A hybrid active damping control method for grid-connected inverter

Abstract

The present invention discloses a hybrid active damping control method for a grid-connected inverter, belonging to the technical field of grid-connected inverter control. The method comprises: collecting capacitor voltage and inverter-side current, a phase-locked loop (PLL) obtaining phase angle information of the control system by locking the capacitor voltage, and using the information collected by the inverter-side current sensor to determine whether the device is in an overcurrent state and to perform closed-loop control of the output current; the inverter-side current and capacitor voltage are used for feedback and feedforward, respectively, and are superimposed with the output of the current loop to control the modulation process of the system. Compared to existing active damping control methods, the proposed method only requires capacitor voltage and inverter-side current sensors. Since both are basic sensor elements for grid-connected system phase information detection, output active and reactive power control, and overcurrent protection, there is no need for additional hardware, which effectively reduces design costs. At the same time, stable operation can be maintained under complex weak grid conditions.

Description

Technical Field

[0001] The present invention belongs to the technical field of grid-connected inverter control, and in particular relates to a hybrid active damping control method for a grid-connected inverter. Background Art

[0002] With energy consumption increasing and energy challenges becoming increasingly prominent, the development and utilization of renewable energy is gaining significant attention, with distributed generation technology becoming a key approach to addressing these challenges. However, due to the volatility, randomness, and intermittency of renewable energy sources such as photovoltaics and wind power, their integration into power systems can cause power quality and stability issues. Consequently, reliable grid-connected inverter control technology is gaining significant attention to efficiently utilize distributed power sources and improve power supply reliability.

[0003] Grid-connected photovoltaic inverters typically use a current control mode, tracking the maximum power point of renewable energy generation equipment via a controlled current source. However, with the continuous increase in installed renewable energy capacity, current source inverters face the risk of instability in weak grid conditions. Control loops, including the current loop, DC voltage loop, and phase-locked loop, can all resonate. Active damping control technology can reshape the inverter's output impedance, effectively suppressing the risk of high-frequency oscillations caused by the interaction between the inverter's current loop and the grid impedance.

[0004] A literature search revealed that researchers have proposed various approaches to active damping control technology. A widely adopted active damping strategy is based on capacitor current feedback, which is equivalent to connecting a virtual resistor in parallel with the filter capacitor to suppress resonant spikes. Furthermore, methods based on inverter-side current, capacitor voltage, and grid-side current feedback, as well as capacitor voltage and PCC voltage feedforward, have been proposed and demonstrated to have some damping effectiveness. To address the low control freedom inherent in single feedback or feedforward variables, hybrid damping methods that employ both state variable feedback and feedforward have been proposed in recent years. A hybrid damping method that utilizes capacitor current feedback and PCC voltage feedforward has also been proposed. However, in addition to capacitor current and PCC voltage sensors, this method also requires a grid-side current sensor for grid-connected power regulation and an inverter-side current sensor for hardware overcurrent protection. This leads to high costs and a complex hardware system. Summary of the Invention

[0005] In order to achieve output impedance reconstruction of the grid-connected inverter, a suitable active damping control method is required. The present invention proposes a hybrid active damping control method for the grid-connected inverter. The present invention requires a lower number of sensors, has a higher degree of freedom in system control, has a significant damping effect, and can ensure strong robustness under both strong and weak power grids.

[0006] The object of the present invention is achieved through the following technical solutions:

[0007] A hybrid active damping control method for a grid-connected inverter, comprising:

[0008] The capacitor voltage and inverter side current are collected. The phase-locked loop obtains the phase angle information of the control system by locking the capacitor voltage. The information collected by the inverter side current sensor is used to determine whether the device is in an overcurrent state and to perform closed-loop control of the output current.

[0009] The inverter side current and capacitor voltage are used for feedback and feedforward respectively, and are superimposed with the output of the current loop to control the modulation process of the system.

[0010] As a further improvement of the present invention, the inverter is a three-phase grid-connected inverter based on an LCL filter;

[0011] The DC side of the inverter is connected to a constant DC voltage source V dc After the three-phase grid-connected inverter is driven by the modulation signal, the output port voltage u oa 、 u ob and u oc Contains high-order harmonics, through three-phase LCL After the filter removes the switching ripple, the output three-phase AC current i L2 , through the AC bus and the large power grid v g Connected to the main power grid v g Equivalent line impedance of connected power grid L g .

[0012] As a further improvement of the present invention, the three-phase grid-connected inverter includes a switch tube S1, a switch tube S2, a switch tube S3, a switch tube S4, a switch tube S5, and a switch tube S6.

[0013] As a further improvement of the present invention, the control system is established in a dq coordinate system. After the inverter side current passes through the PI regulator, it is superimposed with the output after capacitor voltage feedforward and inverter side current feedback. After digital control delay, coordinate transformation and space vector modulation are performed to generate six PWM drive signals to drive the bridge arm switch tubes of the three-phase full-bridge inverter on and off.

[0014] As a further improvement of the present invention, the maximum output duty cycle of the inverter current loop does not exceed 1.15.

[0015] As a further improvement of the present invention, the duty ratios of the d-axis and q-axis of the inverter satisfy equations (1) and (2):

[0016] (1)

[0017] (2)

[0018] in, V d and V q Indicates the PCC point voltage, I Ld and I Lq represents the inverter current, V dc Represents a constant DC voltage source.

[0019] As a further improvement of the present invention, the output of the current loop PI regulator needs to meet the interval ( G c_low , G c_up ), satisfying the following formula:

[0020] (3)

[0021] (4).

[0022] in, G c_low and G c_up Respectively represent the upper and lower limit values ​​of the current loop controller, H i1 and K ff are the coefficients of inverter side current feedback and capacitor voltage feedforward, K pwm is the inverter gain, which is half of the constant DC voltage source.

[0023] As a further improvement of the present invention, the current outer loop controller parameter K p and K i , specifically satisfying equations (5) and (6):

[0024] (5)

[0025] (6)

[0026] in, w c is a multiple of the LCL resonant frequency, L 1 and L 2 represents the inverter-side filter inductance and grid-side filter inductance values.

[0027] As a further improvement of the present invention, the inverter side current feedback should satisfy H i1 ≥0, the capacitor voltage feedforward coefficient should satisfy 0.5≤ K ff ≤1.0.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] The hybrid active damping control method for grid-connected inverters proposed in the present invention only requires the acquisition of capacitor voltage and inverter-side current. The phase-locked loop obtains the phase angle information of the control system by locking the capacitor voltage. The information acquired by the inverter-side current sensor is directly used to determine whether the device is in an overcurrent state, and is also used for closed-loop control of the output current. This combination of the inverter-side current and capacitor voltage for feedback and feedforward, respectively, has been proven to be able to provide damping in weak power grids, effectively ensuring the reliable operation of the grid-connected system. Compared to existing active damping methods for grid-connected inverters, the control strategy proposed in the present invention requires fewer sensors, has a higher degree of freedom in system control, and has a significant damping effect. It can ensure strong robustness in both strong and weak power grids. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The drawings described herein are for illustrative purposes only and are not intended to limit the scope of the present invention in any way. In addition, the shapes and proportional dimensions of the components in the drawings are only schematic and are used to help understand the present invention, and are not intended to specifically limit the shapes and proportional dimensions of the components of the present invention. In the drawings:

[0031] Figure 1 Schematic diagram of the proposed active damping control method for grid-connected inverter;

[0032] Figure 2 This is a schematic diagram of the software control principle of a three-phase phase-locked loop;

[0033] Figure 3 This is the equivalent control block diagram of the proposed active damping control method;

[0034] Figure 4 Design a flow chart for the proposed system parameters;

[0035] Figure 5 is the parameter feasible region obtained according to the proposed control strategy and design method;

[0036] Figure 6 Schematic diagram of simulation waveforms of the control method proposed in the present invention under different power grids; (a) is the capacitor voltage simulation waveform; (b) is the grid-connected current simulation waveform.

[0037] Specific implementation

[0038] In order to make the purpose and technical solution of the present invention clearer and easier to understand, the present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0039] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first" and "second" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "multiple" means two or more. In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they may refer to fixed, detachable, or integral connections; mechanical or electrical connections; direct or indirect connections through an intermediate medium; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0040] In order to achieve output impedance reconstruction of the grid-connected inverter, a suitable active damping control method is required. However, existing methods have the following limitations: (1) To simultaneously achieve output power control, overcurrent protection, and damping, a large number of state variables need to be sampled, resulting in high system hardware costs; (2) During the control parameter design process, the saturation effect of the software control link and the impact of the steady-state operating point on the system are not considered; (3) It is difficult to ensure the robustness of the control system when the grid impedance varies over a wide range, especially in weak grid conditions, where there is a risk of instability.

[0041] Based on the limitations of the above existing methods, the present invention proposes a hybrid active damping control method for a grid-connected inverter to achieve reconstruction of the inverter output impedance.

[0042] The main purpose of the present invention is to provide a hybrid active damping control method for a grid-connected inverter, comprising:

[0043] The capacitor voltage and inverter side current are collected. The phase-locked loop obtains the phase angle information of the control system by locking the capacitor voltage. The information collected by the inverter side current sensor is used to determine whether the device is in an overcurrent state and to perform closed-loop control of the output current.

[0044] The inverter side current and capacitor voltage are used for feedback and feedforward respectively, and are superimposed with the output of the current loop to control the modulation process of the system.

[0045] This method is a hybrid active damping control method for three-phase grid-connected inverters based on LCL filters. By real-time sampling of capacitor voltage and inverter-side current, feedforward and feedback control are performed, respectively, to suppress oscillations of grid impedance over a wide range of variations, while effectively eliminating the current surge at the moment of grid connection. Compared to existing active damping control methods, the proposed method only requires capacitor voltage and inverter-side current sensors. Since both are essential sensor elements for grid-connected system phase information detection, output active and reactive power control, and overcurrent protection, no additional hardware is required, effectively reducing design costs while maintaining stable operation even under complex weak grid conditions.

[0046] The grid-connected inverter described in the present invention operates in a current control mode. The system includes a current loop, a phase-locked loop, and a grid pre-synchronization unit. First, the phase-locked loop detects the filter capacitor voltage to obtain the grid phase. Then, the three-phase inverter uses a grid pre-synchronization algorithm to determine whether the grid connection conditions are met. Finally, the current loop controls the output grid current to be consistent with the command value, completing the power output. In this method, the system only needs to collect capacitor voltage and inverter-side current. The phase-locked loop obtains phase angle information of the control system by locking the capacitor voltage. The information collected by the inverter-side current sensor is directly used to determine whether the device is in an overcurrent state and is also used for closed-loop control of the output current. In addition, the inverter-side current and capacitor voltage are used for feedback and feedforward, respectively, and are superimposed with the output of the current loop for modulation. This combination of state variable feedback and feedforward has been verified to provide damping in weak grid conditions, effectively ensuring the reliable operation of the grid-connected system. Compared with existing methods, the control strategy proposed in the present invention requires fewer sensors, has higher system control freedom, has a significant damping effect, and can ensure strong robustness in both strong and weak grid conditions.

[0047] The control technology solution proposed in this invention is mainly divided into the following parts:

[0048] The active damping strategy proposed in this invention is aimed at the grid-connected inverter, as shown in Figure 1. Since the dynamics of the photovoltaic array and the energy storage unit are not considered, the DC side is controlled by a constant DC voltage source. V dc Inverter LCL The filter output is connected to the large power grid through the AC bus v g connected, among which L gRepresents the equivalent line impedance of the power grid. The control system is established in the dq coordinate system. The outer loop is the inverter side current. After passing through the PI regulator, it is superimposed with the output after capacitor voltage feedforward and inverter side current feedback. G d It represents the 1-beat control delay and 0.5-beat modulation delay of the digital system. Finally, after coordinate transformation and space vector modulation, six PWM drive signals are generated to drive the bridge arm switches of the three-phase full-bridge inverter. Among them, the capacitor voltage feedforward coefficient is K ff , the inverter side current feedback coefficient is H i1 In order to achieve grid pre-synchronization, the inverter needs to track the phase of the capacitor voltage. The control block diagram of the three-phase software phase-locked loop is shown in Figure 2.

[0049] like Figure 1 As shown, an optional solution of the present invention is: the inverter is a three-phase grid-connected inverter based on an LCL filter;

[0050] The DC side of the inverter is connected to a constant DC voltage source V dc , switch tube S 1- S 6 After being driven by the modulated signal, the output port voltage of the three-phase inverter u oa 、 u ob and u oc Contains high-order harmonics, through three-phase LCL After the filter removes the switching ripple, the output three-phase AC current i L2 , through the AC bus and the large power grid v g Connected to the main power grid v g Equivalent line impedance of connected power grid L g .

[0051] The control block diagram of the inverter current loop is as follows: Figure 3 As shown in Figure 1, due to the use of space vector modulation, its maximum output duty cycle is limited to 1.15.

[0052] In addition, under steady-state conditions, the d-axis and q-axis duty cycles of the three-phase inverter satisfy equations (1) and (2), where V d and V q Indicates the PCC point voltage, I Ld and I Lqrepresents the inverter current, V dc Indicates a constant DC voltage source. In addition, the output of the current loop PI regulator must meet the interval ( G c_low , G c_up ). Before the startup process, the PI controller will reach its upper limit due to the code running idle, and it is necessary to ensure that Equation (3) holds true to avoid overmodulation. After entering the steady state, it is necessary to ensure Equation (4) to prevent the PI controller from entering the nonlinear region due to saturation, thereby losing the closed-loop control effect.

[0053] (1)

[0054] (2)

[0055] (3)

[0056] (4)

[0057] in, G c_low and G c_up Respectively represent the upper and lower limit values ​​of the current loop controller, H i1 and K ff are the coefficients of inverter side current feedback and capacitor voltage feedforward, D d 、 V d 、 I Ld The meaning of is described in formulas (1) and (2). K pwm is the inverter gain, which is half of the constant DC voltage source.

[0058] Figure 4 The following is the design process of the control system. Figure 5 The figure shows the feasible region of parameters obtained after design. By selecting within this range, the control system can avoid entering the nonlinear region due to saturation and maintain stability under different power grid conditions. The specific simulation results are shown in Figure 6 As shown in (a) and (b).

[0059] In order to make those skilled in the art better understand the present invention, Figure 4 The present invention is described in further detail. The specific steps are as follows:

[0060] Step 1: Design the initial steady-state operating point, including determining the grid-connected inverter's rated power and current. Then, design the LCL filter based on the inverter-side current ripple requirements, the reactive power requirements absorbed by the filter capacitor, and the injected grid current harmonics per IEEE 2014-1547.

[0061] Step 2: Design the current outer loop controller parameters K p and K i , as shown in equations (5) and (6). In addition, the upper limit of the current outer loop controller is calibrated G c_low and lower limit G c_up .

[0062] (5)

[0063] (6)

[0064] in, w c It is usually designed to be 0.3 times the LCL resonant frequency, that is, 0.3 w r . L 1 and L 2 represents the inverter-side filter inductance and grid-side filter inductance values.

[0065] Step 3: According to the constraints (1) to (4), determine K ff and H i1 In addition, the inverter side current feedback should satisfy H i1 ≥0 to avoid positive feedback effect, the capacitor voltage feedforward coefficient should meet 0.5≤ K ff ≤1.0 to ensure sufficient damping effect. The final design result is as follows Figure 5 shown.

[0066] Step 4: By applying the designed control structure and control parameters in the software code, a good grid-connected control effect can be achieved.

[0067] The above are only preferred embodiments of the present invention and do not limit the present invention in any way. Any simple modifications, changes and equivalent structural changes made to the above embodiments based on the technical essence of the present invention are still within the scope of protection of the technical solution of the present invention.

[0068] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the same. Although the present invention has been described in detail with reference to the above embodiments, a person skilled in the art may still modify or make equivalent substitutions for the specific implementation schemes of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention are within the scope of protection of the claims of the present invention.

[0069] The above content is a further detailed description of the present invention, and it cannot be considered that the specific implementation methods of the present invention are limited to these. For ordinary technicians in the technical field to which the present invention belongs, they can make several simple deductions or substitutions without departing from the concept of the present invention, which should be regarded as belonging to the scope of protection of the present invention determined by the submitted claims.

Claims

1. A hybrid active damping control method for a grid-connected inverter, characterized in that: include: The capacitor voltage and inverter side current are collected. The phase-locked loop obtains the phase angle information of the control system by locking the capacitor voltage. The information collected by the inverter side current sensor is used to determine whether the device is in an overcurrent state and to perform closed-loop control of the output current. The inverter side current and capacitor voltage are used for feedback and feedforward respectively, and are superimposed with the output of the current loop to control the modulation process of the system; The control system is built in a dq coordinate system. After the inverter-side current passes through the PI regulator, it is superimposed with the output of capacitor voltage feedforward and inverter-side current feedback. After a digital control delay, coordinate transformation and space vector modulation, six PWM drive signals are generated to drive the bridge arm switches of the three-phase full-bridge inverter on and off. The duty cycle of the inverter's d-axis and q-axis satisfies equations (1) and (2): （1） （2） in, V d represents the d-axis PCC point voltage of the inverter, V q represents the q-axis PCC point voltage of the inverter, I Ld represents the inverter d-axis current, I Lq represents the inverter q-axis current, V dc represents a constant DC voltage source; The output of the current loop PI regulator must meet the range ( G c_low , G c_up ), satisfying the following formula: （3） （4） in, G c_low and G c_up Respectively represent the upper and lower limit values ​​of the current loop controller, H i1 and K ff are the coefficients of inverter side current feedback and capacitor voltage feedforward, K pwm is the inverter gain, which is half of the constant DC voltage source.

2. A hybrid active damping control method for a grid-connected inverter according to claim 1, characterized in that: The inverter is a three-phase grid-connected inverter based on an LCL filter; The DC side of the inverter is connected to a constant DC voltage source V dc After the three-phase grid-connected inverter is driven by the modulation signal, the output port voltage u oa 、 u ob and u oc , through three phases LCL After the filter removes the switching ripple, the output three-phase AC current i L2 , through the AC bus and the large power grid v g Connected to the main power grid v g Equivalent line impedance of connected power grid L g .

3. A hybrid active damping control method for a grid-connected inverter according to claim 2, characterized in that: The three-phase grid-connected inverter includes a switch tube S1, a switch tube S2, a switch tube S3, a switch tube S4, a switch tube S5, and a switch tube S6.

4. The hybrid active damping control method for a grid-connected inverter according to claim 1, characterized in that: The maximum output duty cycle of the inverter current loop does not exceed 1.

15.

5. The hybrid active damping control method for a grid-connected inverter according to claim 1, characterized in that: Current outer loop controller parameters K p and K i , specifically satisfying equations (5) and (6): (5) (6) in, is a multiple of the LCL resonant frequency, L 1 and L 2 represents the inverter-side filter inductance and grid-side filter inductance values.

6. The hybrid active damping control method for a grid-connected inverter according to claim 1, characterized in that: The current feedback coefficient on the inverter side should satisfy H i1 ≥0, the capacitor voltage feedforward coefficient should satisfy 0.5≤ K ff ≤1.0.

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