Phasor closed optimization control method for secondary harmonic suppression of direct-current micro-grid

Abstract

The invention discloses a phasor closed optimization control method for secondary harmonic suppression of a direct-current micro-grid, and the method comprises the steps: taking the square of a total harmonic current amplitude as a measurement standard, and searching an optimal harmonic reference phase through a gradient descent method; adjusting the harmonic phase injected into the direct current system by adjusting the phase of the voltage of the alternating current side inverter or changing the harmonic phase generated by the single-phase inverter by adjusting the voltage phase of the single-phase inverter; and an alternating current reference current generating ring is constructed by adopting a bidirectional DC-DC converter in a step-up or step-down mode to ensure that the absorption current and the residual harmonic waves are synchronous in phase, so that the residual harmonic waves are filtered out. According to the invention, under the condition of not damaging the electric energy direct current of the alternating current side, the second harmonic current injected into the direct current micro-grid is reduced, the configuration capacity of the filter is reduced, and meanwhile, the complete absorption of the amplitude and phase difference between the compensation current of the parallel filter and the actual second harmonic current is realized.

Description

Technical Field

[0001] The present invention relates to a technology in the field of microgrid control, and in particular to a phasor closure optimization control method for second harmonic suppression in a DC microgrid. Background Art

[0002] Unbalanced AC loads generate second harmonic currents, which induce second harmonic voltages on the common bus of the DC microgrid. This second harmonic voltage not only causes DC bus voltage instability but can also affect the efficiency and service life of devices like batteries and fuel cells, as well as the safe operation of sensitive loads. Summary of the Invention

[0003] In response to the above-mentioned deficiencies in the prior art, the present invention proposes a phasor closure optimization control method for second harmonic suppression in a DC microgrid. By improving the control strategy of the inverter, the second harmonic current injection that would otherwise deteriorate the power quality on the AC side is reduced. This method can reduce the second harmonic current injected into the DC microgrid without damaging the DC power on the AC side, thereby reducing the configuration capacity of the filter and achieving complete absorption of the amplitude and phase difference between the compensation current of the parallel filter and the actual second harmonic current.

[0004] The present invention is achieved through the following technical solutions:

[0005] The present invention relates to a phasor closure optimization control method for second harmonic suppression in a DC microgrid. The method uses the square of the total harmonic current amplitude as a metric, searches for the optimal harmonic reference phase using a gradient descent method, and then adjusts the phase of the harmonics injected into the DC system by adjusting the phase of the AC-side inverter voltage, or changes the phase of the harmonics generated by the single-phase inverter by adjusting the voltage phase of the single-phase inverter. Furthermore, an AC reference current generation loop is constructed by using a bidirectional DC-DC converter in boost or buck mode to ensure that the absorbed current is synchronized with the residual harmonics in phase, thereby filtering out the residual harmonics.

[0006] The present invention relates to a system for implementing the above-mentioned method, comprising: an analog-to-digital conversion module, a Fourier transform module, a harmonic phase reference phase generation module, a harmonic phase tracking module, and a residual harmonic absorption module, wherein: the analog-to-digital conversion module converts an analog signal from an actual circuit into a digital signal; the Fourier transform module converts the digital signal from a time domain value into a phasor value; the harmonic phase reference phase generation module searches for a harmonic phase reference phase based on the phasor value by a gradient descent method; the harmonic phase tracking module performs harmonic phase tracking control of a three-phase or single-phase inverter based on the harmonic phase reference phase to obtain a control signal for an inverter switch tube; and the residual harmonic absorption module performs homodyne tracking of DC and AC references using a boost or buck filter through a PI controller and a PIR controller based on the harmonic phase reference phase to obtain a control signal for an active filter switch tube.

[0007] Technical Effects

[0008] The present invention uses a harmonic phase optimization control strategy to enable the second harmonic current phase tracking to achieve the phase of optimal cancellation of the second harmonic source, thereby achieving optimal cancellation of the harmonic phasor and generating a compensation current reference value based on the characteristics of the boost mode and buck mode of the filter. Compared with the existing technology, the present invention can reduce the total amount of harmonics input to the DC microgrid without compromising the power quality on the AC side, while ensuring that the absorption current and the residual harmonics are synchronized in phase, and their amplitudes are consistent with the set ratio, thereby achieving harmonic sharing and precise absorption of residual harmonics. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 is a schematic diagram of the present invention;

[0010] In the figure: (a) is the system principle diagram; (b) is the principle diagram of this method;

[0011] FIG2 is a schematic diagram of the harmonic reference phase generation process;

[0012] FIG3 is a schematic diagram of harmonic phase tracking control of a three-phase inverter;

[0013] FIG4 is a schematic diagram of harmonic phase tracking control of a single-phase inverter;

[0014] FIG5 is a schematic diagram of an equivalent model of a boost filter;

[0015] FIG6 is a schematic diagram of precise absorption control of residual harmonics for a boost filter;

[0016] FIG7 is a schematic diagram of an equivalent model of a boost filter;

[0017] FIG8 is a schematic diagram of precise absorption control of residual harmonics for a step-down filter;

[0018] Figure 9 shows the results of scenario 1: bus voltage, bus capacitor current, and the amplitude of the second harmonic on the bus voltage;

[0019] Figure 10 shows the results of scenario 1: the amplitude of the second harmonic on the bus voltage, the amplitude of the harmonic current source, and the phase diagram of the harmonic current source;

[0020] Figure 11 is the result of scenario 1: a schematic diagram of the phase tracking process of the harmonic current source;

[0021] Figure 12 shows the results of scenario 1: AC voltage measurement schematic;

[0022] Figure 13 shows the results of scenario 2: bus voltage, bus capacitor current, and the amplitude of the second harmonic on the bus voltage;

[0023] In the figure: (a) is a schematic diagram of the present invention; (b) is a schematic diagram of a method based on virtual impedance; (c) is a schematic diagram of a method based on a metaheuristic algorithm;

[0024] Figure 14 shows the results of scenario 2: a schematic diagram of the amplitude and phase of the first filter, the second filter, and the total absorption current;

[0025] In the figure: (a) is a schematic diagram of the present invention; (b) is a schematic diagram of a method based on virtual impedance; (c) is a schematic diagram of a method based on a metaheuristic algorithm;

[0026] Figure 15 shows the results of scenario 2: a comparison diagram of residual harmonics and filter absorption current. DETAILED DESCRIPTION

[0027] As shown in FIG1 , this embodiment relates to a phasor closure optimization control method for second harmonic suppression in a DC microgrid, including: a harmonic phase optimization stage and a residual harmonic absorption stage. The harmonic phase optimization stage optimizes the phase of the second harmonic current by generating a harmonic reference phase and tracking the harmonic phase. The residual harmonic absorption stage filters out the residual harmonics through a filter, specifically including:

[0028] Step 1: Generate a harmonic reference phase, specifically including:

[0029] 1.1 To measure the effect of harmonic cancellation, the square of the total harmonic current amplitude is used as a metric, so that the optimal elimination of harmonics is equivalent to minimizing the square of the total harmonic current amplitude. , specifically:

[0030] ,in: represents N second harmonic current sources, .

[0031] 1.2 As shown in Figure 2, the gradient descent method is used to search for the optimal harmonic reference phase to minimize the square of the total harmonic current amplitude. , that is, multiple iterative calculations: , that is, a decrease Output reference phase after step, where: is the regulation rate, ,as well as , after descending n steps, we get: ,in: is the harmonic reference phase, is declining The result after step.

[0032] 1.3 After several control intervals, repeat step 1.2 several times to find the optimal harmonic reference phase.

[0033] The gradient descent method mentioned above saturates the gradient to prevent the sudden change of the gradient from causing a large phase change. Specifically, the saturation function is set as follows: ,in: Represent the lower and upper thresholds respectively.

[0034] Step 2: Harmonic phase tracking control of three-phase inverter and single-phase inverter.

[0035] The harmonic phase tracking control of the three-phase inverter is to adjust the phase of the harmonics injected into the DC system by adjusting the phase of the AC-side inverter voltage. As shown in FIG3 , the three-phase inverter adopts Vf control based on dq decoupling, and the relationship between the abc-axis voltage and the dq-axis voltage satisfies: ,in: 、 and is the three-phase voltage, 、 are the d-axis and q-axis voltages, is the voltage amplitude; by controlling and Adjust the phase of the three-phase voltage to meet , that is, using space vector and The relationship between simplifies this problem into a single-input unconstrained control task. The angle between it and the voltage space vector is ,but After substituting, the voltage of axis abc is expressed as , that is, adjust The phase of the three-phase voltage can be changed. is a DC variable, the PI controller can achieve the first The harmonic phase generated by each inverter arrive The error-free tracking and It is projected from the triangle Generated and then tracked by voltage-current closed loop control and .

[0036] The harmonic phase tracking control of the single-phase inverter is to change the harmonic phase generated by the single-phase inverter by adjusting the voltage phase of the single-phase inverter. As shown in Figure 4, a PR controller is used to track the sinusoidal reference voltage. , is the phase of the sinusoidal reference voltage, is a single-phase voltage, It is the reference phase of the optimization generation loop output of the harmonic reference phase.

[0037] Step 3: Filter out the residual harmonics using a bidirectional DC-DC converter in boost or buck mode, including:

[0038] 3.1 Accurate Absorption Control of Residual Harmonics in Boost Mode: As shown in Figure 5, a bidirectional DC-DC converter model in boost mode is shown, where the capacitor is located on the high voltage side and the DC bus is located on the low voltage side. Indicates the harmonics, Represents the output current of the filter, and its second harmonic component In order to prevent harmonics from flowing into bus capacitors and other branches, represents the inductor current, represents the capacitor voltage, is the current flowing into the bus capacitor, is the bus voltage, the inductor of the bidirectional DC-DC converter model in boost mode is in the absorption branch, and the residual harmonics are used. As a reference for harmonic compensation After incorporating it into the inner loop control, the PR controller is used to realize right Homodyne tracking, specifically:

[0039] As shown in Figure 6, the capacitor voltage of the bidirectional DC-DC converter Including the second harmonic voltage, a double frequency reference current is obtained through the PI controller; in order to prevent the voltage loop from generating unnecessary AC reference signals, the second harmonic voltage is removed from the voltage feedback. , ensuring that the voltage loop only outputs DC reference, and the current inner loop uses PIR controller to achieve DC reference and exchange references The control signal d of the active filter switch tube is obtained by homodyne tracking.

[0040] 3.2 Accurate absorption control of residual harmonics in buck mode: As shown in Figure 7, it is a bidirectional DC-DC converter model in buck mode. The capacitor of the filter is located on the low voltage side and the DC bus is located on the high voltage side. 、 、 As a state variable, After establishing the state space equation as input, at the steady-state point Build a small signal model , after converting to the Laplace domain, we get the Laplace model , and then deduce and The transfer function between ,in: is the inductance value, is the capacitance value, is the busbar capacitance value, , Contains ① proportional response items ,②Integral response term , ③ differential response term ;when hour, , It is the superposition of AC and DC signals; the PR controller shown in Figure 8 is used to generate a complete tracking The required AC reference is adjusted by Produced and Synchronous Achieve closed-loop control to ensure AC reference The control signal d of the active filter switch tube is obtained by homodyne tracking.

[0041] As shown in Figure 8, the PR controller is used to automatically generate a Required AC Reference , to achieve closed-loop control, the inner loop uses a PIR controller to ensure the AC reference The control signal d of the active filter switch tube is obtained by homodyne tracking.

[0042] After specific practical experiments, the system was implemented using an STM32 microcontroller and modeled on the OP5700 (OPAL-RT) hardware-in-the-loop (HIL) simulation platform. The system includes three three-phase inverters, a single-phase inverter, a boost active power filter, a buck active power filter, and a photovoltaic energy storage system (to maintain the bus voltage at 700V). Table 1 lists the system and control parameters related to second harmonic suppression.

[0043] The parameter settings for optimizing the generated harmonic reference phase are as follows: , , ,and .

[0044] Table 1 System parameters and control parameters

[0045] The simulation was carried out in the following scenarios:

[0046] Scenario 1) Initially, all four inverters generate harmonics;

[0047] Scenario 2) At 0.2 seconds, harmonic elimination control is activated, but precise absorption control of residual harmonics is not started.

[0048] The results are summarized in Table 2.

[0049] Table 2 Results of Scenario 1

[0050] As can be clearly seen in Table 2, the total harmonic amplitude was large before harmonic phase adjustment. However, after phase adjustment, the total harmonic amplitude was significantly reduced: 1) The first three scenarios achieved complete harmonic cancellation; 2) The last two scenarios achieved optimal cancellation by shifting the largest harmonic out of phase with the other harmonics, although not completely eliminating them.

[0051] As shown in Figure 9, before harmonic elimination control is activated, the second harmonic voltage on the bus is as high as 27 V. After activating harmonic elimination control and adjusting the harmonic phase, the harmonics flowing into the bus capacitors gradually decrease to zero, and the second harmonic voltage on the bus gradually disappears.

[0052] As shown in Figure 10, 0.2 seconds prior, the phase of each harmonic source was set to 0°, resulting in very high harmonic output to the DC system, reaching up to 62A. At this point, the second harmonic voltage on the bus surged to 3.8%. After phase adjustment, the harmonic phases were adjusted to -113.79°, 141.52°, -80.25°, and 48.11°, respectively, achieving optimal harmonic cancellation. Figure 11 shows the process of harmonic phase tracking the reference phase. After 0.4 seconds, the phase can track the reference phase, demonstrating rapid adjustment speed.

[0053] Figure 12 shows the AC voltage variation. During the transient phase, from 0.2 to 0.6 seconds, no noticeable distortion is observed in the AC voltage. Subsequently, during the steady-state phase, after 0.6 seconds, the AC voltage remains stable. This is because simply adjusting the AC voltage phase does not disrupt its waveform. Furthermore, adjusting the voltage phase does not affect normal user experience, as users are primarily concerned with the voltage magnitude and frequency, not the phase. Therefore, the present invention has no impact on the AC side.

[0054] The present invention was further compared with existing virtual resistor-based methods and existing metaheuristic algorithms. Simulations were performed for the following scenario: optimal harmonic cancellation was achieved before 0.1 seconds, and precise absorption control of residual harmonics was initiated at 0.1 seconds. Multiple simulations were performed, and the amplitude and phase of the residual harmonics, the absorbed current of each filter, and the total absorbed current are summarized in Table 3.

[0055] Table 3 Results of Scenario 2

[0056] As shown in Table 3, the virtual resistor-based and metaheuristic-based methods fail to achieve proportional absorption of residual harmonics. This limitation stems from the fact that these methods can only distribute harmonics among devices of the same type. Under the control of the virtual resistor-based or metaheuristic-based methods, there are phase and amplitude discrepancies between the total absorbed current and the residual harmonics, indicating an inability to accurately absorb the residual harmonics. In contrast, under the control of the present invention, the residual harmonics are evenly shared between the two filters, and the total absorbed current is fully synchronized with the residual harmonics.

[0057] As shown in Figure 13, the present invention completely and accurately absorbs the residual harmonics and prevents them from flowing into the bus capacitors. In contrast, under the control of the virtual resistor-based method or the metaheuristic algorithm-based method, some harmonics still flow into the bus capacitors, resulting in the presence of second harmonic voltage after absorption.

[0058] As can be observed in Figure 14, the meta-heuristic algorithm-based method can only ensure that the absorption current of the boost filter is aligned with the residual harmonics in phase and that the amplitude conforms to a predetermined ratio. Although the virtual resistor-based method has a certain filtering effect, it is completely unable to achieve the matching of the absorption current and the residual harmonics in phase and amplitude. In contrast, under the control of the present invention, the phase of the absorption current of the boost and buck filters is precisely matched with the phase of the residual harmonics. In addition, the absorption current amplitude of the two devices is half the amplitude of the residual harmonics. Therefore, the present invention accurately absorbs the residual harmonics and ensures that the absorption current and the residual harmonics are not deviated in amplitude and phase.

[0059] As shown in Figure 15 , under the control of the present invention, the total absorption current is completely synchronized with the residual harmonics in both amplitude and phase. Consequently, residual harmonics are prevented from flowing into other branches. However, under control methods based on virtual resistors or metaheuristic algorithms, significant phase and amplitude differences exist between the absorption current and the residual harmonics, making it impossible to fully absorb the residual harmonics.

[0060] Compared with the prior art, the present invention effectively adjusts the harmonic phase and achieves optimal harmonic elimination without affecting the power quality on the AC side. It can also accurately absorb residual harmonics and distribute them proportionally among filters.

[0061] The above-mentioned specific implementation can be partially adjusted in different ways by those skilled in the art without departing from the principles and purpose of the present invention. The scope of protection of the present invention shall be based on the claims and shall not be limited by the above-mentioned specific implementation. All implementation schemes within its scope shall be subject to the constraints of the present invention.

Claims

1. A phasor closure optimization control method for second harmonic suppression in a DC microgrid, characterized in that: The square of the total harmonic current amplitude is used as a metric. After searching for the optimal harmonic reference phase using the gradient descent method, the phase of the harmonics injected into the DC system is adjusted by adjusting the phase of the AC-side inverter voltage, or the phase of the harmonics generated by the single-phase inverter is changed by adjusting the voltage phase of the single-phase inverter. Then, an AC reference current generation loop is constructed by using a bidirectional DC-DC converter in boost or buck mode to ensure that the absorbed current and the residual harmonics are synchronized in phase, thereby filtering out the residual harmonics.

2. The phasor closure optimization control method for second harmonic suppression in a DC microgrid according to claim 1 is characterized in that: The optimal harmonic reference phase is obtained by: 1.1 To measure the effect of harmonic cancellation, the square of the total harmonic current amplitude is used as a metric, so that the optimal elimination of harmonics is equivalent to minimizing the square of the total harmonic current amplitude. , specifically: ,in: represents N second harmonic current sources, ; 1.2 Use the gradient descent method to search for the optimal harmonic reference phase to minimize the square of the total harmonic current amplitude , that is, multiple iterative calculations: , that is, a decrease Output reference phase after step, where: is the regulation rate, ,as well as , after descending n steps, we get: ,in: is the harmonic reference phase, is declining The result after the step; 1.3 After several control intervals, repeat step 1.2 several times to find the optimal harmonic reference phase.

3. The phasor closure optimization control method for second harmonic suppression in a DC microgrid according to claim 2 is characterized in that: The gradient descent method mentioned above saturates the gradient to prevent the sudden change of the gradient from causing a large phase change. Specifically, the saturation function is set as follows: ,in: Represent the lower and upper thresholds respectively.

4. The phasor closure optimization control method for second harmonic suppression in a DC microgrid according to claim 1 is characterized in that: The phase adjustment of the AC side inverter voltage adjusts the harmonic phase injected into the DC system. Specifically, the three-phase inverter adopts Vf control based on dq decoupling, and the relationship between the abc axis voltage and the dq axis voltage satisfies: ,in: 、 and is the three-phase voltage, 、 are the d-axis and q-axis voltages, is the voltage amplitude; by controlling and Adjust the phase of the three-phase voltage to meet , that is, using space vector and The relationship between simplifies this problem into a single-input unconstrained control task. The angle between it and the voltage space vector is ,but After substituting, the voltage of axis abc is expressed as , that is, adjust The phase of the three-phase voltage can be changed. Since the reference phase of the harmonic reference phase generation link output is a DC variable, and the PI controller implements the The harmonic phase generated by each inverter arrive The error-free tracking and It is projected from the triangle Generated and then tracked by voltage-current closed loop control and .

5. The phasor closure optimization control method for second harmonic suppression in a DC microgrid according to claim 1 is characterized in that: The voltage phase of the single-phase inverter is adjusted to change the harmonic phase generated by the single-phase inverter. , specifically: Use PR controller to track the sinusoidal reference voltage , is the phase of the sinusoidal reference voltage, is a single-phase voltage, It is the reference phase of the optimization generation loop output of the harmonic reference phase.

6. The phasor closure optimization control method for second harmonic suppression in a DC microgrid according to claim 1 is characterized in that: The residual harmonics specifically include: 3.1 Accurate Absorption Control of Residual Harmonics in Boost Mode: In the boost mode, the capacitor of the bidirectional DC-DC converter model is located on the high voltage side, and the DC bus is located on the low voltage side. Indicates the harmonics, Represents the output current of the filter, and its second harmonic component In order to prevent harmonics from flowing into bus capacitors and other branches, represents the inductor current, represents the capacitor voltage, is the current flowing into the bus capacitor, is the bus voltage, the inductor of the bidirectional DC-DC converter model in boost mode is in the absorption branch, and the residual harmonics are used. As a reference for harmonic compensation After incorporating it into the inner loop control, the PR controller is used to realize right Homodyne tracking, specifically: the capacitor voltage of the bidirectional DC-DC converter Including the second harmonic voltage, a double frequency reference current is obtained through the PI controller; in order to prevent the voltage loop from generating unnecessary AC reference signals, the second harmonic voltage is removed from the voltage feedback. , ensuring that the voltage loop only outputs DC reference, and the current inner loop uses PIR controller to achieve DC reference and exchange references The control signal d of the active filter switch tube is obtained by homodyne tracking; 3.2 Accurate absorption control of residual harmonics in buck mode: In the buck mode, the capacitor of the filter of the bidirectional DC-DC converter model is located on the low voltage side and the DC bus is located on the high voltage side. 、 、 As a state variable, After establishing the state space equation as input, at the steady-state point Build a small signal model , after converting to the Laplace domain, we get the Laplace model , and then deduce and The transfer function between ,in: is the inductance value, is the capacitance value, is the busbar capacitance value, , Contains ① proportional response items ,②Integral response term , ③ differential response term ; when hour, The response is: ,in: It is the superposition of AC and DC signals; PR controller is used to generate full tracking The required AC reference is adjusted by Produced and Synchronous Achieve closed-loop control to ensure AC reference The control signal d of the active filter switch tube is obtained by homodyne tracking.

7. A phasor closure optimization control system for DC microgrid second harmonic suppression implementing the method according to any one of claims 1 to 6, characterized in that: include: An analog-to-digital conversion module, a Fourier transform module, a harmonic phase reference phase generation module, a harmonic phase tracking module and a residual harmonic absorption module, wherein: the analog-to-digital conversion module converts an analog signal from an actual circuit into a digital signal; the Fourier transform module converts the digital signal from a time domain value into a phasor value; the harmonic phase reference phase generation module obtains a harmonic phase reference phase by searching the phasor value through a gradient descent method; the harmonic phase tracking module performs harmonic phase tracking control of a three-phase or single-phase inverter based on the harmonic phase reference phase to obtain a control signal for the inverter switch tube; the residual harmonic absorption module uses a boost or buck filter to perform homodyne tracking of DC and AC references through a PI controller and a PIR controller based on the harmonic phase reference phase to obtain a control signal for the active filter switch tube.

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