Fine anti-interference-based voltage fluctuation and harmonic suppression method for grid-forming inverter

Abstract

A fine anti-interference-based voltage fluctuation and harmonic suppression method for a network inverter belongs to the field of network inverter control, and adopts a voltage and current double-closed-loop control strategy, an outer loop is a voltage loop, an inner loop is a current loop, and the current loop adopts fine anti-interference control; firstly, a compensation function observer is designed, the anti-interference performance of a controller is improved, and the voltage fluctuation of a grid-connected point is reduced; secondly, a repetitive controller is designed, a zero-phase filter is arranged outside an inner ring of the repetitive controller, deviation of notch frequency is prevented, and therefore the harmonic suppression capacity is improved. The method can improve the voltage fluctuation and current harmonic problems of the grid-connected point of the power system at the same time, thereby improving the electric energy quality of the power system.

Description

Technical Field

[0001] The present invention belongs to the technical field of grid-connected control of grid-type inverters, and in particular relates to a method for suppressing voltage fluctuations and harmonics of grid-type inverters based on fine anti-interference. Background Art

[0002] New power systems often have low inertia and insufficient anti-interference capabilities due to a high proportion of power electronic equipment. New energy equipment based on power electronics is mostly distributed in areas with poor grid conditions. The accompanying large number of nonlinear loads will cause power quality problems such as current harmonics, thereby reducing power utilization efficiency. In addition, the load characteristics of new power systems are significantly different from traditional loads. The switching operation of loads will lead to increased voltage fluctuations at the grid connection point, thereby increasing the power loss of the grid.

[0003] The current loop of traditional inverters usually uses a linear PI controller. However, the new power system is a complex, strongly coupled and variable nonlinear system. Traditional PI control cannot effectively solve the problems of voltage fluctuations and current harmonics. It is necessary to introduce more advanced control strategies to improve the power quality of the new power system. Summary of the invention

[0004] In order to overcome the shortcomings of the prior art, the present invention proposes a method for suppressing voltage fluctuations and harmonics of a grid-connected inverter based on fine anti-interference, which is used for current loop control in a grid-connected inverter grid-connected system, aiming to take into account both dynamic performance and anti-interference capability, effectively suppress voltage fluctuations at the grid-connected point, and improve the current harmonic problem on the grid side. An extended state observer with a compensation function is designed to eliminate the effects of non-periodic interference and internal parameter uncertainty; at the same time, a repetitive controller based on the internal model principle is designed, which can reduce phase delay by moving the zero-phase low-pass filter to the outside of the inner loop, thereby improving the ability to suppress harmonics.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions: A method for suppressing voltage fluctuations and harmonics of grid-connected inverters based on fine anti-interference is proposed. An extended state observer with a compensation function is designed to eliminate the influence of non-periodic interference and internal parameter uncertainty. At the same time, a repetitive controller based on the internal model principle is designed. By moving the zero-phase low-pass filter to the outside of the inner loop, the phase delay can be reduced, thereby improving the harmonic suppression capability.

[0006] Further, the method comprises the following steps: 1) Real-time acquisition of inverter output voltage V abc , current i abc , and the inverter side current i fabcAfter the coordinate transformation from the three-phase stationary coordinate system to the two-phase rotating coordinate system, the actual output active and reactive power are obtained through power calculation, and input into the power loop together with the active and reactive power command values ​​to generate the amplitude V ref and phase angle θ; 2) With voltage reference value V ref The actual value of the voltage is used as the input signal of the voltage loop, and its output signal is the reference value of the current dq axis component; 3) Take the current d or q axis component as reference value i dref or qref The actual current d or q axis component is used as the input signal of the current loop fine anti-interference, and its output signal is the voltage d or q axis component reference value or ; 4) and After the two-phase rotating coordinate system is converted to the three-phase stationary coordinate system, the PWM signal is used to control the switching of the IGBT.

[0007] Furthermore, in 3), the current loop design process based on fine anti-interference control is described as follows: 3.1) First, an observer based on compensation function is designed to solve the voltage fluctuation problem at the grid connection point. The expression of the observer with compensation function is: (1); Where z1 is the estimated value of the current integral; z2 is the current i d or q The estimated value of; h1 and h2 are the observer gains; e1 and e2 are the current integral and the estimated error of the current respectively; the control gain b0 is the inverse of the inductance value; u is the error feedback control rate of the controller; is the estimated value of the total disturbance D; is the compensation function introduced; due to the estimated value With compensation function The goal is to approximate the total disturbance D and use a low-pass filter to link the estimated value to the compensation function. is the filtering factor, f c is the cutoff frequency of the filter; The error feedback control rate u of the controller is designed as, (2); Among them, r is the reference signal of current, K p-EDA is the proportional gain; 3.2) The transfer function of the designed current loop zero-phase repetitive controller is expressed as, (3); Among them, k r is the repetitive controller gain, F(z) is the compensator of the controlled object, Q(z) is the low-pass filter, and N is f s / f r , N is the number of sampling points per cycle, where f r is the fundamental frequency, and f s is the sampling frequency; The expression of the zero-phase low-pass filter Q(z) is (4); Among them, a and b need to satisfy 2a < b, where both a and b are positive numbers; The compensating function observer designed in 3.1) is combined with the zero-phase repetitive control designed in 3.2) in a parallel manner and applied to the voltage fluctuation and harmonic suppression of the grid-connected inverter; 3.3) Design a refined anti-interference controller. For the d-axis or q-axis current, the transfer function from the disturbance to the output current is (5); Among them, P(z) is the transfer function of the inverter.

[0008] The beneficial effects of the present invention are as follows: It can simultaneously improve the voltage fluctuation and current harmonic problems at the grid connection point of the power system, thereby improving the power quality of the power system. Description of the Drawings

[0009] Figure 1 This is a structural diagram of a grid-connected inverter grid-connected system based on a refined anti-interference controller provided by an embodiment of the present invention.

[0010] Figure 2 This is a current controller block diagram of a linear observer based on a compensation function.

[0011] Figure 3 This is a block diagram of a current controller based on zero-phase repetitive control.

[0012] Figure 4 This is a block diagram of a current controller based on refined anti-interference.

[0013] Figure 5 This is a comparison diagram of the voltage fluctuation suppression effects of refined anti-interference and active disturbance rejection control under sudden load addition.

[0014] Figure 6 This is the effect diagram of grid connection point current harmonic suppression under refined anti-interference control rectification. Detailed Embodiments

[0015] The present invention will be described in detail below with reference to the drawings.

[0016] Figure 1The structure diagram of the grid-connected inverter system based on the fine anti-interference controller is shown in Figure 1. The system mainly consists of three-phase grid-connected inverter, LC filter and control structure. Among them, the control structure includes active frequency control loop, reactive voltage control loop and voltage and current double closed-loop control.

[0017] Reference Figure 1 to Figure 6 A method for suppressing voltage fluctuation and harmonics of a grid-connected inverter based on fine anti-interference includes the following steps: 1) Real-time acquisition of inverter output voltage V abc , current i abc , and the inverter side current i fabc After the coordinate transformation from the three-phase stationary coordinate system to the two-phase rotating coordinate system, the actual output active and reactive power are obtained through power calculation, and input into the power loop together with the active and reactive power command values ​​to generate the amplitude V ref and phase angle θ; 2) With voltage reference value V ref The actual value of the voltage is used as the input signal of the voltage loop, and its output signal is the reference value of the current dq axis component; 3) Take the current d or q axis component as reference value i dref or qref The actual current d or q axis component is used as the input signal of the current loop fine anti-interference, and its output signal is the voltage d or q axis component reference value or ; That is, when the current d-axis component reference value i dref The actual current d-axis component is used as the input signal of the current loop fine anti-interference, and its output signal is the voltage d-axis component reference value ; When the current q-axis component reference value i qref The actual current q-axis component is used as the input signal of the current loop fine anti-interference, and its output signal is the voltage q-axis component reference value ; In 3), the current loop design process based on fine anti-interference control is as follows: 3.1) First, an observer based on compensation function is designed and applied to the current loop. According to Kirchhoff's law, the current loop equation of the LC filter is expressed as follows: (6); Where V dq is the inverter side voltage V abc The dq axis component of V g(dq) is the dq axis component of the grid-connected point voltage; d is the total disturbance of the system; the control gain b0 is the inverse of the inductance value; the traditional linear observer is a non-derivative form. When the integral of the current has a certain observation error, it will further cause the current i d orq The observation accuracy of the sum total disturbance D decreases. Therefore, a pure integral type observer is designed to solve this problem. (7); Among them, z1 is the estimated value of the current integral, and z2 is the estimated value of the current i d or i q ; h1 and h2 are the observer gains. From Equation (7), we get (8); Among them, is the estimated value of D. The observation error is obtained by subtracting Equation (7) from Equation (6). (9); When is sufficiently close to D, the estimation error of the observer will decrease significantly. Since the estimated value and the compensation function both aim to approximate the total disturbance D, a filter is used to connect the estimated value and the compensation function. In the filter, is the filtering factor, and f c is the cut-off frequency of the filter. (1); The error feedback control rate u of the controller is designed as (2); Among them, r is the reference signal of the current, and K p-EDA is the proportional gain; The block diagram of the current controller based on the compensation function linear observer is as Figure 2 shown; 3.2) The transfer function of the designed current loop zero-phase repetitive controller is expressed as (3); Among them, k r is the repetitive controller gain, F(z) is the compensator of the controlled object, Q(z) is the low-pass filter, and N is f s / f r , N is the number of sampling points per cycle, where f r is the fundamental frequency, and f s is the sampling frequency; The expression of the zero-phase low-pass filter Q(z) is (4); Among them, a and b need to satisfy 2a < b, where both a and b are positive numbers. The block diagram of the current controller based on zero-phase repetitive control is as Figure 3 shown; The compensation function observer designed in 3.1) is combined with the zero-phase repetitive control designed in 3.2) in parallel and applied to the voltage fluctuation and harmonic suppression of the grid inverter. 3.3) The block diagram of the current controller based on fine anti-interference is as follows Figure 4 As shown in this figure, the transfer function from disturbance to output current is, (5); Where P(z) is the transfer function of the inverter.

[0018] The tracking error of the fine anti-interference control system is expressed as, (10); Therefore, the characteristic equation of the system is, (11); Therefore, if the control system based on fine anti-interference is to be stable, the following two conditions must be met: (Condition 1) The roots of are inside the unit circle; (Condition 2) .

[0019] Combining equations (1) and (2), the transfer function describing the system output and disturbance is: (12); For the system to be stable, all roots of the characteristic equation must be located in the left half plane of the complex plane. According to equation (12), we have: (13); According to the pole placement method, the roots of the characteristic equation are -3ω c , -12ω c , -12ω c , as long as ω is satisfied c >0, all roots of the characteristic equation are located in the left half plane of the complex plane; , , , so condition 1 can be solved by choosing a suitable ω c and k p To meet; Condition 2 can be met by selecting a suitable low-pass filter Q(z) and phase compensator C(z); 4) and After the two-phase rotating coordinate system is converted to the three-phase stationary coordinate system, the PWM signal is used to control the switching of the IGBT.

[0020] In this embodiment, an IEEE three-machine nine-node system model is established on the RT1000 real-time simulation platform to verify the effectiveness of the proposed fine anti-interference control strategy. The system consists of a synchronous generator with a rated power of 100MVA and two grid-connected inverters with a rated capacity of 100MVA. Under the condition of sudden load interference, the control algorithm is tested to improve the voltage fluctuation and current harmonics at the grid connection point. Figure 5 The comparison of the voltage fluctuation suppression performance of the grid connection point by the self-disturbance rejection control and the fine anti-disturbance control under the sudden load disturbance is demonstrated; the results show that the fine anti-disturbance control method can significantly reduce the voltage fluctuation and improve the tracking performance of the output power. In order to further verify the ability of the fine anti-disturbance control strategy in suppressing the current harmonics of the grid connection point, the 5th, 7th, 11th and 13th harmonics were injected into the simulation system. The simulation results are shown in Figure 2. Figure 6 Compared with the traditional control method, the proposed fine anti-interference control scheme significantly enhances the suppression effect of current harmonics and voltage fluctuations, and shows excellent dynamic response characteristics and strong robustness.

[0021] The contents described in the embodiments of this specification are merely enumerations of implementation forms of the inventive concept and are for illustrative purposes only. The protection scope of the present invention should not be considered to be limited to the specific forms described in this embodiment, and the protection scope of the present invention also extends to equivalent technical means that can be thought of by ordinary technicians in this field based on the inventive concept.

Claims

1. A method for suppressing voltage fluctuation and harmonics of grid-connected inverters based on fine anti-interference, characterized in that: An extended state observer with a compensation function is designed to eliminate the influence of non-periodic disturbances and internal parameter uncertainties; meanwhile, a repetitive controller based on the internal model principle is designed. By moving the zero-phase low-pass filter to the outside of the inner loop, the phase delay can be reduced, thereby improving the ability of harmonic suppression.

2. The method for suppressing voltage fluctuation and harmonics of grid-connected inverters based on fine anti-interference according to claim 1, characterized in that: The method includes the following steps: 1) Real-time acquisition of inverter output voltage V abc , current i abc , and the inverter side current i fabc After the coordinate transformation from the three-phase stationary coordinate system to the two-phase rotating coordinate system, the actual output active and reactive power are obtained through power calculation, and input into the power loop together with the active and reactive power command values ​​to generate the amplitude V ref and phase angle θ; 2) With voltage reference value V ref The actual value of the voltage is used as the input signal of the voltage loop, and its output signal is the reference value of the current dq axis component; 3) Take the current d or q axis component as reference value i dref or qref The actual current d or q axis component is used as the input signal of the current loop fine anti-interference, and its output signal is the voltage d or q axis component reference value or ; 4) and After the two-phase rotating coordinate system is converted to the three-phase stationary coordinate system, the PWM signal is used to control the switching of the IGBT.

3. The method for suppressing voltage fluctuation and harmonics of grid-connected inverters based on fine anti-interference according to claim 2, characterized in that: In step 3), the design process of the current loop based on the refined anti-disturbance control is described as follows: 3.1) First, an observer based on a compensation function is designed. The expression of the observer with the compensation function is （1）； Where z1 is the estimated value of the current integral; z2 is the current i d or q The estimated value of; h1 and h2 are the observer gains; e1 and e2 are the current integral and the estimated error of the current respectively; the control gain b0 is the inverse of the inductance value; u is the error feedback control rate of the controller; is the estimated value of the total disturbance D; is the compensation function introduced; due to the estimated value With compensation function The goal is to approximate the total disturbance D and use a low-pass filter to link the estimated value to the compensation function. is the filtering factor, f c is the cutoff frequency of the filter; The error feedback control law u of the controller is designed as （2）； Among them, r is the reference signal of current, K p-EDA is the proportional gain; 3.2) The transfer function of the designed zero-phase repetitive controller of the current loop is expressed as （3）； Among them, k r is the repetitive controller gain, F(z) is the controlled object compensator, Q(z) is the low-pass filter, and N is f s / f r , N is the number of sampling points per cycle, where f r is the fundamental frequency, f s is the sampling frequency; The expression of the zero-phase low-pass filter Q(z) is （4）； Among them, a and b need to satisfy 2a < b, where both a and b are positive numbers; The compensation function observer designed in 3.1) is combined with the zero-phase repetitive control designed in 3.2) in a parallel manner and applied to the voltage fluctuation and harmonic suppression of the grid-connected inverter; 3.3) Design a refined anti-disturbance controller. For the d-axis or q-axis current, the transfer function from the disturbance to the output current is （5）； Among them, P(z) is the transfer function of the inverter.

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