CN113708613A - Movable AC/DC multi-port flexible controller and ring closing and opening starting and stopping method - Google Patents

Movable AC/DC multi-port flexible controller and ring closing and opening starting and stopping method Download PDF

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Publication number
CN113708613A
CN113708613A CN202110831630.3A CN202110831630A CN113708613A CN 113708613 A CN113708613 A CN 113708613A CN 202110831630 A CN202110831630 A CN 202110831630A CN 113708613 A CN113708613 A CN 113708613A
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port
power
control
interconnection
flexible controller
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CN113708613B (en
Inventor
马洲俊
黄文焘
张明
朱红
邰能灵
王杰
高聪哲
洪露
�田�浩
许洪华
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Shanghai Jiaotong University
State Grid Jiangsu Electric Power Co Ltd
Nanjing Power Supply Co of State Grid Jiangsu Electric Power Co Ltd
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Shanghai Jiaotong University
State Grid Jiangsu Electric Power Co Ltd
Nanjing Power Supply Co of State Grid Jiangsu 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
    • H02M1/00—Details of apparatus for conversion
    • H02M1/36—Means for starting or stopping converters
    • 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
    • H02J5/00—Circuit arrangements for transfer of electric power between ac networks and dc networks
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/10—Arrangements incorporating converting means for enabling loads to be operated at will from different kinds of power supplies, e.g. from ac or dc
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
    • H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
    • H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
    • H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/3353—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having at least two simultaneously operating switches on the input side, e.g. "double forward" or "double (switched) flyback" converter
    • 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
    • H02M7/003—Constructional details, e.g. physical layout, assembly, wiring or busbar connections
    • 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
    • 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
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

Abstract

The invention relates to the technical field of electric power, in particular to a movable alternating current-direct current multi-port flexible controller and a ring closing and opening method, wherein the flexible controller comprises two Modular Multilevel Converters (MMC), the two modular multilevel converters are connected back to back, and the alternating current sides of the two modular multilevel converters are respectively connected with an interconnection switch S1 and an interconnection switch S2 for switching so as to move two alternating current ports AC1 and AC 2; the direct current sides of the two modular multilevel converters MMC are subjected to DC/DC conversion through a plurality of double active bridge converters DAB which are connected in series, input and output in parallel, and the output end of the DAB is used for being connected with a direct current port DC after being switched through an interconnection switch S3; MMC adopts a power decoupling control strategy, and DAB adopts a single phase-shifting control mode. According to the invention, the access position in the power distribution network is freely changed by switching at the contact switch, and the flexible connection between all or any two ports is realized by matching with a start-stop control method, so that the flexibility, stability and reliability of the system are improved.

Description

Movable AC/DC multi-port flexible controller and ring closing and opening starting and stopping method
Technical Field
The invention relates to the technical field of electric power, in particular to a movable alternating current-direct current multi-port flexible controller and a ring closing and opening starting and closing starting method.
Background
The power distribution network is used as the last ring in the whole power transmission system and bears the important task of distributing electric energy to users, but the power distribution network in China has the problems of inflexible operation structure, insufficient control capability for the power distribution network per se and the like in the development process, and the problems are more prominent along with the access of more Distributed Generation (DG) in the future, and the requirements of the development of the power distribution network in the future on high reliability, safety and flexibility cannot be met. With the development of power electronic technology in recent years, a plurality of novel flexible power distribution devices appear in a power distribution system, the concept of a Flexible Control Device (FCD) also comes from the beginning, and by replacing primary devices such as an original interconnection switch and a transformer in a power distribution network, not only can the network structure be optimized, but also the overall regulation and control capability of the power distribution network can be improved. The flexible controller serving as a power distribution network flexible connection device has the functions of flexibly regulating and controlling the power of the flexible controller, optimizing the power flow distribution of a power distribution network and improving the electric energy quality of a system, and the device design and the control design are important research contents. At present, the flexible controller device is mostly designed in a fixed two-port mode, a back-to-back modular multilevel converter (BTB-MMC) topology is most concerned, and the BTB-MMC structure has the advantages of flexible power control, alternating current and direct current decoupling, strong fault ride-through capability and the like. However, the BTB-MMC structure can only be used for double-ended ac interconnection, and the application scenarios are limited. In order to adapt to more complex power distribution network flexible interconnection scenes such as multi-terminal multi-voltage-level interconnection, alternating current-direct current hybrid connection and the like, a novel flexible controller topology with more ports, more comprehensive functions, higher reliability and wider application range needs to be further researched. The documents "Zhang national colt, Wei, Shen, Lining, Dengdeng" triangular AC/AC converter applied to three-end flexible interconnection of power distribution network [ J ] power system automation, 2021,45(08):32-40 "propose a triangular AC/AC converter which can realize three-end AC flexible interconnection and reduce half of the number of bridge arms, but can not be used in AC/DC mixed connection scene and the port current has coupling. The document "flexible interconnection strategy of a direct current micro-grid and an alternating current distribution network [ J ] electric power automation equipment 2021,41(05): 254-.
For the flexible controller device, the following problems are common:
(1) the conventional flexible controller device has the advantages of fixed position, incapability of freely selecting an installation position, simple topological structure, single function, weak control flexibility, difficulty in adapting to complex and changeable actual power distribution network operation scenes and narrow applicable scenes.
(2) Most of the existing flexible controller control modes are designed aiming at double-end alternating current or multi-end alternating current topology, the research on how to cooperate and control between alternating current and direct current ports of a flexible controller with a composite multi-port structure is less, an opening and closing ring opening and closing control method of an alternating current and direct current hybrid multi-port is more complex, and the research on the opening and closing ring opening and closing control method of a mobile flexible controller accessing a power distribution network is lacked at present.
Disclosure of Invention
The invention aims to provide a movable AC/DC multi-port flexible controller and a ring closing and opening method, which realize flexible interconnection of AC/DC multi-ports, have wide applicable scene, freely change the access position in a power distribution network through a contact switch, reduce the impact of the ring closing and opening process by matching with the ring closing and opening control method and improve the stability of the power distribution network.
In order to solve the technical problems, the technical scheme of the invention is as follows: the movable AC-DC multi-port flexible controller is of a three-port structure and comprises two modular multilevel converters MMC, wherein the two modular multilevel converters MMC are connected back to back, and the AC sides of the two modular multilevel converters MMC are respectively connected with interconnection switches S1 and S2 for switching to move two AC ports AC1 and AC 2; the direct current sides of the two modular multilevel converters MMC are subjected to DC/DC conversion through a plurality of double active bridge converters DAB which are connected in series, input and output in parallel, and the output end of the DAB is used for being connected with a direct current port DC after being switched through an interconnection switch S3;
alternating current ports AC1 and AC2 on two sides of the MMC are used for being connected with an alternating current power grid feeder line, a direct current port DC on the DAB side is used for being connected with a direct current power grid feeder line, interconnection switches S1, S2 and S3 are connected with different ports, the positions of the flexible controller among the feeder lines are moved, the switching positions of the ports are changed by controlling the switching actions of the interconnection switches S1, S2 and S3, and the change of the switching positions of the flexible controller in a power distribution network is achieved;
when the two feeder lines which are in an originally disconnected state have interconnection requirements, the flexible controller is respectively connected to the connection points of the two feeder lines through the interconnection switches of the corresponding ports and closes the interconnection switches to close the loop to realize flexible interconnection; when the two feeders only need to be interconnected normally and do not need power control, the switch cabinet is closed, the flexible controller is quitted from operation, and the interconnection switch of the corresponding port of the flexible controller is disconnected;
when the interconnection requirement of the feeder lines changes, the flexible controller changes at the feeder line access position of the power distribution network, moves the flexible controller between the other two feeder lines with the interconnection requirement, and respectively accesses the connection points of the other two feeder lines through the contact switches of the corresponding ports and closes the contact switches to realize flexible interconnection;
when two feeder lines interconnected through a switch cabinet need to be switched to an off state, corresponding ports of a flexible controller are respectively connected to connecting points of the two feeder lines and a contact switch is closed, the power flowing through the switch cabinet is zero through power control, then the switch cabinet is switched off, and loop-off control is carried out on the flexible controller to realize smooth switching of the two feeder lines from the interconnected state to the off state;
the MMC adopts a power decoupling control strategy, which comprises power outer loop control and current inner loop control; the power outer loop control is used for providing a dq axis current reference value for the current inner loop control, wherein the power outer loop control of the MMC at one side adopts constant direct current voltage and reactive power control, namely Udc-Q control, with constant active and reactive power control on the other side, i.e. P-Q control; the current inner loop control adopts a direct current control mode to generate a switching signal for driving each submodule;
DAB adopts single phase shift control mode.
According to the scheme, the single-ended modular multilevel converter MMC adopts a three-phase half-bridge modular multilevel converter, the three-phase half-bridge modular multilevel converter comprises three phase units with the same structure, each phase unit is composed of 2 bridge arms which are symmetrical up and down, and the single-ended MMC is of a three-phase six-bridge-arm structure as a whole; each bridge arm comprises N sub-modules SM which are connected in series and have the same structure, 1 bridge arm reactor Larm and 1 bridge arm equivalent resistor Rarm.
According to the scheme, the sub-module SM comprises 2 power tubes, 2 corresponding anti-parallel diodes, 1 direct-current energy storage capacitor C0, 1 rapid bypass switch K1 and 1 bypass protection diode K2; the 2 power tubes are respectively a power tube VT1 and a power tube VT2, and the anti-parallel diodes are respectively a diode VD1 and a diode VD 2; wherein, 5 devices of VT1, VT2, VD1, VD2 and C0 form a half-bridge structure topology;
an emitter of the power tube VT1 is connected with a collector of the power tube VT2, a collector of the power tube VT1 is connected with the anode of the direct-current energy storage capacitor C0, an emitter of the power tube VT2 is connected with the cathode of the direct-current energy storage capacitor C0, the power tube VT1 is reversely connected with the diode VD1 in parallel, and the power tube VT2 is reversely connected with the diode VD2 in parallel;
the fast bypass switch K1 and the bypass protection diode K2 are connected with the power tube VT2 in parallel; the K1 and K2 devices can be timely cut off and connected when the connected AC/DC system or the submodule per se fails, so that converter equipment is protected; when the submodule fails, the K1 can quickly bypass the failed submodule and ensure the continuity of bridge arm current; when serious faults such as short circuit of a direct current bus of the converter or short circuit of a transmission line occur, the K2 has good current impact resistance and can protect a freewheeling diode connected with the K2 in parallel; therefore, K1 and K2 greatly enhance the safety and utility of the sub-modules.
According to the scheme, the double-active-bridge converter DAB comprises two symmetrical H bridges and a high-frequency transformer containing leakage inductance; the left H bridge and the right H bridge are connected with each other through a high-frequency transformer.
According to the scheme, the single phase-shifting control mode adopted by the DAB specifically comprises the following steps: in DAB, the driving signals of all the switch tubes are PWM signals with the duty ratio of 50%, and the primary side signal and the secondary side signal are staggered by a phase shift angle
Figure BDA0003175790070000031
To control the primary and secondary side voltages.
According to the scheme, the DAB control also comprises the step of carrying out current sharing control on a plurality of DAB powers by adopting a power balance control strategy.
The invention also provides a movable AC/DC multiport flexible controller ring closing and opening control method, which adopts the movable AC/DC multiport flexible controller and comprises a ring closing starting control method and a ring opening stopping control method;
the loop closing starting control method specifically comprises the following steps:
when two AC ports are closed, one side of MMC adopts Udc-Q control, and the MMC on the other side adopts fixed P-Q control; first, the U of the AC1 AC port MMCdcSetting the control command to be a rated direct-current voltage value, setting the power control command of an AC2 AC port MMC to be 0, enabling the flexible controller to have no power transmission, then switching on the connecting switches S1 and S2, and controlling the power reference value of the AC2 AC port to be changed into an operation value after loop closing; limiting the change rate of the power reference value to k1, and limiting the value of k1 according to the current maximum current, so as to avoid sudden change of the power reference value and make the power reference value rise linearly;
when the AC port AC1 and the DC port DC are looped, the MMC adopts Udc-Q control, setting the DC bus voltage reference to nominal voltage, DAB control active reference PrefWhen the voltage is 0, the flexible controller has no power transmission, then the connection switches S1 and S3 are switched on, and DAB is switched to the voltage control of a DC port DC after loop closing; limiting the change rate of the active reference value to k2, and limiting the value of k2 according to the current maximum current to avoid sudden change of the power reference value;
when the AC port AC2 and the DC port DC loop-closing, the control method is the same as the control method when the AC port AC1 and the DC port DC loop-closing;
when the three ports are closed and put into operation simultaneously, the AC port on one side adopts UdcQ control, setting the voltage of the direct current bus as a rated value, controlling the active reference value to be 0 by adopting P-Q control at the alternating current port on the other side, controlling the active reference value to be 0 by the direct current port, enabling the flexible controller to have no power transmission, switching on the connecting switches S1, S2 and S3, and switching on the port power parameter of the alternating current port on the other side after loop closingAnd changing the reference value into an operation value, limiting the change rate of the power reference value to k1, switching the DC port into fixed port voltage control, limiting the change rate of the power reference value to k2, and realizing alternating current power control and direct current port voltage control after the flexible controller stably operates.
The method for controlling the ring opening stop is characterized in that the flexible control of ring opening and operation quitting is the reverse process of ring closing starting, and specifically comprises the following steps:
when the flexible controller is used for realizing the ring opening of the direct current port from the three-port interconnection state, the direct current port is changed into an alternating current port AC1 and AC2 interconnection state; firstly, controlling the active reference value of the direct current port to linearly drop to 0, limiting the change rate of the active reference value to k2, and disconnecting the tie switch S3 to realize the ring-off of the direct current port when the power of the direct current port linearly drops to 0; in the AC1 and AC2 interconnected state, if the AC1 and AC2 loop release is further realized and the flexible controller is made to exit the operation, firstly controlling the active reference value of the P-Q control side to be 0, limiting the change rate of the reference value to be k1, and disconnecting the interconnection switches S1 and S2 when the active power between the AC ports AC1 and AC2 linearly drops to 0, so as to realize the loop release of the three ports and make the flexible controller exit the operation;
when the flexible controller is used for unlocking the alternating current port on one side from the three-port interconnection state, the alternating current port is changed into an alternating current port AC1 and a direct current port DC interconnection state; before the ring-opening operation, the AC port AC2 to be opened needs to be switched to P-Q control, and the other AC port AC1 needs to be switched to Udc-Q control; firstly, controlling an active reference value of an AC2 port to be 0, limiting a change rate of the reference value to be k1, and disconnecting a tie switch S2 to realize the ring-off of the AC2 port when the power of the AC2 port is linearly reduced to 0; in the AC1 and DC interconnection state, if an AC1 and a DC loop are further realized and the flexible controller is made to exit the operation, firstly, the active reference value at the DC side is controlled to be 0, the change rate of the reference value is limited to be k2, and when the active power between the ports AC1 and DC is linearly reduced to 0, the interconnection switches S1 and S3 are disconnected, the loop of the three ports is realized, and the flexible controller exits the operation.
When the flexible controller is in the state of disconnecting the alternating current port at the side of the three-port interconnection state and changing the state into the state of interconnecting the alternating current port AC2 and the direct current port DC, the control method is the same as the control method of changing the state of disconnecting the alternating current port at the side of the three-port interconnection state and changing the state into the state of interconnecting the alternating current port AC1 and the direct current port DC.
The invention has the following beneficial effects:
the topological structure of the movable AC/DC multiport flexible controller has the characteristics of movable design, multiple ports, AC/DC hybrid connection and cross-voltage grade interconnection; the controller is of a three-port structure, the two sides of the MMC are connected with the AC distribution network feeder lines, the DAB side is connected with the DC feeder line, the controller is flexibly connected with different ports by moving the position of the flexible controller according to the actual requirement of the distribution network and matching with the contact switch to start and stop according to the ports to be accessed and matching with the corresponding contact switch, and the actual application effect and the access position flexibility of the controller are greatly improved, so that the controller is called as movable; compared with the existing flexible controller topology, the access position in the power distribution network can be flexibly selected, the utilization rate of the device is improved, more functions can be realized, the device is suitable for various application requirements of alternating current interconnection, alternating current and direct current hybrid connection, cross-voltage interconnection, multi-port interconnection and the like of the power distribution network, and the device can be suitable for wider flexible interconnection scenes of the power distribution network;
the flexible controller is movably designed, each port of the flexible controller can be flexibly accessed and independently controlled, and the access position and the control mode are more flexible; aiming at different specific application scenes and control requirements of the power distribution network, the access position in the power distribution network is freely changed by switching at the interconnection switch, flexible connection between all or any two ports is realized by matching with a start-stop control method, current impact in the ring closing and opening process is effectively reduced by limiting the amplitude of the power change rate in the ring closing and opening process, and the flexibility, stability and reliability of the system are improved.
Drawings
FIG. 1 is a schematic diagram of the overall structure of an embodiment of the present invention;
FIG. 2 is a schematic diagram of the mobile principle of the present embodiment;
FIG. 3 is a block diagram of a modular multilevel converter topology according to the present embodiment;
fig. 4 is a view of a topology structure of a submodule SM in the present embodiment;
FIG. 5 is a DAB topology structure diagram of the dual active bridge converter in this embodiment;
FIG. 6 is a schematic diagram of the structure of multiple DAB serial inputs and parallel outputs in this embodiment;
fig. 7 is a schematic diagram of a power decoupling control strategy of the MMC in the present embodiment;
fig. 8 is a schematic diagram of the DAB power balance control strategy in this embodiment.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.
Referring to fig. 1 to 8, the present invention provides a movable ac/dc multiport flexible controller for controlling power among different feeders of a power grid, wherein feeders with interconnection requirements are connected through a normally open switch cabinet, and the switch cabinet realizes interconnection or disconnection of feeders. The flexible controller is of a three-port structure, and a movable AC/DC multi-port flexible controller topological structure is shown in figure 1 and comprises two Modular Multilevel Converters (MMC), wherein the two Modular Multilevel Converters (MMC) are connected back to back, namely alternating current sides are interconnected through a BTB-MMC structure, so that decoupling interconnection between two alternating current ports is realized, the alternating current ports on two sides can operate at different voltage levels, and the alternating current sides of the two Modular Multilevel Converters (MMC) are respectively connected with interconnection switches S1 and S2 to switch and move the two alternating current ports AC1 and AC 2; the direct current sides of the two modular multilevel converters MMC are subjected to DC/DC conversion through a plurality of double active bridge converters DAB which are connected in series, input and output in parallel, the DAB output end is used for being connected with a direct current port DC after being switched through an interconnection switch S3, and direct current loads of electric automobiles and the like or direct current power supplies of photovoltaics and the like can be connected. Alternating current ports AC1 and AC2 on two sides of the MMC are used for being connected with an alternating current power grid feeder line, a direct current port DC on the DAB side is used for being connected with a direct current power grid feeder line, interconnection switches S1, S2 and S3 are connected with different ports, the positions of the flexible controller among the feeder lines are moved, the connection positions of the ports are changed by controlling the switching actions of the interconnection switches S1, S2 and S3, and the flexible change of the connection positions of the flexible controller in a power distribution network is achieved.
As shown in fig. 2, the feeders with interconnection requirements are connected through a normally open switch cabinet, and the switch cabinet can realize interconnection or disconnection of the feeders, but cannot suppress current impact in the process of ring closing and ring opening, and cannot actively perform decoupling control on active power and reactive power. When two original feeder lines in a disconnected state have an interconnection requirement, such as the feeder line 1 and the feeder line 2 in fig. 2, the flexible controller is firstly respectively connected to connection points at the feeder line 1 and the feeder line 2 through the interconnection switch, and then the feeder line 1 and the feeder line 2 are flexibly interconnected through a closed-loop control strategy, so that free power control between the feeder line 1 and the feeder line 2 is realized. When the feeder line 1 and the feeder line 2 are only connected with each other normally and power control is not needed, the switch cabinet 1 is closed, the flexible controller is quitted from operation, and the interconnection switch of the alternating current port of the flexible controller is disconnected. At this time, the feeder 1 and the feeder 2 complete the switching from the disconnection state to the interconnection state. If the interconnection between the feeder lines 2 and 3 is further realized, the flexible control can be moved to the position between the feeder lines 2 and 3 through the transportation device, the interconnection switches of the flexible controller are respectively connected to the connection points at the feeder lines 2 and 3, and the interconnection between the feeder lines 2 and 3 is realized by repeating the steps. When two feeder lines such as the feeder line 1 and the feeder line 2 which are interconnected through a switch cabinet need to be switched to a disconnected state, firstly, the alternating current ports AC1 and AC2 of the flexible controller are respectively connected to connection points at the positions of the feeder line 1 and the feeder line 2, the power flowing through the switch cabinet is enabled to be zero through power control, then the switch cabinet is disconnected, loop-releasing control is carried out on the flexible controller, and the feeder line 1 and the feeder line 2 are smoothly switched to the disconnected state from the interconnected state.
The topologies of MMC and DAB are explained below:
the single-ended MMC topological structure is shown in fig. 3, a three-phase half-bridge modular multilevel converter is adopted, the three-phase half-bridge modular multilevel converter comprises three phase units with the same structure, each phase unit is composed of 2 bridge arms which are symmetrical up and down, and the single-ended MMC is of a three-phase six-bridge arm structure as a whole; each bridge arm comprises N sub-modules SM which are connected in series and have the same structure, 1 bridge arm reactor Larm and 1 bridge arm equivalent resistor Rarm.
As shown in fig. 4, the single sub-module topology structure is that the sub-module SM includes 2 power transistors IGBT (power transistor VT1, power transistor VT2), 2 corresponding anti-parallel diodes (diode VD1, diode VD2), 1 direct-current energy storage capacitor C0, 1 fast bypass switch K1, and 1 bypass protection diode K2; wherein, 5 devices of VT1, VT2, VD1, VD2 and C0 form a half-bridge structure topology;
an emitter of the power tube VT1 is connected with a collector of the power tube VT2, a collector of the power tube VT1 is connected with the anode of the direct-current energy storage capacitor C0, an emitter of the power tube VT2 is connected with the cathode of the direct-current energy storage capacitor C0, the power tube VT1 is reversely connected with the diode VD1 in parallel, and the power tube VT2 is reversely connected with the diode VD2 in parallel;
the fast bypass switch K1 and the bypass protection diode K2 are connected with the power tube VT2 in parallel; the K1 and K2 devices can be timely cut off and connected when the connected AC/DC system or the submodule per se fails, and converter equipment is protected. When the sub-module breaks down, K1 can bypass the fault sub-module quickly and ensure the continuity of bridge arm current. When serious faults such as short circuit of a direct current bus of the converter or short circuit of a transmission line occur, the K2 has good current impact resistance, and can protect the freewheeling diode connected with the K2 in parallel. Therefore, K1 and K2 greatly enhance the safety and utility of the sub-modules.
The DAB topology of the dual-active-bridge converter is shown in fig. 5 and 6, and comprises two symmetrical H-bridges and a high-frequency transformer containing leakage inductance, wherein the left H-bridge and the right H-bridge are connected with each other through the high-frequency transformer. The two symmetrical H-bridges ensure the bidirectional flow of energy, the high-frequency transformer is coupled and mainly completes energy transfer and voltage grade conversion, and leakage inductance is a core element for energy transfer of a system. The structure is easy to realize soft switching, the stress born by the power device is small, the power supply and the load are isolated by the high-frequency transformer, the volume of the transformer can be reduced, and the power density of the converter is improved.
The following describes the operation control scheme of each port respectively:
port commutation for multi-port flexible controllersThe MMC and the DAB can be respectively and independently controlled, wherein the MMC adopts a power decoupling control strategy, namely d-q decoupling double closed-loop control is adopted, and as shown in figure 7, the power outer-loop control and the current inner-loop control are included; wherein the dynamic performance of the outer ring control is influenced by the dynamic performance of the current inner ring control, the power outer ring control is used for providing a dq axis current reference value for the current inner ring control, the power outer ring control is used for controlling an active quantity and a passive quantity, the active quantity controls active power or direct current voltage, the passive quantity controls reactive power or alternating current voltage, the MMC at one side adopts constant direct current voltage and reactive power control, namely UdcAnd the other side adopts constant active power and reactive power control, namely P-Q control. The current inner loop control adopts a direct current control mode to realize the control of active and reactive current and output modulation waves, thereby generating switching signals for driving switching devices in each submodule; the current inner loop control directly passes through a current negative feedback closed loop, so that the controller can quickly respond, the dynamic characteristic of the converter is improved, and the robustness of the system is enhanced.
The DAB structure adopts a single phase-shift control mode, the driving signals of all switching tubes in the DAB are PWM signals with the duty ratio of 50%, and a phase shift angle is staggered between a primary side signal and a secondary side signal
Figure BDA0003175790070000071
To control the primary and secondary side voltages. Because a plurality of DAB outputs are connected in parallel, current sharing control needs to be carried out on a plurality of DAB powers by adopting a power balance control strategy, so that each DAB power is balanced, as shown in FIG. 8, for example, two DAB series input and parallel output are taken as an example, VdcL_ref,VdcLReference voltage and actual voltage, I, of the parallel DAB respectivelyDAB1，IDAB1Output currents of DAB1 and DAB2, V, respectivelydcH1,VdcH2Respectively the output voltages of DAB1 and DAB2, obtaining the average power of each DAB through power calculation, comparing the average power with a power reference value, and obtaining the phase shift angle of each DAB module after PI regulation
Figure BDA0003175790070000072
Thereby realizing current sharing control.
In this embodiment, for the multi-port flexible controller shown in fig. 1, the following control scheme is adopted: the MMC adopts a power decoupling control strategy shown in figure 7, and the MMC on the left side adopts UdcAnd (4) Q control, namely maintaining the voltage stability of the direct current bus, optimizing the power of the alternating current port by adopting P-Q control for the MMC at the right side, and controlling the voltage stability of the direct current port by adopting a power balance control strategy shown in figure 8 for DAB.
The invention also provides a method for controlling the on-off loop of the movable AC/DC multiport flexible controller, wherein the on-off loop control of the multiport flexible controller is realized by connecting power grid feeders with the same or different voltage grades through the flexible controller on the basis of MMC technology and DAB technology under the voltage grade of a power distribution network, so that the loop closing operation is realized, the power supply reliability and the serious fault resistance of the power grid can be effectively improved, and the power exchange among different power distribution network feeders is realized. The flexible controller is movable, and as shown in fig. 1, the flexible controller is flexibly connected to different positions of a power distribution network through interconnection switches S1, S2 and S3, so that flexible interconnection and power transmission of access points are realized. When the corresponding port of the flexible controller is connected to or disconnected from the power distribution network, a reasonable on-off loop starting and stopping control method needs to be designed to smoothly control the power in order to avoid generating power impact. When the flexible controller is used for starting and stopping a closed loop, firstly, the power distribution network at the port access position is ensured to be in a light-load running state, the load ratio of the alternating-current power distribution network and the source load ratio of the direct-current port are taken as starting and stopping criteria, and the flexible controller can be flexibly set according to the running condition. The closed-loop start control method and the open-loop stop control method are further described below.
(1) Closed loop starting control method
As shown in fig. 1, the flexible controller has a three-port structure, and can be used for loop closing at any two ports, or loop closing at all three ports, and is selected according to the control requirements of the power distribution network in practical application. When two AC ports AC1 and AC2 are looped, it is necessary to adopt U from MMC on one sidedcAnd the MMC on the other side adopts constant P-Q control. U determination is adopted by AC1 port MMCdc-Q control, AC2 Port MMC taking P-Q control as an example, first the U of AC1 Port MMCdcControl command is set to amountThe method comprises the steps of fixing a direct-current voltage value, setting a power control command of an MMC (modular multilevel converter) with an AC2 port to be 0, enabling a flexible controller to have no power transmission, switching on a connecting switch S1 and a connecting switch S2, controlling a power reference value of an AC2 port to be an operation value after loop closing, and improving current fluctuation through power command regulation and control aiming at the problem that the current exceeds the standard caused by sudden change of the power reference value after loop closing. And the power instruction linear control is provided, the change rate of the power reference value is limited to k1(k1 can be limited according to the current maximum current, so that the sudden change of the power reference value is avoided, the power reference value is linearly increased, and the loop closing current impact is reduced. When the flexible controller operates stably, the power flow distribution of the distribution network can be controlled, the power flow optimization of the alternating current power grid is carried out, and the emergency active support function is realized when a fault occurs.
When AC port AC1 (taking AC1 as an example, the same principle as AC2) and DC port DC are looped, MMC adopts Udc-Q control, setting the DC bus voltage reference to nominal voltage, DAB control active reference PrefAnd 0, enabling the flexible controller to have no power transmission, switching on the connection switches S1 and S3, switching the DAB to the voltage control of the direct current outlet end in the figure 8 after loop closing, but in order to avoid current impact, limiting the change rate of the active reference value to k2 (the k2 can be limited according to the current maximum current amplitude limiting value), avoiding sudden change of the power reference value, and realizing stable control of the voltage of the direct current port after the flexible controller operates stably.
When the three-port simultaneous loop closing operation is performed, it is necessary to adopt a U-type one-side AC port (for example, AC 1) as the above-mentioned two-port loop closing casedcAnd Q control, setting the voltage of a direct current bus as a rated value, adopting P-Q control for the rest alternating current port (AC2), controlling an active reference value to be 0, controlling the active reference value to be 0 by the direct current port, enabling the flexible controller to have no power transmission, switching on the connecting switches S1, S2 and S3, changing the power reference value of the AC2 port into an operating value after loop closing, limiting the change rate of the power reference value to k1, switching the DC port into fixed port voltage control, limiting the change rate of the power reference value to k2, and realizing flexible control of alternating current power and stable control of the voltage of the direct current port after the flexible controller is stably operated.
When the flexible controller is in a two-port interconnection operation state, if loop closing is carried out on the remaining third port, referring to the method, firstly, the active reference value to be input into the port controller is set to be 0, under the condition that the port has no power exchange, the tie switch is switched on, the amplitude limiting is carried out on the change rate of the power reference value after loop closing, and the three-port flexible interconnection state can be realized after the power is stable.
(2) Ring-opening stop control method
The flexible control of ring opening and operation quitting is the reverse process of ring closing starting, the control method is similar, and the impact caused by sudden change of the power in the ring opening process is also required to be avoided.
When the flexible controller is disconnected from the DC port from the three-port interconnection state, the AC port is changed into the AC1 and AC2 interconnection states. Firstly, the active reference value of the direct current port is controlled to linearly drop to 0, the change rate of the active reference value is limited to k2, and when the power of the direct current port linearly drops to 0, the tie switch S3 is disconnected to realize the ring-opening of the direct current port. In the interconnected state of AC1 and AC2, if the loop of AC1 and AC2 is to be further realized and the flexible controller is to be quitted from running, the active reference value of the P-Q control side is controlled to be 0, the change rate of the reference value is limited to be k1, and when the active power between AC1 and AC2 is linearly reduced to 0, the interconnection switches S1 and S2 are disconnected, the loop of three ports is realized, and the flexible controller is quitted from running.
When the flexible controller is disconnected from the AC port on one side from the three-port interconnection state, the AC port is changed into an AC1 (the same as AC2) and a DC port is changed into a DC interconnection state. Before the ring opening operation, firstly, the AC port to be opened needs to be switched to P, Q control, and the other AC port needs to be switched to Udc-Q control. In this example, AC1 is Udc-Q control, AC2 is P, Q control. Firstly, the active reference value of the AC2 is controlled to be 0, the change rate of the reference value is limited to be k1, and when the power of the AC2 port linearly drops to 0, the tie switch S2 is opened to realize the ring-opening of the AC 2. In the AC1 and DC interconnection state, if an AC1 and DC loop release is further realized to enable the flexible controller to quit operation, firstly controlling the active reference value at the DC side to be 0, limiting the change rate of the reference value to be k2, and disconnecting the port AC1 and the port DC when the active power linearly drops to 0The switches S1, S3 are communicated to effect three port de-looping and the flexible controller exits service.
And similarly to the above situation, firstly controlling the power reference values of all the ports to be 0, and disconnecting the interconnection switches S1, S2 and S3 when the power among all the ports of the flexible controller linearly decreases to 0 according to the change rate of the maximum current limit reference value, so as to realize the loop disconnection among the ports, and the flexible controller exits the operation.
The invention relates to a movable AC/DC multi-port flexible controller, which can be freely moved and applied among feeders of distribution networks with different voltage levels and different positions. Furthermore, the flexible access of the controller is embodied in a controller on-off loop starting and stopping control method, different alternating current and direct current ports are accessed, and the switching actions of the interconnection switch are different; the invention accesses different AC/DC ports according to different port control strategies to realize flexible interconnection and power transmission of the access point.
The parts not involved in the present invention are the same as or implemented using the prior art.
The foregoing is a more detailed description of the present invention that is presented in conjunction with specific embodiments, and the practice of the invention is not to be considered limited to those descriptions. For those skilled in the art to which the invention pertains, several simple deductions or substitutions can be made without departing from the spirit of the invention, and all shall be considered as belonging to the protection scope of the invention.

Claims (7)

1. The movable AC/DC multi-port flexible controller is used for controlling power among different feeders of a power grid, the feeders with interconnection requirements are connected through a switch cabinet in a normally open state, and the switch cabinet realizes interconnection or disconnection of the feeders; the method is characterized in that: the flexible controller is of a three-port structure and comprises two modular multilevel converters MMC, the two modular multilevel converters MMC are connected back to back, and the alternating current sides of the two modular multilevel converters MMC are respectively connected with the communication switches S1 and S2 for switching to move the two alternating current ports AC1 and AC 2; the direct current sides of the two modular multilevel converters MMC are subjected to DC/DC conversion through a plurality of double active bridge converters DAB which are connected in series, input and output in parallel, and the output end of the DAB is used for being connected with a direct current port DC after being switched through an interconnection switch S3;
alternating current ports AC1 and AC2 on two sides of the MMC are used for being connected with an alternating current power grid feeder line, a direct current port DC on the DAB side is used for being connected with a direct current power grid feeder line, interconnection switches S1, S2 and S3 are connected with different ports, the positions of the flexible controller among the feeder lines are moved, the switching positions of the ports are changed by controlling the switching actions of the interconnection switches S1, S2 and S3, and the change of the switching positions of the flexible controller in a power distribution network is achieved;
when the two feeder lines which are in an originally disconnected state have interconnection requirements, the flexible controller is respectively connected to the connection points of the two feeder lines through the interconnection switches of the corresponding ports and closes the interconnection switches to close the loop to realize flexible interconnection; when the two feeders only need to be interconnected normally and do not need power control, the switch cabinet is closed, the flexible controller is quitted from operation, and the interconnection switch of the corresponding port of the flexible controller is disconnected;
when the interconnection requirement of the feeder lines changes, the flexible controller changes at the feeder line access position of the power distribution network, moves the flexible controller between the other two feeder lines with the interconnection requirement, and respectively accesses the connection points of the other two feeder lines through the contact switches of the corresponding ports and closes the contact switches to realize flexible interconnection;
when two feeder lines interconnected through a switch cabinet need to be switched to an off state, corresponding ports of a flexible controller are respectively connected to connecting points of the two feeder lines and a contact switch is closed, the power flowing through the switch cabinet is zero through power control, then the switch cabinet is switched off, and loop-off control is carried out on the flexible controller to realize smooth switching of the two feeder lines from the interconnected state to the off state;
the MMC adopts a power decoupling control strategy, which comprises power outer loop control and current inner loop control; the power outer loop control is used for providing a dq axis current reference value for the current inner loop control, wherein the power outer loop control of the MMC at one side adopts constant direct current voltage and reactive power control, namely Udc-Q control, with constant active and reactive power control on the other side, i.e. P-Q control; the current inner loop control adopts a direct current control mode to generate a switching signal for driving each submodule;
DAB adopts single phase shift control mode.
2. The portable ac/dc multiport flexible controller of claim 1, wherein: the single-ended modular multilevel converter MMC adopts a three-phase half-bridge modular multilevel converter, the three-phase half-bridge modular multilevel converter comprises three phase units with the same structure, each phase unit consists of 2 bridge arms which are symmetrical up and down, and the single-ended MMC is integrally of a three-phase six-bridge-arm structure; each bridge arm comprises N sub-modules SM which are connected in series and have the same structure, 1 bridge arm reactor Larm and 1 bridge arm equivalent resistor Rarm.
3. The portable ac/dc multiport flexible controller of claim 2, wherein: the sub-module SM comprises 2 power tubes, 2 corresponding anti-parallel diodes, 1 direct-current energy storage capacitor C0, 1 rapid bypass switch K1 and 1 bypass protection diode K2; the 2 power tubes are respectively a power tube VT1 and a power tube VT2, and the anti-parallel diodes are respectively a diode VD1 and a diode VD 2; wherein, 5 devices of VT1, VT2, VD1, VD2 and C0 form a half-bridge structure topology;
an emitter of the power tube VT1 is connected with a collector of the power tube VT2, a collector of the power tube VT1 is connected with the anode of the direct-current energy storage capacitor C0, an emitter of the power tube VT2 is connected with the cathode of the direct-current energy storage capacitor C0, the power tube VT1 is reversely connected with the diode VD1 in parallel, and the power tube VT2 is reversely connected with the diode VD2 in parallel;
the fast bypass switch K1 and the bypass protection diode K2 are connected in parallel with the power tube VT 2.
4. The portable ac/dc multiport flexible controller of claim 1, wherein: the double-active-bridge converter DAB comprises two symmetrical H-bridges and a high-frequency transformer containing leakage inductance; the left H bridge and the right H bridge are connected with each other through a high-frequency transformer.
5. The portable ac/dc multiport flexible controller of claim 1, wherein: the single phase-shifting control mode adopted by DAB specifically comprises the following steps: in DAB, the driving signals of all the switch tubes are PWM signals with the duty ratio of 50%, and the primary side signal and the secondary side signal are staggered by a phase shift angle
Figure FDA0003175790060000021
To control the primary and secondary side voltages.
6. The portable ac/dc multiport flexible controller of claim 5, wherein: the control of the DAB also comprises the step of carrying out current sharing control on a plurality of DAB powers by adopting a power balance control strategy.
7. A movable AC/DC multiport flexible controller ring closing and opening control method adopts the movable AC/DC multiport flexible controller of any one of claims 1 to 6, and is characterized in that: comprises a closed loop starting control method and an open loop stopping control method;
the loop closing starting control method specifically comprises the following steps:
when two AC ports are closed, one side of MMC adopts Udc-Q control, and the MMC on the other side adopts fixed P-Q control; first, the U of the AC1 AC port MMCdcSetting the control command to be a rated direct current voltage value, setting the power control command of an AC2 AC port MMC to be 0 to enable the flexible controller to have no power transmission, then switching on the contact switches S1 and S2, and controlling after loop closingChanging the power reference value of the AC2 AC port into a running value; limiting the change rate of the power reference value to k1, and limiting the value of k1 according to the current maximum current, so as to avoid sudden change of the power reference value and make the power reference value rise linearly;
when the AC port AC1 and the DC port DC are looped, the MMC adopts Udc-Q control, setting the DC bus voltage reference to nominal voltage, DAB control active reference PrefWhen the voltage is 0, the flexible controller has no power transmission, then the connection switches S1 and S3 are switched on, and DAB is switched to the voltage control of a DC port DC after loop closing; limiting the change rate of the active reference value to k2, and limiting the value of k2 according to the current maximum current to avoid sudden change of the power reference value;
when the AC port AC2 and the DC port DC loop-closing, the control method is the same as the control method when the AC port AC1 and the DC port DC loop-closing;
when the three ports are closed and put into operation simultaneously, the AC port on one side adopts Udc-Q control, setting the DC bus voltage as a rated value, using P-Q control for the ac port on the other side, controlling the active reference value to 0, and controlling the active reference value to 0 by the DC port, so that the flexible controller has no power transmission, then switching on the tie switches S1, S2, S3, changing the port power reference value of the ac port on the other side to an operational value after loop closing, and limiting the power reference value variation rate to k1, switching the DC port DC to fixed port voltage control, and limiting the power reference value variation rate to k2, and implementing ac power control and DC port voltage control after the flexible controller is stably operated;
the method for controlling the ring opening stop is characterized in that the flexible control of ring opening and operation quitting is the reverse process of ring closing starting, and specifically comprises the following steps:
when the flexible controller is used for realizing the ring opening of the direct current port from the three-port interconnection state, the direct current port is changed into an alternating current port AC1 and AC2 interconnection state; firstly, controlling the active reference value of the direct current port to linearly drop to 0, limiting the change rate of the active reference value to k2, and disconnecting the tie switch S3 to realize the ring-off of the direct current port when the power of the direct current port linearly drops to 0; in the AC1 and AC2 interconnected state, if the AC1 and AC2 loop release is further realized and the flexible controller is made to exit the operation, firstly controlling the active reference value of the P-Q control side to be 0, limiting the change rate of the reference value to be k1, and disconnecting the interconnection switches S1 and S2 when the active power between the AC ports AC1 and AC2 linearly drops to 0, so as to realize the loop release of the three ports and make the flexible controller exit the operation;
when the flexible controller is used for unlocking the alternating current port on one side from the three-port interconnection state, the alternating current port is changed into an alternating current port AC1 and a direct current port DC interconnection state; before the ring-opening operation, the AC port AC2 to be opened needs to be switched to P-Q control, and the other AC port AC1 needs to be switched to Udc-Q control; firstly, controlling an active reference value of an AC2 port to be 0, limiting a change rate of the reference value to be k1, and disconnecting a tie switch S2 to realize the ring-off of the AC2 port when the power of the AC2 port is linearly reduced to 0; in the AC1 and DC interconnection state, if an AC1 and a DC loop are further realized to enable the flexible controller to exit the operation, firstly controlling an active reference value at a DC side to be 0, limiting the change rate of the reference value to be k2, and disconnecting the interconnection switches S1 and S3 to realize the loop opening of three ports when the active power between the ports AC1 and DC is linearly reduced to 0;
when the flexible controller is in the state of disconnecting the alternating current port at the side of the three-port interconnection state and changing the state into the state of interconnecting the alternating current port AC2 and the direct current port DC, the control method is the same as the control method of changing the state of disconnecting the alternating current port at the side of the three-port interconnection state and changing the state into the state of interconnecting the alternating current port AC1 and the direct current port DC.
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