Active damping compensation method for LCL type grid-connected converter
By combining capacitor current feedback and inverter-side current high-pass filtering scheme in the LCL type grid-connected converter, the automatic adjustment of the damping algorithm and feedback current proportional coefficient is solved, and the problems of resonance instability and low-frequency resonance suppression in the existing technology are achieved, and effective suppression of different frequency points and system stability are improved.
Patent Information
- Application Number
- CN202510329673.X
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-03-20
- Publication Date
- 2025-05-02
AI Technical Summary
The existing LCL type grid-connected converters have problems such as instability and poor high-frequency resonance suppression in the resonance phenomenon, especially in low-frequency resonance, which is difficult to effectively suppress.
By setting up capacitive current feedback and inverter-side current high-pass filtering schemes in the LCL type grid-connected converter, combining automatic detection of sudden current and adaptive debugging technology, the proportional coefficients of the damping algorithm and feedback current are automatically adjusted according to the on-site situation, and effective suppression of different resonant frequencies is achieved.
It realizes that active damping can work at different resonant frequencies, and automatically adjusts the proportional coefficients of feedback current and high-pass filtering, effectively reducing the occurrence of resonance phenomena and improving the stability and adaptability of the system.
Smart Images
Figure CN119921328A_ABST
Abstract
Description
Technical Field
[0001] The invention relates to the technical field of power grid connection, and in particular to an active damping compensation method for an LCL type grid-connected converter. Background Art
[0002] In LCL filters, when the frequency increases to a certain level, the inductive reactance of the inductor increases, while the capacitive reactance of the capacitor decreases. When the two are equal, resonance occurs, which can easily lead to system instability and increased harmonic content. Therefore, there are currently two ways to perform damping compensation. One is hardware compensation, which uses resistors in series / parallel to the LCL filter to suppress the resonance spike through passive damping. The other is to increase the damping term through software strategy control and perform active damping compensation through algorithms.
[0003] Currently, active damping compensation is more commonly used through capacitor current feedforward, as shown in the attached Figure 1 The figure shows the mathematical model of single-phase capacitor current feedback active damping in Ruan Xinbo's "Control Technology of LCL Grid-connected Inverter". Under different power usage environments, the resonance point of this model will shift due to the change of grid impedance. The active damping debugging parameters cannot be applied and need to be adjusted in time according to the on-site environment. Generally, the parameters are debugged on-site through the local touch screen, which is inconvenient and inefficient.
[0004] In addition, for the capacitor current feedback active damping method, since the capacitor current is high-frequency components, the capacitor current feedback can play a good role in suppressing high-frequency components, but it has little effect on damping low-frequency components. Often, due to different grid impedances at different sites, the resonance point will shift toward low frequency or high frequency. Especially when the resonance point shifts toward low frequency, it is difficult to use this method to effectively suppress resonance. Summary of the invention
[0005] The purpose of the present invention is to overcome the defects existing in the prior art and provide an active damping compensation method for an LCL type grid-connected converter. The program is compatible with two algorithms, namely, capacitor current feedback and inverter side current high-pass filtering. The damping algorithm is automatically adjusted according to the on-site conditions to achieve active damping at different resonant frequencies. The proportional coefficient of the feedback current and the high-pass filter can be automatically adjusted, thereby achieving effective damping compensation and reducing the occurrence of resonance.
[0006] To achieve the above object, the present invention designs an active damping compensation method for an LCL type grid-connected converter, by simultaneously setting a capacitor current feedback and an inverter side current high-pass filtering scheme in the converter, the compensation method comprises: The converter uses the capacitor current feedback scheme by default when it leaves the factory. At the same time, since the output current will suddenly change when resonance occurs, in order to protect the circuit components, overcurrent protection will be performed. Therefore, the software performs current mutation detection. If the current change exceeds the set current first threshold, the adaptive debugging of the capacitor current feedback proportional parameter is prioritized. If the current mutation value is detected to be smaller after the proportional parameter is increased or decreased, and the overcurrent protection is not triggered, the adjustment is successful, or the proportional parameter that makes the current mutation value smaller is debugged in this direction until the current mutation disappears or is less than the current second threshold. If the current mutation value does not decrease after adjusting the feedback ratio, or the overcurrent protection is still triggered multiple times, the software will automatically switch to the inverter side high-pass filtering solution and adjust the inverter side current feedforward ratio to reduce the current mutation value; The converter retains the inverter side high-pass filter scheme setting until a new current mutation value exceeding the current first threshold value appears and cannot be suppressed by the inverter side high-pass filter scheme, and is replaced with the capacitor current feedback scheme for resonance suppression debugging. The judgment of the specific debugging success can be consistent with the judgment standard or method when using the capacitor current feedback scheme.
[0007] Furthermore, the capacitor current feedback scheme model includes:
[0008] in, is the inverter side voltage, C is the LCL filter capacitor, L1 is the first equivalent inductor, L2 is the second equivalent inductor, represents the current corresponding to the second inductor; s is the symbol of the complex variable of the transfer function, representing the complex frequency; G is the transfer function of the plant, is the capacitor current feedback coefficient.
[0009] Furthermore, the inverter side current is fed forward to the current loop input, and the inverter side current high-pass filtering solution model includes:
[0010] in, is the cutoff frequency of the first-order high-pass filter, and a is the gain coefficient of the first-order high-pass filter.
[0011] Furthermore, the number of adaptive debugging is greater than or equal to 6 times and after the debugging is invalid, the capacitor current feedback and the inverter side high-pass filtering scheme are switched. The debugging includes debugging at least once in the direction of increasing and decreasing the feedback coefficient respectively. The debugging determines the direction of reducing the feedback output. The debugging feedback coefficient change step should ensure that the system overshoot is less than 1.2 times the rated current. Therefore, the feedback coefficient is limited in the program, and is generally adjusted up and down with the smallest step.
[0012] Furthermore, after the adaptive debugging is invalid for 6 times, the capacitor current feedback and the inverter side high-pass filtering scheme are switched. The 6 debuggings are generally based on the current resonance suppression frequency. If it is determined that the feedback coefficient adjustment direction can reduce the current mutation, it can continue to be adjusted in this direction until the feedback output meets the required resonance suppression point.
[0013] Furthermore, after the adaptive debugging fails for 12 times, the capacitor current feedback and the inverter side high-pass filtering scheme are switched.
[0014] Furthermore, the first current threshold is greater than the rated current of the power grid and less than or equal to the smaller of 4 / 5 of the maximum allowable current or 2 times of the daily working current.
[0015] Furthermore, the second current threshold is less than or equal to the larger one of 1.2 times the rated current of the grid and 1.2 times the daily working current.
[0016] Furthermore, the adaptive debugging method also includes a current mutation slope auxiliary judgment method, which includes calculating the rising slope by measuring the time it takes for the current to rise to the first threshold value. If the slope becomes larger between two adjacent debuggings, it indicates that the debugging resonant frequency change should be carried out in the opposite direction, or the capacitor current feedback and the inverter side high-pass filtering scheme should be switched and then re-debugging is performed.
[0017] The advantages and beneficial effects of the present invention are as follows: the active damping compensation method of the LCL type grid-connected converter of the present invention can automatically detect the sudden change current of the grid-connected converter, automatically adjust the circuit resonance suppression parameters, and select a suitable resonance suppression method, thereby reducing the after-sales labor cost. Because the on-site power grids are not the same, the adaptive algorithm and the debugging parameters can adapt to most sites, reducing the labor cost of on-site debugging; improving the adaptability of the converter power grid, the power grid impedance at the same site will fluctuate according to other factors such as load, and the method of the present invention can adaptively perform real-time adjustments, reducing the risk of power grid tripping. BRIEF DESCRIPTION OF THE DRAWINGS
[0018] Figure 1 It is the transfer function block diagram of the capacitor current feedback scheme; Figure 2 This is the simplified circuit schematic of LCL; Figure 3 It is the block diagram of the inverter side current high-pass filter transfer function; Figure 4 It is the resonance spectrum diagram during the test; Figure 5 It is the resonance suppression spectrum diagram. DETAILED DESCRIPTION
[0019] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and embodiments. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present invention.
[0020] Embodiment 1: The present invention discloses an active damping compensation method for an LCL type grid-connected converter, which automatically detects sudden current changes by simultaneously setting a capacitor current feedback and an inverter side current high-pass filtering scheme in the converter. The compensation method comprises: The converter uses the capacitor current feedback scheme by default when it leaves the factory. At the same time, since the output current will suddenly change when resonance occurs, in order to protect the circuit components, overcurrent protection will be performed. Therefore, the software performs current mutation detection. If the current change exceeds the set current first threshold, the adaptive debugging of the capacitor current feedback proportional parameter is prioritized. If the current mutation value is detected to be smaller after the proportional parameter is increased or decreased, and the overcurrent protection is not triggered, the adjustment is successful, or the proportional parameter that makes the current mutation value smaller is debugged in this direction until the current mutation disappears or is less than the current second threshold. If the current mutation value does not decrease after adjusting the feedback ratio, or the overcurrent protection is still triggered multiple times, the software will automatically switch to the inverter side high-pass filtering solution and adjust the inverter side current feedforward ratio to reduce the current mutation value; The converter retains the inverter side high-pass filter scheme setting until a new current mutation value exceeding the current first threshold value appears and cannot be suppressed by the inverter side high-pass filter scheme, and is replaced with the capacitor current feedback scheme for resonance suppression debugging. The judgment of the specific debugging success can be consistent with the judgment standard or method when using the capacitor current feedback scheme.
[0021] The commonly used capacitor current feedback scheme has a good suppressing effect on high-frequency resonance, but when low-frequency resonance occurs due to on-site reasons, this method cannot effectively deal with it; the inverter side high-pass filtering scheme has a good suppressing effect on low-frequency resonance, but has a strengthening effect on high-frequency signals and is generally rarely used.
[0022] In view of this situation, when resonance is about to occur or has occurred, since its resonance frequency cannot be accurately predicted, according to the conventional high-frequency resonance characteristics, a capacitor current feedback scheme is first tried to suppress it. If the suppression is unsuccessful, the inverter side high-pass filtering scheme can be automatically switched.
[0023] The corresponding APF or LCL type grid-connected converter parameters are set to the inverter side high-pass filtering scheme until a new current mutation value appears that cannot be effectively suppressed (generally at least 2 or more attempts to suppress with different parameters are made), and the software automatically resets to the capacitor current feedback scheme.
[0024] Preferably, the capacitor current feedback scheme model includes: like Figure 1 As shown, according to the model structure
[0025]
[0026] Then there is
[0027] as well as
[0028] in, =Inverter side voltage - capacitor feedback value, is the inverter side voltage, is the grid voltage; C is the LCL filter capacitor, L1 is the first equivalent inductor (equivalent capacitor front end), L2 is the second equivalent inductor (equivalent capacitor back end), Z L1 , Z L2 , Z C They represent the circuit equivalent transfer functions corresponding to inductance and capacitance respectively, , , They represent the corresponding currents respectively; s is the symbol of the complex variable of the transfer function, representing the complex frequency; G is the transfer function of the plant, is the capacitor current feedback coefficient, Represents the grid-connected current feedback function; K PWM is the inverter equivalent gain.
[0029] According to the transfer function, the selection of active damping and capacitor current feedback According to the Bode diagram, it can be seen that the appropriate selection of feedback ratio can effectively suppress resonance. The selection of feedback ratio can be achieved by drawing the root locus of the transfer function and selecting the parameters of the imaginary axis of the left half plane principle.
[0030] like Figure 2 As shown, according to the LCL simplified circuit and Kirchhoff's law, the LCL inverter transfer function is:
[0031] From the transfer function, we can analyze that there is a pair of conjugate poles. From the Bode diagram, we can also see that when When it is 0, infinite, That is the resonant angular frequency. Active damping and other processing methods are all aimed at reducing the amplitude and phase angle mutations around this frequency, thereby making the system stable.
[0032] In the actual power grid, due to different impedances, the resonance point is no longer a theoretical value, so the feedback ratio selection method should be based on the current power grid analysis: by adjusting the grid voltage U g The feedforward coefficient is adjusted to gradually increase the output reactive power. The FFT of the converter output current is analyzed by an oscilloscope to find the resonance point. The gain at this frequency point is close to 0dB. Then the capacitor current feedback ratio is adjusted, for example, the proportional coefficient is adjusted to -1. If the gain at this resonance point is obviously attenuated, continue to adjust the value up and down until the best attenuation is found. This parameter is the capacitor current feedback ratio under the current power grid.
[0033] This embodiment takes =0.2mH, =0.1mH, C=7.5μF, then =7118Hz, that is, after the signal of about 7kHz passes through the LCL hardware circuit, the current value near this frequency will suddenly change upward. Figure 4 As shown in the figure, when it is resonant, the gain at 8kHz is close to 0dB. Figure 5 When the capacitor current feedback ratio is adjusted to -1.4, the best suppression is achieved, and the gain reduction is obvious at 8kHz.
[0034] Preferably, the inverter side current is fed forward to the current loop input, and the inverter side current high-pass filtering scheme model includes: like Figure 3 As shown, according to the model structure, there are
[0035] in, is the cutoff frequency of the first-order high-pass filter, and a is the gain coefficient of the first-order high-pass filter.
[0036] Figure 3 middle
[0037] is the first-order high-pass filter transfer function. The goal is to make the active damping work in the high-frequency band within the resonant frequency without affecting the low-frequency band of the LCL. From the Bode diagram, it can be analyzed that after introducing the inverter side current high-pass filter, it can also work at the resonance. In order to ensure that the LCL low-frequency signal can continue to pass, Much larger than the cutoff frequency without damping , generally should meet ; From the resonance peak The active damping starts from 0dB and keeps working to attenuate the resonance point. < , that is, when the inverter side high-pass filter solution is used to suppress low-frequency resonance, the high-pass filter should meet Debugging within the scope of
[0038] In actual testing, the inverter-side high-pass filtering solution is used to pass high-frequency signals, thereby weakening the attenuation of high-frequency signals by LCL.
[0039] Preferably, the number of adaptive debugging is greater than or equal to 6 times and after the debugging is invalid, the capacitor current feedback and the inverter side high-pass filtering scheme are switched. The debugging includes debugging at least once in the direction of increasing and decreasing the feedback coefficient respectively. The debugging determines the direction of reducing the feedback output. The debugging feedback coefficient change step should ensure that the system overshoot is less than 1.2 times the rated current. Therefore, the feedback coefficient and the adjustment amount are limited in the program, and generally adjusted up and down with the minimum step. The invalidity means that the current mutation value does not decrease or triggers the overcurrent protection.
[0040] Preferably, the first current threshold is greater than the rated current of the grid and is less than or equal to the smaller of 4 / 5 of the maximum allowable current or 2 times the daily working current. This setting is mainly to ensure that there is a certain amount of time for grid adjustment before the circuit trips, reduce the probability of tripping, and protect the stable operation of the converter.
[0041] Preferably, the second current threshold is less than or equal to the larger of 1.2 times the rated current of the grid and 1.2 times the daily working current. This setting is mainly considered that the compensation parameter adjustment may be a long process, especially the optimal parameter adjustment time will be relatively long, and the grid load is in dynamic change. The parameter adjustment itself will also have a certain impact on the grid. Therefore, it is controlled within a certain time range. Even if the result is not completely ideal, the system is still in normal working state.
[0042] Preferably, the adaptive debugging method also includes a current mutation slope auxiliary judgment method, which includes calculating the rising slope by measuring the time it takes for the current to rise to the first threshold. If the slope becomes larger between two consecutive debuggings, it indicates that the debugging feedback coefficient change should be carried out in the opposite direction, or the capacitor current feedback and inverter side high-pass filtering schemes should be switched and then re-debugging should be performed (the scheme switching should be carried out when the original scheme is invalid in both the increasing and decreasing directions of the feedback coefficient). A larger slope generally means that the resonance phenomenon is more obvious or aggravated, so debugging should be carried out in the opposite direction to reduce the impact of the resonance; the rise time measurement can start from 0A or from a certain minimum current value. In short, the standards should be unified.
[0043] In this embodiment, the rated current of the power grid is 1200A, the maximum allowable working current is 1000A, and the average current of daily operation is 200A; the maximum output current of this device is 100A, the average current of daily operation is 50A, the first current threshold is set to 80A, the second current threshold is set to 60A, and the capacitor current feedback and inverter side high-pass filtering scheme are switched after 6 adaptive debugging are invalid. Regarding the comparison of the change in the slope of the sudden current rise, the two debuggings are compared with the slope of the initial current sudden change.
[0044] Embodiment 2: The difference from Example 1 is that after the adaptive debugging described in this embodiment is invalid for 12 times, the capacitor current feedback and the inverter side high-pass filtering scheme are switched. This setting is mainly to further confirm the direction of the sudden increase or decrease of the current. After the direction is confirmed, a better feedback coefficient value is further debugged to obtain a better resonance suppression effect.
[0045] In this embodiment, the first current threshold is set to 70A and the second current threshold is set to 55A.
[0046] Embodiment 3: The difference from Example 2 is that this embodiment is debugged twice in the direction of increasing and decreasing the resonance suppression frequency respectively. When the next debugging direction or the need for the actual resonance frequency to occur cannot be accurately determined by relying solely on the current mutation, resulting in changes in the capacitor current feedback and the inverter side high-pass filtering scheme switching (such as the mutation situations are basically the same), the magnitudes of the rising slopes of the current mutations in the aforementioned two adjacent increasing and two adjacent decreasing directions can be compared at the same time, and the direction in which the latter slope becomes smaller should be the main target resonance suppression frequency debugging direction; if there is a direction in which the slope becomes smaller, continue to debug according to the minimum compensation, otherwise switch the capacitor current feedback and the inverter side high-pass filtering scheme.
[0047] The above is only part of the more systematic and comprehensive implementation scheme of the active damping compensation method of the LCL type grid-connected converter of the present invention. In fact, for the capacitor current feedback and the inverter side current high-pass filtering scheme, there can be more debugging rules and threshold selection methods to determine the change direction of the actual resonant frequency, so that the actual resonant frequency can be determined more accurately and timely, and the grid fluctuation can be controlled within a limited range. These combinations or preferred schemes should also be regarded as the protection scope of the present invention and will not be listed one by one here.
Claims
1. An active damping compensation method for an LCL type grid-connected converter, characterized in that: Capacitor current feedback and inverter side current high-pass filtering scheme are set at the same time, and the compensation method includes: The converter uses the capacitor current feedback scheme by default and performs current mutation detection at the same time. If the current change exceeds the set current first threshold, the capacitor current feedback proportional parameter is adaptively debugged first. If the current mutation value is detected to be smaller after the proportional parameter is increased or decreased, and the overcurrent protection is not triggered, the adjustment is successful, or the proportional parameter that makes the current mutation value smaller is continuously debugged until the current mutation disappears or is less than the current second threshold. If the current mutation value does not decrease after adjusting the feedback ratio, or the overcurrent protection is still triggered, switch to the inverter side high-pass filtering solution and adjust the inverter side current feedforward ratio to reduce the current mutation value; The converter retains the inverter side high-pass filtering scheme setting until a new current mutation value exceeding the first current threshold value appears and cannot be suppressed by the inverter side high-pass filtering scheme, and is replaced with a capacitor current feedback scheme for resonance suppression debugging.
2. The active damping compensation method for an LCL type grid-connected converter according to claim 1, characterized in that: The capacitor current feedback scheme model includes: in, is the inverter side voltage, C is the LCL filter capacitor, L1 is the first equivalent inductor, L2 is the second equivalent inductor, represents the current corresponding to the second inductor; s is the symbol of the complex variable of the transfer function, representing the complex frequency; G is the mechanism transfer function, is the capacitor current feedback coefficient.
3. The active damping compensation method for an LCL type grid-connected converter according to claim 1, characterized in that: The inverter side current is fed forward to the current loop input, and the inverter side current high-pass filtering solution model includes: in, is the cut-off frequency of the first-order high-pass filter, and a is the gain coefficient of the first-order high-pass filter.
4. The active damping compensation method for an LCL type grid-connected converter according to claim 1, characterized in that: The number of adaptive debugging is greater than or equal to 6 times and after the debugging is invalid, the capacitor current feedback and inverter side high-pass filtering schemes are switched, and the debugging includes debugging at least once in the direction of increasing and decreasing the feedback coefficient respectively.
5. The active damping compensation method for an LCL type grid-connected converter according to claim 4, characterized in that: After the adaptive debugging fails for 6 times, the capacitor current feedback and the inverter side high-pass filtering scheme are switched.
6. The active damping compensation method for an LCL type grid-connected converter according to claim 4, characterized in that: After the adaptive debugging fails for 12 times, the capacitor current feedback and inverter side high-pass filtering schemes are switched.
7. The active damping compensation method for an LCL type grid-connected converter according to claim 1, characterized in that: The first current threshold is greater than the rated current of the power grid and less than or equal to the smaller of 4 / 5 of the maximum allowable current or 2 times of the daily working current.
8. The active damping compensation method for an LCL type grid-connected converter according to claim 1, characterized in that: The second current threshold is less than or equal to the larger one of 1.2 times the rated current of the power grid and 1.2 times the daily working current.
9. The active damping compensation method for an LCL type grid-connected converter according to claim 1, characterized in that: The method for adaptive debugging also includes a current mutation slope auxiliary judgment method, which includes calculating the rising slope by measuring the time it takes for the current to rise to the first threshold value. If the slope becomes larger between two consecutive debuggings, it indicates that the debugging resonant frequency change should be carried out in the opposite direction, or the capacitor current feedback and the inverter side high-pass filtering scheme should be switched and then debugged again.