CN107171540B - MMC system with rapid starting and direct-current voltage drop restraining capability and working method - Google Patents

MMC system with rapid starting and direct-current voltage drop restraining capability and working method Download PDF

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CN107171540B
CN107171540B CN201710592865.5A CN201710592865A CN107171540B CN 107171540 B CN107171540 B CN 107171540B CN 201710592865 A CN201710592865 A CN 201710592865A CN 107171540 B CN107171540 B CN 107171540B
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CN107171540A (en
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岳有军
杨立
王红君
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Tianjin Huawei Zhisheng Information Technology Co ltd
Tianjin University of Technology
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Tianjin University of Technology
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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
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • 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]

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Abstract

An MMC system with the capability of quick start and suppression of direct-current voltage drop discloses a modular multilevel converter topological structure, which comprises a main controller, a valve base controller VBC and a three-phase six-bridge-arm circuit topological structure; the combination of a plurality of MMC systems can be applied to the high-voltage direct-current flexible power transmission system, the starting method is simple and easy to implement, and the drop of direct-current voltage in the starting process can be effectively inhibited; the MMC can be quickly started; the requirement on power electronic devices is low, and good expandability is achieved; the method can also be applied to overhead line flexible direct current transmission, multi-terminal flexible direct current transmission and other occasions.

Description

MMC system with rapid starting and direct-current voltage drop restraining capability and working method
The technical field is as follows:
the invention relates to the technical field of power electronic direct current transmission, in particular to a starting method and a working method of an MMC (Modular Multilevel Converter) with the capability of quickly starting and restraining direct current voltage drop.
(II) background technology:
with the development and application of new energy power generation technology, the scale of the new energy power generation technology is gradually enlarged. Therefore, how to realize the long-distance transmission of large-capacity power is a very practical and urgent subject. The flexible direct-current transmission technology has obvious advantages in the aspect of realizing large-capacity and long-distance power transmission.
The modular multilevel converter has the technical advantages of independent control of active power and reactive power, low content of multiple harmonics of output voltage level number, good output voltage waveform, low switching frequency, high modularization, easy expansion, redundant control, capability of being used as a black start power supply and the like, and is a hotspot of research in academic circles and industrial circles at home and abroad in recent years. The method is successfully applied to occasions such as wind power integration, long-distance high-power transmission and the like, and can be widely applied to the fields such as renewable energy integration, high-voltage direct-current power transmission, multi-terminal direct-current power transmission and the like in the future.
In recent years, research into the use of voltage source converter based high voltage direct current transmission systems has received much attention. The starting of the flexible direct current converter is taken as the premise and the basis of the normal operation of the system, and the starting has 2 main targets: and establishing the AC outlet voltage of the converter and establishing the rated DC voltage. The establishment of the rated direct current voltage is a central link of the pre-charging process of the converter, and the essence of the establishment is the establishment of the rated voltage of the capacitor. Therefore, proper topology and start-up control are essential for normal operation and equipment safety of modular multi-level based high voltage direct current transmission systems (MMC-HVDC).
The MMC-HVDC system converter station is started by a pre-charging strategy of the capacitor in the sub-module. The capacitors of the MMC are distributed in each submodule, and compared with a two-level converter, the dynamic process of capacitor charging is more complex. When a single converter station is started, at the current uncontrollable stage, overcurrent at the starting stage is limited mainly by connecting a current limiting resistor in series, and the setting and switching time of the current limiting resistor have critical influence on smooth starting of a system.
(III) the invention content:
the invention aims to provide an MMC system with the capability of quick start and restraining DC voltage drop and a working method aiming at the defects in the prior art, and the MMC system has a simple structure; the working method is easy to realize, has low requirements on power electronic devices and has good expandability.
The technical scheme of the invention is as follows: an MMC system with the capability of quickly starting and restraining direct-current voltage drop is characterized by comprising a main controller, a valve base controller VBC and a three-phase six-bridge-arm circuit topological structure; each of the three phases respectively comprises two bridge arms, and each bridge arm is formed by connecting a sub-module SM and a bridge arm reactance in series; the main controller has the same function as the VSC, and inverts and rectifies current through the switch conducting state of the device; and the VBC converts the output voltage instruction value of the MMC system into the number of conducting modules of each bridge arm and determines which sub-modules are conducted so as to apply trigger pulses to each bridge arm of the MMC system.
The number of the sub-modules SM is not less than one, and the number of the bridge arm reactances is not less than one.
The sub-module SM is composed of a port I, a port II, 5 IGBTs, 3 diodes, a capacitor C1 and a capacitor C2; the port I of the sub-module SM is connected with the port II of the other sub-module SM, the port II of the sub-module SM is connected with the port I of the next sub-module SM, and all the sub-modules SM are sequentially connected in series; 3 IGBTs in the 5 IGBTs are respectively connected with 3 diodes in reverse parallel to form three groups of follow current units, which play a role in protecting and follow current on the circuit and are respectively recorded as a follow current unit I, a follow current unit II and a follow current unit III, and the other 2 IGBTs are connected in reverse parallel to form a bidirectional controllable unit for controlling the level output state of the submodule; the follow current unit I and the follow current unit II are connected in series, the follow current unit III and the bidirectional controllable unit are connected in series, and the two series combinations are connected in parallel; the port I is connected between the follow current unit I and the follow current unit II which are connected in parallel; the port II is connected with the follow current unit II, the bidirectional controllable unit and the capacitor C2; one end of the capacitor C1 is connected with one end of the follow current unit I, and the other end of the capacitor C1 is connected to the connection point of the follow current unit III and the bidirectional controllable unit which are connected in series; the capacitor C2 is connected in parallel across the freewheeling unit III and the bi-directional controllable unit connected in series with each other.
The capacitance values of the capacitor C1 and the capacitor C2 can be equal.
The submodule SM has three output voltage states of 0, Uc and 2Uc, wherein Uc is the voltage at two ends of each capacitor of the capacitor C1 and the capacitor C2; and the IGBT tube is turned on by 1, and the IGBT tube is turned off by 0.
The working states of the sub-module SM are as follows, and it is specified that the flow from port I to port II is positive, otherwise, it is negative, and then:
(1) the submodule SM works in the blocked state: when the current direction is positive, the output voltage of the submodule SM is 2UC(ii) a When the current direction is negative, the output voltage of the sub-module SM is 0; when in a closed state, all the IGBTs are in an off state, at this time, 5 IGBT transistors in the sub-module SM are respectively marked as an IGBT transistor T1, an IGBT transistor T2, an IGBT transistor T3, an IGBT transistor T4, and an IGBT transistor T5, and respectively correspond to the IGBT transistor T1, the IGBT transistor T2, the IGBT transistor T3, the IGBT transistor T4, and the IGBT transistor T5The on-off states are 00000 in sequence;
(2) the sub-module SM works in an I working state: when the current direction is positive, the output voltage of the sub-module SM is 0; when the current direction is negative, the output voltage of the sub-module SM is 0; at this time, the switching states of the IGBT tube T1, the IGBT tube T2, the IGBT tube T3, the IGBT tube T4, and the IGBT tube T5 are 01000 in sequence;
(3) the submodule SM works in a II working state: when the current direction is positive, the output voltage of the submodule SM is UC(ii) a When the current direction is negative, the output voltage of the submodule SM is UC(ii) a At this time, the switching states corresponding to the IGBT transistor T1, the IGBT transistor T2, the IGBT transistor T3, the IGBT transistor T4, and the IGBT transistor T5 are 10011 in sequence;
(4) the submodule SM works in a III working state: when the current direction is positive, the output voltage of the submodule SM is 2UC(ii) a When the current direction is negative, the output voltage of the submodule SM is 2UC(ii) a At this time, the switching states of the IGBT tube T1, the IGBT tube T2, the IGBT tube T3, the IGBT tube T4, and the IGBT tube T5 are 10100 in this order.
The MMC system with the rapid starting and Direct-Current Voltage drop restraining capability is applied to a High-Voltage Direct-Current flexible power Transmission system, and is characterized in that the MMC system is an MMC-HVDC (High Voltage Direct Current Transmission Based on modular multilevel Converter-MMC-HVDC power Transmission system) power Transmission system which is composed of not less than 2 MMC systems with the rapid starting and Direct-Current Voltage drop restraining capability; one MMC system with the rapid starting and direct-current voltage drop restraining capabilities is connected with a main power grid, the other MMC systems with the rapid starting and direct-current voltage drop restraining capabilities are respectively connected with a passive network or an active network, and the MMCs are connected through direct-current lines.
The line connection between the MMC system with the capability of quickly starting and inhibiting the direct-current voltage drop and the main power grid is realized by a circuit breaker; the MMC system with the capability of quickly starting and restraining the direct-current voltage drop is connected with a passive network or an active network through a circuit breaker.
A starting method of an MMC system with rapid starting and direct-current voltage drop restraining capability applied to an MMC-HVDC system is characterized by comprising the following steps:
(1) in the starting stage, an MMC system which is connected with the main power grid side and has the capability of quickly starting and restraining the direct-current voltage drop is defined as an MMC1 subsystem, and an MMC which is connected with a passive network or an active network is defined as an MMC2 subsystem; meanwhile, defining 'a valve base controller VBC' to be unlocked, indicating that the MMC1 subsystem and the MMC2 subsystem have the capability of applying trigger pulses to the IGBT, defining 'a main controller to be unlocked', and indicating that the MMC1 subsystem and the MMC2 subsystem start to work normally;
(2) closing a circuit breaker between a main power grid and the MMC1 subsystem, and charging the sub-modules of the MMC1 subsystem and the MMC2 subsystem through uncontrolled rectification of diodes in the sub-module SM by the alternating current power grid; at this time, all the IGBTs are in a locking state; when the steady state is reached, the formula (1) is established;
Figure GDA0002319990930000051
in the formula: vPN-1The voltage of the direct current polar line in the uncontrolled rectification stage; um is the effective value of the alternating-current side phase voltage; uc-MMC1-1 and uc-MMC2-1 are respectively the sub-module capacitor voltage of the MMC1 subsystem and the MMC2 subsystem in the uncontrolled rectification stage;
(3) defining a half-control rectification stage after an MMC2 subsystem 'unlocks a valve base controller VBC'; at the moment, when the direct-current voltage is stable, the MMC-HVDC power transmission system enters a semi-controlled rectification starting stage;
(4) after the semi-controlled rectification stage is finished, the MMC-HVDC system enters a high-frequency rectification stage; when the dc voltage is stable and equation (2) is satisfied, that is:
Figure GDA0002319990930000061
in the formula: vPN-2The voltage of a direct current polar line in a semi-control rectification stage; um is the effective value of the alternating-current side phase voltage; u. ofc-MMC1-2 and uc-MMC2-2 are respectively electricity of a submodule SM in an MMC1 subsystem and an MMC2 subsystem in a half-control rectification stageA capacitance voltage;
(5) the MMC1 subsystem unlocks a main controller, sub-module capacitors of the MMC1 subsystem and the MMC2 subsystem are continuously charged through constant direct-current voltage control, meanwhile, the running state of the MMC2 subsystem is kept unchanged, and the direct-current voltage is ensured to rise to a rated value synchronously; when the direct-current voltage rises to a rated value, starting the passive inversion control of the MMC2 subsystem to establish a stable no-load output voltage, and when the alternating-current output voltage of the MMC2 subsystem is stable, closing a circuit breaker connected to the MMC2 subsystem side, and ending the starting process; when the sub-module capacitor voltage is modulated at this stage, each sub-module SM outputs the required voltage according to the voltage required by the MMC system, and at the moment, each sub-module SM outputs one state of 0, Uc and 2Uc according to the condition; that is, at this stage each submodule SM may operate in any of states I, II, III.
The specific operation of the step (3) comprises the following steps:
1) sequencing submodules of each bridge arm of the MMC2 subsystem from large to small according to output voltages of the submodules;
2) after sequencing, for each bridge arm, enabling 1 submodule with the highest output voltage of the submodules to be in an I working state, namely recording 5 IGBT tubes in the submodule SM as an IGBT tube T1, an IGBT tube T2, an IGBT tube T3, an IGBT tube T4 and an IGBT tube T5 respectively, wherein the corresponding switch states are 01000 in sequence, and at the moment, the output voltage of the submodules is 0; all other sub-modules SM still work in a locking state, are charged through a direct current line and wait for the direct current voltage to be stable;
3) after the direct current voltage is stabilized, sequencing the sub-modules SM in the step 1) to enable 2 sub-modules with the highest output voltage of the sub-modules to work in an I working state, enabling other sub-modules to still work in a locking state, charging through a direct current line, and waiting for the direct current voltage to be stabilized;
4) similarly, after the direct-current voltage is stabilized, sequencing the output voltages of all the sub-modules SM in each step to enable n +1 sub-modules with the highest voltage to work in the I working state, wherein n is the number of the sub-modules working in the I working state in the step 3), and the other sub-modules SM still work in the locking state and are charged through a direct-current circuit;
5) when the number of the sub-modules SM working in the I working state is half of the total number of the sub-modules SM and the direct-current voltage is stable, the formula (2) is established.
The invention has the advantages that: the drop of the direct current voltage in the starting process can be effectively inhibited; the MMC can be quickly started; the submodule can output 0U and 2U according to different current directions in a locked stateCTwo voltage states; the submodule can output 0, U in normal operationC，2UCThree voltage states; the requirement on power electronic devices is low, and good expandability is achieved; the method can be applied to overhead line flexible direct current transmission, multi-terminal flexible direct current transmission and other occasions.
(IV) description of the drawings:
fig. 1 is a schematic view of a topology structure of an MMC system having the capability of fast start and suppressing dc voltage sag according to the present invention.
Fig. 2 is a structural diagram of a submodule SM in an MMC system having the capability of fast start and suppressing dc voltage sag according to the present invention.
Fig. 3(a) and fig. 3(b) are schematic diagrams of current flowing when a sub-module in an MMC system having the capability of fast start and suppressing dc voltage drop according to the present invention is in a locked state (where fig. 3(a) is a schematic diagram when a current direction is positive, and fig. 3(b) is a schematic diagram when the current direction is negative).
Fig. 4(a) and fig. 4(b) are schematic diagrams of current flowing when the MMC neutron module with the capability of fast start and suppressing dc voltage drop according to the present invention is in the state I (where fig. 4(a) is a schematic diagram of the current direction being positive, and fig. 4(b) is a schematic diagram of the current direction being negative).
Fig. 5(a) and 5(b) are schematic diagrams of current flowing when the MMC neutron module with the capability of fast start and dc voltage drop suppression according to the present invention is in the state II (where fig. 5(a) is a schematic diagram of the current direction being positive, and fig. 5(b) is a schematic diagram of the current direction being negative).
Fig. 6(a) and fig. 6(b) are schematic diagrams of current flowing when the MMC neutron module with the capability of fast start and suppressing dc voltage drop according to the present invention is in the state III (where fig. 6(a) is a schematic diagram of the current direction being positive, and fig. 6(b) is a schematic diagram of the current direction being negative).
Fig. 7 shows a high-voltage direct-current flexible power transmission system (MMC-HVDC) composed of MMCs with fast start and dc voltage sag suppression capabilities according to the present invention.
Fig. 8 is a starting control flow chart of an MMC with fast starting and dc voltage sag restraining capabilities in a high voltage dc flexible power transmission system (MMC-HVDC) according to the present invention.
The input from port I to port II is positive, and the output from port I to port II is negative, wherein the dashed lines in fig. 3(a) -6 (b) are the sub-module charging current paths.
(V) specific embodiment:
example (b): an MMC system (shown in figure 1) with the capability of quick start and restraining direct-current voltage drop is characterized by comprising a main controller, a valve base controller VBC and a three-phase six-bridge arm circuit topological structure; each of the three phases respectively comprises two bridge arms, and each bridge arm is formed by connecting a sub-module SM and a bridge arm reactance in series; the main controller has the same function as the VSC, and inverts and rectifies current through the switch conducting state of the device; and the VBC converts the output voltage instruction value of the MMC system into the number of conducting modules of each bridge arm and determines which sub-modules are conducted so as to apply trigger pulses to each bridge arm of the MMC system.
Each bridge arm is formed by connecting not less than 1 submodule SM and 1 bridge arm reactance in series (see figure 1).
The sub-module SM is composed of a port I, a port II, 5 IGBTs, 3 diodes, a capacitor C1 and a capacitor C2; wherein, the port I of the sub-module SM is connected with the port II of another sub-module SM, the port II of the sub-module SM is connected with the port I of the next sub-module SM, and all the sub-modules SM are connected in series in sequence (see figure 1); 3 IGBTs in the 5 IGBTs are respectively connected with 3 diodes in reverse parallel to form three groups of follow current units, which play a role in protecting and follow current on the circuit and are respectively recorded as a follow current unit I, a follow current unit II and a follow current unit III, and the other 2 IGBTs are connected in reverse parallel to form a bidirectional controllable unit for controlling the level output state of the submodule; the follow current unit I and the follow current unit II are connected in series, the follow current unit III and the bidirectional controllable unit are connected in series, and the two series combinations are connected in parallel; the port I is connected between the follow current unit I and the follow current unit II which are connected in parallel; the port II is connected with the follow current unit II, the bidirectional controllable unit and the capacitor C2; one end of the capacitor C1 is connected with one end of the follow current unit I, and the other end of the capacitor C1 is connected to the connection point of the follow current unit III and the bidirectional controllable unit which are connected in series; the capacitor C2 is connected in parallel across the freewheeling unit III and the bi-directional controllable unit connected in series with each other (see fig. 2).
The capacitance values of the capacitor C1 and the capacitor C2 are equal.
The submodule SM has three output voltage states of 0, Uc and 2Uc, wherein Uc is the voltage at two ends of each capacitor of the capacitor C1 and the capacitor C2; and the IGBT tube is turned on by 1, and the IGBT tube is turned off by 0.
The working states of the sub-module SM are as follows, and it is specified that the flow from port I to port II is positive, otherwise, it is negative, and then:
(1) the submodule SM works in the blocked state: when the current direction is positive, the output voltage of the submodule SM is 2UC(ii) a When the current direction is negative, the output voltage of the sub-module SM is 0; when in a locked state, all the IGBTs are in an off state, at this time, 5 IGBT transistors in the sub-module SM are respectively marked as an IGBT transistor T1, an IGBT transistor T2, an IGBT transistor T3, an IGBT transistor T4, and an IGBT transistor T5, and the respective corresponding switching states are 00000 (see fig. 3(a) and 3 (b));
(2) the sub-module SM works in an I working state: when the current direction is positive, the output voltage of the sub-module SM is 0; when the current direction is negative, the output voltage of the sub-module SM is 0; at this time, the switching states of the IGBT tube T1, the IGBT tube T2, the IGBT tube T3, the IGBT tube T4, and the IGBT tube T5 are 01000 in sequence (see fig. 4(a) and 4 (b));
(3) submodule SM workerWorking in a working state II: when the current direction is positive, the output voltage of the submodule SM is UC(ii) a When the current direction is negative, the output voltage of the submodule SM is UC(ii) a At this time, the switching states corresponding to the IGBT transistor T1, the IGBT transistor T2, the IGBT transistor T3, the IGBT transistor T4, and the IGBT transistor T5 are 10011 in sequence (see fig. 5(a) and 5 (b));
(4) the submodule SM works in a III working state: when the current direction is positive, the output voltage of the submodule SM is 2UC(ii) a When the current direction is negative, the output voltage of the submodule SM is 2UC(ii) a At this time, the switching states of the IGBT transistor T1, the IGBT transistor T2, the IGBT transistor T3, the IGBT transistor T4, and the IGBT transistor T5 are 10100 in this order (see fig. 6(a) and 6 (b)).
The MMC system with the rapid starting and direct-current voltage drop inhibiting capabilities is applied to a high-voltage direct-current flexible power transmission system, and is characterized by comprising not less than 2 MMC systems with the rapid starting and direct-current voltage drop inhibiting capabilities; one of the MMC systems with the rapid starting and direct-current voltage drop restraining capabilities is connected with a main power grid, the other MMC systems with the rapid starting and direct-current voltage drop restraining capabilities are respectively connected with a passive network or an active network, and the MMCs are connected through direct-current lines (see figure 7).
The line connection between the MMC system with the capability of quickly starting and inhibiting the direct-current voltage drop and the main power grid is realized by a circuit breaker; the MMC system with the capability of quickly starting and restraining the direct-current voltage drop is connected with a passive network or an active network through a circuit breaker.
A starting method of an MMC system with rapid starting and direct-current voltage drop restraining capability applied to an MMC-HVDC system is characterized by comprising the following steps:
(1) in the starting stage, an MMC system which is connected with the main power grid side and has the capability of quickly starting and restraining the direct-current voltage drop is defined as an MMC1 subsystem, and an MMC which is connected with a passive network or an active network is defined as an MMC2 subsystem; meanwhile, defining 'a valve base controller VBC' to be unlocked, indicating that the MMC1 subsystem and the MMC2 subsystem have the capability of applying trigger pulses to the IGBT, defining 'a main controller to be unlocked', and indicating that the MMC1 subsystem and the MMC2 subsystem start to work normally;
(2) closing a circuit breaker between a main power grid and the MMC1 subsystem, and charging the sub-modules of the MMC1 subsystem and the MMC2 subsystem through uncontrolled rectification of diodes in the sub-module SM by the alternating current power grid; at this time, all the IGBTs are in a locking state; when the steady state is reached, the formula (1) is established;
Figure GDA0002319990930000121
in the formula: vPN-1The voltage of the direct current polar line in the uncontrolled rectification stage; um is the effective value of the alternating-current side phase voltage; uc-MMC1-1 and uc-MMC2-1 are respectively the sub-module capacitor voltage of the MMC1 subsystem and the MMC2 subsystem in the uncontrolled rectification stage;
(3) defining a half-control rectification stage after an MMC2 subsystem 'unlocks a valve base controller VBC'; at the moment, when the direct-current voltage is stable, the MMC-HVDC power transmission system enters a semi-controlled rectification starting stage;
(4) after the semi-controlled rectification stage is finished, the MMC-HVDC system enters a high-frequency rectification stage; when the dc voltage is stable and equation (2) is satisfied, that is:
Figure GDA0002319990930000122
in the formula: vPN-2The voltage of a direct current polar line in a semi-control rectification stage; um is the effective value of the alternating-current side phase voltage; u. ofc-MMC1-2 and uc-MMC2-2 are capacitor voltages of a submodule SM in an MMC1 subsystem and an MMC2 subsystem in a half-control rectification stage respectively;
(5) the MMC1 subsystem unlocks a main controller, sub-module capacitors of the MMC1 subsystem and the MMC2 subsystem are continuously charged through constant direct-current voltage control, meanwhile, the running state of the MMC2 subsystem is kept unchanged, and the direct-current voltage is ensured to rise to a rated value synchronously; when the direct-current voltage rises to a rated value, starting the passive inversion control of the MMC2 subsystem to establish a stable no-load output voltage, and when the alternating-current output voltage of the MMC2 subsystem is stable, closing a circuit breaker connected to the MMC2 subsystem side, and ending the starting process; when the sub-module capacitor voltage is modulated at this stage, each sub-module SM outputs the required voltage according to the voltage required by the MMC system, and at the moment, each sub-module SM outputs one state of 0, Uc and 2Uc according to the condition; that is, at this stage each submodule SM may operate in any of states I, II, III.
The specific operation of the step (3) comprises the following steps:
1) sequencing submodules of each bridge arm of the MMC2 subsystem from large to small according to output voltages of the submodules;
2) after sequencing, for each bridge arm, enabling 1 submodule with the highest output voltage of the submodules to be in an I working state, namely recording 5 IGBT tubes in the submodule SM as an IGBT tube T1, an IGBT tube T2, an IGBT tube T3, an IGBT tube T4 and an IGBT tube T5 respectively, wherein the corresponding switch states are 01000 in sequence, and at the moment, the output voltage of the submodules is 0; all other sub-modules SM still work in a locking state, are charged through a direct current line and wait for the direct current voltage to be stable;
3) after the direct current voltage is stabilized, sequencing the sub-modules SM in the step 1) to enable 2 sub-modules with the highest output voltage of the sub-modules to work in an I working state, enabling other sub-modules to still work in a locking state, charging through a direct current line, and waiting for the direct current voltage to be stabilized;
4) similarly, after the direct-current voltage is stabilized, the output voltages of all the sub-modules SM are sequenced in each step, so that n +1 sub-modules with the highest voltage work in the I working state (n is the number of sub-modules working in the I working state in step 3), and other sub-modules SM still work in the locked state and are charged through the direct-current line;
5) when the number of the sub-modules SM working in the I working state is half of the total number of the sub-modules SM and the direct-current voltage is stable, the formula (2) is established.
Fig. 8 is a start control flow chart of the MMC-HVDC system based on MMC, wherein the left side virtual frame is the control mode of MMC1 during start, and the right side virtual frame is the control mode of MMC2 during start. The uncontrolled rectification start-up phase, the half-controlled rectification phase and the high-frequency rectification phase are respectively marked by dashed boxes.
Finally, it should be noted that the described embodiments are only some of the embodiments of the present application, and not all of them. 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 application.

Claims (5)

1. An MMC system with the capability of quickly starting and restraining direct-current voltage drop is characterized by comprising a main controller, a valve base controller VBC and a three-phase six-bridge-arm circuit topological structure; each of the three phases respectively comprises two bridge arms, and each bridge arm is formed by connecting N/2 submodules SM and a bridge arm reactance in series; the main controller has the same function as the VSC, and inverts and rectifies current through the switch conducting state of the device; the VBC converts the output voltage instruction value of the MMC system into the number of conducting modules of each bridge arm and determines which sub-modules are conducted so as to apply trigger pulses to each bridge arm of the MMC system; the number of the sub-modules SM is not less than one, and the number of the bridge arm reactances is not less than one; the sub-module SM is composed of a port I, a port II, 5 IGBTs, 3 diodes, a capacitor C1 and a capacitor C2; the port I of the sub-module SM is connected with the port II of the next sub-module SM, the port II of the sub-module SM is connected with the port I of the next sub-module SM, and all the sub-modules SM are sequentially connected in series; 3 IGBTs in the 5 IGBTs are respectively connected with 3 diodes in reverse parallel to form three groups of follow current units which are respectively marked as a follow current unit I, a follow current unit II and a follow current unit III, and the other 2 IGBTs are connected in reverse parallel to form a bidirectional controllable unit; the IGBT tube T1 and the reverse parallel diode form a follow current unit I, the IGBT tube T2 and the reverse parallel diode form a follow current unit II, the IGBT tube T3 and the reverse parallel diode form a follow current unit III, and the rest 2 IGBT tubes T4 and the IGBT tube T5 are connected in reverse parallel to form a bidirectional controllable unit; the follow current unit I and the follow current unit II are connected in series, and the follow current unit III and the bidirectional controllable unit are connected in series; one end of the follow current unit I is connected with one end of the follow current unit II to serve as a port I, the other end of the follow current unit II serves as a port II, the other end of the follow current unit I is connected with one end of a capacitor C1, and the other end of a capacitor C1 is connected with a connection point between the follow current unit III and the bidirectional controllable unit; the port I is connected between a follow current unit I and a follow current unit II which are connected in series; the port II is connected with the follow current unit II, the bidirectional controllable unit and the capacitor C2; one end of the capacitor C1 is connected with the cathode end of the diode in the follow current unit I, and the other end of the capacitor C1 is connected with the connection point of the follow current unit III and the bidirectional controllable unit which are connected in series; the capacitor C2 is connected in parallel at two ends of the follow current unit III and the bidirectional controllable unit which are connected in series; the submodule SM has three output voltage states of 0, Uc and 2Uc, wherein Uc is the voltage at two ends of each capacitor of the capacitor C1 and the capacitor C2; and "1" indicates that the IGBT is on, and "0" indicates that the IGBT is off; the working states of the sub-module SM are as follows, and it is specified that the flow from port I to port II is positive, otherwise, it is negative, and then:
(1) the submodule SM works in the blocked state: when the current direction is positive, the output voltage of the submodule SM is 2UC(ii) a When the current direction is negative, the output voltage of the sub-module SM is 0; when the sub-module SM is in a closed state, all the IGBTs are in a closed state, at the moment, 5 IGBTs in the sub-module SM are respectively marked as an IGBT tube T1, an IGBT tube T2, an IGBT tube T3, an IGBT tube T4 and an IGBT tube T5, and the corresponding switch states are 00000 in sequence;
(2) the sub-module SM works in an I working state: when the current direction is positive, the output voltage of the sub-module SM is 0; when the current direction is negative, the output voltage of the sub-module SM is 0; at this time, the switching states of the IGBT tube T1, the IGBT tube T2, the IGBT tube T3, the IGBT tube T4, and the IGBT tube T5 are 01000 in sequence;
(3) the submodule SM works in a II working state: when the current direction is positive, the output voltage of the submodule SM is UC(ii) a When the current direction is negative, the output voltage of the submodule SM is UC(ii) a At this time, the switching states corresponding to the IGBT transistor T1, the IGBT transistor T2, the IGBT transistor T3, the IGBT transistor T4, and the IGBT transistor T5 are 10011 in sequence;
(4) submodule SM operationIn the III working state: when the current direction is positive, the output voltage of the submodule SM is 2UC(ii) a When the current direction is negative, the output voltage of the submodule SM is 2UC(ii) a At this time, the switching states of the IGBT tube T1, the IGBT tube T2, the IGBT tube T3, the IGBT tube T4, and the IGBT tube T5 are 10100 in this order.
2. The MMC system with fast start-up and DC voltage droop suppression capability of claim 1, wherein said capacitor C1 and said capacitor C2 have equal capacitance values.
3. The MMC system with quick start and DC voltage drop suppression capability of claim 1, wherein the MMC system with quick start and DC voltage drop suppression capability is applied to a high voltage DC flexible power transmission system, and is an MMC-HVDC power transmission system consisting of not less than 2 MMC systems with quick start and DC voltage drop suppression capability; one of the MMC systems with the rapid starting and direct-current voltage drop restraining capabilities is connected with a main power grid, the other MMC systems with the rapid starting and direct-current voltage drop restraining capabilities are respectively connected with a passive network or an active network, and the MMC systems with the rapid starting and direct-current voltage drop restraining capabilities are connected through a direct-current line.
4. The MMC system with quick start and DC voltage drop suppression capability of claim 3, wherein the line connection between the MMC system with quick start and DC voltage drop suppression capability and the main power grid is realized by means of a circuit breaker; the MMC system with the capability of quickly starting and restraining the direct-current voltage drop is connected with a passive network or an active network through a circuit breaker.
5. A starting method of the MMC system with fast start-up and dc voltage sag suppression capability as claimed in claim 4 applied to an MMC-HVDC system, characterized in that it comprises the following steps:
(1) in the starting stage, an MMC system which is connected with the main power grid side and has the capability of quickly starting and restraining the direct-current voltage drop is defined as an MMC1 subsystem, and an MMC which is connected with a passive network or an active network is defined as an MMC2 subsystem; meanwhile, defining 'a valve base controller VBC' to be unlocked, indicating that the MMC1 subsystem and the MMC2 subsystem have the capability of applying trigger pulses to the IGBT, defining 'a main controller to be unlocked', and indicating that the MMC1 subsystem and the MMC2 subsystem start to work normally;
(2) closing a circuit breaker between a main power grid and the MMC1 subsystem, and charging the sub-modules of the MMC1 subsystem and the MMC2 subsystem through uncontrolled rectification of diodes in the sub-module SM by the alternating current power grid; at this time, all the IGBTs are in a locking state; when the steady state is reached, the formula (1) is established;
Figure FDA0002319990920000041
in the formula: vPN-1The voltage of the direct current polar line in the uncontrolled rectification stage; um is the effective value of the alternating-current side phase voltage; u. ofc-MMC1-1And uc-MMC2-1Sub-module capacitor voltages of the MMC1 subsystem and the MMC2 subsystem in an uncontrolled rectification stage respectively;
(3) defining a half-control rectification stage after an MMC2 subsystem 'unlocks a valve base controller VBC'; at the moment, when the direct-current voltage is stable, the MMC-HVDC power transmission system enters a semi-controlled rectification starting stage;
(4) after the semi-controlled rectification stage is finished, the MMC-HVDC system enters a high-frequency rectification stage; when the dc voltage is stable, equation (2) holds, i.e.:
Figure FDA0002319990920000051
in the formula: vPN-2The voltage of a direct current polar line in a semi-control rectification stage; um is the effective value of the alternating-current side phase voltage; u. ofc-MMC1-2And uc-MMC2-2The capacitance voltage of a submodule SM in an MMC1 subsystem and an MMC2 subsystem in a half-control rectification stage respectively;
(5) the MMC1 subsystem unlocks a main controller, sub-module capacitors of the MMC1 subsystem and the MMC2 subsystem are continuously charged through constant direct-current voltage control, meanwhile, the running state of the MMC2 subsystem is kept unchanged, and the direct-current voltage is ensured to rise to a rated value synchronously; when the direct-current voltage rises to a rated value, starting the passive inversion control of the MMC2 subsystem to establish a stable no-load output voltage, and when the alternating-current output voltage of the MMC2 subsystem is stable, closing a circuit breaker connected to the MMC2 subsystem side, and ending the starting process; when the sub-module capacitor voltage is modulated at this stage, each sub-module SM outputs the required voltage according to the voltage required by the MMC system, and at the moment, each sub-module SM outputs one state of 0, Uc and 2Uc according to the condition; that is, at this stage each submodule SM operates in any of the states I, II, III;
the specific operation of the step (3) comprises the following steps:
1) sequencing submodules of each bridge arm of the MMC2 subsystem from large to small according to output voltages of the submodules;
2) after sequencing, for each bridge arm, enabling 1 submodule with the highest output voltage of the submodules to be in an I working state, namely recording 5 IGBT tubes in the submodule SM as an IGBT tube T1, an IGBT tube T2, an IGBT tube T3, an IGBT tube T4 and an IGBT tube T5 respectively, wherein the corresponding switch states are 01000 in sequence, and at the moment, the output voltage of the submodules is 0; all other sub-modules SM still work in a locking state, are charged through a direct current line and wait for the direct current voltage to be stable;
3) after the direct current voltage is stabilized, sequencing the sub-modules SM in the step 1) to enable 2 sub-modules with the highest output voltage of the sub-modules to work in an I working state, enabling other sub-modules to still work in a locking state, charging through a direct current line, and waiting for the direct current voltage to be stabilized;
4) similarly, after the direct-current voltage is stabilized, sequencing the output voltages of all the sub-modules SM in each step to enable n +1 sub-modules with the highest voltage to work in an I working state, wherein n is the number of sub-modules working in the I working state in the previous step, and other sub-modules SM still work in a locked state and are charged through a direct-current circuit;
5) when the number of the sub-modules SM working in the I working state is half of the total number of the sub-modules SM and the direct-current voltage is stable, the formula (2) is established.
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