Control method of network-forming type active filter

Abstract

The invention discloses a control method of a network-forming type active filter, the active filter comprises a power outer loop control unit and a voltage and current inner loop control unit, and the power outer loop control unit comprises a direct current bus voltage loop, an active power outer loop and a reactive power outer loop. The voltage and current inner loop control unit comprises an AC voltage loop and an AC current loop. The active filter is operated in a network construction mode, and the voltage source characteristic of the active filter can suppress the source load resonance problem to the greatest extent; for a balanced load and an unbalanced load, an instantaneous reactive power theory and a TTA harmonic detection algorithm are respectively adopted to obtain a harmonic voltage instruction, so that load-side harmonic suppression is realized; a harmonic voltage instruction is obtained by carrying out harmonic detection calculation on load current, and load side harmonic treatment is carried out, so that the function of actively carrying out harmonic treatment on the network construction type converter is realized; the harmonic impedance characteristic reconstruction of the active filter is realized through harmonic voltage source control, and the problem of multi-machine parallel resonance can be effectively solved.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and specifically to a control method for a grid-type active filter. Background Technology

[0002] Grid-type converters, by simulating the dynamic characteristics of a synchronous generator rotor, possess voltage and frequency regulation capabilities. They actively support the system during grid disturbances, preventing system instability or even collapse caused by insufficient inertia and damping. However, while providing inertia and damping, grid-type converters also generate second harmonic currents in response to distorted grid voltages.

[0003] Meanwhile, the system's common connection point is connected to a large number of industrial and residential loads, and the harmonic interactions generated by the sources and loads exacerbate the distortion of the grid current. The distorted grid current generates harmonic voltage drops across the grid impedance, resulting in increased PCC harmonic voltage content and increasing the difficulty of stable grid connection for new energy converters.

[0004] Currently, most studies focus on the voltage source characteristics of grid-connected converters at the fundamental frequency, while neglecting their excellent characteristic of low harmonic voltage in the frequency domain. There is also little analysis and research on grid-connected converters actively participating in the power quality management of distorted power grids. Therefore, an APF that operates in a grid-connected manner is the core solution to this problem.

[0005] Existing power quality management solutions include: 1. Using current-source active power filters to eliminate harmonics; 2. Increasing the converter output impedance through a series virtual impedance control strategy to reduce the impact of grid harmonic voltage on the converter; 3. Using a combination of passive and active methods to eliminate harmonics; 4. Multi-resonant compensators and grid voltage feedforward control. Among these,

[0006] The disadvantages of current source type active power filters are that the stability of multi-parallel systems is significantly reduced and the phase-locked loop response speed is slow (the system stability is poor when the phase-locked loop response speed is fast);

[0007] The disadvantage of the series virtual impedance control strategy is that the output impedance of the converter after reshaping is larger than that of the grid impedance, the proportion of nonlinear load harmonics flowing into the grid increases, and the harmonic content of the grid current increases.

[0008] The disadvantage of combining passive and active methods is that it adds large-capacity inductors and capacitors, which are prone to resonance with the mains impedance. It can only filter out specific harmonics and may cause overcompensation or undercompensation.

[0009] The disadvantages of multi-resonant compensators and grid voltage feedforward control are that multi-resonant compensators are not suitable for grid-connected LCL inverters because grid-connected inverters usually adopt a dual-inner-loop control method of voltage and current, and the controlled object is not the grid current to achieve grid harmonic suppression effect; grid voltage feedforward is for current-source VSG inverters and does not take into account the application of voltage-source VSG inverters.

[0010] Therefore, how to provide a grid-type converter that can suppress source-carrier resonance to the greatest extent and simultaneously achieve harmonic current control is a research direction that those skilled in the art are committed to. Summary of the Invention

[0011] The purpose of this invention is to overcome the shortcomings of the prior art and provide a control method for a mesh-type active filter.

[0012] To achieve the above objectives, the technical solution adopted by this invention is: a grid-type active control method, wherein the active filter includes a power outer loop control unit and a voltage and current inner loop control unit, the power outer loop control unit includes a DC bus voltage loop, an active power outer loop, and a reactive power outer loop, and the voltage and current inner loop control unit includes an AC voltage loop and an AC current loop, and the control method includes the following steps:

[0013] 1) The DC bus voltage loop uses PI control, and its output serves as the reference for the active power outer loop. The active power outer loop uses P control, and its output is related to the grid frequency ω. gLPF The control output frequency is obtained by superposition, and the reference voltage phase θ is obtained by integration of the control output frequency.

[0014] 2) The reactive power outer loop uses PI control, and its output is superimposed with the fundamental voltage amplitude as the reference voltage amplitude V for the AC voltage loop. ref ;

[0015] 3) Reference voltage amplitude V ref The three-phase voltage loop command V is obtained by transforming the phase θ of the reference voltage using dq / abc. aref V bref V cref ;

[0016] 4) Sample the three-phase load current I aLoad I bLoad I cLoad For balanced loads, an instantaneous reactive power algorithm is used, and the nth harmonic is determined based on the grid angular velocity ω. gLPF The angular velocities nωt of each harmonic are obtained, and finally sin(nωt) and cos(nωt) are obtained. After abc / dq transformation, the I of each harmonic is obtained. d I q The I0 component, where n = 1, 2, 3...n, is specifically expressed as follows:

[0017]

[0018] 5) I obtained through step 4) d I q The components, after filtering, are inversely transformed by dq / abc to obtain the harmonic commands V. hanref V hbnref V hcnref ;

[0019] 6) For unbalanced loads, the TTA algorithm is used to extract each harmonic in each phase. The Ip and Iq components of each phase of the three-phase load current are obtained through sin(nωt) and cos(nωt).

[0020] 7) Regarding the three-phase active power component I pa I pb I pc and reactive component I qa I qb I qc Filtering is performed separately, and the filtered values ​​are then used to calculate the harmonic commands for each phase using sin(nωt) and cos(nωt). The specific expressions are as follows:

[0021]

[0022] 8) The total harmonic voltage command V is obtained by superimposing the voltage commands of each harmonic. haref V hbref V hcref The expression is as follows:

[0023]

[0024] 9) The total harmonic voltage command V of the three phases haref V hbref V hcref With the three-phase fundamental voltage loop command V aref V bref V cref The summations are used to obtain the total three-phase voltage loop command;

[0025] 10) The three-phase voltage loop setpoint command and the high-pass capacitor voltage feedback value are processed by the voltage loop VR controller to output the AC current loop setpoint command;

[0026] 11) The AC current loop input and the inductor current feedback are modulated by the output signal of the current loop CR controller to generate three-phase drive signals Sa, Sb, and Sc.

[0027] As a specific implementation method, the specific calculation process of step 5) is as follows:

[0028] V d =I d ×Z d ,

[0029] V q =I q ×Z q ,

[0030]

[0031] Among them, V d V q Zd and Zq represent the components of the harmonic voltage in the d- and q-phase rotating coordinate system; Zd and Zq represent the grid-side impedance; V α V β This represents the components of the harmonic voltage in the α and β phase stationary coordinate systems.

[0032] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0033] 1) When an active filter is operated in network mode, its voltage source characteristics can suppress source-carrier resonance problems to the greatest extent.

[0034] 2) The technical solution of the present invention uses instantaneous reactive power theory and TTA harmonic detection algorithm to obtain harmonic voltage commands for balanced loads and unbalanced loads respectively, thereby realizing harmonic control on the load side.

[0035] 3) The technical solution of the present invention obtains harmonic voltage commands by performing harmonic detection and calculation on the load current, and performs harmonic mitigation on the load side, thereby realizing the function of active harmonic mitigation of the grid-type converter;

[0036] 4) The technical solution of the present invention realizes the reconstruction of the harmonic impedance characteristics of the active filter through harmonic voltage source control, which can effectively solve the problem of multi-machine parallel resonance. Attached Figure Description

[0037] Figure 1 This is a control block diagram of the mesh-type active filter described in this invention;

[0038] Figure 2 This is a control block diagram of the phase-locked loop in the mesh-type active filter described in this invention;

[0039] Figure 3 A calculation block diagram for harmonic voltage command detection under balanced load;

[0040] Figure 4 The calculation block diagram for harmonic voltage command detection under unbalanced load. Detailed Implementation

[0041] The following is in conjunction with the appendix Figure 1-4The technical solution of the present invention will be further described in detail with reference to specific embodiments.

[0042] This invention provides a control method for a grid-type active filter. The active filter includes a power outer loop control unit and a voltage and current inner loop control unit. The power outer loop control unit includes a DC bus voltage loop, an active power outer loop, and a reactive power outer loop. The voltage and current inner loop control unit includes an AC voltage loop and an AC current loop.

[0043] The control method includes the following steps:

[0044] 1) The DC bus voltage loop uses PI control, and its output serves as the reference for the active power outer loop. The active power outer loop uses P control, and its output is related to the grid frequency ω. gLPF The control output frequency is obtained by superposition, and the reference voltage phase θ is obtained by integration of the control output frequency.

[0045] 2) The reactive power outer loop uses PI control, and its output is superimposed with the fundamental voltage amplitude as the reference voltage amplitude V for the AC voltage loop. ref ;

[0046] 3) Reference voltage amplitude V ref The three-phase voltage loop command V is obtained by transforming the phase θ of the reference voltage using dq / abc. aref V bref V cref ;

[0047] 4) Sample the three-phase load current I aLoad I bLoad I cLoad For balanced loads, an instantaneous reactive power algorithm is used, and the nth harmonic is determined based on the grid angular velocity ω. gLPF The angular velocities nωt of each harmonic are obtained, and finally sin(nωt) and cos(nωt) are obtained. After abc / dq transformation, the I of each harmonic is obtained. d I q The I0 component, where n = 1, 2, 3...n, is specifically expressed as follows:

[0048]

[0049] 5) I obtained through step 4) d I q The components, after filtering, are inversely transformed by dq / abc to obtain the harmonic commands V. hanref V hbnref V hcnref The specific expression is as follows:

[0050] V d =I d ×Zd ,

[0051] V q =I q ×Z q ,

[0052]

[0053]

[0054] Among them, V d V q Zd and Zq represent the components of the harmonic voltage in the d- and q-phase rotating coordinate system; Zd and Zq represent the grid-side impedance; V α V β This represents the components of harmonic voltage in the α and β phase stationary coordinate systems.

[0055] 6) For unbalanced loads, the TTA algorithm is used to extract each harmonic in each phase. The Ip and Iq components of each phase of the three-phase load current are obtained through sin(nωt) and cos(nωt).

[0056] 7) Regarding the three-phase active power component I pa I pb I pc and reactive component I qa I qb I qc Filtering is performed separately, and the filtered values ​​are then used to calculate the harmonic commands for each phase using sin(nωt) and cos(nωt). The specific expressions are as follows:

[0057]

[0058] 8) The total harmonic voltage command V is obtained by superimposing the voltage commands of each harmonic. haref V hbref V hcref The expression is as follows:

[0059]

[0060] 9) The total harmonic voltage command V of the three phases haref V hbref V hcref With the three-phase fundamental voltage loop command V aref V bref V cref The summations are used to obtain the total three-phase voltage loop command;

[0061] 10) The three-phase voltage loop setpoint command and the high-pass capacitor voltage feedback value are processed by the voltage loop VR controller to output the AC current loop setpoint command;

[0062] 11) The AC current loop input and the inductor current feedback are modulated by the output signal of the current loop CR controller to generate three-phase drive signals Sa, Sb, and Sc.

[0063] Here, the active filter operates in a network mode, and its voltage source characteristics can suppress the source-load resonance problem to the greatest extent. For balanced loads and unbalanced loads, instantaneous reactive power theory and TTA harmonic detection algorithm are used to obtain harmonic voltage commands, thereby realizing load-side harmonic mitigation. Harmonic voltage source control realizes the reconstruction of the harmonic impedance characteristics of the active filter, which can effectively solve the problem of multi-machine parallel resonance.

[0064] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A control method for a grid-type active power filter, wherein the active power filter includes a power outer loop control unit and a voltage and current inner loop control unit, the power outer loop control unit includes a DC bus voltage loop, an active power outer loop, and a reactive power outer loop, and the voltage and current inner loop control unit includes an AC voltage loop and an AC current loop, characterized in that, The control method includes the following steps: 1) The DC bus voltage loop uses PI control, and its output serves as the reference for the active power outer loop. The active power outer loop uses P control, and its output is related to the grid frequency ω. gLPF The control output frequency is obtained by superposition, and the reference voltage phase θ is obtained by integration of the control output frequency. 2) The reactive power outer loop uses PI control, and its output is superimposed with the fundamental voltage amplitude as the reference voltage amplitude V for the AC voltage loop. ref ; 3) Reference voltage amplitude V ref The three-phase voltage loop command V is obtained by transforming the phase θ of the reference voltage using dq / abc. aref V bref V cref ; 4) Sample the three-phase load current I aLoad I bLoad I cLoad For balanced loads, an instantaneous reactive power algorithm is used, and the nth harmonic is determined based on the grid angular velocity ω. gLPF The angular velocities nωt of each harmonic are obtained, and finally sin(nωt) and cos(nωt) are obtained. After abc / dq transformation, the I of each harmonic is obtained. d I q The I0 component, where n = 1, 2, 3...n, is specifically expressed as follows: 5) I obtained through step 4) d I q The components, after filtering, are inversely transformed by dq / abc to obtain the harmonic commands V. hanref V hbnref V hcnref ; 6) For unbalanced loads, the TTA algorithm is used to extract each harmonic in each phase. The Ip and Iq components of each phase of the three-phase load current are obtained through sin(nωt) and cos(nωt). 7) Regarding the three-phase active power component I pa I pb I pc and reactive component I qa I qb I qc Filtering is performed separately, and the filtered values ​​are then used to calculate the harmonic commands for each phase using sin(nωt) and cos(nωt). The specific expressions are as follows: V hanref =(I pa *sin(nωt)+I qa *cos(nωt))*Z a V hbnref =(I pb *sin(nωt)+I qb *cos(nωt))*Z b ; V hcnref =(I pc *sin(nωt)+I qc *cos(nωt))*Z c 8) The total harmonic voltage command V is obtained by superimposing the voltage commands of each harmonic. haref V hbref V hcref The expression is as follows: 9) The total harmonic voltage command V of the three phases haref V hbref V hcref With the three-phase fundamental voltage loop command V aref V bref V cref The summations are used to obtain the total three-phase voltage loop command; 10) The three-phase voltage loop setpoint command and the high-pass capacitor voltage feedback value are processed by the voltage loop VR controller to output the AC current loop setpoint command; 11) The AC current loop input and the inductor current feedback are modulated by the output signal of the current loop CR controller to generate three-phase drive signals Sa, Sb, and Sc.

2. The control method for a mesh-type active filter according to claim 1, characterized in that, The specific calculation process for step 5) is as follows: V d =I d ×Z d , V q =I q ×Z q , Among them, V d V q Zd and Zq represent the components of the harmonic voltage in the d- and q-phase rotating coordinate system; Zd and Zq represent the grid-side impedance; V α V β This represents the components of the harmonic voltage in the α and β phase stationary coordinate systems.

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