CN109728720B - Bipolar DC/DC converter for flexible direct current transmission and control method thereof - Google Patents

Abstract

A bipolar DC/DC converter for flexible direct current transmission and a control method thereof belong to the field of flexible direct current transmission and solve the problems of more sub-modules and higher cost of the existing hybrid modular DC/DC converter. A converter: for the phases a and b, four IGBT valve banks are connected in series between the high-voltage side anode and the low-voltage side cathode, the common ends of the valve banks 2 and 3 are connected with the low-voltage side anode, the current input end of the half-bridge submodule string is connected into the common ends of the valve banks 1 and 2, and the current output end is connected into the common ends of the valve banks 3 and 4. Unlike the first two phases, for phases c and d, four valve banks are connected in series between the low-side positive electrode and the high-side negative electrode. The method comprises the following steps: and setting the on-off state of the IGBT valve bank of each phase according to the target power transmission direction, and controlling the charging and discharging states of the half-bridge sub-module string of each phase to realize the power transmission in the target direction. The four phases work alternately to ensure the continuous output of the converter.

Description

Bipolar DC/DC converter for flexible direct current transmission and control method thereof

Technical Field

The invention relates to a DC/DC converter and a control method thereof, belonging to the field of flexible direct current transmission.

Background

The flexible direct current transmission technology is rapidly developed in the past decades, and dozens of flexible direct current transmission projects are put into operation worldwide. In the future, direct-current transmission systems with various voltage levels are necessarily connected with each other to form a direct-current power grid in the true sense. However, the problem of voltage level conversion in a direct current power grid must rely on high voltage, high capacity DC/DC converters based on power electronics technology. In order to realize high-voltage and large-capacity dc conversion at the withstand voltage level of the conventional semiconductor power device, a modularized technical route is required.

For the modular DC/DC converter, many reports have appeared in recent years, of which the "face-to-face" type modular multilevel DC/DC converter is the most representative. However, such DC/DC converters belong to an isolated DC/DC converter with an intermediate ac link, and since all power needs to be converted in two stages, such DC/DC converters have large loss and high cost.

In order to reduce loss and cost, various non-isolated DC/DC converters are proposed:

the modular multilevel direct current converter directly outputs direct current voltage in a modular multilevel direct current conversion mode, only one-stage power conversion is needed, and an alternating current isolation transformer is omitted. However, in the bridge arm of such DC/DC converter, high amplitude ac voltage and circulating current need to be injected to maintain the energy balance of the sub-module capacitor voltage, which results in large stress and loss of the semiconductor power device, and a large-capacity filter inductor needs to be configured at the low-voltage side to filter out ac components.

Chinese patent No. CN 103887788B discloses a multi-port DC-DC autotransformer and its application, which belongs to an autotransformer type modular non-isolated DC/DC converter, and because its ac link is only used to transmit balanced power, the efficiency of the multi-port DC-DC autotransformer is effectively improved. However, the multi-port dc-dc autotransformer still cannot avoid using an ac transformer.

Chinese patent application publication No. CN 106160463 a discloses a DC voltage conversion device and a bridge arm control method thereof, and the DC voltage conversion device belongs to a hybrid modular DC/DC converter, which breakthroughs the combination of a device series structure and a sub-module string, thereby realizing DC voltage conversion without an ac transformer, effectively reducing the equipment cost and volume, and greatly improving the efficiency. However, the hybrid modular DC/DC converter is a unipolar converter, and in order to reduce the insulation requirement, the bipolar DC transmission system is adopted in the actual flexible DC transmission project. Therefore, when the hybrid modular DC/DC converter is applied to practical engineering, one converter needs to be respectively configured on the positive electrode and the negative electrode, which causes a large number of sub-modules of the DC/DC converter, and thus certain waste exists in cost.

Disclosure of Invention

The invention provides a bipolar DC/DC converter for flexible direct current transmission and a control method thereof, aiming at solving the problems of more sub-modules and higher cost of the existing hybrid modular DC/DC converter.

The bipolar DC/DC converter for flexible direct current transmission comprises a-phase DC/DC converter topological structure-a-phase DC/DC converter topological structure with the same circuit structure;

the topology structure of the a-phase DC/DC converter comprises a half-bridge submodule string SM_aIGBT valve set S_a1IGBT valve set S_a4And an inductance L_a；

The topology structure of the b-phase DC/DC converter comprises a half-bridge submodule string SM_bIGBT valve set S_b1IGBT valve set S_b4And an inductance L_b；

The topology structure of the c-phase DC/DC converter comprises a half-bridge submodule string SM_cIGBT valve set S_c1IGBT valve set S_c4And an inductance L_c；

The topology structure of the d-phase DC/DC converter comprises a half-bridge submodule string SM_dIGBT valve set S_d1IGBT valve set S_d4And an inductance L_d；

IGBT valve set S_a1IGBT valve set S_a4And IGBT valve set S_b1IGBT valve set S_b4Are all connected in series between the positive pole of the direct current high voltage side and the negative pole of the direct current low voltage side in sequence, and an IGBT valve set S_a2And IGBT valve set S_a3Common terminal and IGBT valve set S_b2And IGBT valve set S_b3The common ends of the two ends are connected with the positive electrode of the direct current low-voltage side;

half-bridge submodule string SM_aThe current input end is connected with an IGBT valve set S_a1And IGBT valve set S_a2The half-bridge submodule string SM_aCurrent output terminal of the inductor L_aAccess IGBT valve set S_a3And IGBT valve set S_a4A common terminal of (a);

half-bridge submodule string SM_bThe current input end is connected with an IGBT valve set S_b1And IGBT valve set S_b2Is disclosedCommon-end, half-bridge sub-module string SM_bCurrent output terminal of the inductor L_bAccess IGBT valve set S_b3And IGBT valve set S_b4A common terminal of (a);

IGBT valve set S_c1IGBT valve set S_c4And IGBT valve set S_d1IGBT valve set S_d4Are all connected in series between the positive pole of the direct current low voltage side and the negative pole of the direct current high voltage side in sequence, and an IGBT valve set S_c2And IGBT valve set S_c3Common terminal and IGBT valve set S_d2And IGBT valve set S_d3The common ends of the two ends are connected with the negative electrode of the direct current low-voltage side;

half-bridge submodule string SM_cCurrent input terminal of via inductor L_cAccess IGBT valve set S_c1And IGBT valve set S_c2The half-bridge submodule string SM_cThe current output end is connected with an IGBT valve set S_c3And IGBT valve set S_c4A common terminal of (a);

half-bridge submodule string SM_dCurrent input terminal of via inductor L_dAccess IGBT valve set S_d1And IGBT valve set S_d2The half-bridge submodule string SM_dThe current output end is connected with an IGBT valve set S_d3And IGBT valve set S_d4A common terminal of (a);

for each phase of DC/DC converter topological structure, a plurality of half-bridge sub-modules contained in a half-bridge sub-module string are enabled to be synchronously charged or discharged according to respective corresponding half-bridge sub-module driving signals and four IGBT valve groups according to respective corresponding IGBT valve group driving signals, so that the balance of self energy is maintained, and further the power transmission between a direct-current low-voltage side and a direct-current high-voltage side is realized;

the topological structures of the a-phase DC/DC converter and the d-phase DC/DC converter work cooperatively to ensure the continuous output of the bipolar DC/DC converter.

Preferably, the half-bridge sub-module string SM_aComprising half-bridge submodules SM having the same circuit configuration_a1Half-bridge submodule SM_aNFor half bridge sub-module SM_a1Half-bridge submodule SM_aNThe current output end of the half-bridge submodule SM is connected with the current input end of the second half-bridge submodule in sequence_a1Current input terminal and SM_aNThe current output ends of the half-bridge sub-module strings are respectively a half-bridge sub-module string SM_aAnd a current input terminal and a current output terminal.

Preferably, the half-bridge submodule SM_a1Comprising an IGBT switching tube S₁IGBT switch tube S₂And a capacitance C;

IGBT switch tube S₁And IGBT switching tube S₂Series connection, capacitor C and series connection IGBT switch tube S₁And IGBT switching tube S₂Parallel connection;

IGBT switch tube S₁And IGBT switching tube S₂The common terminal is a half-bridge submodule SM_a1Current input terminal of, IGBT switching tube S₂The common terminal of the capacitor C is a half-bridge submodule SM_a1The current output terminal of (2).

Preferably, the IGBT valve block S_a1IGBT valve set S_a4All include a plurality of IGBT switch tubes that the quantity is the same, a plurality of IGBT switch tubes are established ties.

The control method for the bipolar DC/DC converter for flexible direct current transmission has the target object of each-phase DC/DC converter topological structure;

the control method comprises the steps of setting the on-off state of each IGBT valve group of each phase of DC/DC converter topological structure according to a target power transmission direction, and controlling the charging and discharging states of a plurality of half-bridge sub-modules of each phase of DC/DC converter topological structure to realize power transmission in the target direction;

the four-phase DC/DC converter topological structure works alternately to ensure the continuous output of the bipolar DC/DC converter.

Preferably, the control method sets the on-off state of each IGBT valve group of each phase DC/DC converter topology according to the target power transmission direction in a specific manner that:

when power is transmitted from the direct current high-voltage side to the direct current low-voltage side:

a-phase DC/DC converter topology: IGBT valve set S_a1And IGBT valve set S_a3When the other two IGBT valve banks are turned off, the half-bridge submodule string SM_aCharging; IGBT valve set S_a1And IGBT valve set S_a3When the other two IGBT valve banks are switched on, the half-bridge submodule string SM_aDischarging;

b-phase DC/DC converter topology: IGBT valve set S_b1And IGBT valve set S_b3When the other two IGBT valve banks are turned off, the half-bridge submodule string SM_bCharging; IGBT valve set S_b1And IGBT valve set S_b3When the other two IGBT valve banks are switched on, the half-bridge submodule string SM_bDischarging;

c-phase DC/DC converter topology: IGBT valve set S_c2And IGBT valve set S_c4When the other two IGBT valve banks are turned off, the half-bridge submodule string SM_cCharging; IGBT valve set S_c2And IGBT valve set S_c4When the other two IGBT valve banks are switched on, the half-bridge submodule string SM_cDischarging;

d-phase DC/DC converter topology: IGBT valve set S_d2And IGBT valve set S_d4When the other two IGBT valve banks are turned off, the half-bridge submodule string SM_dCharging; IGBT valve set S_d2And IGBT valve set S_d4When the other two IGBT valve banks are switched on, the half-bridge submodule string SM_dDischarging;

when power is transmitted from the direct-current low-voltage side to the direct-current high-voltage side, the control mode of the IGBT valve group of each phase of DC/DC converter topological structure is opposite to the control mode when the power is transmitted from the direct-current high-voltage side to the direct-current low-voltage side.

Preferably, the specific way of controlling the charging and discharging states of the plurality of half-bridge sub-modules of the topology structure of each phase DC/DC converter by the control method is as follows:

generating a direct current high-voltage side current reference signal i by adopting a constant direct current power control mode_{H_ref}；

Reference signal i for adjusting difference between direct current high-voltage side current and direct current low-voltage side current by adopting capacitance energy balance control mode_{L-H_ref}Making the average value U of the capacitor voltage of the half-bridge sub-module string_{C_avg}Equal to a given reference capacitor voltage average value U_{C_ref}；

Direct current high-voltage side current reference signal i_{H_ref}Reference to difference between DC high and low side currentsExamination signal i_{L-H_ref}Overlapping, and using the overlapped signal as a half-bridge submodule string current reference signal i_{P_ref}；

Current reference signal i of half-bridge submodule string_{P_ref}With actual current signal i of half-bridge submodule string_PThe difference value of (a) is input into a proportional-integral regulator;

half-bridge submodule string voltage control signal u based on proportional-integral regulator output_{P_ref}And obtaining a driving signal of each half-bridge submodule in the half-bridge submodule string by adopting a half-bridge submodule capacitor voltage balance control and modulation mode.

According to the bipolar DC/DC converter for flexible direct current transmission, when the IGBT valve group of each phase DC/DC converter topological structure is different in switching state, the phase DC/DC converter topological structure outputs different voltage and current, and the four-phase DC/DC converter topological structure is operated in a staggered mode so that the bipolar DC/DC converter outputs continuous direct current voltage and current.

For each phase of DC/DC converter topological structure, the first IGBT valve group and the third IGBT valve group are a pair of valve groups with the same switch state, the second IGBT valve group and the fourth IGBT valve group are a pair of valve groups with the same switch state, and the switch states of the two pairs of IGBT valve groups are complementary.

For the a-phase DC/DC converter topological structure and the b-phase DC/DC converter topological structure, when the first IGBT valve bank and the third IGBT valve bank are conducted, the half-bridge sub-module string and the inductor are connected in series between the positive electrode of the direct-current high-voltage side and the positive electrode of the direct-current low-voltage side, and the difference between the voltages of the high-voltage side and the low-voltage side is borne. When the second IGBT valve bank and the fourth IGBT valve bank are conducted, the half-bridge sub-module string and the inductor are connected in series between the positive pole of the direct-current low-voltage side and the negative pole of the direct-current low-voltage side, and bear double low-voltage side voltage.

For the topology structure of the c-phase DC/DC converter and the topology structure of the d-phase DC/DC converter, when the first IGBT valve bank and the third IGBT valve bank are conducted, the half-bridge sub-module string and the inductor are connected in series between the positive pole of the direct-current low-voltage side and the negative pole of the direct-current low-voltage side to bear double voltage of the low-voltage side. When the second IGBT valve bank and the fourth IGBT valve bank are conducted, the half-bridge sub-module string and the inductor are connected in series between the negative electrode of the direct-current high-voltage side and the negative electrode of the direct-current low-voltage side, and the difference between the voltages of the high-voltage side and the low-voltage side is borne.

The bipolar DC/DC converter for flexible direct-current power transmission can realize bidirectional transmission of power between a direct-current high-voltage side and a direct-current low-voltage side under the control of the control method. Compared with the existing hybrid modular DC/DC converter, the bipolar DC/DC converter for flexible direct current transmission can realize bidirectional power transmission without respectively configuring a converter at the positive pole and the negative pole when being applied to practical engineering. Therefore, the bipolar DC/DC converter for flexible direct current transmission has relatively fewer sub-modules and lower cost.

Drawings

The bipolar DC/DC converter for flexible direct current transmission and the control method thereof according to the present invention will be described in more detail below based on embodiments and with reference to the accompanying drawings, in which:

FIG. 1 is a schematic circuit diagram of a bipolar DC/DC converter for flexible DC power transmission according to an embodiment, wherein + V_HIs a DC high-voltage side anode, -V_HIs a DC high-voltage side cathode, + V_LIs a DC low-voltage side anode, -V_LIs a DC low-voltage side cathode, I_HIs a DC high-voltage side actual current signal, I_LIs an actual current signal of the DC low-voltage side i_Pa、i_Pb、i_PcAnd i_PdRespectively half-bridge sub-module string SM_aHalf-bridge submodule string SM_bHalf-bridge submodule string SM_cAnd half-bridge sub-module string SM_dActual current signal of u_Pa、u_Pb、u_PcAnd u_PdRespectively half-bridge sub-module string SM_aHalf-bridge submodule string SM_bHalf-bridge submodule string SM_cAnd half-bridge sub-module string SM_dActual voltage signals at two ends, SM is a half-bridge submodule, and subscripts thereof sequentially indicate the phase and serial number of the half-bridge submodule;

FIG. 2 is a schematic circuit diagram of an IGBT valve set according to an embodiment, wherein S_a～S_NAre all IGBT switching tubes;

FIG. 3 is a schematic circuit diagram of a half-bridge sub-module according to an embodiment;

FIG. 4 is a control block diagram of the topology of the single-phase DC/DC converter mentioned in the embodiment, wherein P_refFor power reference signals, U_HThe voltage is the high-voltage side voltage, and PI is a proportional-integral regulator;

FIG. 5 is a schematic diagram of the alternate operation of the topology of the four-phase DC/DC converter according to the embodiment, in which U_LIs the low side voltage.

Detailed Description

The bipolar DC/DC converter for flexible DC power transmission and the control method thereof according to the present invention will be further described with reference to the accompanying drawings.

Example (b): the present embodiment is described in detail below with reference to fig. 1 to 5.

The bipolar DC/DC converter for flexible direct current transmission described in this embodiment includes a-phase DC/DC converter topology to d-phase DC/DC converter topology having the same circuit structure;

the topology structure of the a-phase DC/DC converter comprises a half-bridge submodule string SM_aIGBT valve set S_a1IGBT valve set S_a4And an inductance L_a；

The topology structure of the b-phase DC/DC converter comprises a half-bridge submodule string SM_bIGBT valve set S_b1IGBT valve set S_b4And an inductance L_b；

The topology structure of the c-phase DC/DC converter comprises a half-bridge submodule string SM_cIGBT valve set S_c1IGBT valve set S_c4And an inductance L_c；

The topology structure of the d-phase DC/DC converter comprises a half-bridge submodule string SM_dIGBT valve set S_d1IGBT valve set S_d4And an inductance L_d；

IGBT valve set S_a1IGBT valve set S_a4And IGBT valve set S_b1IGBT valve set S_b4Are all connected in series between the positive pole of the direct current high voltage side and the negative pole of the direct current low voltage side in sequence, and an IGBT valve set S_a2And IGBT valve set S_a3Common terminal and IGBT valve set S_b2And IGBT valve set S_b3The common ends of the two ends are connected with the positive electrode of the direct current low-voltage side;

half-bridge submodule string SM_aThe current input end is connected with an IGBT valve set S_a1And IGBT valve set S_a2The half-bridge submodule string SM_aCurrent output terminal of the inductor L_aAccess IGBT valve set S_a3And IGBT valve set S_a4A common terminal of (a);

half-bridge submodule string SM_bThe current input end is connected with an IGBT valve set S_b1And IGBT valve set S_b2The half-bridge submodule string SM_bCurrent output terminal of the inductor L_bAccess IGBT valve set S_b3And IGBT valve set S_b4A common terminal of (a);

IGBT valve set S_c1IGBT valve set S_c4And IGBT valve set S_d1IGBT valve set S_d4Are all connected in series between the positive pole of the direct current low voltage side and the negative pole of the direct current high voltage side in sequence, and an IGBT valve set S_c2And IGBT valve set S_c3Common terminal and IGBT valve set S_d2And IGBT valve set S_d3The common ends of the two ends are connected with the negative electrode of the direct current low-voltage side;

half-bridge submodule string SM_cCurrent input terminal of via inductor L_cAccess IGBT valve set S_c1And IGBT valve set S_c2The half-bridge submodule string SM_cThe current output end is connected with an IGBT valve set S_c3And IGBT valve set S_c4A common terminal of (a);

half-bridge submodule string SM_dCurrent input terminal of via inductor L_dAccess IGBT valve set S_d1And IGBT valve set S_d2The half-bridge submodule string SM_dThe current output end is connected with an IGBT valve set S_d3And IGBT valve set S_d4A common terminal of (a);

for each phase of DC/DC converter topological structure, a plurality of half-bridge sub-modules contained in a half-bridge sub-module string are enabled to be synchronously charged or discharged according to respective corresponding half-bridge sub-module driving signals and four IGBT valve groups according to respective corresponding IGBT valve group driving signals, so that the balance of self energy is maintained, and further the power transmission between a direct-current low-voltage side and a direct-current high-voltage side is realized;

the topological structures of the a-phase DC/DC converter and the d-phase DC/DC converter work cooperatively to ensure the continuous output of the bipolar DC/DC converter.

Half-bridge sub-module string SM of the present embodiment_aComprising half-bridge submodules SM having the same circuit configuration_a1Half-bridge submodule SM_aNFor half bridge sub-module SM_a1Half-bridge submodule SM_aNThe current output end of the half-bridge submodule SM is connected with the current input end of the second half-bridge submodule in sequence_a1Current input terminal and SM_aNThe current output ends of the half-bridge sub-module strings are respectively a half-bridge sub-module string SM_aAnd a current input terminal and a current output terminal.

Half-bridge submodule SM of the embodiment_a1Comprising an IGBT switching tube S₁IGBT switch tube S₂And a capacitance C;

IGBT switch tube S₁And IGBT switching tube S₂Series connection, capacitor C and series connection IGBT switch tube S₁And IGBT switching tube S₂Parallel connection;

IGBT switch tube S₁And IGBT switching tube S₂The common terminal is a half-bridge submodule SM_a1Current input terminal of, IGBT switching tube S₂The common terminal of the capacitor C is a half-bridge submodule SM_a1The current output terminal of (2).

IGBT valve set S of the embodiment_a1IGBT valve set S_a4All include a plurality of IGBT switch tubes that the quantity is the same, a plurality of IGBT switch tubes are established ties.

The target object of the control method for the bipolar DC/DC converter for flexible direct-current transmission according to the present embodiment is a topology structure of each-phase DC/DC converter;

the control method comprises the steps of setting the on-off state of each IGBT valve group of each phase of DC/DC converter topological structure according to a target power transmission direction, and controlling the charging and discharging states of a plurality of half-bridge sub-modules of each phase of DC/DC converter topological structure to realize power transmission in the target direction;

the four-phase DC/DC converter topological structure works alternately to ensure the continuous output of the bipolar DC/DC converter.

The control method comprises the following specific modes of setting the on-off state of each IGBT valve group of each phase of DC/DC converter topological structure according to the target power transmission direction:

when power is transmitted from the direct current high-voltage side to the direct current low-voltage side:

a-phase DC/DC converter topology: IGBT valve set S_a1And IGBT valve set S_a3When the other two IGBT valve banks are turned off, the half-bridge submodule string SM_aCharging; IGBT valve set S_a1And IGBT valve set S_a3When the other two IGBT valve banks are switched on, the half-bridge submodule string SM_aDischarging;

b-phase DC/DC converter topology: IGBT valve set S_b1And IGBT valve set S_b3When the other two IGBT valve banks are turned off, the half-bridge submodule string SM_bCharging; IGBT valve set S_b1And IGBT valve set S_b3When the other two IGBT valve banks are switched on, the half-bridge submodule string SM_bDischarging;

c-phase DC/DC converter topology: IGBT valve set S_c2And IGBT valve set S_c4When the other two IGBT valve banks are turned off, the half-bridge submodule string SM_cCharging; IGBT valve set S_c2And IGBT valve set S_c4When the other two IGBT valve banks are switched on, the half-bridge submodule string SM_cDischarging;

d-phase DC/DC converter topology: IGBT valve set S_d2And IGBT valve set S_d4When the other two IGBT valve banks are turned off, the half-bridge submodule string SM_dCharging; IGBT valve set S_d2And IGBT valve set S_d4When the other two IGBT valve banks are switched on, the half-bridge submodule string SM_dDischarging;

when power is transmitted from the direct-current low-voltage side to the direct-current high-voltage side, the control mode of the IGBT valve group of each phase of DC/DC converter topological structure is opposite to the control mode when the power is transmitted from the direct-current high-voltage side to the direct-current low-voltage side.

The control method controls the charging and discharging states of a plurality of half-bridge sub-modules of each phase of DC/DC converter topological structure in a specific mode as follows:

generating DC by constant DC power controlCurrent reference signal i on high voltage side_{H_ref}；

Reference signal i for adjusting difference between direct current high-voltage side current and direct current low-voltage side current by adopting capacitance energy balance control mode_{L-H_ref}Making the average value U of the capacitor voltage of the half-bridge sub-module string_{C_avg}Equal to a given reference capacitor voltage average value U_{C_ref}；

Direct current high-voltage side current reference signal i_{H_ref}Reference signal i of difference between DC high and low voltage side currents_{L-H_ref}Overlapping, and using the overlapped signal as a half-bridge submodule string current reference signal i_{P_ref}；

Current reference signal i of half-bridge submodule string_{P_ref}With actual current signal i of half-bridge submodule string_PThe difference value of (a) is input into a proportional-integral regulator;

half-bridge submodule string voltage control signal u based on proportional-integral regulator output_{P_ref}And obtaining a driving signal of each half-bridge submodule in the half-bridge submodule string by adopting a half-bridge submodule capacitor voltage balance control and modulation mode.

Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. It should be understood that features described in different dependent claims and herein may be combined in ways different from those described in the original claims. It is also to be understood that features described in connection with individual embodiments may be used in other described embodiments.

Claims (7)

1. The bipolar DC/DC converter for flexible direct current transmission is characterized by comprising a-phase DC/DC converter topological structure-d-phase DC/DC converter topological structure with the same circuit structure;

the topology structure of the a-phase DC/DC converter comprises a half-bridge submodule string SM_aIGBT valve set S_a1IGBT valve set S_a4And an inductance L_a；

The topology structure of the b-phase DC/DC converter comprises a half-bridge submodule string SM_bIGBT valve set S_b1IGBT valve set S_b4And an inductance L_b；

The topology structure of the c-phase DC/DC converter comprises a half-bridge submodule string SM_cIGBT valve set S_c1IGBT valve set S_c4And an inductance L_c；

The topology structure of the d-phase DC/DC converter comprises a half-bridge submodule string SM_dIGBT valve set S_d1IGBT valve set S_d4And an inductance L_d；

IGBT valve set S_a1IGBT valve set S_a4And IGBT valve set S_b1IGBT valve set S_b4Are all connected in series between the positive pole of the direct current high voltage side and the negative pole of the direct current low voltage side in sequence, and an IGBT valve set S_a2And IGBT valve set S_a3Common terminal and IGBT valve set S_b2And IGBT valve set S_b3The common ends of the two ends are connected with the positive electrode of the direct current low-voltage side;

half-bridge submodule string SM_aThe current input end is connected with an IGBT valve set S_a1And IGBT valve set S_a2The half-bridge submodule string SM_aCurrent output terminal of the inductor L_aAccess IGBT valve set S_a3And IGBT valve set S_a4A common terminal of (a);

half-bridge submodule string SM_bThe current input end is connected with an IGBT valve set S_b1And IGBT valve set S_b2The half-bridge submodule string SM_bCurrent output terminal of the inductor L_bAccess IGBT valve set S_b3And IGBT valve set S_b4A common terminal of (a);

IGBT valve set S_c1IGBT valve set S_c4And IGBT valve set S_d1IGBT valve set S_d4Are all connected in series between the positive pole of the direct current low voltage side and the negative pole of the direct current high voltage side in sequence, and an IGBT valve set S_c2And IGBT valve set S_c3Common terminal and IGBT valve set S_d2And IGBT valve set S_d3The common ends of the two ends are connected with the negative electrode of the direct current low-voltage side;

half-bridge submodule string SM_cCurrent ofInput terminal via inductor L_cAccess IGBT valve set S_c1And IGBT valve set S_c2The half-bridge submodule string SM_cThe current output end is connected with an IGBT valve set S_c3And IGBT valve set S_c4A common terminal of (a);

half-bridge submodule string SM_dCurrent input terminal of via inductor L_dAccess IGBT valve set S_d1And IGBT valve set S_d2The half-bridge submodule string SM_dThe current output end is connected with an IGBT valve set S_d3And IGBT valve set S_d4A common terminal of (a);

for each phase of DC/DC converter topological structure, a plurality of half-bridge sub-modules contained in a half-bridge sub-module string are enabled to be synchronously charged or discharged according to respective corresponding half-bridge sub-module driving signals and four IGBT valve groups according to respective corresponding IGBT valve group driving signals, so that the balance of self energy is maintained, and further the power transmission between a direct-current low-voltage side and a direct-current high-voltage side is realized;

the topological structures of the a-phase DC/DC converter and the d-phase DC/DC converter alternately work to ensure the continuous output of the bipolar DC/DC converter.

2. Bipolar DC/DC converter for flexible direct current transmission according to claim 1, characterized in that the half-bridge sub-module string SM_aComprising half-bridge submodules SM having the same circuit configuration_a1Half-bridge submodule SM_aNFor half bridge sub-module SM_a1Half-bridge submodule SM_aNThe current output end of the half-bridge submodule SM is connected with the current input end of the second half-bridge submodule in sequence_a1Current input terminal and SM_aNThe current output ends of the half-bridge sub-module strings are respectively a half-bridge sub-module string SM_aAnd a current input terminal and a current output terminal.

3. Bipolar DC/DC converter for flexible direct current transmission according to claim 2, characterized in that the half-bridge submodule SM_a1Comprising an IGBT switching tube S₁IGBT switch tube S₂And a capacitance C;

IGBT switch tube S₁And IGBT switching tube S₂Series connection, capacitor C and series connection IGBT switch tube S₁And IGBT switching tube S₂Parallel connection;

IGBT switch tube S₁And IGBT switching tube S₂The common terminal is a half-bridge submodule SM_a1Current input terminal of, IGBT switching tube S₂The common terminal of the capacitor C is a half-bridge submodule SM_a1The current output terminal of (2).

4. Bipolar DC/DC converter for flexible direct current transmission according to claim 3, characterized in that the IGBT valve pack S_a1IGBT valve set S_a4All include a plurality of IGBT switch tubes that the quantity is the same, a plurality of IGBT switch tubes are established ties.

5. The control method for a bipolar DC/DC converter for flexible direct current transmission according to claim 1, characterized in that a target of the control method is a per-phase DC/DC converter topology;

the control method comprises the steps of setting the on-off state of each IGBT valve group of each phase of DC/DC converter topological structure according to a target power transmission direction, and controlling the charging and discharging states of a plurality of half-bridge sub-modules of each phase of DC/DC converter topological structure to realize power transmission in the target direction;

the four-phase DC/DC converter topological structure works alternately to ensure the continuous output of the bipolar DC/DC converter.

6. The control method of a bipolar DC/DC converter for flexible direct current transmission according to claim 5, wherein the control method sets the on-off state of each IGBT valve group of each phase DC/DC converter topology according to the target power transmission direction by:

when power is transmitted from the direct current high-voltage side to the direct current low-voltage side:

a-phase DC/DC converter topology: IGBT valve set S_a1And IGBT valve set S_a3When the other two IGBT valve banks are turned off, the half-bridge submodule string SM_aCharging; IGBT valve set S_a1And IGBT valve set S_a3When the other two IGBT valve banks are switched on, the half-bridge submodule string SM_aDischarging;

b-phase DC/DC converter topology: IGBT valve set S_b1And IGBT valve set S_b3When the other two IGBT valve banks are turned off, the half-bridge submodule string SM_bCharging; IGBT valve set S_b1And IGBT valve set S_b3When the other two IGBT valve banks are switched on, the half-bridge submodule string SM_bDischarging;

c-phase DC/DC converter topology: IGBT valve set S_c2And IGBT valve set S_c4When the other two IGBT valve banks are turned off, the half-bridge submodule string SM_cCharging; IGBT valve set S_c2And IGBT valve set S_c4When the other two IGBT valve banks are switched on, the half-bridge submodule string SM_cDischarging;

d-phase DC/DC converter topology: IGBT valve set S_d2And IGBT valve set S_d4When the other two IGBT valve banks are turned off, the half-bridge submodule string SM_dCharging; IGBT valve set S_d2And IGBT valve set S_d4When the other two IGBT valve banks are switched on, the half-bridge submodule string SM_dDischarging;

when power is transmitted from the direct-current low-voltage side to the direct-current high-voltage side, the control mode of the IGBT valve group of each phase of DC/DC converter topological structure is opposite to the control mode when the power is transmitted from the direct-current high-voltage side to the direct-current low-voltage side.

7. The control method of the bipolar DC/DC converter for flexible direct current transmission according to claim 6, wherein the control method controls the charging and discharging states of the plurality of half-bridge sub-modules of each phase DC/DC converter topology by:

generating a direct current high-voltage side current reference signal i by adopting a constant direct current power control mode_{H_ref}；

Reference signal i for adjusting difference between direct current high-voltage side current and direct current low-voltage side current by adopting capacitance energy balance control mode_{L-H_ref}Making the average value U of the capacitor voltage of the half-bridge sub-module string_{C_avg}Equal to a given reference capacitor voltage average value U_{C_ref}；

Direct current high-voltage side current reference signal i_{H_ref}Reference signal i of difference between DC high and low voltage side currents_{L-H_ref}Overlapping, and using the overlapped signal as a half-bridge submodule string current reference signal i_{P_ref}；

Current reference signal i of half-bridge submodule string_{P_ref}With actual current signal i of half-bridge submodule string_PThe difference value of (a) is input into a proportional-integral regulator;

half-bridge submodule string voltage control signal u based on proportional-integral regulator output_{P_ref}And obtaining a driving signal of each half-bridge submodule in the half-bridge submodule string by adopting a half-bridge submodule capacitor voltage balance control and modulation mode.

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