Double frequency ripple suppression control method and system for cascaded energy router

Abstract

The invention discloses a frequency doubling ripple suppression control method and system for a cascade energy router. The method comprises the following steps: acquiring modulation voltage, a modulation degree of a modulation module and a power factor angle; when the modulation degree is greater than a set modulation degree threshold value, according to the modulation degree, calculating to obtain a harmonic injection proportion limit value before and after third harmonic injection and when the H bridge is not over-modulated; according to the harmonic injection proportion limit value, a power factor angle value is obtained through calculation, and the value is a power factor angle minimum inflection point; according to the minimum inflection point of the power factor angle, the limit value of the harmonic injection proportion and a second calculation formula of the harmonic injection proportion, the optimal harmonic injection proportion is obtained through calculation; when the modulation degree is smaller than or equal to a set modulation degree threshold value, an optimal harmonic injection proportion is obtained through calculation according to a first calculation formula of the power factor angle and the harmonic injection proportion; and according to the modulation voltage and the optimal harmonic injection proportion, calculating to obtain a switch driving signal of the H bridge. And the optimal ripple suppression effect can be realized.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and in particular, to a method and system for suppressing double-frequency ripple of a cascaded energy router. Background Art

[0002] Based on the cascaded H-bridge energy router with a three-phase isolated common DC bus, there is a problem of double-frequency power frequency ripple on the DC side of the H-bridge. This problem not only brings the phenomenon of inductor current envelope ripple in the DAB module, but also poses challenges to the value and volume of the electrolytic capacitor on the DC side of the H-bridge.

[0003] Current strategies for suppressing double-frequency power frequency ripple mainly include hardware suppression strategies, ripple suppression strategies based on harmonic injection, and strategies for reducing the impedance of the DAB module to naturally couple and eliminate the ripple on the common DC bus side.

[0004] However, the hardware suppression strategy requires adding active or passive devices and circuits on the basis of the original hardware topology. These methods will add additional devices, thus bringing an increase in volume, weight, and cost. The ripple suppression strategy based on harmonic injection does not consider the operating conditions of non-unit power factor grid connection when injecting third harmonics, and there are calculation blind spots under actual grid operating conditions, which will lead to problems such as poor ripple suppression effect and increased risk of overmodulation of the module. In the strategy of reducing the impedance of the DAB module to naturally couple and eliminate the ripple on the common DC bus side, all the double-frequency power frequency ripple flowing through the DAB module will cause adverse effects such as an increase in the current stress requirement of the high-frequency transformer, an increase in the thermal effect of the phase-shifted inductor, and an increase in the voltage stress of the switching tube.

[0005] In summary, the existing double-frequency power frequency ripple suppression strategies for the three-phase isolated common DC bus cascaded H-bridge energy router all have corresponding drawbacks and cannot minimize the negative impact brought by the ripple in the actual application scenario. Summary of the Invention

[0006] In order to solve the above problems, the present invention proposes a method and system for suppressing double-frequency ripple of a cascaded energy router. One part realizes the optimal suppression of the double-frequency power frequency ripple on the DC side of the H-bridge by adaptively injecting third harmonics, and the other part completely avoids the negative impact of the ripple on the front-stage DAB converter by increasing the output impedance of the DAB.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] In the first aspect, a method for suppressing double-frequency ripple of a cascaded energy router is proposed, including:

[0009] Obtain the three-phase modulation voltage of the inverter, the modulation degree of the modulation module, and the power factor angle;

[0010] When the modulation index is greater than the set modulation index threshold, according to the modulation index, calculate the harmonic injection ratio limit value when the H-bridge is not over-modulated before and after the third harmonic injection;

[0011] According to the harmonic injection ratio limit value, calculate the power factor angle value, which is the minimum inflection point of the power factor angle;

[0012] According to the minimum inflection point of the power factor angle, the harmonic injection ratio limit value, and the second calculation formula of the harmonic injection ratio, calculate the optimal harmonic injection ratio;

[0013] When the modulation index is less than or equal to the set modulation index threshold, according to the power factor angle and the first calculation formula of the harmonic injection ratio, calculate the optimal harmonic injection ratio;

[0014] According to the three-phase modulation voltage and the optimal harmonic injection ratio, calculate the switching drive signals of the H-bridge.

[0015] Furthermore, according to the optimal harmonic injection ratio, calculate the third harmonic injection amount;

[0016] After superimposing the third harmonic injection amount on the three-phase modulation voltage and dividing by the number of H-bridges, obtain the modulation voltage of the converter;

[0017] Divide the modulation voltage of the converter by the DC bus voltage of the H-bridge to obtain the modulation wave;

[0018] According to the modulation wave, calculate the switching drive signals of each H-bridge.

[0019] Furthermore, obtain the DC side voltage of the H-bridge and the common DC bus voltage;

[0020] After processing the DC side voltage of the H-bridge through a notch filter, obtain the voltage after notch filter processing;

[0021] Divide the common DC bus voltage by the transformer turns ratio to obtain the common DC bus voltage after turns ratio processing;

[0022] Taking the common DC bus voltage after turns ratio processing as the target, perform PI control on the voltage after notch filter processing to obtain the phase shift ratio between phases of the DAB converter;

[0023] According to the phase shift ratio between phases, determine the DC bus voltage of the H-bridge.

[0024] Further, the first calculation formula for the harmonic injection ratio is as follows: when the power factor angle is greater than or equal to 0 and less than the first set modulation threshold, the optimal harmonic injection ratio is -1; when the power factor angle is greater than or equal to the first set modulation threshold and less than the second set modulation threshold, the harmonic injection ratio is calculated based on the power factor angle and used as the optimal harmonic injection ratio; when the power factor angle is greater than or equal to the second set modulation threshold, the optimal harmonic injection ratio is 0.

[0025] The second calculation formula for the harmonic injection ratio is as follows: when the power factor angle is greater than or equal to 0 and less than the minimum inflection point of the power factor angle, the limit value of the harmonic injection ratio is used as the optimal harmonic injection ratio; when the power factor angle is greater than or equal to the minimum inflection point of the power factor angle and less than the second set modulation threshold, the harmonic injection ratio is calculated based on the power factor angle and used as the optimal harmonic injection ratio; when the harmonic injection ratio is greater than or equal to the second set modulation threshold, the optimal harmonic injection ratio is 0.

[0026] Further, obtain the reference value of the active current and the reference value of the reactive current of the three-phase power grid.

[0027] Based on the reference value of the active current and the reference value of the reactive current, calculate the power factor angle.

[0028] Further, obtain the phase voltage and phase current of the three-phase power grid.

[0029] Convert the phase voltage and phase current to the dq coordinate system to obtain the d-axis current and q-axis current.

[0030] Taking the reference value of the active current and the reference value of the reactive current as the target, perform PI control on the d-axis current and q-axis current to obtain the active modulation voltage and the reactive modulation voltage.

[0031] Convert the active modulation voltage and the reactive modulation voltage to the abc coordinate system to obtain the three-phase modulation voltage of the inverter.

[0032] In the second aspect, a double-frequency ripple suppression control system for a cascaded energy router is proposed, including:

[0033] A sampling unit for obtaining the three-phase modulation voltage of the inverter, the modulation degree of the modulation module, and the power factor angle.

[0034] An optimal harmonic injection ratio calculation unit is used to calculate the limit value of the harmonic injection ratio when the H-bridge is not over-modulated before and after the third harmonic injection according to the modulation degree when the modulation degree is greater than the set modulation degree threshold; calculate the power factor angle value according to the limit value of the harmonic injection ratio, and this value is the minimum inflection point of the power factor angle; calculate the optimal harmonic injection ratio according to the minimum inflection point of the power factor angle, the limit value of the harmonic injection ratio and the second calculation formula of the harmonic injection ratio; when the modulation degree is less than or equal to the set modulation degree threshold, calculate the optimal harmonic injection ratio according to the power factor angle and the first calculation formula of the harmonic injection ratio.

[0035] A switch drive signal determination unit is used to calculate the switch drive signal of the H-bridge according to the modulation voltage and the optimal harmonic injection ratio.

[0036] In a third aspect, a single-chip microcomputer device is proposed, and the device includes:

[0037] A single-chip microcomputer is suitable for executing calculation-related programs;

[0038] A readable storage medium for a single-chip microcomputer, in which a related program is stored. When the single-chip microcomputer program is executed by the processor, it realizes a double-frequency ripple suppression control method for a cascaded energy router proposed in the first aspect.

[0039] In a fourth aspect, a readable storage medium for a single-chip microcomputer is proposed. The readable storage medium for a single-chip microcomputer stores a computer program, and the single-chip microcomputer program is suitable for being loaded and executed by a processor to implement a double-frequency ripple suppression control method for a cascaded energy router proposed in the first aspect.

[0040] In a fifth aspect, a single-chip microcomputer program product is proposed. The single-chip microcomputer program product includes a computer program, and when the single-chip microcomputer program is executed by a processor, it realizes a double-frequency ripple suppression control method for a cascaded energy router proposed in the first aspect.

[0041] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0042] A double-frequency ripple suppression control method and system for a cascaded energy router proposed by the present invention adaptively determine the optimal harmonic injection ratio according to the modulation degree and the power factor angle; when using this optimal harmonic injection ratio for the third harmonic injection, it is ensured that the H-bridge is not over-modulated before and after the third harmonic injection, and the double-frequency power frequency ripple suppression effect is the best. Compared with the prior art, this strategy can achieve adaptive optimal double-frequency power frequency ripple suppression regardless of whether the system operates at unity power factor.

[0043] In addition, the control method proposed by the present invention, in addition to determining the optimal harmonic injection ratio and then determining the third harmonic injection amount according to the optimal harmonic injection ratio, also processes the DC-side voltage of the H-bridge through a notch filter, and uses the common DC bus voltage divided by the transformer turns ratio to obtain the common DC bus voltage after turns ratio processing as the target, and performs PI control on the voltage after notch filter processing to obtain the phase shift ratio between phases of the DAB converter; finally, according to the phase shift ratio between phases, the switching drive signal of the DAB converter is determined. The two parts work together to completely avoid the negative impact of the double power frequency ripple on the DAB module, and at the same time optimize the suppression of the double power frequency ripple under complex system conditions. Description of the Drawings

[0044] The accompanying drawings forming a part of this application are used to provide a further understanding of this application. The schematic embodiments of this application and their descriptions are used to explain this application and do not constitute an improper limitation of this application.

[0045] Figure 1 Total control block diagram of a double-frequency ripple suppression control method for a cascaded energy router disclosed in the embodiment;

[0046] Figure 2 Overall flowchart of a double-frequency ripple suppression control method for a cascaded energy router disclosed in the embodiment;

[0047] Figure 3 Three-phase isolated common DC bus CHB energy router topology structure disclosed in the embodiment;

[0048] Figure 4 Schematic diagrams of current ripples disclosed in the embodiment;

[0049] Figure 5 Flowchart for determining the optimal harmonic injection ratio disclosed in the embodiment;

[0050] Figure 6 Three impedance reshaping strategies disclosed in the embodiment, Figure 6 In (1) is the small-signal control block diagram without strategy addition; Figure 6 In (2) is the impedance reshaping strategy based on the notch filter; Figure 6 In (3) is the impedance reshaping strategy based on the current inner loop; Figure 6 In (4) is the hybrid impedance reshaping strategy;

[0051] Figure 7 Amplitude-frequency characteristic curves of the DAB output impedance under various strategies disclosed in the embodiment;

[0052] Figure 8 Schematic diagram of the safe injection range disclosed in the embodiment, Figure 8 In (a) is m AiThree-dimensional relationship diagram of (ωt, -M) Figure 8 In (b), it is m Ai The three-dimensional relationship diagram of (ωt, -M) is projected onto the (ωt, -M) plane to obtain a figure;

[0053] Figure 9 Schematic diagram of the critical injection ratio disclosed in the embodiment Figure 9 In (a), it is MAX_m Ai Three-dimensional relationship diagram of Figure 9 In (b), it is MAX_m Ai The three-dimensional relationship diagram of is projected onto the (M, kM) plane to obtain a figure;

[0054] Figure 10 Schematic diagram of the optimal suppression effect region disclosed in the embodiment Figure 10 In (a), it is the three-dimensional surface diagram of MAX_Δu Figure 10 In (b), it is the three-dimensional surface diagram of MAX_Δu projected onto plane to obtain a figure;

[0055] Figure 11 Optimal injection curve when M < 0.65 disclosed in the embodiment Specific implementation manner

[0056] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0057] It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present application belongs.

[0058] It should be noted that the terms used herein are only for describing specific implementation manners and are not intended to limit the exemplary implementation manners according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0059] The cascaded H-bridge energy router has a modular multilevel structure. It can not only achieve maximum power point tracking (MPPT) at the component level, output a low harmonic content of the voltage, but also be connected to the grid without a bulky step-up transformer, having good practical application prospects in the photovoltaic and energy storage integrated system. To further improve the system stability and reliability, reduce the common-mode interference, and fundamentally solve the leakage current problem, the three-phase isolated common DC bus CHB energy router has more application value in actual production and life. Among various types of isolated topologies, the dual active bridge (DAB) type DC / DC converter has become one of the research hotspots of domestic and foreign scholars because of its unique characteristic of bidirectional energy flow and easy implementation of soft switching for all devices.

[0060] The topology diagram of the three-phase isolated common DC bus CHB energy router is as Figure 3 shown. The first part consists of an array of photovoltaic panels, which are composed of n groups of photovoltaic panels, corresponding Boost boost converters adopting the MPPT algorithm, energy storage devices, and their DC / DC converters respectively. The second part is the three-phase isolated common DC bus CHB energy router, which is composed of N modules in each of the three phases. Each module consists of a DAB converter and an H-bridge. Taking the i-th module in phase A as an example for a detailed introduction, C PAi is the capacitor on the common DC bus side; the switching tubes S Ai1 ~S Ai4 together constitute the primary bridge arm of the DAB; C rAi is the high-frequency DC-blocking capacitor; L rAi is the sum of the phase-shifted inductor and the leakage inductance of the transformer (transformed to the primary side); T Ai is an ideal high-frequency transformer, and the turns ratio of its primary and secondary sides is N T :1; S Ai5 ~S Ai8 together form the secondary bridge arm of the DAB; C dcAi is the sum of the DC-side capacitors of the H-bridge; VT Ai1 ~VT Ai4 together form the upper and lower bridge arms of the H-bridge. u com is the voltage of the common DC bus; i inAi is the primary input current of the DAB; u busAi and i oAi are the secondary output voltage and output current of the DAB, that is, the DC bus voltage and current of the H-bridge respectively; i CAi is the current flowing into C dcAi ; i dcAi is the current flowing into the switching tubes of the H-bridge.

[0061] After analysis, as Figure 4 shown, under the stable operating state, the grid-side current i gAIt presents a power frequency (50Hz) sinusoidal fluctuation. At this time, the DC-side current of CHB under CPS-SPWM presents a double power frequency (100Hz) sinusoidal fluctuation, and part of this fluctuation flows into the DC-side capacitor C dcAi This will cause the problem of double power frequency voltage ripple on the DC side. The other part will couple to the DAB, resulting in an envelope ripple phenomenon in the phase-shifted inductor. This phenomenon is not only unfavorable for the design of magnetic components, but also affects the heating and loss of switching devices, and more importantly, poses a challenge to the value and volume of the H-bridge DC-side capacitor. Based on the above analysis, suppressing the double power frequency voltage ripple will greatly improve the stability and reliability of the system.

[0062] A series of strategies for suppressing the double-frequency power supply ripple have been proposed in the existing literature to address this issue. Taking the literature: "Centralized Active Power Decoupling Method for the CHB Converter With Reduced Components and Simplified Control" as an example, a bidirectional power path is constructed by adding switching devices and proper modulation to transfer the energy carried by the ripple to the newly added thin-film capacitor. However, this method will increase the control difficulty of the system and reduce the reliability of the control system. The literature: "Analysis and Control of a Single-Inductor-QAB-Based Modular PV Inverter Considering Effects of Parasitic Circuits" reduces the negative impact of the double-frequency ripple through the method of three-phase flux cancellation and superimposing the corresponding control strategy. However, this strategy will increase the volume of the magnetic components in the high-frequency transformer, and the resulting parasitic parameter oscillation is also not conducive to the normal operation of the system. The literature: "A High Power Density Single-Phase PWM Rectifier With Active Ripple Energy Storage" proposes a bidirectional buck-boost converter as an auxiliary ripple energy storage circuit to reduce the system volume. However, the control of the active devices also requires an additional control loop, which poses higher requirements for the hardware cost. The literature: "Modeling, Impedance Design, and Efficiency Analysis of Quasi-Z Source Module in Cascaded Multilevel Photovoltaic Power System" proposes a quasi-Z source model to analyze and suppress the double-frequency power supply ripple. However, abandoning the isolation stage will bring leakage current and other derivative problems. The literature: "An Optimized Third Harmonic Injection Method for Reducing DC-Link Voltage Fluctuation and Alleviating Power Imbalance of Three-Phase Cascaded H-Bridge Photovoltaic Inverter" proposes a third harmonic injection method. Without changing the module topology, only the control strategy is superimposed. However, the analysis in this paper is based on a non-isolated structure, and the considered grid conditions are relatively ideal.Literature: "Research on capacitor voltage ripple suppression method for power electronic transformer considering fluctuating power coupling" proposes to introduce ripple into the DC bus to achieve natural elimination by reducing the twice power frequency impedance of the DAB module. However, the adverse effects of the ripple on the DAB module are amplified, which is not conducive to the system cost and stability.

[0063] In summary, the existing twice power frequency ripple suppression strategies for three-phase isolated common DC bus H-bridge photovoltaic grid-connected inverters all have corresponding drawbacks and cannot minimize the negative impacts brought by the ripple in actual application scenarios.

[0064] To effectively suppress the twice power frequency ripple of a three-phase isolated common DC bus H-bridge photovoltaic grid-connected inverter, this embodiment discloses a control method for suppressing the twice frequency ripple of a cascaded energy router, as Figures 1 - 6 shown, including:

[0065] Obtain the three-phase modulation voltages of the inverter, the modulation degree of the modulation module, and the power factor angle;

[0066] When the modulation degree is greater than the set modulation degree threshold, calculate the limit value of the harmonic injection ratio when the H-bridge is not over-modulated before and after the third harmonic injection according to the modulation degree;

[0067] Calculate the power factor angle value according to the limit value of the harmonic injection ratio, and this value is the minimum inflection point of the power factor angle;

[0068] Calculate the optimal harmonic injection ratio according to the minimum inflection point of the power factor angle, the limit value of the harmonic injection ratio, and the second calculation formula of the harmonic injection ratio;

[0069] When the modulation degree is less than or equal to the set modulation degree threshold, calculate the optimal harmonic injection ratio according to the power factor angle and the first calculation formula of the harmonic injection ratio;

[0070] Calculate the switching drive signals of the H-bridge according to the three-phase modulation voltages and the optimal harmonic injection ratio.

[0071] Among them, calculate the third harmonic injection amount according to the optimal harmonic injection ratio;

[0072] After superimposing the third harmonic injection amount and the three-phase modulation voltages, divide by the number of H-bridges to obtain the modulation voltage of the converter;

[0073] Divide the modulation voltage of the converter by the DC bus voltage of the H-bridge to obtain the modulation wave;

[0074] Based on the modulation wave, the switching drive signals for each H-bridge are calculated and obtained.

[0075] The determination process of the DC bus voltage of the H-bridge includes:

[0076] Obtain the DC-side voltage of the H-bridge and the common DC bus voltage;

[0077] After processing the DC-side voltage of the H-bridge through a notch filter, the voltage after notch filter processing is obtained;

[0078] Divide the common DC bus voltage by the transformer turns ratio to obtain the common DC bus voltage after turns ratio processing;

[0079] Taking the common DC bus voltage after turns ratio processing as the target, perform PI control on the voltage after notch filter processing to obtain the phase shift ratio between phases of the DAB converter;

[0080] Determine the DC bus voltage of the H-bridge according to the phase shift ratio between phases.

[0081] A double-frequency ripple suppression control method for a cascaded energy router disclosed in this embodiment consists of two newly proposed strategies. One is the adaptive third-harmonic injection strategy, which adaptively determines the optimal harmonic injection ratio, and then determines the third-harmonic injection amount according to the optimal harmonic injection ratio. The other is the impedance reshaping strategy of the DAB module based on a notch filter, which processes the DC-side voltage of the H-bridge through a notch filter, and takes the common DC bus voltage divided by the transformer turns ratio to obtain the common DC bus voltage after turns ratio processing as the target, performs PI control on the voltage after notch filter processing to obtain the phase shift ratio between phases of the DAB converter; finally, determines the switching drive signals of the DAB converter according to the phase shift ratio between phases; through the combined action of the two parts, finally, the negative impact of the double-power-frequency ripple on the DAB module is completely avoided, and at the same time, the double-power-frequency ripple is optimally suppressed under complex system conditions.

[0082] Among them, the adaptive third-harmonic injection strategy is:

[0083] Add k times the third harmonic to the modulation wave of the H-bridge module. The normalized modulation wave expression can be expressed by Equation (1):

[0084] m Ai = Mcos(ωt + α) + kMcos(3ωt + 3α) (1)

[0085] In the formula, m Ai is the system modulation wave after injection; M is the modulation degree of the H-bridge; k is the third-harmonic injection ratio; kMcos(3ωt + 3α) is the third-harmonic injection amount.

[0086] Further derivation gives the DC-side current i of the H-bridge after third-harmonic injectiondcAi The expression, after ignoring the irrelevant sideband harmonic components, obtains Equation (2):

[0087]

[0088] The voltage fluctuation generated by the current ripple acting on C dcAi is shown in Equation (3):

[0089]

[0090] where ω is the grid steady-state operating angular frequency; C Ai is the total equivalent value of the DC-side capacitors; I gA is the amplitude of the grid phase current. The above three components are all constant values. Therefore, the controllable components affecting the voltage fluctuation are the modulation index M of the HB module (modulation module), the third-harmonic injection ratio k, and the system power factor angle

[0091] In addition, the third-harmonic injection should satisfy the following two principles:

[0092] (1) Before and after the third-harmonic injection, the H-bridge is not over-modulated;

[0093] (2) After injecting the third-harmonic with an appropriate ratio k, the effect of suppressing the double-frequency ripple is the best.

[0094] In this embodiment, the optimal harmonic injection ratio is calculated and determined according to the first calculation formula for the harmonic injection ratio and the second calculation formula for the harmonic injection ratio. Among them:

[0095] The first calculation formula for the harmonic injection ratio is that when the power factor angle is greater than or equal to 0 and less than the first set modulation index threshold, the optimal harmonic injection ratio is -1. When the power factor angle is greater than or equal to the first set modulation index threshold and less than the second set modulation index threshold, the harmonic injection ratio is calculated according to the power factor angle and used as the optimal harmonic injection ratio. When the power factor angle is greater than or equal to the second set modulation index threshold, the optimal harmonic injection ratio is 0;

[0096] The second calculation formula for the harmonic injection ratio is that when the power factor angle is greater than or equal to 0 and less than the minimum inflection point of the power factor angle, the limit value of the harmonic injection ratio is used as the optimal harmonic injection ratio; when the power factor angle is greater than or equal to the minimum inflection point of the power factor angle and less than the second set modulation index threshold, the harmonic injection ratio is calculated according to the power factor angle and used as the optimal harmonic injection ratio; when the harmonic injection ratio is greater than or equal to the second set modulation index threshold, the optimal harmonic injection ratio is 0.

[0097] Quantify the value ranges of the variables in Equation (3), as shown in Equation (4).

[0098]

[0099] The third - harmonic injection method is constrained by the following two principles, namely: 1) Before and after the third - harmonic injection, the H - bridge is not over - modulated; 2) After injecting the third - harmonic with an appropriate ratio k, the suppression effect of the double - frequency ripple is the best.

[0100] First, let the power grid operate at unity power factor (i.e., ), when the injection ratio of the reverse third - harmonic (-k) is the largest, determine the constraint relationship expression of the modulation wave amplitude M with respect to the modulation degree m i in the full - time domain. This expression is shown in Equation (5).

[0101]

[0102] Further, based on Equation (5), draw a three - dimensional relationship diagram as shown in Figure 8 (a). Project the three - dimensional diagram onto the (ωt, -M) plane to obtain the diagram shown in Figure 8 (b).

[0103] From the values of the boundary points in the figure, it can be seen that when and only when the modulation wave amplitude M is less than 0.65, after injecting the third - harmonic with k = - 1, there is no over - modulation risk in the system.

[0104] For the case where M is greater than 0.65, further study shows that the constraint relationship between the third - harmonic amplitude kM and the modulation degree of the H - bridge module is shown in Equation (6). Among them, MAX_m Ai is the maximum value of the modulation degree in the full - time domain under the current modulation degree M and the harmonic injection ratio k.

[0105]

[0106] First, based on Equation (6), draw a three - dimensional relationship diagram of M, kM, and MAX_m Ai as shown in Figure 9 (a). Among them, the orange - yellow plane is MAX_m Ai = 1, which represents the boundary condition of the system without over - modulation. Further, based on Figure 9 (a), only keep the part below this plane and project it onto the (M, kM) plane to obtain the diagram shown in Figure 9 (b).

[0107] Fit the boundary curve and further decouple the fitting result to obtain the boundary relationship expression between the third - harmonic injection ratio k min and the modulation wave amplitude M. This expression is shown in Equation (7). Among them, the coefficient L i takes the values shown in Table (1).

[0108]

[0109] Assume coefficient MI A / (4ωC H ) = 1 and 0 < M < 0.65, draw the three-dimensional surface diagram of k and the amplitude of voltage fluctuation MAX_Δu based on Equation (7), as shown in (a) below. Intercept and retain the part of the surface below the plane by the plane MAX_Δu = 1, and project it onto the Figure 10 plane to obtain the diagram shown in (b) below, where the blue area is the injection range for effectively suppressing the DC-side voltage fluctuation. shown in (b) below. Execute the traversal optimization algorithm in the blue area of (b) below, that is, take Figure 10 as the independent variable, find the k value corresponding to the best suppression effect, and further summarize to obtain the optimal suppression effect curve, as

[0110]

[0111] In Figure 10 shown in (b). In the blue area of (b), execute the traversal optimization algorithm, that is, take as the independent variable, find the k value corresponding to the best suppression effect, and further summarize to obtain the optimal suppression effect curve, as Figure 11 shown. Fit the optimal suppression curve in Figure 11 with an eighth-order polynomial, and the obtained functional relationship is as shown in Equation (9).

[0112] Among them, the values of the coefficients P i obtained by the eighth-order polynomial fitting are shown in Table 1.

[0113]

[0114] When M ≥ 0.65, the boundary value k min of the third-harmonic injection ratio can be obtained from Equation (7). In addition, the boundary value of the power factor angle is obtained from the inverse function of Equation (9), as shown in Equation (10).

[0115] The piecewise function of the optimal injection ratio k will be jointly determined by k min and The calculation expression is as shown in Equation (11).

[0116]

[0117] After systematic theoretical derivation and fitting analysis, constructing a three-dimensional relationship diagram and performing data processing, the following steps for determining the optimal harmonic injection ratio are finally obtained, as Figure 5 shown. Taking the set modulation threshold of 0.65 as an example, the process of determining the optimal harmonic injection ratio is described. At the beginning of the calculation, extract the current modulation ratio M and power factor angle of the HB module Further determine whether M is greater than 0.65. If M is less than or equal to 0.65, directly calculate the value of the third-harmonic injection ratio k according to the first calculation formula of the harmonic injection ratio, that is, Equation (13), and this value is the optimal harmonic injection ratio. If M is greater than 0.65, first calculate the limit value of k when principle (1) is satisfied through Equation (12), denoted as k min . Secondly, substitute the obtained k min into Equation (13) to inversely deduce and obtain the power factor angle value, and this value is the minimum inflection point of the power factor

[0118] angle, denoted as . After that, according to and the obtained power factor angle , use the second calculation formula of the harmonic injection ratio, that is, Equation (14) to calculate and obtain the value of the third-harmonic injection ratio k, and this value is the optimal harmonic injection ratio.

[0119]

[0120] The coefficients of formulas (12)-(13) are shown in Table 1.

[0121] Table 1 Coefficient value table of the optimal suppression strategy

[0122]

[0123] In this embodiment, a mathematical model of the output impedance of the DAB converter is established. Although it is based on the relatively simple single-phase-shift modulation strategy (SPS), its model cannot be directly linearly derived due to its inherent complexity. Therefore, in this embodiment, based on the transmission power within one cycle of the steady state, the state of the DAB module is averaged using the generalized averaging method, and then a small-signal perturbation is introduced into the system variables to construct a small-signal model. The perturbation components corresponding to the strong anti-perturbation structure in the topology are ignored, and finally, the small-signal model of the DAB output impedance is obtained by decoupling. The small-signal model is shown in Equation (15).

[0124]

[0125] In the formula, Z c , G d and G PI are shown by Equation (16).

[0126]

[0127] In Equations (15) and (16), Z out is the DAB output impedance; is the small-signal component of the output voltage; Z c is the complex frequency-domain impedance of the parallel capacitor on the DC side of the H-bridge; G PIis the transfer function of the PI controller; G d is the transfer function of the mathematical model processed by the state - space averaging method; N T is the turns ratio of the primary and secondary windings of the transformer; u com is the common DC bus voltage; D is the phase - shift ratio of the DAB converter; L rAi is the sum of the phase - shift inductance and leakage inductance of the DAB; f s is the switching frequency of the DAB switch tube; C dcAi is the sum of the parallel capacitors on the output side of the DAB; k p is the proportional coefficient of the PI controller; k i is the integral coefficient of the PI controller; s is the Laplace operator.

[0128] According to the small - signal model, draw the small - signal control block diagram as Figure 6 shown in (1) below.

[0129] This embodiment proposes three impedance reshaping strategies. After making a horizontal comparison of the three strategies, considering both the impedance improvement effect and control complexity, finally, the impedance reshaping strategy based on the notch filter is selected. The three impedance reshaping strategies are as Figure 6 shown.

[0130] Among them, Figure 6 in (2) is the impedance reshaping strategy based on the notch filter. The specific implementation strategy is to add notch filters with frequencies of twice the power frequency and four times the power frequency on the negative feedback branch of the output voltage. The final effect is to improve the impedance of the DAB at twice the power frequency and four times the power frequency.

[0131] Figure 6 in (3) is the impedance reshaping strategy based on the current inner loop. The specific implementation strategy is to sample the current ripples at twice the power frequency and four times the power frequency in the output current i o . To enhance the tracking effect of the signals at the above two specific frequencies, a quasi - PR controller is introduced, and a current inner loop is further added between the voltage loop and the transfer function to improve the impedance of the DAB at the corresponding frequencies.

[0132] Figure 6 in (4) is the hybrid impedance reshaping strategy. The specific implementation strategy is to adopt a double closed - loop of voltage and current. Notch filters with frequencies of twice the power frequency and four times the power frequency are added to the voltage outer loop, and a quasi - PR regulator with the corresponding frequency is introduced into the current inner loop. The final effect is to improve the impedance of the DAB at twice the power frequency and four times the power frequency.

[0133] The three impedance reshaping strategies are plotted in Bode diagrams based on the experimental data in Table 2. According to the drawn amplitude - frequency characteristic curves, the suppression effects of the three methods are horizontally compared.

[0134] Table 2 Variable value table of the active damping matching strategy

[0135]

[0136] The drawn amplitude-frequency characteristic curve is as Figure 7 shown. First, when no additional strategies are added to the system, the amplitudes of the second and fourth power frequencies are 22.6 dB and 22.9 dB respectively; the impedance reshaping strategy based on the current inner loop increases the output impedance at these two frequencies to 46 dB; the impedance reshaping strategy based on the notch filter increases the output impedance of the DAB module at the second and fourth power frequencies to 260 dB; the hybrid impedance reshaping strategy increases the output impedance to 179 dB and 159 dB respectively. Based on the above quantitative analysis, compared with other strategies, the impedance reshaping strategy based on the notch filter has the best impedance improvement effect, which can increase the impedance at the second and fourth power frequencies to about 11.5 times the original impedance.

[0137] Comparing from the difficulty of strategy implementation, using a notch filter only needs to be implemented through software programming, while adding a current inner loop will require additional current sampling components. Therefore, the control complexity of the current inner loop and the hybrid strategy is higher than that of the damping matching strategy based on the notch filter. Considering both the damping effect and complexity, the active damping matching strategy based on the notch filter is optimal.

[0138] The specific active damping matching strategy based on the notch filter is as follows:

[0139] Process the DC-side voltage of the H-bridge through a notch filter to obtain the voltage after notch filter processing; take the common DC bus voltage as the target, perform PI control on the voltage after notch filter processing to obtain the phase shift ratio between phases of the DAB converter; determine the DC bus voltage of the H-bridge according to the phase shift ratio between phases.

[0140] In this embodiment, the active power current reference value and the reactive power current reference value of the three-phase power grid are obtained;

[0141] According to the active power current reference value and the reactive power current reference value, the power factor angle is calculated and obtained.

[0142] By obtaining the phase voltage and phase current of the three-phase power grid;

[0143] Convert the phase voltage and phase current to the dq coordinate system to obtain the d-axis current and q-axis current;

[0144] Taking the active power current reference value and the reactive power current reference value as the target, perform PI control on the d-axis current and q-axis current, and further correct the active modulation voltage and reactive modulation voltage to obtain the active modulation voltage and reactive modulation voltage;

[0145] Convert the active modulation voltage and reactive modulation voltage to the abc coordinate system to obtain the three-phase modulation voltage of the inverter.

[0146] Among them, by obtaining the average value of all the DC-side voltages of the H-bridges;

[0147] Taking the voltage reference value as the target, perform PI control on the average value of the DC-side voltages of all H-bridges to obtain the reference value of the active current.

[0148] The flow chart of a double-frequency ripple suppression method for a cascaded energy router proposed in this embodiment is as Figure 6 shown. First, the three-phase grid phase voltages u gA , u gB , and u gC , the phase currents i gA , i gB , i gC , and the phase angle ωt are obtained in real time through a phase-locked loop. Based on the phase angle ωt, the phase voltages u gA , u gB , u gC and the phase currents i gA , i gB , i gC are transformed from the abc coordinate system to the dq coordinate system to obtain the d-axis voltage e d , the q-axis voltage e q , and the d-axis current i d , the q-axis current i q . Secondly, according to the reference value of the active current i d * and the reference value of the reactive current i q * set according to the active and reactive power demands of the system, the power factor angle of the power grid is calculated According to the design of the DAB controller, the DC-side voltage u busAi of the H-bridge is controlled to be the common DC bus voltage u com divided by the transformer turns ratio N T through a notch filter and a PI regulator. The output of the PI regulator is the phase shift ratio D between the phases of the DAB converter. By adopting the single-phase shift modulation strategy, the switching drive signal of the DAB converter can be obtained. The H-bridge controller consists of four parts, namely, a voltage outer loop, a current inner loop, an H-bridge modulation link, and an additional adaptive third-harmonic injection link. First, the voltage outer loop controls the average value u ave of the DC-side voltages of all H-bridge modules to the voltage reference value u ref through a PI regulator. The output of the controller is the reference value i d * of the active current. The current inner loop combines i d and i q through the joint action of a PI regulator and a feedforward decoupling link, and superimposes e d , e q respectively to calculate the active modulation voltage u d and the reactive modulation voltage u q . For u d and uq Perform the dq - to - abc transformation to obtain the three - phase modulation voltages u ca , u cb and u cc of the three - phase inverter. Based on the three - phase modulation voltages, superimpose the third - harmonic injection amount u3 obtained by judgment and calculation, and then divide by the number 2N of H - bridges respectively to obtain the modulation voltages of the converters. This process can ensure the average power distribution of all modules. Then divide by the DC - bus voltages u Figure 5 of the corresponding H - bridges respectively, u busAi , u busBi and u busCi to obtain the modulation waves of the modules, and then the switching drive signals of each H - bridge can be obtained through carrier - phase - shifted sinusoidal pulse - width modulation. Among them, the amplitude of the third - harmonic injection amount is equal to the amplitude of the modulation voltage multiplied by the optimal harmonic injection ratio, and the phase is three times the phase angle of the modulation voltage.

[0149] In the current double - frequency ripple hardware suppression strategy, the H - bridge DC - side double - frequency power - frequency ripple suppression strategy requires adding active or passive devices and circuits on the basis of the original hardware topology. These methods will additionally increase the devices, thus bringing about an increase in volume, weight, and cost. However, the method proposed in this embodiment will not affect the topology structure, and only by improving the control strategy, the optimal ripple suppression effect can be achieved on the basis of the original topology. Furthermore, the value of the H - bridge DC - side capacitor can be reduced, and the economic improvement effect is significant;

[0150] In the current ripple suppression strategy based on harmonic injection, the existing third - harmonic injection strategies do not consider the grid operation conditions of non - unity power factor, and there are calculation blind spots under actual grid operation conditions, which will lead to problems such as poor ripple suppression effect and increased risk of module over - modulation. However, the method provided in this embodiment comprehensively considers complex system conditions, completely avoids the risk of H - bridge over - modulation, and achieves the best ripple suppression effect in all states;

[0151] In the current strategy of reducing the impedance of the DAB module to make the ripple naturally coupled and eliminated on the common DC - bus side, all the double - frequency power - frequency ripple flowing through the DAB module will cause adverse effects such as an increase in the current stress requirement of the high - frequency transformer, an increase in the thermal effect of the phase - shifted inductor, and an increase in the voltage stress of the switching tube. However, the method provided in this embodiment will avoid the adverse effects of the ripple flowing through, and thus improve the reliability and stability of the DAB module.

[0152] Embodiment 2

[0153] In this embodiment, a double - frequency ripple suppression control system for a cascaded energy router is disclosed, including:

[0154] A sampling unit for obtaining the three - phase modulation voltages of the inverter, the modulation degrees of the modulation modules, and the power - factor angles;

[0155] The optimal harmonic injection ratio calculation unit is used to calculate the limit value of the harmonic injection ratio when the H-bridge is not over-modulated before and after the injection of the third harmonic according to the modulation degree when the modulation degree is greater than the set modulation degree threshold; calculate the power factor angle value according to the limit value of the harmonic injection ratio, and this value is the minimum inflection point of the power factor angle; calculate the optimal harmonic injection ratio according to the minimum inflection point of the power factor angle, the limit value of the harmonic injection ratio and the second calculation formula of the harmonic injection ratio; when the modulation degree is less than or equal to the set modulation degree threshold, calculate the optimal harmonic injection ratio according to the power factor angle and the first calculation formula of the harmonic injection ratio.

[0156] The switch drive signal determination unit is used to calculate the switch drive signal of the H-bridge according to the modulation voltage and the optimal harmonic injection ratio.

[0157] The present invention also discloses a computer device, which includes:

[0158] A processor, adapted to execute a computer program;

[0159] A computer-readable storage medium, in which a computer program is stored. When the computer program is executed by the processor, it implements a double-frequency ripple suppression control method for a cascaded energy router disclosed in Embodiment 1.

[0160] The present invention also discloses a computer-readable storage medium, which stores a computer program. The computer program is adapted to be loaded and executed by a processor to implement a double-frequency ripple suppression control method for a cascaded energy router disclosed in Embodiment 1.

[0161] The present invention also discloses a computer program product, which includes a computer program. When the computer program is executed by a processor, it implements a double-frequency ripple suppression control method for a cascaded energy router disclosed in Embodiment 1.

[0162] The method disclosed in Embodiment 1 can be directly embodied as being executed by a hardware processor, or by a combination of hardware and software modules in the processor. The software module can be located in a mature storage medium in the art such as a random access memory, a flash memory, a read-only memory, a programmable read-only memory, or an electrically erasable programmable memory, a register, etc. This storage medium is located in the memory, and the processor reads the information in the memory and combines its hardware to complete the steps of the above method. To avoid repetition, it will not be described in detail here.

[0163] Those of ordinary skill in the art will appreciate that the units and algorithm steps of the examples described in conjunction with this embodiment can be implemented by electronic hardware or a combination of computer software and electronic hardware. Whether these functions are executed in hardware or software depends on the specific application and design constraints of the technical solution. Professional technicians can use different methods to implement the described functions for each specific application, but such implementation should not be considered to exceed the scope of this application.

[0164] Although the specific implementation manners of the present invention have been described above in conjunction with the accompanying drawings, it is not a limitation on the protection scope of the present invention. Those skilled in the art should understand that various modifications or deformations that can be made without creative efforts by those skilled in the art on the basis of the technical solution of the present invention are still within the protection scope of the present invention.

Claims

1. A control method for suppressing the double-frequency ripple of a cascaded energy router, characterized in that, Including: Obtain the three-phase modulation voltage of the inverter, the modulation degree of the modulation module, and the power factor angle; When the modulation degree is greater than the set modulation degree threshold, according to the modulation degree, calculate the harmonic injection ratio limit value when the H-bridge is not over-modulated before and after the third harmonic injection; According to the harmonic injection ratio limit value, calculate the power factor angle value, which is the minimum inflection point of the power factor angle; According to the minimum inflection point of the power factor angle, the harmonic injection ratio limit value, and the second calculation formula of the harmonic injection ratio, calculate the optimal harmonic injection ratio; When the modulation degree is less than or equal to the set modulation degree threshold, according to the power factor angle and the first calculation formula of the harmonic injection ratio, calculate the optimal harmonic injection ratio; According to the three-phase modulation voltage and the optimal harmonic injection ratio, calculate the switching drive signal of the H-bridge; 2. The double-frequency ripple suppression control method of a cascaded energy router according to claim 1, characterized in that According to the optimal harmonic injection ratio, calculate the third harmonic injection amount; After superimposing the third harmonic injection amount on the three-phase modulation voltage and dividing by the number of H-bridges, obtain the modulation voltage of the converter; Divide the modulation voltage of the converter by the DC bus voltage of the H-bridge to obtain the modulation wave; According to the modulation wave, calculate the switching drive signal of each H-bridge; 3. The double-frequency ripple suppression control method of a cascaded energy router according to claim 2, wherein, Obtain the DC side voltage of the H-bridge and the common DC bus voltage; After processing the DC side voltage of the H-bridge through a notch filter, obtain the voltage after notch filter processing; Divide the common DC bus voltage by the transformer turns ratio to obtain the common DC bus voltage after turns ratio processing; Taking the common DC bus voltage after turns ratio processing as the target, perform PI control on the voltage after notch filter processing to obtain the phase shift ratio between phases of the DAB converter; According to the phase shift ratio between phases, determine the DC bus voltage of the H-bridge; 4. The double-frequency ripple suppression control method for a cascaded energy router according to claim 1, wherein, The first calculation formula of the harmonic injection ratio is that when the power factor angle is greater than or equal to 0 and less than the first set modulation degree threshold, the optimal harmonic injection ratio is -1; when the power factor angle is greater than or equal to the first set modulation degree threshold and less than the second set modulation degree threshold, calculate the harmonic injection ratio according to the power factor angle as the optimal harmonic injection ratio; when the power factor angle is greater than or equal to the second set modulation degree threshold, the optimal harmonic injection ratio is 0; The second calculation formula of the harmonic injection ratio is that when the power factor angle is greater than or equal to 0 and less than the minimum inflection point of the power factor angle, use the harmonic injection ratio limit value as the optimal harmonic injection ratio; when the power factor angle is greater than or equal to the minimum inflection point of the power factor angle and less than the second set modulation degree threshold, calculate the harmonic injection ratio according to the power factor angle as the optimal harmonic injection ratio; when the harmonic injection ratio is greater than or equal to the second set modulation degree threshold, the optimal harmonic injection ratio is 0; 5. The double-frequency ripple suppression control method of a cascaded energy router according to claim 1, characterized in that Obtain the reference value of the active current and the reference value of the reactive current of the three-phase power grid; According to the reference value of the active current and the reference value of the reactive current, calculate the power factor angle; 6. The double-frequency ripple suppression control method of a cascaded energy router according to claim 1, characterized in that Obtain the phase voltage and phase current of the three-phase power grid; Convert the phase voltage and phase current to the dq coordinate system to obtain the d-axis current and q-axis current; Taking the reference value of the active current and the reference value of the reactive current as the target, perform PI control on the d-axis current and q-axis current to obtain the active modulation voltage and the reactive modulation voltage; Convert the active modulation voltage and the reactive modulation voltage to the abc coordinate system to obtain the three-phase modulation voltage of the inverter; 7. A control system for suppressing the second - harmonic ripple of a cascaded energy router, characterized in that, Including: A sampling unit, configured to obtain the three-phase modulation voltage of the inverter, the modulation degree of the modulation module, and the power factor angle; An optimal harmonic injection ratio calculation unit, configured to, when the modulation degree is greater than the set modulation degree threshold, calculate, according to the modulation degree, the limit value of the harmonic injection ratio when the H-bridge is not over-modulated before and after the injection of the third harmonic; calculate, according to the limit value of the harmonic injection ratio, the power factor angle value, which is the minimum inflection point of the power factor angle; calculate, according to the minimum inflection point of the power factor angle, the limit value of the harmonic injection ratio, and the second calculation formula of the harmonic injection ratio, the optimal harmonic injection ratio; when the modulation degree is less than or equal to the set modulation degree threshold, calculate, according to the power factor angle and the first calculation formula of the harmonic injection ratio, the optimal harmonic injection ratio; A switching drive signal determination unit, configured to calculate the switching drive signal of the H-bridge according to the modulation voltage and the optimal harmonic injection ratio.

8. An electronic device, characterized in that, The device includes: A processor, adapted to execute a computer program; A computer-readable storage medium, in which a computer program is stored, and when the computer program is executed by the processor, a double-frequency ripple suppression control method for a cascaded energy router according to any one of claims 1-6 is implemented.

9. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program, and the computer program is adapted to be loaded and executed by the processor to implement a double-frequency ripple suppression control method for a cascaded energy router according to any one of claims 1-6.

10. A computer program product, characterized in that, The computer program product includes a computer program, and when the computer program is executed by the processor, a double-frequency ripple suppression control method for a cascaded energy router according to any one of claims 1-6 is implemented.

Images