Modular multilevel converter modulation method considering driver dead-time compensation

Abstract

The present invention discloses a modular multi-level converter modulation method considering driving dead zone compensation. In the present invention, the modular multi-level converter adopts quasi-square wave modulation, and the upper and lower switch tubes of the submodule have a driving dead zone when the switching state is switched, and the upper and lower switch tubes of the submodule are locked during the dead zone. In the present invention, the original submodule modulation signal is delayed or maintained according to the polarity of the upper and lower bridge arm currents. The modulation method realizes the complete complementarity of the upper and lower bridge arm voltages of the quasi-square wave modulation, can eliminate the overshoot of the common mode voltage during the dead zone while maintaining the high-frequency operation of the converter, reduce the di / dt of the bridge arm current, avoid the waveform distortion of the bridge arm current, and reduce the interference of the bridge arm voltage dv / dt on the sampling, communication and other modules in the converter control system, thereby improving the stability of the system operation.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronic control, and in particular relates to a modular multi-level converter modulation method considering driving dead zone compensation. Background Art

[0002] With the large-scale access of new energy to the grid, the construction of large-scale data centers and the promotion of large-scale electric vehicles, medium-voltage DC distribution systems have attracted extensive research interest. Among them, medium-voltage large-capacity DC-DC converters are important devices for interconnecting DC grids of different voltage levels or converting medium-voltage DC (MVDC) into low-voltage DC (LVDC) power supply. In recent years, modular multilevel converters have become the preferred solution for medium-voltage large-capacity DC-DC converters. They consist of modular multilevel circuits, filter circuits, medium- and high-frequency transformers and output side circuits. Among them, the modular multilevel circuit on the input side is realized by connecting multiple half-bridge sub-modules in series, which can be applied to high voltage levels and flexibly adjust the output voltage. At the same time, the modular structure is easy to expand the voltage and power levels; different types of DC-DC converters can be constructed according to the selected filter circuits, such as inductor filter circuits, LC resonant circuits or LLC resonant circuits, etc. The design of the filter circuit can realize the soft switching of sub-module power devices under the full load range. These advantages of modular multilevel converters make it have broad application prospects in the field of medium-voltage power supply.

[0003] In the traditional modulation method, the modular multi-level structure inverter generates a square wave voltage to act on the filter circuit, and in order to reduce dv / dt, a quasi-square wave modulation strategy is usually adopted. The modulation signal of each submodule is a square wave with a duty cycle of 50%, and the high and low levels of the modulation signal correspond to the input and removal of the submodule respectively; the modulation signals of each submodule in the same bridge arm differ from each other by a fixed phase shift angle, and the modulation signal of the entire bridge arm is a quasi-square wave, and the modulation signals of the upper and lower bridge arms are complementary. In order to improve the utilization rate of the DC voltage, the phase shift angle is set to be small, so that the quasi-square wave is as close to the ideal square wave as possible, and the phase shift angle can be further reduced as the number of submodules increases. In the half-bridge submodule, in order to prevent the upper and lower tubes from being directly connected and causing a capacitor short circuit, it is necessary to set a driving dead zone for the state switching of the switch tube. During the dead zone time, the driving signals of the upper and lower switch tubes are locked, and the bridge arm current flows through its anti-parallel diode. The output voltage of the submodule is not controlled by the original modulation signal, but is determined by the polarity of the bridge arm current. Compared with the original modulation signal, the driving dead zone will cause a delay in the input or removal of the submodule, which may cause the output voltage of the upper and lower bridge arms to be non-complementary, thus generating an overshoot of the common-mode voltage output of the upper and lower bridge arms. The voltage overshoot acts on the bridge arm inductance, generating a large dv / dt and causing bridge arm current distortion. At the same time, the voltage overshoot dv / dt caused by the driving dead zone will interfere with the sampling, communication and other modules in the converter control system, seriously endangering the safe and stable operation of the system. Summary of the invention

[0004] In view of the above problems existing in the prior art, the present invention proposes a modular multi-level converter modulation method taking driving dead zone into consideration.

[0005] The modular multi-level converter of the present invention is composed of a modular multi-level circuit, a filter circuit, a medium-high frequency transformer and an output side circuit; wherein the modular multi-level circuit comprises one or more phase units, each phase unit is divided into an upper and a lower bridge arm, each bridge arm is composed of N identical half-bridge sub-modules and a bridge arm inductor connected in series; the output end of the modular multi-level circuit is connected to the primary side of the medium-high frequency transformer through the filter circuit, and the secondary side of the transformer is connected to the output side circuit; the original modulation signals of the sub-modules in the same bridge arm differ from each other by a fixed phase shift angle, the modulation signal of the entire bridge arm is a quasi-square wave, the original modulation signals of the upper and lower bridge arms of the same phase unit are complementary, and there is a driving dead zone between the upper and lower switch tubes of the sub-module when switching the switching state, and the upper and lower switch tubes of the sub-module are both locked during the dead zone time.

[0006] The modular multi-level converter modulation method considering driving dead zone compensation provided by the present invention comprises the following steps:

[0007] The dead time is defined as t d ; The reference direction of the upper and lower bridge arm currents is defined as the DC positive pole pointing to the DC negative pole;

[0008] At the time of switching the submodule, when the upper and lower bridge arm current polarity is positive at the top and negative at the bottom or negative at the top and positive at the bottom, the original modulation signal of the submodule is maintained; when the upper and lower bridge arm current polarity is positive at the same time, the input time in the original modulation signal of the submodule is delayed by the dead time t d When the polarity of the upper and lower bridge arm currents is negative at the same time, the removal time in the original modulation signal of the submodule is delayed by the dead time t d Re-resection.

[0009] As a preferred solution of the present invention, the original modulation signal of each submodule of the modular multilevel converter is a square wave with a duty cycle of 50%, and the high and low levels of the modulation signal correspond to the input and output of the submodule respectively.

[0010] As an optional solution of the present invention, the polarity of the current of the upper and lower bridge arms at the time of switching the switching state of the submodule is obtained through real-time detection.

[0011] As an optional solution of the present invention, the polarities of the upper and lower bridge arm currents at the time of switching the switching state of the submodule are obtained by offline calculation according to the working conditions.

[0012] As a preferred solution of the present invention, at the moment of switching the switching state of the submodule, when the polarities of the upper and lower bridge arm currents are both positive, the removal moment in the original modulation signal of the submodule remains unchanged; when the polarities of the upper and lower bridge arm currents are both negative, the input moment in the modulation signal of the submodule remains unchanged.

[0013] As a preferred solution of the present invention, the filter circuit is an inductor filter circuit, an LC resonant circuit or an LLC resonant circuit.

[0014] As a preferred solution of the present invention, the output side circuit is a diode rectifier circuit, a fully controlled switch circuit or a modular multi-level circuit.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] At the switching state switching moment of the submodule, the present invention delays or maintains the original submodule modulation signal according to the polarity of the upper and lower bridge arm currents, thereby realizing complete complementarity of the upper and lower bridge arm voltages of the quasi-square wave modulation, eliminating the overshoot of the upper and lower bridge arm common mode voltages within the dead time under the condition of maintaining the high-frequency operation of the converter, reducing the di / dt of the bridge arm current, avoiding the waveform distortion of the bridge arm current, and reducing the interference of the bridge arm voltage dv / dt on the sampling, communication and other modules in the converter control system, thereby improving the stability of the system operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a topology diagram of a modular multilevel converter.

[0018] Figure 2 It is the ideal working waveform for quasi-square wave modulation of the upper and lower bridge arms.

[0019] Figure 3 It is the state switching process of the submodule when the bridge arm current is positive during the dead time.

[0020] Figure 4 It is the actual output voltage waveform of the upper and lower bridge arms of the existing quasi-square wave modulation method.

[0021] Figure 5 This is the actual output voltage waveform of the upper and lower bridge arms of the quasi-square wave modulation method considering the driving dead zone described in the present invention.

[0022] Figure 6 It is the waveform of the common mode voltage output by the front and rear upper and lower bridge arms using the modulation method of the present invention.

[0023] Figure 7 The waveforms of the upper and lower bridge arm currents before and after the modulation method of the present invention is adopted. DETAILED DESCRIPTION

[0024] In order to describe the method proposed by the present invention more specifically and clearly, the technical implementation scheme of the present invention is described in detail below with reference to the accompanying drawings and actual cases.

[0025] like Figure 1 As shown, the modular multilevel converter consists of a modular multilevel circuit, a filter circuit, a medium and high frequency transformer and an output side circuit; wherein the modular multilevel circuit includes one or more phase units, each phase unit is divided into two upper and lower bridge arms, each bridge arm is composed of N identical half-bridge sub-modules and a bridge arm inductor in series; the output end of the modular multilevel circuit is connected to the primary side of the medium and high frequency transformer through the filter circuit, and the secondary side of the transformer is connected to the output side circuit; the filter circuit is an inductor filter circuit, an LC resonant circuit or an LLC resonant circuit; the output side circuit can be a diode rectifier circuit, a fully controlled switch circuit or a modular multilevel circuit. The modular multilevel converter is used to realize the conversion of medium voltage direct current (MVDC) to low voltage direct current (LVDC) voltage.

[0026] In the prior art, modular multi-level converters usually adopt a quasi-square wave modulation strategy. The modulation signal of each submodule is a square wave with a 50% duty cycle. The high and low levels of the modulation signal correspond to the input and removal of the submodule respectively. The modulation signals of each submodule in the same bridge arm differ from each other by a fixed phase shift angle. The modulation signal of the entire bridge arm is a quasi-square wave, and the modulation signals of the upper and lower bridge arms are complementary. Figure 2 As shown in the figure, under ideal conditions without considering the driving dead zone, the upper and lower bridge arms output complementary quasi-square waves. Quasi-square wave modulation improves the utilization rate of DC input voltage, while increasing the switching frequency of the converter, reducing the capacitance of the submodule and the volume of the inductor transformer, which is beneficial to the improvement of the power density of the converter; on the other hand, it can reduce the impact of dv / dt on the transformer and reduce the difficulty of insulation design of medium and high frequency transformers.

[0027] In the half-bridge submodule, in order to prevent the capacitor from short-circuiting, it is necessary to set a driving dead zone for the state switching of the switch tube. During the dead zone, the driving signals of the upper and lower switch tubes are locked, and the bridge arm current flows through its anti-parallel diode. The output voltage of the submodule is not controlled by the original modulation signal, but is determined by the polarity of the bridge arm current. The driving dead zone needs to be selected according to the characteristics of the switch tube itself. It is usually larger than the phase shift angle of the submodule. Therefore, during the dead zone, the state switching of multiple submodules is uncontrolled, and as the number of submodules in the bridge arm increases and the phase shift angle decreases, the number of uncontrolled submodules increases during the dead zone. Compared with the original modulation signal, the driving dead zone will cause a delay in the input or removal of the submodule, which may cause the output voltage of the upper and lower bridge arms to be non-complementary, thus generating an overshoot of the common mode voltage of the upper and lower bridge arms. The voltage overshoot acts on the bridge arm inductance, generating a large dv / dt and causing bridge arm current distortion. At the same time, the voltage overshoot dv / dt caused by the driving dead zone will interfere with the sampling, communication and other modules in the converter control system, seriously endangering the safe and stable operation of the system.

[0028] like Figure 3 As shown, the reference direction of the upper and lower bridge arm currents are defined as the DC positive pole pointing to the DC negative pole. During the switching process of the submodule state, if the bridge arm current is positive, the bridge arm current flows through the anti-parallel diode D1 of the upper tube of the submodule during the dead time, and the submodule is in a passive on state. In this case, the submodule is passively delayed by a dead time when changing from the on state to the off state, and there is no delay when the submodule changes from the off state to the on state; if the bridge arm current is negative, the bridge arm current flows through the anti-parallel diode D2 of the lower tube of the submodule during the dead time, and the submodule is in a passive off state. In this case, the submodule is passively delayed by a dead time when changing from the off state to the on state, and there is no delay when the submodule changes from the on state to the off state. On this basis, Figure 4 As shown in the figure, if the polarity of the upper and lower bridge arm currents is positive at the same time when the submodule switching state is switched, the submodule is passively delayed by a dead time when changing from the switched-in state to the removed state, and there is no delay when the submodule changes from the removed state to the switched-in state. Therefore, the output voltages of the upper and lower bridge arms are not completely complementary, and the common-mode voltage they output is greater than the DC side voltage, resulting in a voltage overshoot. The voltage overshoot will act on the bridge arm inductance, and the duration is the sum of the phase shift time and the dead time; if the polarity of the upper and lower bridge arm currents is negative at the same time when the submodule switching state is switched, the submodule is passively delayed by a dead time when changing from the removed state to the switched-in state, and there is no delay when the submodule changes from the switched-in state to the removed state. Therefore, the output voltages of the upper and lower bridge arms are not completely complementary, and the common-mode voltage they output is less than the DC side voltage. A voltage overshoot is generated on the bridge arm inductance, and the duration is the sum of the phase shift time and the dead time.

[0029] When the current polarity of the upper and lower bridge arms is positive on the top and negative on the bottom or negative on the top and positive on the bottom, the driving dead zone has the same effect on the upper and lower bridge arm sub-modules, and the switching states are delayed at the same time, or there is no delay at all, and the output voltage waveforms are complementary, so only the original half-bridge sub-module modulation signal needs to be maintained.

[0030] In order to solve the voltage overshoot problem caused by the above-mentioned driving dead zone, the present invention proposes a method for actively delaying the original modulation signal to construct the complementary output voltage of the upper and lower bridge arms. Figure 5 As shown in the figure, if in the original modulation signal, the polarity of the upper and lower bridge arm currents is positive at the same time when the submodule switching state is switched, the original submodule modulation signal is delayed by the dead time t d If the original modulation signal, the upper and lower bridge arm current polarities are both negative at the time of switching the submodule switching state, the original submodule modulation signal will delay the cutting time by the dead zone time t d Then cut off, while the submodule modulation signal is always kept in its original state. Among them, the current polarity of the upper and lower bridge arms can be obtained through real-time detection or offline calculation according to the working conditions. Under the joint action of passive delay and active delay, the output voltages of the upper and lower bridge arms are completely complementary, the common mode voltage is constant to the DC side voltage, and the voltage overshoot caused by the driving dead zone is eliminated.

[0031] In order to demonstrate the implementation effect of the modular multi-level converter modulation method considering the driving dead zone proposed in the present invention, a simulation model was built in the PLEC electrical simulation software for verification. The simulation model is a modular multi-level resonant converter with the following main parameters: DC input voltage 8kV, output voltage is constant 330V, rated output power is 40kW, the number of bridge arm submodules is 16, the resonant frequency is 12kHz, the resonant converter operating frequency is 10kHz, and the upper and lower bridge arm current polarities are both positive at the same time when the switching state of the submodule is switched.

[0032] Table 1

[0033] Design parameters of the present invention DC input voltage 8kV DC output voltage 330V Rated output power 40kW Number of single bridge arm submodules N 16 Submodule rated voltage 500V Bridge arm inductance 1.2mH Transformer ratio 12:1 Transformer magnetizing inductance 800mH Resonant capacitor 293nF Resonant frequency 12kHz Operating frequency 10kHz

[0034] According to the voltage regulation method proposed by the present invention, Figure 6 The waveforms of the common-mode voltage output by the upper and lower bridge arms under the traditional quasi-square wave modulation method and the quasi-square wave modulation method considering the driving dead zone proposed by the present invention are shown; Figure 7The waveforms of the upper and lower bridge arm currents under the traditional quasi-square wave modulation method and the quasi-square wave modulation method proposed in the present invention that takes the driving dead zone into consideration are shown. In the traditional modulation method, the driving dead zone will cause a delay in the removal of the submodule, resulting in the non-complementarity of the output voltages of the upper and lower bridge arms, thereby generating an overshoot of the output common-mode voltage of the upper and lower bridge arms. The voltage overshoot acts on the bridge arm inductance, causing the bridge arm current to be distorted. After adopting the modulation method proposed in the present invention, the complete complementarity of the output voltages of the upper and lower bridge arms is achieved, the overshoot of the output common-mode voltage is eliminated, and the waveform distortion of the bridge arm current is avoided. The simulation results verify the excellent effect of the voltage regulation method proposed in the present invention.

Claims

1. A modular multilevel converter modulation method considering drive dead zone compensation, wherein the modular multilevel converter is composed of a modular multilevel circuit, a filter circuit, a medium-high frequency transformer and an output side circuit; wherein the modular multilevel circuit comprises one or more phase units, each phase unit is divided into an upper and a lower bridge arm, each bridge arm is composed of N identical half-bridge sub-modules and a bridge arm inductor connected in series; the output end of the modular multilevel circuit is connected to the primary side of the medium-high frequency transformer through the filter circuit, and the secondary side of the transformer is connected to the output side circuit; the original modulation signals of each sub-module in the same bridge arm differ from each other by a fixed phase shift angle, the modulation signal of the entire bridge arm is a quasi-square wave, the original modulation signals of the upper and lower bridge arms of the same phase unit are complementary, and there is a drive dead zone between the upper and lower switch tubes when the sub-module is switching the switching state, and the upper and lower switch tubes of the sub-module are locked during the dead zone time; Features: The modulation method comprises the following steps: The dead time is defined as t d ; The reference direction of the upper and lower bridge arm currents is defined as the DC positive pole pointing to the DC negative pole; At the time of switching the submodule, when the upper and lower bridge arm current polarity is positive at the top and negative at the bottom or negative at the top and positive at the bottom, the original modulation signal of the submodule is maintained; when the upper and lower bridge arm current polarity is positive at the same time, the input time in the original modulation signal of the submodule is delayed by the dead time t d When the polarity of the upper and lower bridge arm currents is negative at the same time, the removal time in the original modulation signal of the submodule is delayed by the dead time t d Re-resection.

2. The modular multi-level converter modulation method considering driving dead zone compensation according to claim 1, characterized in that: The original modulation signal of each submodule of the modular multilevel converter is a square wave with a duty cycle of 50%, and the high and low levels of the modulation signal correspond to the input and output of the submodule respectively.

3. The modular multi-level converter modulation method considering driving dead zone compensation according to claim 1, characterized in that: The current polarities of the upper and lower bridge arms at the time of switching the switching state of the submodule are obtained through real-time detection.

4. The modular multi-level converter modulation method considering driving dead zone compensation according to claim 1, characterized in that: The polarity of the upper and lower bridge arm currents at the time of switching the submodule switching state is obtained by offline calculation according to the working conditions.

5. The modular multi-level converter modulation method considering driving dead zone compensation according to claim 1, characterized in that: At the switching moment of the submodule, when the polarities of the upper and lower bridge arm currents are both positive, the cutting moment in the original modulation signal of the submodule remains unchanged; When the polarities of the upper and lower bridge arm currents are both negative, the input time in the modulation signal of the submodule remains unchanged.

6. The modular multi-level converter modulation method considering driving dead zone compensation according to claim 1, characterized in that: The filter circuit is an inductor filter circuit, an LC resonant circuit or an LLC resonant circuit.

7. The modular multi-level converter modulation method considering driving dead zone compensation according to claim 1, characterized in that: The output side circuit is a diode rectifier circuit, a fully controlled switch circuit or a modular multi-level circuit.

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