A fundamental frequency modulation method for reducing switching losses in modular multilevel converters

Abstract

The present invention discloses a fundamental frequency modulation method for reducing switching losses in a modular multilevel converter, belonging to the field of power electronics modulation technology. The method includes determining, based on the voltage modulation ratio, the number of submodules in the upper or lower bridge arm of the converter that are activated with a 100% duty cycle and the number of submodules that are activated with a 50% duty cycle within a switching cycle. At each moment within each switching cycle, a total of N+K submodules are always activated in the upper and lower bridge arms. The drive signal time bases between the upper and lower bridge arms are controlled to differ by half a switching cycle, and the waveforms of the drive signals of the upper and lower bridge arms are symmetrical. The drive signals of each submodule in a single bridge arm that are activated with a 50% duty cycle are obtained by sequentially shifting the waveforms of the drive signal with a 50% duty cycle, so that the waveforms generated by the upper and lower bridge arms complement each other. The present invention fully symmetrically modulates the fundamental frequency switching signals of the submodules in the upper and lower bridge arms of the converter, reducing submodule switching losses and improving the operating efficiency of the modular multilevel converter.

Description

Technical Field

[0001] This invention belongs to the field of power electronic modulation technology, specifically relating to a fundamental frequency modulation method for reducing switching losses in a modular multilevel converter. Background Technology

[0002] With the large-scale integration of new energy sources into the power grid, the rise of large-scale data centers, and the booming development of rail transit traction, medium-voltage high-capacity converters are receiving widespread attention and application. In existing technologies, to enable converters to meet the demands of high voltage levels and large power capacity, the common approach is to connect low-voltage power semiconductor devices or low-voltage modules in series and parallel. In practical engineering applications, it is often necessary to convert medium- and high-voltage electrical energy (which can reach tens of kilovolts) into low-voltage electrical energy (typically hundreds of volts) to provide power support for downstream loads.

[0003] Traditional power conversion schemes typically employ a topology where a single low-voltage module is connected in series on the input side and in parallel on the output side. However, as the input and output voltages increase, this structure necessitates a corresponding increase in the rating of the isolation transformer within each module. This leads to a larger isolation transformer size, which in turn negatively impacts the converter's conversion efficiency and power density.

[0004] Alternatively, a centralized transformer topology is available, which reduces the size of magnetic components such as transformers in the system and lowers their losses. Modular multilevel converters employ a multi-module series connection on the input side, leveraging modularity to flexibly apply to different voltage levels. LLC resonant circuit voltage regulation also has some applications in medium-voltage power supply; however, in medium-voltage applications with a wide input voltage range, relying solely on frequency modulation for voltage regulation presents significant challenges in hardware circuit parameter design. Specifically, a small transformer magnetizing inductance leads to a large magnetizing current in medium-voltage applications, increasing converter losses. Furthermore, LLC resonant circuits typically regulate output voltage by adjusting the frequency of switching devices, but their frequency adjustment range is limited, making it difficult to adapt to wide input voltage variations. Additionally, since the efficiency of resonant converters peaks near the resonant frequency, changing the frequency adjustment method cannot simultaneously achieve wide-range voltage regulation while ensuring maximum operating efficiency. Summary of the Invention

[0005] To address the problems in the prior art, this invention discloses a fundamental frequency modulation method for reducing switching losses in modular multilevel converters.

[0006] The technical solution of the present invention is as follows:

[0007] This invention provides a fundamental frequency modulation method for reducing switching losses in a modular multilevel converter, comprising the following steps:

[0008] At the beginning of each switching cycle, the voltage modulation ratio m of the modular multilevel converter is adjusted according to the magnitude of the DC input voltage. Based on the voltage modulation ratio m, the number K of sub-modules with 100% duty cycle and the number NK of sub-modules with 50% duty cycle in the upper or lower arm of the modular multilevel converter are determined within one switching cycle; where N is the total number of sub-modules in the upper or lower arm; the drive signal of the sub-module with 100% duty cycle is always in the active state within one switching cycle, and the drive signal of the sub-module with 50% duty cycle is in the active state for 50% of the time and in the deactivated state for the remaining 50% of the time within one switching cycle; at any moment in each switching cycle, there are always a total of N+K sub-modules in the upper and lower arms in the active state.

[0009] The time base of the drive signals between the upper and lower bridge arms is controlled to be half a switching cycle apart, and the waveforms of the drive signals of the upper and lower bridge arms are symmetrical. The drive signals of each sub-module with a 50% duty cycle in a single bridge arm are obtained by sequentially shifting the waveforms of the drive signals with a 50% duty cycle. Finally, the waveforms generated by the upper and lower bridge arms are complementary to each other.

[0010] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0011] (1) Compared with the constant voltage modulation ratio control strategy of simultaneously engaging N sub-modules in the upper and lower bridge arms, the present invention changes the number of sub-modules continuously engaged in one cycle as the input voltage changes, while ensuring that at every moment in each switching cycle, a total of N+K sub-modules in the upper and lower bridge arms are always engaged. The N+K sub-modules jointly support the DC input voltage, which can reduce the operating voltage of the sub-modules, thereby reducing the construction cost and operating loss of the modular multilevel converter.

[0012] (2) Variable module number voltage regulation has the characteristics of strong regulation capability but discontinuous regulation, while variable phase shift duration regulation has the characteristics of continuous regulation but limited regulation capability. This invention combines the two control degrees of freedom. By adjusting the number of sub-modules engaged with 100% duty cycle in one switching cycle, the number of modules can achieve wide-range voltage regulation. Then, through feedback closed loop (i.e. PI closed loop control), the phase shift duration can be autonomously adjusted to achieve continuous and precise voltage regulation, thereby achieving wide-range continuous voltage regulation and overcoming the problem that existing technologies cannot adapt to the wide range of input voltage changes.

[0013] (3) Compared with the traditional centralized transformer topology scheme frequency and voltage regulation strategy, the voltage regulation strategy proposed in this invention, which simultaneously adjusts the number of modules and the phase shift time, does not depend on the parameters of the LLC resonant cavity. It can avoid a small transformer magnetizing inductance, thereby reducing the transformer magnetizing current, reducing transformer losses and bridge arm submodule conduction losses, and improving system operating efficiency, especially operating efficiency under light load conditions. This enables efficient conversion under wide input and wide load conditions, overcoming the problem in the prior art that when the transformer magnetizing inductance is small, the converter will have a large magnetizing current in medium voltage application scenarios, thereby increasing the converter losses.

[0014] (4) Compared with the traditional baseband modulation strategy, the fully symmetrical modulation strategy proposed in this invention has a completely symmetrical baseband switching signal between the upper and lower bridge arm sub-module chains. This avoids the extra switching frequency caused by the switching mode in the traditional baseband modulation strategy. Therefore, the switching frequency of the device in this invention is lower than that of the traditional baseband modulation strategy, which can avoid the extra switching frequency of the sub-module when the switching mode is switched between adjacent switching cycles. This reduces the switching loss of the sub-module, improves the operating efficiency of the modular multilevel converter, and overcomes the problem that the existing technology cannot ensure the maximum operating efficiency while achieving wide-range voltage regulation. Attached Figure Description

[0015] Figure 1 This is a topology diagram of a modular multilevel DC-DC converter.

[0016] Figure 2 This is a diagram of the voltage modulation strategy for the bridge arm of a modular multilevel circuit.

[0017] Figure 3 This diagram shows the generation of drive signals for the upper and lower bridge arm submodules of the modular multilevel circuit in the existing scheme.

[0018] Figure 4 This is a phase-shifting generation diagram of the driving signals of the upper and lower bridge arm sub-modules of the modular multilevel circuit of the present invention.

[0019] Figure 5 This is a diagram illustrating the switching behavior using the existing drive signal generation method.

[0020] Figure 6 The diagram shows the switching behavior effect of the driving signal phase shift generation method of the present invention. Detailed Implementation

[0021] The present invention will be further described and illustrated below with reference to specific embodiments. The embodiments described are merely examples of the content of this disclosure and do not limit the scope of the invention. The technical features of each embodiment in the present invention can be combined accordingly, provided that there is no mutual conflict.

[0022] To address the aforementioned problems in existing technologies, this invention proposes a fundamental frequency modulation method to reduce switching losses in modular multilevel converters (MMCs), overcoming the shortcomings of existing technologies. This invention utilizes the voltage modulation ratio of the modular multilevel circuit to control the switching signals and phase shift duration of the harmonic submodules, enabling continuous and precise adjustment of the output voltage across a wide input voltage range. Compared to traditional frequency and voltage modulation schemes, this invention fixes the operating frequency at the resonant frequency, improving converter efficiency and enabling soft switching of power devices across the entire load range. Furthermore, by configuring the time reference phase difference between the driving modulation waves of the upper and lower bridge arms of the MMC to be half a switching cycle, the upper and lower bridge arms can generate completely complementary bridge arm voltages, thereby avoiding switching losses caused by additional switching. This method can adapt to a wide range of input voltage fluctuations without adjusting the switching frequency, ensuring high-efficiency output voltage control under a wide input range.

[0023] The fundamental frequency modulation method for reducing switching losses in a modular multilevel converter according to the present invention includes the following steps:

[0024] At the beginning of each switching cycle, the voltage modulation ratio m of the modular multilevel converter is adjusted according to the magnitude of the DC input voltage. Based on the voltage modulation ratio m, the number K of sub-modules with 100% duty cycle and the number NK of sub-modules with 50% duty cycle in the upper or lower arm of the modular multilevel converter are determined within one switching cycle; where N is the total number of sub-modules in the upper or lower arm; the drive signal of the sub-module with 100% duty cycle is always in the active state within one switching cycle, and the drive signal of the sub-module with 50% duty cycle is in the active state for 50% of the time and in the deactivated state for the remaining 50% of the time within one switching cycle; at any moment in each switching cycle, there are always a total of N+K sub-modules in the upper and lower arms in the active state.

[0025] The time base of the drive signals between the upper and lower bridge arms is controlled to be half a switching cycle apart, and the waveforms of the drive signals of the upper and lower bridge arms are symmetrical. The drive signals of each sub-module with a 50% duty cycle in a single bridge arm are obtained by sequentially shifting the waveforms of the drive signals with a 50% duty cycle. Finally, the waveforms generated by the upper and lower bridge arms are complementary to each other.

[0026] like Figure 1 As shown, this invention proposes a fundamental frequency modulation method to reduce switching losses in a modular multilevel converter. The single-phase modular multilevel resonant converter includes a DC input capacitor C. in The system includes a modular multilevel circuit (MMC), a resonant circuit, a medium-to-high frequency transformer, and a rectifier circuit. The modular multilevel circuit is divided into upper and lower bridge arms, each consisting of N identical half-bridge submodules SM1 to SM2. N and a bridge arm inductor L p / Ln The upper and lower bridge arm inductors are connected in series. They can be wound in a coupled manner on the same magnetic core, ensuring equal inductance between the upper and lower arms and making the equivalent inductance of the bridge arm inductor on the AC side zero. This achieves decoupling between the bridge arm inductors and the inductors in the resonant circuit. The voltage and current of the upper and lower bridge arms are represented by u. p i p ,u n i n This indicates that the resonant circuit consists of a resonant capacitor C. r Transformer leakage inductance L k And excitation inductance L m Composition. The secondary rectifier circuit of the transformer can be a half-bridge structure, a full-bridge structure, or a modular multilevel structure.

[0027] Figure 2 The diagram illustrates the specific modulation strategy for voltage modulation ratio control. Each upper and lower bridge arm has N sub-modules. K sub-modules in either the upper or lower bridge arm maintain a 100% duty cycle during one switching cycle, remaining continuously engaged. The remaining NK sub-modules maintain a 50% duty cycle during one switching cycle. At any given moment within each switching cycle, a total of N+K sub-modules in both the upper and lower bridge arms are always engaged. There is a phase shift between the drive signals of adjacent sub-modules with a 50% duty cycle engaged within a single bridge arm (either the upper or lower arm). This allows it to generate multi-level quasi-square waves, trapezoidal waves, sine waves, or triangular waves, with varying phase shift times. The method for determining m and K is as follows: after m and K are determined, the difference between the real-time output voltage of the modular multilevel converter and the target output voltage of the converter is used to autonomously adjust the voltage through PI closed-loop control.

[0028] The minimum value should be greater than the switching time t of the submodule device. on To achieve the minimum voltage change rate during the switching process of the bridge arm cascaded submodules; The maximum value depends on the required shape of the modulation wave; the quasi-square wave requires... Smaller, trapezoidal waves and sine waves Larger, triangular wave The maximum and upper limit are:

[0029]

[0030] therefore,

[0031] When a set of complementary drive signals is applied to the upper and lower bridge arms, the relationship between the DC input voltage and the voltage modulation ratio is as follows:

[0032]

[0033] Among them, V in V is the DC input voltage of the modular multilevel converter. in0 This is the preset minimum DC input voltage for the modular multilevel converter.

[0034] The calculation relationship between voltage modulation ratio and the number of switching modules is as follows:

[0035]

[0036] Where K is an integer and is rounded to the nearest integer; therefore

[0037]

[0038] When the DC input voltage is V in0 At that time, the voltage modulation ratio m = 1, and the value of K is 0. When the DC input voltage increases to V... in1 At that time, the voltage modulation ratio is K switches from 0 to 1, V in1 The following relationship must be satisfied:

[0039]

[0040] When the DC input voltage increases to V in2 When the voltage modulation ratio becomes K switches from 1 to 2, V in2 The following relationship must be satisfied:

[0041]

[0042] Similarly, when the DC input voltage increases to V... ink At that time, the voltage modulation ratio is K increases from k-1 to k (k is 1, 2, 3, ...), V ink The following relationship must be satisfied:

[0043]

[0044] The DC input voltage gradually increases until it reaches V. ink It is greater than the maximum DC input voltage.

[0045] Using [nTs, (n+1)Ts] as the time reference for one switching cycle of the upper bridge arm, then [(n+0.5)Ts, (n+1.5)Ts] is the time reference for one switching cycle of the lower bridge arm, where T... sThe duration of the switching cycle is n, which is an integer. It can achieve symmetrical consistency between the upper and lower bridge arm drive signals and their switching process. The selected switching cycle time reference [nTs, (n+1)Ts] must contain a complete drive signal that is always in the active state or a drive signal that is in the active state for 50% of the time and in the deactivated state for the remaining 50% of the time.

[0046] The drive signals of NK sub-modules with 50% duty cycles within the upper or lower bridge arm are in the time domain t∈[0,T] of one switching cycle. s The expression within the brackets is:

[0047]

[0048] Where i is the number of the submodule that is put into operation with a 50% duty cycle in the upper or lower bridge arm, i = 0, 1, 2, ..., NK-1; G(i) = 0 indicates that the submodule with number i is in the cut-off state, and G(i) = 1 indicates that the submodule with number i is in the put-in state.

[0049] To demonstrate the effectiveness of the proposed fundamental frequency modulation method for reducing switching losses in modular multilevel converters, a simulation model was built and verified using PLEC electrical simulation software. The main parameters of the simulation model are as follows: DC input voltage 9kV~18kV, output voltage constant 375V, maximum output power 100kW, number of bridge arm sub-modules 18, resonant frequency 12kHz, and the phase shift time of a single sub-module adjusted based on the modulation waveform and K value. Table 1 shows the parameter values ​​for the implementation case.

[0050] Table 1

[0051]

[0052]

[0053] When the input voltage is the minimum V in0 When the input voltage is 9kV, the feedforward circuit sets the voltage modulation ratio to a maximum of 1. This means that within one switching cycle, 18 sub-modules in either the upper or lower bridge arm participate in square wave switching until the number of sub-modules K = 0, and the sub-module phase shift duration is a relatively small value. As the input voltage increases, the feedback circuit adjusts the sub-module phase shift duration to maintain the stability of the converter's output voltage. When the input voltage increases to... At that time, the feedforward circuit sets the voltage modulation ratio to... That is, when K=1, the phase shift time of the submodule is actively adjusted to the minimum to reduce output voltage overshoot. Subsequently, as the input voltage changes, the feedback closed loop autonomously adjusts the phase shift time of the submodule to make the real-time output voltage of the modular multilevel converter equal to the target output voltage of the converter. Similarly, for K=2, 3, 4, 5, 6, the voltage switching points of the feedforward submodule adjustment settings are calculated according to the formula. Table 2 shows the voltage switching points of the feedforward submodule adjustment settings.

[0054] Table 2

[0055]

[0056] Figure 3 This demonstrates an existing method for generating drive signals. The upper and lower bridge arms have identical time bases, and the switching drive signals are completely complementary, thus generating separate drive signals for the upper and lower bridge arms. The drive signals and their corresponding bridge arm voltage waveforms are shown below. Figure 5 As shown, the submodule drive signals of the upper and lower bridge arms are complementary but not completely symmetrical, which causes the submodules of the lower bridge arm, which should be continuously engaged in one cycle, to have additional switching times, increasing switching losses. At the same time, the bridge arm voltages are not complementary, increasing the current fluctuation of the bridge arm.

[0057] Figure 4 This invention demonstrates the phase-shifting generation method for the drive signals proposed in this invention. The time bases of the upper and lower bridge arms differ by half a cycle, i.e., 0.5T, and the switching drive signals are completely complementary, thereby generating drive signals for the upper and lower bridge arms respectively. The drive signals and their corresponding bridge arm voltage waveforms are shown below. Figure 6 As shown, the submodule drive signals of the upper and lower bridge arms are complementary and completely symmetrical, avoiding additional switching times and significantly reducing switching losses. At the same time, the bridge arm voltages are completely complementary, reducing bridge arm current fluctuations.

[0058] In summary, this invention eliminates the need to adjust the switching frequency, allowing the modular DC-DC converter to operate at its resonant point. This broadens the input and output voltage range, enabling wide-range, high-efficiency conversion. The phase-shifting drive signal design method achieves complementary and completely symmetrical drive signals, avoiding additional switching cycles in the low-voltage bridge arm. The resulting bridge arm voltages are completely complementary. This design method reduces the number of sub-modules required for the modular multilevel circuit, lowers system construction costs, significantly reduces switching losses, and improves waveform quality, making this converter of significant application value in the field of medium- and high-voltage DC-DC conversion.

[0059] The above-described embodiments are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. Those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A fundamental frequency modulation method for reducing switching losses in a modular multilevel converter, characterized in that, The steps include: At the beginning of each switching cycle, the voltage modulation ratio m of the modular multilevel converter is adjusted according to the magnitude of the DC input voltage. Based on the voltage modulation ratio m, the number K of sub-modules with 100% duty cycle and the number NK of sub-modules with 50% duty cycle in the upper or lower arm of the modular multilevel converter are determined within one switching cycle; where N is the total number of sub-modules in the upper or lower arm; the drive signal of the sub-module with 100% duty cycle is always in the active state within one switching cycle, and the drive signal of the sub-module with 50% duty cycle is in the active state for 50% of the time and in the deactivated state for the remaining 50% of the time within one switching cycle; at any moment in each switching cycle, there are always a total of N+K sub-modules in the upper and lower arms in the active state. The time base of the drive signal between the upper and lower bridge arms is half a switching cycle apart, and the waveforms of the drive signals of the upper and lower bridge arms are symmetrical. The drive signal of each sub-module with a 50% duty cycle in a single bridge arm is obtained by sequentially shifting the waveform of the 50% duty cycle drive signal. Finally, the waveforms generated by the upper and lower bridge arms are complementary to each other. Within the upper or lower bridge arm, the phase shift time difference between the drive signals of adjacent sub-modules with 50% duty cycle input is equal. Phase shift duration The determination method is as follows: after m and K are determined, the difference between the real-time output voltage of the modular multilevel converter and the target output voltage of the converter is used for autonomous adjustment through PI closed-loop control; where, t on T is the time of the switching process of the submodule device. s The duration of the switching cycle; The drive signals of the NK sub-modules with 50% duty cycles within the upper or lower bridge arm are in the time domain t∈[0,T] of one switching cycle. s The expression within the brackets is: Where i is the number of the submodule that is put into operation with a 50% duty cycle in the upper or lower bridge arm, i = 0, 1, 2, ..., NK-1; G(i) = 0 indicates that the submodule with number i is in the cut-off state, and G(i) = 1 indicates that the submodule with number i is in the put-in state.

2. The fundamental frequency modulation method for reducing switching losses in a modular multilevel converter as described in claim 1, characterized in that, The relationship between the DC input voltage and the voltage modulation ratio is as follows: Among them, V in V is the DC input voltage of the modular multilevel converter. in0 The preset minimum DC input voltage for the modular multilevel converter; The calculation relationship between the voltage modulation ratio and the number of switching modules is as follows: Where K is an integer and is rounded to the nearest integer; therefore When the DC input voltage is V in0 At this time, the voltage modulation ratio is 1, and the K value is 0; When the DC input voltage increases to V in1 When the voltage modulation ratio becomes K switches from 0 to 1, V in1 The following relationship must be satisfied: When the DC input voltage increases to V in2 When the voltage modulation ratio becomes K switches from 1 to 2, V in2 The following relationship must be satisfied: Similarly, when the DC input voltage increases to V... ink When the voltage modulation ratio becomes K changes from k-1 to k, V ink The following relationship must be satisfied: The DC input voltage gradually increases until it reaches V. ink It is greater than the maximum DC input voltage.

3. The fundamental frequency modulation method for reducing switching losses in a modular multilevel converter according to claim 1, characterized in that, A switching cycle consists of a complete drive signal that is always in the ON state or a drive signal that is in the ON state for 50% of the time and in the OFF state for the remaining 50% of the time.

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