CN109256964B - MMC model prediction control method for mixing half-bridge submodule and full-bridge submodule - Google Patents

MMC model prediction control method for mixing half-bridge submodule and full-bridge submodule Download PDF

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CN109256964B
CN109256964B CN201811196255.4A CN201811196255A CN109256964B CN 109256964 B CN109256964 B CN 109256964B CN 201811196255 A CN201811196255 A CN 201811196255A CN 109256964 B CN109256964 B CN 109256964B
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bridge arm
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state
submodules
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CN109256964A (en
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公铮
郑曦
戴鹏
杨连胜
李康
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China University of Mining and Technology CUMT
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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
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Abstract

The invention discloses a half-bridge and full-bridge submodule mixed MMC model predictive control method, which utilizes a specified selection rule of upper and lower bridge arm level combinations to select an optimal level combination, and combines a bridge arm submodule capacitor voltage sequencing result and a bridge arm current direction to determine the switching state of each submodule. The invention does not need a PI controller, omits the complicated parameter setting process, can realize multi-target control by only utilizing one system cost function, does not need to increase various control rings, simplifies the control, and simultaneously improves the dynamic response capability of the system, thereby improving the working efficiency of the system.

Description

MMC model prediction control method for mixing half-bridge submodule and full-bridge submodule
Technical Field
The invention relates to a model prediction control method in the field of multilevel power electronic converters, in particular to an MMC model prediction control method with a half-bridge submodule and a full-bridge submodule mixed.
Background
A direct current transmission technology based on a Voltage Source Converter (VSC) is called as a flexible direct current transmission technology in China. Because the fully-controlled switch device is adopted, the method has the characteristics of no need of phase-change voltage, independent control of active power and reactive power output, capability of supplying power to a passive network and the like, and has wide application prospect in the fields of new energy power generation grid connection, urban power grid capacity expansion transformation, offshore platform isolated load power supply and the like. The Modular Multilevel Converter (MMC) becomes the first-choice technical scheme of the current flexible direct current transmission project with a plurality of advantages of modular design, low switching frequency, good harmonic performance and the like, but the MMC based on a half-bridge submodule does not have direct current fault ride-through capability, cannot block fault current, and needs to rely on an alternating current breaker or a direct current breaker to clear faults.
Based on the mixed MMC of half-bridge and full-bridge submodule piece, when satisfying direct current fault current scavenging ability, can utilize the negative level output of full-bridge submodule piece, realize the overmodulation operation, compare in full-bridge type MMC moreover, reducible switching device's quantity, reduce loss. The normal operation of a half-bridge and full-bridge sub-module mixed MMC requires that 3 control objectives are met: output current control, bridge arm circulation control and submodule capacitor voltage balance control. At present, the mixed MMC is mainly controlled by adopting the closed-loop control of a PI controller, although the steady-state precision of the PI controller is high, the number of the PI controller is increased along with the increase of control targets, so that the difficulty is increased for the setting of PI parameters, the PI parameter setting process is complicated due to more control targets, the workload of workers is increased, in addition, the PI parameters are required to be readjusted for different working conditions of the mixed MMC, and extra burden is also increased for the workers.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides the MMC model prediction control method with the mixed half-bridge and full-bridge sub-modules, which does not need a large number of PI controllers, omits a complicated parameter setting process, can improve the dynamic response capability of a system while omitting the traditional PI control parameter setting process, and reduces the workload and burden of workers.
In order to achieve the above object, the present invention provides a half-bridge and full-bridge submodule mixed MMC model predictive control method, which utilizes a specified selection rule of upper and lower bridge arm level combinations to select an optimal level combination, and determines the switch state of each submodule by combining a bridge arm submodule capacitor voltage sequencing result and a bridge arm current direction, and specifically comprises the following steps:
(1) according to the active power and reactive power instruction value P*And Q*Calculating the given value of output current
Figure GDA0002421992350000021
Wherein j is a, b, c, and represents a, b, c triphase, isIs an output current;
(2) according to the active power instruction value P*And a reactive power command value Q*Calculating the given value of the bridge arm circulation
Figure GDA0002421992350000022
(3) Determining the change range of the bridge arm voltage: setting the maximum number of full-bridge submodules working at a negative level state as M, determining that the variation range of the bridge arm voltage is expanded to be [ -MU (multi-user) according to the requirement of capacitance-voltage balance and M is less than or equal to N/3c，-(M-1)Uc,…，(N-1)Uc，NUc]While the DC voltage is Udc＝(N-M)Uc；
Wherein N is the number of submodules of each bridge arm, UcIs the rated submodule capacitor voltage;
(4) controlling output current: calculating a predicted value i of the output current at the next moment according to a discretization equation of the output current of the modular multilevel converter with the mixed half-bridge and full-bridge submodulessj(k +1), the discretization equation for the output current is:
Figure GDA0002421992350000023
where k is the kth time, LdIs the equivalent output inductance of the system, Ld＝L/2+Ls(ii) a L is bridge arm inductance, LsFor the output inductance, RdIs the equivalent output resistance of the system, Rd＝R/2+Rs(ii) a R is bridge arm resistance, RsFor the output resistance, Ts is the sampling frequency, usjIs the grid voltage upj、unjFor the output voltage of the upper and lower bridge arms, setting the capacitor voltage balance of the module, then ucjim≈Uc，i＝p,n，m＝1,2,…,N，ucjimFor the capacitance voltage of each submodule, p is the upper bridge arm of the converter, N is the lower bridge arm of the converter, m represents N different submodules of each bridge arm, and u ispj＝NpjUc，unj＝NnjUc；Npj，NnjThe number and the state of the submodules respectively communicated with the upper bridge arm and the lower bridge arm if Npj，NnjPositive, indicating that the selected sub-module output + UcIf N is presentpj，NnjThe value of the negative value is the negative value,representing the selected full bridge submodule output-UcThe sum of the voltages of the submodules for ensuring that the upper bridge arm and the lower bridge arm of each phase are simultaneously conducted is (N-M) Uc，Npj，NnjThe rule of selection is:
(Npj,Nnj)＝[(-M,N),(-(M-1),N-1),...,(N-1,-(M-1)),(N,-M)]；
(5) determining a cost function of the output current: according to the given value of the output current obtained in the step (1) and the step (4)
Figure GDA0002421992350000024
And a predicted value isj(k +1), the cost function of the output current is obtained as:
Figure GDA0002421992350000025
(6) bridge arm circulation control: calculating a predicted value i of the bridge arm circulation at the next moment under different switch combinations according to a discretization equation of the bridge arm circulation of the modular multilevel converter with the mixed half-bridge and full-bridge submodules and by combining the selection rules of the number and the states of the submodules conducted by the upper and lower bridge arms in the step (4)zj(k +1), the discretization equation of the bridge arm circulation is as follows:
Figure GDA0002421992350000031
wherein izj＝(ipj+inj)/2，ipj，injBridge arm currents of an upper bridge arm and a lower bridge arm of the converter;
(7) determining a value function of bridge arm circulation: according to the given value of the bridge arm circulation obtained in the step (1) and the step (5)
Figure GDA0002421992350000032
And a predicted value izj(k +1), the cost function for obtaining the predicted output current is:
Figure GDA0002421992350000033
(8) determining a cost function of the whole system: for simultaneously controlling output current and bridge arm circulation of converter, an output current weight coefficient lambda is introducedsAnd bridge arm circulating current weight coefficient lambdazAnd obtaining a value function of the whole system:
Jj＝λsJsj+λzJzj
selecting a system cost function J according to the selection rule of the levels of the upper bridge arm and the lower bridge arm of the converter in the step (4)jThe smallest combination of switches is a given combination of levels, denoted
Figure GDA0002421992350000034
(9) And (3) capacitor voltage balance control and submodule input state determination: performing balance control on the capacitance and voltage of the bridge arm by adopting a sequencing algorithm, and combining the upper bridge arm level and the lower bridge arm level given in the step (8) according to the bridge arm current direction of the converter
Figure GDA0002421992350000035
And determining the switch state of each submodule by the following method:
① determining the switching state of each submodule of the upper bridge arm
If ipj>0，
Figure GDA0002421992350000036
Sequencing all the N sub-module capacitor voltages, and selecting the capacitor voltage with small value
Figure GDA0002421992350000037
Sub-modules to make them operate at + UcState, other submodules are working in the cutting-off state; if ipj>0，
Figure GDA0002421992350000038
Sequencing capacitor voltages of F full-bridge submodules in the bridge arm (F is the number of the full-bridge submodules in the bridge arm), and selecting the capacitor voltage with large value
Figure GDA0002421992350000039
Full bridge sub-modules, making them work at-UcState, other submodules are working in the cutting-off state;
if ipj≤0，
Figure GDA00024219923500000310
Sequencing all the N sub-module capacitor voltages, and selecting the capacitor voltage with large value
Figure GDA00024219923500000311
Sub-modules to make them operate at + UcState, other submodules are working in the cutting-off state; if ipj≤0，
Figure GDA00024219923500000312
Sequencing F full-bridge sub-module capacitor voltages in a bridge arm, and selecting capacitor voltages with small capacitor voltages
Figure GDA00024219923500000313
Full bridge sub-modules, making them work at-UcState, other submodules are working in the cutting-off state;
② determining the switching state of each submodule of the lower bridge arm
If ipj>0，
Figure GDA0002421992350000041
Sequencing all the N sub-module capacitor voltages, and selecting the capacitor voltage with small value
Figure GDA0002421992350000042
Sub-modules to make them operate at + UcState, other submodules are working in the cutting-off state; if ipj>0，
Figure GDA0002421992350000043
Sequencing F full-bridge sub-modules in the bridge arm, and selecting the sub-modules with large capacitor voltage
Figure GDA0002421992350000044
Full bridge sub-modules, making them work at-UcState, other submodules are working in the cutting-off state;
if ipj≤0，
Figure GDA0002421992350000045
Sequencing all the N sub-module capacitor voltages, and selecting the capacitor voltage with large value
Figure GDA0002421992350000046
Sub-modules to make them operate at + UcState, other submodules are working in the cutting-off state; if ipj≤0，
Figure GDA0002421992350000047
Sequencing F full-bridge sub-module capacitor voltages in a bridge arm, and selecting capacitor voltages with small capacitor voltages
Figure GDA0002421992350000048
Full bridge sub-modules, making them work at-UcStatus, other submodules operate in the cut-off state.
Compared with the prior art, the invention does not need a PI controller, omits the complicated parameter setting process, can realize multi-target control by only utilizing one system cost function, does not need to increase various control rings, simplifies the control, and simultaneously improves the dynamic response capability of the system, thereby improving the working efficiency of the system.
Drawings
FIG. 1 is a flow chart of the present invention;
FIG. 2 is a main circuit topology of a modular multilevel converter of the present invention having a mix of half-bridge and full-bridge sub-modules;
FIG. 3 is a half-bridge sub-module circuit topology of the present invention;
FIG. 4 is a full bridge sub-module circuit topology of the present invention;
FIG. 5 is a flowchart of the input of the upper bridge arm submodule of the present invention;
FIG. 6 is a flow chart of the lower arm submodule investment of the present invention.
Detailed Description
The invention will be further explained with reference to the drawings.
As shown in FIG. 2, the modular multilevel converter circuit with mixed half-bridge and full-bridge submodules, which applies the predictive control method, comprises a three-phase circuit a, b and c, wherein each phase consists of an upper bridge arm, a lower bridge arm, a reactor L and a resistor R which are connected in series, and the upper bridge arm comprises N Submodules (SM)p1-SMpF-SMpN) The lower bridge arm comprises N sub-modules (SM)n1-SMnF-SMnN) And the submodules 1-F are full-bridge submodules, and the submodules F-N are half-bridge submodules.
Each phase of the modular multilevel converter with the mixed half-bridge and full-bridge submodules is composed of 2N submodules, as shown in fig. 3 and 4, diodes D1, D2, D3 and D4 are anti-parallel diodes of power switches VT1, VT2, VT3 and VT4 respectively; c is a DC capacitor with a voltage uc(ii) a In the submodule of the half-bridge structure, power switches VT1 and VT2 are connected in series and then are connected in parallel with a direct current capacitor C, and A, B is an input/output end of the submodule; in the submodule of the full-bridge structure, power switches VT1 and VT2 are connected in series, VT3 and VT4 are connected in series, and then are connected with a direct current capacitor C in parallel, A, B is an input end and an output end of the submodule. As shown in fig. 2, the upper bridge arm and the lower bridge arm are each formed by connecting N sub-modules in series, that is, the output end B of the previous sub-module is connected to the input end a of the next sub-module. Upper bridge arm top sub-module SMp1The input end A of the bridge arm is connected to the positive pole of a direct current power supply, and the lowest sub-module SM of the lower bridge armnNThe output end B of the DC power supply is connected to the cathode of the DC power supply. Meanwhile, the submodules of the modular multilevel converter with the mixed half-bridge and full-bridge submodules contain independent control units.
For convenience of description, the operation state of the sub-module in the modular multilevel converter with the mixed half-bridge and full-bridge sub-modules in normal operation will be described first. As shown in FIG. 3, for the half-bridge submodule, when power switch VT1 is on and power switch VT2 is off, the current charges capacitor C through diode D1, or discharges capacitor C through power switch VT1, and the submodule output voltage is + ucCalled half-bridge submodule input state; when the power switch VT1 is turned off and the power is turned onWhen the off VT2 is turned on, current passes through the power switch VT2 or the diode D2, the capacitor C is always in a bypass state, the voltage of the capacitor C does not change, and the output voltage of the submodule is 0, which is called a half-bridge submodule cutting state. As shown in fig. 4, for the full-bridge submodule, when the power switches VT1 and VT4 are turned on and the power switches VT2 and VT3 are turned off, the current charges the capacitor C through the diodes D1 and D4, or discharges the capacitor C through the power switches VT1 and VT4, and the output voltage of the submodule is + ucThe state is called a forward input state of the full-bridge submodule; when the power switches VT2, VT3 are on and the power switches VT1, VT4 are off, the current charges the capacitor C through the diodes D3, D2, or discharges the capacitor C through the power switches VT2, VT3, and the sub-module output voltage is-ucThe state is called a full-bridge submodule reverse input state; when the power switches VT1 and VT3 are turned on and the power switches VT2 and VT4 are turned off, the current passes through the diode D1 and the power switch VT3 or the diode D3 and the power switch VT1, the capacitor C is always bypassed, the sub-module output voltage is 0, which is called a full-bridge sub-module cut-off state, and similarly, when the power switches VT2 and VT4 are turned on and the power switches VT1 and VT3 are turned off, the current passes through the power switch VT2 and the diode D4 or the power switch VT4 and the diode D2, the capacitor C is always bypassed, and the sub-module output voltage is 0, which is also called a full-bridge cut-off state.
The influence of the bridge arm current direction on the sub-module capacitor voltage is illustrated by taking a phase a of a modular multilevel converter with a mixed half-bridge and full-bridge sub-modules as an example. As shown in fig. 2, upper arm current ipaAnd lower arm current inaAll the positive directions of (1) are downward. For half-bridge submodule, current i of upper bridge armpaAnd lower arm current inaWhen the value of the voltage is greater than 0, the sub-module capacitor C in the switching state is charged for charging current, and the voltage of the capacitor C is increased; when upper bridge arm current ipaAnd lower arm current inaWhen the value of (d) is less than 0, the sub-module capacitor C in the on state is discharged as a discharge current, and the voltage of the capacitor C is reduced. For the full-bridge submodule, when the upper bridge arm current ipaAnd lower arm current inaIf the sub-module is in the forward input state, the sub-module capacitor C is charged and the capacitor C is charged when the value of (D) is greater than 0C, the voltage rises, if the sub-module is in a reverse input state, the sub-module capacitor C discharges, and the voltage of the capacitor C decreases; when upper bridge arm current ipaAnd lower arm current inaWhen the value of (1) is less than 0, if the submodule is in a forward input state, the submodule capacitor C discharges and the voltage of the capacitor C is reduced, and if the submodule is in a reverse input state, the submodule capacitor C charges and the voltage of the capacitor C is increased.
As shown in fig. 1, the present invention utilizes a rule for selecting a predetermined upper and lower bridge arm level combination to select an optimal level combination, and determines the on-off state of each sub-module by combining the sorting result of capacitance and voltage of the bridge arm sub-modules and the bridge arm current direction, and specifically describes the model predictive control method of the present invention by taking the above circuit as an example, which comprises the following steps:
(1) according to the active power and reactive power instruction value P*And Q*Calculating the given value of output current
Figure GDA0002421992350000061
Firstly, calculating an output current given value under an αβ coordinate system, wherein the calculation formula is as follows:
Figure GDA0002421992350000062
wherein u issαAnd usβIs the component of the grid voltage in the αβ coordinate system,
Figure GDA0002421992350000063
and
Figure GDA0002421992350000064
the component of the given value of the output current under αβ coordinate system, the Clark transformation, the power before and after transformation are unchanged, and then the Clark inverse transformation is used to obtain the given value of the output current
Figure GDA0002421992350000065
j is a, b, c, and represents a, b, c triphase, isTo output a current.
2) According to the active power instruction value P*And a reactive power command value Q*Calculating bridge arm loop current given
Figure GDA0002421992350000066
Figure GDA0002421992350000067
Wherein izIs a circular current of a bridge arm,
Figure GDA0002421992350000068
for a given dc side current, neglecting the converter internal losses,
Figure GDA0002421992350000069
Udcis a dc voltage.
(3) Determining the change range of the bridge arm voltage: if the maximum number of the full-bridge sub-modules working at the negative level is M, the direct-current voltage is Udc＝(N-M)Uc，UcThe voltage of the capacitor of the rated submodule is obtained according to the principle that the power of the AC side and the DC side is not changed,
Figure GDA00024219923500000610
Usm、Ismis the amplitude of the ac grid voltage,
Figure GDA00024219923500000611
is the ac power factor; consider Usm＝1/2mUdcAnd m is the modulation degree of the system,
Figure GDA00024219923500000612
thus, the fundamental component of the leg current is (for example phase a):
Figure GDA00024219923500000613
according to the capacitor voltage balance requirement, the current direction must have positive or negative in a complete bridge arm current period, so the above formula must be satisfied, Im/2≥mImAnd/4, i.e. m is less than or equal to 2.Because of mUdc/2≤UdcTherefore, the maximum number M of the full-bridge submodules working in the negative level state must be satisfied, wherein M is less than or equal to N/3, and the change range of the bridge arm voltage is determined to be expanded to be [ -MUc，-(M-1)Uc,…，(N-1)Uc，NUc]。
(4) Controlling output current: calculating a predicted value i of the output current at the next moment according to a discretization equation of the output current of the modular multilevel converter with the mixed half-bridge and full-bridge submodulessj(k +1), the discretization equation for the output current is:
Figure GDA0002421992350000071
where k is the kth time, LdIs the equivalent output inductance of the system, Ld＝L/2+Ls(ii) a L is bridge arm inductance, LsFor the output inductance, RdIs the equivalent output resistance of the system, Rd＝R/2+Rs(ii) a R is bridge arm resistance, RsFor the output resistance, Ts is the sampling frequency, usjIs the grid voltage upj、unjFor the output voltage of the upper and lower bridge arms, setting the capacitor voltage balance of the module, then ucjim≈Uc，i＝p,n，m＝1,2,…,N，ucjimFor the capacitance voltage of each submodule, p is the upper bridge arm of the converter, N is the lower bridge arm of the converter, m represents N different submodules of each bridge arm, and u ispj＝NpjUc，unj＝NnjUc；Npj，NnjThe number and the state of the submodules respectively communicated with the upper bridge arm and the lower bridge arm if Npj，NnjPositive, indicating that the selected sub-module output + UcIf N is presentpj，NnjNegative, indicating the selected full bridge sub-module output-UcThe sum of the voltages of the submodules for ensuring that the upper bridge arm and the lower bridge arm of each phase are simultaneously conducted is (N-M) Uc，Npj，NnjThe rule of selection is:
(Npj,Nnj)＝[(-M,N),(-(M-1),N-1),...,(N-1,-(M-1)),(N,-M)]；
(5) determining a cost function of the output current: according to the given value of the output current obtained in the step (1) and the step (4)
Figure GDA0002421992350000072
And a predicted value isj(k +1), obtaining a cost function of the output current as:
Figure GDA0002421992350000073
(6) bridge arm circulation control: calculating a predicted value i of the bridge arm circulation at the next moment under different switch combinations according to a discretization equation of the bridge arm circulation of the modular multilevel converter with the mixed half-bridge and full-bridge submodules and by combining the selection rules of the number and the states of the submodules conducted by the upper and lower bridge arms in the step (4)zj(k +1), the discretization equation for the bridge arm circulation is:
Figure GDA0002421992350000074
wherein izj＝(ipj+inj)/2，ipj，injThe bridge arm current of the upper bridge arm and the lower bridge arm of the converter.
(7) Determining a value function of bridge arm circulation: according to the given value of the bridge arm circulation current obtained in the step (2) and the step (6)
Figure GDA0002421992350000075
And a predicted value izj(k +1), the cost function for obtaining the predicted output current is:
Figure GDA0002421992350000081
(8) determining a cost function of the whole system: for simultaneously controlling output current and bridge arm circulation of converter, an output current weight coefficient lambda is introducedsAnd bridge arm circulating current weight coefficient lambdazAnd obtaining a value function of the whole system:
Jj＝λsJsj+λzJzj
selecting a value function J of the system according to the selection rule of the number and the state of the submodules communicated with the upper bridge arm and the lower bridge arm in the step (4)jThe smallest combination of switches is a given combination of levels, denoted
Figure GDA0002421992350000082
(9) And (3) capacitor voltage balance control and submodule input state determination: as shown in fig. 5 and 6, the bridge arm capacitance and voltage are subjected to balance control by adopting a sequencing algorithm, and according to the bridge arm current direction of the converter and the level combination of the upper bridge arm and the lower bridge arm given in the step (8)
Figure GDA0002421992350000083
And determining the switching condition of each submodule by the following method:
① determining the switching state of each submodule of the upper bridge arm
If ipj>0，
Figure GDA0002421992350000084
Sequencing all the N sub-module capacitor voltages, and selecting the capacitor voltage with small value
Figure GDA0002421992350000085
Sub-modules to make them operate at + UcState, other submodules are working in the cutting-off state; if ipj>0，
Figure GDA0002421992350000086
Sequencing capacitor voltages of F full-bridge submodules in the bridge arm (F is the number of the full-bridge submodules in the bridge arm), and selecting the capacitor voltage with large value
Figure GDA0002421992350000087
Full bridge sub-modules, making them work at-UcState, other submodules are working in the cutting-off state;
if ipj≤0，
Figure GDA0002421992350000088
Sequencing all the N sub-module capacitor voltages, and selecting the capacitor voltage with large value
Figure GDA0002421992350000089
Sub-modules to make them operate at + UcState, other submodules are working in the cutting-off state; if ipj≤0，
Figure GDA00024219923500000810
Sequencing capacitor voltages of F full-bridge sub-modules in a bridge arm, and selecting a capacitor with a small voltage
Figure GDA00024219923500000811
Full bridge sub-modules, making them work at-UcState, other submodules are working in the cutting-off state;
② determining the switching state of each submodule of the lower bridge arm
If ipj>0，
Figure GDA00024219923500000812
Sequencing all the N sub-module capacitor voltages, and selecting the capacitor voltage with small value
Figure GDA00024219923500000813
Sub-modules to make them operate at + UcState, other submodules are working in the cutting-off state; if ipj>0，
Figure GDA00024219923500000814
Sequencing F full-bridge sub-modules in the bridge arm, and selecting the sub-modules with large capacitor voltage
Figure GDA00024219923500000815
Full bridge sub-modules, making them work at-UcState, other submodules are working in the cutting-off state;
if ipj≤0，
Figure GDA00024219923500000816
Sequencing all the N sub-module capacitor voltages, and selecting the capacitor voltage with large value
Figure GDA00024219923500000817
Sub-modules to make them operate at + UcState, other submodules are working in the cutting-off state; if ipj≤0，
Figure GDA00024219923500000818
Sequencing F full-bridge sub-module capacitor voltages in a bridge arm, and selecting capacitor voltages with small capacitor voltages
Figure GDA0002421992350000091
Full bridge sub-modules, making them work at-UcStatus, other submodules operate in the cut-off state.
Compared with PI control, the model predictive control of the modular multilevel converter with the mixed half-bridge and full-bridge sub-modules realizes the simultaneous control of 3 control targets by only one cost function, does not need a plurality of PI controllers, omits a fussy parameter setting process, and has faster dynamic response in the control effect.

Claims (4)

1. The MMC model prediction control method for mixing a half-bridge submodule and a full-bridge submodule is characterized in that an optimal level combination is selected by utilizing a specified selection rule of an upper bridge arm level combination and a lower bridge arm level combination, and the switching state of each submodule is determined by combining a capacitor voltage sequencing result of the bridge arm submodule and a current direction of the bridge arm, and specifically comprises the following steps:
(1) according to the active power and reactive power instruction value P*And Q*Calculating the given value of output current
Figure FDA0002421992340000011
Wherein j is a, b, c, and represents a, b, c triphase, isIs an output current;
(2) according to the active power instruction value P*And a reactive power command value Q*Calculating bridge arm ringFlow set point
Figure FDA0002421992340000012
(3) Determining the change range of the bridge arm voltage: setting the maximum number of full-bridge submodules working at a negative level state as M, determining that the variation range of the bridge arm voltage is expanded to be [ -MU (multi-user) according to the requirement of capacitance-voltage balance and M is less than or equal to N/3c，-(M-1)Uc,…，(N-1)Uc，NUc]While the DC voltage is Udc＝(N-M)Uc；
Wherein N is the number of submodules of each bridge arm, UcIs the rated submodule capacitor voltage;
(4) controlling output current: calculating a predicted value i of the output current at the next moment according to a discretization equation of the output current of the modular multilevel converter with the mixed half-bridge and full-bridge submodulessj(k +1), the discretization equation for the output current is:
Figure FDA0002421992340000013
where k is the kth time, LdIs the equivalent output inductance of the system, Ld＝L/2+Ls(ii) a L is bridge arm inductance, LsFor the output inductance, RdIs the equivalent output resistance of the system, Rd＝R/2+Rs(ii) a R is bridge arm resistance, RsFor the output resistance, Ts is the sampling frequency, usjIs the grid voltage upj、unjFor the output voltage of the upper and lower bridge arms, setting the capacitor voltage balance of the module, then ucjim≈Uc，i＝p,n，m＝1,2,…,N，ucjimFor the capacitance voltage of each submodule, p is the upper bridge arm of the converter, N is the lower bridge arm of the converter, m represents N different submodules of each bridge arm, and u ispj＝NpjUc，unj＝NnjUc；Npj，NnjThe number and the state of the submodules respectively communicated with the upper bridge arm and the lower bridge arm if Npj，NnjPositive, indicating that the selected sub-module output + UcIf N is presentpj，NnjNegative, indicating the selected full bridge sub-module output-UcThe sum of the voltages of the submodules for ensuring that the upper bridge arm and the lower bridge arm of each phase are simultaneously conducted is (N-M) Uc，Npj，NnjThe rule of selection is:
(Npj,Nnj)＝[(-M,N),(-(M-1),N-1),...,(N-1,-(M-1)),(N,-M)]；
(5) determining a cost function of the output current: according to the given value of the output current obtained in the step (1) and the step (4)
Figure FDA0002421992340000014
And a predicted value isj(k +1), the cost function of the output current is obtained as:
Figure FDA0002421992340000021
(6) bridge arm circulation control: calculating a predicted value i of the bridge arm circulation at the next moment under different switch combinations according to a discretization equation of the bridge arm circulation of the modular multilevel converter with the mixed half-bridge and full-bridge submodules and by combining the selection rules of the number and the states of the submodules conducted by the upper and lower bridge arms in the step (4)zj(k +1), the discretization equation of the bridge arm circulation is as follows:
Figure FDA0002421992340000022
wherein izj＝(ipj+inj)/2，ipj，injBridge arm currents of an upper bridge arm and a lower bridge arm of the converter;
(7) determining a value function of bridge arm circulation: according to the given value of the bridge arm circulation obtained in the step (1) and the step (5)
Figure FDA0002421992340000023
And a predicted value izj(k +1), obtaining a value function of the predicted bridge arm circulation as follows:
Figure FDA0002421992340000024
(8) determining a cost function of the whole system: for simultaneously controlling output current and bridge arm circulation of converter, an output current weight coefficient lambda is introducedsAnd bridge arm circulating current weight coefficient lambdazAnd obtaining a value function of the whole system:
Jj＝λsJsj+λzJzj
selecting a system cost function J according to the selection rule of the levels of the upper bridge arm and the lower bridge arm of the converter in the step (4)jThe smallest combination of switches is a given combination of levels, denoted
Figure FDA0002421992340000025
(9) And (3) capacitor voltage balance control and submodule input state determination: performing balance control on the capacitance and voltage of the bridge arm by adopting a sequencing algorithm, and combining the upper bridge arm level and the lower bridge arm level given in the step (8) according to the bridge arm current direction of the converter
Figure FDA0002421992340000026
And determining the switch state of each submodule by the following method:
① determining the switching state of each submodule of the upper bridge arm
If ipj>0，
Figure FDA0002421992340000027
Sequencing all the N sub-module capacitor voltages, and selecting the capacitor voltage with small value
Figure FDA0002421992340000028
Sub-modules to make them operate at + UcState, other submodules are working in the cutting-off state; if ipj>0，
Figure FDA0002421992340000029
Sequencing F full-bridge sub-module capacitor voltages in bridge armSelecting a capacitor with a large voltage
Figure FDA00024219923400000210
Full bridge sub-modules, making them work at-UcState, other submodules are working in the cutting-off state;
if ipj≤0，
Figure FDA00024219923400000211
Sequencing all the N sub-module capacitor voltages, and selecting the capacitor voltage with large value
Figure FDA00024219923400000212
Sub-modules to make them operate at + UcState, other submodules are working in the cutting-off state; if ipj≤0，
Figure FDA00024219923400000213
Sequencing F full-bridge sub-module capacitor voltages in a bridge arm, and selecting capacitor voltages with small capacitor voltages
Figure FDA0002421992340000031
Full bridge sub-modules, making them work at-UcState, other submodules are working in the cutting-off state;
② determining the switching state of each submodule of the lower bridge arm
If ipj>0，
Figure FDA0002421992340000032
Sequencing all the N sub-module capacitor voltages, and selecting the capacitor voltage with small value
Figure FDA0002421992340000033
Sub-modules to make them operate at + UcState, other submodules are working in the cutting-off state; if ipj>0，
Figure FDA0002421992340000034
Sequencing F full-bridge sub-modules in the bridge arm, and selecting the sub-modules with large capacitor voltage
Figure FDA0002421992340000035
Full bridge sub-modules, making them work at-UcState, other submodules are working in the cutting-off state;
if ipj≤0，
Figure FDA0002421992340000036
Sequencing all the N sub-module capacitor voltages, and selecting the capacitor voltage with large value
Figure FDA0002421992340000037
Sub-modules to make them operate at + UcState, other submodules are working in the cutting-off state; if ipj≤0，
Figure FDA0002421992340000038
Sequencing F full-bridge sub-module capacitor voltages in a bridge arm, and selecting capacitor voltages with small capacitor voltages
Figure FDA0002421992340000039
Full bridge sub-modules, making them work at-UcState, other submodules are working in the cutting-off state;
and F is the number of full-bridge submodules in the bridge arm.
2. The MMC model predictive control method of claim 1, wherein the output current set point in step (1) is set as the output current
Figure FDA00024219923400000310
The method comprises the following steps:
1) calculating the given value of the output current under an αβ coordinate system, wherein the calculation formula is as follows:
Figure FDA00024219923400000311
wherein u issαAnd usβIs the component of the grid voltage in the αβ coordinate system,
Figure FDA00024219923400000312
and
Figure FDA00024219923400000313
is the component of the given value of the output current in αβ coordinate system;
2) the Clark transformation is used, the power before and after transformation is unchanged, and the Clark inverse transformation is used for obtaining the given value of the output current
Figure FDA00024219923400000314
3. The MMC model predictive control method of claim 1 or 2, wherein the bridge-arm loop current set point in step (2)
Figure FDA00024219923400000315
The calculation formula is as follows:
Figure FDA00024219923400000316
wherein izIs a circular current of a bridge arm,
Figure FDA0002421992340000041
for a given dc side current, neglecting the converter internal losses,
Figure FDA0002421992340000042
Udcis a dc voltage.
4. The MMC model predictive control method for half-bridge and full-bridge sub-module mixture of claim 1 or 2, wherein the discretization equation of the bridge arm circulation in step (4) is as follows:
Figure FDA0002421992340000043
wherein izj＝(ipj+inj)/2，ipj，injThe bridge arm current of the upper bridge arm and the lower bridge arm of the converter.
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