Grid-connected converter operation control method based on new synchronous phase locked loop structure

Abstract

Disclosed in the present invention is a grid-connected converter operation control method based on a new synchronous phase locked loop structure. The method comprises: according to a new synchronous phase locked loop structure, performing phase-locked tracking on the voltage of a point of common coupling of a power grid, so as to obtain a common voltage phase; performing a first coordinate transformation on a grid-connected current to obtain dq-axis current components of the grid-connected current; performing a difference calculation on the dq-axis current components and dq-axis current reference values of the grid-connected current to obtain dq-axis current deviation values, and inputting same into a corresponding PI controller to obtain dq-axis modulation signals; according to the common voltage phase, performing a second coordinate transformation on the dq-axis modulation signals, so as to obtain a three-phase modulation signal; and, according to a pulse width modulation coefficient, performing comparative modulation on the three-phase modulation signal and a carrier to obtain a driving signal of an inverter bridge switch device, so as to perform operation control on a grid-connected converter. The present embodiment achieves controlling the tracking of the phase and the amplitude between the grid-connected inverter and the power grid during grid connection, thereby improving the stability of grid-connected converters and grid-connected systems.

Description

A grid-connected converter operation control method based on symmetrical phase-locked loop structure Technical Field

[0001] The present invention relates to the field of grid-connected converters, and in particular to an operation control method of a grid-connected converter based on a symmetrical phase-locked loop structure. Background Art

[0002] With the continuous innovation of renewable energy generation technologies, power electronic devices using power semiconductors as switching elements are widely used across the generation, transmission, and consumption sides of power systems. This has led to a gradual increase in the degree of power electronics in the "source-grid-load" system, further promoting the rapid development of renewable energy generation towards high penetration. The new characteristics of the power system have also led to the grid gradually exhibiting weak grid characteristics, meaning that grid impedance is not negligible. At the same time, within the power electronics power system, renewable energy generation systems, primarily centralized and distributed wind turbines and photovoltaics, have experienced rapid development. The development of these diverse renewable energy generation systems has led to a diversity of control objectives within power electronic converters, complex control structures, and varying control timescales, leading to interactions between converters and the grid. Furthermore, the intermittent, random, and volatile output of wind and photovoltaic power can lead to imbalances in source-load supply and demand, disrupting system voltage and frequency stability.

[0003] Most existing technologies improve the conventional SRF-PLL phase-locked loop structure according to different application scenarios to meet the needs of specific working conditions. For example, by adding a filtering link to the front stage of the traditional phase-locked loop, the influence of the power grid background harmonics on the output performance of the phase-locked loop is suppressed. Although there are various forms of improved phase-locked structures based on the traditional SRF-PLL, the control structure involved is relatively complex. At the same time, most of the existing improved phase-locked loops have not conducted research on their applicability to weak power grids, and are unable to solve the problem of the fluctuation amplitude of the phase-locked loop output frequency amplitude when the inverter operating state suddenly changes in a weak power grid. With the large-scale access to new energy photovoltaic and wind power generation, as well as the access to random loads such as energy electric vehicles, it is bound to bring greater challenges to the safe and stable operation of the power grid.

[0004] Summary of the Invention

[0005] The present invention provides a grid-connected converter operation control method based on a symmetrical phase-locked loop structure, which realizes the tracking of phase and amplitude between the grid-connected inverter and the power grid during grid connection, thereby improving the stability of the grid-connected converter and the grid-connected system.

[0006] In order to solve the above technical problems, an embodiment of the present invention provides a grid-connected converter operation control method based on a symmetrical phase-locked loop structure, comprising:

[0007] According to the symmetrical phase-locked loop structure, the grid common point voltage is phase-locked and tracked to obtain the common voltage phase; wherein the common voltage phase includes the true voltage amplitude and the true voltage phase estimation value;

[0008] Performing a first coordinate transformation on the grid-connected current according to the common voltage phase to obtain dq-axis current components of the grid-connected current; wherein the dq-axis current components include a d-axis current component and a q-axis current component;

[0009] Performing a difference operation on the dq-axis current components of the grid-connected current and the dq-axis current reference value to obtain a dq-axis current deviation, inputting the dq-axis current deviation into a corresponding PI controller, obtaining a dq-axis modulation signal at the output end of the PI controller, and performing a second coordinate transformation on the dq-axis modulation signal according to the common voltage phase to obtain a three-phase modulation signal; wherein the dq-axis current reference value includes a d-axis current reference value and a q-axis current reference value;

[0010] According to the pulse width modulation coefficient, the three-phase modulation signal and the carrier are compared and modulated to obtain the driving signal of the inverter bridge switch device, and the grid-connected converter is controlled according to the driving signal of the inverter bridge switch device.

[0011] In an embodiment of the present invention, a symmetrical phase-locked loop structure is used to phase-lock the grid common point voltage and track it to obtain a common voltage phase. The common voltage phase includes a true voltage amplitude and an estimated true voltage phase. The grid-connected current is subjected to a first coordinate transformation based on the common voltage phase to obtain a dq-axis current component of the grid-connected current. The dq-axis current component includes a d-axis current component and a q-axis current component. The dq-axis current component of the grid-connected current and a dq-axis current reference value are subjected to a difference operation to obtain a dq-axis current deviation. The dq-axis current deviation is input into a corresponding PI controller to obtain a dq-axis modulation signal at the output of the PI controller. The dq-axis modulation signal is subjected to a second coordinate transformation based on the common voltage phase to obtain a three-phase modulation signal. The dq-axis current reference value includes a d-axis current reference value and a q-axis current reference value. The three-phase modulation signal and the carrier are compared and modulated based on a pulse width modulation coefficient to obtain a drive signal for an inverter bridge switch device. The grid-connected converter is controlled based on the drive signal for the inverter bridge switch device. The symmetrical phase-locked loop (PLL) structure improves the operational stability of the grid-connected converter and suppresses the frequency coupling component introduced by the traditional SRF-PLL phase-locked loop (SRF-PLL), thereby increasing grid-connected system stability. Furthermore, by introducing additional damping and inertia, these additional damping and inertia characteristics ensure that the PLL output frequency maintains robustness when the grid-connected converter's operating state changes.

[0012] As a preferred solution, based on a symmetrical phase-locked loop structure, the grid common point voltage is phase-locked and tracked to obtain the common voltage phase, specifically:

[0013] Among them, the symmetrical phase-locked loop structure includes a coordinate transformation link, an amplitude tracking link and a phase tracking link;

[0014] According to the current dq-axis phase estimation value, the grid common point voltage is subjected to Park transformation through a coordinate transformation link to obtain the current d-axis voltage and the current q-axis voltage; wherein the current dq-axis phase estimation value is obtained based on the current d-axis phase estimation value and the current q-axis phase estimation value;

[0015] Determine whether the current d-axis voltage and the current q-axis voltage meet a preset voltage condition; wherein the preset voltage condition is that the current d-axis voltage has the same amplitude as the grid common point voltage and the current q-axis voltage is 0;

[0016] If satisfied, the current d-axis voltage is taken as the true voltage amplitude, and the current d-axis phase estimation value is taken as the true voltage phase estimation value;

[0017] If not, the current d-axis voltage and the current q-axis voltage are tracked in amplitude and phase to obtain an updated dq-axis phase estimate. Through negative feedback, the updated dq-axis phase estimate is used as the current dq-axis phase estimate. According to the current dq-axis phase estimate, the grid common point voltage is subjected to Park transformation through the coordinate transformation link to obtain the current d-axis voltage and the current q-axis voltage. It is judged whether the current d-axis voltage and the current q-axis voltage meet the preset voltage conditions. Through continuous feedback correction, until the current d-axis voltage and the current q-axis voltage meet the preset voltage conditions, the voltage true amplitude and voltage true phase estimate are obtained.

[0018] As a preferred solution, the current d-axis voltage and the current q-axis voltage are subjected to amplitude and phase tracking to obtain updated dq-axis phase estimation values, specifically:

[0019] The updated dq-axis phase estimation value includes an updated d-axis phase estimation value and an updated q-axis phase estimation value;

[0020] Through the amplitude tracking link, according to the amplitude of the grid common point voltage, the current d-axis voltage is used as the controlled variable of the amplitude tracking link to perform amplitude tracking to obtain the updated d-axis phase estimation value;

[0021] Through the phase tracking link, the output angular frequency is obtained according to the grid angular frequency, the damping coefficient of the symmetrical phase-locked loop structure and the amplitude of the grid common point voltage. Based on the output angular frequency, the current q-axis voltage is used as the controlled variable of the phase tracking link for phase tracking to obtain the updated q-axis phase estimation value.

[0022] As a preferred solution, the current dq-axis phase estimation value is obtained according to the current d-axis phase estimation value and the current q-axis phase estimation value, specifically: θ dq =θ d +jθ q

[0023] Among them, θ dq is the current dq axis phase estimate, θ d is the current d-axis phase estimate, θ q is the current q-axis phase estimate.

[0024] As a preferred solution, the output angular frequency is obtained according to the grid angular frequency, the damping coefficient of the symmetrical phase-locked loop structure, and the amplitude of the grid common point voltage, specifically:

[0025] Among them, ω o (s) is the output angular frequency, ω g is the grid angular frequency, U m is the amplitude of the grid common point voltage, k f is the damping coefficient of the symmetrical phase-locked loop structure, T f is the inertia time constant.

[0026] As a preferred solution, the transfer function of the symmetrical phase-locked loop structure is specifically:

[0027] Among them, H PLL (s) is the transfer function of the symmetrical phase-locked loop structure, U m is the amplitude of the grid common point voltage, ξ is the damping ratio, ω n is the undamped natural oscillation angular frequency, k f is the damping coefficient of the symmetrical phase-locked loop structure, T f is the inertia time constant.

[0028] As a preferred solution, the grid-connected current is subjected to a first coordinate transformation according to the common voltage phase to obtain the dq-axis current components of the grid-connected current, specifically:

[0029] According to the common voltage phase, the grid-connected current is transformed from a three-phase stationary coordinate system to a two-phase rotating coordinate system to obtain the dq-axis current components of the grid-connected current.

[0030] As a preferred solution, the modulation signal of the dq axis is subjected to a second coordinate transformation according to the common voltage phase to obtain a three-phase modulation signal, specifically:

[0031] According to the common voltage phase, the modulation signal of the dq axis is transformed from the two-phase rotating coordinate system to the three-phase stationary coordinate system to obtain the modulation signal in the three-phase stationary coordinate system, thereby obtaining the three-phase modulation signal;

[0032] The dq-axis modulation signal includes a d-axis modulation signal and a q-axis modulation signal.

[0033] To solve the same technical problem, an embodiment of the present invention further provides a grid-connected converter operation control device based on a symmetrical phase-locked loop structure, comprising: a phase-locked tracking module configured to perform phase-locked tracking on the grid common point voltage according to the symmetrical phase-locked loop structure to obtain a common voltage phase; wherein the common voltage phase includes a true voltage amplitude and an estimated true voltage phase;

[0034] The coordinate transformation module is used to perform a first coordinate transformation on the grid-connected current according to the common voltage phase to obtain the dq axis current components of the grid-connected current; wherein the dq axis current components include the d axis current component and the q axis current component;

[0035] The deviation modulation module is used to perform a difference operation on the dq-axis current components of the grid-connected current and the dq-axis current reference value to obtain a dq-axis current deviation, and input the dq-axis current deviation into the corresponding PI controller. A dq-axis modulation signal is obtained at the output end of the PI controller. The dq-axis modulation signal is subjected to a second coordinate transformation based on the common voltage phase to obtain a three-phase modulation signal. The dq-axis current reference value includes a d-axis current reference value and a q-axis current reference value.

[0036] The operation control module is used to compare and modulate the three-phase modulation signal and the carrier according to the pulse width modulation coefficient to obtain the driving signal of the inverter bridge switching device, and control the operation of the grid-connected converter according to the driving signal of the inverter bridge switching device.

[0037] In order to solve the same technical problem, an embodiment of the present invention also provides a computer device, including a processor and a memory, the memory is used to store a computer program, and when the computer program is executed by the processor, a grid-connected converter operation control method based on a symmetrical phase-locked loop structure is implemented. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] FIG1 is a flow chart showing an embodiment of a method for controlling operation of a grid-connected converter based on a symmetrical phase-locked loop structure provided by the present invention;

[0039] FIG2 is a control structure diagram of a grid-connected inverter in the dq domain according to a grid-connected converter operation control method based on a symmetrical phase-locked loop structure provided by the present invention;

[0040] FIG3 is a control structure diagram of a symmetrical phase-locked loop structure according to an embodiment of a method for controlling operation of a grid-connected converter based on a symmetrical phase-locked loop structure provided by the present invention;

[0041] FIG4 is a unit step response diagram of an NS-PLL structure according to an embodiment of a method for controlling operation of a grid-connected converter based on a symmetrical phase-locked loop structure provided by the present invention;

[0042] FIG5 is a diagram showing the stability analysis results of a conventional SRF-PLL structure according to an embodiment of a grid-connected converter operation control method based on a symmetrical phase-locked loop structure provided by the present invention; wherein (a) is a grid impedance Lg=8mH, and (b) is a grid impedance Lg=9mH;

[0043] FIG6 is a diagram showing the stability analysis results of an NS-PLL structure according to an embodiment of a method for controlling operation of a grid-connected converter based on a symmetrical phase-locked loop structure provided by the present invention; wherein (a) is a grid impedance Lg=10.5 mH, and (b) is a grid impedance Lg=11.8 mH;

[0044] FIG7 is a diagram showing the output electrical results of the power grid system under a conventional SRF-PLL structure, according to an embodiment of a method for controlling the operation of a grid-connected converter based on a symmetrical phase-locked loop structure provided by the present invention; wherein (a) shows the output current of the grid-connected converter under the conventional SRF-PLL structure, and (b) shows the output frequency and phase of the phase-locked loop under the conventional SRF-PLL structure;

[0045] FIG8 is a diagram showing the electrical output results of the power grid system under the NS-PLL structure according to an embodiment of a method for controlling the operation of a grid-connected converter based on a symmetrical phase-locked loop structure provided by the present invention; wherein (a) is the output current of the grid-connected converter with the NS-PLL structure, and (b) is the output frequency and phase of the phase-locked loop with the NS-PLL structure;

[0046] FIG9 is a schematic structural diagram of an embodiment of a grid-connected converter operation control device based on a symmetrical phase-locked loop structure provided by the present invention. DETAILED DESCRIPTION

[0047] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0048] Example 1

[0049] Please refer to Figure 1, which is a flow chart of a method for controlling the operation of a grid-connected converter based on a symmetrical phase-locked loop structure according to an embodiment of the present invention. The method for controlling the operation of a grid-connected converter according to this embodiment is applicable to grid-connected power conversion in a grid-connected system. This embodiment utilizes a symmetrical phase-locked loop structure to improve the stability of the grid-connected converter operation and the stability of the grid-connected system. The method for controlling the operation of a grid-connected converter includes steps 101 to 104, each of which is described as follows:

[0050] Step 101: According to a symmetrical phase-locked loop structure, phase-locked tracking is performed on the common point voltage of the power grid to obtain a common voltage phase; wherein the common voltage phase includes a true voltage amplitude and an estimated true voltage phase.

[0051] In this embodiment, a symmetrical phase-locked loop structure NS-PLL is used to control the grid common point voltage u PCC Phase-locked tracking is performed to obtain the common voltage phase. The control structure diagram of the grid-connected inverter in the dq domain is shown in Figure 1. The main circuit and control circuit of a 14kW three-phase LCL grid-connected inverter under unity power factor grid-connected current control are shown in Figure 1. In Figure 1, PCC represents the converter grid connection point, dq / abc represents the transformation from the two-phase rotating coordinate system (dq coordinate system) to the three-phase stationary coordinate system (abc coordinate system), and abc / dq represents the transformation from the three-phase stationary coordinate system (abc coordinate system) to the two-phase rotating coordinate system (dq coordinate system); θ dq is the phase angle of the phase-locked loop in the dq coordinate system; u dc is the DC bus voltage; the inverter side filter inductor L1, filter capacitor C f and the grid-side filter inductor L2 form an LCL filter; R d is the damping resistance; u inv is the output voltage of the inverter bridge arm; i2 is the grid-connected current, i 2d 、i 2q are the grid-connected current components on the d and q axes respectively; u PCC is the grid connection point voltage (grid common point voltage); u g is the grid voltage; Z g is the grid impedance; the current inner loop adopts PI controller, and its transfer function is G i (s) = k ip +k ii / s,k ip is the proportional coefficient of the PI controller, k ii is the integral coefficient of the PI controller; I 2dref , I 2qref are the reference values ​​of the grid-connected current on the d-axis and q-axis, respectively; v Md 、v Mq are the three-phase modulation signals output by the current loop PI controller on the d-axis and q-axis respectively; kPWM is the pulse width modulation coefficient.

[0052] It should be noted that a converter is an electrical device that changes the voltage, frequency, number of phases, and other electrical quantities or characteristics of a power system. This category includes rectifiers (converting AC to DC), inverters (converting DC to AC), AC converters, and DC converters. In addition to the main circuits (rectifier, inverter, AC conversion, and DC conversion), converters also require trigger circuits to control the on / off switching of power switching elements and control circuits to regulate and control electrical energy. An inverter converts DC power (from batteries or storage batteries) into AC power (typically 220V, 50Hz, sinusoidal or square waves). An inverter is an electronic device that converts low-voltage (12, 24V, or 48V) DC power into 220V AC.

[0053] In this embodiment, the New Synchronous Phase Locked Loop (NS-PLL) structure includes a coordinate transformation link, an amplitude tracking link, and a phase tracking link. To simplify the description, the NS-PLL structure is used to represent the symmetrical phase-locked loop structure. The control structure of the symmetrical phase-locked loop structure is shown in Figure 2. The symmetrical phase-locked loop structure is used to achieve phase and amplitude tracking between the inverter and the grid. Fixed d-axis control is adopted, that is, when the grid common point voltage U pcc Completely coincides with the d-axis, at this time U pccd =U m 、U pccq =0, U m is the voltage amplitude of the grid common point, U pccd is the d-axis voltage of the grid common point voltage, U pccq is the q-axis voltage of the grid common point voltage. Therefore, for the NS-PLL structure, the d-axis voltage U pccd (the d-axis voltage of the grid common point voltage) is the controlled quantity of the amplitude tracking link, and the q-axis voltage U pccq (the q-axis voltage of the grid common point voltage) is used as the controlled variable of the phase tracking link; then the d-axis phase estimation value θ d and the q-axis phase estimate θ q For three-phase voltage (grid common point voltage U PCC ) to perform Park transformation and continuously correct θ through negative feedback d ,θ q , until U pccd =U m 、 U pccq =0; Finally, we get a stable U pccd and θ d , which is the estimated value of the true amplitude and phase of the voltage at the PCC point.

[0054] Optionally, step 101 specifically includes steps 1011 to 1014, and each step is specifically as follows:

[0055] Step 1011: Based on the current dq-axis phase estimation value, the grid common point voltage is subjected to Park transformation through a coordinate transformation link to obtain the current d-axis voltage and the current q-axis voltage; wherein the current dq-axis phase estimation value is obtained based on the current d-axis phase estimation value and the current q-axis phase estimation value.

[0056] It should be noted that Park transform is to transform i a、 i b、 i c The projection of the current on the α and β axes is equivalent to the d and q axes, and the current on the stator is equivalent to the direct axis and the quadrature axis.

[0057] Step 1012: Determine whether the current d-axis voltage and the current q-axis voltage meet a preset voltage condition; wherein the preset voltage condition is that the current d-axis voltage has the same amplitude as the grid common point voltage, and the current q-axis voltage is 0.

[0058] Step 1013: If satisfied, the current d-axis voltage is used as the voltage true amplitude, and the current d-axis phase estimation value is used as the voltage true phase estimation value.

[0059] Step 1014: If not satisfied, the current d-axis voltage and the current q-axis voltage are subjected to amplitude and phase tracking to obtain updated dq-axis phase estimation values. Through negative feedback, the updated dq-axis phase estimation values ​​are used as the current dq-axis phase estimation values. According to the current dq-axis phase estimation values, the grid common point voltage is subjected to Park transformation through the coordinate transformation link to obtain the current d-axis voltage and the current q-axis voltage. It is determined whether the current d-axis voltage and the current q-axis voltage meet the preset voltage conditions. Through continuous feedback correction, until the current d-axis voltage and the current q-axis voltage meet the preset voltage conditions, the voltage true amplitude and voltage true phase estimation values ​​are obtained.

[0060] Optionally, the current dq-axis phase estimation value is obtained based on the current d-axis phase estimation value and the current q-axis phase estimation value, specifically: θ dq =θ d +jθ q

[0061] Among them, θ dq is the current dq axis phase estimate, θ d is the current d-axis phase estimate, θ q is the current q-axis phase estimate.

[0062] In this embodiment, θdq =θ d +jθ q Used to track the phase angle and amplitude of the grid common point (PCC) voltage.

[0063] Optionally, the current d-axis voltage and the current q-axis voltage are subjected to amplitude and phase tracking to obtain updated dq-axis phase estimation values, specifically:

[0064] The updated dq-axis phase estimation value includes an updated d-axis phase estimation value and an updated q-axis phase estimation value;

[0065] Through the amplitude tracking link, the current d-axis voltage is used as the controlled variable of the amplitude tracking link for amplitude tracking according to the amplitude of the grid common point voltage, and an updated d-axis phase estimation value is obtained; through the phase tracking link, the output angular frequency is obtained according to the grid angular frequency, the damping coefficient of the symmetrical phase-locked loop structure and the amplitude of the grid common point voltage, and based on the output angular frequency, the current q-axis voltage is used as the controlled variable of the phase tracking link for phase tracking to obtain an updated q-axis phase estimation value.

[0066] Optionally, the output angular frequency is obtained according to the grid angular frequency, the damping coefficient of the symmetrical phase-locked loop structure, and the amplitude of the grid common point voltage, specifically:

[0067] Among them, ω o (s) is the output angular frequency, ω g is the grid angular frequency, U m is the amplitude of the grid common point voltage, k f is the damping coefficient of the symmetrical phase-locked loop structure, T f is the inertia time constant.

[0068] In this embodiment, the output angular frequency of the NS-PLL structure is:

[0069] Among them, U m is the voltage amplitude of the common point of the power grid (the voltage amplitude of the PCC point), k f is the damping coefficient of the NS-PLL structure, T f is the inertia time constant, ω g is the grid angular frequency.

[0070] Optionally, the transfer function of the symmetrical phase-locked loop structure is specifically:

[0071] Among them, H PLL (s) is the transfer function of the symmetrical phase-locked loop structure, U mis the amplitude of the grid common point voltage, ξ is the damping ratio, ω n is the undamped natural oscillation angular frequency, k f is the damping coefficient of the symmetrical phase-locked loop structure, T f is the inertia time constant.

[0072] In this embodiment, the transfer function of the phase-locked loop structure is a typical second-order oscillation link. For a typical second-order oscillation system, in order to take into account the dynamic and static characteristics of the system, the damping ratio is generally set to ξ = 0.707. At the same time, the present invention selects the phase-locked loop bandwidth to be 70Hz, and calculates k f =0.182, T f =0.005628, from which the unit step response curves of the conventional SRF-PLL structure and the NS-PLL structure of the present invention can be obtained. The unit step response diagram of the NS-PLL structure is shown in Figure 4. As shown in Figure 4, the NS-PLL structure provides a certain amount of damping and inertia in the process of tracking the grid frequency. Therefore, it can be predicted that when the grid-connected converter jumps from half load to full load, the component U of the PCC point voltage in the dq coordinate system PCCd 、U PCCq A mutation also occurs; and at this time U PCCd 、U PCCq There is a deviation from the steady-state value before the mutation, and the fluctuation of the frequency amplitude is suppressed by a regulator with additional damping.

[0073] Step 102: performing a first coordinate transformation on the grid-connected current according to the common voltage phase to obtain dq-axis current components of the grid-connected current; wherein the dq-axis current components include a d-axis current component and a q-axis current component.

[0074] Optionally, step 102 is specifically: according to the common voltage phase, the grid-connected current is subjected to a first coordinate transformation to obtain the dq-axis current component of the grid-connected current, specifically: according to the common voltage phase, the grid-connected current is transformed from a three-phase stationary coordinate system to a two-phase rotating coordinate system to obtain the dq-axis current component of the grid-connected current.

[0075] In this embodiment, according to the common voltage phase, the grid-connected current i2 is subjected to abc / dq coordinate transformation (first coordinate transformation) to obtain its components on the d and q axes, that is, the current component i on the d axis. 2d and the q-axis current component i 2q abc / dq represents the transformation from the three-phase stationary coordinate system (abc coordinate system) to the two-phase rotating coordinate system (dq coordinate system).

[0076] Step 103: Perform a difference operation on the dq-axis current components of the grid-connected current and the dq-axis current reference value to obtain a dq-axis current deviation, and input the dq-axis current deviation into the corresponding PI controller to obtain a dq-axis modulation signal at the output end of the PI controller. According to the common voltage phase, perform a second coordinate transformation on the dq-axis modulation signal to obtain a three-phase modulation signal; wherein the dq-axis current reference value includes a d-axis current reference value and a q-axis current reference value.

[0077] In this embodiment, the dq axis current components are respectively subtracted from the dq axis current reference value (the current given value under the dq axis) to obtain the dq axis current deviation. The dq axis current deviation includes the d axis current deviation and the q axis current deviation. That is, the d axis current deviation is obtained by subtracting the d axis current component from the d axis current reference value (the current given value under the d axis), and the q axis current deviation is obtained by subtracting the q axis current component from the q axis current reference value (the current given value under the q axis). The dq axis current deviation is then passed through the corresponding PI controller to obtain the components of the modulation signal on the d and q axes (dq axis modulation signal), that is, the modulation signal v output by the current loop PI controller on the d axis Md (modulation signal on the d-axis) and the modulation signal v on the q-axis Mq (q-axis modulation signal). Then, the dq-axis modulation signal is transformed through the second coordinate transformation to obtain the modulation signal v in the abc three-phase coordinate system. Mabc , that is, three-phase modulated signal.

[0078] Optionally, according to the common voltage phase, the modulation signal of the dq axis is subjected to a second coordinate transformation to obtain a three-phase modulation signal, specifically: according to the common voltage phase, the modulation signal of the dq axis is transformed from a two-phase rotating coordinate system to a three-phase stationary coordinate system to obtain the modulation signal in the three-phase stationary coordinate system, and the three-phase modulation signal is obtained; wherein, the modulation signal of the dq axis includes the modulation signal of the d axis and the modulation signal of the q axis.

[0079] In this embodiment, according to the common voltage phase, the modulation signal of the dq axis is subjected to the dq / abc coordinate transformation (second coordinate transformation) to obtain the modulation signal v in the abc three-phase coordinate system. Mabc , which is a three-phase modulated signal. dq / abc represents the transformation from a two-phase rotating coordinate system (dq coordinate system) to a three-phase stationary coordinate system (abc coordinate system).

[0080] Step 104: Compare and modulate the three-phase modulation signal and the carrier according to the pulse width modulation coefficient to obtain a drive signal for the inverter bridge switch device, and control the operation of the grid-connected converter according to the drive signal for the inverter bridge switch device.

[0081] In this embodiment, according to the pulse width modulation coefficient k PWM , the three-phase modulated signal vMabc By comparing with the carrier wave, the driving signal of the inverter bridge switching device is obtained, and the operation control of the grid-connected converter is realized according to the driving signal of the inverter bridge switching device.

[0082] In this embodiment, to verify the effectiveness and feasibility of the NS-PLL structure of the present invention, a conventional SRF-PLL structure and the NS-PLL structure of the present invention are compared. A stability analysis of a grid-connected converter system using the conventional SRF-PLL structure under different grid impedance conditions is performed, as shown in FIG5 . A stability analysis of a grid-connected converter system using the NS-PLL structure of the present invention under different grid impedance conditions is performed, as shown in FIG6 . As shown in FIG5 , when the conventional SRF-PLL structure is used, the grid-connected converter can operate stably at a grid impedance of Lg = 8 mH. When the grid impedance increases to 9 mH, the grid-connected converter becomes unstable. However, as shown in FIG6 , when the symmetrical phase-locked loop structure (NS-PLL) with added damping and inertia of the present invention is used, the grid-connected converter can maintain stable operation at a grid impedance of Lg = 10.5 mH. The grid-connected converter does not become unstable until the grid impedance further increases to 11.8 mH. By comparing the stability analysis results of grid-connected converters using two different phase-locked loop structures, it can be seen that the NS-PLL structure proposed in the present invention can improve the adaptability of the grid-connected converter to the grid impedance, thereby improving the stability of the grid-connected converter.

[0083] In this embodiment, to illustrate the added damping and inertia characteristics of the NS-PLL structure of the present invention, the phase-locked loop output frequency and phase of the conventional SRF-PLL structure and the NS-PLL structure are compared after a sudden change in the operating condition of the grid-connected converter under the conventional SRF-PLL structure when the grid impedance Lg = 5mH. When the operating condition changes, the electrical output of the grid system under the conventional SRF-PLL structure is shown in Figure 7, which shows the system output current and phase when the conventional SRF-PLL structure is used. Figure 7(a) shows the output current of the grid-connected converter under the conventional SRF-PLL structure, and Figure 7(b) shows the output frequency and phase of the phase-locked loop of the conventional SRF-PLL structure. When the operating condition changes, the electrical output of the grid system under the NS-PLL structure is shown in Figure 8, which shows the system output current and phase when the NS-PLL structure is used. Figure 8(a) shows the output current of the grid-connected converter under the NS-PLL structure, and Figure 8(b) shows the output frequency and phase of the phase-locked loop of the NS-PLL structure. As shown in Figure 7(a), when the traditional SRF-PLL structure is used, the converter jumps from full load to half load, and the output current transition process fluctuates severely. At the same time, it can be seen from the output frequency and phase angle of the phase-locked loop under the SRF-PLL structure that when the operating conditions of the grid-connected converter change, the system output frequency fluctuates by about 15Hz. The fluctuation amplitude is large, which is not conducive to the safe and stable operation of the grid-connected converter. By comparing Figures 7 and 8, it can be seen that the symmetrical phase-locked loop structure (NS-PLL structure) with additional damping and inertia proposed in the present invention, under the same operating conditions as the traditional SRF-PLL, has a smooth grid-connected current transition process, and the frequency fluctuation of the phase-locked loop output is only 1.7Hz and 5.2Hz. The fluctuation amplitude has been suppressed to a great extent, which is much smaller than the traditional SRF-PLL phase-locked loop.

[0084] The above theory and simulation verify that the symmetrical phase-locked loop structure with additional damping and inertia proposed in the present invention can effectively suppress the problem of large fluctuations in the system output frequency caused by sudden changes in the system operating conditions while improving the stability of the grid-connected converter.

[0085] In an embodiment of the present invention, a symmetrical phase-locked loop structure is used to phase-lock the grid common point voltage and track it to obtain a common voltage phase. The common voltage phase includes a true voltage amplitude and an estimated true voltage phase. The grid-connected current is subjected to a first coordinate transformation based on the common voltage phase to obtain a dq-axis current component of the grid-connected current. The dq-axis current component includes a d-axis current component and a q-axis current component. The dq-axis current component of the grid-connected current and a dq-axis current reference value are subjected to a difference operation to obtain a dq-axis current deviation. The dq-axis current deviation is input into a corresponding PI controller to obtain a dq-axis modulation signal at the output of the PI controller. The dq-axis modulation signal is subjected to a second coordinate transformation based on the common voltage phase to obtain a three-phase modulation signal. The dq-axis current reference value includes a d-axis current reference value and a q-axis current reference value. The three-phase modulation signal and the carrier are compared and modulated based on a pulse width modulation coefficient to obtain a drive signal for an inverter bridge switch device. The grid-connected converter is controlled based on the drive signal for the inverter bridge switch device. The symmetrical phase-locked loop (PLL) structure improves the operational stability of the grid-connected converter and suppresses the frequency coupling component introduced by the traditional SRF-PLL phase-locked loop (SRF-PLL), thereby increasing grid-connected system stability. Furthermore, by introducing additional damping and inertia, these additional damping and inertia characteristics ensure that the PLL output frequency maintains robustness when the grid-connected converter's operating state changes.

[0086] Example 2

[0087] Accordingly, referring to FIG9 , FIG9 is a schematic diagram of a second embodiment of a grid-connected converter operation control device based on a symmetrical phase-locked loop structure provided by the present invention. As shown in FIG9 , the grid-connected converter operation control device based on a symmetrical phase-locked loop structure includes a phase-locked tracking module 901, a coordinate transformation module 902, a deviation modulation module 903, and an operation control module 904;

[0088] The phase-locked tracking module 901 is used to perform phase-locked tracking on the common point voltage of the power grid according to a symmetrical phase-locked loop structure to obtain a common voltage phase; wherein the common voltage phase includes a true voltage amplitude and an estimated true voltage phase;

[0089] The coordinate transformation module 902 is used to perform a first coordinate transformation on the grid-connected current according to the common voltage phase to obtain the dq-axis current components of the grid-connected current; wherein the dq-axis current components include the d-axis current component and the q-axis current component;

[0090] The deviation modulation module 903 is used to perform a difference operation on the dq-axis current components of the grid-connected current and the dq-axis current reference value to obtain a dq-axis current deviation, and input the dq-axis current deviation into the corresponding PI controller. A dq-axis modulation signal is obtained at the output of the PI controller. The dq-axis modulation signal is subjected to a second coordinate transformation based on the common voltage phase to obtain a three-phase modulation signal. The dq-axis current reference value includes a d-axis current reference value and a q-axis current reference value.

[0091] The operation control module 904 is used to compare and modulate the three-phase modulation signal and the carrier according to the pulse width modulation coefficient to obtain the driving signal of the inverter bridge switch device, and control the operation of the grid-connected converter according to the driving signal of the inverter bridge switch device.

[0092] The aforementioned symmetrical phase-locked loop (PLL)-based grid-connected converter operation control device can implement the aforementioned method embodiment. The optional options in the aforementioned method embodiment also apply to this embodiment and are not described in detail here. The remaining contents of the present application embodiment can be referenced to the aforementioned method embodiment and are not further described in this embodiment.

[0093] By implementing the embodiments of the present invention, a symmetrical phase-locked loop structure is established, which suppresses the frequency coupling phenomenon of the grid-connected converter caused by the asymmetry of the phase-locked loop in the dq coordinate system, thereby improving the stability of the grid-connected system. While suppressing the frequency coupling component of the traditional SRF-PLL phase-locked loop and thus enhancing the stability of the grid-connected system, additional damping and inertia are introduced to ensure that the output frequency of the phase-locked loop is stable when the operating state of the grid-connected converter changes, thereby suppressing frequency fluctuations caused by sudden changes in the converter operating conditions, and improving the robustness of the grid-connected system.

[0094] In addition, an embodiment of the present application further provides a computer device, which includes a processor and a memory, wherein the memory is used to store a computer program, and when the computer program is executed by the processor, the steps in any of the above method embodiments are implemented.

[0095] The above specific embodiments further illustrate the objectives, technical solutions, and beneficial effects of the present invention. It should be understood that the above are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. In particular, it should be noted that any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention for those skilled in the art.

Claims

1. A grid-connected converter operation control method based on a symmetrical phase-locked loop structure, characterized in that: include: According to the symmetrical phase-locked loop structure, the common point voltage of the power grid is phase-locked and tracked to obtain a common voltage phase; wherein the common voltage phase includes a true voltage amplitude and a true voltage phase estimation value; According to the common voltage phase, the grid-connected current is subjected to a first coordinate transformation to obtain a dq-axis current component of the grid-connected current; wherein the dq-axis current component includes a d-axis current component and a q-axis current component; Performing a difference operation on the dq-axis current component of the grid-connected current and the dq-axis current reference value to obtain a dq-axis current deviation, and inputting the dq-axis current deviation into a corresponding PI controller, obtaining a dq-axis modulation signal at the output end of the PI controller, and performing a second coordinate transformation on the dq-axis modulation signal according to the common voltage phase to obtain a three-phase modulation signal; wherein the dq-axis current reference value includes a d-axis current reference value and a q-axis current reference value; According to the pulse width modulation coefficient, the three-phase modulation signal and the carrier are compared and modulated to obtain the driving signal of the inverter bridge switch device, and the grid-connected converter is operated and controlled according to the driving signal of the inverter bridge switch device.

2. The grid-connected converter operation control method based on a symmetrical phase-locked loop structure according to claim 1, characterized in that: According to the symmetrical phase-locked loop structure, the common point voltage of the power grid is phase-locked and tracked to obtain the common voltage phase, specifically: Wherein, the symmetrical phase-locked loop structure includes a coordinate transformation link, an amplitude tracking link and a phase tracking link; According to the current dq-axis phase estimation value, the grid common point voltage is subjected to Park transformation through the coordinate transformation link to obtain the current d-axis voltage and the current q-axis voltage; wherein the current dq-axis phase estimation value is obtained according to the current d-axis phase estimation value and the current q-axis phase estimation value; Determine whether the current d-axis voltage and the current q-axis voltage meet a preset voltage condition; wherein the preset voltage condition is that the current d-axis voltage has the same amplitude as the grid common point voltage, and the current q-axis voltage is 0; If satisfied, the current d-axis voltage is used as the voltage true amplitude, and the current d-axis phase estimation value is used as the voltage true phase estimation value; If not satisfied, the current d-axis voltage and the current q-axis voltage are subjected to amplitude and phase tracking to obtain an updated dq-axis phase estimation value, and the updated dq-axis phase estimation value is used as the current dq-axis phase estimation value through negative feedback. According to the current dq-axis phase estimation value, the grid common point voltage is subjected to Parker transformation through the coordinate transformation link to obtain the current d-axis voltage and the current q-axis voltage, and it is determined whether the current d-axis voltage and the current q-axis voltage meet the preset voltage condition, and continuous feedback correction is performed until the current d-axis voltage and the current q-axis voltage meet the preset voltage condition, thereby obtaining the voltage true amplitude and the voltage true phase estimation value.

3. The grid-connected converter operation control method based on a symmetrical phase-locked loop structure according to claim 2, characterized in that: The current d-axis voltage and the current q-axis voltage are subjected to amplitude phase tracking to obtain an updated dq-axis phase estimation value, specifically: Wherein, the updated dq-axis phase estimation value includes an updated d-axis phase estimation value and an updated q-axis phase estimation value; Through the amplitude tracking link, according to the amplitude of the common point voltage of the power grid, the current d-axis voltage is used as the controlled variable of the amplitude tracking link to perform amplitude tracking to obtain the updated d-axis phase estimation value; Through the phase tracking link, the output angular frequency is obtained according to the grid angular frequency, the damping coefficient of the symmetrical phase-locked loop structure and the amplitude of the grid common point voltage, and according to the output angular frequency, the current q-axis voltage is used as the controlled quantity of the phase tracking link for phase tracking to obtain the updated q-axis phase estimation value.

4. The grid-connected converter operation control method based on a symmetrical phase-locked loop structure according to claim 2, characterized in that: The current dq-axis phase estimation value is obtained according to the current d-axis phase estimation value and the current q-axis phase estimation value, specifically: θ dq =θ d +jθ q Among them, θ dq is the current dq axis phase estimation value, θ d is the current d-axis phase estimate, θ q is the current q-axis phase estimation value.

5. The grid-connected converter operation control method based on a symmetrical phase-locked loop structure according to claim 3, characterized in that: The output angular frequency is obtained according to the grid angular frequency, the damping coefficient of the symmetrical phase-locked loop structure and the amplitude of the grid common point voltage, specifically: Among them, ω o (s) is the output angular frequency, ω g is the grid angular frequency, U m is the voltage amplitude at the common point of the power grid, k f is the damping coefficient of the symmetrical phase-locked loop structure, T f is the inertia time constant.

6. The grid-connected converter operation control method based on a symmetrical phase-locked loop structure according to claim 5, characterized in that: The transfer function of the symmetrical phase-locked loop structure is specifically: Among them, H PLL (s) is the transfer function of the symmetrical phase-locked loop structure, U m is the voltage amplitude at the common point of the power grid, ξ is the damping ratio, ω n is the undamped natural oscillation angular frequency, k f is the damping coefficient of the symmetrical phase-locked loop structure, T f is the inertia time constant.

7. The grid-connected converter operation control method based on a symmetrical phase-locked loop structure according to claim 1, characterized in that: According to the common voltage phase, the grid-connected current is subjected to a first coordinate transformation to obtain the dq-axis current components of the grid-connected current, specifically: According to the common voltage phase, the grid-connected current is transformed from a three-phase stationary coordinate system to a two-phase rotating coordinate system to obtain dq-axis current components of the grid-connected current.

8. The grid-connected converter operation control method based on a symmetrical phase-locked loop structure according to claim 7, characterized in that: According to the common voltage phase, the modulation signal of the dq axis is subjected to a second coordinate transformation to obtain a three-phase modulation signal, specifically: According to the common voltage phase, the modulation signal of the dq axis is transformed from the two-phase rotating coordinate system to the three-phase stationary coordinate system to obtain the modulation signal in the three-phase stationary coordinate system, thereby obtaining a three-phase modulation signal; The dq-axis modulation signal includes a d-axis modulation signal and a q-axis modulation signal.

9. A grid-connected converter operation control device based on a symmetrical phase-locked loop structure, characterized in that: include: Phase-locked tracking module, coordinate transformation module, deviation modulation module and operation control module; The phase-locked tracking module is used to perform phase-locked tracking on the common point voltage of the power grid according to the symmetrical phase-locked loop structure to obtain a common voltage phase; wherein the common voltage phase includes a voltage real amplitude and a voltage real phase estimation value; The coordinate transformation module is used to perform a first coordinate transformation on the grid-connected current according to the common voltage phase to obtain a dq-axis current component of the grid-connected current; wherein the dq-axis current component includes a d-axis current component and a q-axis current component; The deviation modulation module is used to perform a difference operation on the dq axis current component of the grid-connected current and the dq axis current reference value to obtain a dq axis current deviation, and input the dq axis current deviation into a corresponding PI controller, obtain a dq axis modulation signal at the output end of the PI controller, and perform a second coordinate transformation on the dq axis modulation signal according to the common voltage phase to obtain a three-phase modulation signal; wherein the dq axis current reference value includes a d axis current reference value and a q axis current reference value; The operation control module is used to compare and modulate the three-phase modulation signal and the carrier according to the pulse width modulation coefficient to obtain the driving signal of the inverter bridge switch device, and control the operation of the grid-connected converter according to the driving signal of the inverter bridge switch device.

10. A computer device, characterized in that: It comprises a processor and a memory, wherein the memory is used to store a computer program, and when the computer program is executed by the processor, the grid-connected converter operation control method based on a symmetrical phase-locked loop structure according to any one of claims 1 to 8 is implemented.

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