Method and device for realizing soft switching of three-phase T-type three-level inverter

Abstract

The invention provides a method and a device for realizing soft switching of a three-phase T-type three-level inverter, and belongs to the field of inverters. The method comprises the steps of calculating a first critical switching frequency of an unclamped phase in each sector under minimum pulse width modulation and a second critical switching frequency of the unclamped phase in each sector under maximum pulse width modulation when the three-phase T-type three-level inverter operates at a non-unity power factor; further, according to the first critical switching frequency and the second critical switching frequency of the non-clamping phase in each sector, determining the final critical switching frequency of the non-clamping phase in each sector; and finally, according to the final critical switching frequency of the non-clamping phase in each sector, determining a modulation wave and a carrier wave of the three-phase T-type three-level inverter, and controlling a switching device based on the modulation wave and the carrier wave. Zero-voltage switching of a switching device is realized when the three-phase T-type three-level inverter operates at a non-unity power factor, switching loss can be eliminated, the switching frequency is greatly improved, and the power density is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of inverters, in particular to a control method and device for realizing soft switching of a three-phase T-type three-level inverter. BACKGROUND

[0002] In the development history of T-type three-level inverters, reducing the size of the converter and improving the power density of the system has always been the focus of research. Limited by the performance of traditional silicon-based power devices, the switching frequency of the T-type three-level inverter is generally below 20 kHz, which cannot fully utilize the high-speed switching characteristics of silicon-based power devices. To solve this problem, the zero voltage switching (ZVS) technology in the soft switching technology is usually used to eliminate the turn-on loss, thereby greatly improving the switching frequency, reducing electromagnetic interference, and reducing the size of the filter and the overall cost of the inverter.

[0003] However, the prior art lacks research on realizing soft switching of a T-type three-level inverter when operating at a non-unit power factor. SUMMARY

[0004] The embodiments of the present application provide a soft switching method and device for a three-phase T-type three-level inverter to solve the problem that the prior art lacks research on realizing soft switching of a T-type three-level inverter when operating at a non-unit power factor.

[0005] In a first aspect, the embodiments of the present application provide a soft switching method for a three-phase T-type three-level inverter, comprising:

[0006] obtaining a non-clamped phase of each sector through which a three-phase voltage synthesis vector passes under minimum pulse width modulation and maximum pulse width modulation;

[0007] calculating a first critical switching frequency of the non-clamped phase in each sector under minimum pulse width modulation and a second critical switching frequency of the non-clamped phase in each sector under maximum pulse width modulation when the three-phase T-type three-level inverter operates at a non-unit power factor; wherein the critical switching frequency is the minimum switching frequency at which the switching device of the corresponding non-clamped phase realizes soft switching;

[0008] determining a final critical switching frequency of the non-clamped phase in each sector according to the first critical switching frequency and the second critical switching frequency of the non-clamped phase in each sector;

[0009] determining a modulation wave and a carrier wave of the three-phase T-type three-level inverter according to the final critical switching frequency of the non-clamped phase in each sector, and controlling the switching device based on the modulation wave and the carrier wave.

[0010] In a possible implementation, the first critical switching frequency of the first non-clamped phase under the minimum pulse width modulation when the three-phase T-type three-level inverter operates at a non-unit power factor in each sector comprises:

[0011] For each sector, a critical switching frequency of a first non-clamped phase under the minimum pulse width modulation when the three-phase T-type three-level inverter operates at a non-unit power factor is calculated, and a critical switching frequency of a second non-clamped phase under the minimum pulse width modulation is calculated; wherein each sector comprises the first non-clamped phase and the second non-clamped phase;

[0012] If the critical switching frequency of the first non-clamped phase is less than the critical switching frequency of the second non-clamped phase, the critical switching frequency of the first non-clamped phase is determined as the first critical switching frequency of the non-clamped phase under the minimum pulse width modulation in the sector;

[0013] If the critical switching frequency of the first non-clamped phase is greater than or equal to the critical switching frequency of the second non-clamped phase, the critical switching frequency corresponding to the second non-clamped phase is determined as the first critical switching frequency of the non-clamped phase under the minimum pulse width modulation in the sector.

[0014] In a possible implementation, the critical switching frequency of the first non-clamped phase under the minimum pulse width modulation when the three-phase T-type three-level inverter operates at a non-unit power factor comprises:

[0015] The output voltage and the output current of the first non-clamped phase are obtained, and a corresponding zero voltage switching boundary condition is determined; wherein the zero voltage switching boundary condition is used to indicate a relationship between an inductance current of the non-clamped phase and a preset bias current when the three-phase T-type three-level inverter operates at a non-unit power factor;

[0016] Based on the zero voltage switching boundary condition, the critical switching frequency of the first non-clamped phase under the minimum pulse width modulation is obtained according to a direct current voltage, a modulation wave of the minimum pulse width modulation, and the output voltage, the output current, and an inductance of the first non-clamped phase.

[0017] In a possible implementation, the critical switching frequency of the first non-clamped phase under the minimum pulse width modulation is obtained based on the zero voltage switching boundary condition, the direct current voltage, the modulation wave of the minimum pulse width modulation, and the output voltage, the output current, and the inductance of the first non-clamped phase, comprises:

[0018] The inductance voltage of the first non-clamped phase is determined according to the direct current voltage and the output voltage, and the first change amount information of the inductance current of the first non-clamped phase is determined according to the inductance voltage and the inductance;

[0019] determining second variation information of the first unclamped phase inductance current with respect to time according to the zero-voltage switching boundary condition and the output current;

[0020] obtaining a critical switching frequency of the first unclamped phase according to the first variation information, the second variation information and a minimum pulse width modulation wave.

[0021] In a possible implementation, the determining of the final critical switching frequency of the unclamped phase in each sector according to the first critical switching frequency and the second critical switching frequency of the unclamped phase in each sector comprises:

[0022] if the first critical switching frequency of the unclamped phase in a sector is greater than the second critical switching frequency of the unclamped phase in the sector, determining the first critical switching frequency as the final critical switching frequency of the unclamped phase in the sector;

[0023] if the first critical switching frequency of the unclamped phase in a sector is less than or equal to the second critical switching frequency of the unclamped phase in the sector, determining the second critical switching frequency as the final critical switching frequency of the unclamped phase in the sector.

[0024] In a possible implementation, the determining of the modulation wave and the carrier wave of the three-phase T-type three-level inverter according to the final critical switching frequency of the unclamped phase in each sector comprises:

[0025] determining a modulation mode corresponding to the final critical switching frequency of each sector as the modulation mode of the sector;

[0026] determining the modulation wave of the three-phase T-type three-level inverter according to the modulation mode corresponding to each sector, and determining the carrier wave according to the final critical switching frequency of each sector; wherein the carrier wave is used to control the turn-on and turn-off of the switching device.

[0027] In a possible implementation, the controlling of the switching device based on the modulation wave and the carrier wave comprises:

[0028] performing pulse width modulation of a signal based on the modulation wave and the carrier wave with variable switching frequency of each sector to obtain a control signal for each switching device in the three-phase T-type three-level inverter; wherein the control signal is used to control the turn-on and turn-off of the switching device.

[0029] controlling the switching device in the three-phase T-type three-level inverter based on the control signal.

[0030] In a possible implementation, the obtaining of the unclamped phase of each sector of the three-phase voltage resultant vector under minimum pulse width modulation and maximum pulse width modulation comprises:

[0031] obtaining clamping mode information of each sector through which the three-phase voltage resultant vector passes under minimum pulse width modulation and maximum pulse width modulation;

[0032] determining, according to the clamping mode information, unclamped phases of each sector through which the three-phase voltage resultant vector passes under minimum pulse width modulation and maximum pulse width modulation.

[0033] In a second aspect, an embodiment of the present application provides a soft switching device for implementing a three-phase T-type three-level inverter, comprising:

[0034] an obtaining unit configured to obtain unclamped phases of each sector through which the three-phase voltage resultant vector passes under minimum pulse width modulation and maximum pulse width modulation;

[0035] a first processing unit configured to calculate a first critical switching frequency of an unclamped phase in each sector under minimum pulse width modulation and a second critical switching frequency of the unclamped phase under maximum pulse width modulation when the three-phase T-type three-level inverter is operated at a non-unit power factor, wherein the critical switching frequency is a minimum switching frequency at which a switching device of the corresponding unclamped phase realizes soft switching;

[0036] a second processing unit configured to determine a final critical switching frequency of the unclamped phase in each sector according to the first critical switching frequency and the second critical switching frequency of the unclamped phase in each sector;

[0037] a third processing unit configured to determine a modulation wave and a carrier wave of the three-phase T-type three-level inverter according to the final critical switching frequency of the unclamped phase in each sector, and control the switching device based on the modulation wave and the carrier wave.

[0038] In a possible implementation, the first processing unit is specifically configured to:

[0039] for each sector, calculate a critical switching frequency of a first unclamped phase under minimum pulse width modulation when the three-phase T-type three-level inverter is operated at a non-unit power factor, and calculate a critical switching frequency of a second unclamped phase under minimum pulse width modulation, wherein each sector comprises the first unclamped phase and the second unclamped phase;

[0040] if the critical switching frequency of the first unclamped phase is less than the critical switching frequency of the second unclamped phase, determine that the critical switching frequency of the first unclamped phase is the first critical switching frequency of the unclamped phase in the sector under minimum pulse width modulation;

[0041] if the critical switching frequency of the first unclamped phase is greater than or equal to the critical switching frequency of the second unclamped phase, determine that the critical switching frequency corresponding to the second unclamped phase is the first critical switching frequency of the unclamped phase in the sector under minimum pulse width modulation.

[0042] The embodiment of the present application provides a soft switching method and device for realizing a three-phase T-type three-level inverter. The first critical switching frequency of a non-clamped phase in each sector under minimum pulse width modulation and the second critical switching frequency of the non-clamped phase under maximum pulse width modulation are calculated when the three-phase T-type three-level inverter is operated at a non-unit power factor. The critical switching frequency is the minimum switching frequency for realizing soft switching of the corresponding non-clamped phase. Then, the final critical switching frequency of the non-clamped phase in each sector is determined according to the first critical switching frequency and the second critical switching frequency of the non-clamped phase in each sector. Finally, the modulation wave and the carrier wave of the three-phase T-type three-level inverter are determined according to the final critical switching frequency of the non-clamped phase in each sector, and the switching device is controlled based on the modulation wave and the carrier wave. The method realizes zero voltage turn-on of the switching device of the three-phase T-type three-level inverter when the three-phase T-type three-level inverter is operated at a non-unit power factor by reasonably selecting the critical switching frequency of the non-clamped phase under different modulation modes of the three-phase T-type three-level inverter when the three-phase T-type three-level inverter is operated at a non-unit power factor, determining the final critical switching frequency of the non-clamped phase in each sector, and based on this, the turn-on loss can be eliminated, the switching frequency can be greatly improved, the electromagnetic interference can be reduced, the cost and the volume of the system can be reduced, and the power density of the system can be improved. BRIEF DESCRIPTION OF DRAWINGS

[0043] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0044] Figure 1 is the implementation flowchart of the soft switching method for realizing a three-phase T-type three-level inverter provided by the embodiment of the present application;

[0045] Figure 2 is the implementation flowchart of another soft switching method for realizing a three-phase T-type three-level inverter provided by the embodiment of the present application;

[0046] Figure 3 is the main circuit topology structure diagram of a three-phase T-type three-level inverter provided by the embodiment of the present application;

[0047] Figure 4 is the structural schematic diagram of the soft switching device for realizing a three-phase T-type three-level inverter provided by the embodiment of the present application. DETAILED DESCRIPTION

[0048] In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular architectures, technologies, techniques, etc. in order to provide a thorough understanding of the embodiments of the present application. However, it will be apparent to those skilled in the art that the present application can be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known systems, devices, circuits, and methods are omitted so as not to obscure the description of the present application with unnecessary detail. SUMMARY

[0050] In the development history of three-phase T-type three-level inverters, reducing the volume of the converter and improving the power density of the system has always been the focus of research. However, due to the performance of traditional silicon-based power devices such as Insulated Gate Bipolar Transistor (IGBT) and the like, the switching frequency of the commonly used three-phase T-type three-level inverters on the market is generally below 20 kHz, which cannot fully exert the high-speed switching characteristics of silicon carbide (SiC) devices. The reasons are as follows: (1) the turn-on loss of SiC devices is much larger than the turn-off loss under high-speed switching; (2) the high voltage rate of change and current rate of change generated under high-speed switching greatly increase the design difficulty of the electromagnetic interference (EMI) filter and affect the operation of the sampling, control and driving circuits inside the inverter.

[0051] The inventors have found through research that, in actual application scenarios, in order to meet the demand for reactive power control and grid stability, the inverter often needs to operate at a non-unit power factor. However, the prior art lacks research on the implementation of soft switching of three-phase T-type three-level inverters when operating at a non-unit power factor, resulting in the inability to increase the switching frequency, reduce the volume of the filter and the overall cost of the inverter when the three-phase T-type three-level inverter operates at a non-unit power factor.

[0052] In order to improve the switching frequency of the three-phase T-type three-level inverter when operating at a non-unit power factor, reduce the volume of the filter and the overall cost of the inverter, in the embodiments of the present application, the critical switching frequency of the non-clamped phase in each sector is determined by reasonably selecting the critical switching frequency of the non-clamped phase under different modulation modes when the three-phase T-type three-level inverter operates at a non-unit power factor. Based on this, zero-voltage turn-on of the switching device of the three-phase T-type three-level inverter when operating at a non-unit power factor is achieved, which can eliminate the turn-on loss, thereby greatly improving the switching frequency, reducing electromagnetic interference, reducing the cost and volume of the system, and improving the power density of the system.

[0053] In order to make the purpose, technical scheme and advantages of the present application clearer, specific embodiments will be described below with reference to the accompanying drawings.

[0054] Figure 1 The implementation flowchart of the soft switching method for implementing a three-phase T-type three-level inverter provided by the embodiment of the present application is described in detail as follows:

[0055] In step 101, the non-clamped phase of each sector through which the three-phase voltage synthesis vector passes under minimum pulse width modulation and maximum pulse width modulation is obtained.

[0056] Exemplarily, the embodiment determines the sector through which the three-phase voltage synthesis vector passes and determines the clamping mode information of each sector through which the synthesis vector passes under minimum pulse width modulation and maximum pulse width modulation by the angle and size of the three-phase voltage synthesis vector, and then obtains the non-clamped phase information of each sector through which the three-phase voltage synthesis vector passes under minimum pulse width modulation and maximum pulse width modulation.

[0057] The non-clamped phase refers to the output voltage of the phase under a certain switching state, which can be switched between multiple levels, instead of being fixed at a certain specific clamped level. For example, when the switching state of a certain phase allows the output voltage thereof to change between a positive level, a zero level and a negative level, the phase is a non-clamped phase.

[0058] Minimum pulse width modulation (DPWMMIN) is an optimized pulse width modulation strategy, which aims to reduce the pulse width as much as possible to reduce the switching loss of the power device and improve the system efficiency on the premise of meeting the system performance requirements, but the minimum pulse width limit needs to be noted to avoid problems such as output waveform distortion; maximum pulse width modulation (DPWMMAX) is a modulation mode that makes the pulse width as large as possible, but it is also limited by system parameters and performance requirements, such as DC bus voltage and voltage withstand of the power device.

[0059] In step 102, the first critical switching frequency of the non-clamped phase in each sector under minimum pulse width modulation and the second critical switching frequency under maximum pulse width modulation of the three-phase T-type three-level inverter when operating at a non-unit power factor are calculated; the critical switching frequency is the minimum switching frequency at which the switching device of the corresponding non-clamped phase achieves soft switching.

[0060] The three-phase T-type three-level inverter can output three levels of states: positive level, zero level and negative level. When operating at a non-unit power factor, there is an exchange of reactive power between the input and output of the inverter, which will affect the switching state and voltage and current waveform of the inverter. Due to the existence of reactive power, the current waveform is no longer in phase with the voltage waveform.

[0061] Soft switching refers to the power switching device being turned on or turned off under ZVS condition, which can significantly reduce switching loss and electromagnetic interference. In a three-phase T-type three-level inverter, achieving soft switching is very important to improve the efficiency and reliability of the inverter. The critical switching frequency is a key parameter for achieving soft switching. When the switching frequency is lower than this critical value, the switching device may not be turned on and off under ideal zero voltage or zero current conditions, resulting in increased switching loss.

[0062] Exemplarily, the embodiment analyzes the voltage and current variation law of the switching device of the non-clamped phase under the maximum pulse width modulation and the minimum pulse width modulation, and calculates the first critical switching frequency of the non-clamped phase under the minimum pulse width modulation and the second critical switching frequency of the non-clamped phase under the maximum pulse width modulation in each sector of the three-phase T-type three-level inverter when operating at a non-unit power factor.

[0063] Step 103, according to the first critical switching frequency and the second critical switching frequency of the non-clamped phase in each sector, determine the final critical switching frequency of the non-clamped phase in each sector.

[0064] Exemplarily, the embodiment is to find a suitable switching frequency, so that the switching device of the non-clamped phase can achieve soft switching in the entire modulation process (including minimum and maximum pulse width modulation). When the first critical switching frequency is greater than the second critical switching frequency, it means that a higher switching frequency is required to achieve soft switching under the minimum pulse width modulation condition. In order to ensure that soft switching can be achieved under any pulse width modulation condition, the higher frequency should be selected as the final critical switching frequency. Therefore, the final critical switching frequency is. On the contrary, when the first critical switching frequency is less than the second critical switching frequency, the maximum pulse width modulation condition requires a higher switching frequency. Therefore, the final critical switching frequency is.

[0065] Step 104, according to the final critical switching frequency of the non-clamped phase in each sector, determine the modulation wave and carrier wave of the three-phase T-type three-level inverter, and control the switching device based on the modulation wave and carrier wave.

[0066] Exemplarily, after determining the final critical switching frequency of the non-clamped phase in each sector, the embodiment determines the corresponding modulation wave and carrier wave of the three-phase T-type three-level inverter, and then determines the turn-on and turn-off time of the switching device by comparing the size relationship between the modulation wave and the carrier wave. The switching device is controlled.

[0067] To sum up, the embodiment calculates the first critical switching frequency of the non-clamped phase in each sector under the minimum pulse width modulation and the second critical switching frequency of the non-clamped phase in each sector under the maximum pulse width modulation when the three-phase T-type three-level inverter is operated at a non-unit power factor, wherein the critical switching frequency is the minimum switching frequency at which the switching device of the corresponding non-clamped phase achieves soft switching, then determines the final critical switching frequency of the non-clamped phase in each sector according to the first critical switching frequency and the second critical switching frequency of the non-clamped phase in each sector, and finally determines the modulation wave and the carrier wave of the three-phase T-type three-level inverter according to the final critical switching frequency of the non-clamped phase in each sector, and controls the switching device based on the modulation wave and the carrier wave. The method can achieve zero voltage switching of the switching device of the three-phase T-type three-level inverter when the three-phase T-type three-level inverter is operated at a non-unit power factor by reasonably selecting the critical switching frequency of the non-clamped phase under different modulation modes, determining the final critical switching frequency of the non-clamped phase in each sector, and based on this, can eliminate the switching loss, greatly improve the switching frequency, reduce electromagnetic interference, reduce the cost and volume of the system, and improve the power density of the system.

[0068] In order for the non-clamped phases to achieve ZVS, the embodiment selects the smaller critical switching frequency of the two non-clamped phases in the same sector under the same modulation mode as the critical switching frequency of the non-clamped phase in the sector under this modulation mode, and for the two critical switching frequencies of the non-clamped phase in the same sector under two modulation modes, the embodiment selects the larger critical switching frequency as the final critical switching frequency of the sector, and determines the modulation mode corresponding to the final critical switching frequency as the modulation mode of the sector. In addition, based on the corresponding zero voltage switching boundary condition, the critical switching frequency of the non-clamped phase under the minimum pulse width modulation can be obtained according to the direct current voltage, the modulation wave of the minimum pulse width modulation, and the output voltage, the output current and the inductance of the first non-clamped phase, and the critical switching frequency of the non-clamped phase when operated at a non-unit power factor can be obtained.

[0069] Figure 2 Another implementation flowchart of the soft switching method for the three-phase T-type three-level inverter provided by the embodiment is provided, and is described in detail as follows:

[0070] In step 201, the non-clamped phase of each sector through which the three-phase voltage synthesis vector passes under the minimum pulse width modulation and the maximum pulse width modulation is obtained.

[0071] In one example, step 201 includes the following steps:

[0072] In step 201, the non-clamped phase of each sector through which the three-phase voltage synthesis vector passes under the minimum pulse width modulation and the maximum pulse width modulation is obtained.

[0073] According to the clamping mode information, the non-clamping phases of each sector through which the three-phase voltage resultant vector passes are determined under the minimum pulse width modulation and the maximum pulse width modulation.

[0074] Exemplarily, the embodiment determines the sector position through which the resultant vector passes according to the direct current voltage and the output voltage, and determines the clamping mode of each sector through which the resultant vector passes under the DPWMMIN and DPWMMAX modulation, as shown in Table 1.

[0075] Table 1 DPWMMIN and DPWMMAX clamping mode

[0076]

[0077] In Table 1, a represents the a phase, b represents the b phase, and c represents the c phase; O represents a zero level or a floating state, so that the output phase voltage is at a zero level; P represents a positive level, indicating that the phase voltage is connected to the positive direct current bus voltage; and N represents a negative level, indicating that the phase voltage is connected to the negative direct current bus voltage.

[0078] In one example, if the three-phase voltage resultant vector passes through the second small sector of the first large sector, the a phase in the sector is a clamping phase, and the b phase and the c phase are non-clamping phases.

[0079] In step 202, the first critical switching frequency of the non-clamping phase in each sector under the minimum pulse width modulation and the second critical switching frequency under the maximum pulse width modulation are calculated when the three-phase T-type three-level inverter is operated at a non-unit power factor; wherein the critical switching frequency is the minimum switching frequency at which the switching device of the corresponding non-clamping phase realizes soft switching.

[0080] In one example, each sector includes a first non-clamping phase and a second non-clamping phase; and step 202 includes the following steps:

[0081] For each sector, the critical switching frequency of the first non-clamping phase under the minimum pulse width modulation and the critical switching frequency of the second non-clamping phase under the minimum pulse width modulation are calculated when the three-phase T-type three-level inverter is operated at a non-unit power factor; if the critical switching frequency of the first non-clamping phase is less than the critical switching frequency of the second non-clamping phase, the critical switching frequency of the first non-clamping phase is determined as the first critical switching frequency of the non-clamping phase in the sector under the minimum pulse width modulation; and if the critical switching frequency of the first non-clamping phase is greater than or equal to the critical switching frequency of the second non-clamping phase, the critical switching frequency corresponding to the second non-clamping phase is determined as the first critical switching frequency of the non-clamping phase in the sector under the minimum pulse width modulation.

[0082] In one possible implementation, the first non-clamped phase output voltage and output current are obtained, and a corresponding zero-voltage switching boundary condition is determined, wherein the zero-voltage switching boundary condition is used to indicate a relationship between an inductance current of the non-clamped phase and a preset bias current when the three-phase T-type three-level inverter is operated at a non-unit power factor; and then, based on the zero-voltage switching boundary condition, the critical switching frequency of the first non-clamped phase under the minimum pulse width modulation is obtained according to the DC voltage, the minimum pulse width modulation wave, and the output voltage, the output current and the inductance of the first non-clamped phase.

[0083] In one example, the inductance voltage of the first non-clamped phase is determined according to the DC voltage and the output voltage, and the first change amount information of the inductance current of the first non-clamped phase is determined according to the inductance voltage and the inductance; then, the second change amount information of the inductance current of the first non-clamped phase with time is determined according to the zero-voltage switching boundary condition and the output current; and finally, the critical switching frequency of the first non-clamped phase is obtained according to the first change amount information, the second change amount information and the minimum pulse width modulation wave.

[0084] As shown in Table 1, there are two non-clamped phases in each sector, and in order to achieve ZVS for the non-clamped phases, the smaller critical switching frequency of the two non-clamped phases of the same sector under the same modulation mode is selected as the critical switching frequency of the non-clamped phase of the sector under the modulation mode.

[0085] In the calculation of the critical switching frequency of each non-clamped phase, the output voltage and the output current of the non-clamped phase are first obtained, and the corresponding zero-voltage switching boundary condition is determined, which reflects the relationship between the inductance current of the non-clamped phase and the preset bias current when the three-phase T-type three-level inverter is operated at a non-unit power factor.

[0086] For the three-phase T-type three-level inverter, to achieve soft switching of the non-clamped phase, the parasitic parameters (such as inductance and capacitance) in the circuit and the load characteristics need to be considered. According to the DC voltage V dc and the output voltage u x , the voltage v Lx across the inductance of the non-clamped phase under different vector states can be determined, and therefore the change amount Δi Lx of the inductance current of the non-clamped phase within Δt time can be represented as v Lx / L. The change amount Δi Lx of the inductance current of the non-clamped phase within Δt time is determined by the output current i x and the boundary condition at the turn-on time of the switching device. Based on the boundary condition and the output current i x , the change amount Δi Lx of the inductance current of the non-clamped phase within Δt time can be obtained. Δt is determined by the switching frequency f sxand non-clamped phase in modulation wave m of DPWM MAX or DPWM MIN x According to the geometric relationship. Therefore, the critical switching frequency of the non-clamped phase can be determined according to the direct voltage V dc , output voltage u x , modulation wave m x , output current i x and bias current I bias , inductance L

[0087] Where, due to the operation of non-unit power factor, the voltage and current exist the same and opposite signs in a power frequency cycle. Since the analysis of the boundary condition is true for any phase, the analysis can be unified.

[0088] In a feasible implementation, Figure 3 The main circuit topology diagram of the three-phase T-type three-level converter provided by the embodiment of the application, the DC side of the circuit is composed of two capacitors C1 and C2, which are connected in series across the DC power supply V dc . These two capacitors are used for voltage division, providing two different DC voltage levels for the converter. The AC side of the circuit has three phases, marked as phases a, b and c respectively. Each phase has an inductor L and a resistor R connected in series, for filtering and limiting current. The output current of each phase is marked as i a , i b , i c , and the corresponding phase voltage is u a , u b , u c . Each phase has four switching tubes, marked as Q 1x , Q 2x , Q 3x , Q 4x .

[0089] (1) ZVS boundary condition when voltage and current signs are the same: when the voltage and current of the phase are the same, Q 1x and Q 4x need to achieve ZVS. When the voltage is greater than 0, the current is also greater than 0, at this time Q 1x switching action, Q 4x switching does not act. In order to achieve ZVS of Q 1x , at the time of its turn-on, the corresponding inductor current i Lx <-I bias . Similarly, when the voltage is less than 0, the current is also less than 0, at this time Q 4x switching action, Q 1x switching does not act. In order to achieve ZVS of Q 4x , at the time of its turn-on, the corresponding inductor current i Lx >I bias(2) ZVS boundary condition when voltage and current signs are different: when the voltage and current of a phase are different, Q 2x and Q 3x ZVS needs to be realized. When the voltage is greater than 0, the current is less than 0, at this time Q 3x switching is in action, Q 2x switching is not in action. In order to realize ZVS of Q 3x , at the time when it is turned on, the corresponding inductor current i Lx > I bias . Similarly, when the voltage is less than 0, the current is greater than 0, at this time Q 2x switching is in action, Q 3x switching is not in action. In order to realize ZVS of Q 2x , at the time when it is turned on, the corresponding inductor current i Lx < -I bias .

[0090] wherein I bias is a preset bias current. x represents a phase, and takes a, b, or c.

[0091] In step 203, if the first critical switching frequency of the non-clamped phase in the sector is greater than the second critical switching frequency of the non-clamped phase in the sector, it is determined that the first critical switching frequency is the final critical switching frequency of the non-clamped phase in the sector.

[0092] In step 204, if the first critical switching frequency of the non-clamped phase in the sector is less than or equal to the second critical switching frequency of the non-clamped phase in the sector, it is determined that the second critical switching frequency is the final critical switching frequency of the non-clamped phase in the sector.

[0093] Exemplarily, when the first critical switching frequency is greater than the second critical switching frequency, it means that a higher switching frequency is required to realize soft switching in the minimum pulse width modulation case. In order to ensure that soft switching can be realized in any pulse width modulation case, a higher frequency should be selected as the final critical switching frequency. Therefore, the final critical switching frequency is the first critical switching frequency.

[0094] Conversely, when the first critical switching frequency is less than the second critical switching frequency, the maximum pulse width modulation case requires a higher switching frequency. Therefore, the final critical switching frequency is the second critical switching frequency.

[0095] In step 205, the modulation wave and the carrier wave of the three-phase T-type three-level inverter are determined according to the final critical switching frequencies of the non-clamped phases in each sector.

[0096] In one example, step 205 includes the following steps:

[0097] It is determined that the modulation mode corresponding to the final critical switching frequency of each sector is the modulation mode of the sector.

[0098] Based on the modulation scheme corresponding to each sector, the modulation wave of the three-phase T-type three-level inverter is determined, and the carrier wave is determined based on the final critical switching frequency corresponding to each sector; wherein, the carrier wave is used to control the switching devices to turn on and off.

[0099] For example, in this embodiment, the modulation scheme corresponding to the final critical switching frequency of each sector is determined as the modulation scheme of that sector. Then, based on the modulation scheme corresponding to each sector, the modulation wave of the three-phase T-type three-level inverter is determined, and based on the determined final critical switching frequency corresponding to each sector, the carrier wave is determined.

[0100] In this embodiment, the modulation wave for ZVS is a combination of DPWMMAX and DPWMMIN.

[0101] Step 206: Based on the modulation wave of each sector and the carrier wave of the variable switching frequency, pulse width modulation of the signal is performed to obtain the control signal for each switching device in the three-phase T-type three-level inverter; wherein, the control signal is used to control the switching device to turn on and off.

[0102] For example, in this embodiment, pulse-width modulation (PWM) is performed based on the modulation wave of each sector and the carrier wave with a variable switching frequency to obtain the turn-on and turn-off signals for controlling each switching device.

[0103] Step 207: Based on the control signal, control the switching devices in the three-phase T-type three-level inverter.

[0104] For example, in this embodiment, the dual carriers use concave triangular carriers with the same phase. The upper carrier range is 0 to 1, and the lower carrier range is -1 to 0. When the modulating wave is positive, the switching state is determined by the comparison result between the modulating wave and the upper carrier; that is, when the modulating wave is greater than the upper carrier, Q... 1x On, Q 3x When the x-phase is off, the x-phase output is high; when the modulated wave is less than the upper carrier wave, Q... 1x Off, Q 3x When the transistor is turned on, phase x outputs a low level. At this time, the switching transistor Q... 2x Always maintain conduction, Q 4x Always keep it off, so that the output level of phase x is at V. dc The switching state alternates between / 2 and 0. When the modulation wave is negative, the switching state is determined by the comparison between the modulation wave and the download wave; that is, when the modulation wave is greater than the download wave, Q... 2x On, Q 4x When the x-phase is off, the x-phase output is high; when the modulated wave is smaller than the download wave, Q... 2x Off, Q 4x When the circuit is on, the lower arm of phase x outputs a low level. At this time, the output level of phase x is between 0 and -V.dc switching between the first mode and the second mode.

[0105] In summary, the present application can realize ZVS of all 12 switching tubes of the three-phase T-type three-level inverter by improving the modulation strategy. The difficulty of circuit control is greatly reduced, the size of the filter is reduced, the switching frequency is improved, and the power density of the converter is greatly improved. Only the information of the output power frequency voltage, current and input DC voltage needs to be sampled to complete the pulse prediction calculation and control of the inductor current, without sampling high-frequency signals, and the reliability is high.

[0106] It should be understood that the size of the serial number of each step in the above embodiment does not mean the order of execution, and the execution order of each process should be determined according to its function and internal logic, and should not constitute any limitation on the implementation process of the embodiment of the present application.

[0107] The following is an apparatus embodiment of the present application. For details not described in detail, reference can be made to the corresponding method embodiments described above.

[0108] Figure 4 A structure diagram of a soft switching device for realizing a three-phase T-type three-level inverter is shown. For the convenience of description, only the part related to the embodiment of the present application is shown, and the details are as follows:

[0109] As shown in Figure 4 The soft switching device 3 for realizing a three-phase T-type three-level inverter includes:

[0110] The acquisition unit 31 is configured to acquire the non-clamped phase of each sector through which the three-phase voltage synthesis vector passes under the minimum pulse width modulation and the maximum pulse width modulation.

[0111] The first processing unit 32 is configured to calculate the first critical switching frequency of the non-clamped phase in each sector under the minimum pulse width modulation and the second critical switching frequency of the non-clamped phase in each sector under the maximum pulse width modulation when the three-phase T-type three-level inverter is operated at a non-unit power factor; wherein the critical switching frequency is the minimum switching frequency at which the switching device of the corresponding non-clamped phase realizes soft switching.

[0112] The second processing unit 33 is configured to determine the final critical switching frequency of the non-clamped phase in each sector according to the first critical switching frequency and the second critical switching frequency of the non-clamped phase in each sector.

[0113] The third processing unit 34 is configured to determine the modulation wave and the carrier wave of the three-phase T-type three-level inverter according to the final critical switching frequency of the non-clamped phase in each sector, and control the switching device based on the modulation wave and the carrier wave.

[0114] In one possible implementation, the first processing unit 32 is specifically configured to:

[0115] For each sector, a critical switching frequency of the first unclamped phase under the minimum pulse width modulation is calculated when the three-phase T-type three-level inverter is operated at a non-unit power factor, and a critical switching frequency of the second unclamped phase under the minimum pulse width modulation is calculated; wherein each sector includes the first unclamped phase and the second unclamped phase.

[0116] If the critical switching frequency of the first unclamped phase is less than the critical switching frequency of the second unclamped phase, the critical switching frequency of the first unclamped phase is determined as the first critical switching frequency of the unclamped phase under the minimum pulse width modulation in the sector.

[0117] If the critical switching frequency of the first unclamped phase is greater than or equal to the critical switching frequency of the second unclamped phase, the critical switching frequency corresponding to the second unclamped phase is determined as the first critical switching frequency of the unclamped phase under the minimum pulse width modulation in the sector.

[0118] In a possible implementation, the first processing unit 32 is specifically configured to:

[0119] The output voltage and the output current of the first unclamped phase are obtained, and a corresponding zero-voltage switching boundary condition is determined; wherein the zero-voltage switching boundary condition is used to indicate a relationship between the inductance current of the unclamped phase and a preset bias current when the three-phase T-type three-level inverter is operated at a non-unit power factor.

[0120] Based on the zero-voltage switching boundary condition, the critical switching frequency of the first unclamped phase under the minimum pulse width modulation is obtained according to the direct current voltage, a modulation wave of the minimum pulse width modulation, and the output voltage, the output current, and the inductance of the first unclamped phase.

[0121] In a possible implementation, the first processing unit 32 is specifically further configured to:

[0122] Based on the zero-voltage switching boundary condition, the critical switching frequency of the first unclamped phase under the minimum pulse width modulation is obtained according to the direct current voltage, a modulation wave of the minimum pulse width modulation, and the output voltage, the output current, and the inductance of the first unclamped phase, including:

[0123] According to the direct current voltage and the output voltage, the inductance voltage of the first unclamped phase is determined, and according to the inductance voltage and the inductance, the first change amount information of the inductance current of the first unclamped phase is determined.

[0124] According to the zero-voltage switching boundary condition and the output current, the second change amount information of the inductance current of the first unclamped phase changing over time is determined.

[0125] According to the first change amount information, the second change amount information, and the modulation wave of the minimum pulse width modulation, the critical switching frequency of the first unclamped phase is obtained.

[0126] In a possible implementation, the second processing unit 33 is specifically configured to:

[0127] The final critical switching frequency of the non-clamped phase in each sector is determined according to the first critical switching frequency and the second critical switching frequency of the non-clamped phase in each sector, including:

[0128] If the first critical switching frequency of the non-clamped phase in the sector is greater than the second critical switching frequency of the non-clamped phase in the sector, the first critical switching frequency is determined as the final critical switching frequency of the non-clamped phase in the sector.

[0129] If the first critical switching frequency of the non-clamped phase in the sector is less than or equal to the second critical switching frequency of the non-clamped phase in the sector, the second critical switching frequency is determined as the final critical switching frequency of the non-clamped phase in the sector.

[0130] In a possible implementation, the third processing unit 34 is specifically configured to:

[0131] The modulation mode corresponding to the final critical switching frequency of each sector is determined as the modulation mode of the sector.

[0132] The modulation wave of the three-phase T-type three-level inverter is determined according to the modulation mode corresponding to each sector, and the carrier wave is determined according to the final critical switching frequency corresponding to each sector; wherein the carrier wave is used to control the turn-on and turn-off of the switching device.

[0133] In a possible implementation, the third processing unit 34 is specifically configured to:

[0134] The pulse width modulation of the signal is performed based on the modulation wave of each sector and the variable switching frequency carrier wave, to obtain the control signal for each switching device in the three-phase T-type three-level inverter; wherein the control signal is used to control the turn-on and turn-off of the switching device.

[0135] The switching device in the three-phase T-type three-level inverter is controlled based on the control signal.

[0136] In a possible implementation, the acquisition unit 31 is specifically configured to:

[0137] The clamping mode information of each sector through which the three-phase voltage synthesis vector passes under minimum pulse width modulation and maximum pulse width modulation is acquired.

[0138] The non-clamped phase of each sector through which the three-phase voltage synthesis vector passes under minimum pulse width modulation and maximum pulse width modulation is determined according to the clamping mode information.

[0139] In the above embodiments, the description of each embodiment has its own focus, and the part not described or recorded in a certain embodiment can be referred to the related description of other embodiments.

[0140] Those skilled in the art can understand that the templates, units and algorithm steps of each example described in combination with the embodiments disclosed herein can be realized by electronic hardware or a combination of computer software and electronic hardware. Whether the functions are realized in hardware or software depends on the specific application and design constraints of the technical solution. A person skilled in the art can use different methods to realize the described functions for each specific application, but such implementation should not be considered beyond the scope of the present application.

[0141] If the modules / units are realized in the form of software function units and sold or used as independent products, they can be stored in a computer readable storage medium. Based on such understanding, all or part of the processes in the above-mentioned embodiment methods can also be completed by instructing related hardware through a computer program, and the computer program can be stored in a computer readable storage medium. When the processor executes the computer program, the steps of each method embodiment described above can be implemented. The computer program includes computer program code, which can be in the form of source code, object code, executable file or some intermediate form. The computer readable medium can include any entity or device capable of carrying the computer program code, recording medium, U disk, mobile hard disk, magnetic disk, optical disk, computer memory, read-only memory, random access memory, electric carrier wave signal, telecommunication signal and software distribution medium, etc.

[0142] The above embodiments are only used to illustrate the technical solutions of the present application, but not limit it; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that the technical solutions recorded in the foregoing embodiments can be modified, or some technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application, and should be included in the protection scope of the present application.

Claims

1. A method for implementing soft switching in a three-phase T-type three-level inverter, characterized in that, The method includes: obtaining the non-clamped phase of each sector traversed by the three-phase voltage synthesis vector under minimum pulse width modulation and maximum pulse width modulation; Calculate the first critical switching frequency of the non-clamped phase in each sector under minimum pulse width modulation and the second critical switching frequency under maximum pulse width modulation for a three-phase T-type three-level inverter operating at non-unity power factor; wherein, the critical switching frequency is the minimum switching frequency that enables the switching device of the corresponding non-clamped phase to achieve soft switching. The final critical switching frequency of the unclamped phase in each sector is determined based on the first and second critical switching frequencies of the unclamped phase in each sector. Based on the final critical switching frequency of the non-clamped phase in each sector, the modulation wave and carrier of the three-phase T-type three-level inverter are determined, and the switching devices are controlled based on the modulation wave and the carrier.

2. The soft-switching method for a three-phase T-type three-level inverter according to claim 1, characterized in that, The calculation of the first critical switching frequency of the non-clamped phase in each sector under minimum pulse width modulation for a three-phase T-type three-level inverter operating at a non-unity power factor includes: For each sector, calculate the critical switching frequency of the first unclamped phase under minimum pulse width modulation for a three-phase T-type three-level inverter operating at non-unity power factor, and calculate the critical switching frequency of the second unclamped phase under minimum pulse width modulation; wherein each sector includes the first unclamped phase and the second unclamped phase. If the critical switching frequency of the first unclamped phase is less than the critical switching frequency of the second unclamped phase, then the critical switching frequency of the first unclamped phase is determined to be the first critical switching frequency of the unclamped phase in the sector under minimum pulse width modulation. If the critical switching frequency of the first unclamped phase is greater than or equal to the critical switching frequency of the second unclamped phase, then the critical switching frequency corresponding to the second unclamped phase is determined to be the first critical switching frequency of the unclamped phase in that sector under minimum pulse width modulation.

3. The soft-switching method for a three-phase T-type three-level inverter according to claim 2, characterized in that, The calculation of the critical switching frequency of the first unclamped phase under minimum pulse width modulation for a three-phase T-type three-level inverter operating at a non-unity power factor includes: The output voltage and output current of the first unclamped phase are obtained, and the corresponding zero-voltage switching boundary conditions are determined; wherein, the zero-voltage switching boundary conditions are used to indicate the relationship between the inductor current of the unclamped phase and the preset bias current when the three-phase T-type three-level inverter is running at a non-unity power factor. Based on the zero-voltage switching boundary condition, and according to the DC voltage, the modulation wave of minimum pulse width modulation, and the output voltage, output current, and inductance of the first unclamped phase, the critical switching frequency of the first unclamped phase under minimum pulse width modulation is obtained.

4. The soft-switching method for a three-phase T-type three-level inverter according to claim 3, characterized in that, The method of obtaining the critical switching frequency of the first unclamped phase under minimum pulse width modulation based on the zero-voltage switching boundary condition, the DC voltage, the modulation wave of minimum pulse width modulation, and the output voltage, output current, and inductance of the first unclamped phase includes: Based on the DC voltage and the output voltage, the inductor voltage of the first unclamped phase is determined, and based on the inductor voltage and the inductance, the first change information of the inductor current of the first unclamped phase is determined. Based on the zero-voltage switching boundary conditions and the output current, determine the second change information of the first non-clamped inductor current over time; Based on the first change information, the second change information, and the modulation wave of minimum pulse width modulation, the critical switching frequency of the first non-clamped phase is obtained.

5. The method for implementing soft switching of a three-phase T-type three-level inverter according to any one of claims 1-4, characterized in that, The step of determining the final critical switching frequency of the unclamped phase in each sector based on the first and second critical switching frequencies of the unclamped phase in each sector includes: If the first critical switching frequency of the unclamped phase in a sector is greater than the second critical switching frequency of the unclamped phase in that sector, then the first critical switching frequency is determined to be the final critical switching frequency of the unclamped phase in that sector. If the first critical switching frequency of the unclamped phase in a sector is less than or equal to the second critical switching frequency of the unclamped phase in that sector, then the second critical switching frequency is determined to be the final critical switching frequency of the unclamped phase in that sector.

6. The method for implementing soft switching of a three-phase T-type three-level inverter according to any one of claims 1-4, characterized in that, The step of determining the modulation wave and carrier wave of the three-phase T-type three-level inverter based on the final critical switching frequency of the non-clamped phases in each sector includes: The modulation scheme corresponding to the final critical switching frequency of each sector is determined as the modulation scheme of that sector; Based on the modulation scheme corresponding to each sector, the modulation wave of the three-phase T-type three-level inverter is determined, and the carrier wave is determined based on the final critical switching frequency corresponding to each sector; wherein, the carrier wave is used to control the switching devices to turn on and off.

7. The method for implementing soft switching of a three-phase T-type three-level inverter according to any one of claims 1-4, characterized in that, The control of the switching device based on the modulation wave and the carrier wave includes: Based on the modulation wave of each sector and the carrier wave of the variable switching frequency, pulse width modulation of the signal is performed to obtain control signals for each switching device in the three-phase T-type three-level inverter; wherein, the control signals are used to control the switching devices to turn on and off. Based on the control signal, the switching devices in the three-phase T-type three-level inverter are controlled.

8. The method for implementing soft switching of a three-phase T-type three-level inverter according to any one of claims 1-4, characterized in that, The acquisition of the non-clamped phase of each sector traversed by the three-phase voltage synthesis vector under minimum pulse width modulation and maximum pulse width modulation includes: Obtain clamping mode information for each sector traversed by the three-phase voltage synthesis vector under minimum pulse width modulation and maximum pulse width modulation. Based on the clamping mode information, the unclamped phase of each sector traversed by the three-phase voltage synthesis vector under minimum pulse width modulation and maximum pulse width modulation is determined.

9. A soft-switching device for implementing a three-phase T-type three-level inverter, characterized in that, The device includes: The acquisition unit is used to acquire the non-clamped phase of each sector traversed by the three-phase voltage synthesis vector under minimum pulse width modulation and maximum pulse width modulation. The first processing unit is used to calculate the first critical switching frequency of the non-clamped phase in each sector under minimum pulse width modulation and the second critical switching frequency under maximum pulse width modulation when the three-phase T-type three-level inverter is running at non-unity power factor; wherein, the critical switching frequency is the minimum switching frequency that enables the switching device of the corresponding non-clamped phase to achieve soft switching. The second processing unit is used to determine the final critical switching frequency of the unclamped phase in each sector based on the first critical switching frequency and the second critical switching frequency of the unclamped phase in each sector. The third processing unit is used to determine the modulation wave and carrier of the three-phase T-type three-level inverter based on the final critical switching frequency of the non-clamped phase in each sector, and to control the switching devices based on the modulation wave and the carrier.

10. The soft-switching device for realizing a three-phase T-type three-level inverter according to claim 9, characterized in that, The first processing unit is specifically used for: For each sector, calculate the critical switching frequency of the first unclamped phase under minimum pulse width modulation for a three-phase T-type three-level inverter operating at non-unity power factor, and calculate the critical switching frequency of the second unclamped phase under minimum pulse width modulation; wherein each sector includes the first unclamped phase and the second unclamped phase. If the critical switching frequency of the first unclamped phase is less than the critical switching frequency of the second unclamped phase, then the critical switching frequency of the first unclamped phase is determined to be the first critical switching frequency of the unclamped phase in the sector under minimum pulse width modulation. If the critical switching frequency of the first unclamped phase is greater than or equal to the critical switching frequency of the second unclamped phase, then the critical switching frequency corresponding to the second unclamped phase is determined to be the first critical switching frequency of the unclamped phase in that sector under minimum pulse width modulation.

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