CN113992031A - Neutral point offset voltage control method of three-port nonagon MMC - Google Patents

Neutral point offset voltage control method of three-port nonagon MMC Download PDF

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CN113992031A
CN113992031A CN202111204666.5A CN202111204666A CN113992031A CN 113992031 A CN113992031 A CN 113992031A CN 202111204666 A CN202111204666 A CN 202111204666A CN 113992031 A CN113992031 A CN 113992031A
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voltage
alternating current
power
bridge arm
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CN113992031B (en
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黄文慧
刘飞
庄一展
刁晓光
黄艳辉
查晓明
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Wuhan University WHU
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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
    • H02M5/00—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases
    • 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
    • H02J3/34—Arrangements for transfer of electric power between networks of substantially different frequency
    • 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
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • 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
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/381—Dispersed generators
    • 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
    • H02M5/00—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases
    • H02M5/40—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc
    • H02M5/42—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters
    • H02M5/44—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac
    • H02M5/453—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M5/458—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • 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
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20—The dispersed energy generation being of renewable origin
    • H02J2300/28—The renewable source being wind energy
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/70—Wind energy
    • Y02E10/76—Power conversion electric or electronic aspects

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Abstract

The invention relates to the technical field of power electronic converters, in particular to a neutral point offset voltage control method of a three-port nonagon MMC, wherein the converter comprises a first alternating current port, a second alternating current port, a third alternating current port and nine nonagon structures of end-to-end bridge arms; the first alternating current port is connected with an alternating current power grid, and the second alternating current port and the third alternating current port are respectively connected with the two low-frequency wind power systems; the control method is based on the condition of power balance among three-port nonagon MMC bridge arms, a neutral point offset voltage control method is adopted, the power constant which causes the boost of the direct-current pump of the sub-module on each bridge arm is restrained, and the aim of tracking a reference value by the capacitance and voltage of the sub-module is fulfilled; the vertical compression pump is prevented from being lifted, and the stability of the system is improved. Through voltage-sharing control, the fluctuation amount of the direct voltage of the sub-module is inhibited, and the purpose of optimizing the capacitance parameter selection is achieved.

Description

Neutral point offset voltage control method of three-port nonagon MMC
Technical Field
The invention belongs to the technical field of power electronic converters, and particularly relates to a neutral point offset voltage control method of a three-port nonagon MMC.
Background
At present, three main schemes are provided for realizing offshore wind power collection: high-voltage alternating current transmission, high-voltage direct current transmission and frequency division transmission. The offshore frequency division power transmission has certain economic advantages in an offshore wind power transmission scheme with the distance of 100km-150 km. The offshore frequency division power transmission improves the transmission capacity of the system by reducing the frequency of the system and reducing transmission loops, does not need an offshore converter station, and reduces the investment cost and the maintenance cost.
The core device of offshore frequency division power transmission is a frequency converter, and a matrix converter is usually adopted to realize interconnection of a low-frequency system and a power-frequency system. When the transmission capacity is improved, a plurality of independent frequency converters are needed for grid connection of the multi-end offshore frequency division power transmission system, and the size and the cost of the device are increased. The multi-port converter is more valuable in consideration of the utilization rate of the device and the smaller conversion stage number.
A hexagonal Modular Multilevel Converter (MMC) can be regarded as a simplified form of a matrix converter, and a three-port nonagon MMC is formed by adding three sets of bridge arms. The three-port nonagon MMC has high bridge arm reuse rate, greatly reduces the use of switching devices, has good low-frequency characteristics, and can realize interconnection of two low-frequency systems and a power frequency system. However, the low-frequency power fluctuation from the offshore wind power generation system affects the stability of the system, such as the voltage rise of the sub-module capacitor vertical compression pump, and the system stability. For the problem of uneven direct voltage between bridge arms, there are two common ideas, namely, increasing the capacitance value of the capacitor of the sub-module and designing the control, however, the increase of the capacitance value can increase the volume and the cost of the device.
Disclosure of Invention
The invention aims to provide a three-port nonagon MMC circulating current model and a neutral point offset voltage control method, and solves the technical problem that bridge arm current is increased due to the fact that two low-frequency systems and a power frequency system are interconnected by analyzing the nonagon MMC, and low-frequency components of offshore low-frequency wind power are generated.
In order to solve the technical problems, the invention adopts the following technical scheme: a neutral point offset voltage control method of a three-port nonagon MMC comprises a first alternating current port, a second alternating current port and a third alternating current port, and nine bridge arms which are connected end to end are of a nonagon structure, each bridge arm comprises n H-bridge inverter modules and 1 inductor, and n is a positive integer which is larger than or equal to 1; the nine vertexes of the nonagon structure are R, W, X, S, U, Y, T, V, Z clockwise, wherein XYZ is a first alternating current port, RST is a second alternating current port, UVW is a third alternating current port, the first alternating current port is connected with an alternating current power grid, and the second alternating current port and the third alternating current port are respectively connected with two low-frequency wind power systems; the first alternating current port, the second alternating current port and the third alternating current port transmit active power through a bridge arm; neutral point offset voltage control is realized based on the power balance condition among bridge arms; the control method comprises the following steps:
step 1, port power control: the power balance condition comprises active power balance of the first alternating current port, the second alternating current port and the third alternating current port, and the formula is as follows:
Figure BDA0003306398750000021
in the formula, instantaneous active and instantaneous reactive power P of three portsxyzIs the instantaneous active power, P, of the first AC portrstInstantaneous active power, P, for the second AC portuvwInstantaneous active power, Q, for the third AC portxyzInstantaneous reactive power, Q, for the first AC portrstInstantaneous reactive power, Q, for the second AC portuvwInstantaneous reactive power for the third ac port; using the first AC port as a reference point, vN1The second AC port is offset from the neutral point of the first AC port by a voltage vN2Neutral bias of the second AC port compared to the first AC portPressing; i.e. icirIs a circular flow;
the total direct voltage stability is increased at the front end of the power balance control, and the direct voltage mean value v of each sub-module capacitor is controlled by adopting PI (proportional integral)dc_avgConverge on the reference value v of the capacitor voltagedc_ref；
The output of the controller is the current amplitude I of the first alternating current port1 *The current amplitude I of the second alternating current port2 *And third AC port current magnitude I3 *；
Step 2, port current control: according to the current amplitude I of the first alternating current port1 *The current amplitude I of the second alternating current port2 *And third AC port current magnitude I3 *And generating a current signal with a port of the phase generation module, performing difference with the acquired current, and generating a voltage signal v of a first alternating current port by adopting QPR controlxyz *A second AC port voltage signal vrst *And a third AC port voltage signal vuvw *；
Step 3, neutral point offset voltage control: the neutral point offset voltage expression is obtained according to the power balance expression as follows:
Figure BDA0003306398750000031
wherein v isN1 *And vN2 *In order to offset the voltage at the neutral point,
Figure BDA0003306398750000032
Pxyzactive power of the first AC port, PrstActive power, P, for the second AC portuvwActive power, Q, for the second AC portxyzIs the reactive power, Q, of the first AC portrstIs the reactive power, Q, of the first AC portuvwIs the reactive power of the third ac port; k is a radical of1And k2Is a proportional parameter;
Figure BDA0003306398750000033
and
Figure BDA0003306398750000034
is the average value of the capacitance and the voltage corresponding to the three groups of bridge arms;
synthesizing the neutral point offset voltage expression and the voltage signals of all ports into bridge arm voltage signals;
step 4, bridge arm voltage-sharing control: the average value of the capacitor voltage of each bridge arm is differed from the average value of the voltage of each group of bridge arms, and the item with the largest output deviation is used as the input of PI control, so that the voltage sharing among the bridge arms is realized, and the regulating quantity v of the voltage of the bridge arms is outputb147 *、vb258 *、vb369 *；
Step 5, bridge arm inner voltage-sharing control: the charge and discharge of the capacitor are controlled by finely adjusting the bridge arm voltage modulation wave, the voltage sharing of the capacitor in the bridge arm is ensured, and the adjustment value of each module of the bridge arm voltage is output;
and 6, superposing the modulation wave of each unit and the adjusting signal to generate a modulation signal, and realizing the phase shift of the modulation signal in the bridge arm by adopting horizontal carrier phase shift to serve as a switching signal.
Compared with the prior art, the invention has the beneficial effects that: according to the invention, based on the power balance condition among three-port nonagon MMC bridge arms, the power constant of the sub-module direct-current pump, which is caused by the voltage rise of each bridge arm, is restrained through a neutral point offset voltage control method, so that the aim of tracking the reference value by the sub-module capacitor voltage is achieved, the sub-module capacitor direct-current pump is prevented from rising, and the stability of the system is improved. The fluctuation amount of the direct voltage of the sub-modules is restrained through voltage-sharing control, and the purpose of optimizing the capacitance parameter selection is achieved.
Drawings
FIG. 1 is a block diagram of a three-port MMC of an embodiment of the present invention;
FIG. 2 is an equivalent schematic diagram of a three-port nonagon MMC according to an embodiment of the present invention;
FIG. 3 is a control flow diagram of a three-port nonagon MMC according to an embodiment of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be described clearly and completely with reference to the following embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
It should be noted that the embodiments and features of the embodiments may be combined with each other without conflict.
The present invention is further illustrated by the following examples, which are not to be construed as limiting the invention.
For the problem of uneven direct voltage between bridge arms, there are two common ideas, and by increasing the capacitance value of the sub-module capacitor and the control design, the increase of the capacitance value increases the volume and cost of the device, so that the embodiment provides a neutral point offset voltage control method to suppress the fluctuation amount of the direct voltage of the sub-module.
Active power is transmitted among three ports of the three-port nonagon MMC through a bridge arm, so that the accumulation of a power constant on the bridge arm is easily caused, and the direct-pressure pump of the sub-module is increased; because the power component expressions among the bridge arms have differences, the average values of the capacitance and the voltage among the bridge arms are different, and the stability of the system is influenced.
In order to meet the condition that the power constant between bridge arms is zero and prevent the voltage of the capacitor direct-current pump from rising, the circulating current and the neutral point offset voltage need to be decoupled. The embodiment provides an idea of decoupling the circulating current and the neutral point offset voltage to meet the condition of power balance between bridge arms and realize neutral point offset voltage control. Aiming at the problem of the difference of power constant expressions among bridge arms, the voltage-sharing control among the bridge arms is adopted to realize the convergence of direct voltage among the bridge arms.
The embodiment is realized by the following technical scheme that the MMC comprises a first alternating current port, a second current port, a third alternating current port and a nonagon structure formed by nine bridge arms in an end-to-end connection mode, wherein each bridge arm comprises n H-bridge inverter modules and 1 bridge arm inductor, and n is a positive integer larger than or equal to 1;
the three-port nonagon MMC has nine vertexes, as shown in fig. 1, R, W, X, S, U, Y, T, V, Z are sequentially arranged, wherein XYZ is a first alternating current port, RST is a second alternating current port, UVW is a third alternating current port, the first alternating current port is connected with a power frequency alternating current power grid, and the second alternating current port and the third alternating current port are respectively connected with an offshore wind power generation system.
An equivalent schematic diagram of the three-port nonagon MMC is shown in fig. 2, and the equivalent principle is that each bridge arm is equivalent to the series connection of an ideal voltage source and an inductor. As shown in FIG. 2, the bridge arm voltage is defined as vbi(i ═ 1,2, …,9), bridge arm current defines ibi. The first ac port, the second ac port and the third ac port correspond to three neutral points O, N respectively1And N2When using O as a reference point, vN1And vN2Respectively, the neutral point offset voltages of the first, second and third ac ports.
Taking bridge arm 1, bridge arm 2 and bridge arm 3 as examples, the power balance among the bridge arms needs to meet the following requirements:
Figure BDA0003306398750000051
wherein the neutral point offset voltage vN1And vN2To control the degree of freedom, vN1、vN2And a circulating current icirCoupling relation exists, the power balance condition cannot be directly met, and a decoupling method needs to be searched.
The expression of instantaneous active power and reactive power between ports is shown as the formula (2):
Figure BDA0003306398750000052
in the above formula, V1Is the effective value of the voltage of the first AC port, V2Is the effective value of the voltage of the second AC port, V3Is the effective value of the voltage of the third AC port, I1Is the effective value, I, of the first AC port current2Is a second crossEffective value of current at current port, I3Is the effective value of the third ac port current.
The formula (2) is taken into the formula (1), and the power balance between bridge arms needs to meet the following requirements:
Figure BDA0003306398750000053
in the formula (3), PxyzActive power of the first AC port, PrstActive power, P, for the second AC portuvwActive power, Q, for the second AC portxyzIs the reactive power, Q, of the first AC portrstIs the reactive power, Q, of the first AC portuvwIs the reactive power of the third ac port.
The bridge arm is equivalent to a series connection of a voltage source and an inductor, so that the bridge arm voltage expression is as follows:
Figure BDA0003306398750000061
wherein v iso(bi) Is the output voltage of the cascade module, vo(bin) Is the output voltage of the nth module. Due to the ring structure of the topology, the sum of the voltage vectors of the nine bridge arms is:
Figure BDA0003306398750000062
the vector sum of the nine bridge arm currents is circulating current icir(iii) carry-over (5):
Figure BDA0003306398750000063
in a switching period TcAnd the output voltages of the n sub-modules are as follows:
Figure BDA0003306398750000064
vm(bi)and VcmThe amplitudes of the modulated wave and the triangular wave of the bridge arm i. When the switching frequency is high enough, the modulated waves of each module can be considered identical:
Figure BDA0003306398750000065
as shown in fig. 1, the adjacent bridge arms of different phases of the same port have power similarity, and nine sets of bridge arms can be combined into three sets, wherein bridge arm 1, bridge arm 4 and bridge arm 7 are divided into a set of bridge arms 147, bridge arm 2, bridge arm 5 and bridge arm 8 are divided into a set of bridge arms 258, and bridge arm 3, bridge arm 6 and bridge arm 9 are divided into a set of bridge arms 369. For the bridge arms with similar power, the voltage waveforms of the capacitors are also similar. Thus, vo(bi)The vector sum of (c):
Figure BDA0003306398750000071
in the formula (9), the reaction mixture,
Figure BDA0003306398750000072
Figure BDA0003306398750000073
Figure BDA0003306398750000074
and
Figure BDA0003306398750000075
is the mean value of the capacitance voltages corresponding to bridge arm 147, bridge arm 258, and bridge arm 369. Δ vdc_(b1),Δvdc_(b2)And Δ vdc_(b3)Mean deviation.
The sum of the voltages of the bridge arms in the same group is as follows:
Figure BDA0003306398750000076
combining equations (9) and (10), one can obtain:
Figure BDA0003306398750000077
in formula (11), vN1And vN2In order to control the degree of freedom, the first half of the formula is direct pressure, and the value of the second half is small, so that neglect treatment is carried out.
Neutral point offset voltage v according to equations (6) and (11)N1And vNAnd a circulation flow icirThe relationship of (a) to (b) is in proportion:
Figure BDA0003306398750000078
combining equations (3) and (12), the neutral point offset voltage expression can be obtained:
Figure BDA0003306398750000079
Figure BDA0003306398750000081
the proportional parameter k in equation (14)1And k2Prevents overmodulation.
In summary, the neutral offset voltage is decoupled from the circulating current.
The general control block diagram of a three-port nonagon MMC is shown in fig. 3.
Port power control: as can be seen from the formula (3), the power balance condition also needs to satisfy the active power balance of the three ac ports; the total direct voltage stability is increased at the front end of the power balance control, and the direct voltage mean value v of each sub-module capacitor is controlled by adopting PI (proportional integral)dc_avgConverge on the reference value v of the capacitor voltagedc_ref. The output of the controller is the current amplitude I of the first alternating current port1 *The current amplitude I of the second alternating current port2 *And third AC port current magnitude I3 *。
Port current control: according to the current amplitude I of the first alternating current port1 *The current amplitude I of the second alternating current port2 *And third AC port current magnitude I3 *And the phase generating module carries out difference with the acquired current, and QPR control is adopted to ensure the current output characteristic of the port and the voltage signal v of the first alternating current portxyz *A second AC port voltage signal vrst *And a third AC port voltage signal vuvw *。
Neutral point offset voltage control: according to the formulas (13) and (14), an expression of neutral point offset voltage for preventing the direct-current pump from boosting is obtained, wherein the neutral point offset voltage meets the power balance condition between the bridge arms. And synthesizing the bridge arm voltage signal with the port voltage signal.
And (3) bridge arm voltage-sharing control: due to the difference of power components among the bridge arms, the voltage imbalance of the capacitors of the bridge arms is reflected. The average value of the capacitance of each bridge arm is differed from the average value of the voltage of each group of bridge arms, and the item with the largest output deviation is used as the input of PI control, so that the voltage sharing among the bridge arms is realized, and the regulating quantity v of the voltage of the bridge arms is outputb147 *、vb258 *、vb369 *。
Voltage-sharing control in the bridge arm: the charging and discharging of the capacitor are controlled by finely adjusting the bridge arm voltage modulation wave, the voltage sharing of the capacitor in the bridge arm is ensured, and the adjustment value of each module of the bridge arm voltage is output.
And the modulated waves of each unit and the regulating signals are superposed to generate modulated signals, the phase shift of the modulated signals in the bridge arm is realized by adopting horizontal carrier phase shift, and the port characteristics are optimized to be used as switching signals.
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.

Claims (1)

1. A neutral point offset voltage control method of a three-port nonagon MMC comprises a first alternating current port, a second alternating current port and a third alternating current port, and nine bridge arms which are connected end to end are of a nonagon structure, each bridge arm comprises n H-bridge inverter modules and 1 inductor, and n is a positive integer which is larger than or equal to 1; the nine vertexes of the nonagon structure are R, W, X, S, U, Y, T, V, Z clockwise, wherein XYZ is a first alternating current port, RST is a second alternating current port, UVW is a third alternating current port, the first alternating current port is connected with an alternating current power grid, and the second alternating current port and the third alternating current port are respectively connected with two low-frequency wind power systems; the first alternating current port, the second alternating current port and the third alternating current port transmit active power through a bridge arm; the method is characterized in that: neutral point offset voltage control is realized based on the power balance condition among bridge arms; the control method comprises the following steps:
step 1, port power control: the power balance condition comprises active power balance of the first alternating current port, the second alternating current port and the third alternating current port, and the formula is as follows:
Figure FDA0003306398740000011
in the formula, instantaneous active and instantaneous reactive power P of three portsxyzIs the instantaneous active power, P, of the first AC portrstInstantaneous active power, P, for the second AC portuvwInstantaneous active power, Q, for the third AC portxyzInstantaneous reactive power, Q, for the first AC portrstInstantaneous reactive power, Q, for the second AC portuvwInstantaneous reactive power for the third ac port; using the first AC port as a reference point, vN1The second AC port is offset from the neutral point of the first AC port by a voltage vN2A neutral offset voltage of the third ac port relative to the first ac port; i.e. icirIs a circular flow;
the total direct voltage stability is increased at the front end of the power balance control, and the direct voltage mean value v of each sub-module capacitor is controlled by adopting PI (proportional integral)dc_avgConverge on the reference value v of the capacitor voltagedc_ref；
Control ofThe output of the device is the current amplitude I of the first alternating current port1 *The current amplitude I of the second alternating current port2 *And third AC port current magnitude I3 *；
Step 2, port current control: according to the current amplitude I of the first alternating current port1 *The current amplitude I of the second alternating current port2 *And third AC port current magnitude I3 *And generating a current signal with a port of the phase generation module, performing difference with the acquired current, and generating a voltage signal v of a first alternating current port by adopting QPR controlxyz *A second AC port voltage signal vrst *And a third AC port voltage signal vuvw *；
Step 3, neutral point offset voltage control: obtaining a neutral point offset voltage expression according to the power balance expression:
Figure FDA0003306398740000021
wherein v isN1 *And vN2 *In order to offset the voltage at the neutral point,
Figure FDA0003306398740000022
Pxyzactive power of the first AC port, PrstActive power, P, for the second AC portuvwActive power, Q, for the second AC portxyzIs the reactive power, Q, of the first AC portrstIs the reactive power, Q, of the first AC portuvwIs the reactive power of the third ac port; k is a radical of1And k2Is a proportional parameter;
Figure FDA0003306398740000023
and
Figure FDA0003306398740000024
is the average value of the capacitance and the voltage corresponding to the three groups of bridge arms;
synthesizing the neutral point offset voltage expression and the voltage signals of all ports into bridge arm voltage signals;
step 4, bridge arm voltage-sharing control: the average value of the capacitor voltage of each bridge arm is differed from the average value of the voltage of each group of bridge arms, and the item with the largest output deviation is used as the input of PI control, so that the voltage sharing among the bridge arms is realized, and the regulating quantity v of the voltage of the bridge arms is outputb147 *、vb258 *、vb369 *；
Step 5, bridge arm inner voltage-sharing control: the charge and discharge of the capacitor are controlled by finely adjusting the bridge arm voltage modulation wave, the voltage sharing of the capacitor in the bridge arm is ensured, and the adjustment value of each module of the bridge arm voltage is output;
and 6, superposing the modulation wave of each unit and the adjusting signal to generate a modulation signal, and realizing the phase shift of the modulation signal in the bridge arm by adopting horizontal carrier phase shift to serve as a switching signal.
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