CN110912434B - MMC topology containing partial energy storage element and steady-state operation control method thereof - Google Patents

MMC topology containing partial energy storage element and steady-state operation control method thereof Download PDF

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CN110912434B
CN110912434B CN201911226206.5A CN201911226206A CN110912434B CN 110912434 B CN110912434 B CN 110912434B CN 201911226206 A CN201911226206 A CN 201911226206A CN 110912434 B CN110912434 B CN 110912434B
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sub
bess
voltage
bridge arm
bridge
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CN110912434A (en
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郭思源
严亚兵
陈远扬
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State Grid Corp of China SGCC
Electric Power Research Institute of State Grid Hunan Electric Power Co Ltd
State Grid Hunan Electric Power Co Ltd
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State Grid Corp of China SGCC
Electric Power Research Institute of State Grid Hunan Electric Power Co Ltd
State Grid Hunan Electric Power Co Ltd
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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
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/28—Arrangements for balancing of the load in a network by storage of energy
    • H02J3/32—Arrangements for balancing of the load in a network by storage of energy using batteries with converting means
    • 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
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H02M7/4835—Converters with outputs that each can have more than two voltages levels comprising two or more cells, each including a switchable capacitor, the capacitors having a nominal charge voltage which corresponds to a given fraction of the input voltage, and the capacitors being selectively connected in series to determine the instantaneous output voltage

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Abstract

The invention discloses an MMC topology containing a part of energy storage elements and a steady-state operation control method thereof, wherein the MMC topology comprises the following steps: each bridge arm comprises a plurality of sub-modules and a bridge arm inductor which are connected in series; the sub-modules comprise HBSM, FBSM and FBSM-BESS, namely three types of half-bridge sub-modules, common full-bridge sub-modules and full-bridge sub-modules containing energy storage elements. The number of HBSM, FBSM and FBSM-BESS in each bridge arm is respectively N/2, N/4 and N/4. The converter not only has the power conversion capability of an alternating current side and a direct current side, the capability of inhibiting power oscillation and good frequency supporting capability, but also has low requirement on the capacity of an energy storage device, and has small influence on the efficiency of the converter.

Description

MMC topology containing partial energy storage element and steady-state operation control method thereof
Technical Field
The invention belongs to the technical field of multi-level power electronic converters, and particularly relates to an MMC topology containing partial energy storage elements and a steady-state operation control method thereof.
Background
By means of good direct-current short-circuit fault ride-through capability, low cost and high operation efficiency, a hybrid MMC (modular multilevel converter) formed by a Full Bridge Sub-module (FBSM) and a Half Bridge Sub-module (HBSM) 1:1 in a Bridge arm becomes one of main choices of flexible direct-current transmission converter topologies. However, the hybrid MMC cannot decouple the ac-dc side power, cannot provide sufficient damping to suppress the power oscillation in a transient state, and has a limited ability to support the ac grid frequency.
The energy storage device has the functions of smooth transition, peak clipping and valley filling, frequency and voltage regulation and the like due to the fact that the energy storage device is provided with the single battery pack; the solar energy and wind energy can be smoothly output, and the impact on a power grid and users caused by randomness, intermittence and fluctuation of the solar energy and wind energy can be reduced; the electricity charge expenditure of the user can be reduced by charging in the valley price period and discharging in the peak price period; when a large power grid is powered off, the power grid can operate in an isolated island mode, and good frequency supporting capability for uninterrupted power supply of users is guaranteed. However, the ac-dc side power conversion capability is not available.
Therefore, it is necessary to design an MMC topology and a steady-state operation control method thereof that have both ac/dc power conversion capability, sufficient power oscillation suppression capability, and good frequency support capability.
Disclosure of Invention
The invention solves the technical problem that aiming at the defects of the prior art, the invention provides an MMC topology containing partial energy storage elements and a steady-state operation control method thereof, which not only have AC/DC side power conversion capability, power oscillation inhibition capability and good frequency support capability, but also have low requirement on the capacity of an energy storage device, and have little influence on the efficiency of a converter.
To achieve the above object, the present invention provides
An MMC topology containing partial energy storage elements is disclosed, wherein each bridge arm comprises a plurality of sub-modules and a bridge arm inductor which are connected in series; the sub-modules comprise HBSM, FBSM and FBSM-BESS, namely three types of half-bridge sub-modules, common full-bridge sub-modules and full-bridge sub-modules containing energy storage elements.
Furthermore, the number of HBSM, FBSM and FBSM-BESS in each bridge arm is respectively N/2, N/4 and N/4, and N is the total number of sub-modules of each bridge arm.
Further, the FBSM-BESS comprises a battery pack, a DC/DC converter and a full-bridge submodule; and one side of the DC/DC converter is connected with the battery pack, and the other side of the DC/DC converter is connected with the direct current side of the full-bridge submodule.
Further, the DC/DC converter adopts a buck/boost non-isolated converter, a phase-shifting type double-active-bridge converter or a resonant type double-active-bridge converter.
The invention also discloses a method for controlling the stable operation of the MMC with a part of energy storage elements, wherein the MMC with the part of energy storage elements adopts the topological structure of any one of claims 1 to 5; the control method comprises the following steps: and determining control signals of the submodules based on a voltage-sharing strategy of capacitor voltage sequencing to ensure that the capacitor voltages of all the submodules on each bridge arm are balanced.
Further, a voltage-sharing strategy based on capacitor voltage sequencing determines control signals of each submodule so that the MMC works in a direct-current power transmission mode, and the steps are as follows:
firstly, calculating an output voltage command u of a j-phase k bridge arm of an MMCkjrefWherein j ═ a, B and C respectively represent three phases A, B and C; k is p, n represents upper and lower bridge arms;
then, calculating the number of the submodules selected by the j-phase k bridge arm of the MMC according to the following formula:
Nkj＝round(ukjref/Uc)
where round (x) is a function of nearest rounding (rounding function); u shapecRated voltage for the sub-module;
finally, controlling switches in a DC/DC converter in the FBSM-BESS to be in a locking state, and enabling the full-bridge submodule with the energy storage element to work in a common full-bridge submodule working mode;
for j-phase k bridge arm, according to the current capacitor voltage of each submodule and bridge arm current ikjThe direction to guarantee that the capacitor voltage is balanced to obtain the control signal of each submodule, specifically:
when the set output voltage command u of the bridge arm submodule iskjref,smWhen the sub-module is more than or equal to 0, N is selected from all the sub-moduleskjThe sub-modules output positive levels, and the rest sub-modules bypass or output zero levels; the selection method comprises the following steps: bridge arm current ikjWhen the capacitance voltage is more than or equal to 0, selecting N with the lowest capacitance voltagekjA sub-module; i.e. ikjWhen the voltage is less than 0, selecting N with the highest capacitance voltagekjA sub-module;
when u is setkjref,smIf < 0, selecting N from all sub-moduleskjThe sub-modules output negative levels, and the rest sub-modules bypass or output zero levels; selecting methodThe method comprises the following steps: bridge arm current ikjWhen the capacitance voltage is more than or equal to 0, selecting N with the highest capacitance voltagekjA sub-module; i.e. ikjWhen the voltage is less than 0, selecting N with the lowest capacitance voltagekjAnd a sub-module.
Further, a voltage-sharing strategy based on capacitor voltage sequencing determines control signals of each submodule so that the MMC works in an energy absorption mode of the energy storage element, and the steps are as follows:
firstly, calculating an output voltage command u of a j-phase k bridge arm of an MMCkjrefWherein j ═ a, B and C respectively represent three phases A, B and C; k is p, n represents upper and lower bridge arms;
then, an output voltage command u of FBSM-BESS in a j-phase k bridge arm of the MMC is calculated according to the following formulakjref,bessAnd the output voltage instruction u of the common submodule in the same bridge armkjref,sm：
Figure GDA0002640524060000031
ukjref,sm＝ukjref-ukjref,bess
Wherein, PbessFor energy absorbed by the energy storage element, Pbess＝|Pac-PdcAnd which satisfies 0. ltoreq.Pbess≤Pbess,max，Pbess,maxAn upper limit value of the energy absorbed by the energy storage element,
Figure GDA0002640524060000032
in the formula PacThe amplitude of the interaction energy of the MMC and the alternating-current side power grid is obtained; pdcAmplitude of the alternating energy of MMC and DC side, theta1For bridge arm current ikjPhase, theta, corresponding to zero-crossing from negative to positive2The phase corresponding to the zero crossing point of the bridge arm current from positive to negative is shown, N is the total number of sub-modules of each bridge arm, omega1Is the angular frequency; u shapecRated voltage for the sub-module;
then, the number N of the FBSM-BESS selected by the j-phase k bridge arm is calculated according to the following formulakj,bessAnd the number N of the selected common sub-moduleskj,sm：
Figure GDA0002640524060000033
Wherein, the common sub-modules comprise HBSM and FBSM;
finally, for the j-phase k bridge arm, according to the current capacitor voltage of each submodule and the current i of the bridge armkjThe direction to guarantee that the capacitor voltage is balanced to obtain the control signal of each submodule, specifically:
when the output voltage command u of the common submodule in the bridge armkjref,smWhen the sub-module is more than or equal to 0, N is selected from 3N/4 common sub-moduleskj,smThe sub-modules output positive levels, and the other sub-modules bypass or output zero levels; the selection method comprises the following steps: bridge arm current ikjWhen the capacitance voltage is more than or equal to 0, selecting N with the lowest capacitance voltagekj,smA sub-module; i.e. ikjWhen the voltage is less than 0, selecting N with the highest capacitance voltagekj,smA sub-module;
when u iskjref,smWhen < 0, N is selected from N/4 FBSMskj,smThe submodule outputs a negative level, other FBSM bypasses or outputs a zero level, and all HBSM bypasses or outputs a zero level; the selection method comprises the following steps: bridge arm current ikjWhen the capacitance voltage is more than or equal to 0, selecting N with the highest capacitance voltagekj,smA sub-module; i.e. ikjWhen the voltage is less than 0, selecting N with the lowest capacitance voltagekj,smA sub-module;
the output voltages of the DC/DC converters for controlling the N/4 FBSM-BESS are all Uc(ii) a When u iskjref,bessWhen the number is more than or equal to 0, selecting N from N/4 FBSM-BESSkj,bessThe submodules enable the full-bridge submodules to output positive levels, and the full-bridge submodules in the rest FBSM-BESS to output zero levels; when u iskjref,bessWhen the number is less than 0, selecting N from N/4 FBSM-BESSkj,bessAnd the submodules enable the full-bridge submodules in the submodules to output negative levels, and the full-bridge submodules in the rest FBSM-BESS to output zero levels.
Further, a voltage-sharing strategy based on capacitor voltage sequencing determines control signals of each submodule so that the MMC works in an energy-releasing mode of the energy storage element, and the steps are as follows:
firstly, calculating an output voltage command u of a j-phase k bridge arm of an MMCkjrefWherein j ═ a, B and C respectively represent three phases A, B and C; k is p, n represents upper and lower bridge arms;
then, an output voltage command u of FBSM-BESS in a j-phase k bridge arm of the MMC is calculated according to the following formulakjref,bessAnd the output voltage instruction u of the common submodule in the same bridge armkjref,sm：
Figure GDA0002640524060000041
ukjref,sm＝ukjref-ukjref,bess
Wherein, PbessFor energy absorbed by the energy storage element, Pbess＝|Pac-PdcAnd which satisfies 0. ltoreq.Pbess≤Pbess,max，Pbess,maxAn upper limit value of the energy absorbed by the energy storage element,
Figure GDA0002640524060000042
in the formula PacThe amplitude of the interaction energy of the MMC and the alternating-current side power grid is obtained; pdcAmplitude of the alternating energy of MMC and DC side, theta1For bridge arm current ikjPhase, theta, corresponding to zero-crossing from negative to positive2The phase corresponding to the zero crossing point of the bridge arm current from positive to negative is shown, N is the total number of sub-modules of each bridge arm, omega1Is the angular frequency; u shapecRated voltage for the sub-module;
then, the number N of the FBSM-BESS selected by the j-phase k bridge arm is calculated according to the following formulakj,bessAnd the number N of the selected common sub-moduleskj,sm：
Figure GDA0002640524060000043
Wherein, the common sub-modules comprise HBSM and FBSM;
finally, for the j-phase k bridge arm, according to the current capacitor voltage of each submodule and the current i of the bridge armkjDirection to ensure the capacitor voltage balanceThe control signals to each submodule are specifically:
when the output voltage command u of the common submodule in the bridge armkjref,smWhen the sub-module is more than or equal to 0, N is selected from 3N/4 common sub-moduleskj,smThe sub-modules output positive levels, and the other sub-modules bypass or output zero levels; the selection method comprises the following steps: bridge arm current ikjWhen the capacitance voltage is more than or equal to 0, selecting N with the lowest capacitance voltagekj,smA sub-module; i.e. ikjWhen the voltage is less than 0, selecting N with the highest capacitance voltagekj,smA sub-module;
when u iskjref,smWhen < 0, N is selected from N/4 FBSMskj,smThe submodule outputs a negative level, other FBSM bypasses or outputs a zero level, and all HBSM bypasses or outputs a zero level; the selection method comprises the following steps: bridge arm current ikjWhen the capacitance voltage is more than or equal to 0, selecting N with the highest capacitance voltagekj,smA sub-module; i.e. ikjWhen the voltage is less than 0, selecting N with the lowest capacitance voltagekj,smA sub-module;
the output voltages of the DC/DC converters for controlling the N/4 FBSM-BESS are all Uc(ii) a When u iskjref,bessWhen the number is more than or equal to 0, selecting N from N/4 FBSM-BESSkj,bessThe submodules enable the full-bridge submodules to output positive levels, and the full-bridge submodules in the rest FBSM-BESS to output zero levels; when u iskjref,bessWhen the number is less than 0, selecting N from N/4 FBSM-BESSkj,bessAnd the submodules enable the full-bridge submodules in the submodules to output negative levels, and the full-bridge submodules in the rest FBSM-BESS to output zero levels.
Has the advantages that:
the invention integrates the good direct current short circuit fault ride-through capability of the full-bridge-half-bridge mixed MMC and the good power decoupling capability of the energy storage device, thereby not only realizing the normal energy transmission of the alternating current-direct current side, but also realizing the short-term dynamic support of the frequency; when short-time and permanent faults occur on the direct current side, the power shortage of the sending end and the receiving end is reduced, and the transient stability of the connected alternating current and direct current system is further improved. The invention has the power conversion capability of the AC/DC side, the capability of inhibiting power oscillation and good frequency supporting capability. Compared with the scheme that the energy storage converters in the hybrid MMC and the energy storage device are connected in parallel at the direct current side, the direct current side of the energy storage converter is not required to output high voltage, and the cost and the volume of the energy storage converter are not increased; compared with the scheme that each battery pack in the energy storage device is connected to the direct current side of each submodule in the hybrid MMC through the DC/DC converter, the problem that the capacity of the energy storage device needs to be matched with the transmission power of the alternating current side and the direct current side of the hybrid MMC, so that the requirement on the capacity of the energy storage device is high is solved, and the problem that the normal operation efficiency of the hybrid MMC is reduced due to the fact that the DC/DC converter is configured on each submodule is solved.
Drawings
FIG. 1 is a diagram of an MMC topology including a portion of the energy storage element;
FIG. 2 is a topology structure diagram of a full-bridge sub-module including an energy storage element according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of the operation of an embodiment of the present invention;
fig. 4 is a structural diagram after the structural diagrams of a half-bridge sub-module, a full-bridge sub-module and an energy storage element in each phase of upper and lower bridge arms in the MMC are equivalent to a direct-current voltage source;
FIG. 5 is a diagram illustrating the calculation of the three-phase AC voltage reference value u in the present embodimentjrefA control block diagram of (1);
FIG. 6 is a diagram illustrating a calculation of the output voltage reference u of the circulating current suppressor according to the present embodimentjdifrefA control block diagram of (1);
fig. 7 is a schematic diagram illustrating selective switching of the common sub-modules according to the bridge arm voltage command and the bridge arm current in this embodiment.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features related to the embodiments of the present invention described below may be combined with each other to form a new embodiment as long as they do not conflict with each other.
Example 1:
the embodiment discloses an MMC topology containing partial energy storage elements, wherein each bridge arm comprises three submodules, namely a half-bridge submodule (HBSM), a common full-bridge submodule (FBSM) and a full-bridge submodule (FBSM-BESS) containing energy storage elements (a battery energy storage system, BESS), and the number of the three submodules is N/2, N/4 and N/4 respectively; the HBSM and FBSM are collectively called as common sub-modules, and the MMC concrete topology is shown in FIG. 1.
Example 2:
in this embodiment, on the basis of embodiment 1, the full-bridge sub-module including the energy storage element adopts a topology structure as shown in fig. 2, and includes a battery pack, a full-bridge sub-module, and a DC/DC converter in the middle; the DC/DC converter can be a buck/boost non-isolated converter, a phase-shift dual-active-bridge converter (DAB) composed of two full-bridge units and a DC/DC transformer, and a resonance dual-active-bridge converter composed of two full-bridge units, a DC/DC transformer, an energy storage capacitor and a resonance inductor.
Example 3:
the embodiment discloses a method for controlling the steady-state operation of an MMC containing a part of energy storage elements, wherein the MMC containing a part of energy storage elements adopts the topological structure in embodiment 1 or 2, and through the method for controlling the steady-state operation, the MMC containing a part of energy storage elements can work in three modes, namely a direct-current power transmission mode, an energy storage element energy absorption mode in the MMC topology, and an energy storage element energy release mode in the MMC topology, and further can be divided into five modes, namely a direct-current power transmission mode, an energy storage element energy absorption mode from an alternating-current side, an energy storage element energy absorption mode from a direct-current side, an energy storage element energy release mode from a direct-current side, and an energy storage element energy release mode from an alternating-current side, as shown in fig. 3. For the convenience of subsequent analysis, according to the difference of the specific topologies of the modules, the MMC topology including part of the energy storage elements in this embodiment is equivalent to the structure shown in fig. 4.
In the direct-current power transmission mode, the specific control steps are as follows:
1.1) calculating M according to the transmitted power according to the control block diagram (namely, constant power control mode) shown in FIG. 5The reference value of the alternating voltage output by each phase of the MC; marking the reference value of the alternating voltage output by the j phase in the MMC as ujrefWherein j ═ a, B and C respectively represent three phases A, B and C;
1.2) calculating the output voltage reference value of the circulating current suppressor according to a control block diagram (constant current control mode) shown in FIG. 6; the j phase voltage reference value output by the circulating current suppressor is recorded as ujdifref；
1.3) calculating the output voltage command u of the j-phase upper bridge arm and the j-phase lower bridge arm according to the following formulapjrefAnd unjref：
Figure GDA0002640524060000061
1.4) calculating the number of the submodules selected by the upper bridge arm and the lower bridge arm of the j phase according to the following formula as follows:
Figure GDA0002640524060000062
where round (x) is a function of nearest rounding (rounding function); u shapecFor sub-module rated voltage (rated voltage of three sub-modules is U)c)；
1.5) setting j-phase k bridge arm submodule output voltage instruction ukjref,smWherein k is p, n represents an upper bridge arm and a lower bridge arm respectively;
1.6) switches in a DC/DC converter in the full-bridge sub-module containing the energy storage element are all in a locked state, so that the full-bridge sub-module with the energy storage element works in a common full-bridge sub-module working mode;
according to the capacitor voltage of each submodule in the current bridge arm and the current direction of the bridge arm, a control signal of each submodule is obtained by ensuring the capacitor voltage to be balanced, taking an A-phase upper bridge arm as an example, the method specifically comprises the following steps:
when the bridge arm submodule outputs a voltage instruction uparef,smWhen the sub-module is more than or equal to 0, N is selected from all the sub-modulespaSubmodule (bridge arm current i)paWhen the capacitance voltage is more than or equal to 0, selecting N with the lowest capacitance voltagepaA sub-module; i.e. ipaWhen the voltage is less than 0, selecting N with the highest capacitance voltagepaA submodule) to output a positive level, and the remaining submodules bypass (i.e. short-circuit the submodule to make the submodule inoperative) or output a zero level (i.e. control the submodule to work normally, but output is zero);
when u isparef,smIf < 0, selecting N from all sub-modulespaSubmodule (bridge arm current i)paWhen the capacitance voltage is more than or equal to 0, selecting N with the highest capacitance voltagepaA sub-module; i.e. ipaWhen the voltage is less than 0, selecting N with the lowest capacitance voltagepaSub-modules) to output a negative level, with the remaining sub-modules bypassing or outputting a zero level.
The energy absorption mode of the energy storage element in the MMC topology comprises an energy absorption mode of the energy storage element from the alternating current side and an energy absorption mode of the energy storage element from the direct current side, namely two modes that the MMC absorbs energy from the alternating current side and outputs energy to the direct current side and the energy storage element and absorbs energy from the direct current side and outputs energy to the alternating current side and the energy storage element; in which the energy P absorbed by the energy storage elementbessSatisfies the following conditions:
Figure GDA0002640524060000071
wherein, PacThe amplitude of the interaction energy of the MMC and the alternating-current side power grid is obtained; pdcAmplitude of the alternating energy of MMC and DC side, theta1For the phase position, θ, corresponding to the zero crossing of the bridge arm current (taking the current of the upper or lower bridge arm) from negative to positive2The phase corresponding to the zero crossing point of the bridge arm current from positive to negative;
the specific control steps of the energy absorption mode of the energy storage element in the MMC topology are as follows:
2.1) calculating the AC voltage reference value output by each phase according to the MMC and the AC side interactive power and the control block diagram shown in FIG. 5; marking the reference value of the alternating voltage output by the j phase in the MMC as ujrefWherein j ═ a, B and C respectively represent three phases A, B and C;
2.2) calculating the output voltage reference value of the circulation current suppressor according to the control block diagram shown in FIG. 6; the j phase voltage reference value output by the circulating current suppressor is recorded as ujdifref；
2.3) calculating the output voltage command u of the j-phase upper bridge arm and the j-phase lower bridge arm according to the following formulapjrefAnd unjref：
Figure GDA0002640524060000072
2.4) calculating an output voltage instruction u of a full-bridge submodule containing an energy storage element in a j-phase k bridge arm according to the following formulakjref,bessAnd the output voltage instruction u of the common submodule in the bridge armkjref,sm：
Figure GDA0002640524060000081
ukjref,sm＝ukjref-ukjref,bess
Wherein, k is p, n represents an upper bridge arm and a lower bridge arm respectively;
2.5) calculating the number of full-bridge sub-modules containing energy storage elements and the number of common sub-modules selected by the j-phase k bridge arm according to the following formula as follows:
Figure GDA0002640524060000082
wherein N iskj,bessSelecting the number N of full-bridge submodules with energy storage elements for a j-phase k-bridge armkj,smSelecting the number of the common submodules of the j-phase k bridge arm;
2.6) according to the current capacitor voltage of each submodule and the current direction of a bridge arm, ensuring the capacitor voltage to be balanced to obtain a control signal of each submodule; taking the phase a upper bridge arm as an example, a control block diagram thereof is shown in fig. 7, and specifically includes:
when the output voltage command u of the common submodule in the bridge armparef,smWhen the number of the common full-bridge sub-modules is more than or equal to 0, the common full-bridge sub-modules work in a half-bridge sub-module mode, and N is selected from 3N/4 common sub-modulespa,smSubmodule (bridge arm current i)paWhen the capacitance voltage is more than or equal to 0, selecting N with the lowest capacitance voltagepa,smA sub-module; i.e. ipaWhen the voltage is less than 0, selecting N with the highest capacitance voltagepa,smA sub-module) to output a positive level, and the remaining sub-modules bypass or output a zero level;
when u isparef,smWhen the number is less than 0, N is selected from N/4 common full-bridge sub-modules (FBSM)pa,smSubmodule (bridge arm current i)paWhen the capacitance voltage is more than or equal to 0, selecting N with the highest capacitance voltagepa,smA sub-module; i.e. ipaWhen the voltage is less than 0, selecting N with the lowest capacitance voltagepa,smSub-modules) to output negative level, the other common full-bridge sub-modules bypass or output zero level, and all half-bridge sub-modules bypass or output zero level;
the output voltage of the DC/DC converter controlling the N/4 full-bridge sub-modules with energy storage elements (FBSM-BESS) (in the full-bridge sub-module with energy storage elements (FBSM-BESS) structure as shown in fig. 2, the voltage across the capacitor, i.e. the output voltage of the DC/DC converter) is Uc(ii) a When u isparef,bessWhen the number is more than or equal to 0, selecting N from N/4 FBSM-BESSpa,bessSub-modules, which make the full-bridge sub-module output a positive level (in the full-bridge sub-module (FBSM-BESS) structure with energy storage elements as shown in fig. 2, the voltage between X, Y is the full-bridge sub-module output voltage), and the full-bridge sub-modules in the rest FBSM-BESS output a zero level; when u isparef,bessWhen the number is less than 0, selecting N from N/4 FBSM-BESSpa,bessAnd the submodules enable the full-bridge submodules in the submodules to output negative levels, and the full-bridge submodules in the rest FBSM-BESS to output zero levels.
Preferably, the energy storage element in the MMC topology releases energy modes including an energy storage element to dc side release energy mode and an energy storage element to ac side release energy mode, that is, two modes of the MMC absorbing energy from the ac side and the energy storage element to output energy from the dc side and the MMC absorbing energy from the dc side and the energy storage element to output energy from the ac side; in which the energy P released by the energy storage elementbessSatisfies the following conditions:
Figure GDA0002640524060000091
wherein, PacThe amplitude of the interaction energy of the MMC and the alternating-current side power grid is obtained; pdcAmplitude of the alternating energy of MMC and DC side, theta1For the phase corresponding to the zero crossing of the bridge arm current from negative to positive, θ2The phase corresponding to the zero crossing point of the bridge arm current from positive to negative; u shapecRated voltage for the sub-module;
further, the concrete control steps are as follows:
3.1) calculating the AC voltage reference value output by each phase according to the MMC and the AC side interaction power and the control block diagram shown in FIG. 5; marking the reference value of the alternating voltage output by the j phase in the MMC as ujrefWherein j ═ a, B and C respectively represent three phases A, B and C;
3.2) calculating the output voltage reference value of the circulating current suppressor according to the control block diagram shown in FIG. 6; the reference value of the j-th phase voltage output by the loop current suppressor is recorded as ujdifref；
3.3) calculating the output voltage command u of the j-phase upper bridge arm and the j-phase lower bridge arm according to the following formulapjrefAnd unjref：
Figure GDA0002640524060000092
3.4) calculating an output voltage instruction u of a full-bridge submodule containing an energy storage element in a j-phase k bridge arm according to the following formulakjref,bessAnd the output voltage instruction u of the common submodule in the same bridge armkjref,sm：
Figure GDA0002640524060000093
ukjref,sm＝ukjref-ukjref,bess
Wherein, k is p, n represents an upper bridge arm and a lower bridge arm respectively;
3.5) calculating the number of full-bridge submodules containing energy storage elements selected by the j-phase k bridge arm and the number of common submodules according to the following formulas:
Figure GDA0002640524060000094
wherein N iskj,bessSelecting the number N of full-bridge submodules with energy storage elements for a j-phase k-bridge armkj,smSelecting the number of the common submodules of the j-phase k bridge arm;
3.6) according to the current capacitor voltage of each submodule and the current direction of a bridge arm, ensuring the capacitor voltage to be balanced to obtain a control signal of each submodule; taking the phase a upper bridge arm as an example, the control block diagram is shown in fig. 7, and specifically includes:
when the output voltage command u of the common submodule in the bridge armparef,smWhen the number of the common full-bridge sub-modules is more than or equal to 0, the common full-bridge sub-modules work in a half-bridge sub-module mode, and N is selected from 3N/4 common sub-modulespa,smSubmodule (bridge arm current i)paWhen the capacitance voltage is more than or equal to 0, selecting N with the lowest capacitance voltagepa,smA sub-module; i.e. ipaWhen the voltage is less than 0, selecting N with the highest capacitance voltagepa,smA sub-module) to output a positive level, and the remaining sub-modules bypass or output a zero level;
when u isparef,smWhen the number is less than 0, N is selected from N/4 common full-bridge sub-modules (FBSM)pa,smSubmodule (bridge arm current i)paWhen the capacitance voltage is more than or equal to 0, selecting N with the highest capacitance voltagepa,smA sub-module; i.e. ipaWhen the voltage is less than 0, selecting N with the lowest capacitance voltagepa,smSub-modules) to output negative level, the other common full-bridge sub-modules bypass or output zero level, and all half-bridge sub-modules bypass or output zero level;
the output voltage of the DC/DC converter for controlling N/4 full-bridge sub-modules (FBSM-BESS) containing energy storage elements is Uc(ii) a When u isparef,bessWhen the number is more than or equal to 0, selecting N from N/4 FBSM-BESSpa,bessThe submodules enable the full-bridge submodules to output positive levels, and the full-bridge submodules in the rest FBSM-BESS to output zero levels; when u isparef,bessWhen the number is less than 0, selecting N from N/4 FBSM-BESSpa,bessAnd the submodules enable the full-bridge submodules in the submodules to output negative levels, and the full-bridge submodules in the rest FBSM-BESS to output zero levels.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims (6)

1. The MMC with partial energy storage elements adopts an MMC topology with partial energy storage elements; the control method comprises the following steps: determining control signals of all the sub-modules based on a voltage-sharing strategy of capacitor voltage sequencing to ensure that the capacitor voltages of all the sub-modules on each bridge arm are balanced;
each bridge arm comprises a plurality of sub-modules and a bridge arm inductor which are connected in series; the sub-modules comprise HBSM, FBSM and FBSM-BESS, namely three types of half-bridge sub-modules, common full-bridge sub-modules and full-bridge sub-modules containing energy storage elements;
the FBSM-BESS comprises a battery pack, a DC/DC converter and a full-bridge submodule; one side of the DC/DC converter is connected with the battery pack, and the other side of the DC/DC converter is connected with the direct current side of the full-bridge submodule;
determining control signals of each submodule based on a voltage-sharing strategy of capacitor voltage sequencing so that the MMC works in an energy absorption mode of an energy storage element, and the steps are as follows:
firstly, calculating an output voltage command u of a j-phase k bridge arm of an MMCkjrefWherein j ═ a, B and C respectively represent three phases A, B and C; k is p, n represents upper and lower bridge arms;
then, an output voltage command u of FBSM-BESS in a j-phase k bridge arm of the MMC is calculated according to the following formulakjref,bessAnd the output voltage instruction u of the common submodule in the same bridge armkjref,sm：
Figure FDA0002640524050000011
ukjref,sm＝ukjref-ukjref,bess
Wherein, PbessFor energy absorbed by the energy storage element, Pbess＝|Pac-PdcAnd which satisfies 0. ltoreq.Pbess≤Pbess,max，Pbess,maxAn upper limit value of the energy absorbed by the energy storage element,
Figure FDA0002640524050000012
in the formula PacThe amplitude of the interaction energy of the MMC and the alternating-current side power grid is obtained; pdcAmplitude of the alternating energy of MMC and DC side, theta1For bridge arm current ikjPhase, theta, corresponding to zero-crossing from negative to positive2The phase corresponding to the zero crossing point of the bridge arm current from positive to negative is shown, N is the total number of sub-modules of each bridge arm, omega1Is the angular frequency; u shapecRated voltage for the sub-module;
then, the number N of the FBSM-BESS selected by the j-phase k bridge arm is calculated according to the following formulakj,bessAnd the number N of the selected common sub-moduleskj,sm：
Figure FDA0002640524050000013
Wherein, round (x) is a nearby integer function, and the common sub-modules comprise HBSM and FBSM;
finally, for the j-phase k bridge arm, according to the current capacitor voltage of each submodule and the current i of the bridge armkjThe direction to guarantee that the capacitor voltage is balanced to obtain the control signal of each submodule, specifically:
when the output voltage command u of the common submodule in the bridge armkjref,smWhen the sub-module is more than or equal to 0, N is selected from 3N/4 common sub-moduleskj,smThe sub-modules output positive levels, and the other sub-modules bypass or output zero levels; the selection method comprises the following steps: bridge arm current ikjWhen the capacitance voltage is more than or equal to 0, selecting N with the lowest capacitance voltagekj,smA sub-module; i.e. ikjWhen the voltage is less than 0, selecting N with the highest capacitance voltagekj,smA sub-module;
when u iskjref,smWhen < 0, N is selected from N/4 FBSMskj,smThe submodule outputs a negative level, other FBSM bypasses or outputs a zero level, and all HBSM bypasses or outputs a zero level; the selection method comprises the following steps: bridge arm current ikjWhen the capacitance voltage is more than or equal to 0, selecting N with the highest capacitance voltagekj,smA sub-module; i.e. ikjWhen the voltage is less than 0, selecting N with the lowest capacitance voltagekj,smA sub-module;
the output voltages of the DC/DC converters for controlling the N/4 FBSM-BESS are all Uc(ii) a When u iskjref,bessWhen the number is more than or equal to 0, selecting N from N/4 FBSM-BESSkj,bessThe submodules enable the full-bridge submodules to output positive levels, and the full-bridge submodules in the rest FBSM-BESS to output zero levels; when u iskjref,bessWhen the number is less than 0, selecting N from N/4 FBSM-BESSkj,bessAnd the submodules enable the full-bridge submodules in the submodules to output negative levels, and the full-bridge submodules in the rest FBSM-BESS to output zero levels.
2. The method for controlling the steady-state operation of the MMC with a part of energy storage components according to claim 1, wherein a voltage-sharing strategy based on capacitor voltage sequencing is used to determine the control signals of each submodule so that the MMC can work in a DC power transmission mode, and the steps are as follows:
firstly, calculating an output voltage command u of a j-phase k bridge arm of an MMCkjrefWherein j ═ a, B and C respectively represent three phases A, B and C; k is p, n represents upper and lower bridge arms;
then, calculating the number of the submodules selected by the j-phase k bridge arm of the MMC according to the following formula:
Nkj＝round(ukjref/Uc)
wherein round (x) is a function of rounding nearby; u shapecRated voltage for the sub-module;
finally, controlling switches in a DC/DC converter in the FBSM-BESS to be in a locking state, and enabling the full-bridge submodule with the energy storage element to work in a common full-bridge submodule working mode;
for j-phase k bridge arm, according to the current capacitor voltage of each submodule and bridge arm current ikjThe direction to guarantee that the capacitor voltage is balanced to obtain the control signal of each submodule, specifically:
when the set output voltage command u of the bridge arm submodule iskjrefWhen the sub-module is more than or equal to 0, N is selected from all the sub-moduleskjA sub-module for outputting positive level, and othersThe sub-module of (a) bypasses or outputs a zero level; the selection method comprises the following steps: bridge arm current ikjWhen the capacitance voltage is more than or equal to 0, selecting N with the lowest capacitance voltagekjA sub-module; i.e. ikjWhen the voltage is less than 0, selecting N with the highest capacitance voltagekjA sub-module;
when u is setkjrefIf < 0, selecting N from all sub-moduleskjThe sub-modules output negative levels, and the rest sub-modules bypass or output zero levels; the selection method comprises the following steps: bridge arm current ikjWhen the capacitance voltage is more than or equal to 0, selecting N with the highest capacitance voltagekjA sub-module; i.e. ikjWhen the voltage is less than 0, selecting N with the lowest capacitance voltagekjAnd a sub-module.
3. The method for controlling the steady-state operation of the MMC with a part of energy storage components according to claim 1, wherein the control signals of each submodule are determined based on a voltage-sharing strategy of capacitor voltage sequencing so that the MMC works in an energy-releasing mode of the energy storage components, and the steps are as follows:
firstly, calculating an output voltage command u of a j-phase k bridge arm of an MMCkjrefWherein j ═ a, B and C respectively represent three phases A, B and C; k is p, n represents upper and lower bridge arms;
then, an output voltage command u of FBSM-BESS in a j-phase k bridge arm of the MMC is calculated according to the following formulakjref,bessAnd the output voltage instruction u of the common submodule in the same bridge armkjref,sm：
Figure FDA0002640524050000031
ukjref,sm＝ukjref-ukjref,bess
Wherein, PbessFor energy absorbed by the energy storage element, Pbess＝|Pac-PdcAnd which satisfies 0. ltoreq.Pbess≤Pbess,max，Pbess,maxAn upper limit value of the energy absorbed by the energy storage element,
Figure FDA0002640524050000032
in the formula PacThe amplitude of the interaction energy of the MMC and the alternating-current side power grid is obtained; pdcAmplitude of the alternating energy of MMC and DC side, theta1For bridge arm current ikjPhase, theta, corresponding to zero-crossing from negative to positive2The phase corresponding to the zero crossing point of the bridge arm current from positive to negative is shown, N is the total number of sub-modules of each bridge arm, omega1Is the angular frequency; u shapecRated voltage for the sub-module;
then, the number N of the FBSM-BESS selected by the j-phase k bridge arm is calculated according to the following formulakj,bessAnd the number N of the selected common sub-moduleskj,sm：
Figure FDA0002640524050000033
Wherein, the common sub-modules comprise HBSM and FBSM;
finally, for the j-phase k bridge arm, according to the current capacitor voltage of each submodule and the current i of the bridge armkjThe direction to guarantee that the capacitor voltage is balanced to obtain the control signal of each submodule, specifically:
when the output voltage command u of the common submodule in the bridge armkjref,smWhen the sub-module is more than or equal to 0, N is selected from 3N/4 common sub-moduleskj,smThe sub-modules output positive levels, and the other sub-modules bypass or output zero levels; the selection method comprises the following steps: bridge arm current ikjWhen the capacitance voltage is more than or equal to 0, selecting N with the lowest capacitance voltagekj,smA sub-module; i.e. ikjWhen the voltage is less than 0, selecting N with the highest capacitance voltagekj,smA sub-module;
when u iskjref,smWhen < 0, N is selected from N/4 FBSMskj,smThe submodule outputs a negative level, other FBSM bypasses or outputs a zero level, and all HBSM bypasses or outputs a zero level; the selection method comprises the following steps: bridge arm current ikjWhen the capacitance voltage is more than or equal to 0, selecting N with the highest capacitance voltagekj,smA sub-module; i.e. ikjWhen the voltage is less than 0, selecting N with the lowest capacitance voltagekj,smA sub-module;
the output voltages of the DC/DC converters for controlling the N/4 FBSM-BESS are all Uc(ii) a When u iskjref,bessIs more than or equal to 0Selecting N from N/4 FBSM-BESSkj,bessThe submodules enable the full-bridge submodules to output positive levels, and the full-bridge submodules in the rest FBSM-BESS to output zero levels; when u iskjref,bessWhen the number is less than 0, selecting N from N/4 FBSM-BESSkj,bessAnd the submodules enable the full-bridge submodules in the submodules to output negative levels, and the full-bridge submodules in the rest FBSM-BESS to output zero levels.
4. The MMC steady-state operation control method containing part of energy storage elements of any claim 1-3, characterized in that, calculate the output voltage instruction u of j looks k bridge arm of MMC according to following stepkjref：
1.1) calculating an alternating voltage reference value output by each phase of the MMC according to the transmitted power; marking the reference value of the alternating voltage output by the j phase in the MMC as ujrefWherein j ═ a, B and C respectively represent three phases A, B and C;
1.2) calculating an output voltage reference value of the circulating current suppressor; the j phase voltage reference value output by the circulating current suppressor is recorded as ujdifref；
1.3) calculating the output voltage command u of the j-phase upper bridge arm and the j-phase lower bridge arm according to the following formulapjrefAnd unjref：
Figure FDA0002640524050000041
Wherein, UdcIs the dc bus voltage.
5. The MMC steady-state operation control method of claim 1, wherein the number of HBSM, FBSM and FBSM-BESS in each bridge arm is N/2, N/4 and N/4 respectively, N is the total number of sub-modules in each bridge arm.
6. The MMC steady-state operation control method of claim 5, wherein the DC/DC converter is a buck/boost non-isolated converter, a phase-shifted dual-active bridge converter or a resonant dual-active bridge converter.
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