A system and method for improving the grid-connected capability of L-type inverters in weak power grids

Abstract

The present invention discloses a system and method for improving the grid-connected capability of an L-type inverter under a weak power grid, belonging to the technical field of grid-connected inverter control. The system comprises the following steps: subtracting a measured grid-connected current from a reference current through a first subtractor module to obtain a current deviation; passing the obtained current deviation through a proportional controller module, an integral controller module, and a differential controller module, and then adding the current deviation through a first adder module to generate a first output signal; passing a measured common coupling point voltage through a virtual capacitor module and a repetitive control prediction module to obtain a second output signal; adding the first output signal and the second output signal through a second adder module, and then passing the sum of ...

Description

Technical Field

[0001] The present invention belongs to the technical field of grid-connected inverter control, and particularly relates to a system and method for improving the grid-connected capability of an L-type inverter under a weak power grid. Background Art

[0002] Existing research on how to improve the quality of the inverter's grid-connected current and system stability in weak grids mostly targets LC / LCL inverters, and rarely studies are conducted in extremely weak grids. In extremely weak grids, the system's stability margin decreases significantly as the grid impedance increases, and the use of conventional PI controllers can no longer guarantee stable system operation. Although existing literature has shown that operating the inverter in voltage source mode or adding additional reactive power compensation devices in extremely weak grids can increase the system's stability margin, the system's dynamic characteristics and grid-connected current quality need to be further improved when the inverter operates in voltage source mode, and adding reactive power compensation devices will increase system costs. To this end, a system and method for improving the grid-connected capability of L-type inverters in weak grids are now proposed. Summary of the Invention

[0003] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a system and method for improving the grid-connected capability of L-type inverters under weak power grids, thereby solving the technical problem of low system stability margin of L-type inverters under weak power grids in the existing technology.

[0004] The object of the present invention can be achieved by the following technical solutions: A system for improving the grid-connected capability of an L-type inverter under a weak power grid, comprising a first subtractor module, a proportional controller module, an integral controller module, a differential controller module, a first adder module, a virtual capacitor module, a repetitive control prediction module, a second adder module, an inverter bridge gain inverse module, and a PWM module;

[0005] The measured grid-connected current and the reference current are subtracted through the first subtractor module to obtain a current deviation, and the obtained current deviation is respectively used as the input of the proportional controller module, the integral controller module and the differential controller module. The outputs of the proportional controller module, the integral controller module and the differential controller module are added through the first adder module to generate a first output signal; the measured common coupling point voltage is respectively passed through the virtual capacitor module and the repetitive control prediction module to obtain a second output signal, the first output signal and the second output signal are added through the second adder module, and then passed through the inverter bridge gain inverse module to generate a modulation wave, and the generated modulation wave is passed through the PWM pulse width modulation module to generate a drive signal to control the inverter grid-connected current.

[0006] Preferably, the current deviation is connected to the input ends of the proportional controller module, the integral controller module and the differential controller module respectively.

[0007] Preferably, the output ends of the proportional controller module, the integral controller module and the differential controller module are connected to the input end of the first adder.

[0008] Preferably, the measured common coupling point voltage is connected to an input end of a virtual capacitor module, and an output end of the virtual capacitor module is connected to an input end of a repetitive control prediction module.

[0009] Preferably, the output end of the first adder module and the output end of the repetitive control prediction module are both connected to the input end of the second adder.

[0010] Preferably, the output end of the second adder is connected to the input end of the inverter bridge gain inverse module, and the output end of the inverter bridge gain inverse module is connected to the input end of the PWM module.

[0011] Preferably, the differential controller module, the proportional controller module and the integral controller module constitute a PID controller.

[0012] Preferably, the PID controller is used to increase the stability margin of the inverter under a weak power grid.

[0013] Preferably, a method for improving the grid-connected capability of an L-type inverter under a weak power grid comprises the following steps:

[0014] The measured grid current is subtracted from the reference current to obtain a current deviation, and the current deviation is then used as an input for proportional control, integral control, and differential control, respectively. The outputs of the proportional control, integral control, and differential control are added together by a first adder to obtain a first output signal;

[0015] Then, the measured common coupling point voltage is predicted through virtual capacitance and repeated control to obtain a second output signal;

[0016] The first output signal and the second output signal are added together and then passed through the inverter bridge gain to obtain a modulated wave, and then the modulated wave is passed through PWM to generate a drive signal to control the inverter grid current.

[0017] Beneficial effects of the present invention:

[0018] During use of the present invention, the measured grid-connected current and the reference current are subtracted through a first subtractor module to obtain a current deviation, and the obtained current deviation is respectively used as an input to a proportional controller module, an integral controller module and a differential controller module. The outputs of the proportional controller module, the integral controller module and the differential controller module are added through a first adder module to generate a first output signal; the measured common coupling point voltage is respectively passed through a virtual capacitor module and a repetitive control prediction module to obtain a second output signal, the first output signal and the second output signal are added through a second adder module, and then a modulation wave is generated through an inverter bridge gain inverse module. The generated modulation wave is passed through a PWM pulse width modulation module to generate a drive signal to control the inverter grid-connected current, thereby realizing the function of the L-type inverter to improve the system stability margin under a weak power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0020] Figure 1 This is a schematic diagram of the power circuit and control loop connection of the L-type grid-connected inverter;

[0021] Figure 2 (a) is a waveform diagram of the voltage at the access point of the L-type inverter under a weak power grid according to an embodiment of the present invention. Figure 2 (b) is a spectrum diagram of the voltage at the access point of the L-type inverter under a weak power grid according to an embodiment of the present invention. Figure 2 (c) is a waveform diagram of the modulation wave of the L-type inverter under the weak power grid according to an embodiment of the present invention;

[0022] Figure 3 (a) is a schematic diagram of a circuit after adding a virtual capacitor according to an embodiment of the present invention. Figure 3 (b) is a simplified schematic diagram of the circuit after adding virtual capacitors according to an embodiment of the present invention;

[0023] Figure 4 (a) is the virtual capacitor of the embodiment of the present invention when G is different RLC Bode plot of (s), Figure 4 (b) is the damping coefficient of the embodiment of the present invention when G is different RLC Bode plot of (s);

[0024] Figure 5 This is a simplified circuit diagram of a single-phase inverter in a weak power grid according to an embodiment of the present invention;

[0025] Figure 6A Bode diagram of the lead correction and repetitive control prediction transfer function according to an embodiment of the present invention;

[0026] Figure 7 This is a system control block diagram of an embodiment of the present invention;

[0027] Figure 8 The relationship between the inverter output impedance and the grid impedance when a conventional PI controller is used in an embodiment of the present invention;

[0028] Figure 9 The relationship between the inverter output impedance and the grid impedance when the PID controller is used in the embodiment of the present invention;

[0029] Figure 10 (a) shows the PCC voltage waveform and grid current waveform when the PCC voltage is directly fed forward according to an embodiment of the present invention; Figure 10 (b) shows the PCC voltage waveform and grid current waveform after adding virtual capacitor and repeated control prediction module in the present invention;

[0030] Figure 11 (a) is a current waveform diagram of the inverter instability when a conventional PI controller is used in a weak power grid according to an embodiment of the present invention; Figure 11 (b) is a current waveform diagram showing the inverter maintaining stability when a PID controller is used in a weak power grid according to an embodiment of the present invention. DETAILED DESCRIPTION

[0031] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0032] like Figure 1 As shown, a system for improving the grid-connected capability of an L-type inverter under a weak power grid includes a first subtractor module, a proportional controller module, an integral controller module, a differential controller module, a first adder module, a virtual capacitor module, a repetitive control prediction module, a second adder module, an inverter bridge gain inverse module, and a PWM module;

[0033] The measured grid-connected current and the reference current are subtracted through the first subtractor module to obtain a current deviation, and the obtained current deviation is respectively used as the input of the proportional controller module, the integral controller module and the differential controller module. The outputs of the proportional controller module, the integral controller module and the differential controller module are added through the first adder module to generate a first output signal; the measured common coupling point voltage is respectively passed through the virtual capacitor module and the repetitive control prediction module to obtain a second output signal, the first output signal and the second output signal are added through the second adder module, and then passed through the inverter bridge gain inverse module to generate a modulation wave, and the generated modulation wave is passed through the PWM pulse width modulation module to generate a drive signal to control the inverter grid-connected current.

[0034] Example: Combination Figure 1 , and gives a design example to improve the quality and stability of the grid current of the L-type inverter under weak grid conditions. Figure 2 The PCC voltage waveform, spectrum, and modulation waveform of an L-type inverter under weak grid conditions are shown. It can be seen that under weak grid conditions, the voltage at the L-type inverter's access point is severely distorted and rich in harmonics. Feeding this voltage forward also severely distorts the modulation waveform, which in turn affects the quality of the incoming current.

[0035] Considering the LC / LCL type inverter, since it contains a filter capacitor C, there is no problem of serious PCC voltage distortion. Therefore, a virtual filter capacitor C is constructed in the circuit, such as Figure 3 As shown in (a) in Figure 2. At the same time, considering that the grid impedance is much larger than the inductive reactance corresponding to the L-type filter in a weak grid, Figure 3 (a) can be simplified to Figure 3 At this point, we can find the expression for the PCC voltage and the transfer function from the inverter voltage to the grid current:

[0036]

[0037]

[0038] Drawing on the parameters of the LC filter, and considering that the increase of the virtual capacitance C is beneficial to stabilizing the voltage, the increase of the damping coefficient R in formula (2) can effectively increase the damping, which is beneficial to G RLC (s) Suppression of harmonics in PCC voltage. Figure 4 When different virtual capacitance C and damping coefficient R are given, G RLC (s) of the Bode diagram. Figure 4 , select C=20uF, R=20Ω, then G RLC (s) It has a good suppression effect on harmonics containing the third and above.

[0039] from Figure 4As can be seen from (b) in the figure, although G RLC (s) has a good suppression effect on harmonics, but it will cause a 27° phase lag in the PCC voltage fundamental component, requiring phase compensation of the PCC voltage fundamental phase. Conventional phase compensation methods include advance correction and interpolation prediction, but both methods have differential characteristics and are prone to amplifying the harmonic components in the PCC voltage. Repetitive control prediction, due to its internal model link, can theoretically achieve zero-static error prediction of the fundamental and each harmonic. The transfer function of advance correction and repetitive predictive control can be expressed as:

[0040]

[0041]

[0042] In the above formula, T represents the differential link constant, αT represents the integral link constant, Q is generally a low-pass filter or a constant less than 1; N represents the number of switching cycles in a power grid cycle; k represents the number of switching cycles to be predicted; m represents the gain coefficient, which is generally less than 1. Taking the prediction of 29° as an example, we can calculate α = 0.347, T = 0.0054, and set Q = 0.98, m = 0.96, k = 29, and N = 360. Draw the Bode diagram of formulas (3) and (4), as shown in the following figure: Figure 5 As shown in Figure 2, it can be seen that both the advance correction and repeated prediction can achieve the goal of compensating the fundamental wave by 29°, but the advance correction will amplify the amplitude of each harmonic, while the repeated prediction will not amplify the amplitude of each harmonic.

[0043] In a weak power grid, in addition to the distorted PCC voltage affecting the quality of the incoming current, the stability of the system will also be affected as the grid impedance increases. Figure 6 The equivalent circuit of a single-phase inverter under weak power grid is given. s (s) is the reference command current, Z op (s) represents the inverter output impedance, Z g (s) is the grid impedance. If Z op (s) Amplitude-frequency curve and Z g The amplitude-frequency curve of (s) has an intersection point. To ensure the stability of the system, Z should be ensured at the intersection point. op The phase of (s) is greater than -90°.

[0044] Figure 7 The control block diagram of the system is given, where G i (s) represents the current loop controller, K pwm Indicates the gain of the inverter; G d (s) represents the system control delay. Figure 7 , we can find the grid current expression:

[0045]

[0046] Then the output impedance of the inverter can be calculated:

[0047]

[0048] From formula (6), it can be seen that if the control delay can be completely eliminated, the inverter output impedance is infinite, and the influence of the grid impedance can be eliminated. However, in actual situations, the control delay cannot be completely eliminated. When the control delay becomes 0.1T s , the current loop controller is a conventional PI controller as an example, draw Z op (s) and Z g (s) relationship, such as Figure 8 shown.

[0049] It can be seen that with the Z g (s) continues to increase, Z g (s) and Z op At the intersection of the amplitude-frequency curve of (s), the corresponding phase margin is getting closer to -90°, which proves that the stability margin of the system is constantly decreasing. g When the voltage increases to 51mH, SCR = 1, and the phase margin is less than 30°, the system can be considered unstable. Although significantly reducing the gain of the PI controller can increase the system stability margin, it will undoubtedly reduce the bandwidth of the inverter and the quality of the grid current.

[0050] Conventional PI controllers are most widely used in practice due to their simplicity of implementation and strong robustness. However, the PI controller will cause phase lag due to the existence of an integral link. Figure 8 It can be seen that this is the fundamental reason for the inverter to become unstable under weak power grid. In order to improve the adaptability of the inverter under weak power grid, the essence is to solve the problem of phase lag caused by PI controller. Considering that the differential link can compensate for the phase, adding a differential link to the PI controller can effectively solve the phase lag problem caused by the integral link. Therefore, the present invention adopts PID controller. When the PID controller is used, the inverter output impedance Bode diagram is drawn, as shown in Figure 9 shown.

[0051] It can be seen that after adding the differential link, the amplitude and phase angle of the inverter's output impedance have been greatly improved. Moreover, under weak power grid conditions, there is no intersection point between the inverter output impedance and the grid impedance, so the stability of the system can be guaranteed. Regarding the selection of the coefficient of the differential link, we can plot the Z op (s) and Z under weak power grid gThe integral coefficient is determined when there is no intersection of the amplitude-frequency characteristic curves of the two and there is at least a 3dB margin.

[0052] Figure 10 (a) shows the PCC voltage waveform and grid current waveform when the PCC voltage is directly fed forward; Figure 10 Figure (b) shows the PCC voltage and grid current waveforms after adding the virtual capacitor module and the repetitive control prediction module. This shows that virtual capacitor fuzzy and repetitive predictive control can effectively address the issue of PCC voltage distortion affecting grid current quality.

[0053] Figure 11 Figure (a) shows the grid current waveforms when the SCR is reduced to 1.64 using both the PI and PID controllers. It can be seen that the system becomes unstable using the PI controller, while the inverter remains stable using the PID controller. This demonstrates that the present invention can improve the stability margin of the L-type inverter in weak grid conditions.

[0054] A method for improving the grid-connected capability of an L-type inverter under a weak power grid, the method comprising the following steps:

[0055] The measured grid current is subtracted from the reference current to obtain a current deviation, and the current deviation is then used as an input for proportional control, integral control, and differential control, respectively. The outputs of the proportional control, integral control, and differential control are added together by a first adder to obtain a first output signal;

[0056] Then, the measured common coupling point voltage is predicted through virtual capacitance and repeated control to obtain a second output signal;

[0057] The first output signal and the second output signal are added together and then passed through the inverter bridge gain to obtain a modulated wave, and then the modulated wave is passed through PWM to generate a drive signal to control the inverter grid current.

[0058] Throughout this specification, references to terms such as "one embodiment," "example," or "specific example" indicate that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0059] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention, and such changes and modifications are intended to fall within the scope of the present invention.

Claims

1. A system for improving the grid-connected capability of an L-type inverter under a weak power grid, characterized in that: It includes a first subtractor module, a proportional controller module, an integral controller module, a differential controller module, a first adder module, a virtual capacitor module, a repetitive control prediction module, a second adder module, an inverter bridge gain reciprocal module and a PWM module; The measured grid-connected current and the reference current are subtracted through a first subtractor module to obtain a current deviation, and the obtained current deviation is respectively used as the input of the proportional controller module, the integral controller module and the differential controller module. The outputs of the proportional controller module, the integral controller module and the differential controller module are added through a first adder to generate a first output signal; the measured common coupling point voltage is respectively passed through a virtual capacitor module and a repetitive control prediction module to obtain a second output signal, the first output signal and the second output signal are added through a second adder module, and then a modulation wave is generated through an inverter bridge gain inverse module. The generated modulation wave is passed through a PWM pulse width modulation module to generate a drive signal to control the inverter grid-connected current; Transfer function G of the virtual capacitor module RLC (s) are as follows: Where C is the virtual capacitance and R is the damping coefficient; The transfer coefficient G of the repetitive control prediction module RP as follows: Wherein, Q is a low-pass filter or a constant less than 1, k represents the number of switching cycles to be predicted; m represents a gain coefficient, and N represents the number of switching cycles in one grid cycle.

2. A system for improving the grid-connected capability of an L-type inverter under a weak power grid according to claim 1, characterized in that: The current deviation is connected to the input ends of the proportional controller module, the integral controller module and the differential controller module respectively.

3. The system for improving the grid-connected capability of an L-type inverter under a weak power grid according to claim 2, characterized in that: The output ends of the proportional controller module, the integral controller module and the differential controller module are connected to the input end of the first adder.

4. The system for improving the grid-connected capability of an L-type inverter under a weak power grid according to claim 1, characterized in that: The measured common coupling point voltage is connected to the input end of the virtual capacitor module, and the output end of the virtual capacitor module is connected to the input end of the repetitive control prediction module.

5. The system for improving the grid-connected capability of an L-type inverter under a weak power grid according to claim 1, characterized in that: The output end of the first adder module and the output end of the repetitive control prediction module are both connected to the input end of the second adder.

6. The system for improving the grid-connected capability of an L-type inverter under a weak power grid according to claim 1, characterized in that: The output end of the second adder is connected to the input end of the inverter bridge gain reciprocal module, and the output end of the inverter bridge gain reciprocal module is connected to the input end of the PWM module.

7. The system for improving the grid-connected capability of an L-type inverter under a weak power grid according to claim 1, characterized in that: The differential controller module, the proportional controller module and the integral controller module constitute a PID controller.

8. The system for improving the grid-connected capability of an L-type inverter under a weak power grid according to claim 7, characterized in that: The PID controller is used to increase the stability margin of the inverter under a weak power grid.

9. A method for improving the grid-connected capability of an L-type inverter under a weak power grid, using the system for improving the grid-connected capability of an L-type inverter under a weak power grid according to any one of claims 1 to 8, characterized in that: The method comprises the following steps: Subtracting the measured grid current from the reference current to obtain a current deviation, and then using the current deviation as input for proportional control, integral control, and differential control, respectively. The outputs of the proportional control, integral control, and differential control are added together to obtain a first output signal. Then, the measured common coupling point voltage is predicted through virtual capacitance and repeated control to obtain a second output signal; The first output signal and the second output signal are added together and then passed through the inverter bridge gain to obtain a modulated wave, and then the modulated wave is passed through PWM to generate a drive signal to control the inverter grid current.

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