to complex grid environments, and enhances the overall operation efficiency and reliability of the system.

Description

Technical Field

[0001] The present application relates to the technical field of new energy grid-connected converter control, and particularly to a grid-connected converter control method based on hybrid control of grid-following and grid-forming. Background Art

[0002] With the continuous growth of the global demand for renewable energy, new energy power generation technologies, especially grid-connected power generation systems based on converters, have been widely used. These systems convert the direct current generated by new energy (such as solar energy, wind energy, etc.) into alternating current through a converter and safely and efficiently connect to the grid. As a key device connecting new energy power generation equipment and the grid, the control strategy of the grid-connected converter is crucial for the stability and reliability of the system.

[0003] Currently, the current closed-loop vector control method based on rotational coordinate transformation and phase-locked loop (PLL) is widely used in wind power and photovoltaic grid-connected systems. The converter adopting this control method is often called a grid-following (GFL) converter. For example, the patent with the publication number CN115483843A provides an open-loop synchronization method that can improve the global stability of the grid-connected converter. By providing a grid synchronization current reference for the grid-following control of the grid-connected converter, the non-linear structure of the grid-connected inverter is greatly simplified, and the robustness of the grid-connected inverter system is significantly improved. The grid-following control has good stability and fast power response speed under a strong grid. It can not only maximize the utilization rate of new energy through maximum power point tracking, with good economy, but also has relatively mature technology, and is the mainstream control technology for new energy grid-connected converters at the present stage.

[0004] However, when a large amount of new energy is connected to the power grid, the proportion of synchronous generators decreases relatively, the power grid strength drops, and the power grid exhibits the characteristics of a "weak power grid", with the system showing low inertia and weak damping. In the case of a weak power grid, the input of the phase-locked loop is usually the voltage at the point of common coupling (PCC) or the voltage of the filter capacitor, which causes mutual coupling between the inverter, the phase-locked loop, and the power grid, easily leading to system instability and restricting the power transmission capacity. Therefore, the concept of Grid forming (GFM) converter control is proposed. Because of its external characteristics of a voltage source, it can provide voltage and frequency support for weak power systems and is widely used in new power systems with a high proportion of new energy and a high degree of power electronics. For example, the patent with the publication number CN115513995A discloses a GFM converter control method and system based on PQ compensation for a weak power grid, which reduces the active power output by the converter during a fault to reduce the current on the converter side, thereby suppressing the overcurrent in the system during the fault, achieving low-voltage ride-through, and improving the stability of the weak power system.

[0005] However, GFM control has problems such as slow power regulation speed, strong multi-machine coupling, poor stability and economy in a strong power grid. In an actual power system, changes in power generation equipment of the power grid will cause significant changes in the power grid strength, which is usually measured by the short-circuit ratio (SCR) at the point of common coupling. It is difficult for new energy grid-connected converters with a single grid-following or GFM control mode to meet the stability requirements under large SCR fluctuations in an actual power system. Summary of the Invention

[0006] Based on this, it is necessary to provide a grid-connected converter control method based on a hybrid control of grid-following and GFM to address the problem that new energy grid-connected converters with a single grid-following or GFM control mode are difficult to meet the stability requirements under large SCR fluctuations in an actual power system.

[0007] The present application provides a grid-connected converter control method based on a hybrid control of grid-following and GFM, and the method includes:

[0008] Step S1, establishing an adaptive hybrid synchronization control model, a voltage-current double-loop control model, and a virtual impedance control model; the adaptive hybrid synchronization control model includes a phase-locked loop control model, a power synchronization control model, and a ratio adaptive adjustment model;

[0009] Step S2, acquiring the voltage and current of the AC bus;

[0010] Step S3: Obtain the output angular velocity of the phase-locked loop according to the phase-locked loop control model and the voltage of the AC bus; obtain the grid synchronization angular velocity and the excitation electromotive force according to the power synchronization control model and the voltage and current of the AC bus; obtain the grid-following characteristic ratio and the grid-forming characteristic ratio according to the ratio adaptive regulation model and the voltage and current of the AC bus.

[0011] Step S4: Obtain the converter synchronization angle according to the grid-following characteristic ratio, the grid-forming characteristic ratio, the output angular velocity of the phase-locked loop, and the grid synchronization angular velocity.

[0012] Step S5: Obtain the voltage outer loop control parameter according to the converter synchronization angle, the excitation electromotive force, and the virtual impedance control model.

[0013] Step S6: Generate the working control parameter of the grid-connected converter according to the voltage outer loop control parameter and the voltage-current double-loop control model.

[0014] In one embodiment, the expression of the phase-locked loop control model is:

[0015]

[0016] In the formula, U o is the amplitude of the AC bus voltage, u oq is the q-axis component of the AC bus voltage, k pllp is the proportional coefficient of the phase-locked loop PI controller, k plli is the integral coefficient of the phase-locked loop PI controller, ω pll is the output angular velocity of the phase-locked loop, θ pll is the output phase angle corresponding to the output angular velocity ω pll θ pllref is the reference value of the output phase angle.

[0017] In one embodiment, the expression of the power synchronization control model is:

[0018]

[0019] In the formula, ω is the mechanical angular velocity, ω PBS represents the grid synchronization angular velocity, ω0 represents the grid reference angular velocity, θ PBS represents the virtual rotor position angle corresponding to the grid synchronization angular velocity ω PBS J is the moment of inertia, D is the damping torque, T m is the given value of the virtual synchronous generator torque, T e is the actual output value of the virtual synchronous generator torque, P m is the mechanical power, P e is the active power, T d is the damping torque, K Qis the PI regulation coefficient, U ref is the reference voltage, Q ref is the reference reactive power, Q e is the reactive power, Q d is the reactive power with primary voltage regulation function, E is the excitation electromotive force, U n is the amplitude of the terminal voltage, U0 is the amplitude of the AC bus voltage, P ref is the reference active power, K p is the active - frequency regulation coefficient, K q is the reactive - voltage regulation coefficient; among them, the expressions of the AC bus voltage amplitude U0 and the grid reference angular velocity ω0 are respectively:

[0020] ω o ≈ω pll =k pllp u oq +k plli ∫u oq dt

[0021]

[0022] In the formula, ω pll is the output angular velocity of the phase - locked loop, u od is the d - axis component of the AC bus voltage, u oq is the q - axis component of the AC bus voltage, k pllp is the proportional coefficient of the phase - locked loop PI controller, k plli is the integral coefficient of the phase - locked loop PI controller.

[0023] In one of the embodiments, the ratio adaptive regulation model includes an SCR identification unit and a ratio adjustment unit. The expression of the SCR identification unit is:

[0024]

[0025] In the formula, SCR is the short - circuit ratio, S sc is the short - circuit capacity, S N is the rated capacity of the generator, Z * is the per - unit value of the equivalent impedance of the power grid;

[0026] The expression of the ratio adjustment unit is:

[0027]

[0028] In the formula, SCR high represents the short - circuit ratio when the power grid shows strong grid characteristics, SCR low represents the short - circuit ratio when the power grid shows weak grid characteristics, r PLL is the grid - following characteristic ratio, α is a real parameter, 0 ≤ α ≤ 2π.

[0029] In one embodiment, the expression of the equivalent impedance of the power grid is as follows:

[0030]

[0031] In the formula, R g represents the power grid resistance, L g represents the power grid inductance, R gs represents the total impedance, X gs represents the total inductive reactance, L gg represents the grid-side inductor of the LCL filter; among them, the expressions of the total impedance and the total inductive reactance are respectively:

[0032]

[0033] In the formula, P e is the active power transmitted from the grid-connected converter to the power grid, Q e is the reactive power transmitted from the grid-connected converter to the power grid, v gf_ref represents the reference voltage of the converter output voltage, Δv gf_ref represents the voltage amplitude disturbance.

[0034] In one embodiment, the ratio adjustment unit includes a grid-following characteristic ratio look-up table, and the calculation steps of the grid-following characteristic ratio include:

[0035] Step S31, obtaining the grid-following characteristic ratio according to the short-circuit ratio of the point of common coupling and the grid-following characteristic ratio look-up table.

[0036] In one embodiment, the calculation formula of the converter synchronization angle is:

[0037]

[0038] In the formula, θ is the converter synchronization angle, ω0 is the grid reference angular velocity, θ0 is the integral of the grid reference angular velocity, ω PLL is the output angular velocity of the phase-locked loop, θ PLL is the integral of the output angular velocity of the phase-locked loop, ω PBS is the grid synchronization angular velocity, θ PBS is the integral of the grid synchronization angular velocity, r PLL is the grid-following characteristic ratio, r PBS is the grid-forming characteristic ratio, and the sum of the grid-following characteristic ratio and the grid-forming characteristic ratio is 1, and S is the Laplace operator.

[0039] In one embodiment, the voltage-current double-loop control model includes a voltage outer-loop control loop and a current inner-loop control loop, and the expression of the voltage outer-loop control loop is:

[0040]

[0041] In the formula, i dref represents the d-axis component of the reference current, and i qref represents the q-axis component of the reference current. u dref represents the d-axis component of the reference voltage, and u qref represents the q-axis component of the reference voltage. K PU1 and K PU2 are both the proportionality coefficients of the voltage-loop PI controller. K IU1 and K IU are both the integral coefficients of the voltage-loop PI controller. ω g is the angular frequency of the sending-end power grid, C f is the filter capacitor, S is the Laplace operator, and i od is the d-axis component of the AC bus current, and i oq is the q-axis component of the AC bus current;

[0042] The expression of the current inner-loop control loop is:

[0043]

[0044] In the formula, i Ld is the d-axis component of the output current of the grid-connected converter, and i Lq is the q-axis component of the output current of the grid-connected converter. L f is the filter inductor, and K PI1 and K PI2 are both the proportionality coefficients of the current-loop PI controller. K II1 and K II2 are both the integral coefficients of the current-loop PI controller.

[0045] In one embodiment, the expression of the virtual impedance control model is:

[0046] u dref = -R s i Ld + e d + wLi Lq

[0047] u qref = -R s i Ld + e q - wLi Lq

[0048] In the formula, e d , e q are the dq-axis components of the excitation electromotive force, R s is the virtual resistance value, and wL is the virtual reactance value.

[0049] The above grid-connected converter control method based on the hybrid control of grid-following and grid-forming flexibly adjusts the ratio of grid-following and grid-forming characteristics by constructing an adaptive hybrid synchronization control model, ensuring a smooth and efficient grid connection process. A voltage-current double-loop control model is constructed to accurately regulate the output voltage and current, meeting the high-standard power quality requirements of the power grid for new energy power generation. At the same time, grid impedance estimation is introduced for virtual impedance control, further enhancing the anti-interference ability and response speed of the system. In summary, the grid-connected converter control method based on the hybrid control of grid-following and grid-forming provided by this application significantly improves the adaptability and stability of the system to complex grid environments, and enhances the overall operation efficiency and reliability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 FIG. is a schematic structural diagram of a new energy grid-connected power generation system in an embodiment;

[0051] Figure 2 FIG. is a schematic flow diagram of a grid-connected converter control method based on the hybrid control of grid-following and grid-forming in an embodiment;

[0052] Figure 3 FIG. is a schematic structural diagram of an adaptive hybrid synchronization control model in an embodiment;

[0053] Figure 4 (a) FIG. is a schematic diagram of equivalent impedance estimation of a new energy grid-connected power generation system in an embodiment;

[0054] Figure 4 (b) FIG. is a phasor diagram of equivalent impedance estimation of a new energy grid-connected power generation system in an embodiment;

[0055] Figure 5 FIG. is a schematic diagram of the relationship between the grid-following characteristic ratio and the short-circuit ratio in an embodiment;

[0056] Figure 6 FIG. is a schematic structural diagram of a voltage-current double-loop control model in an embodiment;

[0057] Figure 7 FIG. is a schematic structural diagram of a virtual impedance control model in an embodiment;

[0058] Figure 8 FIG. is a schematic diagram of the relationship between the grid-following characteristic ratio and the short-circuit ratio in another embodiment. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0059] In order to make the objectives, technical solutions and advantages of this application clearer, the following further elaborates on this application in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely used to explain this application and are not intended to limit this application.

[0060] A grid-connected inverter control method based on hybrid control of following the grid and forming the grid provided by an embodiment of the present application can be applied to, for example, Figure 1 the control device in the new energy grid-connected power generation system shown in the figure. The system includes a new energy grid-connected inverter (hereinafter simply referred to as the grid-connected inverter 101), a power grid 102, and a control device 103. Among them, the grid-connected inverter 101 is connected to the power grid 102 through an AC bus, Figure 1 where Lf is the inductor on the converter side of the LCL filter, Rf is the filter resistor, Cf is the filter capacitor, Lgg is the inductor on the grid side of the LCL filter, Rg is the grid resistor, Lg is the grid inductor, and Vg represents the grid voltage source. The first end of the control device 103 is connected to the power grid 102, and the second end is connected to the grid-connected inverter 101.

[0061] Among them, the control device 103 includes an abc-dq coordinate conversion module, a power calculation module, a hybrid control module 113, a dq-abc coordinate conversion module, and a PWM modulator. The abc-dq coordinate conversion module is used to convert the AC bus voltage U o(abc) into the voltage components U od and U oq in the dq coordinate system, convert the AC bus current I o(abc) into the current components I od and I oq in the dq coordinate system, and convert the output current i L (abc) of the grid-connected inverter into the current components i Ld and i Lq in the dq coordinate system. The power calculation module is used to calculate the active power P od and the reactive power Q oq transmitted from the grid-connected inverter 101 to the power grid according to U od , U oq , I e and I e .

[0062] The hybrid control module 113 first establishes an adaptive hybrid synchronization control model, a virtual impedance control model, and a voltage-current double-loop control model. The adaptive hybrid synchronization control model includes a phase-locked loop control model, a power synchronization control model, and a ratio adaptive regulation model. Then, it respectively obtains the phase-locked loop output angular velocity according to the phase-locked loop control model and the voltage of the AC bus, obtains the grid synchronization angular velocity and the excitation electromotive force according to the power synchronization control model and the voltage and current of the AC bus, and obtains the grid-following characteristic ratio and the grid-forming characteristic ratio according to the ratio adaptive regulation model and the voltage and current of the AC bus. Then, it obtains the converter synchronization angle according to the grid-following characteristic ratio, the grid-forming characteristic ratio, the phase-locked loop output angular velocity, and the grid synchronization angular velocity, and obtains the voltage outer-loop control parameter according to the converter synchronization angle, the excitation electromotive force, and the virtual impedance control model. Finally, it generates the working control parameter of the grid-connected converter according to the voltage outer-loop control parameter and the voltage-current double-loop control model, and the working control parameter is used to control the grid-connected converter to perform adaptive grid connection.

[0063] The control method of the grid-connected converter based on the hybrid control of grid-following and grid-forming of the present application will be described in detail below with reference to some specific embodiments.

[0064] Embodiment 1

[0065] An embodiment of the present disclosure provides a control method for a grid-connected converter based on the hybrid control of grid-following and grid-forming, as Figure 2 shown, which specifically includes the following steps:

[0066] Step S1, establish an adaptive hybrid synchronization control model, a voltage-current double-loop control model, and a grid impedance estimation model. The adaptive hybrid synchronization control model includes a phase-locked loop control model, a power synchronization control model, and a ratio adaptive regulation model.

[0067] The adaptive hybrid synchronization control model realizes grid synchronization through the phase-locked loop control model. The power synchronization control model simulates the characteristics of a virtual synchronous generator (VSG). The ratio adaptive regulation model dynamically adjusts the ratio of the grid-following and grid-forming characteristics according to the change of the grid strength. Among them, the specific structure of the adaptive hybrid synchronization control model is as Figure 3 shown. The phase-locked loop control model includes a voltage coordinate transformation unit and a PI controller, which are used to track the voltage phase of the AC bus and output the corresponding angular velocity. Its mathematical expression is as shown in Equation (1):

[0068]

[0069] In the formula, U o is the amplitude of the AC bus voltage, u oq is the q-axis component of the AC bus voltage, k pllp is the proportional coefficient of the phase-locked loop PI controller, k plliis the integral coefficient of the PLL PI controller, ω pll is the output angular velocity of the PLL, θ pll is the output angular velocity ω of the PLL pll corresponding output phase angle, θ pllref is the preset reference value of the output phase angle.

[0070] The power synchronization control model includes an active power control loop and a reactive power control loop. The active power control loop is used to output the grid synchronization angular velocity ω e according to the active power P transmitted by the grid-connected converter to the grid PBS , and the reactive power control loop is used to output the excitation electromotive force E according to the reactive power Q transmitted by the grid-connected converter to the grid e . Among them, the active power P e and the reactive power Q e can both be calculated according to the dq components (u o(abc) , u od , u oq ) of the AC bus voltage U o(abc) and the dq components (i od , i oq ) of the AC bus current I

[0071]

[0072] In the formula, ω is the mechanical angular velocity, ω PBS represents the grid synchronization angular velocity, ω0 represents the grid reference angular velocity, and the grid reference angular velocity in this embodiment is the AC bus voltage frequency, θ PBS represents the virtual rotor position angle corresponding to the grid synchronization angular velocity ω PBS , J is the moment of inertia, D is the damping torque, T m is the virtual synchronous generator torque given value, T e is the actual output value of the virtual synchronous generator torque, which can be obtained from P m is the mechanical power, P e is the electromagnetic power, that is, the active power, T d is the damping torque, K Q is the PI regulation coefficient, U ref is the reference voltage, specifically the reference voltage of the excitation electromotive force, Q ref is the reference reactive power, Q e is the reactive power, Q dFor the reactive power with the primary voltage regulation function, E is the excitation electromotive force, and U n is the amplitude of the terminal voltage of the machine, U0 is the amplitude of the AC bus voltage, and P ref is the reference active power, and K p is the active-power frequency regulation coefficient, and K q is the reactive-power voltage regulation coefficient. Among them, the amplitude U0 of the AC bus voltage and the frequency ω0 of the AC bus voltage are obtained through the phase-locked loop control model, and the specific expressions are shown in Equations (4) and (5) respectively:

[0073] ω o ≈ω pll =k pllp u oq +k plli ∫u oq dt (4)

[0074]

[0075] In the formula, ω pll is the output angular velocity of the phase-locked loop, u od is the d-axis component of the AC bus voltage, and u oq is the q-axis component of the AC bus voltage, and k pllp is the proportional coefficient of the phase-locked loop PI controller, and k plli is the integral coefficient of the phase-locked loop PI controller.

[0076] The ratio adaptive regulation model includes an SCR recognition unit and a ratio adjustment unit. The SCR recognition unit is used to calculate the short-circuit ratio SCR of the point of common coupling, and its mathematical expression is shown in Equation (6):

[0077]

[0078] In the formula, SCR is the short-circuit ratio, S sc is the short-circuit capacity, which can be calculated from the voltage and current of the AC bus. The specific calculation formula is In this formula, U PCC and I PCC respectively represent the voltage and current of the point of common coupling. The voltage and current of the point of common coupling in this embodiment are the voltage U0 and current I0 of the AC bus. S N is the rated capacity of the generator, and Z * is the per-unit value of the equivalent impedance of the power grid.

[0079] Furthermore, this embodiment also provides an impedance estimation method for calculating the equivalent impedance of the power grid. Specifically, Figure 4 (a) is a simplified schematic diagram of the power grid impedance estimation. In the figure, the grid-connected converter is modeled as a controllable AC voltage source V gf ∠0°, and the converter output voltage Vgf The phase of ∠0° is used as the reference angle, I gg is the grid current phasor, V s ∠-δ is the grid voltage phasor, Z gs ∠θ is the impedance connected between the two voltage sources. This impedance is composed of the grid-side inductor L of the LCL filter gg , the grid resistance R g and the grid inductor L g . Therefore, the impedance Z gs ∠θ can be expressed by Equation (7):

[0080] Z gs ∠θ = R gs + jX gs = R g + jω0(L gg + L g ) (7)

[0081] In the formula, R gs is the total impedance, R gs = R g , X gs is the total inductive reactance, X gs = L gg + L g .

[0082] Based on Figure 4 (a), the grid resistance and the grid inductor are estimated for impedance. To more intuitively reflect the relationship between the phasors, Figure 4 (b) shows a phasor diagram including each phasor. According to Figure 4 (b), the expression of the grid current phasor can be derived, as shown in Equation (8) specifically:

[0083]

[0084] In addition, the active power Pe and the reactive power Qe transmitted by the grid-connected converter to the grid are obtained by the complex product of the converter output voltage and the conjugate current phasor, as shown in Equations (9)-(10):

[0085]

[0086] Generally speaking, the grid-connected converter can operate under voltage or power control. In the case of voltage control, the power control part should be ignored, and at this time, the voltage amplitude V gf and the phase angle θ of the grid-connected converter can be directly adjusted. When the control device is working properly, the voltage amplitude V gf and the phase angle δ of its output voltage are as shown in Equations (11)-(12):

[0087] Vgf = v gf_ref = V gf_ref + rv gf_ ref(11)

[0088] δ = θ PBS (12)

[0089] wherein, v gf_ref represents the reference voltage of the converter output voltage, that is, the reference value of the output voltage after the converter is equivalent to a controllable AC voltage source, V gf_ref is the reference voltage amplitude, and its value is equal to the grid voltage amplitude, that is, V gf_ref = V s , V s is obtained before grid connection to avoid inrush current, Δv _gf_ref represents the voltage amplitude disturbance, and its value is equal to Figure 3 the output value of the PI controller in the reactive power control loop in gf_ref that is, Δv

[0090]

[0091] Furthermore, according to equations (13) and (14), the mathematical expressions of the total impedance R gs and the total impedance X gs can be derived, as shown in equations (15)-(16) specifically:

[0092]

[0093] Finally, the equivalent impedance of the grid obtained is as shown in equations (17)-(18), including the grid resistance shown in equation (17) and the grid inductance shown in equation (18):

[0094] R g = R gs (17)

[0095]

[0096] In the formula, R g represents the grid resistance, L g represents the grid inductance, and L gg represents the grid-side inductance of the LCL filter.

[0097] The ratio adjustment unit obtains an approximate relationship between the SCR and the grid-following characteristic ratio as shown in Figure 5 by simulating the hysteresis loop of the magnetic material, and its expression is as shown in equation (19):

[0098]

[0099] where α is a real parameter (0 ≤ α ≤ 2π), and SCR high represents the short-circuit ratio when the power grid exhibits the characteristics of a "strong power grid", and SCR low represents the short-circuit ratio when the power grid exhibits the characteristics of a "weak power grid". The hysteresis loop characteristic helps to reduce the sensitivity to small fluctuations of SCR, thereby preventing rapid changes in the grid-following or grid-forming characteristics.

[0100] The voltage-current double-loop control model includes a voltage outer-loop control circuit and a current inner-loop control circuit. By precisely controlling the output voltage and current of the grid-connected converter, it ensures that the new energy power generation system can supply power to the power grid according to the preset power and power quality requirements. Figure 6 shows the specific structure of the voltage-current double-loop control model. From Figure 6 it can be seen that the expression of the voltage outer-loop control circuit is shown in Equations (20)-(21), and the expression of the current inner-loop control circuit is shown in Equations (22)-(23):

[0101]

[0102] where i dref represents the d-axis component of the reference current, and i qref represents the q-axis component of the reference current, u dref represents the d-axis component of the reference voltage, and u qref represents the q-axis component of the reference voltage, K PU1 and K PU2 are both the proportionality coefficients of the voltage-loop PI controller, K IU1 and K IU are both the integral coefficients of the voltage-loop PI controller, ω g is the angular frequency of the sending-end power grid, C f is the filter capacitor, S is the Laplace operator, i od is the d-axis component of the AC bus current, and i oq is the q-axis component of the AC bus current.

[0103]

[0104] where i Ld is the d-axis component of the output current of the grid-connected converter, i Lq is the q-axis component of the output current of the grid-connected converter, L f is the filter inductor, K PI1 and K PI2 are both the proportionality coefficients of the current-loop PI controller, K II1 and K II2 are both the integral coefficients of the current-loop PI controller.

[0105] Figure 7 shows the specific structure of the virtual impedance control model, and its mathematical expression is:

[0106] u dref =-R s i Ld +e d +wLi Lq (24)

[0107] u qref =-R s i Ld +e q -wLi Lq

[0108] In the formula, e d , e q are the dq-axis components of the excitation electromotive force, R s is the virtual resistance value, corresponding to R in Figure 1 , and wL is the virtual reactance value, corresponding to L in f . Figure 1 f .

[0109] Step S2, obtain the voltage and current of the AC bus.

[0110] Among them, the voltage of the AC bus is the three-phase voltage U o(abc) , and the current of the AC bus is the three-phase current I o(abc) .

[0111] Step S3, obtain the output angular velocity of the phase-locked loop according to the phase-locked loop control model and the voltage of the AC bus, obtain the grid synchronization angular velocity and the excitation electromotive force according to the power synchronization control model and the voltage and current of the AC bus, and obtain the grid-following characteristic ratio and the grid-forming characteristic ratio according to the ratio adaptive regulation model and the voltage and current of the AC bus.

[0112] Specifically, first, obtain the output angular velocity ω pll of the phase-locked loop according to the phase-locked loop control model shown in Equation (1) with the AC bus voltage Uo. Then, calculate the excitation electromotive force E and the grid synchronization angular velocity ω e and the reactive power Q e according to the active power P PBS and the power synchronization control model shown in Equations (2)-(3). Then, obtain the grid-following characteristic ratio r PLL according to the voltage and current of the AC bus and the ratio adaptive regulation model shown in Equations (6) and (19). The value of the grid-following characteristic ratio r PLL is usually less than 1. Finally, use the result of subtracting the grid-following characteristic ratio r PLL from 1 as the grid-forming characteristic ratio r​PBS value

[0113] Step S4: Obtain the converter synchronization angle based on the grid-following characteristic ratio, grid-forming characteristic ratio, PLL output angular velocity, and grid synchronization angular velocity.

[0114] Specifically, the calculation formula for the converter synchronization angle is shown in Equation (25):

[0115]

[0116] In the formula, θ is the converter synchronization angle, ω0 is the grid reference angular velocity, θ0 is the integral of the grid reference angular velocity, ω PLL is the PLL output angular velocity, r PLL is the grid-following characteristic ratio, θ PLL is the PLL output phase angle, ω PBS is the grid synchronization angular velocity, θ PBS represents the virtual rotor position angle, which is the integral of ω PBS , r PBS is the grid-forming characteristic ratio, and S is the Laplace operator.

[0117] Step S5: Obtain the voltage outer-loop control parameters based on the converter synchronization angle, excitation electromotive force, and virtual impedance control model.

[0118] Among them, the voltage outer-loop control parameters include the dq-axis components u dref and u qref . Specifically, as Figure 7 shown, the virtual impedance control model first performs a coordinate transformation on the converter synchronization angle θ and the excitation electromotive force E to obtain the d-axis component u d and the q-axis component u q , and then calculates u f and u Ld and the q-axis component i Lq of the current transmitted to the filter inductor L d and e q and the dq-axis components e dref and u qref .

[0119] Step S6: Generate the working control parameters of the grid-connected converter based on the voltage outer-loop control parameters and the voltage-current double-loop control model. The working control parameters are used to control the grid-connected converter for adaptive grid connection.

[0120] Among them, the working control parameters of the grid-connected converter include the components u d and u q, i.e., the output value of the current inner-loop control loop.

[0121] Specifically, first, according to u obtained in step S5 dref and u qref and the voltage-current double-loop control model shown in equations (20)-(23), obtain u d and u q , then perform coordinate transformation on u d and u q , convert the target working voltage from the dq rotating coordinate system to the abc three-phase stationary coordinate system, so as to obtain the control voltage component u abc in the abc three-phase stationary coordinate system. Then, based on this control voltage component u abc , further generate the drive signals of each switch tube in the grid-connected converter. These drive signals regulate the voltage amplitude and phase angle of the grid-connected converter output by controlling the on-off of the switch tubes, so as to achieve precise tracking of the power grid and adaptive grid connection.

[0122] The grid-connected converter control method based on the hybrid control of grid following and grid forming disclosed in this embodiment flexibly adjusts the ratio of grid following and grid forming characteristics by constructing an adaptive hybrid synchronization control model, ensuring a smooth and efficient grid connection process. A voltage-current double-loop control model is constructed to precisely regulate the output voltage and current, meeting the high-standard power quality requirements of the power grid for new energy power generation. At the same time, grid impedance estimation is introduced for virtual impedance control, further enhancing the anti-interference ability and response speed of the system. In summary, the grid-connected converter control method using the hybrid control of grid following and grid forming in this embodiment significantly improves the adaptability and stability of the system to complex grid environments, and improves the overall operation efficiency and reliability of the system.

[0123] Embodiment 2

[0124] The difference from Embodiment 1 is that the ratio adjustment unit of this embodiment includes a grid-following characteristic ratio look-up table, and step S3 includes:

[0125] Step S31, obtain the grid-following characteristic ratio according to the short-circuit ratio of the point of common coupling and the grid-following characteristic ratio look-up table.

[0126] Among them, the grid-following characteristic ratio look-up table includes the grid-following characteristic ratios corresponding to the short-circuit ratios of different points of common coupling. Table 1 shows a preferred implementation of the grid-following characteristic ratio look-up table. When SCR ≤ 2, r PLL = 0. When 2 < SCR ≤ 5, r PLL = 10%. When 5 < SCR ≤ 10, r PLL = 20%. When 10 < SCR ≤ 15, r PLL = 90%. When SCR > 15, r PLL= 100%. It can be understood that those skilled in the art can adjust the relationship between the short - circuit ratio and the characteristic ratio in the table according to actual needs to apply to different application scenarios.

[0127] Table 1

[0128] SCR ≤ 2 2 < SCR ≤ 5 5 < SCR ≤ 10 10 < SCR ≤ 15 SCR > 15 <![CDATA[r PLL = 0]]> <![CDATA[r PLL = 10%]]> <![CDATA[r PLL = 20%]]> <![CDATA[r PLL = 90%]]> <![CDATA[r PLL = 100%]]>

[0129] Specifically, after completing the impedance estimation, calculate the short - circuit ratio SCR of the common coupling point according to Equation (5), then look up the corresponding grid - following characteristic ratio from the grid - following characteristic ratio look - up table based on the value of SCR, and then obtain the grid - forming characteristic ratio based on the grid - following characteristic ratio. Particularly, Figure 8 shows a preferred embodiment of the short - circuit ratio SCR and the grid - following characteristic ratio r PLL From the figure, it can be seen that the relationship between SCR and r PLL satisfies the grid - following characteristic ratio look - up table shown in Table 1.

[0130] In this embodiment, by obtaining the corresponding grid - following characteristic ratio from the grid - following characteristic ratio look - up table according to the value of SCR, the calculation amount is greatly reduced, thereby further improving the control efficiency of the grid - following and grid - forming hybrid control.

[0131] The technical features of the above - described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above - described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.

[0132] The above - described embodiments only represent several implementation manners of the present application. Their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several deformations and improvements can still be made, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A grid-connected converter control method based on hybrid control of grid following and grid building, characterized in that: The method comprises: Step S1, establishing an adaptive hybrid synchronous control model, a voltage and current dual-loop control model and a virtual impedance control model; the adaptive hybrid synchronous control model includes a phase-locked loop control model, a power synchronous control model and a ratio adaptive adjustment model; the ratio adaptive adjustment model includes an SCR identification unit and a ratio adjustment unit, and the expression of the SCR identification unit is: Where SCR is the short circuit ratio, S sc is the short-circuit capacity, S N is the rated capacity of the generator, Z * is the per unit value of the equivalent impedance of the power grid; wherein the short-circuit capacity is calculated from the voltage and current of the AC bus; The expression of the ratio adjustment unit is: In the formula, SCR high Indicates the short-circuit ratio when the power grid is strong, SCR low Indicates the short-circuit ratio when the power grid is weak. PLL is the ratio of the network type characteristics, α is a real parameter, 0≤α≤2π; Step S2, obtaining the voltage and current of the AC bus; Step S3, obtaining a phase-locked loop output angular velocity according to the phase-locked loop control model and the voltage of the AC bus, obtaining a power grid synchronization angular velocity and an excitation electromotive force according to the power synchronization control model and the voltage and current of the AC bus, obtaining the grid-following characteristic ratio according to the ratio adaptive adjustment model and the voltage and current of the AC bus, and then subtracting the grid-following characteristic ratio from 1 as a result of obtaining a grid-forming characteristic ratio; Step S4, obtaining a converter synchronization angle according to the grid-following characteristic ratio, the grid-forming characteristic ratio, the phase-locked loop output angular velocity, and the grid synchronization angular velocity; Step S5, obtaining voltage outer loop control parameters according to the converter synchronization angle, the excitation electromotive force and the virtual impedance control model; Step S6, generating working control parameters of the grid-connected converter according to the voltage outer loop control parameters and the voltage-current dual loop control model.

2. The grid-connected converter control method based on hybrid control of grid following and grid building according to claim 1 is characterized in that: The expression of the phase-locked loop control model is: Where U o is the AC bus voltage amplitude, u oq is the q-axis component of the AC bus voltage, k pllp is the proportional coefficient of the phase-locked loop PI controller, k plli is the integral coefficient of the phase-locked loop PI controller, ω pll is the phase-locked loop output angular velocity, θ pll The phase-locked loop outputs an angular velocity ω pll The corresponding output phase angle, θ pllref is the output phase angle reference value.

3. The grid-connected converter control method based on hybrid control of grid following and grid building according to claim 1 is characterized in that: The expression of the power synchronization control model is: Where ω is the mechanical angular velocity, ω PBS represents the grid synchronization angular velocity, ω0 represents the grid reference angular velocity, θ PBS represents the grid synchronous angular velocity ω PBS The corresponding virtual rotor position angle, J is the moment of inertia, D is the damping torque, T m is the torque given value of the virtual synchronous generator, T e is the actual output value of the virtual synchronous generator torque, P m is the mechanical power, P e is the active power, T d is the damping torque, K Q is the PI adjustment coefficient, U ref is the reference voltage, Q ref is the reference reactive power, Q e is the reactive power, Q d is the reactive power with primary voltage regulation function, E is the excitation electromotive force, U n is the amplitude of the terminal voltage, U0 is the amplitude of the AC bus voltage, P ref is the reference active power, K p is the active power-frequency regulation coefficient, K q is the reactive power-voltage regulation coefficient; wherein the expressions of the AC bus voltage amplitude U0 and the grid reference angular velocity ω0 are respectively: ω o ≈ω pll =k pllp u oq +k plli ∫u oq dt In the formula, ω pll is the phase-locked loop output angular velocity, u od is the d-axis component of the AC bus voltage, u oq is the q-axis component of the AC bus voltage, k pllp is the proportional coefficient of the phase-locked loop PI controller, k plli is the integral coefficient of the phase-locked loop PI controller.

4. The grid-connected converter control method based on hybrid control of grid following and grid building according to claim 1 is characterized in that: The expression of the equivalent impedance of the power grid is: In the formula, R g Indicates the grid resistance, L g Represents the grid inductance, R gs Represents the total impedance, X gs Indicates total inductive reactance, L gg represents the grid-side inductance of the LCL filter; wherein the expressions of the total impedance and the total inductive reactance are respectively: Where P e is the active power transmitted from the grid-connected converter to the grid, Q e is the reactive power transmitted from the grid-connected converter to the grid, v gf_ref The reference voltage of the converter output voltage, Δv gf_ref Represents voltage amplitude disturbance.

5. The grid-connected converter control method based on hybrid control of grid following and grid building according to claim 1 is characterized in that: The ratio adjustment unit includes a network-type characteristic ratio lookup table, and the calculation step of the network-type characteristic ratio includes: Step S31, obtaining the grid-following type characteristic ratio by looking up a table according to the short-circuit ratio of the common coupling point and the grid-following type characteristic ratio.

6. The grid-connected converter control method based on hybrid control of grid following and grid building according to claim 1 is characterized in that: The calculation formula of the converter synchronization angle is: Where θ is the converter synchronization angle, ω0 is the grid reference angular velocity, θ0 is the integral of the grid reference angular velocity, ω PLL is the phase-locked loop output angular velocity, θ PLL is the integral of the phase-locked loop output angular velocity, ω PBS is the grid synchronous angular velocity, θ PBS is the integral of the grid synchronization angular velocity, r PLL is the following network characteristic ratio, r PBS is the networking type characteristic ratio, and the sum of the following networking type characteristic ratio and the networking type characteristic ratio is 1, and S is the Laplace operator.

7. The grid-connected converter control method based on hybrid control of grid following and grid building according to claim 1 is characterized in that: The voltage-current dual-loop control model includes a voltage outer loop control loop and a current inner loop control loop. The expression of the voltage outer loop control loop is: In the formula, i dref represents the d-axis component of the reference current, i qref represents the q-axis component of the reference current, u dref represents the d-axis component of the reference voltage, u qref Represents the q-axis component of the reference voltage, K PU1 and K PU2 are the proportional coefficients of the voltage loop PI controller, K IU1 and K IU are the integral coefficients of the voltage loop PI controller, ω g is the angular frequency of the power grid at the sending end, C f is the filter capacitor, S is the Laplace operator, i od is the d-axis component of the AC bus current, i oq is the q-axis component of the AC bus current; The expression of the current inner loop control loop is: In the formula, u d is the d-axis component of the target operating voltage of the grid-connected converter, u q is the q-axis component of the target operating voltage of the grid-connected converter, i Ld is the d-axis component of the grid-connected converter output current, i Lq is the q-axis component of the grid-connected converter output current, L f is the filter inductance, K PI1 and K PI2 are the proportional coefficients of the current loop PI controller, K II1 and K II2 They are the integral coefficients of the current loop PI controller.

8. The grid-connected converter control method based on hybrid control of grid following and grid building according to claim 7 is characterized in that: The expression of the virtual impedance control model is: u dref -R s i Ld +e d +wLi Lq u qref -R s i Ld +e q -wLi Lq In the formula, e d 、e q is the dq axis component of the excitation electromotive force, R s is the virtual resistance value, and wL is the virtual reactance value.

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