Active filter control method for suppressing low-frequency interharmonics and flicker

Abstract

The present invention proposes an active filter control method for suppressing low-frequency interharmonics and flicker. First, the quantitative conversion relationship between low-frequency interharmonics and voltage fluctuations is analyzed to determine the suppression interval frequency of the interharmonics. A low-pass filter is designed with the upper limit frequency of the interval as the cutoff frequency. Then, the reactive component and interharmonic component of the current signal to be compensated are detected by an instantaneous reactive power method based on the IP-IQ method. Subsequently, the interharmonic component is processed using a low-pass filter to generate a target current for the interharmonic compensation link of the active filter. Then, the target voltage of the dynamic reactive compensation link of the active filter is obtained through PI control, and the target voltage of the interharmonic compensation link is obtained through quasi-proportional resonant control. Finally, the target voltage signals of the two compensation links are superimposed to form a control target signal for the active filter.

Description

Technical Field

[0001] The present invention relates to the technical field of power quality, and in particular to an active filter control method for suppressing low-frequency interharmonics and flicker. Background Art

[0002] With the increasing capacity and number of impact loads such as large scientific facilities and high-power arc furnaces, as well as fluctuating loads such as distributed photovoltaic and wind power generation, connected to the power grid, the interharmonics, voltage fluctuations, and flicker generated by these loads are increasingly impacting grid power quality. Interharmonics not only cause similar hazards as harmonics, such as eddy current losses, hysteresis losses, and resonant overvoltages, disrupting the proper functioning of metering instruments and interfering with control, protection, and communication signals on power lines, but also cause voltage fluctuations, low-frequency oscillations, and impact the stability of power electronics. Voltage fluctuations and flicker can affect the proper functioning of equipment such as motors, electronic instruments, and automatic control devices, and can cause malfunctions in household appliances such as televisions and fluorescent lamps, impacting both business operations and daily life.

[0003] In the research on interharmonics, voltage fluctuations, and flicker suppression technologies in power systems, only a few experts and scholars have conducted research and analysis on the interaction mechanism between interharmonics and voltage fluctuations. However, existing research rarely involves the management of interharmonics and flicker. There are no mature analysis methods or replicable management cases for interharmonic and flicker suppression technologies. Therefore, it is necessary to develop a method that can effectively suppress interharmonics and flicker in power systems, promote the upgrading of related power quality management equipment industry, and provide technical support for the safe and reliable operation of power systems. Summary of the Invention

[0004] In order to fill the deficiencies and gaps in existing interharmonic and flicker control devices, promote the upgrading of related power quality control equipment industry, and support the safe and reliable operation of the power system, the present invention provides an active filter control method for suppressing low-frequency interharmonics and flicker, and provides an effective solution to the flicker control problem of impact loads such as AC arc furnaces, filling the deficiencies and gaps in existing flicker control technology, and providing technical support for the safe and reliable operation of the power system.

[0005] Firstly, the quantitative conversion relationship between low-frequency interharmonics and voltage fluctuations is analyzed to determine the suppression interval frequency of interharmonics, and a low-pass filter is designed with the upper limit frequency of the interval as the cut-off frequency; then, the low-pass filter is designed based on i p -i qThe instantaneous reactive power method of the method is used to detect the reactive component and interharmonic component of the current signal to be compensated; the interharmonic component is then processed by a low-pass filter to generate the target current of the interharmonic compensation link of the active filter; the target voltage of the dynamic reactive compensation link of the active filter is then obtained through PI control, and the target voltage of the interharmonic compensation link is obtained through quasi-proportional resonant control; finally, the target voltage signals of the two compensation links are superimposed to form the control target signal of the active filter.

[0006] The invention has novel principles, reliable technology and versatility. It can be used as a control strategy for active filters, while compensating for low-frequency interharmonics and dynamic reactive power, thereby suppressing flicker. It can provide an effective solution to the interharmonic and flicker control problems of impact loads such as AC arc furnaces and fluctuating loads such as distributed photovoltaics, and provide technical support for the safe and reliable operation of power systems.

[0007] The present invention specifically adopts the following technical solutions:

[0008] A method for controlling an active filter for suppressing low-frequency interharmonics and flicker, characterized by comprising the following steps:

[0009] Step S1: Analyze the quantitative conversion relationship between low-frequency interharmonics and voltage fluctuations to determine the suppression interval frequency of interharmonics;

[0010] Step S2: Designing a low-pass filter according to the upper limit frequency of the interharmonic suppression interval determined in step S1;

[0011] Step S3: detecting the reactive component and interharmonic component of the current signal to be compensated by using the instantaneous reactive power method based on the ip-iq method;

[0012] Step S4: using the low-pass filter designed in step S2 to process the interharmonic components obtained in step S3 to generate a target current for the interharmonic compensation link of the active filter;

[0013] Step S5: obtaining the target voltage of the dynamic reactive power compensation link of the active filter through PI control;

[0014] Step S6: obtaining the target voltage of the active filter interharmonic compensation link through quasi-proportional resonance control;

[0015] Step S7: Based on steps S5 and S6, the target voltages of the dynamic reactive power compensation link and the interharmonic compensation link are superimposed to form a control target signal of the active filter.

[0016] Furthermore, in step S1, first, the voltage fluctuation signal is decomposed into the superposition of a fundamental wave and a pair of interharmonic signals, and the modulation of the fundamental wave and the interharmonic signals is represented as the superposition of an assigned modulated AM signal and a phase modulated PM signal, that is, the part of the interharmonic that causes flicker is the superposition of amplitude modulation and phase modulation; then, the phase modulated PM signal is used as the target value for analysis to obtain the frequency parameters of the interharmonic signal that causes voltage fluctuations of different degrees; finally, the suppression interval frequency of the interharmonic is set with the interharmonics in the frequency band that has a more significant impact on the voltage fluctuation as the suppression object.

[0017] Furthermore, in step S2, a Butterworth low-pass filter is designed with the upper limit frequency of the interharmonic suppression interval as the cutoff frequency.

[0018] Further, in step S3, based on i p -i q The instantaneous reactive power method of the method is used to detect the current signal to be compensated, and the reactive component and fundamental frequency component in the signal are obtained. Then, the current signal to be compensated and its fundamental frequency component are subtracted to obtain the frequency components in the signal except the fundamental frequency, including the interharmonic components.

[0019] Furthermore, in step S4, the frequency component obtained in step S3 is processed by the Butterworth low-pass filter designed in step S2 to generate a target current for the interharmonic compensation link of the active filter.

[0020] Since the current mainstream active filters all adopt the voltage source converter structure, the control signal of the voltage source converter is a voltage signal, so it is also necessary to convert the target current signal in the compensation link into a voltage signal.

[0021] Furthermore, in step S6, the quasi-proportional resonant control is composed of a plurality of parallel-connected quasi-proportional resonant controllers, the control frequency interval of each controller is 5 Hz, and does not include fundamental and harmonic frequencies.

[0022] In step S7, flicker is caused by voltage fluctuation, which is strongly correlated with reactive power fluctuation and low-frequency interharmonics. Therefore, by compensating for interharmonics and dynamic reactive components in the signal, flicker can be effectively suppressed.

[0023] Compared with the existing active filter control strategy, the present invention and its preferred embodiment have the following technical advantages and innovations: The present invention provides an active filter control method for suppressing low-frequency interharmonics and flicker: first, the quantitative conversion relationship between low-frequency interharmonics and voltage fluctuations is analyzed to determine the suppression interval frequency of the interharmonics, and a low-pass filter is designed with the upper limit frequency of the interval as the cut-off frequency; then, the reactive component and interharmonic component of the current signal to be compensated are detected by the instantaneous reactive power method based on the ip-iq method; then, the interharmonic component is processed using a low-pass filter to generate the target current of the interharmonic compensation link of the active filter; then, the target voltage of the dynamic reactive compensation link of the active filter is obtained through PI control, and the target voltage of the interharmonic compensation link is obtained through quasi-proportional resonant control; finally, the target voltage signals of the two compensation links are superimposed to form the control target signal of the active filter. The present invention is versatile and can be used as a control strategy for active filters. It can simultaneously compensate for low-frequency interharmonics and dynamic reactive power, thereby suppressing interharmonics and flicker. It can provide a solution for the interharmonic and flicker control of impact loads such as AC arc furnaces and fluctuating loads such as distributed photovoltaics, and provide technical support for the safe and reliable operation of power systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments:

[0025] Figure 1 4 is a flow chart of the control strategy in an embodiment of the present invention.

[0026] Figure 2 Schematic diagram of the interaction between interharmonics and voltage fluctuations in an embodiment of the present invention.

[0027] Figure 3 Graph showing the amplitude-frequency characteristics of the Butterworth low-pass filter designed in an embodiment of the present invention.

[0028] Figure 4 In the embodiment of the present invention, based on i p -i q Schematic diagram of the instantaneous reactive power method.

[0029] Figure 5 Schematic diagram of quasi-proportional resonant control in an embodiment of the present invention. DETAILED DESCRIPTION

[0030] Hereinafter, specific embodiments of the present application will be described in detail with reference to the accompanying drawings. Based on these detailed descriptions, those skilled in the art will be able to clearly understand the present application and implement the present application. Without violating the principles of the present application, the features of different embodiments may be combined to obtain new implementations, or certain features of certain embodiments may be substituted to obtain other preferred implementations.

[0031] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0032] To make the features and advantages of this patent more clearly understood, the following embodiments are specifically described in detail as follows:

[0033] like Figure 1 : is a flow chart of the control strategy in an embodiment of the present invention, which consists of 7 steps.

[0034] like Figure 2 The figure shows the interaction between interharmonics and voltage fluctuations according to an embodiment of the present invention. In step S1, the voltage fluctuation signal can be expressed as the superposition of the fundamental wave and a pair of interharmonics, as shown in the following formula:

[0035]

[0036] The modulation of the fundamental wave and interharmonic signal can be expressed as the superposition of the value-modulated AM signal and the phase-modulated PM signal. That is, the part of the interharmonic that causes flicker is the superposition of amplitude modulation and phase modulation, as shown in the following formula:

[0037]

[0038] Low-frequency interharmonics have a greater impact on voltage fluctuations. As the interharmonic frequency increases, the impact of interharmonics on voltage fluctuations decreases significantly. The frequency band with the most significant impact on voltage fluctuations is concentrated near the fundamental frequency. Therefore, the interharmonic suppression range determined in this embodiment of the present invention is 0 to 100 Hz.

[0039] like Figure 3 The figure shows the amplitude-frequency characteristics of a Butterworth low-pass filter designed according to an embodiment of the present invention. In step S2, the Butterworth low-pass filter is designed with a cutoff frequency of 100 Hz, a bandwidth of 50 Hz, and a stopband attenuation of 80 dB. The designed low-pass filter has an attenuation of 80 dB at a frequency of 150 Hz, which is 10,000 times smaller than the original signal, ensuring that the third harmonic does not interfere with the control strategy.

[0040] like Figure 4 The embodiment of the present invention is shown based on i p -i q Schematic diagram of the instantaneous reactive power method, where i a 、i b 、i care the A, B, and C phase currents of the current signal to be compensated respectively; e a is the grid A phase voltage signal; PLL is a phase-locked loop used to track the grid fundamental frequency 50Hz; coordinate transformation matrices C and C 32 As shown in the following formula, C 23 C 32 The inverse matrix of .

[0041]

[0042]

[0043] In step S3, the three-phase positive sequence fundamental active current and reactive current can be obtained through the low-pass filter LPF and They become DC components on the dq axis. Pass through matrices C and C separately 23 By transforming , the reactive component in the current signal to be compensated can be obtained.

[0044] Will and At the same time, through the matrix C and C 23 By transforming the current signal to be compensated, the fundamental component of the current signal to be compensated can be obtained. Then, by vectorially subtracting the current signal to be compensated from its fundamental component, the frequency components other than the fundamental component of the current signal to be compensated can be obtained, which include harmonics and interharmonics.

[0045] In step S4, the Butterworth low-pass filter designed in step S2 is used to process the frequency components of the current signal to be compensated, excluding the fundamental component. This results in the signal's frequency components in the 0-100 Hz frequency range, excluding the fundamental component. This frequency component, which contains only interharmonic components in the 0-100 Hz frequency range, serves as the target current for the active filter's interharmonic compensation phase. Furthermore, since currently mainstream active filters all utilize a voltage source converter structure, the control signal of which is a voltage signal. Therefore, in subsequent steps, the target current signal in the compensation phase must be converted into a voltage signal.

[0046] In step S5, the target voltage of the dynamic reactive power compensation link of the active filter is obtained through PI control. The expression of the PI controller is shown as follows:

[0047] U q =k p (i ql -i qf )+k i ∫(i ql -i qf )dt

[0048] Among them, U q is the target voltage; i ql is the PI controller command current; i qf is the feedback current of the PI controller; k p is the proportional coefficient; k i is the integral coefficient. Through the PI controller, the reactive component can be converted into the target voltage of the reactive compensation link of the active filter.

[0049] like Figure 5 The figure shows a schematic diagram of quasi-proportional resonance control in an embodiment of the present invention, wherein U h is the target voltage; i hl is the command current of the quasi-proportional resonant controller; i hf is the feedback current of the quasi-proportional resonant controller; the transfer function of the quasi-proportional resonant controller is:

[0050]

[0051] Among them, k p is the proportional coefficient; k i is the integral coefficient; w c is the cutoff frequency; w0 is the fundamental angular frequency; n is the number of interharmonics.

[0052] In step S6, the quasi-proportional resonant control system consists of 18 parallel quasi-proportional resonant controllers. The first controller has a control frequency of 5 Hz, the second has a control frequency of 10 Hz, and each controller has a control frequency interval of 5 Hz, excluding the fundamental and harmonic frequencies. Through these 18 parallel quasi-proportional resonant controllers, the interharmonic components in the frequency range of 0 to 100 Hz (with a frequency resolution of 5 Hz) are converted into the target voltage for the active filter's interharmonic compensation link.

[0053] In step S7, the target voltages of the reactive power compensation link and the interharmonic compensation link obtained in steps S5 and S6 are superimposed to obtain the final control target signal of the active filter:

[0054]

[0055] By controlling the active filter to output the control target signal, the interharmonics and dynamic reactive power of the disturbance source can be compensated, thereby achieving the effect of suppressing flicker. This can provide an effective solution to the interharmonic and flicker control problems of impact loads such as AC arc furnaces and fluctuating loads such as distributed photovoltaics, and provide technical support for the safe and reliable operation of the power system.

[0056] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications are also considered to be within the scope of protection of the present invention.

[0057] This patent is not limited to the above-mentioned best implementation mode. Anyone can derive other forms of active filter control methods for suppressing low-frequency interharmonics and flicker based on the inspiration of this patent. All equivalent changes and modifications made within the scope of the patent application of this invention should fall within the scope of this patent.

Claims

1. A method for controlling an active filter for suppressing low-frequency interharmonics and flicker, characterized in that: The following steps are involved: Step S1: Analyze the quantitative conversion relationship between low-frequency interharmonics and voltage fluctuations to determine the suppression interval frequency of interharmonics: First, the voltage fluctuation signal is decomposed into the superposition of a fundamental wave and a pair of interharmonic signals. The modulation of the fundamental wave and interharmonic signals is represented by the superposition of an assigned modulated AM signal and a phase modulated PM signal. That is, the part of the interharmonic that causes flicker is the superposition of amplitude modulation and phase modulation. Then, the phase modulated PM signal is used as the target value for analysis to obtain the frequency parameters of the interharmonic signals that cause voltage fluctuations of varying degrees. Finally, the interharmonic suppression interval frequency is set, targeting the interharmonics in the frequency range that has a more significant impact on voltage fluctuations. Step S2: Designing a low-pass filter according to the upper limit frequency of the interharmonic suppression interval determined in step S1; Step S3: Based on i p - i q The instantaneous reactive power method of the method is used to detect the reactive component and interharmonic component of the current signal to be compensated, specifically: Through based on i p - i q The instantaneous reactive power method of the method is used to detect the current signal to be compensated, and the reactive component and fundamental frequency component in the signal are obtained. Then, the current signal to be compensated and its fundamental frequency component are subtracted to obtain the frequency components in the signal other than the fundamental frequency, including the interharmonic components. Step S4: using the low-pass filter designed in step S2 to process the interharmonic components obtained in step S3 to generate a target current for the interharmonic compensation link of the active filter; Step S5: obtaining the target voltage of the dynamic reactive power compensation link of the active filter through PI control; Step S6: obtaining the target voltage of the active filter interharmonic compensation link through quasi-proportional resonance control; Step S7: Based on steps S5 and S6, the target voltages of the dynamic reactive power compensation link and the interharmonic compensation link are superimposed to form a control target signal of the active filter.

2. The active filter control method for suppressing low-frequency interharmonics and flicker according to claim 1, characterized in that: In step S2, a Butterworth low-pass filter is designed with the upper limit frequency of the interharmonic suppression interval as the cutoff frequency.

3. The active filter control method for suppressing low-frequency interharmonics and flicker according to claim 1, characterized in that: In step S4, the frequency component obtained in step S3 is processed by the Butterworth low-pass filter designed in step S2 to generate a target current for the interharmonic compensation link of the active filter.

Images