A high power density module, modular medium voltage frequency inverter and method

Abstract

The application discloses a high-power-density module, a modular medium-voltage frequency converter and a method. The high-power-density module is composed of a grid-side circuit, multi-winding high-frequency transformers and a machine-side circuit. The DC sides of three single-phase rectifiers of the grid-side circuit are connected to the DC sides of three high-frequency converters through three DC capacitors respectively. The AC output ports of the three high-frequency converters are connected to three windings of the high-frequency transformers through three LC resonance branches respectively, forming the grid-side circuit. The remaining n windings of the high-frequency transformers are connected to the AC ports of the remaining n high-frequency converters through n LC resonance branches respectively. The DC sides of the n high-frequency converters are connected to the DC sides of n single-phase inverters of the machine-side circuit through n DC capacitors, forming the machine-side circuit. The modular medium-voltage frequency converter is connected in series or parallel through isolation ports by one or more high-power-density modules to be applied to occasions of different voltage grades and power grades. The application has the characteristics of high power density, modularity and high efficiency.

Description

Technical Field

[0001] This invention belongs to the field of power electronics and power conversion technology, and relates to a high power density module, a modular medium-voltage frequency converter, and a method thereof. Background Technology

[0002] With economic and social development, medium-voltage high-power motors are increasingly widely used in wind turbines, water pumps, railway traction, and ship propulsion systems. Adopting medium-voltage speed-regulating drive systems can significantly save energy, optimize control, extend motor life, and improve motor performance. Among these research hotspots, modular high-capacity converter technology is a key area of ​​focus in medium-voltage speed-regulating drive systems.

[0003] Robicon, an American company, introduced the Cascaded H-Bridge (CHB) converter, which achieved great success. Due to its numerous advantages, including multi-level output voltage, low dv / dt, compatibility with low-voltage power devices, low switching frequency, simple redundancy mechanism, and ease of maintenance, the CHB has become one of the mainstream topologies for medium-voltage motor drives. Another key feature of the CHB is the need for a phase-shifting transformer to provide isolated DC power to each submodule, while also ensuring the power factor on the grid side and reducing current harmonics through phase shifting. However, the use of this phase-shifting transformer has many disadvantages: large internal circulating current and high heat generation; low efficiency; high noise; bulky size; difficult transportation; high cost; and complex manufacturing process.

[0004] Professor Marquardt proposed another type of medium-voltage high-power inverter called the Modular Multilevel Converter (MMC). The MMC eliminates the bulky and expensive phase-shifting transformer found in the CHB (Chain Bus) inverter, directly using a common DC bus for power supply, thus simplifying the overall system structure. However, the voltage fluctuations of the capacitors in each MMC submodule are inversely proportional to the AC output frequency. If the MMC is used to directly drive a high-power, low-speed motor, it will cause significant voltage fluctuations in the submodule capacitors, affecting the quality of the AC output waveform and, in severe cases, even the normal operation of the MMC and the motor. To suppress these voltage fluctuations, the capacitor capacity is typically increased, but this results in excessively large capacitor size and weight, and high system construction and maintenance costs.

[0005] In addition, the number of switching devices and capacitors in MMC is increased to 2 times and 4 times that of CHB, respectively. The technical approach of eliminating the need for multi-winding transformers at the power frequency by increasing the number of switching devices and capacitors does not fundamentally solve the cost and power density problems of medium-voltage frequency converter systems. These shortcomings limit the application of MMC in the field of medium-voltage motor drives. Therefore, the field of medium-voltage high-power motor drives urgently needs to develop a new generation of high-power-density converters with smaller capacitor values ​​and transformer sizes. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention proposes a high power density module, a modular medium-voltage frequency converter, and a method. This solution features high power density, modularity, and high efficiency.

[0007] To achieve the above-mentioned objectives, the present invention employs the following technical solution:

[0008] A high power density module for motor drive includes three single-phase rectifiers on the grid side, (3+n) DC capacitors, (3+n) high-frequency converters, (3+n) LC resonant branches, a (3+n) winding high-frequency transformer, and n single-phase inverters on the machine side.

[0009] The DC sides of the three single-phase rectifiers on the grid side are connected to the DC sides of the three high-frequency converters through three DC capacitors respectively. The AC output ports of the three high-frequency converters are connected to the three windings of the high-frequency transformer through three LC resonant branches respectively, forming the grid-side circuit.

[0010] The remaining n windings of the high-frequency transformer are connected to the AC ports of the remaining n high-frequency converters through n LC resonant branches. The DC sides of the n high-frequency converters are connected to the DC sides of the n single-phase inverters on the machine side through n DC capacitors, forming the machine side circuit.

[0011] As a further improvement of the present invention, the grid-side single-phase rectifier is a single-phase full-bridge circuit; the high-frequency converter is a full-bridge circuit or a half-bridge circuit; the machine-side single-phase inverter is a single-phase full-bridge circuit; and the power electronic devices in the high power density module are MOSFETs, IGBTs or other fully controlled switching devices.

[0012] As a further improvement of the present invention, the LC resonant branch is a series resonant circuit consisting of an inductor and a capacitor.

[0013] As a further improvement of the present invention, the frequency of the high-frequency transformer is any frequency in the range of several hundred hertz to several hundred kilohertz; the turns ratio of the high-frequency transformer winding is arbitrary, but it must be ensured that the number of coil turns of the three windings on the grid side and the n windings on the machine side are consistent respectively.

[0014] A modular medium-voltage frequency converter, consisting of one or more of the aforementioned high-power-density modules for motor drive connected in series or parallel through isolation ports, is applicable to motor drive applications with different voltages and power ratings.

[0015] A control method for a modular medium-voltage frequency converter, characterized in that it includes:

[0016] The control method of the high-frequency converter is as follows: the same set of drive signals with the same switching frequency and resonant frequency are used to make all high-frequency converters output synchronous square wave voltage pulses to achieve automatic equalization of the DC voltage of (3+n) capacitors; or square wave signals with switching frequencies close to but not equal to the resonant frequency and phase angles that are independently adjustable are used as gate drive signals, and the power control of the high-frequency converter is achieved by adjusting the phase angle of the square wave voltage.

[0017] The control method for the machine-side single-phase inverter is as follows: a motor control method is used to generate a modulation wave, and a carrier phase-shift modulation method is used to generate the gate drive signal of the machine-side single-phase inverter in each module.

[0018] The control method for the grid-side single-phase rectifier is as follows: A current control method based on DQ-axis decoupling using a synchronous rotating coordinate system is adopted. The required active power of the motor is divided by three times the effective value of the three-phase grid-side voltage to obtain the initial command value of the d-axis of the current decoupling control loop. The average voltage of the DC capacitors in all modules is taken, and the difference is calculated with the capacitor voltage command value. Through a closed-loop controller, the command value compensation amount of the d-axis in the current decoupling control loop is obtained. The initial command value and the command value compensation amount are added to obtain the d-axis command value of the current decoupling control loop. After passing through the current decoupling control loop, the three-phase voltage modulation wave of each module is output. The gate drive signal of each module's grid-side single-phase rectifier is generated using a carrier phase-shift modulation method.

[0019] As a further improvement of the present invention, the voltage coordination control of the DC capacitors between high power density modules in the modular medium-voltage frequency converter includes a software program control method and a hardware automatic coordination control method.

[0020] The software program coordination control method includes: subtracting the average voltage of the DC-side capacitor in each module from the average voltage of all DC-side capacitors in the system, passing the difference through a closed-loop controller and multiplying it by the three single-phase currents on the grid side to obtain the compensation amount of the three single-phase voltage modulation waves for each module; summing the modulation wave and the compensation amount of each phase respectively, and generating the gate drive signal of the grid-side single-phase rectifier of each module using a carrier phase-shift modulation method.

[0021] The hardware automatic coordination control method includes: adding a winding to the high-frequency transformer in each high power density module, the winding being connected in series with an LC resonant circuit to form a high-frequency resonant AC terminal, and the high-frequency resonant AC terminals of each module being connected in parallel to form a common high-frequency AC bus, so that the power spontaneous flow is balanced, thereby achieving self-balancing of the DC capacitor voltage in each module. This topology requires that the gate drive signal of all high-frequency converters must be a square wave signal with the same switching frequency and resonant frequency and the same phase.

[0022] A modular medium-voltage frequency converter including a soft-start section is characterized in that: it includes the aforementioned modular medium-voltage frequency converter; the soft-start section is a low-voltage DC power supply connected to a common AC bus via a low-voltage side high-frequency converter; or a low-voltage AC power supply connected to a common AC bus via a low-voltage side rectifier circuit and a low-voltage side high-frequency converter.

[0023] The common high-frequency AC bus is composed of a high-frequency resonant AC terminal connected in parallel, which is formed by the series LC resonant circuit of the newly added winding of each module.

[0024] The low-voltage side rectifier circuit is single-phase, three-phase, or multi-phase. This part operates in diode rectification or PWM rectification mode, and the corresponding low-voltage AC power supply is single-phase AC, three-phase AC, or multi-phase AC power supply.

[0025] A soft-start method for a modular medium-voltage frequency converter includes:

[0026] 1) During the pre-charging stage of the capacitor, the switching transistors in each power module are in a locked state. The low-voltage side high-frequency converter outputs voltage pulses with a duty cycle that increases linearly from 0% to 50%, which are supplied to the common high-frequency AC bus. The common AC bus charges the DC capacitor through the multi-winding transformers in each power module and the anti-parallel diodes of the switching devices in the high-frequency converter.

[0027] 2) After the capacitor is precharged to the rated voltage, the grid-side circuit is closed, the high-frequency converters in each power module start working, and the grid-side single-phase rectifier is closed-loop controlled. Then the motor side is closed, and finally the motor-side single-phase inverter is closed-loop controlled, and the circuit enters the normal operation mode. In the normal operation mode, the low-voltage high-frequency converter can continue to operate synchronously to realize the automatic charging of the DC battery, or enter the lockout mode.

[0028] The present invention, by adopting the above technical solution, has the following beneficial effects:

[0029] Compared with existing frequency converters of the same type, the technical advantages of this invention are high power density, modularity, and high efficiency.

[0030] 1) The core feature of this invention is high power density, manifested in increasing the transformer frequency from power frequency to high frequency to reduce transformer size, and reducing capacitor volume and weight by converting low-frequency fluctuation power to high-frequency fluctuation power. The high power density module consists of grid-side circuitry, a multi-winding high-frequency transformer, and machine-side circuitry. Topologically, it solves the problems of excessively large transformers in CHB and excessive capacitor voltage fluctuations when the MMC directly drives a high-power, low-speed motor. Since the voltage fluctuation of each MMC sub-module is inversely proportional to the AC output frequency, directly driving a high-power, low-speed motor with the MMC will result in significant voltage fluctuations in the sub-module capacitors. Existing solutions increase capacitor capacity to suppress sub-module capacitor voltage fluctuations within acceptable limits, but this also leads to excessively large capacitor volume and weight, and high system construction and maintenance costs. The inverter system proposed in this invention features a multi-winding high-frequency transformer in the inverter module with a much smaller volume and weight than a power frequency transformer of the same specifications. Furthermore, the 2 times harmonic frequency fluctuation power of the three-phase AC is canceled out at the high-frequency transformer, resulting in a significant reduction in the capacitance of the DC capacitors within the module, further reducing the size and weight of the equipment.

[0031] 2) The inverter system is highly modular, facilitating system expansion based on capacity requirements. This invention utilizes a modular approach, allowing the number of high-power-density modules connected in series and parallel to determine the specific capacity requirements of the actual drive system, resulting in a highly modular system with greater practical engineering value. Simultaneously, by increasing the number of modules, the voltage and current withstand requirements of individual power devices can be significantly reduced. This is a visual difference between this invention and other existing high-power low-speed motor drive solutions.

[0032] 3) This invention reduces losses at the high-frequency converter by introducing resonant technology, and achieves high efficiency by lowering the switching frequency of the grid-side single-phase rectifier and the machine-side single-phase inverter, thereby reducing losses in the low-frequency circuit. Furthermore, each power channel in the high-power-density module is connected in series with an LC resonant branch, enabling soft-switching or full-resonant control, reducing power losses during turn-on and turn-off. Simultaneously, the grid-side single-phase rectifier and the machine-side single-phase inverter employ phase-shift modulation, and each switch operates at a lower switching frequency, significantly reducing switching losses while ensuring overall spectral quality. Attached Figure Description

[0033] The accompanying drawings described herein are for illustrative purposes only and are not intended to limit the scope of the invention in any way. Furthermore, the shapes and proportions of the components in the drawings are merely schematic to aid in understanding the invention and do not specifically limit the shapes and proportions of the components. In the drawings:

[0034] Figure 1 This is a schematic diagram of the high power density module topology of the present invention;

[0035] Figure 2 This is a schematic diagram of the modular medium-voltage frequency converter topology of the present invention;

[0036] Figure 3 This is a schematic diagram of the modular medium-voltage frequency converter grid-side single-phase rectifier control method including DC capacitor voltage software program coordinated control according to the present invention;

[0037] Figure 4 This is a schematic diagram of the topology of the modular medium-voltage frequency converter with soft-start function of the present invention;

[0038] Figure 5 This is a schematic diagram of a modular medium-voltage frequency converter topology based on a seven-winding high-frequency transformer, according to a preferred embodiment of the present invention.

[0039] Figure 6 The simulated waveforms of the motor speed, torque, grid-side three-phase voltage, grid-side three-phase current, machine-side three-phase voltage, machine-side three-phase current, and all DC-side capacitor voltages during the motor starting process, according to a preferred embodiment of the present invention.

[0040] Figure 7 The waveform diagram of the high-frequency converter bridge port voltage and resonant current in a modular medium-voltage frequency converter according to a preferred embodiment of the present invention is shown.

[0041] Figure 8 The diagram shows the DC capacitor voltage, the voltage pulse output by the high-frequency converter in the low-voltage side soft start circuit, the current waveform flowing through the DC capacitor on the high-voltage side of the module, and the voltage pulse waveform output by the high-frequency converter on the high-voltage side during a soft start process, which is a preferred embodiment of the present invention.

[0042] Specific implementation

[0043] To make the objectives and technical solutions of this invention clearer and easier to understand, the invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0044] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more. In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0045] This invention relates to a high power density module for motor drive, a modular medium-voltage frequency converter, a control method for a modular medium-voltage frequency converter, a modular medium-voltage frequency converter including a soft-start section, and a soft-start method for a modular medium-voltage frequency converter. To enhance understanding of this invention, the technical solution is further described below with reference to the accompanying drawings and specific embodiments.

[0046] like Figure 1 As shown, a high power density module for motor drive includes three single-phase rectifiers on the grid side, (3+n) DC capacitors, (3+n) high-frequency converters, (3+n) LC resonant branches, one (3+n) winding high-frequency transformer, and n single-phase inverters on the machine side. The windings shown in the dashed box and one LC resonant branch can be added to the module to form high-frequency resonant AC terminals V1 and V2.

[0047] Unless otherwise specified below, high power density modules do not include a new high-frequency resonant AC winding.

[0048] Among them, the DC side of the three single-phase rectifiers on the grid side is connected to the DC side of the three high-frequency converters through three DC capacitors respectively, and the AC output ports of the three high-frequency converters are connected to the three windings of the high-frequency transformer through three LC resonant branches respectively, forming a complete grid-side circuit.

[0049] The remaining n windings of the high-frequency transformer are connected to the AC ports of the remaining n high-frequency converters through n LC resonant branches, and the DC sides of the n high-frequency converters are connected to the DC sides of the n single-phase inverters on the machine side through n DC capacitors, forming a complete machine-side circuit.

[0050] The grid-side single-phase rectifier is a single-phase full-bridge circuit with resistors R1, R2, and R3. The machine-side single-phase inverter is a single-phase full-bridge circuit with resistors O1, O2, O3...O n The LC resonant branch is a series resonant circuit consisting of an inductor and a capacitor; the frequency of the high-frequency transformer is any frequency in the range of several hundred hertz to several hundred kilohertz; the turns ratio of the high-frequency transformer is arbitrary, but it must be ensured that the number of turns of the three windings on the grid side and the n windings on the machine side are consistent; the power electronic devices in the high power density module are MOSFETs, IGBTs or other fully controlled switching devices.

[0051] As shown in sub-figures (b) and (c), the high-frequency converter is a full-bridge circuit or a half-bridge circuit.

[0052] like Figure 2 As shown, a modular medium-voltage frequency converter consists of multiple high-power-density modules for motor drive connected in series or parallel through isolation ports, applicable to motor drive applications with different voltages and power.

[0053] The control method of the high-frequency converter is as follows: the same set of drive signals with the same switching frequency and resonant frequency are used to make all high-frequency converters output synchronous square wave voltage pulses to achieve automatic equalization of the DC voltage of (3+n) capacitors; or square wave signals with switching frequencies close to but not equal to the resonant frequency and phase angles that are independently adjustable are used as gate drive signals, and the power control of the high-frequency converter is achieved by adjusting the phase angle of the square wave voltage.

[0054] n single-phase inverters on the machine side, O1, O2, O3...O n The control method is as follows: an arbitrary motor control method is used to generate the gate drive signal of the machine-side single-phase inverter;

[0055] The control method for the (3+n) high-frequency converters in each module is as follows: The same set of drive signals with the same switching frequency and resonant frequency are used to synchronize the output of square wave voltage pulses from all high-frequency converters, achieving automatic balancing of the DC voltages of the (3+n) capacitors; or, square wave signals with switching frequencies close to but not equal to the resonant frequency and independently adjustable phase angles are used as gate drive signals, and power control of the high-frequency converters is achieved by adjusting the phase angle of the square wave voltage. If the high-power density module contains high-frequency resonant AC terminals, the high-frequency resonant AC terminals V1 and V2 of each module are connected in parallel to form a common high-frequency resonant AC bus, as shown by the dotted line in the figure. The connection of V1 and V2 enables self-flow balance of power, thereby achieving self-balancing of the DC capacitor voltages in each high-power density module. In this case, the control signal for all high-frequency converters must be a square wave signal with the same switching frequency and resonant frequency and the same phase as the gate drive signal.

[0056] like Figure 3 As shown, the control method for a modular medium-voltage frequency converter grid-side single-phase rectifier, which includes software program-coordinated control of DC capacitor voltage, comprises the following steps:

[0057] (1) A current control method based on the DQ axis decoupling of a synchronous rotating coordinate system is adopted to control the active power required by the motor. Divide the effective value of the three-phase grid voltage U by three times to obtain the initial command value of the d-axis of the current decoupling control loop; the voltage V of the DC capacitors in all modules c1 V c2 …V cn Take the average value Then, the capacitor voltage command value The difference is calculated using the closed-loop controller to obtain the command value compensation amount for the d-axis in the current decoupling control loop. The initial command value is then added to the command value compensation amount to obtain the d-axis command value of the current decoupling control loop.

[0058] (2) After passing through the current decoupling control loop, the output of each module is a three-phase voltage modulation wave V. a V b V c The average voltage of the DC-side capacitors in each module. The average value of the voltages of all DC-side capacitors in the system. The difference is calculated, passed through the closed-loop controller, and multiplied by the three single-phase currents i on the grid side. a i b i c The compensation quantities ΔV of the three single-phase voltage modulation waves of each module are obtained. ai ΔV bi ΔV ci The modulation wave and the modulation wave compensation amount of each phase are summed to obtain V. ai Vbi V ci The gate drive signal of the grid-side single-phase rectifier of each module is generated by carrier phase-shift modulation method.

[0059] Figure 4 The diagram shows a modular medium-voltage frequency converter including a soft-start section. The high-power-density module includes newly added high-frequency resonant AC terminals. The topology of the soft-start section is shown in sub-figures (b) and (c), and the form of the high-frequency converter is shown in sub-figures (d) and (e). As shown in sub-figure (b), the left AC terminals p1 and p2 of the high-frequency converter are connected to the P1 and P2 terminals of the common high-frequency AC bus, and the right DC terminal of the high-frequency converter is connected to the low-voltage DC power supply. Or, as shown in sub-figure (c), the left AC terminals p1 and p2 of the high-frequency converter are connected to the P1 and P2 terminals of the common high-frequency AC bus, and the right terminal of the high-frequency converter is connected to the rectifier circuit through a DC capacitor, and then connected to the low-voltage AC power supply. The low-voltage side rectifier circuit is single-phase, three-phase, or multi-phase. This part of the circuit operates in diode rectification or PWM rectification mode, and the corresponding low-voltage AC power supply is single-phase AC, three-phase AC, or multi-phase AC power supply.

[0060] The soft-start process of the modular medium-voltage frequency converter is as follows:

[0061] (1) During the pre-charging stage of the capacitor, the switching transistors in each power module are in a locked state. The low-voltage side high-frequency converter outputs voltage pulses with a duty cycle that increases linearly from 0% to 50%, which are supplied to the common high-frequency AC bus. The common AC bus charges the DC capacitor through the multi-winding transformers in each power module and the anti-parallel diodes of the switching devices in the high-frequency converter.

[0062] (2) After the capacitor is precharged to the rated voltage, the grid-side circuit is closed, and the high-frequency converters in each power module start working. At the same time, the grid-side single-phase rectifier is closed-loop controlled. Then the motor side is closed, and finally the motor-side single-phase inverter is closed-loop controlled. The circuit enters the normal operation mode. In the normal operation mode, the low-voltage high-frequency converter can continue to operate synchronously to realize the automatic charging of the DC battery, or it can enter the lockout mode.

[0063] The following detailed description is provided in conjunction with specific embodiments.

[0064] Figure 5The diagram shows a modular medium-voltage frequency converter built upon four high-power-density modules, used for four-quadrant frequency conversion drive of medium-voltage motors. The high-power-density modules include newly added high-frequency resonant AC terminals. As shown in sub-figure (a), each modular medium-voltage frequency converter consists of a seven-winding high-frequency transformer, seven LC resonant branches, three grid-side single-phase rectifiers, three machine-side single-phase inverters, six full-bridge high-frequency converters, and six DC-side capacitors. As shown in sub-figure (b), the four high-power-density modules are connected in series to form the modular medium-voltage frequency converter. The input phases of the modular medium-voltage frequency converter are connected in series and then connected to the three-phase AC grid via inductors. The output phases of the modular medium-voltage frequency converter are connected in series and then connected to the three-phase AC motor. The four high-frequency AC resonant terminals are connected in parallel to form a common high-frequency AC bus. A single-phase full-bridge rectifier circuit is connected to the common AC bus formed by the seventh winding, and a low-voltage DC voltage source V is connected to both ends of the bridge arm. DC .

[0065] The parameters of the modular medium-voltage frequency converter designed based on the above approach are summarized in Table 1. The Matlab / Simulink simulation model constructed based on this model fully achieved the expected design goals and realized the control function.

[0066] Table 1 Specific parameters of modular medium-voltage frequency converters

[0067]

[0068] Figure 6 The figure shows the simulated waveforms of the motor operating frequency from 0Hz to the mains frequency of 60Hz. Subfigure (a) shows the motor speed ω. r Waveforms; Sub-figure (b) shows the electromagnetic torque Te waveform of the motor, and sub-figure (c) shows the three-phase grid voltage V. in Waveform, sub-figure (d) shows the three-phase grid current i in Waveform, sub-figure (e) shows the three-phase voltage V on the motor side. out Waveform, sub-figure (f) shows the three-phase current i on the motor side. a i b i c Waveform, sub-figure (g) shows the grid-side DC capacitor voltage V. cp Waveform, sub-graph (h) shows the DC capacitor voltage V on the machine side. csWaveform. At the start of the simulation, the system operates under no-load; at t = 0.5s, the load torque is 0, and the motor starts under no-load to 1189 RPM. Throughout the simulation, the three-phase fluctuating power components are mainly canceled out by the transformer magnetic circuit. The fluctuating power that the capacitor needs to bear is very small. Throughout the entire range of motor speed changes from low speed to full speed, the voltage fluctuation amplitude on the DC-side capacitor is consistently controlled within ±5%. This demonstrates that the modular medium-voltage frequency converter proposed in this invention significantly reduces the power buffered by the DC-side capacitor, thereby reducing the capacitor's capacitance. Furthermore, this scheme uses a high-frequency transformer, which reduces the overall size and weight of the core magnetoelectric components, verifying the high power density advantage of the proposed scheme.

[0069] Figure 7 The diagram shows the waveforms of the resonant voltage of phase A and the square wave voltage at the bridge port of a module in a modular medium-voltage converter. Sub-figure (a) shows the square wave voltage V at the bridge port. r The waveform is shown in sub-figure (b), which shows the current i flowing through the resonant branch. r The waveform is shown in the figure. It can be seen from the figure that the voltage and current are basically in phase. This shows that the resonant branch of the present invention is basically in a fully resonant state. The current at the turn-on and turn-off times of the switching transistor on the high-frequency converter is very small, which verifies the high efficiency advantage of the proposed scheme.

[0070] Figure 8 The system waveforms of the modular medium-voltage converter during soft start-up are shown in sub-figure (a), which shows the voltage V across the DC capacitor. c Waveform, sub-figure (b) shows the voltage pulse V output by the high-frequency converter in the low-voltage side soft-start circuit. lb Waveform, sub-figure (c) shows the current i flowing through the high-voltage side DC capacitor in the module. hb The waveform, sub-figure (d) shows the voltage pulse V output by the high-voltage side high-frequency converter. hb Waveform. The simulation starts at t=0s. The duty cycle of the voltage pulse output by the high-frequency converter on the low-voltage side increases linearly from 0%, and the voltage on the DC capacitor also gradually increases. At t=0.5s, the duty cycle of the voltage pulse output by the high-frequency converter on the low-voltage side reaches 50%, and the voltage on the DC capacitor reaches the rated voltage value. At this time, the switching transistor of the high-frequency converter on the low-voltage side is locked, and the main circuit starts. When t=0.6s, the low-voltage side circuit is disconnected from the common high-frequency AC bus. The DC capacitor voltage remains stable at the rated voltage, verifying the effectiveness of the proposed soft-start scheme.

[0071] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Any simple modifications, alterations, or equivalent structural changes made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

[0072] The above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can still make modifications or equivalent substitutions to the specific implementation schemes of the present invention, and these modifications or equivalent substitutions do not depart from the spirit and scope of the present invention, and are all within the protection scope of the claims of the present invention.

[0073] The above content provides a further detailed description of the present invention. It should not be construed that the specific embodiments of the present invention are limited to this. For those skilled in the art, several simple deductions or substitutions can be made without departing from the concept of the present invention, and all such deductions or substitutions should be considered as falling within the scope of protection of the present invention as defined by the submitted claims.

Claims

1. A control method for a modular medium-voltage frequency converter, characterized in that, The modular medium-voltage frequency converter includes: one or more high-power-density modules for motor drive connected in series or in parallel through isolation ports; The high power density module for motor drive includes three single-phase rectifiers on the grid side, (3+n) DC capacitors, (3+n) high-frequency converters, (3+n) LC resonant branches, one (3+n) winding high-frequency transformer, and n single-phase inverters on the machine side. The DC sides of the three single-phase rectifiers on the grid side are connected to the DC sides of the three high-frequency converters through three DC capacitors respectively. The AC output ports of the three high-frequency converters are connected to the three windings of the high-frequency transformer through three LC resonant branches respectively, forming the grid-side circuit. The remaining n windings of the high-frequency transformer are connected to the AC ports of the remaining n high-frequency converters through n LC resonant branches, and the DC sides of the n high-frequency converters are connected to the DC sides of the n single-phase inverters on the machine side through n DC capacitors, forming the machine side circuit. The control method for the modular medium-voltage frequency converter includes: The control method of the high-frequency converter is as follows: the same set of drive signals with the same switching frequency and resonant frequency are used to make all high-frequency converters output synchronous square wave voltage pulses, thereby realizing the automatic balancing of the DC voltage of (3+n) capacitors.

2. The control method for the modular medium-voltage frequency converter according to claim 1, characterized in that, The control method for the machine-side single-phase inverter is as follows: an arbitrary motor control method is used to generate a modulation wave, and a carrier phase-shift modulation method is used to generate the gate drive signal of the machine-side single-phase inverter in each module; The control method for the grid-side single-phase rectifier is as follows: A current control method based on DQ-axis decoupling using a synchronous rotating coordinate system is adopted. The required active power of the motor is divided by three times the effective value of the three-phase grid-side voltage to obtain the initial command value of the d-axis of the current decoupling control loop. The average voltage of the DC capacitors in all modules is taken, and the difference is calculated with the capacitor voltage command value. Through a closed-loop controller, the command value compensation amount of the d-axis in the current decoupling control loop is obtained. The initial command value and the command value compensation amount are added to obtain the d-axis command value of the current decoupling control loop. After passing through the current decoupling control loop, the three-phase voltage modulation wave of each module is output. The gate drive signal of each module's grid-side rectifier is generated using a carrier phase-shift modulation method.

3. The control method for the modular medium-voltage frequency converter according to claim 1, characterized in that, The high-frequency converter is a full-bridge circuit or a half-bridge circuit.

4. The control method for the modular medium-voltage frequency converter according to claim 1, characterized in that, The LC resonant branch is a series resonant circuit consisting of an inductor and a capacitor.

5. The control method for the modular medium-voltage frequency converter according to claim 1, characterized in that, The frequency of the high-frequency transformer is any frequency in the range of several hundred hertz to several hundred kilohertz; the turns ratio of the high-frequency transformer windings satisfies that the number of coil turns of the three windings on the grid side and the n windings on the machine side are the same.

6. The control method for the modular medium-voltage frequency converter as described in claim 1, characterized in that, The voltage coordination control of DC capacitors between high power density modules in the modular medium-voltage frequency converter includes software program control methods and hardware automatic coordination control methods. The software program control method includes: subtracting the average voltage of the DC-side capacitor in each module from the average voltage of all DC-side capacitors in the system, passing the difference through a closed-loop controller, and multiplying it by the three single-phase currents on the grid side to obtain the compensation amount of the three single-phase voltage modulation wave for each module; summing the three-phase voltage modulation wave and the modulation wave compensation amount of the grid-side single-phase rectifier of each module, and generating the gate drive signal of the grid-side single-phase rectifier of each module using a carrier phase-shift modulation method; The hardware automatic coordination control method includes: adding a winding to the high-frequency transformer in each high power density module, the winding being connected in series with an LC resonant circuit to form a high-frequency resonant AC terminal, and the high-frequency resonant AC terminals of each module being connected in parallel to form a common AC bus, so that the power spontaneous flow is balanced, thereby achieving self-balancing of the DC capacitor voltage in each module. This topology requires that the gate drive signal of all high-frequency converters must be a square wave signal with the same switching frequency and resonant frequency and the same phase.

7. A modular medium-voltage frequency converter including a soft-start section, characterized in that: This includes a modular medium-voltage frequency converter; the soft starter section is a low-voltage DC power supply connected to the common AC bus via a low-voltage side high-frequency converter; or a low-voltage AC power supply connected to the common AC bus via a low-voltage side rectifier circuit and a low-voltage side high-frequency converter. The common AC busbar is composed of a high-frequency resonant AC terminal connected in parallel, which is formed by adding a winding series LC resonant circuit to the high-frequency transformer in each module. The low-voltage side rectifier circuit is single-phase, three-phase, or multi-phase. This part operates in diode rectification or PWM rectification mode, and the corresponding low-voltage AC power supply is single-phase AC, three-phase AC, or multi-phase AC power supply. The modular medium-voltage frequency converter includes: one or more high-power-density modules for motor drive connected in series or in parallel through isolation ports; The high power density module for motor drive includes three single-phase rectifiers on the grid side, (3+n) DC capacitors, (3+n) high-frequency converters, (3+n) LC resonant branches, one (3+n) winding high-frequency transformer, and n single-phase inverters on the machine side. The DC sides of the three single-phase rectifiers on the grid side are connected to the DC sides of the three high-frequency converters through three DC capacitors respectively. The AC output ports of the three high-frequency converters are connected to the three windings of the high-frequency transformer through three LC resonant branches respectively, forming the grid-side circuit. The remaining n windings of the high-frequency transformer are connected to the AC ports of the remaining n high-frequency converters through n LC resonant branches. The DC sides of the n high-frequency converters are connected to the DC sides of the n single-phase inverters on the machine side through n DC capacitors, forming the machine side circuit.

8. A soft-start method for a modular medium-voltage frequency converter as described in claim 7, characterized in that, include: 1) During the pre-charging stage of the capacitor, the switching transistors in each power module are in a locked state. The low-voltage side high-frequency converter outputs voltage pulses with a duty cycle that increases linearly from 0% to 50%, which are supplied to the common AC bus. The common AC bus charges the DC capacitor through the multi-winding transformers in each power module and the anti-parallel diodes of the switching devices in the high-frequency converter. 2) After the capacitor is precharged to the rated voltage, the grid-side circuit is closed, the high-frequency converters in each power module start working, and the grid-side single-phase rectifier is closed-loop controlled. Then the motor side is closed, and finally the motor-side single-phase inverter is closed-loop controlled, and the circuit enters the normal operation mode. In the normal operation mode, the low-voltage high-frequency converter can continue to operate synchronously to realize the automatic charging of the DC battery, or enter the lockout mode.

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