Fundamental frequency modulation method for reducing switching loss of modular multilevel converter

Abstract

The invention discloses a fundamental frequency modulation method for reducing switching loss of a modular multilevel converter, and belongs to the technical field of power electronic modulation. The method comprises the steps that the number of sub-modules input in 100% duty ratio and the number of sub-modules input in 50% duty ratio of an upper bridge arm or a lower bridge arm of the converter in a switching period are determined according to the voltage modulation ratio, and N + K sub-modules in the upper bridge arm and the lower bridge arm are always in an input state at each moment in each switching period; time references of driving signals between the upper bridge arm and the lower bridge arm are controlled to have a half switching period, waveforms of the driving signals of the upper bridge arm and the lower bridge arm are symmetrical, the driving signals of the sub-modules input at the duty ratio of 50% in the single bridge arm are controlled to be obtained by sequentially shifting phases of the waveforms of the driving signals with the duty ratio of 50%, and finally the waveforms generated by the upper bridge arm and the lower bridge arm are mutually complementary. According to the invention, the fundamental frequency switching signals of the upper and lower bridge arm sub-modules of the converter are completely symmetrically modulated, the switching loss of the sub-modules is reduced, and the operation efficiency of the modular multilevel converter is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronic modulation, and in particular relates to a base frequency modulation method for reducing switching loss of a modular multi-level converter. Background Art

[0002] With the large-scale access of new energy to the power grid, the rise of large-scale data centers, and the vigorous development of rail transit traction and other fields, medium-voltage large-capacity converters are receiving widespread attention and application. In the prior art, in order to enable the converter to meet the requirements of high voltage levels and large-capacity power supplies, the commonly used method is to operate low-voltage power semiconductor devices or low-voltage modules in series and parallel. In actual application engineering scenarios, it is often necessary to convert medium- and high-voltage electrical energy (whose voltage can reach tens of kilovolts) into low-voltage electrical energy (usually hundreds of volts) to provide power support for subsequent loads.

[0003] Traditional power conversion solutions generally use a topology structure in which a single low-voltage module is connected in series on the input side and in parallel on the output side. However, when the input and output voltages increase, this structure will cause the level of the isolation transformer inside each module to increase accordingly, which will increase the size of the isolation transformer and have a negative impact on the conversion efficiency and power density of the converter.

[0004] In addition, there is also a centralized transformer topology solution available, which can reduce the volume of magnetic components such as transformers in the system and reduce their losses. Among them, the modular multilevel converter adopts a multi-module series connection on the input side, and the advantage of modularization can be used flexibly in occasions with different voltage levels. The LLC resonant circuit voltage regulation solution also has certain applications in the field of medium voltage power supply. However, in the case of medium voltage and a wide input voltage range, relying solely on the frequency modulation and voltage regulation strategy will bring great difficulties to the parameter design of the hardware circuit. Specifically, when the excitation inductance of the transformer is small, it will cause the converter to have a large excitation current in the medium voltage application scenario, thereby increasing the loss of the converter. In addition, when the LLC resonant circuit adjusts the output voltage, it is usually achieved by adjusting the frequency of the switching device, and its frequency adjustment range is limited, which is difficult to adapt to the needs of a wide range of input voltage changes. At the same time, since the efficiency of the resonant converter reaches its highest point near the resonant frequency, it is impossible to achieve wide range voltage regulation while ensuring maximum operating efficiency by changing the frequency adjustment method. Summary of the invention

[0005] In order to solve the problems in the prior art, the present invention discloses a base frequency modulation method for reducing the switching loss of a modular multi-level converter.

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

[0007] The present invention provides a base frequency modulation method for reducing switching loss of a modular multi-level converter, comprising the following steps:

[0008] According to the DC input voltage of the modular multilevel converter, the voltage modulation ratio m of the converter is adjusted at the beginning of each switching cycle. According to the voltage modulation ratio m, the number K of submodules with 100% duty cycle and the number NK of submodules with 50% duty cycle of the upper bridge arm or the lower bridge arm of the modular multilevel converter in one switching cycle are determined; wherein N is the total number of submodules of the upper bridge arm or the lower bridge arm; the driving signal of the submodule with 100% duty cycle is always in the on state in one switching cycle, and the driving signal of the submodule with 50% duty cycle is in the on state for 50% of the time in one switching cycle, and is in the off state for the remaining 50% of the time; at each moment in each switching cycle, there are always N+K submodules in the upper and lower bridge arms that are in the on state;

[0009] The time base of the driving signal between the upper and lower bridge arms is controlled to differ by half a switching cycle, and the waveforms of the driving signals of the upper and lower bridge arms are symmetrical. The driving signals of the sub-modules with a 50% duty cycle in a single bridge arm are controlled by sequentially shifting the waveforms of the driving signals with a 50% duty cycle. Finally, the waveforms generated by the upper and lower bridge arms complement each other.

[0010] Compared with the prior art, the present invention has the following beneficial effects:

[0011] (1) Compared with the constant voltage modulation ratio control strategy of simultaneously putting N sub-modules into operation in the upper and lower bridge arms, the present invention changes the number of sub-modules continuously put into operation within a cycle as the input voltage changes, while ensuring that at every moment in each switching cycle, a total of N+K sub-modules are always in the put-in state in the upper and lower bridge arms. 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 multi-level converter.

[0012] (2) Variable module number voltage regulation has the characteristics of strong regulation ability but discontinuous regulation, and variable phase shift duration regulation has the characteristics of continuous regulation but limited regulation ability. The present invention combines two control degrees of freedom. By adjusting the number of sub-modules with 100% duty cycle in one switching cycle, the number of modules can be adjusted to achieve wide-range voltage regulation, and then the phase shift duration is autonomously adjusted through a feedback closed loop (i.e., PI closed-loop control) to achieve continuous and precise voltage regulation, thereby achieving wide-range continuous voltage regulation, which overcomes the problem that the prior art is difficult to adapt to the needs of wide-range changes in input voltage.

[0013] (3) Compared with the frequency and voltage regulation strategy of the traditional centralized transformer topology solution, the voltage regulation strategy of simultaneously adjusting the number of modules and the phase shift duration proposed in the present invention does not rely on the parameters of the LLC resonant cavity, and can avoid a smaller transformer excitation inductance, thereby reducing the transformer excitation current, reducing transformer losses and bridge arm submodule conduction losses, and improving system operating efficiency, especially the operating efficiency under light load conditions, thereby achieving high-efficiency conversion under wide input and wide load conditions, and overcoming the problem in the prior art that when the transformer excitation inductance is small, the converter will have a larger excitation current in the medium voltage application scenario, thereby increasing the converter loss.

[0014] (4) Compared with the traditional baseband modulation strategy, the fully symmetrical modulation strategy proposed in the present invention has completely symmetrical baseband switching signals of the upper and lower bridge arm sub-module chains, thereby avoiding the extra switching times caused by the switching mode in the traditional baseband modulation strategy. Therefore, the switching frequency of the device of the present invention is lower than the switching frequency of the traditional baseband modulation strategy, which can avoid the extra switching times of the sub-modules when switching the switching mode in adjacent switching cycles, thereby reducing the switching loss of the sub-modules and improving the operating efficiency of the modular multi-level converter, thereby overcoming the problem that the prior art cannot ensure the maximum operating efficiency while achieving wide range voltage regulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is the topology diagram of the modular multi-level DC converter.

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

[0017] Figure 3 This is a diagram of the generation of drive signals for the upper and lower bridge arm sub-modules of the modular multi-level circuit of the existing solution.

[0018] Figure 4 This is a phase shift generation diagram of the upper and lower bridge arm submodule drive signals of the modular multi-level circuit of the solution of the present invention.

[0019] Figure 5 The figure is a diagram showing the switching behavior effect using the existing drive signal generation method.

[0020] Figure 6 This is a diagram showing the switching behavior effect of the drive signal phase shift generation method of the present invention. DETAILED DESCRIPTION

[0021] The present invention is further described and illustrated below in conjunction with specific embodiments. The embodiments are merely exemplary of the present disclosure and do not define the scope of limitation. The technical features of each embodiment of the present invention may be combined accordingly without conflicting with each other.

[0022] In view of the above problems existing in the prior art, the present invention proposes a base frequency modulation method for reducing the switching loss of a modular multi-level converter to overcome the shortcomings of the prior art. The present invention uses the switching signal and phase shift duration of the modular multi-level circuit voltage modulation ratio to reconcile the sub-module to control the degree of freedom, thereby realizing continuous and precise regulation of the output voltage under a wide input voltage range of the modular multi-level converter. Compared with the traditional frequency modulation and voltage regulation scheme, the present invention fixes the operating frequency point at the resonant frequency, improves the working efficiency of the converter, and can realize soft switching of power devices in the full load range. In addition, by configuring the time base phase difference of the driving modulation wave of the upper and lower bridge arms of the modular multi-level converter to be half a switching cycle, the upper and lower bridge arms can generate completely complementary bridge arm voltages, thereby avoiding the switching loss caused by additional switching. This method can adapt to the wide range of voltage fluctuations on the input side without adjusting the switching frequency, ensuring high-efficiency regulation of the output voltage under a wide range of inputs.

[0023] The base frequency modulation method for reducing the switching loss of a modular multi-level converter of the present invention comprises the following steps:

[0024] According to the DC input voltage of the modular multilevel converter, the voltage modulation ratio m of the converter is adjusted at the beginning of each switching cycle. According to the voltage modulation ratio m, the number K of submodules with 100% duty cycle and the number NK of submodules with 50% duty cycle of the upper bridge arm or the lower bridge arm of the modular multilevel converter in one switching cycle are determined; wherein N is the total number of submodules of the upper bridge arm or the lower bridge arm; the driving signal of the submodule with 100% duty cycle is always in the on state in one switching cycle, and the driving signal of the submodule with 50% duty cycle is in the on state for 50% of the time in one switching cycle, and is in the off state for the remaining 50% of the time; at each moment in each switching cycle, there are always N+K submodules in the upper and lower bridge arms that are in the on state;

[0025] The time base of the driving signal between the upper and lower bridge arms is controlled to differ by half a switching cycle, and the waveforms of the driving signals of the upper and lower bridge arms are symmetrical. The driving signals of the sub-modules with a 50% duty cycle in a single bridge arm are controlled by sequentially shifting the waveforms of the driving signals with a 50% duty cycle. Finally, the waveforms generated by the upper and lower bridge arms complement each other.

[0026] like Figure 1 As shown, the present invention proposes a base frequency modulation method for reducing the switching loss of a modular multi-level converter. The single-phase modular multi-level resonant converter includes a DC input capacitor C in , modular multi-level circuit MMC, resonant circuit, medium and high frequency transformer and rectifier circuit. The modular multi-level circuit is divided into two bridge arms, upper and lower, and each bridge arm consists of N identical half-bridge sub-modules SM1~SM N and a bridge arm inductor L p / Ln The upper and lower bridge arm inductors can be connected in series by coupling winding and wound on the same magnetic core, which can ensure that the inductance of the upper and lower bridge arms is equal and make the equivalent inductance of the bridge arm inductor on the AC side zero, thus realizing the decoupling design of the bridge arm inductor and the inductance in the resonant circuit. The voltage and current of the upper and lower bridge arms are expressed by u p ,i p ,u n ,i n Indicates that the resonant circuit consists of the resonant capacitor C r , Transformer leakage inductance L k and the magnetizing inductance L m The transformer secondary side rectifier circuit can be a half-bridge structure, a full-bridge structure or a modular multi-level structure.

[0027] Figure 2 The figure shows the specific modulation strategy of voltage modulation ratio control. The upper bridge arm and the lower bridge arm each have N submodules, among which K submodules in the upper bridge arm or the lower bridge arm have a duty cycle of 100% in a switching cycle and remain continuously in operation. The remaining NK submodules have a duty cycle of 50% in a switching cycle, and at every moment in each switching cycle, there are always N+K submodules in the upper and lower bridge arms that are in operation. There is a phase shift duration between the drive signals of adjacent 50% duty cycle submodules in a bridge arm, that is, the upper bridge arm or the lower bridge arm. It generates multi-level quasi-square wave, trapezoidal wave, sine wave or triangle wave, and the phase shift duration The determination method is as follows: after m and K are determined, autonomous adjustment is performed through PI closed-loop control based on the difference between the real-time output voltage of the modular multilevel converter and the target output voltage of the converter.

[0028] The minimum value should be greater than the switching time t of the submodule device on , in order to achieve the minimum voltage change rate during the switching process of the bridge arm cascade sub-module; The maximum value depends on the modulation wave shape requirements. The quasi-square wave requires Smaller, trapezoidal and sine waves Larger, triangular wave Maximum, upper limit is:

[0029]

[0030] therefore,

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

[0032]

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

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

[0035]

[0036] Where K is an integer and is rounded off; therefore

[0037]

[0038] When the DC input voltage is V in0 When the voltage modulation ratio m=1, the K value is 0. When the DC input voltage increases to V in1 When the voltage modulation ratio is K switches from 0 to 1, V in1 Satisfies the following relationship:

[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 Satisfies the following relationship:

[0041]

[0042] Similarly, when the DC input voltage increases to V ink When the voltage modulation ratio is K increases from k-1 to k (k is 1, 2, 3, ...), V ink Satisfies the following relationship:

[0043]

[0044] The DC input voltage gradually increases until V ink Greater than the maximum DC input voltage.

[0045] Taking [nTs, (n+1)Ts] as the time reference of a switching cycle of the upper bridge arm, [(n+0.5)Ts, (n+1.5)Ts] is the time reference of a switching cycle of the lower bridge arm, where T sis the duration of the switching cycle, n is an integer, which can achieve symmetry and consistency between the upper and lower bridge arm drive signals and their switching processes. The selected switching cycle time base [nTs, (n+1)Ts] must be able to contain 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.

[0046] The driving signals of NK sub-modules with 50% duty cycle in the upper bridge arm or lower bridge arm are in the time domain t∈[0,T s ] is expressed as:

[0047]

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

[0049] In order to demonstrate the implementation effect of the base frequency modulation method for reducing the switching loss of the modular multi-level converter proposed in the present invention, a simulation model was built in the PLEC electrical simulation software for verification. The main parameters of the simulation model are as follows: DC input voltage 9kV~18kV, output voltage is constant 375V, maximum output power is 100kW, the number of bridge arm submodules is 18, the resonant frequency is 12kHz, and the phase shift duration of a single submodule is adjusted based on the modulation waveform and K value. Table 1 shows the parameter values ​​of the implementation case.

[0050] Table 1

[0051]

[0052]

[0053] When the input voltage is the minimum V in0 =9kV, the feedforward link sets the voltage modulation ratio to the maximum of 1, that is, in one switching cycle, 18 sub-modules in the upper bridge arm or the lower bridge arm participate in square wave switching, and the number of sub-modules K=0 is always put into operation, and the phase shift time of the sub-module is a small value; as the input voltage rises, the feedback link will adjust the phase shift time of the sub-module to maintain the stability of the output voltage of the converter. When the input voltage increases to When the feedforward link sets the voltage modulation ratio to That is, K = 1, and the submodule phase shift duration is actively adjusted to the minimum to reduce the output voltage overshoot. Then, as the input voltage changes, the feedback closed loop autonomously adjusts the submodule phase shift duration to make the real-time output voltage of the modular multilevel converter equal to the target output voltage of the converter. Similarly, when K = 2, 3, 4, 5, 6, the voltage switching point set by the feedforward submodule adjustment is calculated according to the formula. Table 2 shows the voltage switching point set by the feedforward submodule adjustment.

[0054] Table 2

[0055]

[0056] Figure 3 The existing drive signal generation method is shown. The time base of the upper and lower bridge arms is exactly the same, and the switching drive signals are completely complementary, thereby generating drive signals for the upper and lower bridge arms respectively. The drive signal and the corresponding bridge arm voltage waveform are shown in Figure 5 As shown, the sub-module driving signals of the upper and lower bridge arms are complementary but not completely symmetrical, resulting in additional switching times for the sub-module of the lower bridge arm that should be continuously put into use within a cycle, increasing the switching loss. At the same time, the bridge arm voltages are not complementary, which increases the current fluctuation of the bridge arm.

[0057] Figure 4 The driving signal phase shifting generation method proposed by the present invention is shown. The time base of the upper and lower bridge arms differs by half a cycle, that is, 0.5T. The switching driving signals are completely complementary, thereby generating driving signals for the upper and lower bridge arms respectively. The driving signal and the corresponding bridge arm voltage waveform are shown in FIG. Figure 6 As shown, the sub-module driving signals of the upper and lower bridge arms are complementary and completely symmetrical, which avoids additional switching times and significantly reduces switching losses. At the same time, the bridge arm voltages are completely complementary, reducing bridge arm current fluctuations.

[0058] In summary, the present invention does not need to adjust the switching frequency, so that the modular DC converter works at the resonance point, which can widen the input and output voltage range and achieve wide-range high-efficiency conversion; the phase-shifted drive signal design method realizes complementary and completely symmetrical drive signals, avoids the extra switching times of the low-voltage bridge arm, and the obtained bridge arm voltages are completely complementary. The design method of the present invention can reduce the number of sub-modules required for the modular multi-level circuit, reduce the system construction cost, significantly reduce the switching loss, and improve the waveform quality, so that the converter has important application value in the field of medium and high voltage DC conversion.

[0059] The above-mentioned embodiments only express several implementation methods of the present invention, and the description is relatively specific and detailed, but it cannot be understood as limiting the scope of the present invention. For ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A base frequency modulation method for reducing switching loss of a modular multi-level converter, characterized in that: The steps include: According to the DC input voltage of the modular multilevel converter, the voltage modulation ratio m of the converter is adjusted at the beginning of each switching cycle. According to the voltage modulation ratio m, the number K of submodules with 100% duty cycle and the number NK of submodules with 50% duty cycle of the upper bridge arm or the lower bridge arm of the modular multilevel converter in one switching cycle are determined; wherein N is the total number of submodules of the upper bridge arm or the lower bridge arm; the driving signal of the submodule with 100% duty cycle is always in the on state in one switching cycle, and the driving signal of the submodule with 50% duty cycle is in the on state for 50% of the time in one switching cycle, and is in the off state for the remaining 50% of the time; at each moment in each switching cycle, there are always N+K submodules in the upper and lower bridge arms that are in the on state; The time base of the driving signal between the upper and lower bridge arms is controlled to differ by half a switching cycle, and the waveforms of the driving signals of the upper and lower bridge arms are symmetrical. The driving signals of the sub-modules with a 50% duty cycle in a single bridge arm are controlled by sequentially shifting the waveforms of the driving signals with a 50% duty cycle. Finally, the waveforms generated by the upper and lower bridge arms complement each other.

2. The base frequency modulation method for reducing switching loss of a modular multi-level converter according to claim 1, characterized in that: The relationship between the DC input voltage and the voltage modulation ratio is: Among them, V in is the DC input voltage of the modular multilevel converter; V in0 A minimum DC input voltage for a preset modular multilevel converter; The calculation relationship between the voltage modulation ratio and the number of switching modules is: Where K is an integer and is rounded off; therefore When the DC input voltage is V in0 When , 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 Satisfies the following relationship: When the DC input voltage increases to V in2 When the voltage modulation ratio becomes K switches from 1 to 2, V in2 Satisfies the following relationship: Similarly, when the DC input voltage increases to V ink When the voltage modulation ratio becomes K switches from k-1 to k, V ink Satisfies the following relationship: The DC input voltage gradually increases until V ink Greater than the maximum DC input voltage.

3. The base frequency modulation method for reducing switching loss of a modular multi-level converter according to claim 1, characterized in that: A switching cycle includes a complete drive signal that is always in the on state or a drive signal that is in the on state 50% of the time and in the off state the remaining 50% of the time.

4. The base frequency modulation method for reducing switching loss of a modular multi-level converter according to claim 1, characterized in that: In the upper bridge arm or the lower bridge arm, the phase shift duration of the driving signals of the adjacent 50% duty cycle submodules is equal, and both are Phase shift duration The determination method is as follows: after m and K are determined, autonomous adjustment is performed through PI closed-loop control based on the difference between the real-time output voltage of the modular multilevel converter and the target output voltage of the converter; wherein, t on is the switching time of the submodule device, T s is the duration of the switching cycle.

5. The base frequency modulation method for reducing switching loss of a modular multi-level converter according to claim 4, characterized in that: The driving signals of the sub-modules with NK 50% duty cycle in the upper bridge arm or the lower bridge arm are in the time domain t∈[0,T s ] is expressed as: Wherein, i is the number of the submodule with 50% duty cycle in the upper bridge arm or the lower bridge arm, i=0,1,2,…,NK-1; G(i)=0 indicates that the submodule numbered i is in the cut-off state, and G(i)=1 indicates that the submodule numbered i is in the put-in state.

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