A harmonic suppression method of voltage-controlled DG-grid interface converter based on two-step virtual impedance

Abstract

This invention proposes a harmonic suppression method for voltage-controlled DG-Grid interface converters based on a two-step virtual impedance. This method utilizes a two-step virtual impedance control structure to actively suppress DG output current harmonics by reconstructing the harmonic voltage reference command. The effectiveness of the method is visually verified through the output voltage and current waveforms. Theoretical and simulation results show that this method not only effectively improves the DG output current quality under different harmonic severity levels but also possesses advantages such as simple structure and ease of integration, providing an effective harmonic mitigation solution for high-standard grid-connected operation of distributed generation systems.

Description

Technical Field

[0001] This invention belongs to the field of power electronic converter control technology, specifically relating to a harmonic suppression method for a voltage-controlled DG-Grid interface converter based on two-step virtual impedance. Background Technology

[0002] With the increasing severity of global environmental problems and the gradual transformation of the energy structure, traditional synchronous generators in power systems are being gradually replaced by distributed generation (DG) systems such as photovoltaic power generation. However, distributed generation systems inevitably face harmonic pollution caused by nonlinear loads. In response, the power grid has corresponding requirements and standards for the power quality of DG systems. To ensure optimal interconnection between distributed generation and the grid, most interface inverters use voltage source inverters, and in microgrids, voltage control methods are typically employed. To effectively manage grid harmonics, two approaches can be taken: firstly, adding auxiliary equipment such as passive power filters (PFFs) and active power filters (APFs) can suppress harmonics; secondly, controlling the distributed generation (DG) system itself and optimizing control strategies can ensure that it not only avoids introducing additional harmonics but also improves the power quality of the original system. For example, the harmonic voltage of the DG system can be controlled to be proportional to the harmonic voltage at the point of common coupling (PCC). However, during harmonic suppression, due to the limitation of the proportionality coefficient, when the harmonics from nonlinear loads are significant, the harmonics contained in the DG side current are still relatively high, failing to meet the grid connection requirements of the inverter. Summary of the Invention

[0003] To address the problems existing in the background technology, this paper proposes a harmonic suppression method for voltage-controlled DG-Grid interface converters based on two-step virtual impedance. By extracting the system's line current and load current to form a virtual two-step impedance method, the harmonics of the output current on the DG side are effectively suppressed while minimizing the impact on the PCC point. The effectiveness of the method is intuitively verified by the total harmonic distortion (THD) of the output current and voltage waveforms. Theoretical and simulation results show that this method not only achieves similar results to traditional methods when the harmonics caused by nonlinear loads are relatively small, but also reduces the current THD on the DG side to a low value when the harmonics caused by nonlinear loads are severe. To achieve the above objectives, such as Figure 1 The diagram shown is a control structure block diagram. The technical solution of this invention is as follows: 1. Parameter Measurement: Real-time acquisition of key electrical quantities of the DG-Grid system, including grid current, load harmonic current, parallel capacitor voltage, converter output current, and PCC voltage; 2. Control Strategy: A second-order generalized integrator (SOGI) is used to extract harmonic components of a specified order, and a reference harmonic voltage for the parallel capacitor is constructed based on the extracted harmonic information. This reference voltage consists of two terms: one related to the line harmonic current and the other related to the load harmonic current. 3. Closed-loop control implementation: Through a dual-loop structure consisting of outer-loop voltage control and inner-loop current control, namely, the outer-loop voltage control is a parallel P+ resonant controller and the inner-loop current control is a proportional controller, the precise tracking of harmonics of a specified order is achieved; The mechanism of this invention is as follows: by constructing a parallel capacitor reference voltage that includes grid-side harmonic current feedback and load-side harmonic current feedforward, the harmonics of the output current of the drain generator (DG) are effectively suppressed. The former suppresses the harmonic output of the DG itself by actively increasing the equivalent output impedance of the DG at the harmonic frequency, thereby significantly reducing the harmonic content of the DG current; the latter reduces the harmonic components injected into the grid by offsetting the load harmonic current, thus mitigating the pollution of the PCC voltage. In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are: This invention introduces an improved control strategy based on two-step virtual impedance into the harmonic suppression mode of traditional voltage-controlled DG units. It can be directly embedded into droop-controlled voltage-type DG systems. Compared with traditional harmonic suppression methods, it can further reduce current harmonics on the DG side while minimizing pollution to the PCC voltage. Attached Figure Description

[0004] Figure 1 The control block diagram for the proposed harmonic suppression method based on voltage-controlled DG is shown below. Figure 2 For the parallel capacitor voltage based on voltage control DG Response modeling Figure 3 for Closed-loop response Figure 4 for Closed-loop response Figure 5 The attenuation ratio of DG harmonic current when only the line current branch is added. Figure 6 The attenuation ratio of PCC harmonic voltage after adding line current branch and load current. Figure 7 DG current harmonic analysis when nonlinear loads introduce large harmonics Figure 8 DG current harmonic analysis when the harmonics introduced by the nonlinear load are small Detailed Implementation

[0005] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The control method of this invention is designed for voltage-controlled DG systems. The control block diagram of the two-step impedance method for suppressing harmonics is as follows: Figure 1 As shown. The distributed generation system passes through a converter and then an LC filter, with the filter inductance being... The filter capacitor is It is connected to the power grid. The line impedance is... The line current is The grid resistance is The grid current is The current flowing through the nonlinear load is For the control system, the converter output current is measured. Parallel capacitors Voltage at both ends Line current and load current Then the measured signal is transformed from the abc coordinate system to A coordinate system is used to implement the proposed control strategy. This plan includes the following steps: Step 1. Extract the harmonics of the load current and line current using a second-order generalized integrator (SOGI), and multiply them by the corresponding virtual resistors to form harmonic voltages that are injected into the voltage loop. (1) (2) (3) in, The cutoff frequency of SOGI is... This is the fundamental angular frequency. (4) (5) (6) in, These are the harmonic components of the relevant line current. For the harmonic components of the load current, and For virtual resistance, This is a virtual inductance. The fundamental voltage reference value obtained through power synchronization control. and The two ends of the parallel capacitor are obtained by adding them together. Voltage reference quantity . Step 2. Select voltage and current loop parameters to ensure that harmonic voltage tracking can be performed. Parallel capacitor voltage based on voltage-controlled DG Response modeling, such as Figure 2 As shown. Wherein, the reference voltage... For input, line current This is a disturbance. The transfer function of the outer loop voltage controller is given, and a voltage controller based on a parallel P+ resonant controller is adopted. for: (7) in, This refers to the proportional control gain at each frequency. The integral gain at each frequency, This represents the cutoff bandwidth at each frequency. Assuming PWM is ideal, Modeled using delay units: (8) The modulation process of the system is as follows: (9) (10) in The voltage modulated by the converter, It is a proportional controller, the outer control loop output is, and the converter output current is. . according to Figure 2 The control block diagram and (4) to (10) can be used to obtain the parallel capacitor voltage. The closed-loop response is: (11) In the formula The reference voltage across the parallel capacitor The closed-loop gain, The impedance of the harmonic voltage source for the series closed-loop output controller. and The expression is as follows: (12) (13) in, (14) (15) (16) Bird diagram Figure 3 As shown in the figure, the system response is close to 0 dB and 0° at the 5th, 7th, 11th, and 13th harmonics, indicating that the system responds well to these harmonics. It can track very well. But note that... The closed-loop response is only valid for the specified harmonic frequencies. Series output impedance Bird diagram Figure 4 As shown. At the selected harmonic, the amplitude of the response is very low; therefore, the response of the harmonic voltage of the parallel capacitor is mainly determined by the reference voltage. Decision, and The impact on the system is minimal. Similarly, The closed-loop response is only valid for the specified harmonic frequencies. Step 3. Verify the effectiveness of the solution. That is, adding a line current branch can effectively improve the current harmonics on the DG side, and adding a load current branch can reduce the impact on the PCC voltage. Compared to the uncontrolled mode (i.e., without harmonic suppression), the attenuation ratio of the DG harmonic current after adding the line current branch can be calculated as follows: (17) Compared to the uncontrolled mode (i.e., without harmonic suppression), the attenuation ratio of the PCC harmonic voltage after adding the line current branch and the load current branch can be calculated as follows: (18) The Bode diagrams for (17) and (18) are as follows: Figure 5 and Figure 6 As shown, it can be seen that as the virtual resistance of the line current branch increases, the attenuation ratio of the DG harmonic current decreases, and as the virtual resistance of the load current branch increases, the attenuation ratio of the PCC harmonic voltage decreases. To verify the above conclusions, Matlab simulation software was used for verification. The proposed strategy improvement method consists of two aspects. When the harmonics caused by the nonlinear load are severe, the current harmonics on the DG side are still too high even when the traditional method achieves optimal results. This scheme can further reduce the harmonics to within the required range. Figure 7 As shown, traditional methods can significantly reduce current harmonics on the DG side, but the THD value is also relatively large when the effect is best. This solution can further reduce the THD value of traditional solutions. When the harmonics caused by the nonlinear load are small, this solution can achieve the same harmonic suppression effect as traditional methods. For example... Figure 8As shown, this scheme, when added, can reduce the current harmonic THD value on the DG side to a value that is close to that of the traditional method. In summary, this invention proposes a harmonic suppression method for voltage-controlled DG-Grid interface converters based on two-step virtual impedance. The harmonic voltage reference is decomposed into two items related to grid harmonic current and load harmonic current. This method can not only effectively suppress DG output current harmonics and reduce the impact on PCC voltage quality, but also be directly embedded into the existing voltage control architecture without relying on an additional current tracking loop. The above description is merely a specific implementation method of the present invention. Any feature disclosed in this specification may be replaced by other equivalent or similar alternative features unless otherwise specified. All disclosed features, or steps in all methods or processes, may be combined in any way except for mutually exclusive features or steps.

Claims

1. A harmonic suppression method for a voltage-controlled DG-Grid interface converter based on two-step virtual impedance, the proposed control structure of which is shown in Figure 1, includes the following steps: Step 1. Extract the harmonics of the load current and line current using a second-order generalized integrator (SOGI), and multiply them by the corresponding virtual resistors to form harmonic voltages that are injected into the voltage loop. (1) (2) (3) in, The cutoff frequency of SOGI, This is the fundamental angular frequency. (4) (5) (6) in, These are the harmonic components of the relevant line current. For the harmonic components of the load current, and For virtual resistance, This is a virtual inductance. The fundamental voltage reference value obtained through power synchronization control. and Add them together to get the terminals of the parallel capacitor. voltage reference quantity . Step 2. Select voltage and current loop parameters to ensure that harmonic voltage tracking can be performed. Parallel capacitor voltage based on voltage-controlled DG The response modeling is shown in Figure 2. The reference voltage is... For input, line current This is a disturbance. The transfer function of the outer loop voltage controller is given, and a voltage controller based on a parallel P+ resonant controller is adopted. for: (7) in, This refers to the proportional control gain at each frequency. The integral gain at each frequency, This represents the cutoff bandwidth at each frequency. Assuming PWM is ideal, Modeled using delay units: (8) The modulation process of the system is as follows: (9) (10) in The voltage modulated by the converter, It is a proportional controller, the outer control loop output is, and the converter output current is. . Based on the control block diagram in Figure 2 and (4) to (10), the parallel capacitor voltage can be obtained. The closed-loop response is: (11) In the formula The reference voltage across the parallel capacitor The closed-loop gain, The impedance of the harmonic voltage source for the series closed-loop output controller. and The expression is as follows: (12) (13) in, (14) (15) (16) The Bode plot is shown in Figure 3. As can be seen from the figure, the system response is close to 0 dB and 0° at the 5th, 7th, 11th, and 13th harmonics, indicating that the system's response to... It can track very well. However, note that... The closed-loop response is only valid for the specified harmonic frequencies. Series output impedance The Bode plot is shown in Figure 4. At the selected harmonic, the amplitude of the response is very low; therefore, the response of the harmonic voltage of the parallel capacitor is mainly determined by the reference voltage. Decision, and The impact on the system is minimal. Similarly, The closed-loop response is only valid for the specified harmonic frequencies. Step 3. Verify the effectiveness of the solution. That is, adding a line current branch can effectively improve the current harmonics on the DG side, and adding a load current branch can reduce the impact on the PCC voltage. Compared to the uncontrolled mode (i.e., without harmonic suppression), the attenuation ratio of the DG harmonic current after adding the line current branch can be calculated as follows: (17) Compared to the uncontrolled mode (i.e., without harmonic suppression), the attenuation ratio of the PCC harmonic voltage after adding the line current branch and the load current branch can be calculated as follows: (18) The Bode plots for (17) and (18) are shown in Figures 5 and 6, respectively. It can be seen that as the virtual resistance of the line current branch increases, the attenuation ratio of the DG harmonic current decreases, and as the virtual resistance of the load current branch increases, the attenuation ratio of the PCC harmonic voltage decreases.

2. The harmonic suppression method for a voltage-controlled DG-Grid interface converter based on two-step virtual impedance as described in claim 1, characterized in that, Its harmonic voltage reference signal is decomposed into two terms. The first term is related to the grid-side harmonic current, which realizes the harmonic suppression of the DG-side current. The second term is related to the load-side harmonic current, which minimizes the impact on the power quality of the PCC-side.

3. The harmonic suppression method for a voltage-controlled DG-Grid interface converter based on two-step virtual impedance as described in claim 1, characterized in that, The method is applicable to voltage-controlled DG systems based on droop control, and does not require the addition of a separate harmonic current tracking control loop.

4. The harmonic suppression method for a voltage-controlled DG-Grid interface converter based on two-step virtual impedance as described in claim 1, characterized in that, When the harmonic content of the nonlinear load is high, this method can further reduce the total harmonic distortion rate of the DG output current on the basis of the traditional harmonic suppression strategy, so as to meet the grid-connected power quality standards.

5. The harmonic suppression method for a voltage-controlled DG-Grid interface converter based on two-step virtual impedance as described in claim 1, characterized in that, When the harmonic content of the nonlinear load is low, this method can still maintain a harmonic suppression effect similar to that of traditional harmonic suppression strategies.

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