Hybrid active damping control method based on LCL grid-connected inverter

Abstract

The invention provides a hybrid active damping control method based on an LCL grid-connected inverter, which comprises an inverter and an LCL filter composed of an inverter side inductor, a power grid side inductor and a capacitor, the control end of the inverter is connected with a driving module, the input end of the driving module is connected with a sine pulse width modulation module, and the input end of the sine pulse width modulation module is connected with the inverter side inductor. The input end of the sine pulse width modulation module is connected with the Clark-Park conversion module, the time delay module, the PI controller and the sampling module in sequence, and the sampling module collects an inverter side inductive current IL, a capacitor voltage V and a power grid current Ig respectively; the LCL resonance is suppressed by combining inverter current and capacitor voltage double-closed-loop control, the grid-connected current harmonic THD can be effectively reduced to be below 5%, the anti-interference capability and dynamic response of the system are improved, meanwhile, stable operation under a weak power grid can be achieved without an additional sensor, and the cost is low.

Description

Technical Field

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

[0002] With the increasing global demand for renewable energy, such as solar and wind energy, the grid-connected inverter, as a core component of renewable energy, has become increasingly prominent. It converts the DC power generated by clean energy into AC power with the same frequency and phase as the grid, realizing the interaction with the grid power. With the increase of distributed power generation systems, a large number of grid-connected inverters have been added to the distribution network. LCL filters are widely used in three-phase inverter systems due to their superior high-frequency harmonic attenuation characteristics. However, LCL filters have inherent resonance problems, which may cause the system to generate harmonic oscillations at specific frequencies, affecting the stable operation of the system. In addition, in the case of weak grids, the existence of grid impedance reduces the open-loop gain and bandwidth of the system, and the low-order harmonics of the system affect the current quality.

[0003] The active damping control strategy can suppress the resonance of the LCL filter by introducing an additional control signal into the control loop. It is implemented through an algorithm, does not require changing the hardware structure, and can avoid additional system losses. Early active damping control strategies were mainly based on PCC point voltage feedback, but this method may introduce grid background harmonic voltage disturbances into the grid-connected current reference value, affecting the grid-connected current quality. Additional sensors are needed to suppress the resonance of the LCL filter, but adding sensors will increase costs. Therefore, it is urgent to seek new active damping strategies to balance system stability and grid-connected current quality. Summary of the invention

[0004] In view of the problems in the prior art, the present invention provides a hybrid active damping control method based on an LCL grid-connected inverter, aiming to obtain better harmonic control effect and system stability compared with a dual current closed-loop control strategy without the need for additional sensors.

[0005] A hybrid active damping control method based on an LCL grid-connected inverter includes an inverter and an LCL filter composed of an inverter-side inductor, a grid-side inductor and a capacitor. The control end of the inverter is connected to a drive module, the input end of the drive module is connected to a sinusoidal pulse width modulation module, the input end of the sinusoidal pulse width modulation module is sequentially connected to a Clark-Park conversion module, a time delay module, a PI controller and a sampling module. The sampling module collects the inductor current I L , capacitor voltage V, grid current I g , the hybrid active damping control method includes the following steps:

[0006] Step 1: Use the Clark-Park transformation module to convert the inverter side inductor current I L , capacitor voltage V, grid current I g Converted into current I Ld 、Current I Lq , voltage V d , voltage V q 、Current I gd and current I gq ;

[0007] Step 2: Set the current I Ld and current I Lq Subtract the given first reference value Igd_ref and the second reference value Igq_ref respectively to obtain two error signals, and send the two error signals to the PI controller for calculation to obtain the current values ​​Y 2d and current value Y 2q ;

[0008] Step 3: Set the current value Y 2d Subtract the feedback factor H i1 and current I Ld The result after multiplication, plus the feedforward coefficient H ff and voltage V d The result after multiplication is the current value Y 3d At the same time, the current value Y 2q Subtract the feedback factor H i1 and current I Lq The result after multiplication, plus the feedforward coefficient H ff and voltage V q The result after multiplication is the current value Y 3q ;

[0009] Step 4: Set the current value Y 3d and current value Y 3q Send it to the delay module to get the pulse width modulation signal V md and V mq ;

[0010] Step 5: Convert the PWM signal V md 、V mq Perform inverse transformation to obtain a three-phase modulated signal;

[0011] Step 6: Use the sinusoidal pulse width modulation module to generate a triangular carrier, and compare the three-phase modulation signal with the triangular carrier in real time. When the amplitude of the three-phase modulation signal is higher than the triangular carrier, a high level is output; when the amplitude of the three-phase modulation signal is lower than the triangular carrier, a low level is output, thereby generating three-way PWM drive signals;

[0012] Step 7: The driving module controls the working state of the inverter switch tube according to the three-way PWM driving signal.

[0013] Further: The transfer function of the PI controller is Among them, s is a complex variable, K p is the proportional gain, K i is the integral gain; bring the parameters of the PI controller into the open-loop function T(s) and draw the Bode diagram of the function. When the Bode diagram is used to judge the stability of the LCL grid-connected inverter system, determine K p and K i The value is then fed into the PI controller to calculate the current value Y 2d and current value Y 2q ; The expression of the open-loop function T(s) is:

[0014]

[0015] Where, L 1 is the inverter side inductance, L T =L 2 +L g , L 2 is the grid side inductance, L g Represents the equivalent grid-side inductance; K pwm is the gain, C is the capacitance value of the filter, ω r is the resonant angular frequency, K ff is the feedforward gain coefficient, T s is the sampling period, H i1 is the inverter current feedback coefficient.

[0016] Further: The total delay in the delay module is:

[0017] Further: In step 5, the pulse width modulation signal V md 、V mq The three-phase modulation signal is obtained by inverse transformation with the calculated frequency ωt; where ωt=2πf res , f res is the resonant frequency and is obtained by differentiating the phase angle θ, that is,

[0018] Further, the capacitor voltage V is calculated by the phase-locked loop module to obtain the calculation frequency ωt.

[0019] Further, the inverse transformation is specifically the process of converting the signal of the two-phase rotating coordinate system back to the three-phase stationary coordinate system, namely, the inverse Park transformation and the inverse Clarke transformation;

[0020] Among them, the formula for inverse Park transform is:

[0021]

[0022] The formula for the inverse Clarke transform is

[0023]

[0024] Among them, I d and I q is the input of the inverse Park transform, I α and I β is the output of the inverse Park transform or the input of the inverse Clarke transform, I A ,I B and I C is the output of the inverse Clarke transform.

[0025] Furthermore, the sinusoidal pulse width modulation module is a module in the DSP digital signal processor.

[0026] The beneficial effects of the present invention are as follows: combining the inverter current and capacitor voltage dual closed-loop control to suppress LCL resonance, which can effectively reduce the grid-connected current harmonic THD to below 5%, improve the system's anti-interference ability and dynamic response, and at the same time, can operate stably under weak power grids without the need for additional sensors, and has low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a flow chart of the present invention;

[0028] Figure 2 It is a structural block diagram of the weak power grid in the present invention;

[0029] Figure 3 It is the dq axis current control block diagram in the present invention;

[0030] Figure 4 Bode diagram of the strategy used in the present invention;

[0031] Figure 5 This is the simulation result of the strategy used in the present invention when the grid inductance is 3mH. The upper waveform is the grid voltage, and the lower waveform is the grid current.

[0032] Figure 6 This is a graph of total harmonic distortion data for the strategy used in the present invention. DETAILED DESCRIPTION

[0033] The present invention is described in detail below in conjunction with the accompanying drawings. The embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be interpreted as limiting the present invention. The directional terms such as left, middle, right, top, and bottom in the embodiments of the present invention are only relative concepts or are based on the normal use state of the product, and should not be considered as restrictive.

[0034] A hybrid active damping control method based on LCL grid-connected inverter, such as Figure 1 and Figure 2 As shown, it includes an inverter and an inductor on the inverter side and an inductor on the grid side (L a1 ~L a3 , L b1 ~L b3 , L c1 ~L c3 ) and an LCL filter composed of a capacitor, the control end of the inverter is connected to the drive module, the input end of the drive module is connected to the sinusoidal pulse width modulation module, the input end of the sinusoidal pulse width modulation module is connected to the Clark-Park conversion module, the delay module, the PI controller, and the sampling module in sequence, and the sampling module collects the inductor current I L , capacitor voltage V, grid current I g , the hybrid active damping control method includes the following steps:

[0035] Step 1: Use the Clark-Park transformation module to transform the inverter side inductor current I in the three-phase stationary coordinate system L , capacitor voltage V, grid current I g Converted to the current I in the dq axis rotating coordinate system respectively Ld 、Current I Lq , voltage V d , voltage V q 、Current I gd and current I gq ;

[0036] Step 2: Set the current I Ld and current I Lq Subtract the given first reference value Igd_ref and the second reference value Igq_ref respectively to obtain two error signals, and send the two error signals to the PI controller for calculation to obtain the current values ​​Y 2d and current value Y 2q ;

[0037] The transfer function of the PI controller is Among them, s is a complex variable, Kp is the proportional gain, K i is the integral gain. The determination of PI parameters will affect the stability of the system; Figure 4 As shown, the parameters of the PI controller are brought into the open-loop function T(s) and the Bode diagram of the function is drawn. When the Bode diagram is used to judge the stability of the LCL grid-connected inverter system, K is determined. p and K i The value is then fed into the PI controller to calculate the current value Y 2d and current value Y 2q ; The expression of the open-loop function T(s) is:

[0038]

[0039] Where, L 1 is the inverter side inductance, L T =L 2 +L g , L 2 is the grid side inductance, L g Represents the equivalent grid-side inductance; K pwm is the gain, C is the capacitance value of the filter, ω r is the resonant angular frequency, K ff is the feedforward gain coefficient, T s is the sampling period, H i1 is the inverter current feedback coefficient;

[0040] Step 3: Set the current value Y 2d Subtract the feedback factor H i1 and current I Ld The result after multiplication, plus the feedforward coefficient H ff and voltage V d The result after multiplication is the current value Y 3d At the same time, the current value Y 2q Subtract the feedback factor H i1 and current I Lq The result after multiplication, plus the feedforward coefficient H ff and voltage V q The result after multiplication is the current value Y 3q ; Thus, the inverter current and capacitor voltage dual closed-loop control are combined to suppress LCL resonance;

[0041] Step 4: Set the current value Y 3d and current value Y 3q Send it to the delay module to get the pulse width modulation signal V md and V mq ; Calculation delay in inverter synchronous sampling (1T s ) and PWM zero-order hold (0.5Ts ) together lead to 1.5T s Equivalent hysteresis causes resonant frequency shift and phase margin attenuation; where T s is the sampling period, which needs to be compensated in advance by the control algorithm; therefore, the total delay in the delay module is:

[0042] Step 5: Convert the PWM signal V md 、V mq Perform inverse transformation to obtain the three-phase modulated signal (V a 、V b 、V c );Pulse width modulation signal V md 、V mq All are inversely transformed with the calculated frequency ωt, that is, the process of converting the signal of the two-phase rotating coordinate system (dq) back to the three-phase stationary coordinate system (abc), namely, the inverse Park transform and the inverse Clarke transform;

[0043] Among them, the formula for inverse Park transform is:

[0044]

[0045] The formula for the inverse Clarke transform is

[0046]

[0047] Among them, I d and I q is the input of the inverse Park transform, I α and I β is the output of the inverse Park transform or the input of the inverse Clarke transform, I A ,I B and I C is the output of the inverse Clarke transform; the three-phase modulated signal (V a 、V b 、V c ), where ωt = 2πf res , f res is the resonant frequency and is obtained by differentiating the phase angle θ, that is, The capacitor voltage V is calculated by the phase-locked loop module to obtain the calculation frequency ωt;

[0048] Step 6: Use the sinusoidal pulse width modulation module inside the DSP digital signal processor to generate a triangular carrier and convert the three-phase modulation signal (V a 、V b 、V c) is compared with the triangular carrier in real time. When the amplitude of the three-phase modulation signal is higher than the triangular carrier, a high level is output; when the amplitude of the three-phase modulation signal is lower than the triangular carrier, a low level is output, thereby generating three-way PWM drive signals;

[0049] Step 7: The driving module controls the working state of the inverter switch tube according to the three-way PWM driving signal.

[0050] In the simulation example of the present invention, the data of various components and the coefficients in the control module are shown in Table 1 below.

[0051] parameter value parameter value <![CDATA[L 1 ]]> 3.5mH <![CDATA[K pwm ]]> 100 <![CDATA[L 2 ]]> 1.75mH <![CDATA[K ff ]]> 0.5 C 5μF <![CDATA[K p ]]> 0.07 <![CDATA[L g ]]> 3mH <![CDATA[K i ]]> 50 <![CDATA[T s ]]> 1 / 10000 <![CDATA[H i1 ]]> 0.05 Igd_ref 5A Igq_ref 0A

[0052] Combination Figure 5 and Figure 6 It can be seen that the grid-connected current waveform has no obvious distortion or high-frequency oscillation, indicating that the control algorithm proposed in the present invention can effectively suppress high-frequency harmonics, and the total harmonic distortion rate is only 0.93%, which is much lower than the typical grid-connected standard (such as IEEE1547 requires THD<5%). The control algorithm significantly reduces the harmonic content; the control algorithm proposed in the present invention successfully realizes the low harmonic output of the LCL grid-connected inverter, the current waveform is smooth and stable, and the THD is low; the algorithm can meet the grid-connected power quality requirements when the grid impedance changes under weak power grids in terms of harmonic suppression, resonance damping and dynamic response.

[0053] The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A hybrid active damping control method based on an LCL grid-connected inverter, comprising an inverter and an LCL filter composed of an inverter-side inductor, a grid-side inductor and a capacitor, wherein the control end of the inverter is connected to a drive module, and the input end of the drive module is connected to a sinusoidal pulse width modulation module, characterized in that: The input end of the sine pulse width modulation module is connected to the Clark-Park transformation module, the delay module, the PI controller, and the sampling module in sequence. The sampling module collects the inductor current I L , capacitor voltage V, grid current I g , the hybrid active damping control method includes the following steps: Step 1: Use the Clark-Park transformation module to convert the inverter side inductor current I L , capacitor voltage V, grid current I g Converted into current I Ld 、Current I Lq , voltage V d , voltage V q 、Current I gd and current I gq ; Step 2: Set the current I Ld and current I Lq Subtract the given first reference value Igd_ref and the second reference value Igq_ref respectively to obtain two error signals, and send the two error signals to the PI controller for calculation to obtain the current values ​​Y 2d and current value Y 2q ; Step 3: Set the current value Y 2d Subtract the feedback factor H i1 and current I Ld The result after multiplication, plus the feedforward coefficient H ff and voltage V d The result after multiplication is the current value Y 3d At the same time, the current value Y 2q Subtract the feedback factor H i1 and current I Lq The result after multiplication, plus the feedforward coefficient H ff and voltage V q The result after multiplication is the current value Y 3q ; Step 4: Set the current value Y 3d and current value Y 3q Send it to the delay module to get the pulse width modulation signal V md and V mq ; Step 5: Convert the PWM signal V md 、V mq Perform inverse transformation to obtain a three-phase modulated signal; Step 6: Use the sinusoidal pulse width modulation module to generate a triangular carrier, and compare the three-phase modulation signal with the triangular carrier in real time. When the amplitude of the three-phase modulation signal is higher than the triangular carrier, a high level is output; when the amplitude of the three-phase modulation signal is lower than the triangular carrier, a low level is output, thereby generating three-way PWM drive signals; Step 7: The driving module controls the working state of the inverter switch tube according to the three-way PWM driving signal.

2. The hybrid active damping control method based on LCL grid-connected inverter according to claim 1 is characterized in that: The transfer function of the PI controller is Among them, s is a complex variable, K p is the proportional gain, K i is the integral gain; bring the parameters of the PI controller into the open-loop function T(s) and draw the Bode diagram of the function. When the Bode diagram is used to judge the stability of the LCL grid-connected inverter system, determine K p and K i The value is then fed into the PI controller to calculate the current value Y 2d and current value Y 2q ; The expression of the open-loop function T(s) is: Where L1 is the inductance on the inverter side, L T =L2+L g , L2 is the grid side inductance, L g Represents the equivalent grid-side inductance; K pwm is the gain, C is the capacitance value of the filter, ω r is the resonant angular frequency, K ff is the feedforward gain coefficient, T s is the sampling period, H i1 is the inverter current feedback coefficient.

3. The hybrid active damping control method based on LCL grid-connected inverter according to claim 1, characterized in that: The total delay in the delay module is:

4. The hybrid active damping control method based on LCL grid-connected inverter according to claim 1, characterized in that: In step 5, the pulse width modulation signal V md 、V mq The three-phase modulation signal is obtained by inverse transformation with the calculated frequency ωt; where ωt=2πf res , f res is the resonant frequency and is obtained by differentiating the phase angle θ, that is, 5. The hybrid active damping control method based on LCL grid-connected inverter according to claim 4 is characterized in that: The capacitor voltage V is calculated by the phase-locked loop module to obtain the calculation frequency ωt.

6. The hybrid active damping control method based on LCL grid-connected inverter according to claim 4 is characterized in that: The inverse transformation is specifically the process of converting the signal of the two-phase rotating coordinate system back to the three-phase stationary coordinate system, namely the inverse Park transformation and the inverse Clarke transformation; Among them, the formula for inverse Park transform is: The formula for the inverse Clarke transform is Among them, I d and I q is the input of the inverse Park transform, I α and I β is the output of the inverse Park transform or the input of the inverse Clarke transform, I A ,I B and I C is the output of the inverse Clarke transform.

7. The hybrid active damping control method based on LCL grid-connected inverter according to claim 1, characterized in that: The sinusoidal pulse width modulation module is a module in the DSP digital signal processor.

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