Grid-connected control method of active damping LCL inverter based on capacitor current negative feedback

Abstract

The present invention relates to a grid-connected control method for an active damping LCL inverter based on capacitor current negative feedback, which includes a voltage outer loop plus a capacitor current inner loop control, and in the process of switching between grid connection and off-grid connection, switching into grid voltage full feedforward control of the grid-connected inverter to offset grid voltage disturbances. The capacitor current feedback active damping method has lower additional losses and output current total harmonic distortion in the inverter output power. In actual power grids, in addition to the power frequency fundamental component, the grid voltage also contains a large number of low-order harmonics. The grid voltage harmonics also cause the grid-connected current to generate harmonics. The influence of the grid voltage on the grid-connected current can be suppressed by proportional feedforward, first differential feedforward, and second differential of the grid voltage. In order to reduce the distortion of the filter capacitor voltage during the switching between grid connection and off-grid connection, the grid current reference can be linearly reduced to zero within the time required by the specification to cope with the adverse situation of the inverter switching from grid-connected full load to independent no-load. The above method has a very obvious effect in suppressing load voltage oscillation.

Description

Technical Field

[0001] The present invention relates to a grid-connected control technology, and in particular to a grid-connected control method of an active damping LCL inverter based on capacitor current negative feedback. Background Art

[0002] Grid-tied inverters are the interface between distributed power sources and the power grid. By properly controlling the phase and amplitude of the grid-connected current, they can achieve unity power factor grid connection or simultaneously provide active and reactive power to the grid. Key technologies for grid-tied inverters include current control, grid-connected current harmonic suppression, and on-grid and off-grid mode switching. Common methods for suppressing LCL resonance include passive damping, split capacitors, and notch filters. The passive damping method suppresses LCL resonance spikes by connecting actual resistors in series or parallel with the inductor L or capacitor C. However, due to actual resistor losses, this reduces the inverter's grid-connected efficiency. The split capacitor method relies on the precise configuration of the two split capacitors to maintain the same capacitive reactance at all times, which undoubtedly places higher demands on component aging compensation, making it more idealistic and difficult to implement. The notch filter damping method uses real-time adjustment of notch filter parameters to track resonant frequency changes, which increases control complexity and system cost, making it difficult to achieve the desired damping effect. Summary of the Invention

[0003] To address these issues, a grid-connected control method for LCL inverters with active damping based on capacitor current negative feedback is proposed. Active damping with capacitor current proportional feedback is simple to implement and offers excellent damping effectiveness. By negatively feedbacking the capacitor current signal, a parallel resistor is connected across the filter capacitor C. This control algorithm replaces the actual resistor in passive damping, achieving active damping without incurring additional power loss and offering flexible control. Compared to traditional passive damping methods, active damping with capacitor current feedback achieves lower additional losses and output current total harmonic distortion (THD) in the inverter output power.

[0004] The technical solution of the present invention is: an active damping LCL inverter grid-connected control method based on capacitor current negative feedback, an LCL filter circuit composed of an inverter side filter inductor L1, a grid side filter inductor L2 and a filter capacitor C is arranged between the inverter bridge and the grid, and the inverter bridge modulation wave signal V t (s) after voltage gain K pwm Then, it works together with the current feedback of the filter capacitor C to form the LCL filter circuit, and the grid-side filter inductor L2 outputs the grid-connected current I g Feedback to the front end of the filter capacitor C, the filter capacitor C is connected in parallel with the resistor Rd, and the front end of the filter capacitor C i c (s) through the feedback coefficient H i1 To control terminal K pwm Before, Grid-connected current loop control is the given current i* ref (s) and grid-connected current i g (s) through the grid-connected current feedback coefficient H i2 The feedback current is then fed into the current controller G i (s), current controller G i (s) output to control terminal K pwm Previously, inner-loop active damping and grid-connected current loop control were performed, and during the on-grid and off-grid switching process, full feed-forward control of the grid voltage of the grid-connected inverter was switched in to offset the disturbance.

[0005] Furthermore, during the on-grid and off-grid switching process, the grid voltage v g (s) and control terminal K pwm Previously, the grid voltage full feedforward coefficient G ff The voltage feedback is:

[0006]

[0007] Z L1 (s)=s*L1、Z L2 (s)=s*L2、 Achieve smooth transition of filter capacitor voltage and reduce impact current on grid-side filter inductor L2.

[0008] Furthermore, the voltage gain K pwm is the output voltage gain from the inverter bridge modulation wave signal to the bridge arm, V dc is the DC voltage input to the inverter bridge, V tri is the triangular carrier amplitude.

[0009] The active damping LCL inverter grid-connected control method based on capacitor current negative feedback uses outer loop voltage plus capacitor current inner loop control in both grid-connected and off-grid control. When grid-connected, the grid-connected capacitor current inner loop regulator is G inc_1 , the grid-connected capacitor current feedback coefficient is the active damping feedback coefficient; when off-grid, the off-grid capacitor current inner loop regulator is G inc_2 , is the actual calculated controller, and the capacitor current feedback coefficient is the off-grid feedback sampling coefficient.

[0010] The beneficial effects of the present invention are as follows: the present invention is based on the active damping LCL inverter grid-connected control method of capacitor current negative feedback, and adopts the capacitor current feedback active damping method to have lower additional loss and output current total harmonic distortion in the inverter output power; in the actual power grid, in addition to the power frequency fundamental component, the grid voltage also contains a large number of low-order harmonics, and the grid voltage harmonics will also cause the grid-connected current to generate harmonics. The influence of the grid voltage on the grid-connected current can be suppressed by proportional feedforward, first differential feedforward and second differential of the grid voltage; in order to reduce the distortion of the filter capacitor voltage during the grid-on and off-grid switching process, the grid current reference can be linearly reduced to zero within the time required by the specification to cope with the severe situation of the inverter switching from grid-connected full load to independent no-load. The effect of the above method in suppressing load voltage oscillation is very obvious. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figures 1a and 1b The filter control block diagrams are of the conventional filter capacitor and the filter control block diagram of the present invention after resistors are connected in parallel at both ends;

[0012] Figures 2a to 2g This is a derivation diagram of the control block diagram of the grid voltage full feedforward control strategy of the grid-connected inverter of the present invention;

[0013] Figure 3 The Bode diagram of the loop gain when the capacitor current feedback coefficient changes in the present invention;

[0014] Figure 4 This is the main circuit topology of the grid-connected inverter of the present invention;

[0015] Figure 5 This is the inverter output control block diagram;

[0016] Figure 6 This is the control block diagram of the inverter when it is connected to the grid;

[0017] Figure 7 Unified model for grid-connected inverters

[0018] Figure 8 This is a timing diagram of the inverter of the present invention switching from an independent working mode to a grid-connected working mode;

[0019] Figure 9 This is a timing diagram of off-grid switching of the inverter based on current control of the present invention;

[0020] Figure 10 This is the experimental waveform diagram of the inverter of the present invention switching to the grid-connected mode. DETAILED DESCRIPTION

[0021] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0022] The LCL inverter is connected to the grid, that is, there is an LCL filter circuit between the inverter bridge and the grid, which consists of the inverter side filter inductor L1, the grid side filter inductor L2 and the filter capacitor C. In all the following figures, the lowercase current i and voltage v are the actual current and voltage, and the uppercase I and V are the corresponding current and voltage after Laplace transformation. Figure 1a The figure shows the inverter model block diagram based on passive damping control after the damping resistor is connected in parallel across the filter capacitor C, where V t (s) is the modulation wave signal obtained by the control of the inverter bridge; K pwm is the output voltage gain from the inverter bridge modulation wave signal to the bridge arm, ignoring factors such as dead zone delay, and its expression can be simplified to V dc is the DC voltage input to the inverter bridge, V tri is the triangular carrier amplitude; Rd is the damping resistor connected in parallel across the filter capacitor C. Inverter bridge modulation wave signal V t (s) after voltage gain K pwm Then, it works together with the current feedback of the filter capacitor C to form the LCL filter circuit, and the grid-side filter inductor L2 outputs the grid-connected current I g Feedback to the front end of the filter capacitor C, V g (s) is the grid voltage in front of the grid-side filter inductor L2.

[0023] exist Figure 1a Based on the LCL inverter model, active damping and grid-connected current loop control are performed, such as Figure 1b As shown. First Figure 1a The passive damping in the figure, that is, the control block diagram of capacitor C in parallel with resistor Rd, is simplified. The input is from V c (s)Move to Figure 1b shown Front-end i c (s), capacitor current feedback is used to The output from i c (s)Move to Figure 1b Shown K pwm Through the equivalent simplification of the block diagram, the passive damping of the parallel resistor can be obtained, which is equivalent to the passive damping of the parallel resistor at the control end K pwm Before, a proportional feedback of the capacitor current is added, and the feedback coefficient is Active damping. Figure 1b Medium V t (s) Modulated wave signal, which is obtained by subtracting active damping control from grid-connected current loop control. Grid-connected current loop control is the given current i* ref (s) and grid-connected current i g (s) through the grid-connected current feedback coefficient H i2The feedback current is then fed into the current controller G i (s), G i (s) PI controller is generally used.

[0024] In order to suppress the influence of grid voltage on grid current, a grid voltage full feedforward control strategy based on LCL filter is proposed. Figure 1b The control block diagram in is further simplified to obtain the grid-connected current i g (s) and grid voltage disturbance V g (s) the relationship between them.

[0025] Figures 2a to 2g The control block diagram of the grid voltage full feedforward control strategy of the grid-connected inverter is deduced as shown.

[0026] exist Figures 2a to 2d The dashed line in the figure represents the original state diagram of the model, and the solid line represents the equivalent diagram after transformation. Figure 1b The feedback capacitor voltage V c (s) is adjusted to the capacitor current i c (s), at this time its feedback point moves to G i (s) output terminal, we get Figure 2a Then the grid current feedback quantity i g Feedback point (s) The output terminal is moved to G i (s) output terminal, the capacitor current i c (s) The two feedback branches are combined to obtain Figure 2b , and then the grid current i in the previous step is g (s) in G i (s) The feedback point at the output is moved to the capacitor The output of the system is simplified, and the forward channel is obtained Figure 2c , and finally Figure 2c Simplified to get Figure 2d .

[0027] in:

[0028]

[0029]

[0030]

[0031]

[0032] For the convenience of expression, let Z L1 (s)=s*L1、Z L2 (s)=s*L2、 represents the impedance expression in the frequency domain, T A is the loop gain of the system.

[0033] Depend on Figure 2d , we can get the grid-connected current expression of the system as:

[0034]

[0035] From the expression of grid-connected current, we can know that grid-connected current i2(s) is not only affected by grid-connected current instruction The control also receives the grid voltage V g (s) The influence of disturbance, after analysis, we know that the grid voltage V g The fundamental component of (s) will cause the grid-connected current i g (s) lags behind the grid voltage V g (s), affecting its grid-connected power factor, V g The harmonic components of (s) will cause the grid current i g (s) is severely distorted, and the specific calculation and analysis process will not be given here.

[0036] Figures 2e to 2g The figure shows the control method of grid voltage full feedforward control strategy of grid-connected inverter.

[0037] Inspired by feedforward control, Figure 2d In the inverter block diagram, a control loop is added to offset the grid voltage V g (s) impact, such as Figure 2e As shown in the dotted line, a control with opposite sign is used to control V g (s) cancel each other out and become zero. Figure 2e The addition of the dotted line is in the actual model of the inverter, which is difficult to implement in the actual model. It is necessary to lead the dotted line back to the control end, that is, K pwm Before, perform block diagram equivalent transformation, such as Figure 2f As shown, Contains current controller G i (s), whose value is affected by G i (s) control parameter influence, therefore, the block diagram is transformed again, such as Figure 2g As shown, it is fed back to the current controller G i Thus, the final grid voltage full feedforward coefficient Gff is obtained as:

[0038]

[0039] Grid-connected inverters typically have two operating modes: grid-connected and standalone. During grid-connected operation, the grid-connected switch is closed, and current control is employed to align the grid current with a reference current, thereby delivering power to the grid. During standalone operation, the grid-connected switch is open, and voltage control is employed to control the filter capacitor voltage and ensure proper operation of the local load. Due to differences in hardware structure and control algorithms between the two operating modes, safe and reliable mode switching is required to ensure the safe and stable operation of the inverter, the grid, and the local load.

[0040] Figure 3 The main circuit topology of the grid-connected inverter, where the local load Z ld Connected in parallel across the output filter capacitor. During on-grid and off-grid switching, three switches (SW1-SW3) control whether the inverter is connected to the grid voltage, whether full grid voltage feedforward is enabled, and the selection of the on-grid and off-grid control outer loop. A high level indicates a 1, and the switch is closed, indicating on-grid operation. A low level indicates a 0, and the switch is open, indicating off-grid operation.

[0041] Figure 4 Given Figure 3 The inverter output terminal SW1 control model block diagram. Figure 3 The grid-connected switch STS is equivalent to a switch SW1, and its control signal is v sw1 , output high and low levels to control whether the grid is cut in, that is, to select the on-grid and off-grid mode.

[0042] Figure 5 The control block diagram of SW2 during inverter on-grid and off-grid operation is given. Figure 5 In the case of SW1 being closed, the grid-connected operation is carried out. From the above analysis, it can be seen that the grid-connected current is affected by the grid voltage V g (s) disturbance, therefore, using the above grid voltage full feedforward G ff (s) to offset the disturbance, and there is no grid voltage when running off-grid, so there is no need to use G ff (s) to offset the disturbance, so a virtual switch SW2 is added in the feedforward branch, and its control signal is v sw2 When v sw2 When it is high level, SW2 is closed, grid voltage feedforward is added, and grid-connected operation is performed; when v sw2 When it is low level, SW2 is disconnected, no grid voltage feedforward is added, and off-grid operation is performed.

[0043] There are essential differences in the output targets of grid-connected and off-grid inverter control. Grid-connected inverters control current, i.e., power, while off-grid inverters control voltage. Grid-connected inverters use a single current loop, while off-grid inverters use a capacitor voltage outer loop and a capacitor current inner loop. To unify the control block diagrams of the two inverters, and to use capacitor current feedback under LCL active damping in grid-connected mode, it is considered that both grid-connected and off-grid inverters use an outer voltage loop plus a capacitor current inner loop, except that the grid-connected capacitor current inner loop regulator is G. inc_1 =1, grid-connected capacitor current feedback coefficient H i1_1 is the active damping feedback coefficient, and the off-grid capacitor current inner loop regulator is G inc_2 , is the actual calculated controller, the capacitor current feedback coefficient is H i1_2 is the off-grid feedback sampling coefficient.

[0044] According to the characteristics of the above two working modes of the inverter, the double closed-loop control block diagrams of the two are combined to obtain Figure 6 The unified model of the grid-connected inverter is shown. Figure 6 There is also a virtual mode switch SW3, whose control signal is v sw3 When v sw3 When it is high, the grid-connected mode is selected. At this time, the active damping coefficient of the filter capacitor current is H i1_1 , the inner loop regulator is G inc_1 is 1; when v sw3 When it is low level, the off-grid mode is selected, and the sampling feedback coefficient of the capacitor current is H i1_2 , the inner loop regulator is G inc_2 By properly arranging the action sequence of SW1, SW2 and SW3, the inverter can realize independent operation, grid-connected operation and switching between the two working modes.

[0045] As mentioned above, when the inverter is running independently, SW1 is disconnected and the filter capacitor voltage tracks the voltage reference. After being connected to the grid, SW1 is closed. Generally, the grid-side filter inductor L f2 The voltage drop is much smaller than the grid voltage, so the voltage of the filter capacitor is approximately equal to the grid voltage during grid-connected operation. Cf Smooth transition and reduced filter inductance L f2 To prevent inrush current, the filter capacitor voltage amplitude and phase need to be adjusted to match the grid voltage before SW1 is closed. The filter capacitor voltage amplitude can be adjusted by modifying the voltage amplitude reference cycle by cycle, and the filter capacitor voltage phase can be adjusted using a phase-locked loop.

[0046] Figure 7The timing of the inverter switching from independent working mode to grid-connected working mode is given: before time t1, the inverter operates independently, and the amplitude and phase of the filter capacitor have tracked the grid voltage well; at time t1, which is the zero-crossing point of the grid voltage, v sw1 、v sw2 and v sw3 At the same time, the SW1 is closed and the capacitor current reference value of the inner loop is determined by the voltage regulator G v The output of (s) is switched to the current regulator G i The sum of the output of (s) and the grid voltage feedforward, the capacitor current feedback coefficient is given by H i1_2 Switch to H i1_1 , the inner loop regulator is composed of G inc_2 Switch to G inc_1 , the inverter has completed the switch from independent working mode to grid-connected working mode. To avoid the switching instantaneous filter inductor L f2 If an excessively large inrush current occurs, the initial current reference value is set to 0 during the first power frequency cycle starting at time t1, and is then adjusted to the rated value cycle by cycle.

[0047] The timing of the inverter off-grid switching based on current control is as follows Figure 8 As shown, the specific steps are as follows: before time t1, the inverter is connected to the grid; at time t1, the inverter detects that the grid voltage amplitude or frequency exceeds the normal range, and the v sw1 Set to low so that SW1 automatically disconnects after the grid current reaches 0, and at the same time sets the grid current reference to 0. DSP starts to continuously detect whether the grid current reaches 0. During the period t1 to t2, the filter capacitor voltage drops rapidly, causing the grid-side filter inductor L f2 Under the reverse pressure, the grid current drops rapidly and drops to 0 at time t2, and SW1 is disconnected; at time t3, DSP confirms that the grid current has reached zero and switches v sw2 and v sw3 At the same time, it is set to low, that is, the reference value of the inner loop of the capacitor current is determined by the current regulator G i The output of (s) and the grid voltage feedforward quantity are switched to the voltage regulator G v (s) output, the capacitor current feedback coefficient is determined by H i1_1 Switch to H i1_2 , the inner loop regulator is composed of G inc_1 Switch to G inc_2 , the inverter completes the switch from grid-connected mode to standalone mode. To reduce filter capacitor voltage oscillation, the initial value of the voltage reference is taken from the filter capacitor voltage immediately before the switch, and the filter capacitor voltage is then adjusted to the rated value cycle by cycle.

[0048] Table 1 Main parameters of the grid-connected inverter prototype

[0049] <![CDATA[V dc (V)]]> 360 <![CDATA[V g (V)]]> 220 <![CDATA[V tri (V)]]> 3 <![CDATA[f s (kHz)]]> 10 <![CDATA[L f1 (μH)]]> 600 <![CDATA[L f2 (μH)]]> 150 <![CDATA[C f (μF)]]> 10 <![CDATA[H i1 ]]> 0.075 <![CDATA[H i2 ]]> 0.15

[0050] Figure 9 、 10 This is the experimental waveform for switching the inverter working mode. Figure 9 This is the waveform of the inverter grid-connected experiment. It can be seen from the waveform that the inverter switches from independent working mode to grid-connected working mode at the grid voltage zero-crossing point. There is no obvious inrush current at the moment of grid connection. Figure 10 This is the waveform from the inverter off-grid test. The program sets the switching to begin at the peak of the grid current, and the grid current reference is linearly reduced to zero over 2ms. The waveform shows that the grid current tracking reference drops to zero in approximately 2ms. After approximately 1ms of grid current zero confirmation time, the inverter switches from grid-connected mode to standalone mode. The filter capacitor voltage exhibits no noticeable distortion during the switching process, achieving a relatively smooth transition.

[0051] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A grid-connected control method for an active damping LCL inverter based on capacitor current negative feedback, characterized in that: There is a filter inductor on the inverter side between the inverter bridge and the grid. L 1. Grid-side filter inductor L 2 and filter capacitor C The LCL filter circuit composed of the inverter bridge modulation wave signal V t ( s ) after voltage gain K pwm After the filter capacitor C Current feedback acts on the LCL filter circuit and the grid-side filter inductor L 2 Output grid-connected current I g Feedback to filter capacitor C Front end, filter capacitor C parallel resistor Rd, front end of filter capacitor C i c ( s ) through the feedback coefficient To voltage gain K pwm Before, , grid-connected current loop control means given current i* ref ( s ) and grid-connected current i g ( s ) through the grid current feedback coefficient H i2 The feedback current is then fed into the current controller G i ( s ), current controller G i ( s ) output to voltage gain K pwm Previously, the inner loop active damping and grid-connected current loop control were performed, and during the on-grid and off-grid switching process, the grid voltage full feed-forward control of the grid-connected inverter was switched in to offset the disturbance; During the on-grid and off-grid switching process, the grid voltage v g ( s ) and voltage gain K pwm Previously, the grid voltage full feedforward coefficient G ff The voltage feedback is: ， 、 、 , to achieve smooth transition of filter capacitor voltage and reduce grid-side filter inductance L 2. Inrush current.

2. The active damping LCL inverter grid-connected control method based on capacitor current negative feedback according to claim 1, characterized in that: The voltage gain K pwm is the output voltage gain from the inverter bridge modulation wave signal to the bridge arm, , V dc Input DC voltage to the inverter bridge, V tri is the triangular carrier amplitude.

3. The active damping LCL inverter grid-connected control method based on capacitor current negative feedback according to claim 1, characterized in that: Under both grid-connected and off-grid control, the outer loop voltage loop plus the capacitor current inner loop control is used. When grid-connected, the grid-connected capacitor current inner loop regulator is G inc_1 , the grid-connected capacitor current feedback coefficient is the active damping feedback coefficient; when off-grid, the off-grid capacitor current inner loop regulator is G inc_2 , is the actual calculated controller, and the capacitor current feedback coefficient is the off-grid feedback sampling coefficient.

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