CN112054690B - Control method of direct current transformer - Google Patents

Control method of direct current transformer Download PDF

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CN112054690B
CN112054690B CN202010915410.4A CN202010915410A CN112054690B CN 112054690 B CN112054690 B CN 112054690B CN 202010915410 A CN202010915410 A CN 202010915410A CN 112054690 B CN112054690 B CN 112054690B
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voltage side
voltage
low
current transformer
direct
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CN112054690A (en
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张航
李子欣
高范强
徐飞
赵聪
王平
李耀华
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Institute of Electrical Engineering of CAS
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Institute of Electrical Engineering of CAS
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
    • H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
    • H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
    • H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/33569—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements
    • H02M3/33576—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements having at least one active switching element at the secondary side of an isolation transformer
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
    • H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
    • H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
    • H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/33507—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of the output voltage or current, e.g. flyback converters
    • H02M3/33523—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of the output voltage or current, e.g. flyback converters with galvanic isolation between input and output of both the power stage and the feedback loop
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0048—Circuits or arrangements for reducing losses
    • H02M1/0054—Transistor switching losses
    • H02M1/0058—Transistor switching losses by employing soft switching techniques, i.e. commutation of transistors when applied voltage is zero or when current flow is zero
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0083—Converters characterised by their input or output configuration
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B70/00—Technologies for an efficient end-user side electric power management and consumption
    • Y02B70/10—Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Dc-Dc Converters (AREA)

Abstract

A control method of a direct current transformer is characterized in that the direct current transformer is formed by connecting input and output of a multi-unit isolation type series resonance double-active-bridge converter in parallel. In the operation process, partial power modules in the direct-current transformer adopt an intermittent 50% duty ratio square wave voltage open-loop control mode, and partial power modules adopt a variable-frequency phase-shifting voltage closed-loop control mode. Compared with the prior art, the invention can effectively improve the electric energy transmission efficiency of the system while ensuring the flexible regulation and control of the voltage/power of the direct-current transformer.

Description

Control method of direct current transformer
Technical Field
The invention relates to a control method of a direct current transformer.
Background
In recent years, the appearance of various distributed energy sources, energy storage devices and direct current loads has prompted the diversified development of the traditional alternating current power distribution system. Compared with the traditional alternating current power distribution system, the energy interaction between renewable energy sources and direct current loads can be directly realized by constructing the direct current power distribution network, so that a large number of electric energy conversion links are saved, the cost is reduced, the loss is reduced, and the electric energy transmission efficiency is improved. In addition, the direct-current power distribution network has the advantages of larger power supply capacity, longer power supply radius, unobvious power quality problem, no reactive compensation problem and the like.
In a direct current distribution system, a direct current transformer is used as key equipment for connecting a medium-low voltage bus, a power electronic converter and a high-frequency transformer are integrated inside the direct current distribution system, and functions of direct current voltage conversion, electrical isolation, power control and the like can be realized. In addition, different from the traditional alternating current transformer, the direct current transformer also has the functions of automatic protection, fault self-isolation and the like.
The direct current transformer for medium-voltage 10 kV-level power distribution application is limited by the withstand voltage level of a power semiconductor, and generally comprises a plurality of power modules, wherein the modules are connected in parallel according to a high-voltage side cascade low-voltage side. Currently common types of power modules include phase-shifted dual active bridge converters and series resonant dual active bridge converters. Compared with a phase-shifting double-active-bridge converter, the series resonance double-active-bridge converter can effectively isolate direct-current components due to the fact that the resonance capacitor is connected in series in a high-frequency link, and the direct-current magnetic bias phenomenon of the high-frequency transformer cannot occur.
At present, the common control modes for the series resonance dual-active bridge type dc transformer mainly include a synchronous 50% duty cycle square wave voltage open-loop control mode and a variable frequency phase-shift voltage closed-loop control mode. Under the open-loop control mode, the high-voltage side H-bridge converter and the low-voltage side H-bridge converter output same-frequency same-phase 50% duty ratio square wave voltage, voltage difference of the high-voltage side and the low-voltage side is acted on an internal LC resonance network due to loss of internal circuits of the modules, so that same-frequency same-phase sinusoidal current is generated, high-frequency current is zero when the polarity of the square wave voltage is reversed, and zero-current soft switching is achieved. For the open-loop control mode, although the internal switching devices can realize soft switching operation, in the control mode, the system operation efficiency is relatively high, but the voltage and the transmission power of the power module are not controllable. Under the frequency conversion phase-shift voltage closed-loop control mode, although the controllability of the system is improved, the current of internal devices is nonzero when the internal devices are switched off, loss still exists in the switching-off process, and the soft switching operation of all the devices cannot be realized.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a control method of a direct current transformer. The control method is mainly suitable for the input-series output-parallel direct current transformer with multiple power modules, and each power module adopts a series resonance double-active-bridge converter. In the operation process, partial power modules in the direct-current transformer adopt an intermittent 50% duty ratio square wave voltage open-loop control mode, and partial power modules adopt a variable-frequency phase-shifting voltage closed-loop control mode. Compared with the prior art, the invention can effectively improve the electric energy transmission efficiency of the system while ensuring the flexible regulation and control of the voltage/power of the direct-current transformer.
The direct-current transformer is composed of N series resonance type double-active-bridge converters, and the value range of N is 3-15; each series resonance type double-active-bridge converter is used as a power module of the direct-current transformer; in the direct current transformer, N direct current transformer power modules have the same structure and are connected with each otherHas the same structure, and each power module high-voltage side energy storage capacitor CH1The capacitance values are the same, and the low-voltage side energy storage capacitors C of all the power modulesL1High frequency transformer T with same capacitance value for each power moduleFH1The voltage transformation ratio, the leakage inductance and the magnetic core material are the same. Each DC transformer power module is composed of a high-voltage side energy storage capacitor CH1High-voltage side H-bridge unit and high-voltage side resonant capacitor Cr1High frequency transformer TFH1Low voltage side resonant capacitor Cr2A low-voltage side H-bridge unit and a low-voltage side energy storage capacitor CL1And (4) forming. High-voltage side H bridge unit and high-voltage side direct current energy storage unit CH1Parallel connection of low-voltage side H bridge unit and low-voltage side energy storage unit CL1Parallel connection of terminal q of high-voltage side H-bridge unit and high-voltage side resonant capacitor Cr1Is connected with the positive electrode of the low-voltage side H-bridge unit, and the terminal w of the low-voltage side H-bridge unit and the low-voltage side resonance capacitor Cr2Is connected with the negative pole of the high-frequency transformer TFH1High-voltage side upper end s and high-voltage side resonance capacitor Cr1Is connected with the negative pole of the high-frequency transformer TFH1Is connected with a terminal r of the high-voltage side H bridge unit, and a high-frequency transformer TFH1Low voltage side upper end u and low voltage side resonance capacitor Cr2Is connected with the positive pole of the high-frequency transformer TFH1Is connected with a terminal x of the low-voltage side H bridge unit; meanwhile, two ends of the high-voltage side H-bridge unit are respectively connected with a high-voltage side energy storage capacitor C of the power module of the direct-current transformerH1The two ends of the low-voltage side H bridge unit are respectively connected with a low-voltage side energy storage capacitor C of the power module of the direct-current transformerL1Positive terminal y and negative terminal z; each power module of the direct current transformer is connected in series at the high-voltage direct current side and connected in parallel at the low-voltage direct current side; energy storage capacitor C at high-voltage side of each direct-current transformer power moduleH1The positive terminal a of the capacitor is connected to the high-voltage side energy storage capacitor C of the adjacent direct current transformerH1And a negative electrode terminal b. Energy storage capacitor C at low-voltage side of each direct-current transformer power moduleL1The positive terminals y are connected to the energy storage capacitor C at the low voltage side of the adjacent direct current transformer power moduleL1The positive terminal y, and the energy storage capacitor C at the low-voltage side of each direct-current transformer power moduleL1The negative terminals z are all connected to the energy storage capacitor C at the low voltage side of the adjacent direct current transformer power moduleL1The negative terminal z of (a); in the direct current transformer, the high-voltage side energy storage capacitor C of the power module of the first direct current transformerH1Is connected to the first dc port positive terminal P1Energy storage capacitor C at high-voltage side of power module of Nth direct-current transformerH1Is connected to the first dc port negative terminal N1(ii) a Low-voltage side energy storage capacitor C of first direct current transformer power moduleL1Is connected to the positive terminal P of the second dc port2The low-voltage side energy storage capacitor C of the first DC transformer power moduleL1Is connected to the positive terminal N of the second dc port2。
In the running process of the direct-current transformer, first to Mth power modules in the direct-current transformer adopt an intermittent 50% duty ratio square wave voltage open-loop control mode, and M is smaller than the number N of the power modules of the direct-current transformer when taking values; in order to avoid the backflow of the transmission power, the first to Mth power modules need to lock the switching device of the low-voltage side H-bridge converter when the power flows from the high-voltage side to the low-voltage side, and the output working frequency of the high-voltage side converter is fs150% duty cycle square wave voltage; when power flows from the low-voltage side to the high-voltage side, the switching device of the high-voltage side H-bridge converter is locked, and the output working frequency of the low-voltage side H-bridge converter is fs150% duty cycle square wave voltage; operating frequency f of the switching devices1Operating frequency f of LC resonance networkr1Is not the same, and satisfies fs1＝fr1/(1+2fr1TD) Wherein T isDIs the converter dead time; in the control mode, the current flowing into the high-frequency transformer is interrupted in each open loop period for 2TD(ii) a The M +1 th to the Nth power modules in the direct-current transformer adopt a frequency conversion phase-shifting voltage closed-loop control mode, and in the control mode, phase differences phi exist at the output ends of high-low voltage side H-bridge converters in the M +1 th to the Nth power modules, and the frequencies are fs2The phase difference phi of the square wave voltage with the 50 percent duty ratio is controlled by the traditional low-voltage side capacitor voltage closed loopAnd (4) obtaining. When energy is transferred from the high-voltage side to the low-voltage side, the high-voltage side H-bridge converter outputs a 50% duty cycle square wave voltage phase ahead of the low-voltage side H-bridge converter outputs a 50% duty cycle square wave voltage phase; when power flows from the low-voltage side to the high-voltage side, the phase of the square wave voltage with the duty ratio of 50% output by the high-voltage side H-bridge converter lags behind the phase of the square wave voltage with the duty ratio of 50% output by the low-voltage side H-bridge converter; in addition, the operating frequency f of the switching devices2Operating frequency f of LC resonance networkr1Different and the operating frequency f of the switching devices2Higher than the LC resonant network operating frequency fr1。
When the direct current transformer adopts the control method of the invention, the value of M is adjusted according to the actual operation condition of the direct current transformer: when the controlled low voltage DC side voltage uoVoltage transformation ratio k of high-frequency transformer in power moduletfThe number N of power modules of the direct-current transformer and the voltage u on the high-voltage direct-current sideISatisfy uI/(Nktfuo) When the number of the power modules is not equal to 1, M is equal to fix [0.5N ] between M and the number N of the power modules of the direct-current transformer]Wherein fix [ x]Is a rounded down function; when the controlled low voltage DC side voltage uoVoltage transformation ratio k of high-frequency transformer in power moduletfThe number N of power modules of the direct-current transformer and the voltage u on the high-voltage direct-current sideISatisfy uI/NktfuoWhen 1, M is selected from the range of fix [0.5N ]]M is not more than M and is less than N, and M is determined by combining an off-line simulation result during adjustment, and the optimal M value is determined by ensuring that when the direct current transformer works at rated power, the phase shift angle of 50% duty ratio square wave voltage output by H bridges at the high-voltage side and the low-voltage side of the rest N-M power modules is 30 degrees.
Compared with the traditional open-loop or phase-shift closed-loop control strategy, the control strategy of the direct-current transformer provided by the invention can realize that all switching devices in part of power modules work in zero-current soft switching within the allowable range of the economic cost of the system, thereby effectively improving the electric energy transmission efficiency of the system and ensuring the flexibility of voltage and power regulation.
Drawings
Fig. 1 is a schematic structural diagram of a dc transformer according to the present invention.
Detailed Description
The invention is further described below with reference to the accompanying drawings and the detailed description.
The dc transformer of the present invention is shown in fig. 1. The direct-current transformer is composed of N series resonance type double-active-bridge converters, and the value range of N is 3-15; each series resonance type double-active-bridge converter is used as a power module of the direct-current transformer; in the direct current transformer, the structures of N direct current transformer power modules are the same, and the high-voltage side energy storage capacitor C of each power moduleH1The capacitance values are the same, and the low-voltage side energy storage capacitors C of all the power modulesL1High frequency transformer T with same capacitance value for each power moduleFH1The voltage transformation ratio, the leakage inductance and the magnetic core material are the same. Each DC transformer power module is composed of a high-voltage side energy storage capacitor CH1High-voltage side H-bridge unit and high-voltage side resonant capacitor Cr1High frequency transformer TFH1Low voltage side resonant capacitor Cr2A low-voltage side H-bridge unit and a low-voltage side energy storage capacitor CL1And (4) forming. High-voltage side H bridge unit and high-voltage side direct current energy storage unit CH1Parallel connection of low-voltage side H bridge unit and low-voltage side energy storage unit CL1Parallel connection of terminal q of high-voltage side H-bridge unit and high-voltage side resonant capacitor Cr1Is connected with the positive electrode of the low-voltage side H-bridge unit, and the terminal w of the low-voltage side H-bridge unit and the low-voltage side resonance capacitor Cr2Is connected with the negative pole of the high-frequency transformer TFH1High-voltage side upper end s and high-voltage side resonance capacitor Cr1Is connected with the negative pole of the high-frequency transformer TFH1Is connected with a terminal r of the high-voltage side H bridge unit, and a high-frequency transformer TFH1Low voltage side upper end u and low voltage side resonance capacitor Cr2Is connected with the positive pole of the high-frequency transformer TFH1Is connected with a terminal x of the low-voltage side H bridge unit; meanwhile, two ends of the high-voltage side H-bridge unit are respectively connected with a high-voltage side energy storage capacitor C of the power module of the direct-current transformerH1The two ends of the low-voltage side H bridge unit are respectively connected with a low-voltage side energy storage capacitor C of the power module of the direct-current transformerL1Positive terminal y and negative terminalz; each power module of the direct current transformer is connected in series at the high-voltage direct current side and connected in parallel at the low-voltage direct current side; energy storage capacitor C at high-voltage side of each direct-current transformer power moduleH1The positive terminal a of the capacitor is connected to the high-voltage side energy storage capacitor C of the adjacent direct current transformerH1And a negative electrode terminal b. Energy storage capacitor C at low-voltage side of each direct-current transformer power moduleL1The positive terminals y are connected to the energy storage capacitor C at the low voltage side of the adjacent direct current transformer power moduleL1The positive terminal y, and the energy storage capacitor C at the low-voltage side of each direct-current transformer power moduleL1The negative terminals z are all connected to the energy storage capacitor C at the low voltage side of the adjacent direct current transformer power moduleL1The negative terminal z of (a); in the direct current transformer, the high-voltage side energy storage capacitor C of the power module of the first direct current transformerH1Is connected to the first dc port positive terminal P1Energy storage capacitor C at high-voltage side of power module of Nth direct-current transformerH1Is connected to the first dc port negative terminal N1(ii) a Low-voltage side energy storage capacitor C of first direct current transformer power moduleL1Is connected to the positive terminal P of the second dc port2The low-voltage side energy storage capacitor C of the first DC transformer power moduleL1Is connected to the positive terminal N of the second dc port2。
In the running process of the direct-current transformer, first to Mth power modules in the direct-current transformer adopt an intermittent 50% duty ratio square wave voltage open-loop control mode, and M is smaller than the number N of the power modules of the direct-current transformer when taking values; in order to avoid the backflow of the transmission power, the first to Mth power modules need to lock the switching device of the low-voltage side H-bridge converter when the power flows from the high-voltage side to the low-voltage side, and the output working frequency of the high-voltage side converter is fs150% duty cycle square wave voltage; when power flows from the low-voltage side to the high-voltage side, the switching device of the high-voltage side H-bridge converter is locked, and the output working frequency of the low-voltage side H-bridge converter is fs150% duty cycle square wave voltage; operating frequency f of the switching devices1Operating frequency f of LC resonance networkr1Is not the same, and satisfies fs1＝fr1/(1+2fr1TD) Wherein T isDIs the converter dead time; in the control mode, the current flowing into the high-frequency transformer is interrupted in each open loop period for 2TD(ii) a The M +1 th to the Nth power modules in the direct-current transformer adopt a frequency conversion phase-shifting voltage closed-loop control mode, and in the control mode, phase differences phi exist at the output ends of high-low voltage side H-bridge converters in the M +1 th to the Nth power modules, and the frequencies are fs2The phase difference phi of the square wave voltage with the duty ratio of 50 percent is obtained through the traditional low-voltage side capacitor voltage closed-loop control. When energy is transferred from the high-voltage side to the low-voltage side, the high-voltage side H-bridge converter outputs a 50% duty cycle square wave voltage phase ahead of the low-voltage side H-bridge converter outputs a 50% duty cycle square wave voltage phase; when power flows from the low-voltage side to the high-voltage side, the phase of the square wave voltage with the duty ratio of 50% output by the high-voltage side H-bridge converter lags behind the phase of the square wave voltage with the duty ratio of 50% output by the low-voltage side H-bridge converter; in addition, the operating frequency f of the switching devices2Operating frequency f of LC resonance networkr1Different and the operating frequency f of the switching devices2Higher than the LC resonant network operating frequency fr1。
When the direct current transformer adopts the control method of the invention, the value of M is adjusted according to the actual operation condition of the direct current transformer: when the controlled low voltage DC side voltage uoVoltage transformation ratio k of high-frequency transformer in power moduletfThe number N of power modules of the direct-current transformer and the voltage u on the high-voltage direct-current sideISatisfy uI/(Nktfuo) When the number of the power modules is not equal to 1, M is equal to fix [0.5N ] between M and the number N of the power modules of the direct-current transformer]Wherein fix [ x]Is a rounded down function; when the controlled low voltage DC side voltage uoVoltage transformation ratio k of high-frequency transformer in power moduletfThe number N of power modules of the direct-current transformer and the voltage u on the high-voltage direct-current sideISatisfy uI/NktfuoWhen 1, M is selected from the range of fix [0.5N ]]M is less than or equal to N, and M needs to be determined by combining an off-line simulation result during adjustment, and the optimal M value is satisfied when the DC transformer works at rated power, and the sum of the high-voltage sides of the rest N-M power modulesThe phase shift angle of the 50% duty ratio square wave voltage output by the H bridge at the low-voltage side is 30 degrees.

Claims (1)

1. A control method of a direct current transformer is characterized in that the direct current transformer is composed of N series resonance type double-active-bridge converters, and the value range of N is 3-15; each series resonance type double-active-bridge converter is used as a power module of the direct-current transformer; in the direct current transformer, N direct current transformer power modules have the same structure, and each power module high-voltage side energy storage capacitor CH1The capacitance values are the same, and the low-voltage side energy storage capacitors C of all the power modulesL1High frequency transformer T with same capacitance value for each power moduleFH1The voltage transformation ratio, the leakage inductance and the magnetic core material are the same; each DC transformer power module is composed of a high-voltage side energy storage capacitor CH1High-voltage side H-bridge unit and high-voltage side resonant capacitor Cr1High frequency transformer TFH1Low voltage side resonant capacitor Cr2A low-voltage side H-bridge unit and a low-voltage side energy storage capacitor CL1Composition is carried out; high-voltage side H bridge unit and high-voltage side direct-current energy storage capacitor CH1Parallel connection of low-voltage side H-bridge unit and low-voltage side energy-storage capacitor CL1Parallel connection of terminal q of high-voltage side H-bridge unit and high-voltage side resonant capacitor Cr1Is connected with the positive electrode of the low-voltage side H-bridge unit, and the terminal w of the low-voltage side H-bridge unit and the low-voltage side resonance capacitor Cr2Is connected with the negative pole of the high-frequency transformer TFH1High-voltage side upper end s and high-voltage side resonance capacitor Cr1Is connected with the negative pole of the high-frequency transformer TFH1Is connected with a terminal r of the high-voltage side H bridge unit, and a high-frequency transformer TFH1Low voltage side upper end u and low voltage side resonance capacitor Cr2Is connected with the positive pole of the high-frequency transformer TFH1Is connected with a terminal x of the low-voltage side H bridge unit; meanwhile, two ends of the high-voltage side H-bridge unit are respectively connected with a high-voltage side energy storage capacitor C of the power module of the direct-current transformerH1The two ends of the low-voltage side H bridge unit are respectively connected with a low-voltage side energy storage capacitor C of the power module of the direct-current transformerL1Positive terminal y and negative terminal z; the power modules of the DC transformer are connected in series at the high-voltage DC side and at the low voltageThe DC voltage sides are connected in parallel; energy storage capacitor C at high-voltage side of each direct-current transformer power moduleH1Is connected to the high-voltage side energy storage capacitor C of the adjacent direct current transformerH1Negative electrode terminal p of (1); energy storage capacitor C at low-voltage side of each direct-current transformer power moduleL1The positive terminals y are connected to the energy storage capacitor C at the low voltage side of the adjacent direct current transformer power moduleL1The positive terminal y, and the energy storage capacitor C at the low-voltage side of each direct-current transformer power moduleL1The negative terminals z are all connected to the energy storage capacitor C at the low voltage side of the adjacent direct current transformer power moduleL1The negative terminal z of (a); in the direct current transformer, the high-voltage side energy storage capacitor C of the power module of the first direct current transformerH1Is connected to the first dc port positive terminal P1Energy storage capacitor C at high-voltage side of power module of Nth direct-current transformerH1Is connected to the first dc port negative terminal N1(ii) a Low-voltage side energy storage capacitor C of first direct current transformer power moduleL1Is connected to the positive terminal P of the second dc port2The low-voltage side energy storage capacitor C of the first DC transformer power moduleL1Is connected to the negative terminal N of the second dc port2，
The method is characterized in that in the running process of the direct-current transformer, first to Mth power modules in the direct-current transformer adopt an intermittent 50% duty ratio square wave voltage open-loop control mode, and M is smaller than the number N of the power modules of the direct-current transformer when taking values; in order to avoid the backflow of the transmission power, the first to Mth power modules need to lock the switching device of the low-voltage side H-bridge converter when the power flows from the high-voltage side to the low-voltage side, and the output working frequency of the high-voltage side converter isf s150% duty cycle square wave voltage; when power flows from the low-voltage side to the high-voltage side, the switching device of the high-voltage side H-bridge converter is locked, and the output working frequency of the low-voltage side H-bridge converter isf s150% duty cycle square wave voltage; operating frequency of switching devicef s1Operating frequency of LC resonance networkf r1Are not identical and satisfyf s1=f r1/(1+2f r1 T D) WhereinT DIs the converter dead time; in the control mode, the current flowing into the high-frequency transformer is interrupted in each open loop period for an interruption time of 2T D(ii) a The M +1 th to the Nth power modules in the direct-current transformer adopt a frequency conversion phase-shifting voltage closed-loop control mode, and in the control mode, phase differences phi exist at the output ends of high-low voltage side H-bridge converters in the M +1 th to the Nth power modules, and the frequencies are all the samef s2The phase difference phi of the square wave voltage with the duty ratio of 50 percent is obtained by the closed-loop control of the traditional low-voltage side capacitor voltage; when energy is transferred from the high-voltage side to the low-voltage side, the high-voltage side H-bridge converter outputs a 50% duty cycle square wave voltage phase ahead of the low-voltage side H-bridge converter outputs a 50% duty cycle square wave voltage phase; when power flows from the low-voltage side to the high-voltage side, the phase of the square wave voltage with the duty ratio of 50% output by the high-voltage side H-bridge converter lags behind the phase of the square wave voltage with the duty ratio of 50% output by the low-voltage side H-bridge converter; in addition, the operating frequency of the switching devicef s2Operating frequency of LC resonance networkf r1Different and operating frequency of the switching devicef s2Higher than the LC resonance network operating frequencyf r1；
The value of M is adjusted according to the actual operation condition of the direct current transformer: when the controlled low voltage DC side voltageu oVoltage transformation ratio k of high-frequency transformer in power moduletfThe number N of power modules of the direct current transformer and the voltage on the high voltage direct current sideu ISatisfyu I/(Nktf u o) When the number of the power modules is not equal to 1, M = fix [0.5N ] is satisfied between M and the number N of the power modules of the direct-current transformer]Wherein fix [ x]Is a rounded down function; when the controlled low voltage DC side voltageu oVoltage transformation ratio k of high-frequency transformer in power moduletfThe number N of power modules of the direct current transformer and the voltage on the high voltage direct current sideu ISatisfyu I/Nktf u oWhen =1, the value range of M is fix [0.5N ]]M is less than or equal to N, and M needs to be determined by combining an off-line simulation result during adjustment, and the optimal M value is satisfied when the direct current transformer works at rated power and the rest N-M power modules are highThe phase shift angle of the square wave voltage with 50 percent duty ratio output by the H bridge on the voltage side and the low voltage side is 30 degrees.
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