Phase-locked loop control method and device of power converter and power converter

Abstract

The application provides a phase-locked loop control method and device of a power converter and the power converter. The method comprises: in each control cycle of a phase-locked loop control loop, acquiring actual voltage data at a grid connection point of the power converter and a first phase-locked loop angle output by the phase-locked loop control loop in a previous control cycle; performing coordinate transformation on the actual voltage data according to the first phase-locked loop angle to obtain a quadrature-axis voltage component; and based on the quadrature-axis voltage component and the phase-locked loop control loop, obtaining a second phase-locked loop angle output by the phase-locked loop control loop in a current control cycle; wherein a phase compensator is arranged in the phase-locked loop control loop and is used for phase compensation on a preset frequency; when the grid into which the power converter is incorporated is a weak grid, the control coefficient of the phase-locked loop control loop is a first preset value; the first preset value is smaller than a second preset value, and the second preset value is a control coefficient when the incorporated grid is not a weak grid. The application can improve the stability of the power converter operation.

Description

Technical Field

[0001] This application relates to the field of converter technology, and in particular to a phase-locked loop control method, device and power converter for a power converter. Background Technology

[0002] With the rapid development of new power systems, new energy equipment such as photovoltaics, wind power, and energy storage are being widely adopted. As the core equipment for power conversion and power regulation, power converters play a crucial role in voltage and current transformation, bidirectional power regulation, grid voltage and frequency stabilization, and fluctuation smoothing.

[0003] In related technologies, power converters mainly use phase-locked loops (PLLs) to track the actual phase of the grid voltage at the grid connection point, thereby achieving synchronization between the power converter and the actual phase of the grid. Specifically, the PLL uses the phase-locked angle tracked in the previous control cycle to convert the acquired grid voltage to a rotating coordinate system. It then uses the quadrature-axis voltage component of the grid voltage in the rotating coordinate system for adjustment to obtain a new phase-locked angle, thus achieving tracking of the actual phase of the grid voltage.

[0004] However, under weak grid conditions, the equivalent impedance of the grid increases significantly and becomes inductive, making the phase-locked loop prone to phase lag, which can lead to subsynchronous / supersynchronous oscillations and affect the safe and stable operation of the power converter. Summary of the Invention

[0005] This application provides a phase-locked loop control method, device, and power converter for a power converter, in order to reduce the risk of subsynchronous / supersynchronous oscillation instability of the power converter and improve the stability of the power converter operation.

[0006] In a first aspect, embodiments of this application provide a phase-locked loop control method for a power converter, including: In each control cycle of the phase-locked loop of the power converter tracking the actual phase of the voltage at the grid connection point, the actual voltage data at the grid connection point of the power converter and the first phase-locked angle output by the phase-locked loop in the previous control cycle are acquired; based on the first phase-locked angle, the actual voltage data is subjected to coordinate transformation to obtain the quadrature-axis voltage component; based on the quadrature-axis voltage component and the phase-locked loop, the second phase-locked angle output by the phase-locked loop in the current control cycle is obtained; The phase-locked loop includes a phase compensator for performing phase compensation at a preset frequency. When the power grid connected to the power converter is a weak grid, the control coefficient of the phase-locked loop is a first preset value. The first preset value is less than a second preset value, which is the control coefficient when the connected grid is not a weak grid. The control coefficient includes a proportional coefficient and / or an integral coefficient.

[0007] In one possible implementation, when the power grid to which the power converter is connected is a weak grid, the method further includes: Obtain the first short-circuit ratio of the power converter connected to the power grid; Based on the preset correspondence between the short-circuit ratio and the filter coefficient, the first filter coefficient corresponding to the first short-circuit ratio is determined. The filter coefficient of the phase compensator is adjusted to the first filter coefficient to perform phase compensation at the preset frequency.

[0008] In one possible implementation, before determining the first filter coefficient corresponding to the first short-circuit ratio based on the preset correspondence between the short-circuit ratio and the filter coefficient, the method further includes: When the power converter, operating under preset conditions, is connected to a weak grid, impedance frequency sweep is performed on the power converter for each of the various preset short-circuit ratios of the weak grid to obtain the swept Bode plot corresponding to that short-circuit ratio. Based on the swept Bode plot, the oscillation frequency at which the power converter exhibits subsynchronous / supersynchronous oscillations corresponding to that short-circuit ratio is determined. Based on the oscillation frequency, the compensation frequency and compensation phase corresponding to that short-circuit ratio are determined. Based on the compensation frequency and the compensation phase, the filter coefficient corresponding to that short-circuit ratio is calculated. Based on the filter coefficients corresponding to each short-circuit ratio, the correspondence between the short-circuit ratio and the filter coefficients is obtained.

[0009] In one possible implementation, determining the compensation frequency and compensation phase corresponding to the short-circuit ratio based on the oscillation frequency includes: The compensation frequency corresponding to the short-circuit ratio is obtained based on the difference between the oscillation frequency and the preset frequency margin. From the swept Bode plot, determine the impedance phase corresponding to the compensation frequency; The compensation phase corresponding to the short-circuit ratio is determined based on the impedance phase and the preset phase margin.

[0010] In one possible implementation, calculating the filter coefficients corresponding to the short-circuit ratio based on the compensation frequency and the compensation phase includes: Based on the compensation phase, calculate the scale factor of the phase compensator under the short-circuit ratio; Based on the compensation frequency and the indexing coefficient, the transfer function of the phase compensator under this short-circuit ratio is determined; Based on the control period of the phase-locked loop, the transfer function is subjected to a bilinear transformation to obtain the filter coefficient of the phase compensator corresponding to the short-circuit ratio.

[0011] In one possible implementation, based on the quadrature-axis voltage component and the phase-locked loop, the second phase-locked angle output by the phase-locked loop in the current control cycle is obtained, including: The quadrature-axis voltage component is filtered using the phase compensator to obtain the quadrature-axis compensated voltage. The cross-axis error value is determined based on the cross-axis compensation voltage and the preset cross-axis voltage; The frequency adjustment amount is obtained by performing proportional-integral control on the cross-axis error value; Based on the frequency adjustment amount and the rated frequency, the second phase-locked angle output by the phase-locked control loop in the current control cycle is obtained.

[0012] In one possible implementation, when the control coefficient of the phase-locked loop is a first preset value, the method further includes: Based on the quadrature-axis voltage component and the phase-locked control loop, the voltage frequency at the grid connection point of the power converter is determined. Calculate the difference between the voltage frequency and the rated frequency to obtain the frequency deviation value; If the frequency deviation value is greater than the preset deviation threshold, the control coefficient of the phase-locked loop is adjusted to a third preset value, which is greater than the first preset value.

[0013] In one possible implementation, the method further includes: Obtain the second short-circuit ratio of the power converter connected to the power grid; If the second short-circuit ratio is less than or equal to the preset short-circuit ratio, then the power grid into which the power converter is connected is determined to be a weak power grid.

[0014] Secondly, embodiments of this application provide a phase-locked loop control device for a power converter, comprising: The acquisition module is used to acquire the actual voltage data at the grid connection point of the power converter and the first phase-locked angle output by the phase-locked control loop in the previous control cycle during each control cycle of tracking the actual phase of the voltage at the grid connection point in the phase-locked control loop of the power converter. The control module is used to perform coordinate transformation on the actual voltage data according to the first phase-locked angle to obtain the quadrature-axis voltage component; and to obtain the second phase-locked angle output by the phase-locked control loop in the current control cycle based on the quadrature-axis voltage component and the phase-locked control loop. The phase-locked loop includes a phase compensator for performing phase compensation at a preset frequency. When the power grid connected to the power converter is a weak grid, the control coefficient of the phase-locked loop is a first preset value. The first preset value is less than a second preset value, which is the control coefficient when the connected grid is not a weak grid. The control coefficient includes a proportional coefficient and / or an integral coefficient.

[0015] Thirdly, embodiments of this application provide a power converter, including a controller, which is used to implement the method in the first aspect or any possible implementation of the first aspect.

[0016] Fourthly, embodiments of this application provide a computer program product, including a computer program that, when executed by a processor, implements the method described in the first aspect or any possible implementation of the first aspect.

[0017] The beneficial effects of the embodiments in this application compared with the prior art are: This application embodiment obtains the actual voltage data at the grid connection point of the power converter and the first phase-locked angle output by the phase-locked control loop in the previous control cycle. Using this first phase-locked angle, the actual voltage data of the power grid at the grid connection point is transformed into a coordinate system, which transforms the grid voltage from a three-phase stationary coordinate system to a two-phase rotating coordinate system. This yields the quadrature-axis voltage component of the target voltage data in the two-phase rotating coordinate system, which is the phase deviation between the phase-locked angle tracked by the power converter and the actual phase angle of the actual voltage data. Then, using the quadrature-axis voltage component and the phase-locked control loop, the second phase-locked angle output by the phase-locked control loop in the current control cycle can be obtained. Specifically, by activating the phase compensator in the phase-locked loop (PLL), compensation can be provided for frequencies at which there is a risk of subsynchronous / supersynchronous oscillation instability. Furthermore, when the power converter is connected to a weak grid, reducing the control coefficient of the PLL to a first preset value can reduce phase fluctuations caused by disturbance signals. By using the adjusted PLL to control and regulate the quadrature-axis component, the phase margin of the PLL under weak grid conditions can be improved, suppressing subsynchronous / supersynchronous oscillations and thus enhancing the stability of the power converter operation. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of a phase-locked control loop provided in an embodiment of this application; Figure 2 This is a flowchart illustrating the implementation of the phase-locked loop control method for a power converter provided in this application embodiment; Figure 3 This is a schematic diagram of another phase-locked control loop provided in an embodiment of this application; Figure 4 This is a schematic diagram of the phase-locked loop control device for a power converter provided in the embodiments of this application. Detailed Implementation

[0020] In the following description, specific details such as particular system architectures and techniques are set forth for illustrative purposes and not for limitation, in order to provide a thorough understanding of the embodiments of this application. However, those skilled in the art will understand that this application may also be implemented in other embodiments without these specific details. In other instances, detailed descriptions of well-known systems, apparatuses, circuits, and methods have been omitted so as not to obscure the description of this application with unnecessary detail.

[0021] To make the objectives, technical solutions, and advantages of this application clearer, the following description will be provided in conjunction with the accompanying drawings and specific embodiments.

[0022] In the control of power converters, the current loop needs to use the precise phase of the power grid to convert AC current commands into DC commands in a two-phase rotating coordinate system, or to convert current control quantities in a two-phase rotating coordinate system back to a three-phase coordinate system. Simultaneously, the power loop also needs to use the precise phase of the power grid to calculate active and reactive power in order to control the power output of the power converter. Therefore, accurate phase tracking of the power grid is required.

[0023] When tracking the phase of the power grid, a phase-locked loop (PLL) is typically used to track the actual phase of the grid voltage at the grid connection point, thereby synchronizing the power converter with the actual phase of the grid. For example... Figure 1 As shown, in the phase-locked loop, the obtained three-phase grid voltage ( , and Transform to a two-phase stationary coordinate system to obtain the voltage. and Then, by using the phase-locked angle tracked in the previous control cycle, it is transformed into a two-phase rotating coordinate system to obtain the direct-axis voltage component. and cross-axis voltage components When the phase-locked loop angle is aligned with the phase of the grid voltage, all voltages are projected onto the direct axis (d-axis), while no voltage is projected onto the quadrature axis (q-axis). It will be 0.

[0024] Therefore, the reference value 0 and the transformed value can be compared. Subtracting the two values ​​yields the voltage error signal e, which reflects the deviation between the current phase-locked angle and the actual phase of the power grid. Multiplying this voltage error signal by -1 converts the actual error to a positive value. This signal is then input to a proportional-integral (PI) controller for adjustment. The PI controller outputs a frequency adjustment value, which, when added to the initial angular frequency, yields the current actual angular frequency. Integrating this angular frequency gives the phase-locked angle for the current control cycle. This enables the tracking of the actual phase of the grid voltage.

[0025] However, phase lag is prone to occur during the control process of a phase-locked loop (PLL). Especially when the power converter is connected to a weak grid, the equivalent impedance of the grid increases significantly and becomes inductive due to the combined effect of nonlinear loads and line impedance. This amplifies the effect of the phase lag generated by the PLL, easily triggering subsynchronous / supersynchronous oscillations and affecting the safe and stable operation of the power converter.

[0026] To improve the stability of power converter operation and reduce oscillation risks, this application adjusts the phase-locked loop (PLL). The adjusted PLL reduces phase fluctuations caused by disturbance signals, increases the phase margin of the PLL, and suppresses sub- / super-synchronous oscillations, thereby improving the stability of the power converter operation. In this embodiment, when the power converter is connected to a weak grid, the control coefficient of the PLL is reduced to a first preset value to reduce phase fluctuations caused by disturbance signals, resulting in a relatively stable current phase during power converter operation. Furthermore, by activating the phase compensator in the PLL, frequencies with insufficient phase margin and the risk of sub- / super-synchronous oscillation instability can be compensated, improving the phase margin of the PLL under weak grid conditions, suppressing sub- / super-synchronous oscillations, and thus improving the stability of the power converter operation.

[0027] See Figure 2 The document illustrates a flowchart of the phase-locked loop control method for a power converter provided in this embodiment, which is described in detail below: Step 201: In each control cycle during which the phase-locked loop of the power converter tracks the actual phase of the voltage at the grid connection point, acquire the actual voltage data at the grid connection point of the power converter, as well as the first phase-locked angle output by the phase-locked loop in the previous control cycle.

[0028] The phase-locked control loop can be implemented using a phase-locked loop (PLL), which is used to track the frequency and phase of the input voltage signal in real time and output a phase angle synchronized with the input.

[0029] The grid connection point is the connection point between the power converter and the power grid. The voltage and current at this point reflect the interaction state between the power converter and the power grid and serve as the reference for synchronization of the power converter.

[0030] Actual voltage data refers to the instantaneous three-phase voltage values ​​collected at the grid connection point. For example, if the power grid includes phases a, b, and c, then the actual voltage data includes the voltage of phase a. phase b voltage and c-phase voltage .

[0031] The phase-locked angle (PLO) output by the phase-locked loop (PLL) is an angle signal representing a real-time estimate of the grid voltage vector phase. Specifically, the first PLO represents the angle signal obtained in the previous control cycle, while the second PLO, mentioned later, represents the angle signal obtained in the current control cycle. Both the first and second PLOs are angle signals output by the PLO, differing only in the control cycles they represent. Furthermore, the PLO is an angle signal that follows the phase change of the grid voltage and is not a fixed value.

[0032] Step 202: Based on the first phase-locked angle, perform coordinate transformation on the actual voltage data to obtain the quadrature-axis voltage component; based on the quadrature-axis voltage component and the phase-locked control loop, obtain the second phase-locked angle output by the phase-locked control loop in the current control cycle.

[0033] The phase-locked loop includes a phase compensator, which is used to compensate for the phase of a preset frequency. When the power grid connected to the power converter is a weak grid, the control coefficient of the phase-locked loop is a first preset value. The first preset value is less than a second preset value, which is the control coefficient when the connected grid is not a weak grid. The control coefficient includes a proportional coefficient and / or an integral coefficient.

[0034] In this embodiment, as Figure 1 As shown, coordinate transformation can include two transformations. The first transformation converts the actual voltage data in a three-phase stationary coordinate system into components in a two-phase stationary coordinate system. The second transformation converts these components into components in a two-phase rotating coordinate system, resulting in the direct-axis and quadrature-axis voltage components in the two-phase rotating coordinate system. Alternatively, coordinate transformation can be a single transformation, directly converting the actual voltage data from a three-phase stationary coordinate system to a two-phase rotating coordinate system, obtaining the direct-axis and quadrature-axis voltage components in the two-phase rotating coordinate system.

[0035] When the power converter is connected to a weak grid, the control coefficient of the phase-locked loop (PLL) can be reduced to a first preset value, which is less than the control coefficient when the connected grid is not a weak grid. The PLL uses PI control, and the control coefficient can include a proportional coefficient and / or an integral coefficient. Correspondingly, the first preset value includes the values ​​of both the proportional and integral coefficients. The proportional coefficient value in the first preset value is less than the proportional coefficient when the connected grid is not a weak grid, and the integral coefficient value in the first preset value is also less than the integral coefficient when the connected grid is not a weak grid.

[0036] The larger the control coefficient, the faster and more sensitive the phase-locked loop (PLL) response to input disturbances. However, when using an adjusted PLL for control, the response to input disturbances slows down due to the reduced control coefficient. When disturbances exist in the power grid (such as voltage fluctuations, frequency fluctuations, and harmonics), the phase-locked angle output by the PLL will change. It will not quickly follow these disturbances, thus reducing the phase-locked angle. The drastic fluctuations caused the phase-locked angle of the phase control loop output to... The current phase becomes smoother, which in turn makes the current phase relatively stable during the operation of the power converter, thus suppressing oscillations.

[0037] For example, the value of the proportional coefficient in the first preset value can be 0.002, and the value of the integral coefficient in the first preset value can be 0.001.

[0038] In this embodiment, a phase compensator is also connected in series in the phase-locked loop. The phase compensator is used to perform phase compensation at a preset frequency to obtain an adjusted phase-locked loop.

[0039] When using the adjusted phase-locked loop (PLL) for control, phase compensation via a phase compensator reduces phase lag near the preset frequency, increases the phase margin near that frequency, and ensures sufficient phase margin at the original oscillation frequency, thus suppressing oscillation. Simultaneously, by increasing the phase margin at the preset frequency, the second phase-locked angle output by the PLL gradually decreases, mitigating the jitter of the coordinate transformation reference in the current loop and suppressing subsynchronous / supersynchronous oscillations. The preset frequency is the frequency point requiring compensation, and its value can be determined from frequencies where oscillation risk exists.

[0040] Here, the phase compensator can be set in the phase-locked loop (PLL). In grid-connected mode, the phase compensator is typically always on and does not switch between on and off states depending on whether the connected grid is weak, thus ensuring system stability. When the power converter is connected to a weak grid, the filter coefficient of the phase compensator can be adjusted to the required value. Furthermore, when the power converter is connected to a non-weak grid, the filter coefficient of the phase compensator can be adjusted to adapt to the current grid conditions. For example, the filter coefficient of the phase compensator can be 1; in this case, the phase compensator has no substantial effect, and its continuous on / off operation in grid-connected mode is not controlled, reducing its impact on system stability.

[0041] In some embodiments, when the power grid to which the power converter is connected is a weak power grid, the phase-locked loop control method for the power converter provided in this embodiment further includes: obtaining the first short-circuit ratio of the power converter connected to the power grid; determining the first filter coefficient corresponding to the first short-circuit ratio based on a preset correspondence between the short-circuit ratio and the filter coefficient; and adjusting the filter coefficient of the phase compensator to the first filter coefficient to perform phase compensation for the preset frequency.

[0042] Here, the Short Circuit Ratio (SCR) is the ratio of the grid's short-circuit capacity to the equipment's rated capacity. Under weak grid conditions, the oscillation frequency at which sub- / super-synchronous oscillations are at risk is closely related to the SCR: different SCRs indicate different levels of grid weakness, different equivalent grid impedances, and consequently, different oscillation frequencies at which sub- / super-synchronous oscillations are at risk. For example, a smaller SCR indicates a weaker grid, a larger equivalent grid impedance, and a lower oscillation frequency at which sub- / super-synchronous oscillations are at risk. Therefore, different filter coefficients can be set for the phase compensator for different SCRs. When the SCR changes, the filter coefficients are set according to the changed SCR to ensure the compensation effect of the phase compensator.

[0043] In this embodiment, when the power grid connected to the power converter is a weak grid and a phase compensator is provided in the phase-locked loop, the first short-circuit ratio of the power converter connected to the grid can be obtained, the first filter coefficient corresponding to the first short-circuit ratio can be found, and the filter coefficient of the phase compensator can be dynamically adjusted so that the filter coefficient of the phase compensator is adjusted to the first filter coefficient, thereby accurately compensating the oscillation frequency of the subsynchronous / supersynchronous oscillation risk.

[0044] The first short-circuit ratio can be obtained through online estimation, such as by actively injecting disturbances or passively detecting voltage / current changes, to calculate the SCR at the grid connection point of the power converter. The SCR acquisition period can be different from the control period of the phase-locked loop; for example, the duration of the SCR acquisition period can be longer than the duration of the control period of the phase-locked loop.

[0045] The preset correspondence between short-circuit ratios and filter coefficients can be a pre-stored mapping table or a corresponding expression calculated using interpolation formulas. If the correspondence is a mapping table, the filter coefficient corresponding to the first short-circuit ratio can be found. If the first short-circuit ratio is not found in the mapping table, the filter coefficient corresponding to the short-circuit ratio closest to it can be found. If the correspondence is a corresponding expression, the corresponding filter coefficient can be obtained by inputting the first short-circuit ratio.

[0046] In addition, when the power grid to which the power converter is connected is not a weak grid, the filter coefficient corresponding to the short-circuit ratio of the power converter connected to the grid can be determined according to the preset correspondence between the short-circuit ratio and the filter coefficient. The filter coefficient of the phase compensator can then be adjusted to the aforementioned filter coefficient to perform phase compensation.

[0047] In some embodiments, the second phase-locked angle output by the phase-locked control loop in the current control cycle is obtained based on the quadrature-axis voltage component and the phase-locked control loop. This can be achieved by: first, filtering the quadrature-axis voltage component using a phase compensator to obtain a quadrature-axis compensation voltage; then, determining the quadrature-axis error value based on the quadrature-axis compensation voltage and a preset quadrature-axis voltage; performing proportional-integral control on the quadrature-axis error value to obtain a frequency adjustment amount; and finally, obtaining the second phase-locked angle output by the phase-locked control loop in the current control cycle based on the frequency adjustment amount and the rated frequency.

[0048] In this embodiment, when a phase compensator is provided in the phase-locked loop, such as Figure 3 As shown, the phase compensator can be set before the PI control in the phase-locked loop to perform phase compensation on the q-axis voltage component obtained by coordinate transformation, and obtain the quadrature-axis compensation voltage.

[0049] When the phase-locked angle is aligned with the phase of the grid voltage, all voltages will be projected onto the d-axis, and the q-axis voltage will be 0. Therefore, the preset quadrature-axis voltage can be set to 0.

[0050] The quadrature axis error value can be obtained by measuring the difference between the quadrature axis compensation voltage and the preset quadrature axis voltage. This error value is then input to the PI control to obtain the frequency adjustment. The formula for the PI control is as follows: In the formula, This represents the output of the PI control, i.e., the frequency adjustment amount. This represents the proportionality coefficient. Represents the integral coefficient. This indicates the cross-axis error value.

[0051] Here, frequency synthesis is performed by superimposing the frequency adjustment amount and the rated frequency. The frequency adjustment amount is an angular frequency, which can be converted from the rated frequency and then superimposed. When the rated frequency is 50Hz, its angular frequency is... rad / s. Correspondingly, the frequency adjustment and the rated frequency are combined to obtain the total angular frequency. By integrating this total angular frequency, the corresponding phase can be obtained, which is the second phase-locked angle output in the current control cycle.

[0052] In some embodiments, the phase-locked loop control method for the power converter provided in this embodiment further includes: obtaining a second short-circuit ratio of the power converter connected to the power grid; if the second short-circuit ratio is less than or equal to a preset short-circuit ratio, then determining that the power grid connected to the power converter is a weak power grid.

[0053] Short-circuit capacity reflects the maximum short-circuit current capability that the power grid can provide at the grid connection point. When the short-circuit ratio (SCR) is large, the equivalent impedance of the power grid is small, the voltage support capability is strong, and the impact of power converter disturbances on the power grid is small. When the SCR is small, the equivalent impedance of the power grid is large, the voltage support capability is weak, the interaction between the power converter and the power grid is significant, and instability is prone to occur. Therefore, the short-circuit ratio can be used to distinguish whether the power grid to which the power converter is connected is a weak grid.

[0054] The preset short-circuit ratio can be 2, 2.5, or 3, etc. When the second short-circuit ratio is less than or equal to the preset short-circuit ratio, it indicates that the SCR of the power converter connected to the grid is small, the voltage support capability is weak, and the grid is a weak grid.

[0055] If the second short-circuit ratio is greater than the preset short-circuit ratio, it indicates that the SCR of the power converter connected to the grid is large, the voltage rigidity is strong, and the impedance is small, meaning that the grid is not a weak grid.

[0056] Here, the first short-circuit ratio and the second short-circuit ratio can be the same or different. The first short-circuit ratio and the second short-circuit ratio can be expressed using the short-circuit ratio or the effective short-circuit ratio, etc.

[0057] This application embodiment obtains the actual voltage data at the grid connection point of the power converter and the first phase-locked angle output by the phase-locked control loop in the previous control cycle. Using this first phase-locked angle, the actual voltage data of the power grid at the grid connection point is transformed into a coordinate system, which transforms the grid voltage from a three-phase stationary coordinate system to a two-phase rotating coordinate system. This yields the quadrature-axis voltage component of the target voltage data in the two-phase rotating coordinate system, which is the phase deviation between the phase-locked angle tracked by the power converter and the actual phase angle of the actual voltage data. Then, using the quadrature-axis voltage component and the phase-locked control loop, the second phase-locked angle output by the phase-locked control loop in the current control cycle can be obtained. Specifically, by activating the phase compensator in the phase-locked loop (PLL), compensation can be provided for frequencies at which there is a risk of subsynchronous / supersynchronous oscillation instability. Furthermore, when the power converter is connected to a weak grid, reducing the control coefficient of the PLL to a first preset value can reduce phase fluctuations caused by disturbance signals. By using the adjusted PLL to control and regulate the quadrature-axis component, the phase margin of the PLL under weak grid conditions can be improved, suppressing subsynchronous / supersynchronous oscillations and thus enhancing the stability of the power converter operation.

[0058] In some embodiments, before determining the first filter coefficient corresponding to the first short-circuit ratio based on a preset correspondence between the short-circuit ratio and the filter coefficient, the method further includes: When a power converter operating under preset conditions is connected to a weak grid, impedance frequency sweep is performed on the power converter for each of the various preset short-circuit ratios of the weak grid to obtain the swept Bode plot corresponding to that short-circuit ratio. Based on the swept Bode plot, the oscillation frequency of the power converter corresponding to that short-circuit ratio is determined. Based on the oscillation frequency, the compensation frequency and compensation phase corresponding to that short-circuit ratio are determined. Based on the compensation frequency and compensation phase, the filter coefficient corresponding to that short-circuit ratio is calculated.

[0059] Based on the filter coefficients corresponding to each short-circuit ratio, the correspondence between the short-circuit ratio and the filter coefficients is obtained.

[0060] Here, impedance frequency sweep refers to injecting small disturbance currents (or voltages) of different frequencies into the grid connection point of a power converter, measuring the response voltage (or current), and calculating the frequency response of the power converter's output impedance. By performing an impedance frequency sweep on the power converter, a swept-frequency Bode plot can be obtained. The swept-frequency Bode plot is a graph with frequency on the x-axis and impedance amplitude and phase on the y-axis. The swept-frequency Bode plot allows for a direct assessment of the power converter's impedance characteristics and stability margin at different frequencies. Based on the swept-frequency Bode plot, the Nyquist criterion can be used to determine the frequency at which subsynchronous / supersynchronous oscillations are at risk; this frequency is the determined oscillation frequency.

[0061] In this embodiment, impedance frequency sweep is performed on the power converter for each of the various short-circuit ratios in a weak grid. The short-circuit ratio of the weak grid can be experimentally set to achieve impedance frequency sweep of the power converter. Alternatively, impedance frequency sweep can be performed on the power converter when the weak grid is at the desired short-circuit ratio, as long as the Bode plot of the power converter under each short-circuit ratio can be obtained. The various short-circuit ratios can include 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, and 2.2, etc.

[0062] The compensation frequency is the center frequency at which the designed phase compensator performs phase compensation. The phase compensator provides the maximum phase lead angle (compensated phase) at this compensation frequency. The compensation frequency can be the oscillation frequency, or a value slightly lower than the oscillation frequency can be chosen to ensure sufficient phase margin at and above the oscillation frequency after compensation. The compensation phase is the required phase boost at the compensation frequency obtained by frequency sweeping.

[0063] Given the compensation frequency and compensation phase, the filter coefficients required by the phase compensator to perform phase compensation filtering at the compensation frequency can be calculated.

[0064] Finally, a mapping table between different short-circuit ratios and their corresponding filter coefficients can be established. Alternatively, a function fit can be performed using the short-circuit ratio and its corresponding filter coefficient to obtain the corresponding expression.

[0065] Furthermore, when the power grid to which the power converter is connected is not a weak grid, the filter coefficients of the phase compensator can also be determined based on the corresponding relationship. Correspondingly, when establishing the correspondence between the short-circuit ratio and the filter coefficients, impedance sweeping can be performed on the power converter for each of the various short-circuit ratios under the condition of a non-weak grid, and the filter coefficients corresponding to each short-circuit ratio can be calculated using the above method, thus obtaining the correspondence between different short-circuit ratios and filter coefficients.

[0066] Optionally, in this embodiment, the compensation frequency and compensation phase corresponding to the short-circuit ratio are determined based on the oscillation frequency. This can be achieved by: first obtaining the compensation frequency corresponding to the short-circuit ratio based on the difference between the oscillation frequency and a preset frequency margin; determining the impedance phase corresponding to the compensation frequency from the swept-frequency Bode plot; and then determining the compensation phase corresponding to the short-circuit ratio based on the impedance phase and a preset phase margin.

[0067] In this embodiment, to ensure the compensation effect of the phase compensator, a position slightly lower than the oscillation frequency is typically selected as the compensation frequency. The frequency margin is a preset frequency offset between the compensation frequency and the oscillation frequency. The compensation frequency is obtained by the difference between the oscillation frequency and this frequency margin, so that the phase compensator provides the maximum phase lead angle at the compensation frequency.

[0068] Impedance phase refers to the phase angle of the output impedance of a power converter, reflecting the lag / lead characteristics of the power converter at different frequencies.

[0069] The preset phase margin is the target value of the phase margin required for system stability, which can be the difference between the phase and -180°. For example, the phase margin can be 90° to ensure that the compensated phase is not lower than -90° under weak power grid conditions, thereby reducing the risk of subsynchronous / supersynchronous oscillation instability.

[0070] Here, the compensation phase that needs to be compensated can be calculated by using the impedance phase at the compensation frequency and the preset phase margin.

[0071] For example, if the oscillation frequency is 56.5 Hz and the phase of this frequency is -103°, the compensation frequency can be selected as 53 Hz based on the swept Bode plot. The phase of this frequency is -140°. To ensure that the compensated phase is not lower than -90° and to leave a margin, the compensation phase can be determined to be 60°. The phase margin can be 5°, 10°, 15°, etc.

[0072] Optionally, in this embodiment, the filter coefficients corresponding to the short-circuit ratio are calculated based on the compensation frequency and compensation phase. This can be achieved by: first, calculating the scale factor of the phase compensator under the short-circuit ratio based on the compensation phase; then, determining the transfer function of the phase compensator under the short-circuit ratio based on the compensation frequency and scale factor; and finally, performing a bilinear transformation on the transfer function according to the control period of the phase-locked loop to obtain the filter coefficients of the phase compensator corresponding to the short-circuit ratio.

[0073] In this embodiment, the maximum phase lead angle of the phase compensator depends only on the division function. Therefore, the corresponding scale function can be uniquely calculated by compensating for the phase. For example, the expression for calculating the scale function can be: In the formula, Represents the scale function. This indicates the compensation phase. For example, if the compensation phase is 60°, then the scale function... .

[0074] Here, the phase compensator employs a lead compensation network, whose transfer function in the continuous domain is: In the formula, Represents the transfer function in a continuous domain. Represents the time constant. This represents the frequency in the continuous domain. The time constant is also present. Compensation frequency and scale factor The decision can be expressed as: Substitute the compensation frequency and scale factor This will give you the specific transfer function.

[0075] Since the phase-locked loop is implemented discretely in a digital controller, the transfer function in the continuous domain must be... Transfer function converted to discrete domain Through the transfer function of the continuous domain Perform a bilinear transform to convert the frequency in the continuous domain Replace with It can make the whole Plane mapping to Within the unit circle of the plane, the stability of the discretized system is guaranteed, and the frequency response distortion is small in the low-frequency range. Among these, This indicates the control period of the phase-locked loop, which is usually taken as the period corresponding to the switching frequency of the power converter.

[0076] Accordingly, the transfer function in the discrete domain can be obtained. In the formula , and These are the required filter coefficients.

[0077] In some embodiments, when the control coefficient of the phase-locked loop is a first preset value, the phase-locked loop control method for the power converter provided in this embodiment further includes: firstly determining the voltage frequency at the grid connection point of the power converter based on the quadrature-axis voltage component and the phase-locked loop; calculating the difference between the voltage frequency and the rated frequency to obtain the frequency deviation value; if the frequency deviation value is greater than a preset deviation threshold, then adjusting the control coefficient of the phase-locked loop to a third preset value.

[0078] In this embodiment, when the control coefficient of the phase-locked loop is adjusted to the first preset value, the tracking of the frequency phase by the phase-locked loop slows down during the first frequency modulation process, resulting in a longer response time for the first frequency modulation. Therefore, when a first frequency modulation is detected, the control coefficient of the phase-locked loop can be adjusted again to ensure that the control process meets the response time requirement for the first frequency modulation.

[0079] Here, the control coefficient of the phase-locked loop can be increased to a third preset value, which is greater than the first preset value and can meet the requirements of the primary frequency regulation response time. The third preset value can be the same as the second preset value, i.e., the control coefficient used when the power grid to which the power converter is connected is not a weak grid.

[0080] In a phase-locked loop (PLL) control circuit, PI control is used, and the control coefficients can include proportional and / or integral coefficients. When the adjusted control coefficients are proportional and integral coefficients, both the first and third preset values ​​include the values ​​of the proportional and integral coefficients. The value of the proportional coefficient in the first preset value is less than the value of the proportional coefficient in the third preset value, and the value of the integral coefficient in the first preset value is less than the value of the integral coefficient in the third preset value.

[0081] In this embodiment, the frequency deviation value (the difference between the acquired voltage frequency and the rated frequency) determines whether to trigger primary frequency regulation. When the frequency deviation value is less than or equal to a preset deviation value, it indicates that the grid frequency is fluctuating normally with small fluctuations and no frequency regulation is required; therefore, the first preset value can be maintained. When the frequency deviation value is greater than a preset deviation threshold, it indicates that the grid frequency has deviated significantly, triggering a primary frequency regulation requirement. In this case, the control coefficient of the phase-locked loop (PLL) is switched to a third preset value to improve the frequency tracking speed of the PLL and ensure that the power converter responds to the frequency regulation command in a timely manner.

[0082] This application embodiment obtains the actual voltage data at the grid connection point of the power converter and the first phase-locked angle output by the phase-locked control loop in the previous control cycle. Using this first phase-locked angle, the actual voltage data of the power grid at the grid connection point is transformed into a coordinate system, which transforms the grid voltage from a three-phase stationary coordinate system to a two-phase rotating coordinate system. This yields the quadrature-axis voltage component of the target voltage data in the two-phase rotating coordinate system, which is the phase deviation between the phase-locked angle tracked by the power converter and the actual phase angle of the actual voltage data. Then, using the quadrature-axis voltage component and the phase-locked control loop, the second phase-locked angle output by the phase-locked control loop in the current control cycle can be obtained. Specifically, by activating the phase compensator in the phase-locked loop (PLL), compensation can be provided for frequencies at which subsynchronous / supersynchronous oscillations and instability risks exist. When the power converter is connected to a weak grid, reducing the control coefficient of the PLL to a first preset value can reduce phase fluctuations caused by disturbance signals. Furthermore, by using the adjusted PLL to control and regulate the quadrature-axis component, the phase margin of the PLL under weak grid conditions can be improved, suppressing subsynchronous / supersynchronous oscillations and thus enhancing the stability of the power converter operation. Additionally, different filter coefficients for the phase compensator are set for different SCRs. When the SCR changes, the filter coefficient is adjusted for the changed SCR to ensure the compensation effect of the phase compensator. When the control coefficient of the PLL is at the first preset value, if primary frequency regulation is detected, the control coefficient of the PLL can be adjusted again to increase the control coefficient, ensuring that the control process meets the primary frequency regulation response time.

[0083] It should be understood that the sequence number of each step in the above embodiments does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0084] The following are device embodiments of this application. For details not described in detail, please refer to the corresponding method embodiments described above.

[0085] Figure 4 A schematic diagram of the phase-locked loop control device for a power converter according to an embodiment of this application is shown. For ease of explanation, only the parts relevant to the embodiment of this application are shown, and are described in detail below: like Figure 4 As shown, the phase-locked loop control device 40 of the power converter includes: The receiving module 41 is used to acquire the actual voltage data at the grid connection point of the power converter and the first phase-locked angle output by the phase-locked control loop in the previous control cycle during each control cycle of tracking the actual phase of the voltage at the grid connection point in the phase-locked control loop of the power converter.

[0086] Control module 42 is used to perform coordinate transformation on the actual voltage data according to the first phase-locked angle to obtain the quadrature-axis voltage component; and to obtain the second phase-locked angle output by the phase-locked control loop in the current control cycle based on the quadrature-axis voltage component and the phase-locked control loop. The phase-locked loop includes a phase compensator, which is used to compensate for the phase of a preset frequency. When the power grid connected to the power converter is a weak grid, the control coefficient of the phase-locked loop is a first preset value. The first preset value is less than a second preset value, which is the control coefficient when the connected grid is not a weak grid. The control coefficient includes a proportional coefficient and / or an integral coefficient.

[0087] In one possible implementation, when the power grid to which the power converter is connected is a weak grid, the control module 42 is further configured to: Obtain the first short-circuit ratio of the power converter connected to the power grid; Based on the preset correspondence between the short-circuit ratio and the filter coefficient, the first filter coefficient corresponding to the first short-circuit ratio is determined. Adjust the filter coefficient of the phase compensator to the first filter coefficient to perform phase compensation at the preset frequency.

[0088] In one possible implementation, the control module 42 is also used for: When a power converter operating under preset conditions is connected to a weak grid, impedance frequency sweep is performed on the power converter for each of the various preset short-circuit ratios of the weak grid to obtain the swept Bode plot corresponding to that short-circuit ratio. Based on the swept Bode plot, the oscillation frequency of the power converter corresponding to that short-circuit ratio is determined. Based on the oscillation frequency, the compensation frequency and compensation phase corresponding to that short-circuit ratio are determined. Based on the compensation frequency and compensation phase, the filter coefficient corresponding to that short-circuit ratio is calculated. Based on the filter coefficients corresponding to each short-circuit ratio, the correspondence between the short-circuit ratio and the filter coefficients is obtained.

[0089] In one possible implementation, the control module 42 is specifically used for: The compensation frequency corresponding to the short-circuit ratio is obtained based on the difference between the oscillation frequency and the preset frequency margin. Determine the impedance phase corresponding to the compensation frequency from the swept Bode plot; The compensation phase corresponding to the short-circuit ratio is determined based on the impedance phase and the preset phase margin.

[0090] In one possible implementation, the control module 42 is specifically used for: Calculate the phase compensator's indexing factor at this short-circuit ratio based on the compensation phase; Based on the compensation frequency and the scale factor, determine the transfer function of the phase compensator at this short-circuit ratio; Based on the control period of the phase-locked loop, the transfer function is subjected to a bilinear transformation to obtain the filter coefficient of the phase compensator corresponding to the short-circuit ratio.

[0091] In one possible implementation, the control module 42 is specifically used for: The quadrature-axis voltage component is filtered using a phase compensator to obtain the quadrature-axis compensated voltage; The cross-axis error value is determined based on the cross-axis compensation voltage and the preset cross-axis voltage. The frequency adjustment is obtained by performing proportional-integral control on the cross-axis error value; Based on the frequency adjustment and the rated frequency, the second phase-locked angle output by the phase-locked control loop in the current control cycle is obtained.

[0092] In one possible implementation, when the control coefficient of the phase-locked loop is a first preset value, the control module 42 is further configured to: Based on the quadrature-axis voltage component and the phase-locked control loop, the voltage frequency at the grid connection point of the power converter is determined. Calculate the difference between the voltage frequency and the rated frequency to obtain the frequency deviation value; If the frequency deviation value is greater than the preset deviation threshold, the control coefficient of the phase-locked loop will be adjusted to the third preset value, which is greater than the first preset value.

[0093] In one possible implementation, the control module 42 is also used for: Obtain the second short-circuit ratio of the power converter connected to the power grid; If the second short-circuit ratio is less than or equal to the preset short-circuit ratio, then the power grid into which the power converter is connected is determined to be a weak power grid.

[0094] For the sake of convenience and brevity, the above division of functional modules / units is used as an example. In actual applications, the above functions can be assigned to different functional modules / units as needed.

[0095] This application also provides a power converter. For details not described in detail, please refer to the corresponding method embodiments described above.

[0096] In some embodiments, the power converter includes a power converter body and a controller for implementing the methods described in the various embodiments above.

[0097] This application also provides a computer-readable storage medium storing a computer program. When the computer program is executed by a processor, it implements the methods described in the above-described method embodiments.

[0098] This application also provides a computer program product, including a computer program. When the computer program is executed by a processor, it implements the methods in the above-described method embodiments.

[0099] Computer programs include computer program code, which can be in the form of source code, object code, executable files, or certain intermediate forms. Computer-readable media can include: any entity or device capable of carrying computer program code, recording media, USB flash drives, portable hard drives, magnetic disks, optical disks, computer memory, read-only memory (ROM), random access memory (RAM), electrical carrier signals, telecommunication signals, and software distribution media, etc.

[0100] In the above embodiments, the descriptions of each embodiment have their own emphasis. Parts not detailed or described in a particular embodiment can be referred to in the relevant descriptions of other embodiments. Unless otherwise specified or in conflict with logic, the terminology and / or descriptions between different embodiments are consistent and can be referenced interchangeably. Technical features in different embodiments can be combined to form new embodiments based on their inherent logical relationships.

[0101] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A phase-locked loop control method for a power converter, characterized in that, include: In each control cycle of the phase-locked loop of the power converter tracking the actual phase of the voltage at the grid connection point, the actual voltage data at the grid connection point of the power converter and the first phase-locked angle output by the phase-locked loop in the previous control cycle are acquired; based on the first phase-locked angle, the actual voltage data are subjected to coordinate transformation to obtain the quadrature-axis voltage component; Based on the quadrature-axis voltage component and the phase-locked loop, the second phase-locked angle output by the phase-locked loop in the current control cycle is obtained; The phase-locked loop includes a phase compensator for performing phase compensation at a preset frequency. When the power grid connected to the power converter is a weak grid, the control coefficient of the phase-locked loop is a first preset value. The first preset value is less than a second preset value, which is the control coefficient when the connected grid is not a weak grid. The control coefficient includes a proportional coefficient and / or an integral coefficient.

2. The phase-locked loop control method for a power converter according to claim 1, characterized in that, When the power grid to which the power converter is connected is a weak grid, the method further includes: Obtain the first short-circuit ratio of the power converter connected to the power grid; Based on the preset correspondence between the short-circuit ratio and the filter coefficient, the first filter coefficient corresponding to the first short-circuit ratio is determined. The filter coefficient of the phase compensator is adjusted to the first filter coefficient to perform phase compensation at the preset frequency.

3. The phase-locked loop control method for a power converter according to claim 2, characterized in that, Before determining the first filter coefficient corresponding to the first short-circuit ratio based on the preset correspondence between the short-circuit ratio and the filter coefficient, the method further includes: When the power converter, operating under preset conditions, is connected to a weak grid, impedance frequency sweep is performed on the power converter for each of the various preset short-circuit ratios of the weak grid to obtain the swept Bode plot corresponding to that short-circuit ratio. Based on the swept Bode plot, the oscillation frequency at which the power converter exhibits subsynchronous / supersynchronous oscillations corresponding to that short-circuit ratio is determined. Based on the oscillation frequency, the compensation frequency and compensation phase corresponding to that short-circuit ratio are determined. Based on the compensation frequency and the compensation phase, the filter coefficient corresponding to that short-circuit ratio is calculated. Based on the filter coefficients corresponding to each short-circuit ratio, the correspondence between the short-circuit ratio and the filter coefficients is obtained.

4. The phase-locked loop control method for a power converter according to claim 3, characterized in that, The step of determining the compensation frequency and compensation phase corresponding to the short-circuit ratio based on the oscillation frequency includes: The compensation frequency corresponding to the short-circuit ratio is obtained based on the difference between the oscillation frequency and the preset frequency margin. From the swept Bode plot, determine the impedance phase corresponding to the compensation frequency; The compensation phase corresponding to the short-circuit ratio is determined based on the impedance phase and the preset phase margin.

5. The phase-locked loop control method for a power converter according to claim 3, characterized in that, The step of calculating the filter coefficients corresponding to the short-circuit ratio based on the compensation frequency and the compensation phase includes: Based on the compensation phase, calculate the scale factor of the phase compensator under the short-circuit ratio; Based on the compensation frequency and the indexing coefficient, the transfer function of the phase compensator under this short-circuit ratio is determined; Based on the control period of the phase-locked loop, the transfer function is subjected to a bilinear transformation to obtain the filter coefficient of the phase compensator corresponding to the short-circuit ratio.

6. The phase-locked loop control method for a power converter according to any one of claims 1 to 5, characterized in that, Based on the quadrature-axis voltage component and the phase-locked loop, the second phase-locked angle output by the phase-locked loop in the current control cycle is obtained, including: The quadrature-axis voltage component is filtered using the phase compensator to obtain the quadrature-axis compensated voltage. The cross-axis error value is determined based on the cross-axis compensation voltage and the preset cross-axis voltage; The frequency adjustment amount is obtained by performing proportional-integral control on the cross-axis error value; Based on the frequency adjustment amount and the rated frequency, the second phase-locked angle output by the phase-locked control loop in the current control cycle is obtained.

7. The phase-locked loop control method for a power converter according to any one of claims 1 to 5, characterized in that, When the control coefficient of the phase-locked loop is a first preset value, the method further includes: Based on the quadrature-axis voltage component and the phase-locked control loop, the voltage frequency at the grid connection point of the power converter is determined. Calculate the difference between the voltage frequency and the rated frequency to obtain the frequency deviation value; If the frequency deviation value is greater than the preset deviation threshold, the control coefficient of the phase-locked loop is adjusted to a third preset value, which is greater than the first preset value.

8. The phase-locked loop control method for a power converter according to any one of claims 1 to 5, characterized in that, The method further includes: Obtain the second short-circuit ratio of the power converter connected to the power grid; If the second short-circuit ratio is less than or equal to the preset short-circuit ratio, then the power grid into which the power converter is connected is determined to be a weak power grid.

9. A phase-locked loop control device for a power converter, characterized in that, include: The acquisition module is used to acquire the actual voltage data at the grid connection point of the power converter and the first phase-locked angle output by the phase-locked control loop in the previous control cycle during each control cycle of tracking the actual phase of the voltage at the grid connection point in the phase-locked control loop of the power converter. The control module is used to perform coordinate transformation on the actual voltage data according to the first phase-locked angle to obtain the quadrature-axis voltage component; Based on the quadrature-axis voltage component and the phase-locked loop, the second phase-locked angle output by the phase-locked loop in the current control cycle is obtained; The phase-locked loop includes a phase compensator for performing phase compensation at a preset frequency. When the power grid connected to the power converter is a weak grid, the control coefficient of the phase-locked loop is a first preset value. The first preset value is less than a second preset value, which is the control coefficient when the connected grid is not a weak grid. The control coefficient includes a proportional coefficient and / or an integral coefficient.

10. A power converter, characterized in that, Includes a controller for implementing the method as described in any one of claims 1 to 8.

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