High step-up ratio DC-AC modular multilevel converter topology and modulation method

Abstract

The present invention discloses a high-step-up ratio DC-AC modular multi-level converter topology and a modulation method thereof, and relates to the technical field of power electronic converters. The present invention comprises: the DC side voltage is connected to the upper and lower bridge arms of the converter through a bidirectional H-bridge circuit through a transformer; the AC output side of the low-voltage DC to high-voltage AC converter is connected to the AC power grid, and the DC side is connected to the photovoltaic array; the converter is applied to the photovoltaic power generation grid-connected stage, and can directly invert the low-voltage DC into high-voltage AC and connect it to the power grid. Compared with the N+1 level output of the traditional single-phase MMC, the present invention can output 2N+1 levels, significantly increasing the number of step levels and the equivalent switching frequency, and improving the DC voltage utilization rate. At the same time, the present invention integrates the two links of boosting and inversion, saving costs and reducing volume compared with the traditional two-stage photovoltaic grid-connected.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronic converters, and in particular to a high-step-up ratio DC / AC modular multi-level converter topology and a modulation method thereof. Background Art

[0002] Modular multilevel converters (MMCs) offer numerous advantages, including modular structure, high reliability, excellent output characteristics, easy redundant control, and a wide range of current and voltage applications. These characteristics have led to increasing academic interest in MMC topologies, resulting in numerous improvements and enhancements in circuit topology, operating principles, modulation methods, and control strategies. They have rapidly gained widespread application in various medium-, high-voltage, and high-power power conversion scenarios, such as medium- and high-voltage direct current (DC) transmission, AC / DC grid interconnection, smart transformers, and medium-voltage motor drives.

[0003] The traditional photovoltaic grid-connected system adopts a two-stage grid-connected structure (front-stage DC / DC and back-stage DC / AC), consisting of a photovoltaic array, a DC-DC circuit, and a three-phase commutation circuit. Multiple DC-DC circuits are connected in parallel and in series on the output side to achieve the purpose of increasing the photovoltaic output voltage. However, the traditional two-stage photovoltaic grid-connected system is large in size and has low DC voltage utilization. To this end, we propose a high-step-up ratio DC-AC modular multilevel converter topology and its modulation method. Summary of the Invention

[0004] The purpose of the present invention is to provide a high-step-up ratio DC-AC modular multi-level converter topology and its modulation method. The topology output mode is different from the traditional half-bridge MMC, which not only improves the DC voltage utilization rate, but also reduces the volume and design cost compared with the traditional two-stage photovoltaic grid-connected structure.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a high-step-up ratio DC-AC modular multi-level converter topology, comprising:

[0006] It includes a bidirectional H-bridge circuit, the bidirectional H-bridge circuit is electrically connected to a transformer, the transformer is electrically connected to a modular circuit, and the modular circuit is electrically connected to a power grid side;

[0007] Low-voltage DC power is transmitted to the transformer through a bidirectional H-bridge circuit for boosting, and then converted into AC power according to the modular circuit and transmitted to the grid side.

[0008] Furthermore, the modular circuit includes an upper bridge arm and a lower bridge arm, and the upper bridge arm and the lower bridge arm are both formed by cascading multiple half-bridge sub-modules, and the half-bridge sub-modules are connected in sequence through diodes and inductors.

[0009] Furthermore, the half-bridge submodules of the upper bridge arm and the lower bridge arm are arranged symmetrically, and the top of the upper bridge arm is connected to the grid side.

[0010] Furthermore, the two adjacent half-bridge sub-modules are connected via a diode and an inductor.

[0011] According to one aspect of the present invention, a modulation method for a high-step-up ratio DC-AC modular multi-level converter topology is provided, which specifically comprises the following steps:

[0012] Construct a mathematical model of a low-voltage DC to high-voltage AC modular multilevel converter, and perform coordinate transformation on the mathematical model to obtain a dynamic mathematical model under the dq coordinate axis;

[0013] According to the obtained dynamic mathematical model, the current inner loop decoupling control is designed, and the outer loop control adopts the power outer loop control, and the output is the phase modulation wave voltage u vj ;

[0014] The obtained modulation wave voltage u vj Compare it with 0, and the part greater than 0 is used as the modulation voltage wave of all sub-modules in the upper bridge arm. The absolute value of the part less than 0 is taken as the modulation voltage wave of all sub-modules in the lower bridge arm.

[0015] According to the modulated voltage waves obtained by the upper and lower bridge arms, the rounding function is used to obtain the number n required for the upper and lower bridge arms respectively. insert ;

[0016] Number of upper bridge arms put into operation n up =round(u s_up );

[0017] Number of lower bridge arms put into operation n down =round(u s_down );

[0018] According to the capacitor voltage balancing strategy, each bridge arm puts the sub-module with the highest priority into use, and all the remaining sub-modules are removed.

[0019] Furthermore, the mathematical model of the low-voltage DC to high-voltage AC modular multilevel converter is:

[0020]

[0021] Among them, R f Indicates the grid side loss resistance, L f Represents the equivalent loss inductance on the grid side, i j (j=a,b,c) represents the three-phase current, e j (j=a,b,c) represents the three-phase AC voltage on the grid side, u sj(j=a, b, c) represents the sum of the upper and lower arm voltages of the three-phase bridge.

[0022] Furthermore, the dynamic mathematical model under the dq coordinate axis is:

[0023]

[0024] Among them, i d Represents the d-axis component of the three-phase current, i q represents the q-axis component of the three-phase current, e d represents the d-axis component of the three-phase AC voltage on the grid side, e q represents the q-axis component of the three-phase AC voltage on the grid side, u sd It represents the component of the sum of the three-phase upper and lower bridge arm voltages on the d-axis, u sq It represents the component of the sum of the three-phase upper and lower arm voltages on the q-axis.

[0025] Furthermore, when the modulation wave voltage is positive, only the upper bridge arm submodule is put into operation and the lower bridge arm submodule is charged; when the modulation wave voltage is negative, only the lower bridge arm submodule is put into operation and the upper bridge arm submodule is charged.

[0026] Furthermore, the capacitor voltage balancing strategy is specifically as follows:

[0027] Note the current i vj The outflow from the bridge arm is positive, the inflow into the bridge arm is negative, and the current i of each phase is determined. vj positive and negative;

[0028] If i vj If it is positive, the capacitor voltage is sorted from high to low, and the submodule with high capacitor voltage is put into use first. up or n down , all remaining submodules are removed;

[0029] If i vj If negative, the capacitor voltage is sorted from low to high, and the submodule with low capacitor voltage is put into use first. up or n down , all remaining submodules are removed.

[0030] According to another aspect of the present invention, a modulation device for a high-step-up ratio DC-AC modular multi-level converter topology is provided, comprising:

[0031] Mathematical model construction module: used to construct a mathematical model of a low-voltage DC to high-voltage AC modular multi-level converter and perform coordinate transformation on the mathematical model to obtain a dynamic mathematical model under the dq coordinate axis;

[0032] Modulation wave voltage output module: used to design the current inner loop decoupling control according to the obtained dynamic mathematical model, and the outer loop control adopts the power outer loop control to output the modulation wave voltage u of each phase vj ;

[0033] Comparison module: used to obtain the modulation wave voltage u vj Compare it with 0, and the part greater than 0 is used as the modulation voltage wave of all sub-modules in the upper bridge arm. The absolute value of the part less than 0 is taken as the modulation voltage wave of all sub-modules in the lower bridge arm.

[0034] Submodule switching module: used to obtain the modulated voltage wave of the upper bridge arm and the lower bridge arm, and use the rounding function to obtain the number n of upper and lower bridge arms that need to be switched on. insert ;

[0035] Number of upper bridge arms put into operation n up =round(u s_up );

[0036] Number of lower bridge arms put into operation n down =round(u s_down );

[0037] According to the capacitor voltage balancing strategy, each bridge arm puts the sub-module with the highest priority into use, and all the remaining sub-modules are removed.

[0038] The present invention has at least the following beneficial effects:

[0039] 1. The present invention provides a new topology that can directly convert low-voltage direct current to high-voltage alternating current for photovoltaic grid connection. Compared with the traditional two-stage photovoltaic grid connection, it reduces the volume and production cost, and has the characteristics of modularity, large capacity and low switching frequency.

[0040] 2. The output voltage waveform and current waveform of the present invention are both sinusoidal waves with small harmonic components. The capacitor voltage fluctuation is also within a certain range, and can well track the active power and reactive power given by the outside world.

[0041] 3. The present invention can effectively deal with asymmetric faults on the grid side and minimize the impact of the fault on its normal operation by controlling the positive and negative sequence currents.

[0042] Of course, any product implementing the present invention does not necessarily need to achieve all of the advantages described above at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a schematic diagram of the overall structure of the present invention;

[0044] Figure 2This is a high step-up ratio DC-AC modular multi-level converter circuit diagram (single-phase) described in the present invention;

[0045] Figure 3 is an implementation flow chart of the modulation method of the present invention;

[0046] Figure 4 is a flow chart of capacitor voltage balance control according to the present invention;

[0047] Figure 5 This is a topology simulation three-phase AC current waveform diagram of the present invention;

[0048] Figure 6 This is a topology simulation three-phase AC voltage waveform diagram of the present invention;

[0049] Figure 7 This is the capacitor voltage waveform diagram of the three-phase submodule in the topology simulation of the present invention.

[0050] Reference numerals:

[0051] 1. Bidirectional H-bridge circuit; 2. Transformer; 3. Modular circuit; 4. Grid side; 3-1. Half-bridge submodule. DETAILED DESCRIPTION

[0052] The following will be combined with the accompanying drawings in the embodiments of the present disclosure to clearly and completely describe the technical solutions in the embodiments of the present disclosure. Obviously, the embodiments described are only part of the embodiments of the present disclosure, not all of the embodiments. Based on the embodiments of the present disclosure, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present disclosure.

[0053] See also Figure 1-2 The present invention provides a technical solution: a high-step-up ratio DC-AC modular multi-level converter topology, comprising a bidirectional H-bridge circuit 1, the bidirectional H-bridge circuit 1 being electrically connected to a transformer 2, the transformer 2 being electrically connected to a modular circuit 3, and the modular circuit 3 being electrically connected to a grid side 4;

[0054] The low-voltage DC power is transmitted to the transformer 2 through the bidirectional H-bridge circuit 1 for boosting, and then inverted into AC power according to the modular circuit 3 and transmitted to the grid side 4.

[0055] It should be noted that the modular circuit 3 includes an upper bridge arm and a lower bridge arm, and both the upper bridge arm and the lower bridge arm are cascaded by multiple half-bridge sub-modules 3-1, and the half-bridge sub-modules 3-1 are connected in sequence through diodes and inductors. The half-bridge sub-modules 3-1 of the upper bridge arm and the lower bridge arm are arranged symmetrically, and the top of the upper bridge arm is connected to the grid side 4.

[0056] like Figure 1-2As shown, the DC voltage passes through the bidirectional H-bridge circuit 1 and is boosted by the transformer 2 to provide power to the upper and lower bridge arms. The upper and lower bridge arm submodules are symmetrical, and adjacent submodules are connected through diodes and inductors. Finally, the top of the upper bridge arm is output to the grid side 4.

[0057] like Figure 3 As shown, the present invention provides a modulation method for a high-step-up ratio DC-AC modular multi-level converter topology, which adopts the nearest level modulation and includes the following steps:

[0058] S1: The mathematical model of the low-voltage DC to high-voltage AC modular multilevel converter is derived as follows:

[0059]

[0060] Among them, R f Indicates the grid side loss resistance, L f Represents the equivalent loss inductance on the grid side, i j (j=a,b,c) represents the three-phase current, e j (j=a,b,c) represents the three-phase AC voltage on the grid side, u sj (j=a,b,c) represents the sum of the upper and lower bridge arm voltages of the three phases;

[0061] S2: Perform coordinate transformation on the mathematical model to obtain the dynamic mathematical model under the dq coordinate axis:

[0062]

[0063] Among them, i d Represents the d-axis component of the three-phase current, i q represents the q-axis component of the three-phase current, e d represents the d-axis component of the three-phase AC voltage on the grid side, e q represents the q-axis component of the three-phase AC voltage on the grid side, u sd It represents the component of the sum of the three-phase upper and lower bridge arm voltages on the d-axis, u sq It represents the component of the sum of the three-phase upper and lower arm voltages on the q-axis.

[0064] Then the current inner loop decoupling control is designed, and the outer loop control adopts the power outer loop control, and the output is the modulation wave voltage u of each phase vj ;

[0065] S3: The modulated wave voltage u obtained from the output of step S2 vj Compare it with 0, and the part greater than 0 is used as the modulation voltage wave of all sub-modules in the upper bridge arm. The absolute value of the part less than 0 is taken as the modulation voltage wave of all sub-modules in the lower bridge arm.

[0066] S4: Based on the modulated voltage waves obtained by the upper and lower bridge arms in S3, the number n required for each of the upper and lower bridge arms can be obtained using the rounding function. insert .

[0067] The rounding function is a mathematical function that rounds a floating point number to the nearest integer. The rounding function is defined as: round(x) = [x + 0.5];

[0068] Where x is a floating point number, [x] represents the largest integer not greater than x;

[0069] Number of upper bridge arms put into operation n up =round(u s_up ),

[0070] Number of lower bridge arms put into operation n down =round(u s_down );

[0071] Because the modulation wave voltage of all sub-modules in the upper bridge arm is staggered by half a cycle with the modulation wave voltage of the lower bridge arm sub-module, when the modulation wave voltage is positive, only the upper bridge arm sub-module is put into operation and the lower bridge arm sub-module is charged; when the modulation wave voltage is negative, only the lower bridge arm sub-module is put into operation and the upper bridge arm sub-module is charged.

[0072] S5: According to the capacitor voltage balancing strategy, each bridge arm puts the sub-module with the highest priority into use, and all the remaining sub-modules are removed.

[0073] Furthermore, in step S5, Figure 4 As shown, the capacitor voltage balancing strategy is:

[0074] S51: Record current i vj The outflow from the bridge arm is positive, the inflow into the bridge arm is negative, and the current i of each phase is determined. vj positive and negative;

[0075] S52: If i vj If it is positive, the capacitor voltage is sorted from high to low, and the submodule with high capacitor voltage is put into use first. up or n down , all remaining submodules are removed.

[0076] S53: If i vj If negative, the capacitor voltage is sorted from low to high, and the submodule with low capacitor voltage is put into use first. up or n down , all remaining submodules are removed.

[0077] Figure 5 As an embodiment, it shows the output AC current waveform when the number of single-phase sub-modules is 2N=20. At t=0.138s, the active power changes.

[0078] Figure 6 As an embodiment, it shows the output AC voltage waveform when the number of single-phase sub-modules 2N=20. At t=0.138s, the active power changes.

[0079] Figure 7 As an embodiment, it shows the submodule capacitor voltage waveform when the number of single-phase submodules is 2N=20. At t=0.138s, the active power changes.

[0080] According to another aspect of the present invention, a modulation device for a high-step-up ratio DC-AC modular multi-level converter topology is provided, comprising:

[0081] Mathematical model construction module: used to construct a mathematical model of a low-voltage DC to high-voltage AC modular multi-level converter and perform coordinate transformation on the mathematical model to obtain a dynamic mathematical model under the dq coordinate axis;

[0082] Modulation wave voltage output module: used to design the current inner loop decoupling control according to the obtained dynamic mathematical model, and the outer loop control adopts the power outer loop control to output the modulation wave voltage u of each phase vj ;

[0083] Comparison module: used to obtain the modulation wave voltage u vj Compare it with 0, and the part greater than 0 is used as the modulation voltage wave of all sub-modules in the upper bridge arm. The absolute value of the part less than 0 is taken as the modulation voltage wave of all sub-modules in the lower bridge arm.

[0084] Submodule switching module: used to obtain the modulated voltage wave of the upper bridge arm and the lower bridge arm, and use the rounding function to obtain the number n of upper and lower bridge arms that need to be switched on. insert ;

[0085] Number of upper bridge arms put into operation n up =round(u s_up );

[0086] Number of lower bridge arms put into operation n down =round(u s_down );

[0087] According to the capacitor voltage balancing strategy, each bridge arm puts the sub-module with the highest priority into use, and all the remaining sub-modules are removed.

[0088] In summary, the application of the above low-voltage DC to high-voltage AC modular multilevel converter in photovoltaic grid-connected systems can improve DC voltage utilization, have large capacity, and strong fault tolerance. Compared with traditional two-stage photovoltaic grid-connected systems, it is smaller and lowers cost. The output AC voltage and AC current waveforms have small harmonic components, low switching frequency, and lower losses.

[0089] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus.

[0090] For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to the specific circumstances. When an element is referred to as being "assembled on", "installed on", "fixed on" or "set on" another element, it can be directly on the other element or there can be a central element. When an element is considered to be "connected" to another element, it can be directly connected to the other element or there can be a central element at the same time. The terms "vertical", "horizontal", "up", "down", "left", "right" and similar expressions used herein are for illustrative purposes only and are not intended to be the only embodiment.

[0091] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

[0092] Throughout this specification, references to terms such as "one embodiment," "example," or "specific example" indicate that a specific feature, structure, material, or characteristic described in conjunction with that embodiment or example is included in at least one embodiment or example of the present disclosure. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

Claims

1. High step-up ratio DC-AC modular multi-level converter topology, characterized by: It includes a bidirectional H-bridge circuit, the bidirectional H-bridge circuit is electrically connected to a transformer, the transformer is electrically connected to a modular circuit, and the modular circuit is electrically connected to a power grid side; The low-voltage DC power is transmitted to the transformer through a bidirectional H-bridge circuit for boosting, and then converted into AC power according to the modular circuit and transmitted to the grid side; The modular circuit includes an upper bridge arm and a lower bridge arm, and the upper bridge arm and the lower bridge arm are both formed by cascading multiple half-bridge sub-modules, and the half-bridge sub-modules are connected in sequence through diodes and inductors; The half-bridge submodules of the upper bridge arm and the lower bridge arm are arranged symmetrically, and the top of the upper bridge arm is connected to the grid side; A series branch of a diode and an inductor is connected between each two adjacent half-bridge sub-modules connected in series, and the series branch of a diode and an inductor is connected in parallel with the two adjacent half-bridge sub-modules connected in series.

2. A modulation method for a high step-up ratio DC-AC modular multi-level converter topology according to claim 1, characterized in that: The specific steps include: Construct a mathematical model of a low-voltage DC to high-voltage AC modular multilevel converter, and perform coordinate transformation on the mathematical model to obtain a dynamic mathematical model under the dq coordinate axis; According to the obtained dynamic mathematical model, the current inner loop decoupling control is designed, and the outer loop control adopts the power outer loop control, and the output is the modulation wave voltage of each phase. u vj ; The modulated wave voltage obtained u vj Compare it with 0, and the part greater than 0 is used as the modulation voltage wave of all sub-modules in the upper bridge arm. The absolute value of the part less than 0 is taken as the modulation voltage wave of all sub-modules in the lower bridge arm. According to the modulated voltage waves obtained by the upper and lower bridge arms, the rounding function is used to obtain the number of upper and lower bridge arms that need to be invested. n insert ; Number of upper bridge arms put into use n up =round( u s_up ); Number of lower bridge arms put into use n down =round( u s_down ); According to the capacitor voltage balancing strategy, each bridge arm puts the submodule with the highest priority into use, and all the remaining submodules are removed; When the modulation wave voltage is positive, only the upper bridge arm submodule is put into operation and the lower bridge arm submodule is charged; when the modulation wave voltage is negative, only the lower bridge arm submodule is put into operation and the upper bridge arm submodule is charged; The capacitor voltage balancing strategy is specifically as follows: Record current i vj The outflow from the bridge arm is positive, the inflow into the bridge arm is negative, and the current of each phase is judged i vj positive and negative; like i vj If it is positive, the capacitor voltage is sorted from high to low, and the sub-module with high capacitor voltage is put into use first. n up or n down , all remaining submodules are removed; like i vj If it is negative, the capacitor voltage is sorted from low to high, and the sub-module with low capacitor voltage is put into use first. n up or n down , all remaining submodules are removed.

3. The modulation method of a high step-up ratio DC-AC modular multi-level converter topology according to claim 2, characterized in that: The mathematical model of the low-voltage DC to high-voltage AC modular multilevel converter is: in, Represents the grid side loss resistance, represents the equivalent loss inductance on the grid side, Represents three-phase current, Indicates the three-phase AC voltage on the grid side, It represents the sum of the three-phase upper and lower arm voltages.

4. The modulation method of a high step-up ratio DC-AC modular multi-level converter topology according to claim 3, characterized in that: The dynamic mathematical model under the dq coordinate axis is: in, represents the d-axis component of the three-phase current, represents the q-axis component of the three-phase current, represents the d-axis component of the three-phase AC voltage on the grid side, represents the q-axis component of the three-phase AC voltage on the grid side, It represents the component of the sum of the three-phase upper and lower bridge arm voltages on the d-axis. It represents the component of the sum of the three-phase upper and lower arm voltages on the q-axis.

5. A modulation device for a high-step-up ratio DC / AC modular multilevel converter topology, for implementing a modulation method for a high-step-up ratio DC / AC modular multilevel converter topology according to any one of claims 2 to 4, characterized in that: include: Mathematical model construction module: used to construct a mathematical model of a low-voltage DC to high-voltage AC modular multi-level converter and perform coordinate transformation on the mathematical model to obtain a dynamic mathematical model under the dq coordinate axis; Modulation wave voltage output module: used to design the current inner loop decoupling control according to the obtained dynamic mathematical model, and the outer loop control adopts the power outer loop control to output the modulation wave voltage of each phase u vj ; Comparison module: used to obtain the modulated wave voltage u vj Compare it with 0, and the part greater than 0 is used as the modulation voltage wave of all sub-modules in the upper bridge arm. The absolute value of the part less than 0 is taken as the modulation voltage wave of all sub-modules in the lower bridge arm. Submodule switching module: used to obtain the modulated voltage wave of the upper bridge arm and the lower bridge arm, and use the rounding function to obtain the number of upper and lower bridge arms that need to be switched on. n insert ; Number of upper bridge arms put into use n up =round( u s_up ); Number of lower bridge arms put into use n down =round( u s_down ); According to the capacitor voltage balancing strategy, each bridge arm puts the sub-module with the highest priority into use, and all the remaining sub-modules are removed.

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