A virtual admittance method for improving the stability of grid-connected inverters

Abstract

The present invention provides a virtual admittance method for improving the stability of a grid-connected inverter. Aiming at the low-frequency oscillation problem introduced by the control loop under a weak power grid and the high-frequency oscillation problem introduced by the resonance parameters of the AC side filter and the system delay time under a strong power grid, the present invention proposes a grid-connected inverter stability control method based on virtual admittance design. The method feeds forward the dq-axis AC side filter capacitor voltage to the dq-axis current regulator input in the form of proportional-integral-differential, proposes a virtual admittance design method based on a set of system capacity, decouples the detection of the operating point by the admittance parameter, and effectively compensates for the equivalent negative damping effect introduced by the control parameters and the system delay through the design of the virtual admittance, thereby improving the stability of the grid-connected inverter system.

Description

Technical Field

[0001] The present invention belongs to the field of control of a grid-connected inverter system, and in particular relates to a virtual admittance method based on improving the stability of a grid-connected inverter. Background Art

[0002] As a power conversion device, three-phase grid-connected inverter plays a key role in applications such as photovoltaic power generation, wind power generation, and energy storage systems. With the rapid development of new energy power generation grid-connected and energy storage systems, the penetration rate of power electronic power conversion devices in the power grid continues to increase. The interaction between the equivalent output impedance of power electronic devices and the grid impedance causes various oscillation phenomena, which seriously affects the power generation quality of new energy grid-connected systems and the safe operation of the power grid.

[0003] The study found that the oscillation frequency is related to the frequency domain negative damping region of the inverter output admittance. The negative damping region of the inverter output admittance is affected by a variety of control loop parameters and system control delays. For example, the phase-locked loop will introduce low-frequency negative damping in the system, the AC side voltage feedforward loop will introduce mid-frequency negative damping, and the system control delay will introduce high-frequency negative damping. Therefore, through control design, compensating for the negative damping region of the inverter admittance can improve the stability of the system in the face of disturbances in the corresponding frequency band.

[0004] There are many academic papers that analyze and propose solutions to the negative damping effect and oscillation phenomenon of inverter output admittance introduced by control loops and system delays, such as: "Input-Admittance Calculation and Shaping for Controlled Voltage-Source Converters", L. Harnefors, M. Bongiorno, S. Lundberg, IEEE Transactions on Industrial Electronics, vol. 54, pp. 3323-3334, 2007. The article analyzes in detail the negative damping effect introduced by various control loops and system delays in the inverter output admittance, as well as the coupling effects between them, but does not propose a compensation solution; "Passivity-Based Controller Design of Grid-Connected VSCs for Prevention of Electrical Resonance Instability”, L. Harnefors, A. Yepes, A. Vidal, J. Doval-Gandoy, IEEE Transactions on Industrial Electronics, vol. 62, pp. 702-710, 2015. (“Oscillation Suppression of Grid-Connected Converters Based on Passive Design”, 2015 IEEE Industrial Electronics Journal Vol. 62, pp. 702-710) introduces a grid voltage differential link to compensate for the high-frequency negative damping area introduced by the delay, but ignores the influence of the control loop; “Stability Improvement for Three-Phase Grid-Connected Converters Through Impedance Reshaping in Quadrature-Axis”, J. Fang, X. Li, H. Li, Y. Tang, IEEE Transactions on Power Electronics, vol. 33, pp. 8365-8375, 2018.The article (“Improving the stability of three-phase grid-connected converters by orthogonal axis impedance reconstruction”, 2018 IEEE Journal of Power Electronics, Vol. 33, pp. 8365-8375) compensates the low-frequency negative damping effect introduced by the phase-locked loop by introducing a proportional link from the q-axis grid voltage to the current command, but its control parameters depend on the operating point and ignore the influence of the AC side voltage feedforward; the article “Improved Design of PLL Controller for LCL-Type Grid-Connected Converter in Weak Grid”, D. Zhu, S. Zhou, X. Zou, Y. Kang, IEEE Transactions on Power Electronics, vol. 35, pp. 4715-4727. (“Improved Design of PLL Controller for LCL-Type Grid-Connected Converter in Weak Grid”, 2020 IEEE Journal of Power Electronics, Vol. 35, pp. 4715-4727) reduces the negative damping area of ​​the inverter output admittance by designing the parameters of the phase-locked loop, but reduces the bandwidth of the phase-locked loop, affecting the dynamic performance of the system. .

[0005] In summary, the prior art has the following problems:

[0006] (1) There is a coupling effect between the damping and compensation designs of different frequency bands. The design of the voltage feedforward loop can affect the damping characteristics from low frequency to high frequency. The existing scheme fails to unify the damping of different frequency bands under the same framework.

[0007] (2) In view of the nonlinear influence of the inverter control loop, the parameter design of the compensation scheme for low-frequency negative damping mostly depends on the detection of the current and voltage at the converter operating point and needs to be continuously updated. Summary of the invention

[0008] In order to overcome the limitations of the above technical solutions, the present invention proposes a virtual admittance method for improving the stability of the grid-connected inverter, in which the damping characteristics of different frequency bands are summarized into the same framework for design, and the design of control parameters depends on the system parameters and capacity, and does not depend on the detection of the operating point, thereby improving the operating stability of the inverter system.

[0009] In order to achieve the above object, the present invention adopts the following technical solution:

[0010] A virtual admittance method for improving the stability of a grid-connected inverter is disclosed. In an LCL-type grid-connected inverter, a compensator based on AC voltage feedforward is added to construct a parallel virtual admittance. By eliminating or reducing the negative damping region of the inverter frequency domain admittance, the passive performance of the grid-connected inverter system is improved, thereby improving the stability of the grid-connected inverter system. The specific steps are as follows:

[0011] Step 1: Collect the inverter side inductance L 1 Current And the voltage of the AC side filter capacitor C

[0012] Step 2: According to the voltage of the AC side filter capacitor C collected in step 1 The variables in the three-phase stationary coordinate system are converted into the dq axis variables voltage in the two-phase synchronous rotating coordinate system through Park transformation The dq axis variable voltage passes through the phase-locked loop PLL to obtain the phase angle θ of the AC side capacitor voltage;

[0013] Step 3: Based on the AC capacitor voltage phase angle θ obtained in step 2, the inverter side inductance L in the three-phase stationary coordinate system collected in step 1 is converted into 1 The current variable is converted into the dq axis variable current of the two-phase synchronous rotating coordinate system

[0014] Step 4: Set the dq axis grid current command signal The voltage of the AC side filter capacitor C and the inductor L on the inverter side obtained in step 2 and step 3 1 The inverter control signal is obtained through the AC current control loop ACC, the AC side capacitor voltage feedforward VFF and the virtual admittance based on the AC side capacitor voltage. The inverter control signal equation is:

[0015]

[0016]

[0017] Among them, s is the Laplace operator, ω g is the rated angular frequency of the grid voltage, is the current loop PI controller, is the current loop proportional adjustment coefficient, is the current loop integral adjustment coefficient, G VFF (s) = ω VFF / (s+ω VFF ) is the AC side voltage feedforward low-pass filter, ω VFF is the feedforward low-pass filter bandwidth, and They are the virtual admittances of dq axis PID form, and their expressions are:

[0018]

[0019]

[0020] in, is the proportional, integral and differential adjustment coefficient of the d-axis virtual admittance, is the proportional, integral and differential adjustment coefficient of the q-axis virtual admittance, G LPF (s) = ω LPF / (s+ω LPF ) is a low-pass filter with differential term, ω LPF is the low-pass filter bandwidth;

[0021] Step 5: Design the control parameters of the virtual admittance according to the inverter system parameters and capacity;

[0022] Step 6: According to the AC capacitor voltage phase angle θ obtained in step 2, the inverter control signal obtained in step 4 is converted to The three-phase stationary coordinate system control signal is obtained by Park inverse transformation

[0023] Step 7: Based on the control signal obtained in step 5, a switching signal of the power period in the inverter is generated through pulse width modulation PWM, and the opening and closing of the power device is controlled through the driving circuit.

[0024] Furthermore, the calculation formula of the AC capacitor voltage phase angle θ in step 2 is:

[0025]

[0026] in, is a phase-locked loop PI controller, is the phase-locked loop proportional adjustment coefficient, is the phase-locked loop integral adjustment coefficient.

[0027] Furthermore, the control parameters of the designed virtual admittance in step 5 specifically include:

[0028] Step 5.1: Calculate the system impedance reference value Z based on the system capacity B and inductor L B Baseline value:

[0029]

[0030]

[0031] Among them, V g,ll is the rms value of the grid rated line voltage, S n is the rated capacity of the inverter;

[0032] Step 5.2: Design the dq-axis virtual admittance differential adjustment coefficient according to the filter capacitor C:

[0033]

[0034]

[0035] Step 5.3: Based on the inverter system inductance reference value L B Design the dq axis virtual admittance integral adjustment coefficient:

[0036]

[0037]

[0038] Among them, k is the coefficient of virtual inductance applied on the q-axis, and is adjusted according to the frequency domain damping characteristics of the converter equivalent admittance q-axis;

[0039] Step 5.4: Based on the inverter system impedance reference value Z B Design the dq axis virtual admittance proportional adjustment coefficient:

[0040]

[0041]

[0042] Among them, m and n are the coefficients of virtual resistance applied to the dq axis, which are adjusted according to the frequency domain damping characteristics of the converter equivalent admittance dq axis.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] 1. The present invention unifies the low- and medium-frequency negative damping introduced by the control loop and the high-frequency negative damping introduced by the system delay under the same framework, which can effectively improve the grid-connected inverter's ability to suppress oscillations in various frequency bands;

[0045] 2. The virtual admittance method proposed in the present invention includes virtual resistance, virtual inductance and virtual capacitance. By designing the parameters of different virtual electrical components, the negative damping areas of different frequency bands can be flexibly compensated;

[0046] 3. This method can design the virtual admittance of the dq axis respectively according to the output impedance characteristics of the inverter;

[0047] 4. The virtual admittance method proposed in the present invention has its parameter design determined by the system capacity and is independent of the inverter operating point current and voltage, and its implementation is simple and effective. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 A topological structure diagram of a three-phase LCL type grid-connected inverter used in the implementation of the present invention;

[0049] Figure 2The control structure block diagram of the three-phase LCL grid-connected inverter with virtual admittance added;

[0050] Figure 3 It is a waveform diagram of the output current of the grid-connected inverter when the control strategy of the present invention is adopted or not in a weak power grid;

[0051] Figure 4 It is a waveform diagram of the output current of the grid-connected inverter when the control strategy of the present invention is adopted or not under a strong power grid. DETAILED DESCRIPTION

[0052] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0053] An embodiment of the present invention provides a virtual admittance method for improving the stability of a grid-connected inverter. By designing different virtual admittance elements and parameters, the damping characteristics of different frequency bands of the dq axis are improved respectively. The present invention can not only suppress the low-frequency oscillation caused by the negative damping of the phase-locked loop under a weak power grid, but also suppress the high-frequency oscillation caused by the negative damping of the system control delay and the filter parameters under a strong power grid, thereby improving the stability of the grid-connected converter. The technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings.

[0054] The topological structure adopted in the embodiment of the present invention is as follows: Figure 1 As shown. This topology includes a three-phase full-bridge inverter, an inverter-side inductor L 1 , AC side filter capacitor C, AC side inductor L 2 , grid impedance L g In this embodiment, L 1 =120uH, C = 400uF, L 2 =40mH.

[0055] Figure 2 FIG. 1 is a schematic diagram of a control structure of the present invention based on introducing virtual admittance based on AC voltage feedforward at the AC side. Figure 2 As shown, the virtual admittance method for improving the stability of the grid-connected inverter of the present invention consists of the following steps:

[0056] Step 1: Collect the inverter side inductance L 1 Current And the voltage of the AC side filter capacitor C

[0057] Step 2: According to the voltage of the AC side filter capacitor C collected in step 1 The variables in the three-phase stationary coordinate system are converted into the dq axis variables voltage in the two-phase synchronous rotating coordinate system through Park transformation The dq axis variable voltage passes through the phase-locked loop PLL to obtain the AC side capacitor voltage phase angle θ.

[0058] The calculation formula of the AC side capacitor voltage phase angle is:

[0059]

[0060] in, is a phase-locked loop PI controller, is the phase-locked loop proportional adjustment coefficient, is the integral adjustment coefficient of the phase-locked loop, ω g is the rated angular frequency of the grid voltage, and s is the Laplace operator. In this embodiment, ω g =100πrad / s.

[0061] Step 3: Based on the AC capacitor voltage phase angle θ obtained in step 2, the inverter side inductance L in the three-phase stationary coordinate system collected in step 1 is converted into 1 The current variable is converted into the dq axis variable current of the two-phase synchronous rotating coordinate system

[0062] Step 4: Set the dq axis grid current command signal The voltage of the AC side filter capacitor C and the inductor L on the inverter side obtained in step 2 and step 3 1 The inverter control signal is obtained through the AC current control loop ACC, the AC side capacitor voltage feedforward VFF and the virtual admittance based on the AC side capacitor voltage. The inverter control signal equation is:

[0063]

[0064]

[0065] in, is the current loop PI controller, is the current loop proportional adjustment coefficient, is the current loop integral adjustment coefficient, G VFF (s) = ω VFF / (s+ω VFF ) is the AC side voltage feedforward low-pass filter, ω VFF is the bandwidth of the feedforward low-pass filter. In this embodiment, ω VFF =600πrad / s.

[0066] and They are the virtual admittances of dq axis PID form, and their expressions are:

[0067]

[0068]

[0069] in, is the proportional, integral and differential adjustment coefficient of the d-axis virtual admittance, is the proportional, integral and differential adjustment coefficient of the q-axis virtual admittance, G LPF (s) = ω LPF / (s+ω LPF ) is a low-pass filter with differential term, ω LPF is the low-pass filter bandwidth. In this embodiment, ω LPF =3000πrad / s

[0070] Step 5: Design the control parameters of the virtual admittance according to the inverter system parameters and capacity;

[0071] Step 5.1: Calculate the system impedance reference value Z based on the system capacity B and inductor L B Baseline value:

[0072]

[0073]

[0074] Among them, V g,ll is the rms value of the grid rated line voltage, S n is the rated capacity of the inverter; in this embodiment, V g,ll =550V,S n =2MVA, calculate Z B =150mΩ, L B =480uH.

[0075] Step 5.2: Design the dq-axis virtual admittance differential adjustment coefficient according to the AC side filter capacitor C:

[0076]

[0077]

[0078] Step 5.3: Based on the inverter system inductance reference value L B Design the dq axis virtual admittance integral adjustment coefficient:

[0079]

[0080]

[0081] Wherein, k is the coefficient of the virtual inductance applied to the q-axis, which is adjusted according to the q-axis frequency domain damping characteristics of the converter equivalent admittance. In this embodiment, k=1.

[0082] Step 5.4: Based on the inverter system impedance reference value Z B Design the dq axis virtual admittance proportional adjustment coefficient:

[0083]

[0084]

[0085] Wherein, m and n are coefficients of virtual resistance applied to the dq axis, which are adjusted according to the frequency domain damping characteristics of the converter equivalent admittance dq axis. In this embodiment, m=1 / 6, n=0.

[0086] Step 6: According to the AC capacitor voltage phase angle θ obtained in step 2, the inverter control signal obtained in step 4 is converted to The three-phase stationary coordinate system control signal is obtained by Park inverse transformation

[0087] Step 7: Based on the control signal obtained in step 5, a switching signal of the power period in the inverter is generated through pulse width modulation PWM, and the opening and closing of the power device is controlled through the driving circuit.

[0088] Figure 3 Whether the control strategy of the present invention is adopted or not, the output current waveform of the grid-connected converter under weak power grid. Figure 3 It can be found that: if the control strategy of the present invention is adopted, the output current can still remain stable when the power grid strength becomes weak; if the control strategy of the present invention is removed, the output current will experience obvious low-frequency oscillation under a weak power grid.

[0089] Figure 4 Whether the control strategy of the present invention is adopted or not, the output current waveform of the grid-connected converter under a strong power grid. Figure 4 It can be found that: if the control strategy of the present invention is adopted, the output current can still remain stable when the power grid strength becomes stronger; if the control strategy of the present invention is removed, the output current will experience obvious high-frequency oscillation under a strong power grid.

[0090] The grid-connected inverter virtual admittance method proposed in the present invention can not only suppress the low-frequency oscillation introduced by the phase-locked loop under weak power grids, but also suppress the high-frequency oscillation introduced by control delays and filters under strong power grids, and can effectively improve the stability of the grid-connected converter system.

[0091] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A virtual admittance method for improving the stability of a grid-connected inverter, characterized in that: In the LCL type grid-connected inverter, by adding a compensator based on the AC voltage feedforward on the AC side, a parallel virtual admittance is constructed, and by eliminating or reducing the negative damping area of ​​the inverter frequency domain admittance, the passive performance of the grid-connected inverter system is improved, thereby improving the stability of the grid-connected inverter system; the specific steps are as follows: Step 1: Collect the current of the inverter side inductor L1 And the voltage of the AC side filter capacitor C Step 2: According to the voltage of the AC side filter capacitor C collected in step 1 The variables in the three-phase stationary coordinate system are converted into the dq axis variables voltage in the two-phase synchronous rotating coordinate system through Park transformation The dq axis variable voltage passes through the phase-locked loop PLL to obtain the phase angle θ of the AC side capacitor voltage; Step 3: Based on the AC side capacitor voltage phase angle θ obtained in step 2, the current variable of the inverter side inductor L1 in the three-phase stationary coordinate system acquired in step 1 is converted into the dq axis variable current in the two-phase synchronous rotating coordinate system through Park transformation Step 4: Set the dq axis grid current command signal The dq-axis variables of the voltage of the AC side filter capacitor C and the current of the inverter side inductor L1 obtained in step 2 and step 3 are used to obtain the inverter control signal through the AC current control loop ACC, the AC side capacitor voltage feedforward VFF and the virtual admittance based on the AC side capacitor voltage. The inverter control signal equation is: Among them, s is the Laplace operator, ω g is the rated angular frequency of the grid voltage, is the current loop PI controller, is the current loop proportional adjustment coefficient, is the current loop integral adjustment coefficient, G VFF (s) = ω VFF / (s+ω VFF ) is the AC side voltage feedforward low-pass filter, ω VFF is the feedforward low-pass filter bandwidth, and They are the virtual admittances of dq axis PID form, and their expressions are: in, is the proportional, integral and differential adjustment coefficient of the d-axis virtual admittance, is the proportional, integral and differential adjustment coefficient of the q-axis virtual admittance, G LPF (s) = ω LPF / (s+ω LPF ) is a low-pass filter with differential term, ω LPF is the low-pass filter bandwidth; Step 5: Design the control parameters of the virtual admittance according to the inverter system parameters and capacity; Step 6: According to the AC side capacitor voltage phase angle θ obtained in step 2, the inverter control signal obtained in step 4 is converted to The three-phase stationary coordinate system control signal is obtained by Park inverse transformation Step 7: Based on the control signal obtained in step 5, a switching signal of the power period in the inverter is generated through pulse width modulation PWM, and the opening and closing of the power device is controlled through the driving circuit.

2. A virtual admittance method for improving the stability of a grid-connected inverter according to claim 1, characterized in that: The calculation formula of the AC side capacitor voltage phase angle θ in step 2 is: in, is a phase-locked loop PI controller, is the phase-locked loop proportional adjustment coefficient, is the phase-locked loop integral adjustment coefficient.

3. A virtual admittance method for improving the stability of a grid-connected inverter according to claim 2, characterized in that: The control parameters of the designed virtual admittance in step 5 specifically include: Step 5.1: Calculate the system impedance reference value Z based on the system capacity B and inductor L B Baseline value: Among them, V g,ll is the rms value of the grid rated line voltage, S n is the rated capacity of the inverter; Step 5.2: Design the dq-axis virtual admittance differential adjustment coefficient according to the filter capacitor C: Step 5.3: Based on the inverter system inductance reference value L B Design the dq axis virtual admittance integral adjustment coefficient: Among them, k is the coefficient of virtual inductance applied on the q-axis, which is adjusted according to the frequency domain damping characteristics of the converter equivalent admittance q-axis; Step 5.4: Based on the inverter system impedance reference value Z B Design the dq axis virtual admittance proportional adjustment coefficient: Among them, m and n are the coefficients of virtual resistance applied to the dq axis, which are adjusted according to the frequency domain damping characteristics of the converter equivalent admittance dq axis.

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