A control system for improving the weak grid adaptability of a grid-connected inverter

Abstract

The application discloses a method and a control system for improving the weak power grid adaptability of a grid-connected inverter, and belongs to the field of grid-connected inverter control. The method relates to a control system composed of a main control loop of a novel current controller and a point of common coupling voltage feedforward branch. First, reference currents pass through a first proportional controller, a second proportional controller and a delay module respectively, the outputs of the first proportional controller and the delay module are added by a first adder to obtain a first output signal; meanwhile, a sampling point of common coupling voltage passes through a virtual capacitor link and a repetitive prediction link to generate a second output signal. The first output signal and the second output signal are added by an adder, and then divided by an inverter bridge gain coefficient to obtain a modulation wave. The modulation wave passes through a PWM module to generate a driving signal to drive power devices of the inverter, so as to control an in-grid current.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of grid-connected inverter control, in particular to a method and a control system for improving the weak grid adaptability of a grid-connected inverter. BACKGROUND

[0002] With the gradual increase of the proportion of distributed energy in the power grid, its influence on the power grid cannot be ignored. From the point of common coupling (PCC), the power grid can be equivalent to a voltage source in series with a resistive impedance. Moreover, the greater the grid impedance, the more serious the influence on the stability of the grid-connected inverter. Compared with a strong grid, a weak grid has the following two characteristics: (1) the grid impedance cannot be ignored and changes with the operation mode of the grid; (2) the grid contains rich background harmonics.

[0003] Although there have been many literatures on the stability of inverters under weak grids, most of them are for LC / LCL type inverters, and there are few studies on L type inverters. Compared with LC / LCL type inverters, the PCC voltage of L type inverters is not only affected by the grid background harmonics, but also by the inverter voltage. Therefore, under the same power, the degree of distortion of the L type inverter access point voltage is much larger than that of the LC / LCL type filter, which leads to poor grid current quality of the L type filter under a weak grid.

[0004] In addition, most of the literatures do not study under the extreme weak grid. With the continuous weakening of the grid, especially when the short circuit ratio (SCR) is equal to 1, if the grid impedance is purely inductive and the grid current is kept in phase with the PCC voltage, the PCC voltage will drop to zero, which seriously affects the stability of the inverter. Although existing literatures have shown that, under an extremely weak grid, operating the inverter in voltage source mode or adding reactive compensation devices can increase the stability margin of the system, but the dynamic characteristics and grid current quality of the system need to be further improved when the inverter operates in voltage source mode, and adding reactive compensation devices will increase the cost of the system.

[0005] Compared with LC / LCL type inverters, L type inverters have the characteristics of simple structure and control, and are lower in cost in small power household photovoltaic applications, and therefore are still widely used. However, when the grid is a weak grid, especially an extremely weak grid, the L type inverter is affected by the distorted PCC voltage, leading to poor grid current quality, and the grid impedance makes the system prone to instability. Therefore, if the problem of distorted PCC voltage affecting the grid current quality of the L type inverter can be solved, and a method is found to enable the inverter to operate stably under an extremely weak grid, the problem of limited application of L type inverters under an extremely weak grid will be solved, which has important practical value. SUMMARY

[0006] In view of the deficiencies of the prior art, the application provides a method and a control system for improving the weak grid adaptability of a grid-connected inverter.

[0007] The object of the application can be achieved by the following technical solutions.

[0008] A control system for improving the weak grid adaptability of a grid-connected inverter, comprising: a main control loop of a current controller and a PCC voltage feedforward branch.

[0009] The input end of the PCC voltage feedforward branch is connected to the loop of the inverter to collect the PCC voltage.

[0010] The main control loop of the current controller comprises: a first proportional controller, a second proportional controller, a delay module, a first subtraction controller, a first addition controller and an inverter bridge gain reciprocal module; the reference current is input to the input end of the first proportional controller and the second proportional controller, and the output end of the first proportional controller and the second proportional controller is connected to the input end of the first subtraction controller; the output end of the first subtraction controller and the PCC voltage feedforward branch are both connected to the input end of the first addition controller.

[0011] Optionally, the output end of the first addition controller 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 PWM module.

[0012] Optionally, the PCC voltage feedforward branch comprises a virtual capacitor element and a repetitive predictive controller, and the sampled PCC voltage is connected to the input end of the virtual capacitor element; the output end of the virtual capacitor element is connected to the input end of the repetitive predictive controller; the output end of the repetitive predictive controller is connected to the input end of the first addition controller.

[0013] Optionally, a virtual filter capacitor is constructed on the inverter loop, and the PCC voltage is calculated according to the virtual filter capacitor.

[0014] Optionally, a delay module is connected in series between the second proportional controller and the first addition controller.

[0015] Compared with the existing inverter current loop controller, it does not need to feed back the grid current, and one sensor can be saved, and compared with the conventional PI controller, the stability margin of the inverter under the weak grid can be effectively increased.

[0016] The virtual capacitor module effectively filters the rich background harmonics contained in the PCC voltage, and reduces the influence of the PCC voltage on the grid current quality.

[0017] The repetitive control prediction module can solve the problem of phase lag of PCC voltage after passing through the virtual capacitor module, and will not amplify high-order harmonics in the PCC voltage like other phase compensation modules.

[0018] The application can effectively solve the problem of poor grid current quality of the L-type inverter under a weak power grid, and the new current controller can greatly increase the stability margin of the inverter under an extremely weak power grid without affecting the dynamic characteristics of the system and increasing the cost, so as to ensure the stable operation of the inverter under an extremely weak power grid, which has important significance. BRIEF DESCRIPTION OF DRAWINGS

[0019] The application will be further described below with reference to the drawings.

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

[0021] Figure 2 It is a waveform of point voltage of the L-type inverter under a weak power grid.

[0022] Figure 3 It is a schematic diagram of deviation of the modulation wave caused by the distorted PCC voltage.

[0023] Figure 4 It is a circuit schematic diagram after adding a virtual capacitor.

[0024] Figure 5 It is a Bode diagram of GRLC(s).

[0025] Figure 6 It is a Bode diagram of the lead correction and repetitive control prediction transfer function.

[0026] Figure 7 It is a vector relationship between various physical quantities under a weak power grid.

[0027] Figure 8 It is a relationship between the PCC voltage and the grid current amplitude when SCR=1.

[0028] Figure 9 It is a simplified circuit diagram of a single-phase inverter system under a weak power grid.

[0029] Figure 10 It is a system control block diagram when a conventional PI controller is used.

[0030] Figure 11 It is a relationship between the inverter output impedance and the grid impedance when a conventional PI controller is used.

[0031] Figure 12 It is a system control block diagram when the controller proposed in the application is used.

[0032] Figure 13 The relationship between the inverter output impedance and the grid impedance when the controller of the present application is adopted.

[0033] Figure 14 (a) The PCC voltage waveform and the grid current waveform when the PCC voltage is directly fed forward.

[0034] Figure 14 (b) The PCC voltage waveform and the grid current waveform after the virtual capacitor and the repetitive control prediction module are added.

[0035] Figure 15 (a) The current waveform when the inverter is unstable under a weak grid and a conventional PI controller is adopted.

[0036] Figure 15 (b) The current waveform when the inverter is stable under a weak grid and the controller of the present application is adopted.

[0037] Figure 16 The PCC voltage waveform and the grid current waveform when the controller of the present application is adopted and SCR=1. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative labor fall within the protection scope of the present application.

[0039] In combination Figure 1 In some specific embodiments of the present application, a design example for improving the adaptability of an L-type inverter under a weak grid is given. Figure 2 The PCC voltage waveform of an L-type inverter under a weak grid is given. As can be seen, under a weak grid, the voltage at the access point of the L-type inverter is severely distorted and rich in harmonics.

[0040] Figure 3 The schematic diagram of the PWM wave generated by comparing the modulating wave at the peak and the valley with the carrier wave after the PCC voltage is fed forward is given. In order to better compare, the case of comparing the ideal modulating wave with the triangular carrier wave is also given, and the deviation of the PWM wave generated by comparing the actual modulating wave with the carrier wave from the ideal PWM wave is also given. As can be seen, the distorted PCC voltage will generate obvious current deviation, which affects the grid current quality.

[0041] Considering an LC / LCL-type inverter, since the filter capacitor C is contained, there is no problem of severe PCC voltage distortion, and therefore a virtual filter capacitor C is constructed in the circuit, as shown in Figure 4As shown. Because the grid impedance is much greater than the inductive reactance corresponding to the L-type filter under extremely weak grid conditions, the PCC voltage u... PCC The expression can be approximated as:

[0042] (1)

[0043] In the formula, u inv denoted by s, where s represents the inverter voltage, L represents the frequency domain complex frequency, C represents the inverter filter inductance, and R represents the virtual capacitor.

[0044] According to equation (1), the transfer function from the inverter voltage to the PCC voltage can be obtained. G RLC The expression for (s):

[0045] (2)

[0046] Drawing on the parameters of an LC filter, and considering that increasing the virtual capacitance C is beneficial for voltage stability, the damping coefficient in equation (2) is also increased. R The ability to effectively increase damping is beneficial. G RLC ( s Suppression of harmonics in the PCC voltage. It can be observed that when C=20uF and R=20Ω, at this time... G RLC ( s It has a good suppression effect on harmonics containing the third order and above, and the Bode plot in this case is as follows: Figure 5 As shown.

[0047] from Figure 5 It can be seen that, although G RLC ( s While effectively suppressing harmonics, this method causes a 27° phase lag in the fundamental component of the PCC voltage, necessitating phase compensation. Conventional phase compensation methods include lead compensation and interpolation prediction; however, both of these methods exhibit differential characteristics, easily amplifying harmonic components in the PCC voltage. Repetitive predictive control, due to its internal mode element, theoretically allows for zero steady-state error prediction of the fundamental and all harmonics. The transfer functions of lead compensation and repetitive predictive control can be expressed as follows:

[0048] (3)

[0049] (4)

[0050] In the above formula, G LC ( s ) represents the lead compensation transfer function, GRP (s) represents the repetitive prediction transfer function, T represents the differential constant, αT represents the integral constant, Q is generally a low-pass filter or a constant less than 1; N represents the number of switching cycles in one power grid cycle; k represents the number of switching cycles to be predicted; m represents the gain coefficient, generally less than 1. Taking the prediction of 29° as an example, the Bode plots of equations (3) and (4) are drawn as follows. Figure 6 As shown, both lead compensation and repetitive prediction can achieve the goal of compensating the fundamental frequency by 29°. However, lead compensation amplifies the amplitude of each harmonic, while repetitive prediction does not.

[0051] In weak power grids, in addition to the distorted PCC voltage affecting the quality of the incoming current, the stability of the system is also affected by the increase in grid impedance. Figure 7 The vector relationships of various physical quantities are presented for inverters operating at unity power factor under weak grid conditions. It can be seen that as the grid impedance increases, u... PCC The amplitude of the voltage drop decreases continuously as the grid impedance increases. The short-circuit ratio (SCR) is typically used to measure the strength of the power grid, and its expression is:

[0052] (5)

[0053] In equation (5) u g Z represents the grid voltage. g Indicates the power grid impedance. S This represents the apparent power of the inverter. From equation (5), it can be seen that the SCR decreases as the grid impedance increases. Furthermore, according to equation (5), when SCR=1 and the grid impedance is purely inductive, the apparent power of the inverter equals the reactive power, meaning the inverter cannot generate active power at this time. Moreover, the inductive reactance voltage is equal in magnitude but opposite in amplitude to the grid voltage, which will result in the PCC voltage amplitude being zero. According to... Figure 7 The general expression for the PCC voltage can be derived:

[0054] (6)

[0055] In the formula I g X represents the grid current. g R represents the grid inductive reactance. g Express the grid impedance. Draw diagrams based on equations (5) and (6) for different grid impedances. R g / X g Under the condition that SCR=1, the PCC voltage and the amplitude of the grid connection current are... I g Relationships, such as Figure 8It can be seen that if the grid impedance is purely inductive, the inverter will not be able to deliver active power at rated power and the PCC voltage will drop to zero when SCR = 1. Therefore, for the inverter, if SCR = 1 and there is no additional reactive compensation device, the grid is an extremely weak grid.

[0056] Figure 9 The equivalent circuit of a single-phase inverter under a weak grid is given. In which, i 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 ) and the amplitude-frequency curve of Z g ( s ) exist intersection points, to ensure system stability, the phase of Z op ( s ) should be greater than -90° at the intersection points.

[0057] Figure 10 The control block diagram of the system using a conventional PI controller is given, in which G i ( s ) represents the current loop controller, K pwm represents the gain of the inverter; G d ( s ) represents the system control delay. According to Figure 10 , the expression of the grid current i g_b( s) can be obtained:

[0058] (7)

[0059] Further, the expression of the inverter output impedance Z op_b (s) can be obtained:

[0060] (8)

[0061] As can be seen from equation (8), if the control delay can be completely eliminated, the inverter output impedance is infinite, which can eliminate the influence of the grid impedance, but in the actual situation, it is impossible to completely eliminate the control delay. If the control delay is changed to 0.1 T sTaking a conventional PI controller as an example, the current loop controller is drawn as follows: Z op ( s )and Z g ( s The relationship between ) such as Figure 11 As shown.

[0062] It can be seen that, with Z g ( s The continuous increase of ) Z g ( s )and Z op ( s At the intersection of the amplitude-frequency curves, the corresponding phase margin gets closer and closer to -90°, proving that the system's stability margin is continuously decreasing. L g When the gain is increased to 51mH, the SCR=1, and the phase margin is less than 30°, indicating that the system can no longer guarantee stability. Although significantly reducing the gain of the PI controller can increase the system's stability margin, this will undoubtedly reduce the inverter's bandwidth and the quality of the grid-connected current.

[0063] Conventional PI controllers are the most widely used in practice due to their simplicity and robustness. However, the presence of an integral element in a PI controller can cause phase lag. Figure 11 It can be seen that this is the fundamental reason for the inverter's instability under weak power grid conditions. To improve the inverter's adaptability under weak power grid conditions, the essential solution is to address the problem of phase lag in the inverter's output impedance caused by the PI controller. The novel current loop controller proposed in this invention, composed of a first proportional controller, a second proportional controller, and a delay module, can effectively improve... Z op ( s The phase margin of the novel controller proposed in this invention is shown in the figure below. Figure 12 As shown.

[0064] Figure 12 middle K 1 / K The expressions for 2 are as follows:

[0065] (9)

[0066] In the formula, Ts represents the switching period. According to... Figure 12 The expression for the grid-connected current when the proposed controller is used can be derived:

[0067] (10)

[0068] In the formula e -sTs This represents the internal delay of the proposed controller. The inverter output impedance Z can be calculated using equation (11) when the proposed controller is employed. op_PPD The expression for (s):

[0069] (11)

[0070] Draw diagrams for a weak power grid using both a conventional PI controller and the proposed controller. Z op ( s Bode's diagram, such as Figure 13 As shown. It can be seen that, although when using the proposed controller... Z op ( s The amplitude of the SCR decreases in the low-frequency range, but when the SCR approaches 1, the phase angle stability margin of the system using the proposed controller is close to 90°, and there is still a large margin to ensure system stability when the grid impedance continues to increase significantly.

[0071] Figure 14 (a) The waveforms of the PCC voltage and the grid current are given when the PCC voltage is directly fed forward; Figure 14 (b) shows the PCC voltage waveform and grid-connected current waveform after adding the virtual capacitor module and the repetitive predictive control module. It can be seen that the virtual capacitor fuzzy logic and repetitive predictive control can effectively solve the problem of PCC voltage distortion affecting the grid-connected current quality.

[0072] Figure 15 The grid-connected current waveforms are presented for two scenarios when the SCR is reduced to 1.64, using both the PI controller and the controller proposed in this invention. It can be seen that the system becomes unstable when using the PI controller, while the inverter maintains stability when using the controller proposed in this invention. This demonstrates that this invention can improve the stability margin of L-type inverters under weak grid conditions.

[0073] same Figure 16 The grid-connected current and PCC voltage waveforms of the system using the controller proposed in this invention are given when the SCR is reduced to 1. It can be seen that the PCC voltage distortion is very severe at this point, but the system can still maintain stable operation, proving that this invention can ensure the stable operation of the L-type inverter under extremely weak grid conditions.

[0074] In the description of the specification, the description of the terms "one embodiment", "an example", "a specific example" and the like means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Also, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0075] The basic principles, main features and advantages of the present application are shown and described above. Those skilled in the art should understand that the present application is not limited by the above embodiments, and the above embodiments and descriptions in the specification are only to illustrate the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application.

Claims

1. A control system for improving the adaptability of grid-connected inverters to weak power grids, characterized in that, include: The main control circuit of the current controller and the voltage feedforward branch of the PCC; The input terminal of the PCC voltage feedforward branch is connected to the inverter circuit to collect the PCC voltage. The main control loop of the current controller includes: a first proportional controller, a second proportional controller, a delay module, a first subtraction controller, a first addition controller, and an inverter bridge gain reciprocal module; the reference current is input to the input terminals of the first proportional controller and the second proportional controller respectively, and the output terminals of the first proportional controller and the second proportional controller are connected to the input terminal of the first subtraction controller; the output terminals of the first subtraction controller and the PCC voltage feedforward branch are both connected to the input terminal of the first addition controller; The PCC voltage feedforward branch includes a virtual capacitor circuit and a repetitive prediction controller. The sampled PCC voltage is connected to the input terminal of the virtual capacitor circuit, and the output terminal of the virtual capacitor circuit is connected to the input terminal of the repetitive prediction controller. The output terminal of the repetitive prediction controller is connected to the input terminal of the first adder controller. A virtual filter capacitor is constructed on the inverter circuit, and the collected PCC voltage is calculated based on the virtual filter capacitor. The PCC voltage u PCC The expression is: In the formula, u inv denoted by , where s represents the inverter voltage, s represents the complex frequency in the frequency domain, L represents the inverter filter inductance, C represents the virtual capacitor, and R represents the damping coefficient. u inv to u PCC transfer function G RLC The expression for (s) is: ； The transfer function G of the repetitive predictive control RP for: In the formula, z is the mapping of the frequency domain s to the discrete domain, Q is a low-pass filter or a constant less than 1; N represents the number of switching cycles in one power grid cycle; k represents the number of switching cycles to be predicted; and m represents the gain coefficient.

2. The control system for improving the weak grid adaptability of grid-connected inverters according to claim 1, characterized in that, The output of the first adder controller is connected to the input of the inverter bridge gain reciprocal module, and the output of the inverter bridge gain reciprocal module is connected to the PWM module.

3. The control system for improving the weak grid adaptability of grid-connected inverters according to claim 1, characterized in that, A delay module is connected in series between the second proportional controller and the first adder controller.

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