A modular alternating support converter without bridge arm inductors and its modulation method

Abstract

This invention discloses a modular alternating support converter without bridge arm inductors and its modulation method, relating to the field of power electronics technology. This modular alternating support converter proposes a new modular alternating support converter topology by reconstructing the traditional MMC operating mode, including three phase units; each phase unit includes an upper and lower bridge arm connected in series; each bridge arm includes N sub-modules connected in series; the two ends of the phase unit serve as the positive and negative terminals of the DC terminal; the midpoint of the phase unit is connected to an AC filter inductor as the connection terminal for the three-phase power; the positive terminal of the DC terminal is equipped with a DC filter inductor. This modular alternating support converter eliminates the need for filter inductors on the bridge arms, and under the corresponding alternating operation mode, it exhibits a large capacity reduction and power density, small size, high power density, and a fully modular design that reduces engineering implementation difficulty, resulting in good economic efficiency.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a modular alternating support converter without bridge arm inductors and its modulation method. Background Technology

[0002] The continued rise in global energy demand has created an urgent need for efficient, long-distance power transmission technologies. High-performance converters play a crucial role as core equipment for achieving flexible grid interconnection. Among them, the Modular Multilevel Converter (MMC), with its modular structure, excellent output waveform quality, and good scalability, has become the mainstream topology choice in medium- and high-voltage high-power applications, especially in flexible DC transmission systems (hereinafter referred to as the system). However, the widespread application of MMC faces significant challenges. The primary issue is the large number of sub-modules connected in series to achieve high voltage levels, leading to increased system complexity and cost. Secondly, since the three bridge arms of the MMC are equivalent to three voltage sources in parallel during operation, six bridge arm reactors are required to limit circulating current. More importantly, due to the significant fundamental frequency power ripple experienced by the sub-module capacitors, each sub-module requires a large-capacity DC support capacitor, whose volume typically occupies more than 50% of the total space of a single sub-module. In summary, the application of the system in space- and cost-sensitive scenarios is significantly constrained by these factors.

[0003] To overcome the bottlenecks in size and cost of MMCs, existing technologies have proposed various capacitor optimization strategies based on harmonic injection. One method is to inject a second-order circulating current between phases. This method suppresses instantaneous power fluctuations in the arm by modulating the arm current waveform (without changing the arm voltage), thereby reducing the demand for capacitor capacity. However, this method is usually accompanied by increased arm losses, and its capacitor reduction effect is limited, generally below 30%. Furthermore, the complexity of circulating current calculation and control increases significantly under non-ideal operating conditions such as grid faults. Another method is to inject a third-order harmonic component into the phase voltage. By reconstructing the arm voltage waveform (without changing the arm current), energy fluctuations can also be reduced. However, this method raises the peak voltage on the AC side of the converter, placing higher demands on the insulation level of the connecting transformer, and its capacitor reduction effect is also difficult to exceed the 30% limit. To pursue better results, some studies have attempted to combine second-order circulating current injection and third-order harmonic voltage injection. However, these two harmonic injection mechanisms have a coupling effect on energy smoothing, which makes it extremely complex to determine and maintain the optimal harmonic injection amount during the dynamic operation of the system (such as power step or fault ride-through), and greatly increases the difficulty of control.

[0004] Therefore, although existing harmonic injection methods provide a theoretical direction for optimizing the size of MMC capacitors, they generally face core limitations in practical applications, such as insufficient capacity reduction (usually less than 30%), additional losses or increased insulation requirements, and highly complex dynamic control. Furthermore, these methods do not fundamentally change the operating mode of MMC—"AC differential mode-DC common mode power transfer, three-phase bridge arms parallel support DC voltage"—and still rely on bridge arm reactors. Summary of the Invention

[0005] To address the aforementioned technical problems in the existing technology, the present invention aims to provide a modular alternating support converter with a large capacity reduction range and a high power density.

[0006] Specifically, in the first aspect, the present invention provides a modular alternating support converter without bridge arm inductors, comprising three phase units;

[0007] The phase unit includes an upper bridge arm and a lower bridge arm connected in series; each bridge arm includes a series of... Each submodule;

[0008] The two ends of the phase unit serve as the positive and negative terminals of the DC terminal; the midpoint of the phase unit is connected to an AC filter inductor as the connection terminal for the three-phase power.

[0009] The positive terminal of the DC terminal is equipped with a DC filter inductor.

[0010] Optionally, the submodule is a half-bridge submodule.

[0011] Optionally, the submodule is a full-bridge submodule.

[0012] Optional, In each submodule One is a half-bridge submodule, and the rest are full-bridge submodules.

[0013] Secondly, the present invention provides a modulation method for the above-mentioned modular alternating support converter without bridge arm inductors, comprising the following steps:

[0014] Step S1: Calculate the enable signal of the bridge arm and the number of submodules to be added;

[0015] (1) Calculate the enable signals for the upper and lower bridge arms using the following formulas: ; ; In the formula, This is the enable signal for the upper bridge arm. This is the enable signal for the lower bridge arm. For synchronization signal, for The initial phase of the phase voltage;

[0016] (2) Calculate the modulation voltage of the upper and lower bridge arms using the following formula: ; In the formula, This refers to the number of phases in a three-phase electrical system. , for Modulation voltage of the upper bridge arm, for Modulation voltage of the lower bridge arm, This is the DC voltage reference value. for AC phase voltage reference value;

[0017] Calculate the number of sub-modules required for the upper bridge arm. The number of sub-modules required for the lower bridge arm The formula is as follows: ; ; In the formula, This is the rated capacitor voltage for the submodule; Round (•) indicates the rounding operation.

[0018] Step S2: Determine the switching sequence of submodules in each bridge arm;

[0019] right Xiangshang Bridge Arm,

[0020] If the bridge arm current Then choose the capacitor with the lowest voltage. Each sub-module was implemented;

[0021] If the bridge arm current Then choose the capacitor with the highest voltage. Each sub-module was implemented;

[0022] right Lower bridge arm,

[0023] If the bridge arm current Then choose the capacitor with the lowest voltage. Each sub-module was implemented;

[0024] If the bridge arm current Then choose the capacitor with the highest voltage. Each sub-module was implemented;

[0025] in, , ;

[0026] Step S3: For each bridge arm, perform a bitwise AND operation between the bridge arm current and the corresponding enable signal to obtain the corresponding switching signal, control the corresponding sub-module to be put into operation, and realize the state alternation of the three phase units;

[0027] The states of the phase units include:

[0028] State 1: When the enable signal is active and enable signal At this time, both the upper and lower bridge arms of the corresponding phase unit are enabled for output, and the voltage output equation is as follows: ; In the formula, for The bridge arm voltage of the upper bridge arm, for The bridge arm voltage of the lower bridge arm, DC side voltage for AC phase voltage of the phase;

[0029] State 2: When the enable signal is active and enable signal When the corresponding phase unit's lower bridge arm is enabled and output, and the upper bridge arm is locked, the voltage output equation is as follows: ;

[0030] State 3: When the enable signal is active and enable signal When the corresponding phase unit's upper bridge arm is enabled and output, and the lower bridge arm is locked, the voltage output equation is as follows: .

[0031] Compared with existing technologies, the technical solution provided by this invention has a larger capacity reduction range and power density, smaller size, higher power density, and a fully modular design that makes engineering implementation easier and has good economic benefits. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the modular alternating support converter in this invention.

[0033] Figure 2 This is a schematic diagram of the modular alternating support converter bridge arm in this invention.

[0034] Figure 3 This is a schematic diagram of the operation partition of the modular alternating support converter in this invention.

[0035] Figure 4 This is a schematic diagram of the six arm conduction states of the modular alternating support converter in this invention.

[0036] Figure 5 This is a schematic diagram showing three states of the a-phase bridge arm of the modular alternating support converter in this invention.

[0037] Figure 6 This is a key operating waveform diagram of the modular alternating support converter in this invention.

[0038] Figure 7 This is a schematic diagram showing the modulation details of the upper bridge arm of the modular alternating support converter in this invention. Detailed Implementation

[0039] The technical solution provided by the present invention will be further described in detail below with reference to the accompanying drawings.

[0040] like Figure 1 As shown, this invention proposes a novel Modular Alternate Supporting Converter (MASC) topology by reconstructing the traditional MMC operating mode. In MASC, each phase unit is divided into an upper arm and a lower arm, for a total of 6 arms. Each arm consists of... It is composed of a series of submodules (SM). Figure 1 middle, DC side voltage This is the DC side current; AC side voltage, For AC side current, ; for The AC phase voltage on the output side of the phase bridge arm (i.e., the phase voltage between the midpoint of the bridge arm and the neutral point). for Phase upper arm voltage, for The bridge arm current of the upper bridge arm, for The bridge arm voltage of the lower bridge arm, for The current in the lower arm of the bridge; For DC-side filter inductors, This is the AC filter inductor on the AC side.

[0041] like Figure 2 As shown, from left to right, submodules can be half-bridge submodules, full-bridge submodules, or full-half-bridge submodules. When a submodule is a full-half-bridge submodule, the sum of the number of full-bridge submodules and half-bridge submodules is... .

[0042] Optionally, the capacitors in the submodule can be polarized capacitors. When the submodule is a half-bridge submodule, the negative terminal of the polarized capacitor is the output terminal of the submodule.

[0043] Operating principle:

[0044] MASC operates in a completely different manner from traditional MMC. Each phase's upper / lower bridge arm has both enable and lockout modes. The enable signal for the upper bridge arm... The logic settings are as follows: ; In the formula, For time, For synchronization signal, for The initial phase of the phase voltage. When the enable signal... When the bridge arm is enabled, the submodule switching signal is either enabled or bypassed according to the modulation logic; when At that time, all switch signals of the bridge arm are blocked.

[0045] Similarly, the enable signal for the lower bridge arm The logic settings are as follows: ;

[0046] Based on the enable signal states of each phase, we can derive the following: Figure 3 The diagram shows the converter operation zones. The equivalent circuit of the converter in each zone is as follows: Figure 4 As shown. When the enable signal When the upper bridge arm is enabled, the submodule switching signal is either enabled or bypassed according to the modulation logic; when At this time, all switch signals of the bridge arm are blocked. The enable signal of the lower bridge arm is handled similarly.

[0047] Depend on Figure 3 , Figure 4 As can be seen, a sinusoidal cycle is divided into 6 sectors (denoted as sector I, sector II, sector III, sector IV, sector V, and sector VI). In each sector, only one phase's upper and lower bridge arms are simultaneously enabled to output and support DC voltage. In the other two phases, only one bridge arm is enabled to output, while the opposite bridge arm is in a blocked state. For example, in... Figure 4 Within sector I, both the upper and lower arms of phase c output voltage, and the sum of these voltages is a DC voltage; the upper arm of phase a has an enable signal. Output, its output voltage With voltage The combined AC line voltage is achieved, with the lower arm of phase a in a blocked state; the lower arm of phase b is enabled while the upper arm is blocked. The analysis in other sectors is similar. Ultimately, the three-phase arms alternately enable output and support DC voltage in the order cbacba. Therefore, at any given time, there is no situation where two or three phase arms are simultaneously connected in parallel with the DC side, meaning there is no circulating current path between phases, thus eliminating the need for arm reactors.

[0048] Taking phase a as an example, the key voltages and currents of the bridge arm are analyzed. The three operating states of the upper and lower bridge arms within one power frequency cycle are as follows: Figure 5 As shown. State 1 corresponds to... Figure 3 Sectors III and VI are represented in the diagram. State 2 corresponds to sectors IV and V, and state 3 corresponds to sectors I and II. A schematic diagram of the key voltage and current waveforms of phase a in MASC is shown below. Figure 6 As shown.

[0049] Assume phase a AC voltage and alternating current As shown in the following formula: ; In the formula, This represents the peak value of the AC voltage. The peak value of the alternating current. Phase a alternating current The phase angle.

[0050] For state 1, when the enable signal and enable signal When both the upper and lower bridge arms are enabled, AC voltage generation and DC voltage support are achieved, resulting in the following voltage equation: ; In the formula, for The bridge arm voltage of the upper bridge arm, for The bridge arm voltage of the lower bridge arm, DC side voltage for The AC phase voltage of the phase.

[0051] Ignore AC filter inductance When the voltage drop is equal to the phase voltage of phase a, the AC phase voltage is equal to the phase voltage of phase a. Approximately equal to the AC voltage of phase a .according to Figure 4 From sectors III and VI, we know that the current flowing through the upper bridge arm is related to the DC current and the enabling status of the other two phase bridge arms, so we can obtain: ; ;

[0052] For state 2, when the enable signal and enable signal When the lower bridge arm is enabled, the voltage and current of the lower bridge arm can be obtained as shown in the following formula: ;

[0053] At this time, the upper bridge arm will block all switching signals, and the upper bridge arm will be subjected to a reverse pressure, as shown in the following formula: ;

[0054] Combining the two sets of formulas above, we can obtain the voltage of phase a bridge arm. The voltage it can withstand is Therefore, the maximum value of this back pressure is... Since the total capacitor voltage of each bridge arm submodule is generally designed to be the DC side voltage. And the peak value of AC voltage It is generally designed to be less than 0.5 times the DC side voltage. (Related to the adjustment system), therefore the voltage of phase a bridge arm The voltage of phase a bridge arm is always greater than 0, and the diode in the lower switching transistor of the submodule is in the off state. The voltage is constantly less than the total submodule capacitor voltage, so the diodes of the upper switching transistor in the submodule are in the off state. Therefore, the entire upper bridge arm can be equivalent to a group of diodes all in the off state, and the current flowing through the upper bridge arm... .

[0055] The operating mechanism of state 3 is similar to that of state 2, when the enable signal... and enable signal When the corresponding phase unit's upper bridge arm is enabled and outputs, the lower bridge arm is locked and subjected to reverse voltage. The voltage and current of the upper and lower bridge arms are shown in the following formula: ; .

[0056] MASC modulation strategy:

[0057] Nearest Level Modulation (NLM) is the most typical modulation method in MMC, suitable for applications with a large number of modules. This invention still uses NLM modulation, but some modifications are needed based on the operating characteristics of MASC (alternating enable and latch-up). According to the aforementioned principle analysis, the modulation voltage of each bridge arm can be expressed by the following formula: ; In the formula, for Modulation voltage of the upper bridge arm, for Modulation voltage of the lower bridge arm, This is the DC voltage reference value. for The phase voltage reference value for phase AC. The modulation logic for each bridge arm is similar. The following explanation uses the upper bridge arm of phase a as an example. The modulation logic is as follows: Figure 7 As shown. Modulation voltage With the rated capacitor voltage of the submodule The number of sub-modules required for the bridge arm can be obtained by rounding the ratio. The formula is as follows: ;

[0058] After determining the number of submodules to be deployed in each bridge arm, the capacitor voltage order and bridge arm current polarity are also needed to further select a suitable submodule switching sequence to achieve dynamic balance in charging and discharging among the submodules. Specifically, when the bridge arm current... At this time, all submodules are in a charging state, and the module with the lowest capacitor voltage is selected first. Each sub-module is invested; similarly, when At this time, all submodules are in a discharging state, and the module with the highest capacitor voltage is selected first. Each submodule is then deployed. Finally, the obtained signal is logically ANDed with the enable signal of the upper bridge arm to obtain the final switching signal.

[0059] The number of sub-modules required for the upper bridge arm is easily obtained. The number of sub-modules required for the lower bridge arm The formula is as follows: ; ;

[0060] Determine the switching sequence of submodules in each bridge arm, including: Phase upper arm, if the arm current Then choose the capacitor with the lowest voltage. Each submodule is put into operation; if the bridge arm current... Then choose the capacitor with the highest voltage. Each sub-module has been deployed. For If the current in the lower bridge arm is... Then choose the capacitor with the lowest voltage. Each submodule is put into operation; if the bridge arm current... Then choose the capacitor with the highest voltage. Each sub-module has been deployed. Among them, , .

[0061] Specifically, for Phase upper bridge arm, transfer bridge arm current and enable signal Perform an AND operation to obtain the upper bridge arm switch signal, and control the corresponding... Each sub-module has been deployed. For The lower bridge arm will transfer the bridge arm current. and enable signal Perform an AND operation to obtain the lower bridge arm switch signal, and control the corresponding... Each sub-module is deployed. Since the phase difference between phases a, b, and c in a three-phase power supply is a fixed 120°, the phase difference of the switching signals for each phase is also a fixed 120°, enabling the three-phase bridge arms to alternately and simultaneously enable output and support DC voltage in the order of cbacba.

[0062] Compared with existing technologies, the technical solution provided by this invention has a larger capacity reduction range and power density, smaller size, higher power density, and a fully modular design that makes engineering implementation easier and has good economic benefits.

Claims

1. A modular alternating support converter without bridge arm inductors, characterized in that, Includes three phase units; The phase unit includes an upper bridge arm and a lower bridge arm connected in series; each bridge arm includes a series of... Each submodule; The two ends of the phase unit serve as the positive and negative terminals of the DC terminal; the midpoint of the phase unit is connected to an AC filter inductor as the connection terminal for the three-phase power. The positive terminal of the DC terminal is equipped with a DC filter inductor; In the operation zone of the modular alternating support converter, a sinusoidal cycle is divided into 6 sectors. In each sector, only one phase upper and lower bridge arm is enabled to output and support DC voltage at the same time. In the other two phases, only one bridge arm is enabled to output, and the opposite bridge arm is in a locked state. The three-phase bridge arms are alternately enabled to output and support DC voltage at the same time in the order of cbacba. The states of the phase units include: State 1: When the enable signal of the upper arm is activated. And the enable signal of the lower bridge arm At this time, both the upper and lower bridge arms of the corresponding phase unit are enabled for output, and the voltage output equation is as follows: ； In the formula, for The bridge arm voltage of the upper bridge arm, for The bridge arm voltage of the lower bridge arm, DC voltage for AC phase voltage of the phase; State 2: When the enable signal of the upper arm is activated. And the enable signal of the lower bridge arm When the corresponding phase unit's lower bridge arm is enabled and output, and the upper bridge arm is locked, the voltage output equation is as follows: ； State 3: When the enable signal of the upper arm is activated. And the enable signal of the lower bridge arm When the corresponding phase unit's upper bridge arm is enabled and output, and the lower bridge arm is locked, the voltage output equation is as follows: 。 2. The modular alternating support converter without bridge arm inductors as described in claim 1, characterized in that, The submodule is a half-bridge submodule.

3. A modular alternating support converter without bridge arm inductors as described in claim 1, characterized in that, The submodule is a full-bridge submodule.

4. A modular alternating support converter without bridge arm inductors as described in claim 1, characterized in that, The In each submodule One is a half-bridge submodule, and the rest are full-bridge submodules.

5. A modulation method for a modular alternating support converter without bridge arm inductors as described in any one of claims 1 to 4, characterized in that, Includes the following steps: Step S1: Calculate the enable signal of the bridge arm and the number of submodules to be added; (1) Calculate the enable signals for the upper and lower bridge arms using the following formulas: ； ； In the formula, This is the enable signal for the upper bridge arm. This is the enable signal for the lower bridge arm. For synchronization signal, for The initial phase of the phase voltage; (2) Calculate the modulation voltage of the upper and lower bridge arms using the following formula: ； In the formula, This refers to the number of phases in a three-phase electrical system. , for Modulation voltage of the upper bridge arm, for Modulation voltage of the lower bridge arm, This is the DC voltage reference value. for AC phase voltage reference value; Calculate the number of sub-modules required for the upper bridge arm. The number of sub-modules required for the lower bridge arm The formula is as follows: ； ； In the formula, The rated capacitor voltage of the submodule. Indicates the integer operation; Step S2: Determine the switching sequence of submodules in each bridge arm; right Xiangshang Bridge Arm, If the bridge arm current Then choose the capacitor with the lowest voltage. Each sub-module was implemented; If the bridge arm current Then choose the capacitor with the highest voltage. Each sub-module was implemented; right Lower bridge arm, If the bridge arm current Then choose the capacitor with the lowest voltage. Each sub-module was implemented; If the bridge arm current Then choose the capacitor with the highest voltage. Each sub-module was implemented; in, , ; Step S3: For each bridge arm, perform a bitwise AND operation between the bridge arm current and the corresponding enable signal to obtain the corresponding switching signal, control the corresponding sub-module to be put into operation, and realize the state alternation of the three phase units.

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