Harmonic suppression method suitable for wide-range power grid strength

Abstract

The invention discloses a harmonic suppression method suitable for wide-range power grid intensity, which comprises the following steps of: in a control link of a power electronic converter, adding extraction of harmonic current and harmonic voltage, controlling the harmonic current through a harmonic current controller, and adjusting an output signal of the harmonic current controller by using a cross coupling modulation method, so as to realize harmonic suppression of the wide-range power grid intensity. Parameters of the harmonic current controller can be flexibly adjusted, when complete harmonic current suppression is adopted, the parameters designed in advance are adopted, and when harmonic voltage exceeds a harmonic voltage threshold value, the harmonic voltage is adjusted through the harmonic voltage amplitude controller. According to the method, the problem of dynamic adjustment ringing of a traditional method when the power grid intensity of the distorted power grid changes in a wide range, especially under the weak power grid working condition is solved, the harmonic voltage and the harmonic current under the weak power grid can be flexibly adjusted, and the harmonic voltage characteristic is optimized. According to the method, additional circuits are not needed, and multi-target harmonic control of the grid-connected converter under the working condition of wide-range power grid intensity can be realized.

Description

Technical Field

[0001] The invention relates to the field of power electronic converter control, and in particular to a harmonic suppression method adapting to a wide range of power grid strengths. Background Art

[0002] With the rapid development of new energy grid-connected technology, the proportion of distributed energy such as wind power and photovoltaics in the power system is increasing, and power electronic converters are more widely used in various application scenarios such as power generation and energy storage, which makes the new power system present the characteristics of a high proportion of renewable energy and a high proportion of power electronic equipment. On the one hand, the characteristics of distributed energy make the influence of line impedance more and more obvious, the power grid becomes weaker and the strength varies in a wide range, showing a variety of short-circuit ratio characteristics, which easily cause stability problems; on the other hand, the large-scale introduction of nonlinear loads such as power electronic equipment will lead to power quality problems such as voltage and current distortion. Under the above-mentioned conditions of large changes in power grid strength and uncertain distortion, traditional harmonic suppression methods are difficult to achieve stable and flexible harmonic control goals, resulting in harmonic oscillations and deterioration of power supply quality of the power grid. Summary of the invention

[0003] The purpose of the present invention is to provide a harmonic suppression method that is adaptable to a wide range of grid strengths, so that a power electronic converter can stably achieve harmonic suppression when the grid voltage is distorted and the grid strength varies over a wide range, and can achieve flexible harmonic control targets according to the grid distortion and grid strength, while optimizing the harmonic voltage while controlling the harmonic current.

[0004] The present invention provides a harmonic suppression method that is adaptable to a wide range of power grid strengths. The method adds the extraction of harmonic current and harmonic voltage to the control link of a power electronic converter, and uses a harmonic current controller to control the harmonic current. The output signal of the harmonic current controller is adjusted by a cross-coupling modulation method. The parameters of the harmonic current controller are set differently according to different targets. When full harmonic current suppression is adopted, the control parameters of the harmonic current controller adopt pre-designed parameters. When the harmonic voltage exceeds the harmonic voltage threshold, the control parameters of the harmonic current controller are adjusted by a harmonic voltage amplitude controller. Both the harmonic current controller and the harmonic voltage amplitude controller adopt PI controllers.

[0005] Specifically, the control part may include: fundamental wave control module, harmonic current extraction module, harmonic current controller, cross-coupling modulation module, harmonic voltage extraction module, amplitude calculation module, harmonic voltage amplitude controller, threshold comparison module, harmonic current controller parameter adjustment module, inverse Park transformation module, modulation and drive module; sampling output voltage v abc and the output current i abc Input to the fundamental wave control module to generate fundamental wave modulation voltage v1_pwm ; Sample output current i abc Input to the harmonic current extraction module to obtain the d-axis and q-axis harmonic current i dh and i qh ; Harmonic current i dh and i qh Input to the harmonic current controller to get the PI controller output v do and v qo ;PI controller output v do and v qo Input to the cross-coupling modulation module to generate the harmonic modulation voltage v of the synchronous rotating coordinate system mdh and v mqh ; Harmonic modulation voltage v of synchronous rotating coordinate system mdh and v mqh After being input into the inverse Park transformation module, the harmonic modulation voltage v of the stationary coordinate system is generated. h_pwm ; Harmonic modulation voltage v in the stationary coordinate system h_pwm With the fundamental modulation voltage v 1_pwm After phase superposition, it is input to the modulation and drive module to generate a PWM signal to drive the power switch tube of the power electronic converter to operate; the output voltage v abc After inputting into the harmonic voltage extraction module, the d-axis and q-axis harmonic voltages v dh and v qh ; Harmonic voltage v dh and v qh Input to the amplitude calculation module to get the harmonic voltage amplitude v h ; Harmonic voltage amplitude v h On the one hand, it is input into the harmonic voltage amplitude controller, and its output is used as the proportional parameter k obtained by regulation. ph_ctrl On the other hand, the harmonic voltage amplitude v h and harmonic voltage amplitude threshold v h_th The control parameters of the harmonic current controller are adjusted by the harmonic current controller parameter adjustment module according to the comparison result of the threshold comparison module, and the control parameters are adjusted to the proportional parameter k obtained by the adjustment. ph_ctrl , integration parameter k ih_ctrl Or is it a pre-set scale parameter k ph_designed , integration parameter k ih_designed .

[0006] In the above control scheme, the cross-coupling modulation module is implemented as follows:

[0007]

[0008] in, h is the positive and negative sequence harmonic order (h = -5, +7, -11, +13...).

[0009] The harmonic voltage amplitude controller uses a PI controller to set the harmonic voltage amplitude reference v h_ref Tracking, and the output of the PI controller is used as k ph_ctrl , set a low threshold and a high threshold for it, when the output is lower than the low threshold, k ph_ctrl is clamped to the low threshold, and when the output is above the high threshold, k ph_ctrl is clamped to the high threshold.

[0010] The integration parameter k ih_ctrl Can be directly controlled as:

[0011] k ih_ctrl =0

[0012] The harmonic current controller parameter adjustment module is regulated according to the judgment of the threshold comparison module, specifically:

[0013] When v h ≤v h_th When the ratio parameter k is set ph_designed , integration parameter k ih_designed As a control parameter of the harmonic current controller;

[0014] When v h >v h_th When the proportional parameter k is adjusted ph_ctrl , integration parameter k ih_ctrl As the control parameter of the harmonic current controller.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] By adopting the control method of the present invention, the power electronic converter can stably suppress harmonic current under different distortion grids and different grid strengths, and can optimize the control of harmonic voltage when the harmonic voltage exceeds the standard, achieve flexible control targets, and solve the problems of unstable harmonic control and deterioration of harmonic voltage caused by complex grid conditions. At the same time, the method will not affect the normal operation of fundamental wave control and the stability of low-frequency synchronization. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 A schematic diagram of the control method proposed by the present invention;

[0018] Figure 2 A circuit connection for an embodiment of the present invention;

[0019] Figure 3 This is a fundamental wave control example of an embodiment of the present invention;

[0020] Figure 4A method for extracting harmonic voltage and harmonic current according to an embodiment of the present invention;

[0021] Figure 5 To compare the effect of not using this method and using this method when the power grid is strong;

[0022] Figure 6 The following is a comparison chart of the regulation effects of the traditional method and the method used in a weak power grid. DETAILED DESCRIPTION

[0023] The present invention is described in detail below in conjunction with specific embodiments and accompanying drawings. The following embodiments will help those skilled in the art to further understand the invention, but are not intended to limit the present invention in any form. It should be noted that, without departing from the concept of the present invention, the modifications or improvements made are within the scope of protection of the present invention.

[0024] Reference Figure 2 A circuit of a specific embodiment includes a DC side battery to supply power to the DC side of the inverter, the inverter realizes the power conversion between DC and AC, and the filter inductor L f With filter capacitor C f To filter the high frequency signal, the line impedance L g With r g Connected to the power grid. The sampled quantity is the output current i abc and the output voltage v abc The method of the present invention is mainly used to control the power switch tube in the inverter, and the flexible suppression of harmonics is achieved by controlling the opening and closing of the power switch tube.

[0025] For the above typical embodiment system, the following is adopted Figure 1 The control method shown in the figure includes: fundamental wave control module, harmonic current extraction module, harmonic current controller, cross-coupling modulation module, harmonic voltage extraction module, amplitude calculation module, harmonic voltage amplitude controller, threshold comparison module, harmonic current controller parameter adjustment module, inverse Park transformation module, modulation and driving module; sampling output voltage v abc and the output current i abc Input to the fundamental wave control module to generate fundamental wave modulation voltage v 1_pwm ; Sample output current i abc Input to the harmonic current extraction module to obtain the d-axis and q-axis harmonic current i dh and i qh ; Harmonic current i dh and i qh Input to the harmonic current controller to get the PI controller output v do and v qo ;PI controller output v do and v qoInput to the cross-coupling modulation module to generate the harmonic modulation voltage v of the synchronous rotating coordinate system mdh and v mqh ; Harmonic modulation voltage v of synchronous rotating coordinate system mdh and v mqh After being input into the inverse Park transformation module, the harmonic modulation voltage v of the stationary coordinate system is generated. h_pwm ; Harmonic modulation voltage v in the stationary coordinate system h_pwm With the fundamental modulation voltage v 1_pwm After phase superposition, it is input to the modulation and drive module to generate a PWM signal to drive the power switch tube of the power electronic converter to operate; the output voltage v abc After inputting into the harmonic voltage extraction module, the d-axis and q-axis harmonic voltages v dh and v qh ; Harmonic voltage v dh and v qh Input to the amplitude calculation module to get the harmonic voltage amplitude v h ; Harmonic voltage amplitude v h On the one hand, it is input to the harmonic voltage amplitude controller, and its output is used as the PI parameter k of the harmonic current controller obtained by regulation. ph_ctrl On the other hand, the harmonic voltage amplitude v h and harmonic voltage amplitude threshold v h_th Input them together into the threshold comparison module to obtain the switch signal S for controlling Figure 1 The switches S1 and S2 in the circuit are used to control the contact points of the switches S1 and S2, thereby adjusting the proportional parameter k of the harmonic current controller PI. ph_ctrl , Harmonic current controller PI integral parameter k ih_ctrl Or the pre-designed harmonic current controller PI proportional parameter k ph_designed , Harmonic current controller PI integral parameter k ih_designed The harmonic current controller parameter adjustment module is used as the final harmonic current controller PI parameter k ph and k ih .

[0026] In the above scheme, the fundamental wave control module, harmonic current extraction module, harmonic voltage extraction module, amplitude calculation module, inverse Park transformation module, modulation and driving module are respectively used to realize their corresponding functions, and any existing method can be used, for example:

[0027] Reference Figure 3, is a typical fundamental control scheme. The main goal of fundamental control is to maintain the stable operation of the inverter fundamental power control under different grid strengths. This method can also refer to the document entitled "A Universal Controller for Grid-Connected Voltage-Source Converters" published by Lennart Harnefors et al. in "IEEE Journal of Emerging and Selected Topics in Power Electronics", Issue 9, Volume 5, Pages 5761-5770. In addition to the basic control link of traditional droop grid control, the synchronization link of the phase-locked loop PLL and the reference value feedforward link are added. P *, and the coupling integration link K from the d-axis to the q-axis v / s. This fundamental wave controls the final output modulation voltage v 1_pwm and Figure 1 It should be noted that the fundamental wave control scheme is not limited to the one in this specific embodiment, and may be other control methods that can maintain stability under different power grid strengths.

[0028] Reference Figure 4 , is a feasible Figure 1 The specific implementation method of extracting harmonic voltage and harmonic current. It converts the component v of the fundamental wave synchronously rotating coordinate system through the low-pass filter LPF d and v q Perform a rotation transformation with an angle of θ and compare the result with the sampled output voltage v a 、v b 、v c The harmonic component is obtained by subtraction, and then the harmonic component v can be obtained after the rotation transformation of hθ and the low-pass filter LPF. dh and v qh It should be noted that the extraction of harmonic voltage and harmonic current is not limited to the method of the specific embodiment, and may be other solutions to achieve the same goal. The other modules mentioned above may also be implemented by existing methods, which will not be described in detail here.

[0029] The cross-coupling modulation module is implemented as follows:

[0030]

[0031] in, h is the positive and negative sequence harmonic order (h = -5, +7, -11, +13...).

[0032] Optionally, the harmonic voltage amplitude can be calculated using the following formula:

[0033]

[0034] The harmonic voltage amplitude controller uses a PI controller to set the harmonic voltage amplitude reference v h_ref is tracked, and the output of the PI controller is used as k ph_ctrl , set a low threshold and a high threshold, where the low threshold should not be less than 0. The low threshold and the high threshold can be set according to specific needs, and the high and low are relative to each other; when the output is lower than the low threshold, k ph_ctrl is clamped to the low threshold, and when the output is above the high threshold, k ph_ctrl is clamped to the high threshold.

[0035] In addition, during the control process, the harmonic current controller PI parameter k ih_ctrl satisfy

[0036] k ih_ctrl =0.

[0037] Harmonic voltage amplitude threshold v h_th The settings can be set according to specific needs. Figure 1 In the example, the judgment logic of the threshold comparison module is:

[0038] When v h ≤v h_th When the threshold comparison module outputs S=1, the switches S1 and S2 of the harmonic current controller parameter adjustment module are both in the contact "1" position; that is, the preset proportional parameter k ph_designed , integration parameter k ih_designed As a control parameter of the harmonic current controller;

[0039] When v h >v h_th When the threshold comparison module outputs S=2, the switches S1 and S2 of the harmonic current controller parameter adjustment module are both in the contact "2" position, that is, the proportional parameter k obtained by adjustment is ph_ctrl , integration parameter k ih_ctrl As the control parameter of the harmonic current controller.

[0040] Among them, the scale parameter k ph_designed , integration parameter k ih_designed The settings can be done as follows:

[0041] When the target grid-side inductance is set to the minimum value, the crossover frequency is set to 5 Hz, and the phase margin is 90°. The proportional parameter k can be calculated by combining the following system transfer function: ph_designed , integration parameter k ih_designed :

[0042]

[0043] in,

[0044] G g (s)=sL g +r g , G d (s) = e -1.5sTs ,

[0045] G Lf (s)=sL f +r f , G Cf (s)=sC f , G hωLg (s)=hω0L g , G hωLf (s)=hω0L f ,

[0046] ω h is the cutoff frequency of the low-pass filter LPF, L g is the equivalent inductance of the power grid, r g is the equivalent resistance of the power grid, T s is the sampling period, L f is the filter inductance value, r f is the equivalent series resistance of the filter inductor, C f is the capacitance of the filter capacitor, ω0 is the fundamental angular frequency, and s is the complex frequency in the Laplace transform.

[0047] The empirical effects of the present invention are as follows Figure 5 and Figure 6 shown. Figure 5 This is a comparison chart of the effect of not using this method and using this method under strong power grid (active power is 30kW, grid-side inductance is 0), the figure includes the A phase voltage v ca , A phase current i oa , d-axis 5th harmonic current i d5 and q-axis 5th harmonic current i q5 It can be seen that after adopting the method of the present invention, the current THD is significantly reduced. Figure 6 When the power grid is weak (active power is 30kW, grid-side inductance is 2.1mH), the effect comparison diagram of using the traditional method and the method of the present invention is shown. It can be seen that the dynamic adjustment process of the d-axis and q-axis currents produces low-frequency oscillations and cannot be controlled to 0 when the traditional method is used, while the harmonic current can be stably and quickly controlled to 0 when the method of the present invention is used.

[0048] The embodiments of the present application have been shown and described above. It can be understood that the above embodiments are illustrative and the protection scope of the present invention is not limited thereto. Any changes or modifications that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included in the protection scope of the present invention.

Claims

1. A harmonic suppression method adapted to a wide range of power grid strengths, characterized in that: The extraction of harmonic current and harmonic voltage is added to the control link of the power electronic converter, and a harmonic current controller is used to control the harmonic current. The output signal of the harmonic current controller is adjusted by a cross-coupling modulation method. The parameters of the harmonic current controller are set differently according to different targets. When complete harmonic current suppression is adopted, the control parameters of the harmonic current controller adopt pre-designed parameters. When the harmonic voltage exceeds the harmonic voltage threshold, the control parameters of the harmonic current controller are adjusted by a harmonic voltage amplitude controller. Both the harmonic current controller and the harmonic voltage amplitude controller adopt PI controllers.

2. The harmonic suppression method adapted to a wide range of power grid strength according to claim 1, characterized in that: The control link in the method comprises: a fundamental wave control module (101), a harmonic current extraction module (102), a harmonic current controller (103), a cross-coupling modulation module (104), a harmonic voltage extraction module (105), an amplitude calculation module (106), a harmonic voltage amplitude controller (107), a threshold comparison module (108), a harmonic current controller parameter adjustment module (109), an inverse Park transformation module (110), and a modulation and driving module (111); Sample output voltage v abc and the output current i abc Input to the fundamental wave control module (101) to generate the fundamental wave modulation voltage v 1_pwm ; Sample output current i abc Input to the harmonic current extraction module (102) to obtain the d-axis and q-axis harmonic currents i dh and i qh ; Harmonic current i dh and i qh Input to the harmonic current controller (103) to obtain the PI controller output v do and v qo ;PI controller output v do and v qo Input to the cross-coupling modulation module (104) to generate a harmonic modulation voltage v of the synchronous rotating coordinate system mdh and v mqh ; Harmonic modulation voltage v of synchronous rotating coordinate system mdh and v mqh After being input into the inverse Park transformation module (110), a harmonic modulation voltage v of the stationary coordinate system is generated. h_pwm ; Harmonic modulation voltage v in the stationary coordinate system h_pwm With the fundamental modulation voltage v 1_pwm After being superimposed, the phases are input to the modulation and driving module (111) to generate a PWM signal to drive the power switch tube of the power electronic converter to operate; the output voltage v abc After being input into the harmonic voltage extraction module (105), the d-axis and q-axis harmonic voltages v dh and v qh ; Harmonic voltage v dh and v qh Input to the amplitude calculation module (106) to obtain the harmonic voltage amplitude v h ; Harmonic voltage amplitude v h On the one hand, it is input to the harmonic voltage amplitude controller (107), and its output is used as the proportional parameter k obtained by regulation. ph_ctrl On the other hand, the harmonic voltage amplitude v h and harmonic voltage amplitude threshold v h_th The control parameters of the harmonic current controller are adjusted by the harmonic current controller parameter adjustment module (109) according to the comparison result of the threshold value comparison module (108), and the control parameters of the harmonic current controller are adjusted to the proportional parameter k obtained by the adjustment. ph_ctrl , integration parameter k ih_ctrl Or is it a pre-set scale parameter k ph_designed , integration parameter k ih_designed .

3. A harmonic suppression method adapted to a wide range of power grid strength according to claim 2, characterized in that: The cross-coupling modulation module (104) is implemented as follows: in, h is the number of positive and negative sequence harmonics, h = -5, +7, -11, +13...

4. A harmonic suppression method adapted to a wide range of power grid strength according to claim 2, characterized in that: The harmonic voltage amplitude controller (107) uses a PI controller to set the harmonic voltage amplitude reference v h_ref is tracked, and the output of the PI controller is used as k ph_ctrl , set a low threshold and a high threshold for it, when the output is lower than the low threshold, k ph_ctrl is clamped to the low threshold, when the output is above the high threshold then k ph_ctrl is clamped to the high threshold.

5. A harmonic suppression method adapted to a wide range of power grid strength according to claim 2, characterized in that: The integral parameter k obtained by the regulation ih_ctrl The regulation is: k ih_ctrl ＝0。 6. A harmonic suppression method adapted to a wide range of power grid strength according to claim 2, characterized in that: The harmonic current controller parameter adjustment module (109) adjusts the control parameter of the harmonic current controller according to the comparison result of the threshold comparison module (108) to obtain the proportional parameter k obtained by adjustment. ph_ctrl , integration parameter k ih_ctrl Or is it a pre-set scale parameter k ph_designed , integration parameter k ih_designed , specifically: When v h ≤v h_th When the ratio parameter k is set ph_designed , integration parameter k ih_designed As a control parameter of the harmonic current controller; When v h >v h_th When the proportional parameter k is adjusted ph_ctrl , integration parameter k ih_ctrl As the control parameter of the harmonic current controller.

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