Bidirectional modular multi-level direct current converter based on phase-shift cyclic modulation

Abstract

The invention provides a bidirectional modular multi-level direct current converter based on phase-shift cyclic modulation, and relates to the technical field of power electronics. The basis of the adopted topological structure is the combination of a modular multilevel DC-DC converter and a dual-active bridge structure, and the converter has inherent voltage balancing capability and is suitable for high / medium voltage application; the primary side and the secondary side of the converter adopt modular half-bridge structures, so that the voltage stress of the device can be reduced, and meanwhile, the overall framework can be optimized. In the half-bridge circuit, the two switching devices work complementarily, high voltage can be dispersed, and reliability is improved. The flexibility and the expandability are enhanced through the modular design, and the production cost and the production period are reduced.

Description

Technical Field

[0001] The invention relates to the technical field of power electronics, and in particular to a bidirectional modular multi-level direct current converter based on phase-shift cyclic modulation. Background Art

[0002] Modular multilevel converters (MMCs) are widely used in high voltage / medium voltage direct current (HVDC / MVDC) transmission, motor drives, and DC transformers in MVDC systems. MVDC power distribution has the characteristics of eliminating reactive power and low losses, which promotes the development of AC / DC hybrid distribution networks. In contemporary DC systems, DC-DC converters play an important role in voltage matching, power control, and current isolation, ensuring the stability of the system. However, the voltage increase, the tolerance of the device, the insulation performance of the system, and the operational reliability of the converter are all facing challenges.

[0003] The traditional approach meets the needs of medium voltage systems by using an input series output parallel (ISOP) configuration. Series is connected at the input to support high voltage, and parallel is connected at the output to support high current. As a result, the module size may be larger and the power density may be lower. For medium / high voltage applications, a DC-DC converter using a single transformer can reduce size and power loss compared to the ISOP structure.

[0004] In order to support high modularity and reliability under high and medium voltage stress, SM stacking based on MMC structure is adopted. The inherent balancing ability of this capacitor can increase the operating frequency, reduce the workload, help reduce the implementation cost and improve the efficiency and reliability of the converter. Modular multi-level dc-ac-dc converter (MMDAC) based on dual active bridge (DAB) or LLC structure provides the advantage of soft switching operation, thereby improving efficiency.

[0005] The key condition for the stable operation of the MMDAC and the entire DC system is to maintain a good balance between the voltages of the SM capacitors. Similar to traditional MMC DC-AC conversion, the insertion of SMs needs to be distributed according to their positions during the voltage sorting process. However, in order to charge and discharge the SM capacitors within one AC base frequency cycle, the sampling and switching frequency must be higher than the AC frequency, which limits the internal AC frequency and reduces the power density of the AC stage.

[0006] In recent years, some progress has been made in the SM capacitor voltage regulation methods in MMDACs. This involves the use of a dual sorting process to redistribute the gating signal for voltage regulation. The sorting is based on the voltage of the SM capacitor and its net increment in the previous period. These methods all require a central controller with high computing power to sample, store and sort the voltage data from the SM capacitor in real time. At the same time, the sorting and redistribution process introduces unpredictability in the SM switching action, which may lead to lifetime differences due to different switching frequencies and current stresses of individual SMs. Summary of the invention

[0007] The technical problem to be solved by the present invention is to address the deficiencies of the above-mentioned prior art and provide a bidirectional modular multi-level DC converter based on phase-shifted cyclic modulation, which adopts a modular structure on both the primary side and the secondary side to reduce the voltage stress of the device; the voltage balance of the sub-module capacitors in each bridge arm is achieved through cyclic modulation, and the power switches all work in zero-voltage switching mode, thereby improving the efficiency of the converter and realizing bidirectional power flow through phase-shift control.

[0008] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0009] A bidirectional modular multilevel DC converter based on phase-shifted cyclic modulation, wherein both the primary side and the secondary side adopt modular structures, specifically:

[0010] Primary side, DC voltage V in is the voltage of the input port; input capacitance C in1 , C in2 Series connection creates a neutral point for the primary DC bus and is connected in parallel with the input voltage; SM P For the primary side upper bridge arm module, SM N The lower bridge arm module is the primary side. Both the upper and lower bridge arm modules have N half-bridge sub-modules SM connected in series. SMP 、V SMN is the bridge arm voltage of the upper bridge arm and the lower bridge arm on the primary side; the same-name ends of the primary winding of transformer T are connected in turn through the DC blocking capacitor C B , power transfer inductor L k Connected to the connection point of the upper and lower bridge arms on the primary side, the opposite end of the primary winding of the transformer T is connected to the neutral point created by the DC bus capacitor on the primary side;

[0011] On the secondary side, DC voltage V out is the output voltage of the output port; the output capacitor C out1 , C out2 Series connection creates a neutral point for the secondary output DC bus and is connected in parallel with the output voltage; in the secondary side of the topology circuit, SM S For the secondary side upper bridge arm module, SML The lower bridge arm module on the secondary side has N series-connected half-bridge submodules SM. SMS 、V SML is the bridge arm voltage of the upper bridge arm and the lower bridge arm on the secondary side; the same-name end of the secondary winding of the transformer T is connected to the connection point of the upper and lower bridge arms on the secondary side, and the opposite-name end of the secondary winding of the transformer T is connected to the neutral point created by the secondary side output capacitor.

[0012] Furthermore, each half-bridge submodule SM includes a capacitor C SM and two switching devices Q1, Q2; the drain of the switching device Q1 and the capacitor C SM One end is connected, the source is connected to the drain of the switching device Q2; the source of the switching device Q2 is connected to the capacitor C SM the other end.

[0013] Furthermore, the transformer T is equivalent to the excitation inductance L m It is composed of an ideal transformer, all switching tubes are ideal switching tubes and are anti-parallel connected with a diode and a parasitic capacitor.

[0014] Furthermore, the switching frequency of the switching device driving signal of the half-bridge submodule is f c , the duty cycle is D = (2N-1) / 2N, and the frequency of the converter bidirectional transmission is f s , where f s =N*f c ; In the forward working mode, the phase shift angle of each half-bridge sub-module in the upper bridge arm module on the primary side is 2θ, where θ=180 / N, and the phase shift angle of the half-bridge sub-module on the secondary side relative to the primary side is φ, where 0<φ≤180 / N; in the reverse working mode, it is only necessary to make the driving signal on the secondary side lead the driving signal on the primary side.

[0015] Furthermore, the phase shift angle of the half-bridge submodule corresponding to the upper bridge arm and the lower bridge arm in the primary side is θ; the phase shift angle of the half-bridge submodule corresponding to the upper bridge arm and the lower bridge arm in the secondary side is also θ.

[0016] Furthermore, the switching action of each half-bridge submodule is consistent, and the subsequent half-bridge submodule will be delayed by a fundamental wave period T relative to the previous half-bridge submodule. fs ; For each bridge arm, a switching mode will continuously operate N half-bridge sub-modules in N fundamental cycles; the switching action of each half-bridge sub-module will also cycle once in the same N fundamental cycles; the switching action timing within the cycle of a single half-bridge sub-module is called the sub-module switching sequence, and the switching actions of the N half-bridge sub-modules in each bridge arm within one fundamental cycle constitute the switching sequence of the bridge arm.

[0017] Furthermore, the switch sequence of each bridge arm forms a circulant matrix. When the circulant matrix is ​​a full-rank matrix, the circulant matrix has a unique solution, so that the capacitor voltage of each submodule of the modular multi-level DC-DC converter is inherently balanced.

[0018] The beneficial effects of the above technical solution are: the bidirectional modular multi-level DC converter based on phase-shifted cyclic modulation provided by the present invention adopts a topological structure based on the combination of a modular multi-level DC-DC converter and a dual active bridge structure. The converter has an inherent voltage balancing capability and is suitable for high / medium voltage applications; the primary side and secondary side of the converter of the present invention adopt a modular half-bridge structure, which can reduce the voltage stress of the device and optimize the overall architecture. In a half-bridge circuit, the two switching devices work complementary to each other, which can disperse high voltage and improve reliability. The modular design enhances flexibility and scalability, and reduces production costs and cycles. In the present invention, the topological structure can achieve voltage balance of the sub-module capacitors of each bridge arm through phase-shifted cyclic modulation, ensuring the safe operation of the converter, and can also achieve online bidirectional power flow based on phase-shifted control. In the present invention, according to the bridge arm current waveform, before the switch device is turned on, the current flows from the parallel diode of the switch device, so the switch devices in the present invention are all in zero voltage switching mode, which improves the efficiency of the converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A schematic diagram of the structure of a bidirectional modular multi-level DC converter based on phase-shift cyclic modulation provided by an embodiment of the present invention;

[0020] Figure 2 for Figure 1 The commutation analysis diagram of the bidirectional modular multilevel DC converter shown in FIG. 1 when the converter is operating in mode 1;

[0021] Figure 3 for Figure 1 The commutation analysis diagram of the bidirectional modular multilevel DC converter shown in FIG. 1 when operating in mode 2;

[0022] Figure 4 for Figure 1 The commutation analysis diagram of the bidirectional modular multilevel DC converter shown in FIG. 1 when operating in mode 3;

[0023] Figure 5 for Figure 1 The commutation analysis diagram of the bidirectional modular multilevel DC converter shown in FIG. 1 when operating in mode 4;

[0024] Figure 6 for Figure 1 The commutation analysis diagram of the bidirectional modular multilevel DC converter shown in FIG. 5 when the converter is operating in mode 5;

[0025] Figure 7 for Figure 1 The commutation analysis diagram of the bidirectional modular multilevel DC converter shown in FIG. 1 when operating in mode 6;

[0026] Figure 8 for Figure 1 The bidirectional modular multilevel DC converter shown in the figure is driven by a phase-shifted cyclic modulation control method. DETAILED DESCRIPTION

[0027] The specific implementation of the present invention is further described in detail below in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0028] like Figure 1 As shown, the topological structure provided by this embodiment is a combination of a modular multi-level DC-DC converter and a dual active bridge structure, and the primary side and the secondary side of the converter adopt a modular half-bridge structure. The specific structure is as follows.

[0029] Primary side, DC voltage V in is the voltage of the input port; input capacitance C in1 , C in2 Series connection creates a neutral point for the primary DC bus and is connected in parallel with the input voltage; SM P For the primary side upper bridge arm module, SM N The lower bridge arm module is the primary side. Both the upper and lower bridge arm modules have N half-bridge sub-modules SM connected in series. SMP 、V SMN is the bridge arm voltage of the upper bridge arm and the lower bridge arm on the primary side; the same-name ends of the primary winding of transformer T are connected in turn through the DC blocking capacitor C B , power transfer inductor L k Connected to the connection point of the upper and lower bridge arms on the primary side, the opposite end of the primary winding of the transformer T is connected to the neutral point created by the primary side DC bus capacitor.

[0030] On the secondary side, DC voltage V out is the output voltage of the output port; the output capacitor C out1 , C out2 Series connection creates a neutral point for the secondary output DC bus and is connected in parallel with the output voltage; in the secondary side of the topology circuit, SM S For the secondary side upper bridge arm module, SM L The lower bridge arm module on the secondary side has N series-connected half-bridge submodules SM. SMS 、V SMLis the bridge arm voltage of the upper bridge arm and the lower bridge arm on the secondary side; the same-name end of the secondary winding of the transformer T is connected to the connection point of the upper and lower bridge arms on the secondary side, and the opposite-name end of the secondary winding of the transformer T is connected to the neutral point created by the secondary side output capacitor.

[0031] Each half-bridge submodule SM, including capacitor C SM and two switching devices Q1, Q2; the drain of the switching device Q1 and the capacitor C SM One end is connected, the source is connected to the drain of the switching device Q2; the source of the switching device Q2 is connected to the capacitor C SM the other end.

[0032] In this embodiment, the transformer T is equivalent to the excitation inductance L m It is composed of an ideal transformer, all switching tubes are ideal switching tubes and are anti-parallel connected with a diode and a parasitic capacitor.

[0033] This embodiment performs commutation analysis on the proposed modular multi-level DC converter in a forward power transmission working mode. To simplify the analysis, four half-bridge sub-modules are selected for each bridge arm. Figure 8 The driving waveform of the control method based on phase-shifted cyclic modulation is given, where the high level represents that the submodule is in the on state, and the low level represents that the submodule is in the off state. In the forward power transmission working mode, V AB Advanced V CD Phase φ, a complete switching cycle can be divided into 6 working modes, and the circuit parts that do not participate in the work in each mode are drawn with dotted lines. The working waveform of the converter is as follows Figure 8 As shown, where v p1 -v p4 、v s1 -v s4 They are the driving waveforms of the switch devices Q1 of each submodule on the primary and secondary sides, i Lk is the current of the power inductor, and the bridge arm current flowing to each bridge arm is i P 、i N 、i S 、i L. To describe the working mode commutation process, the following assumptions are made:

[0034] (1) The bidirectional modular multilevel DC converter has been operating in a stable state;

[0035] (2) All switches are ideal switches and are anti-parallel connected with a diode and a parasitic capacitor;

[0036] (3) The switching device is regarded as an ideal switch, and the two ends are connected in anti-parallel with parasitic diodes and parasitic capacitance.

[0037] The switching action of each half-bridge submodule is consistent, and the subsequent half-bridge submodule will be delayed by a fundamental wave period T relative to the previous half-bridge submodule. fs ; For each bridge arm, a switching mode will continuously operate N half-bridge submodules in N fundamental cycles; the switching action of each half-bridge submodule will also cycle once in the same N fundamental cycles; the switching action timing within the cycle of a single half-bridge submodule is called the submodule switching sequence, and the switching actions of the N half-bridge submodules in each bridge arm within one fundamental cycle constitute the switching sequence of the bridge arm. The switching sequence of each bridge arm forms a circulant matrix. Due to the theory of circulant matrices, when the circulant matrix is ​​a full-rank matrix, the circulant matrix has a unique solution. Based on this theory, the capacitor voltage of each submodule of the modular multi-level DC-DC converter can be inherently balanced.

[0038] The commutation process is analyzed as follows (taking forward power transmission as an example):

[0039] Mode 1: [t0~t1]

[0040] like Figure 2 As shown, at time t0, all submodules of the primary side upper bridge arm and the secondary side lower bridge arm are in the on-state, and three submodules of the primary side lower bridge arm and the secondary side upper bridge arm are in the on-state, and one submodule is in the off-state. At this time, the transmission inductor current of the bidirectional modular multilevel DC converter is i Lk In this mode, the primary side bridge arm current I P and the lower bridge arm current I N All of them are collected at point A and flow to the inductor, then i Lk =I P +I N , and the inductor current gradually decays. The secondary side current flows from point C to each sub-module of the upper bridge arm and the lower bridge arm respectively.

[0041] Mode 2: [t1~t2]

[0042] like Figure 3 As shown, at time t1, the inductor current decays to 0, under which the inductor current increases in the reverse direction. At this time, the power flows from the secondary side to the primary side. The primary side inductor current flows from point A to each sub-module of the upper bridge arm and the lower bridge arm respectively. The upper bridge arm and the lower bridge arm current of the secondary side converge at point C and flow to the secondary side of the transformer.

[0043] Mode 3: [t2~t3]

[0044] like Figure 4 As shown, in this mode, all the upper bridge arm submodules on the primary side and the secondary side are in the on state, and only three submodules of the lower bridge arm are in the on state and one submodule is in the off state. At this time, the inductor current i LkContinue to increase in the opposite direction.

[0045] Mode 4: [t3~t4]

[0046] like Figure 5 As shown, at time t3, the inductor current i Lk The reverse direction reaches the maximum. At this time, the sub-modules of the upper bridge arm on the primary side are changed from fully put into operation to only 3, and the lower bridge arm is changed from 3 to all put into operation. The sub-module states of the two bridge arms on the secondary side remain unchanged. At this time, the inductor current gradually decreases in the reverse direction.

[0047] Mode 5: [t4~t5]

[0048] like Figure 6 As shown, at time t4, the inductor current decreases to 0. At this time, the inductor current gradually increases, and the currents of the upper bridge arm and the lower bridge arm on the primary side converge at point A and flow to the inductor and the transformer. The transformer current on the secondary side flows from point C to the two bridge arms respectively. At this time, the power flows from the primary side to the secondary side.

[0049] Mode 6: [t5~t6]

[0050] like Figure 7 As shown, in this mode, the current direction of each bridge arm remains unchanged, the upper bridge arm submodules on the secondary side are changed from all put into operation to only three, and the lower bridge arm is changed to be fully put into operation. At this time, the inductor current continues to increase.

[0051] In the next stage, the circuit will change from mode 6 to mode 1. It is worth noting that the lower bridge arm on the primary side and the upper bridge arm on the secondary side of mode 6 are cut off from the original first submodule and become the second submodule. It can be seen that the removal state of the bridge arm submodules on the primary side and the secondary side of the converter is in a fundamental wave period T s A transfer will occur, and in one switching cycle T c The cycle is completed and returns to the initial state. This cyclic modulation can make the capacitors of each submodule in the same charging and discharging state, so that the capacitors of each submodule are in a stable state. AB and V CD The phase shift angle can realize the transmission of different powers. In the reverse power transmission, only V CD Advanced V AB .

[0052] The commutation analysis of the above stages is carried out on the proposed modular multi-level DC converter in the forward power transmission working mode. The specific driving waveform of the phase-shifted cyclic modulation control is shown in the figure below. Figure 8 As shown. The switching frequency of the driving signal of each sub-module switch device is f c, duty cycle D = (2N-1) / 2N, the frequency of the converter bidirectional transmission is f s , where f s =N*f c In the forward working mode, the phase shift angle of each submodule on the upper bridge arm of the primary side is θ, where θ = 180 / N, and the phase shift angle of the submodule on the secondary side relative to the primary side is φ. The control method of phase shift cyclic modulation is adopted, so that the voltage V AB and V CD The phase difference angle is φ, where 0<φ≤180 / N. The signals of the upper and lower bridge arm switching devices on the same side are phase shifted by θ. Since the proposed converter has the same structure, the reverse working mode control method is similar to this, and only needs to make the control signal on the secondary side lead the control signal on the primary side.

[0053] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope defined by the claims of the present invention.

Claims

1. A bidirectional modular multi-level DC converter based on phase-shifted cyclic modulation, characterized in that: The primary side and the secondary side of the converter both adopt a modular structure, specifically: Primary side, DC voltage V in is the voltage of the input port; input capacitance C in1 , C in2 Series connection creates a neutral point for the primary DC bus and is connected in parallel with the input voltage; SM P For the primary side upper bridge arm module, SM N The lower bridge arm module is the primary side. Both the upper and lower bridge arm modules have N half-bridge sub-modules SM connected in series. SMP 、V SMN is the bridge arm voltage of the upper bridge arm and the lower bridge arm on the primary side; the same-name ends of the primary winding of transformer T are connected in turn through the DC blocking capacitor C B , power transfer inductor L k Connected to the connection point of the upper and lower bridge arms on the primary side, the opposite end of the primary winding of the transformer T is connected to the neutral point created by the DC bus capacitor on the primary side; On the secondary side, DC voltage V out is the output voltage of the output port; the output capacitor C out1 , C out2 Series connection creates a neutral point for the secondary output DC bus and is connected in parallel with the output voltage; in the secondary side of the topology circuit, SM S For the secondary side upper bridge arm module, SM L The lower bridge arm module is the secondary side. Both the upper and lower bridge arm modules have N series-connected half-bridge submodules SM. SMS 、V SML is the bridge arm voltage of the upper bridge arm and the lower bridge arm on the secondary side; the same-name end of the secondary winding of the transformer T is connected to the connection point of the upper and lower bridge arms on the secondary side, and the opposite-name end of the secondary winding of the transformer T is connected to the neutral point created by the secondary side output capacitor.

2. The bidirectional modular multi-level DC converter based on phase-shifted cyclic modulation according to claim 1, characterized in that: Each half-bridge submodule SM includes a capacitor C SM and two switching devices Q1, Q2; the drain of the switching device Q1 and the capacitor C SM One end is connected, the source is connected to the drain of the switching device Q2; the source of the switching device Q2 is connected to the capacitor C SM the other end.

3. The bidirectional modular multi-level DC converter based on phase-shifted cyclic modulation according to claim 2, characterized in that: The transformer T is equivalent to the excitation inductance L m It is composed of an ideal transformer, all switching tubes are ideal switching tubes and are anti-parallel connected with a diode and a parasitic capacitor.

4. The bidirectional modular multi-level DC converter based on phase-shifted cyclic modulation according to claim 2, characterized in that: The switching frequency of the switching device driving signal of the half-bridge submodule is f c , the duty cycle is D = (2N-1) / 2N, and the frequency of the converter bidirectional transmission is f s , where f s =N*f c ; In the forward working mode, the phase shift angle of each half-bridge sub-module in the upper bridge arm module on the primary side is 2θ, where θ=180 / N, and the phase shift angle of the half-bridge sub-module on the secondary side relative to the primary side is φ, where 0<φ≤180 / N; in the reverse working mode, it is only necessary to make the driving signal on the secondary side lead the driving signal on the primary side.

5. The bidirectional modular multi-level DC converter based on phase-shifted cyclic modulation according to claim 4, characterized in that: The phase shift angle of the half-bridge submodule corresponding to the upper bridge arm and the lower bridge arm in the primary side is θ; the phase shift angle of the half-bridge submodule corresponding to the upper bridge arm and the lower bridge arm in the secondary side is also θ.

6. The bidirectional modular multi-level DC converter based on phase-shifted cyclic modulation according to claim 5, characterized in that: The switching action of each half-bridge submodule is consistent, and the subsequent half-bridge submodule will be delayed by a fundamental wave period T relative to the previous half-bridge submodule. fs ; For each bridge arm, a switching mode will continuously operate N half-bridge sub-modules in N fundamental cycles; the switching action of each half-bridge sub-module will also cycle once in the same N fundamental cycles; the switching action timing within the cycle of a single half-bridge sub-module is called the sub-module switching sequence, and the switching actions of the N half-bridge sub-modules in each bridge arm within one fundamental cycle constitute the switching sequence of the bridge arm.

7. The bidirectional modular multi-level DC converter based on phase-shifted cyclic modulation according to claim 6, characterized in that: The switching sequence of each bridge arm forms a circulant matrix. When the circulant matrix is ​​a full-rank matrix, the circulant matrix has a unique solution, so that the capacitor voltage of each sub-module of the modular multi-level DC-DC converter can achieve inherent balance.

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