Virtual impedance control method based on double-resonance controller

Abstract

The invention discloses a virtual impedance control method based on a double-resonance controller, which is characterized in that a nonlinear load is connected to a power grid at a point of common coupling (PCC), a converter side and a power grid side are respectively provided with a converter side inductor () and a power grid side inductor (), and a branch is led out from the middle point of the converter side inductor () and the power grid side inductor () and is provided with a capacitor (C); the converter side current is fed back and compared with the fundamental wave current reference to form closed-loop control; the closed-loop control includes a proportional resonant controller that adjusts at a harmonic frequency of interest; and when the system does not need the virtual impedance, the branch where the resonance controller is located is cancelled. When the system needs virtual impedance, the capacitor voltage is fed forward through the resonance controller. The resonance controller is connected in series with the feedback branch of the capacitor voltage, so that the high-frequency influence of the branch on the system is eliminated, namely, the stability of the system is not influenced by the branch, and the system can be used for simulating any complex virtual impedance and ensuring the stability of the system at the same time.

Description

Technical Field

[0001] This invention relates to a virtual impedance control method, and more particularly to a virtual impedance control method based on a dual resonant controller. Background Technology

[0002] Virtual impedance is an advanced control technique that simulates the physical impedance characteristics at the output of a power electronic converter using control algorithms. In traditional power systems, physical impedance (such as the inductive reactance of transmission lines and the leakage reactance of transformers) plays a crucial role in system stability, power distribution, and fault current limiting. However, as renewable energy and energy storage systems are connected to microgrids via power electronic converters, these "rigidly" connected power sources lack the natural inertial damping and intrinsic impedance characteristics of traditional synchronous generators, leading to reduced system damping and making them prone to power oscillations and stability problems.

[0003] Virtual impedance technology has emerged to provide power electronic converters with flexible and programmable impedance characteristics. Its core idea is to introduce a computational loop into the converter's current or power control loop. Based on output current feedback, a voltage drop equivalent to that generated across the virtual impedance is generated in real time and superimposed on the control reference signal. From a system perspective, the equivalent output impedance of the converter "senses" by other devices or networks is the sum of the designed virtual impedance and its inherent impedance.

[0004] The main advantage of this technology lies in its flexibility and controllability. Through software definition, purely inductive, purely resistive, or more complex resistive-inductive impedance characteristics can be easily implemented, and their values ​​can be adjusted online according to system requirements. This effectively solves key problems in microgrids without increasing hardware costs or size.

[0005] Existing virtual impedance techniques are mainly divided into two categories:

[0006] One type is voltage source control, which samples the output current, calculates the output voltage containing virtual impedance, and uses it as a new voltage command for control;

[0007] The second type is current source control, which samples the output voltage, calculates the output current containing the virtual impedance, and uses this as a new current command for control. That is, it uses either a voltage source or a current source to simulate the virtual impedance.

[0008] However, the two virtual impedance schemes mentioned above face challenges when simulating complex impedances. Studies have found that the system becomes unstable when these techniques simulate a negative inductor or capacitor connected in series with a positive resistor. Furthermore, the introduction of new feedback loops complicates the stability analysis of the system.

[0009] In view of this, the present invention is hereby proposed. Summary of the Invention

[0010] The purpose of this invention is to provide a virtual impedance control method based on a dual resonant controller to solve the aforementioned technical problems in the prior art.

[0011] The objective of this invention is achieved through the following technical solution:

[0012] The virtual impedance control method based on a dual resonant controller of this invention connects the nonlinear load to the power grid at the point of common coupling (PCC), and the converter is connected to the power grid at the PCC through an LCL filter. The LCL filter has converter-side inductors on both the converter side and the power grid side. ) and grid-side inductance ( A branch is drawn from the midpoint between the two and a capacitor (C) is provided thereon, with the other end of the capacitor (C) grounded.

[0013] Converter-side current Feedback is received and referenced to the fundamental current. The comparisons are used to form a closed-loop control; this closed-loop control includes a proportional resonant controller. Adjustments are made at the harmonic frequencies of interest, and the expression for this is:

[0014]

[0015] in, For proportional gain, For resonant controllers, tuned to Second harmonic For the corresponding resonant gain, For the corresponding compensation angle, The fundamental angular frequency, capacitor current pass Feedback is used for energized damping. yes The feedback coefficient;

[0016] When the system requires virtual impedance, the capacitor voltage Through the resonant controller Perform feedforward:

[0017]

[0018] in To tune to Resonant controller for subharmonics For the corresponding resonant gain, This is the corresponding compensation angle.

[0019] Compared with existing technologies, the virtual impedance control method based on a dual resonant controller provided by this invention connects the resonant controller in series with the feedback branch of the capacitor voltage, eliminating the high-frequency influence of this branch on the system. In other words, the system stability is unaffected by this branch. Therefore, this scheme can be used to simulate arbitrarily complex virtual impedances while ensuring system stability. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a three-phase LCL grid-connected inverter according to an embodiment of the present invention.

[0021] Figure 2 This is a block diagram of virtual impedance control based on a dual resonant controller according to an embodiment of the present invention.

[0022] Figure 3 This is a schematic diagram of the simulation results when the virtual resistance is set to 0 in an embodiment of the present invention.

[0023] Figure 4 This is a schematic diagram of the simulation results when the virtual resistance is set to 5Ω in an embodiment of the present invention. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them, and do not constitute a limitation on the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the protection scope of the present invention.

[0025] First, the following explanations are provided for the terms that may be used in this article:

[0026] The terms “including,” “contains,” “comprising,” “having,” or other similar semantic descriptions shall be interpreted as non-exclusive inclusion.

[0027] The term "composed of" excludes any technical feature elements not explicitly listed.

[0028] The contents not described in detail in the embodiments of this invention are prior art known to those skilled in the art. Where specific conditions are not specified in the embodiments of this invention, they shall be performed according to conventional conditions in the art or conditions recommended by the manufacturer. Where the manufacturers of the reagents or instruments used in the embodiments of this invention are not specified, they are all conventional products that can be purchased commercially.

[0029] The virtual impedance control method based on a dual resonant controller of this invention connects the nonlinear load to the power grid at the point of common coupling (PCC), and the converter is connected to the power grid at the PCC through an LCL filter. The LCL filter has converter-side inductors on both the converter side and the power grid side. ) and grid-side inductance ( A branch is drawn from the midpoint between the two and a capacitor (C) is provided thereon, with the other end of the capacitor (C) grounded.

[0030] Converter-side current Feedback is received and referenced to the fundamental current. The comparisons are used to form a closed-loop control; this closed-loop control includes a proportional resonant controller. Adjustments are made at the harmonic frequencies of interest, and the expression for this is:

[0031]

[0032] in, For proportional gain, For resonant controllers, tuned to Second harmonic For the corresponding resonant gain, For the corresponding compensation angle, The fundamental angular frequency, capacitor current pass Feedback is used for energized damping. yes The feedback coefficient is the capacitor voltage when the system requires virtual impedance. Through the resonant controller Perform feedforward:

[0033]

[0034] in, To tune to Resonant controller for subharmonics For the corresponding resonant gain, This is the corresponding compensation angle.

[0035] When the system does not require virtual impedance, the resonant controller The branch road in question has been cancelled;

[0036] The resonant controller refers to a function in the control algorithm, wherein: Include , and Located on different branches, these are two controllers that operate simultaneously, forming a dual resonant controller. Together, they achieve the desired virtual impedance.

[0037]

[0038]

[0039] In the above formula, It is the magnitude of the output admittance; It is the output admittance; It is the angle of output admittance;

[0040] By adjusting Adjust the output admittance.

[0041] By adjusting Adjust the angle of the output admittance;

[0042] By adjusting , , and To achieve any output admittance seen from the capacitor, which is also to achieve any virtual impedance seen from the point of common coupling (PCC).

[0043] In summary, the virtual impedance control method based on a dual resonant controller in this invention integrates the resonant controller in series with the feedback branch of the capacitor voltage, eliminating the high-frequency influence of this branch on the system. In other words, the system's stability is unaffected by this branch. Therefore, this scheme can be used to simulate arbitrarily complex virtual impedances while ensuring system stability.

[0044] To more clearly demonstrate the technical solution and its effects provided by the present invention, the embodiments of the present invention will be described in detail below with reference to specific examples.

[0045] Example 1

[0046] 1. Taking a current source with an LCL filter as an example, the proposed virtual impedance control scheme based on a dual resonant controller is illustrated.

[0047] Figure 1 This is a schematic diagram of a grid-connected inverter with a three-phase LCL filter, where the nonlinear load is connected to the grid at the point of common coupling (PCC). , and These are the converter-side inductor, the grid-side inductor, and the capacitor, respectively.

[0048] The other end of capacitor C is grounded, but this is not a physical ground; rather, it is a reference node in the circuit. The other line of the power grid and the other line of the nonlinear load are also connected to ground.

[0049] Figure 2 shows the control block diagram of the proposed virtual impedance. As shown in the figure, the converter-side current... Feedback is received and referenced to the fundamental current. A comparison was made. This closed-loop controller is a proportional-resonant (PR) controller. Adjustments are made at the harmonic frequencies of interest, and the expression for this is:

[0050]

[0051] in, For proportional gain, For resonant controllers, tuned to Second harmonic For the corresponding resonant gain, For the corresponding compensation angle, This is the fundamental angular frequency. Capacitor current. pass Feedback is used for energized damping.

[0052] When the system does not require virtual impedance, the resonant controller The branch road in question can be cancelled;

[0053] When the system requires virtual impedance, the capacitor voltage Through the resonant controller Perform feedforward:

[0054]

[0055] in To tune to Resonant controller for subharmonics For the corresponding resonant gain, This is the corresponding compensation angle.

[0056] The so-called controller mentioned above refers to a transfer function in the control algorithm, not a physical device. Include As shown in formula (1). and Located on different branches, these are two controllers that operate simultaneously; the patent title, "Dual Resonant Controller," refers to this. and Both are necessary for the desired virtual impedance to be achieved; all three are transfer functions. Figure 1 It is located inside the current control module.

[0057] The following will discuss Its role in output admittance shaping. The 1.5-cycle delay caused by the PWM delay is modeled by the following formula:

[0058]

[0059] In the formula, T is the sampling period. Then, the inverter-side current of the grid-connected inverter can be expressed as:

[0060]

[0061]

[0062]

[0063] in From arrive The closed-loop transfer function, It is the corresponding open-loop transfer function. It is the equivalent output admittance seen from the capacitor.

[0064] 2. Below, we will analyze this by simulating the entire grid-connected inverter as a virtual resistor. Then the following equation holds:

[0065]

[0066] The left side of equation (7) is the impedance of the RAPF observed from the PCC, and the right side of equation (7) is the expected virtual resistance. Therefore, the output admittance at the desired harmonic frequency can be derived as:

[0067]

[0068] on the other hand, It can be derived from (6).

[0069]

[0070] As can be seen from equation (9), even if the virtual impedance is not explicitly specified in the control scheme, the proposed scheme can achieve good results by reasonably designing the resonant gain and compensation angle. It can still be controlled as expected. Specifically, The amplitude is and The ratio, The phase is and The difference. This can be determined using the following two formulas.

[0071]

[0072]

[0073] Virtual resistance as seen from PCC It was achieved.

[0074] 3. Testing and experimental verification

[0075] To verify the proposed virtual impedance scheme, relevant simulations were performed. The simulation circuit and... Figure 1 Same. The nonlinear load is an uncontrolled diode rectifier with a load resistance of 50Ω, and its AC side is connected to a 5mH inductor filter.

[0076] Figure 3 Given Simulation results with the impedance set to zero. Zero virtual impedance means that the inverter is short-circuited to harmonic voltages, so the harmonics of the grid current and voltage are effectively attenuated, with THDs of 5.4% and 2.4%, respectively.

[0077] also, Figure 4 Showing Simulation results with the impedance set to 5Ω. Due to the relatively high output impedance of the inverter, the final THD of the grid current is 20.5%.

[0078] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims. The information disclosed in the background section is intended only to enhance the understanding of the overall background technology of the present invention and should not be construed as an admission or implication in any way that such information constitutes prior art known to those skilled in the art.

Claims

1. A virtual impedance control method based on a dual-resonant controller, characterized in that, The nonlinear load is connected to the grid at the point of common coupling (PCC). The converter is connected to the grid at the PCC through an LCL filter. The LCL filter has converter-side inductors on both the converter side and the grid side. ) and grid-side inductance ( A branch is drawn from the midpoint between the two and a capacitor (C) is provided thereon, with the other end of the capacitor (C) grounded. Converter-side current Feedback is received and referenced to the fundamental current. The comparisons are used to form a closed-loop control; this closed-loop control includes a proportional resonant controller. Adjustments are made at the harmonic frequencies of interest, and the expression for this is: in, For proportional gain, For resonant controllers, tuned to Second harmonic For the corresponding resonant gain, For the corresponding compensation angle, The fundamental angular frequency, capacitor current pass Feedback is used for energized damping. yes The feedback coefficient; When the system requires virtual impedance, the capacitor voltage Through the resonant controller Perform feedforward: in To tune to Resonant controller for subharmonics For the corresponding resonant gain, This is the corresponding compensation angle.

2. The virtual impedance control method based on a dual resonant controller according to claim 1, characterized in that, When the system does not require virtual impedance, the resonant controller The branch road in question has been cancelled; The resonant controller refers to a function in the control algorithm, wherein: Include , and Located on different branches, they form a dual resonant controller, which works together to achieve the desired virtual impedance.

3. The virtual impedance control method based on a dual resonant controller according to claim 2, characterized in that, include: In the above formula, It is the magnitude of the output admittance; It is the output admittance; It is the angle of output admittance; By adjusting Adjust the output admittance. By adjusting Adjust the angle of the output admittance; By adjusting , , and To achieve any output admittance seen from the capacitor, which is also to achieve any virtual impedance seen from the point of common coupling (PCC).

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