CN111082465A - Direct current power flow controller, control method and direct current power transmission system - Google Patents

Direct current power flow controller, control method and direct current power transmission system Download PDF

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CN111082465A
CN111082465A CN201911297122.0A CN201911297122A CN111082465A CN 111082465 A CN111082465 A CN 111082465A CN 201911297122 A CN201911297122 A CN 201911297122A CN 111082465 A CN111082465 A CN 111082465A
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direct current
current
voltage
converter station
power flow
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CN111082465B (en
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张家奎
徐千鸣
黄昕昱
席明湘
纪勇
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Hunan University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
    • H02H7/268—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured for dc systems
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
    • H02H9/02—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection responsive to excess current
    • 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
    • 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

The invention discloses a direct current power flow controller, a control method and a direct current power transmission system, and relates to a high and medium voltage direct current power transmission and distribution technology. The direct current power flow controller and the control method realize the adjustment of the current of the power transmission line by controlling the output direct current voltage of each bridge arm, thereby realizing the power flow distribution control; the transfer of the absorbed power to the emitted power is realized through the circulating current power, so that the power balance control among bridge arms is realized, and meanwhile, the power balance control of the submodules is realized through the voltage-sharing and voltage-stabilizing control of each submodule; when short-circuit faults occur, the direct current capacitor of each submodule is charged, the short-circuit fault current limiting function is achieved, then when the upper limit of the safe voltage of the direct current capacitor is reached, the bypass of the submodule is achieved through the overvoltage protection device, the self-protection function of the submodule and the whole direct current power flow controller is achieved, and the reliability is high.

Description

Direct current power flow controller, control method and direct current power transmission system
Technical Field
The invention belongs to a high-medium voltage direct current power transmission and distribution technology, and particularly relates to a multi-terminal current-limiting type direct current power flow controller, a control method and a direct current power transmission system.
Background
In recent years, the world energy development strategy begins to change from the traditional fossil energy to new energy, but the new energy has the problems of uneven distribution, intermittency and the like, and direct current transmission is one of effective schemes for solving the problems. At present, two main factors restricting the development of direct current transmission are: one is the problem of power flow control, and in a direct current transmission network, when the transmission line is more than a converter station, the problem of incomplete controllable power flow exists; the other is the problem of short-circuit fault, the inertia of the direct-current transmission system is small, and when the short-circuit fault occurs, the fault current increases rapidly and the peak value is large. Aiming at the problem of power flow control, the direct current power flow controller can increase the control freedom degree of the direct current power transmission system, so that the power flow of the direct current power transmission system is completely controllable. Aiming at the problem of short-circuit fault, when the fault occurs, the fault needs to be cleared by a direct current breaker, but the direct current breaker with high voltage and large capacity is not developed and developed. At present, the research on direct current control and direct current fault current limiting is carried out as independent fields.
Scholars at home and abroad have conducted a great deal of research on direct current power flow controllers, and the existing direct current power flow controllers are mainly divided into two types: voltage type and resistance type. The resistance type direct current power flow controller realizes the redistribution of the line current by changing the line resistance, the scheme has simple structure and low construction cost, but the loss is overlarge during the operation, a cooling system is required to be matched, and the unidirectional power flow regulation can be carried out only. The voltage type direct current power flow controller changes the voltage drop on the power transmission line, so that the current of the power transmission line is changed, and the voltage type direct current power flow controller mainly comprises two types according to different line voltage changing modes: (1) the voltage of a port is changed by using a direct current transformer to realize the control of the voltage drop of the line, and the direct current transformer has the defects that the whole equipment directly bears the rated voltage of a system, has high requirement on the insulating property, high construction cost and large loss in operation; (2) an adjustable voltage source is connected in series in a circuit to directly adjust the voltage drop of the circuit, but the bus needs to take electricity, the electricity taking part can bear the rated voltage of the system, and the insulation requirement and the construction cost are high.
The line-to-line direct current power flow controller is a new development trend recently, can be connected with direct current voltage in series in a line, does not need external power supply, directly carries out power exchange in the line, and has the advantages of low working voltage, simple equipment, small loss and the like compared with the direct current power flow controller. The line-to-line direct current power flow controller mainly comprises two types: a magnetic coupling type and a loop coupling type. The magnetic coupling type needs a transformer, so that cost and loss are increased, and the loop coupling type does not need a transformer, so that redundancy, reliability and flexibility are realized.
The existing direct current power flow controller only has the power flow control function, and the operation characteristics in the fault state need to be researched.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a direct current power flow controller, a control method and a direct current power transmission system, which have both the power flow control function and the fault current limiting function.
The invention solves the technical problems through the following technical scheme: a dc power flow controller comprising:
three bridge arms in triangular connection;
each bridge arm is formed by connecting a plurality of sub-modules in series, and the number of the sub-modules of each bridge arm is the same;
each submodule comprises an H-bridge circuit, a direct current capacitor and an overvoltage protection device, wherein the direct current capacitor is connected in parallel with the input end of the H-bridge circuit, and the overvoltage protection device is connected in parallel with the output end of the H-bridge circuit.
According to the direct current power flow controller, each bridge arm is formed by connecting the sub-modules with the same number in series, the direct current power flow controller is highly modularized, the structure is easy to expand, and the application range is wide; the three bridge arms are connected in a triangular shape to form a loop, and the loop is used for coupling each bridge arm without a transformer or an inductor, so that the cost and the volume of the equipment are reduced; the power flow controller carries out power balance by internal circulation without adding an external circuit, has the functions of power flow control and fault current limiting, and has large development and utilization space; the power flow controller also has a short-circuit fault self-protection function, and can perform self-protection when the short-circuit fault reaches the upper limit of the short-circuit fault, so that the power flow controller has high reliability.
Furthermore, the overvoltage protection device is an anti-parallel thyristor which is a controlled component, the sub-module can be bypassed through the anti-parallel thyristor, the sub-module or the direct current power flow controller can be switched out when a fault occurs, self protection is achieved, a bypass circuit does not need to be additionally arranged, and meanwhile compared with a voltage dependent resistor, the overvoltage protection selectable range of the anti-parallel thyristor is wider.
Correspondingly, the invention also provides a control method of the direct current power flow controller, which comprises the following steps:
step 1: the direct current power flow controller is connected into a multi-terminal direct current transmission line, the multi-terminal direct current transmission line at least comprises a main converter station 1 and two slave converter stations 2/3, and one bridge arm A of the direct current power flow controller1The other bridge arm A is connected in series in the power transmission line between the main converter station 1 and the slave converter station 22The power transmission line is connected between the main converter station 1 and the slave converter station 3 in series;
step 2: when the power flow is regulated and controlled, the overvoltage protection device of each submodule is in a blocking state, the output direct-current voltage of each bridge arm is controlled, the current flowing through the power transmission line is regulated, the power balance of the direct-current power flow controller is maintained, and the power flow state regulation and control are realized;
and step 3: when short circuit occurs, fault current is absorbed through the direct current capacitor of each submodule, and then bypass of the corresponding submodule is achieved through the overvoltage protection device of each submodule, so that fault current limiting and self-protection functions are achieved.
According to the control method, through the control of the direct-current voltage output by each bridge arm and the control of the overvoltage protection device, not only can the regulation and control of the tidal current state be realized, but also the functions of fault current limiting and self protection can be realized.
Further, the step 2 comprises the following substeps:
step 2.1, controlling the output direct-current voltage of each bridge arm, and adjusting the output direct-current voltage of the direct-current power flow controller, so that the current of the power transmission line is adjusted, and power flow distribution control is realized;
step 2.2 control bridge arm A3Generating AC circulating current, bridge arm A1And A2Generating additional alternating voltage to realize power balance control among bridge arms;
and 2.3, collecting the direct current capacitor voltages of the n sub-modules in each bridge arm, summing the direct current capacitor voltages of the n sub-modules and averaging the direct current capacitor voltages of the n sub-modules, enabling the direct current capacitor voltage of each sub-module to follow the average value, and enabling the sum of the direct current capacitor voltages of the n sub-modules to follow n times of the rated voltage value of the direct current capacitor, so that voltage-sharing and voltage-stabilizing control of the sub-modules is realized.
The load flow distribution control is realized by controlling the direct current voltage output by each bridge arm, and during the load flow distribution control, the direct current power is absorbed from one power transmission line and then sent to the other power transmission line, but the absorbed direct current power cannot be ensured to be sent right through the other power transmission line, so that the load flow distribution control is realized through the bridge arms A3Generating alternating current circulation, and realizing the transfer of absorbed power through circulation power so as to realize the power balance control among bridge arms; and voltage-sharing and voltage-stabilizing control is carried out on each submodule, so that power divergence of each submodule is avoided, and power balance control of the submodules is realized.
Further, in step 2.1, bridge arm a1Output direct current voltage UA1,DCComprises the following steps:
UA1,DC＝I1R12(1-a)[b(1-a)-a-ac]+I2R12(1-a)c
bridge arm A2Output direct current voltage UA2,DCComprises the following steps:
UA2,DC＝-I1R12a[b(1-a)-a-ac]+I2R12ac
third bridge arm A3Output direct current voltage UA3,DCComprises the following steps:
UA3,DC＝-I1·R12[b(1-a)-a-ac]
wherein, I1Is the port current, I, of the main converter station 12For port current from the converter station 2, R12Is the equivalent resistance of the transmission line between the main converter station 1 and the slave converter station 2, a is the branch current proportionality coefficient,
Figure BDA0002319432030000041
b. c is the proportional coefficient of the branch resistance,
Figure BDA0002319432030000042
I12for the current flowing through the transmission line between the master converter station 1 and the slave converter station 2, R13Is the equivalent resistance R of the transmission line between the main converter station 1 and the slave converter station 323Is the equivalent resistance of the transmission line between the converter station 2 and the converter station 3.
By controlling bridge arm A1、A2Any one of the bridge arms outputs positive direct current voltage, the other bridge arm outputs negative direct current voltage, namely, the direct current power flow controller absorbs direct current power from one power transmission line and then sends out direct current power to the other power transmission line to realize the power flow distribution control of the power transmission line, and meanwhile, the bridge arm A outputs positive direct current voltage and the other bridge arm outputs negative direct current voltage, namely, the direct current power flow controller absorbs direct current power from one power3And the direct current voltage is output, so that the direct current power flow controller is ensured to have no direct current circulation.
Further, in step 2.2, bridge arm a3The generated AC circulating current and the bridge arm A1Or A2The generated additional alternating voltage satisfies the following relation:
Figure BDA0002319432030000051
wherein, IACIs a bridge arm A3Generated AC circulating current, UAi,ACIs a bridge arm A1Or A2The generated additional ac voltage, i ═ 1,2 denotes bridge arm a1Or A2，
Figure BDA0002319432030000052
Is an alternating current circulating current IACWith additional AC voltage UAi,ACPhase difference between them, I1Is the port current, I, of the main converter station 12For port current from the converter station 2, R12Is the equivalent resistance of the transmission line between the main converter station 1 and the slave converter station 2, a is the branch current proportionality coefficient,
Figure BDA0002319432030000053
b. c is the proportional coefficient of the branch resistance,
Figure BDA0002319432030000054
I12for the current flowing through the transmission line between the master converter station 1 and the slave converter station 2, R13Is the equivalent resistance R of the transmission line between the main converter station 1 and the slave converter station 323Is the equivalent resistance of the transmission line between the converter station 2 and the converter station 3.
Bridge arm A3The generated AC circulating current and the bridge arm A1Or A2The generated extra alternating voltage forms circulating current power, and power exchange balance among bridge arms is realized through the circulating current power.
Further, the specific operation of step 3 is:
3.1, enabling the power device of the H-bridge circuit of each submodule to be in a blocking state, and enabling the direct current capacitors of all the submodules to be connected into the power transmission line;
3.2, the direct current capacitor absorbs fault current, and when the voltage of the direct current capacitor is greater than the upper limit of the safety voltage of the direct current capacitor, the corresponding overvoltage protection device acts to bypass the submodule, so that self protection of the submodule is realized;
and 3.3, when the voltage of the direct current capacitors of all the submodules is greater than the upper limit of the safety voltage of the corresponding direct current capacitor, bypassing the direct current power flow controller to realize self protection of the direct current power flow controller.
Correspondingly, the invention also provides a direct current transmission system, which comprises the direct current power flow controller and a multi-terminal direct current transmission line, wherein the multi-terminal direct current transmission line at least comprises one main converter station and two slave converter stations, one bridge arm of the direct current power flow controller is connected in series in a transmission line between the main converter station and one of the slave converter stations, and the other bridge arm of the direct current power flow controller is connected in series in a transmission line between the main converter station and the other slave converter station.
Advantageous effects
Compared with the prior art, the invention provides a direct current power flow controller and a control method, which are formed by three bridge arms in triangular connection, wherein the output direct current voltage of the whole direct current power flow controller is controlled by controlling the output direct current voltage of each bridge arm, so that the current of a power transmission line is regulated, and the power flow distribution control is realized; in the process of power flow regulation, the transfer of absorbed power to transmitted power is realized through the circulating current power, so that the power absorbed by a certain bridge arm from a power transmission line is transmitted from another power transmission line through another bridge arm, the power balance control among the bridge arms is realized, meanwhile, the power balance control of the sub-modules is realized through the voltage-sharing and voltage-stabilizing control of each sub-module, and the stability of the power flow regulation of the whole direct current power flow controller is ensured.
According to the direct current power flow controller and the control method, when short-circuit faults occur, the direct current capacitor of each submodule is charged, short-circuit fault current is absorbed, the short-circuit fault current limiting function is achieved, then when the upper limit of the safe voltage of the direct current capacitor is reached, the bypass of the submodule is achieved through the overvoltage protection device, the self-protection function of the submodule and the whole direct current power flow controller is achieved, and the high-reliability direct current power flow controller has high reliability.
According to the direct current power flow controller and the control method, each bridge arm is formed by connecting the sub-modules with the same number in series, the direct current power flow controller is highly modularized, the structure is easy to expand, and the application range is wide.
Drawings
In order to more clearly illustrate the technical solution of the present invention, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are only one embodiment of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on the drawings without creative efforts.
Fig. 1 is a topology structure diagram of a dc power flow controller according to an embodiment of the present invention;
fig. 2 is an equivalent circuit diagram of a dc power flow controller under power flow regulation in the embodiment of the invention;
FIG. 3 is a block diagram of three-layer control under power flow regulation in an embodiment of the invention;
FIG. 4 is a control flow chart under short-circuit fault in the embodiment of the present invention;
fig. 5 is an equivalent circuit diagram under short-circuit fault in the embodiment of the present invention, the left side of fig. 5 is the equivalent circuit diagram of the first stage, and the right side of fig. 5 is the equivalent circuit diagram of the second stage;
FIG. 6 is a diagram of a simulation model of a three-terminal DC power transmission system in an embodiment of the present invention;
fig. 7 is a simulation verification diagram of power flow regulation in the embodiment of the present invention, where fig. 7(a) is a simulation curve of current flowing through each power transmission line, fig. 7(b) is a direct-current capacitor voltage curve of each corresponding bridge arm sub-module, fig. 7(c) is a bridge arm current curve of each corresponding bridge arm, and fig. 7(d) is a bridge arm voltage curve of each corresponding bridge arm;
fig. 8 is a simulation verification diagram of power flow reversal in the embodiment of the present invention, where fig. 8(a) is a simulation curve of current flowing through each power transmission line after reversal, fig. 8(b) is a direct current capacitance voltage curve of each corresponding bridge arm sub-module, fig. 8(c) is a bridge arm current curve of each corresponding bridge arm, and fig. 8(d) is a bridge arm voltage curve of each corresponding bridge arm;
fig. 9 is a diagram of a simulation verification of short-circuit fault current limiting in an embodiment of the present invention, where fig. 9(a) is a transmission line current when an infinite current measure fault short-circuit, fig. 9(b) is a transmission line current under a fault current limiting effect, and fig. 9(c) is a dc capacitor voltage of a corresponding sub-module.
Detailed Description
The technical solutions in the present invention are clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
As shown in fig. 1, the dc power flow controller provided by the present invention includes three bridge arms connected in a triangle; each bridge arm is formed by connecting a plurality of sub-modules in series, and the number of the sub-modules of each bridge arm is the same; each submodule comprises an H-bridge circuit, a direct current capacitor and an overvoltage protection device, wherein the direct current capacitor is connected in parallel with the input end of the H-bridge circuit, and the overvoltage protection device is connected in parallel with the output end of the H-bridge circuit. In this embodiment, the overvoltage protection device is an anti-parallel thyristor, the anti-parallel thyristor is a controlled component, and the sub-module can be bypassed through the anti-parallel thyristor, so that the whole dc power flow controller is bypassed, the sub-module or the dc power flow controller is switched out when a fault occurs, self protection is realized without additionally arranging a bypass circuit, and meanwhile, compared with a voltage dependent resistor, the overvoltage protection selectable range of the anti-parallel thyristor is wider. The inductance in fig. 1 represents the stray parameters of the bridge arm itself, and generates a dc circulating current under the dc circulating voltage, but is very small and can be ignored.
The invention provides a control method of a direct current power flow controller, which has two working states: the power flow regulation and control state and the short-circuit fault state specifically comprise the following steps:
1. the direct current power flow controller is connected into a multi-terminal direct current transmission line, the multi-terminal direct current transmission line at least comprises a main converter station 1 and two slave converter stations 2/3, and one bridge arm A of the direct current power flow controller1The other bridge arm A is connected in series in the power transmission line between the main converter station 1 and the slave converter station 22In series in the power transmission line between the master converter station 1 and the slave converter station 3.
As shown in FIG. 1, I1、I2And I3The port currents of the master converter station 1, the slave converter station 2 and the slave converter station 3 are respectively, I, because the master transmission line is very short1Also current of the main transmission line, U1、U2And U3A master converter station 1, a slave converter station 2 andfrom the port voltage, I, of the converter station 312Is the current flowing through the transmission line between the master converter station 1 and the slave converter station 2, i.e. the current of the secondary transmission line 2, I13Is the current flowing through the transmission line between the master converter station 1 and the slave converter station 3, i.e. the current of the branch transmission line 3.
2. And in a power flow regulation and control state, the overvoltage protection device of each submodule is in a blocking state, the output direct-current voltage of each bridge arm is controlled, the current flowing through the power transmission line is regulated, the power balance of the direct-current power flow controller is maintained, and the power flow regulation and control are realized. The power flow state regulation and control comprises three layers of control, wherein the first layer is power flow distribution control, the second layer is power balance control among bridge arms, and the third layer is sub-module power balance control, so that the power flow distribution control is realized, the power balance of the direct current power flow controller is maintained, and the stability of the direct current power flow controller is ensured.
2.1 flow distribution control
And under the power flow regulation and control state, the overvoltage protection device of the submodule is always sealed. And each bridge arm enables the direct current power flow controller to generate different output voltages by controlling the output direct current voltage of the submodule. When each submodule works in a power flow regulation state, three output levels exist: + Uc、0、-Uc，UcIs the voltage across the dc capacitor. Each bridge arm A provided with direct current power flow controllerx(x is 1,2,3) contains n submodules, which can generate 2n +1 levels at most, and the maximum amplitude of the output level is nUc. Output direct-current voltage generated by the bridge arm is connected into the power transmission line in series, and the voltage drop of the power transmission line is changed, so that the effect of adjusting the current of the power transmission line is achieved, and the power flow distribution control is realized.
In normal dc transmission and distribution, the equivalent circuit of the line is shown in fig. 2. To describe the distribution law of power flows, three variables are defined here: a is branch current proportionality coefficient, b and c are branch resistance proportionality coefficient, and the specific definition formula is as follows:
Figure BDA0002319432030000091
wherein R is12Is the equivalent resistance R of the transmission line between the main converter station 1 and the slave converter station 213Is the equivalent resistance R of the transmission line between the main converter station 1 and the slave converter station 323Is the equivalent resistance of the transmission line between the converter station 2 and the converter station 3, I23Is the current flowing through the transmission line between the slave converter station 2 and the slave converter station 3.
From which the current I of the transmission line can be set12、I13Comprises the following steps:
Figure BDA0002319432030000101
bridge arm A1And A2The DC voltage to be inserted into the corresponding transmission line is UA1,DC、UA2,DCIdeally, the dc power flow controller does not consume power, i.e. the bridge arm a1And A2The net power interacting with two transmission lines is 0, have
Figure BDA0002319432030000102
And according to KVL has
UA1,DC+I12R12+(I12-I2)R23＝UA2,DC+I13R13(4)
The bridge arm A can be obtained by combining the formulas (3) and (4)1And A2Direct-current voltage to be output:
Figure BDA0002319432030000103
as shown in formula (5), UA1,DC、UA2,DCThe value of the voltage difference depends on a branch current proportionality coefficient a, when the branch current proportionality coefficient is adjusted, the power flowing through the transmission line can be adjusted, and then the direct current power flow controller outputs UA1,DC、UA2,DCAnd the power flow distribution of the power transmission line can be realized. Three bridge arms of the direct current power flow controller are connected in a triangular mode, and in order to prevent direct current circulation from occurring, the output of the bridge armsThe direct current voltage must satisfy:
UA1,DC+UA3,DC-UA2,DC＝0 (6)
from FIG. 2 and equation (6), bridge arm A3The dc voltage to be output is:
UA3,DC＝-I1·R12[b(1-a)-a-ac](7)
in order not to affect the overall net power balance and to generate DC circulating current, bridge arm A3The DC voltage is output, but the DC current does not flow. The first layer is controlled by bridge arm A1、A2Any one of the bridge arms outputs positive direct current voltage, the other bridge arm outputs negative direct current voltage, namely, the direct current power flow controller absorbs direct current power from one power transmission line and then sends out direct current power to the other power transmission line to realize the power flow distribution control of the power transmission line, and meanwhile, the bridge arm A outputs positive direct current voltage and the other bridge arm outputs negative direct current voltage, namely, the direct current power flow controller absorbs direct current power from one power transmission3The upper output direct current voltage ensures that the direct current power flow controller has no direct current circulation.
2.2 Power balance control between bridge arms
The bridge arm A is controlled by the first layer to know that a certain period of time1、A2One of the bridge arms absorbs power from one of the transmission lines, the other bridge arm transmits power to the other transmission line, and the power flowing direction depends on the current direction in the transmission lines and the direction of the output direct current voltage of the bridge arm. If the bridge arm only absorbs or emits power, the bridge arm power is diverged. Therefore, the bridge arms need to be matched to realize power balance of the bridge arms, namely when one bridge arm absorbs power, the absorbed power is transferred to the other bridge arm to be sent out by the other bridge arm.
In order to prevent the direct current power flow controller from generating direct current circulation, the second layer of control adopts alternating current circulation to carry out power transmission, and the alternating current circulation passes through the control bridge arm A3Output, simultaneously, at arm A1、A2And extra alternating current voltage components are generated to form circulating current power with alternating current circulating current, so that power exchange balance among bridge arms is realized.
Bridge arm A3The amplitude of the generated AC circulating current is IACArm A of the bridge1、A2The amplitude of the generated extra AC voltage component is U respectivelyA1，AC、UA2，ACAccording to fig. 2, there are:
Figure BDA0002319432030000111
wherein, IA1,AC、IA2,ACAre respectively a bridge arm A1、A2Generated AC current, bridge arm A1、A2In the above circulating current power generated by the extra ac voltage and the corresponding ac current, there are a double frequency ac component and a dc component, and since the average value of the ac component in a half cycle is zero, only the dc component is considered in the circulating current power exchange balance, and the respective circulating current power dc components are:
Figure BDA0002319432030000121
in the formula: pA1,CCDC、PA2,CCDCAre respectively a bridge arm A1、A2The direct-current component of the circulating power,
Figure BDA0002319432030000126
are respectively a bridge arm A1、A2Ac voltage U ofA1，AC、UA2，ACWith alternating current IA1,AC、IA2,ACThe phase difference between them.
The power balance between the bridge arms is realized by the circulating current power, so the bridge arm A is needed1、A2D.c. power PA1,DC、PA2,DCBalancing with the corresponding direct current component of the circulating current power, namely:
Figure BDA0002319432030000122
from formulae (2), (3), (5) and (8) - (10):
Figure BDA0002319432030000123
according to the formula (11), when
Figure BDA0002319432030000124
At this time, the ac voltage, ac circulating current amplitude required for power balance is the smallest. Therefore it is provided with
Figure BDA0002319432030000125
Bridge arm A1、A2Amplitude of output extra alternating voltage component and bridge arm A3The output AC circulating current amplitude satisfies the relation of the formula (11).
Inter-arm power balance control, arm A1、A2The power absorbed by any one of the bridge arms is transmitted to the other bridge arm through the circulating current power and is sent out by the other bridge arm, so that the power balance of the bridge arms in the direct current power flow controller is realized.
2.3 sub-module power balance control
From the first and second layer control, each bridge arm works as a whole. In practice, each leg consists of n submodules. During operation, voltage-sharing and voltage-stabilizing control needs to be carried out on each submodule, and power divergence of each submodule is avoided.
And (3) voltage stabilization control: collecting the direct current capacitor voltages of n sub-modules in the bridge arm, summing the direct current capacitor voltages, and enabling the sum of the direct current capacitor voltages of the n sub-modules to follow the rated voltage value of the direct current capacitor by n times, wherein the controller adopts a PI controller. And after the control instruction value is obtained through calculation, the control instruction value is added to the direct-current voltage modulation instruction calculated in the first-layer control and is sent to each submodule controller.
Pressure equalizing control: and (3) calculating an average value of the acquired n sub-module capacitor voltages, enabling the direct current capacitor voltage of each sub-module on the bridge arm to follow the average value, calculating to obtain a control instruction by adopting a PI (proportional integral) controller, adding the control instruction to the direct current voltage modulation instruction calculated in the first-layer control, and sending the control instruction to each sub-module controller.
And finally, in a steady state, the capacitor voltage convergence of each sub-module on the bridge arm approaches to a rated voltage value, and the integral output direct current voltage of the bridge arm is a given value controlled by the first layer through duty ratio regulation.
Fig. 3 shows a three-layer control block diagram of a dc power flow controller, and in power flow distribution control, a final output bridge arm a1、A2Voltage modulation command of, DA1，DC、DA2，DC、DA3，DCIs a bridge arm A1、A2、A3D of the direct voltage modulation command valueA1，AC、DA2，AC、DA3，ACIs a bridge arm A1、A2、A3The ac voltage modulation command value of (a),
Figure BDA0002319432030000131
for the bridge arm A obtained according to formula (5)1、A2Outputting given value (reference value) of DC voltage, in the bridge arm power balance control, Clark converting to abc coordinate system and αβ coordinate system, Clark inverse converting to αβ coordinate system and converting to abc coordinate system,
Figure BDA0002319432030000132
is a bridge arm AiThe voltages of the direct-current capacitors of the two capacitors are summed,
Figure BDA0002319432030000133
is n times of the rated voltage value of the direct current capacitor; in sub-module power balance control, IAiIs a bridge arm AiThe current value of (a) is set,
Figure BDA0002319432030000134
is a bridge arm AiVoltage modulation signal of jth sub-module of (1), DAiIs a bridge arm AiThe voltage of (2) modulates the signal.
3. And when the sub-modules are in a short-circuit fault state, the direct current capacitor of each sub-module absorbs fault current, and then the overvoltage protection device of each sub-module realizes the bypass of the corresponding sub-module, thereby realizing the functions of fault current limiting and self protection.
Under the short-circuit fault state, four IGBTs of each submodule H bridge circuit are always in a blocking state, and the submodules become an uncontrollable rectifier bridge structure. When short-circuit fault occurs, all the sub-modules are connected into the power transmission line, and the short-circuit fault current is absorbed by charging the direct-current capacitor, so that the increase of the short-circuit fault current is limited. After the direct current capacitor absorbs the short-circuit fault current, the capacitor voltage can be increased, and when the capacitor voltage is higher than the upper limit of the safe voltage, the overvoltage protection device of the sub-module acts to bypass the sub-module and protect the sub-module from being damaged by the short-circuit fault current.
As shown in fig. 4, when in the short-circuit fault state, the dc power flow controller has two stages: the current limiting stage and the self-protection stage output negative voltage through the current limiting of the first stage to play a role of short-circuit current limiting; the direct current capacitor is protected from being damaged through the self-protection of the second stage, and the short-circuit fault current is not influenced.
If a short-circuit fault occurs from the port outlet side of the converter station 2 and the line equivalent circuit is as shown in fig. 5, UA1,cap、UA2,cap、UA3,capAre respectively a bridge arm A1、A2、A3The first stage: the short-circuit current of the power transmission line can be increased sharply, the direct current power flow controller detects a short-circuit fault, the short-circuit fault current needs to be absorbed, at the moment, the IGBTs of all the H-bridge circuits of the submodules are blocked immediately, the direct current capacitors of all the submodules are connected into the power transmission line, at the moment, the overvoltage protection devices of the submodules are in a closed state, and only the anti-parallel diodes of the IGBTs can be conducted continuously. The dc capacitors of the sub-modules absorb the short-circuit fault current from the power line and output a negative voltage, as shown in the left equivalent diagram of fig. 5.
And a second stage: the sub-module DC capacitor absorbs the short-circuit fault current to charge the capacitor, the voltage of the capacitor rises, and when the voltage of the capacitor reaches the upper limit U of the safe voltage of the DC capacitorC,limWhen the sub-module needs to be switched out of the power transmission line, the overvoltage protection device of the sub-module acts to bypass the sub-module, and self-protection of the sub-module is achieved; when all sub-modules reach UC,limWhen the direct current power flow controller is switched out of the power transmission line, self protection of the direct current power flow controller is achieved, and as shown in an equivalent diagram on the right side of fig. 5, the direct current power flow controller is switched out of the power transmission line.
4. Simulation verification
As shown in fig. 6, a three-terminal direct-current transmission system simulation model including the direct-current power flow controller of the present invention is built on a PLECS simulation platform, and an MMCL-DCPFC (modulated multi-terminal current-limiting DC power flow controller) represents the direct-current power flow controller of the present invention. Three-terminal direct current transmission system: the converter station 1(VSC1) is a main converter station and controls the direct current side voltage U 1200 kV; the active power injected into the three-terminal direct-current transmission system from the converter station 2(VSC2) and the active power injected into the three-terminal direct-current transmission system from the converter station 3(VSC3) are respectively-16.5 MW and-7.5 MW, the rated voltage value of the direct-current capacitor of the submodule is 200V, the upper limit of the safe voltage is set to be 1000V, and the main parameters of the direct-current transmission line of the system are shown in Table 1. In this embodiment, the number of the submodules of each bridge arm is 9, and the submodules can be estimated according to the requirement of short-circuit current-limiting control and the amplitude of the additional alternating-current voltage in the exchanged circulating current power.
TABLE 1 DC POWER TRANSMISSION SYSTEM LINE PARAMETERS TABLE
Direct current circuit Distance/km Resistance/omega inductor/H
Line
12 500 5 0.3
Line 13 375 3.75 0.225
Line 23 500 5 0.3
And (3) tidal current regulation and control:
(1) normal operating mode
At first, the direct current power flow controller is not put into the operation of the direct current transmission system, and the direct current transmission system operates in a stable state. The current of a main power transmission line where the direct current power flow controller is located is as follows: i is1120A, the current of the two transmission lines is as follows: i is12＝62.6A，I1357.4A. When the direct current power flow controller works stably, the currents of the two transmission lines are as follows according to the formula (2): i is12＝72A，I13The simulation results are shown in fig. 7, 48A.
As can be seen from fig. 7(a), when the MMCL-DCPFC is put into the dc transmission system and starts the power flow regulation, the currents of the two transmission lines are rapidly adjusted to implement the redistribution of the currents, and the current values are respectively stabilized at the set values I12＝72A，I1348A. As can be seen from fig. 7(b), after the currents of the two transmission lines are redistributed and stabilized, the dc capacitor voltages of the sub-modules are also stabilized and both are maintained at about 200V, and fig. 7(c) and 7(d) are the bridge arm a respectivelyxBridge arm current IAxAnd bridge arm voltage UAx. Therefore, the MMCL-DCFC can realize rapid and accurate line flow distribution, and meanwhile, the power of the MMCL-DCFC can be kept balanced.
(2) Power flow reversal
The MMCL-DCFC is initially put into operation in the DC transmission system and operates in a steady state. At this time, the currents of the two transmission lines are as follows: i is12＝72A，I1348A. When t is 3s, the power flow of the two transmission lines is reversed, namelyBranch current I12、I13While reversing direction. After the power flow is reversed, because the set branch current proportionality coefficient is not changed, the steady state of the system after the power flow is reversed should be: i is12＝-72A；I13＝-48A。
As shown in fig. 8, it can be seen that after the power flows of the two transmission lines are reversed, the currents of the two transmission lines are rapidly adjusted and stabilized again, and are maintained at the steady-state value: i is12＝-72A；I13-48A, as shown in fig. 8 (a). Correspondingly, the dc capacitor voltage of the sub-module fluctuates when the power flow of the two transmission lines reverses, and is rapidly stabilized to be maintained at about 200V, as shown in fig. 8 (b). After the power flows of the two transmission lines are reversed, the currents of the two transmission lines are reversed and the magnitudes of the currents are unchanged, so that the direct-current voltages output by the bridge arms are changed only in direction, but the magnitudes of the direct-current voltages are unchanged, and as can be seen from fig. 8(c) and 8(d), the MMCL-DCFC bidirectional power flow regulation and control capability is verified.
Short-circuit fault current limiting:
(1) short circuit fault to ground
Initially, the MMCL-DCPFC is put into operation in the dc transmission system and operates in a steady state. At this time, the currents of the two transmission lines are as follows: i is12＝72A，I1348A. And 3s, generating a grounding short circuit from the direct current port of the converter station 2, and removing the fault after 8ms of short circuit.
As shown in fig. 9, it can be seen from fig. 9(a) that when a short-circuit fault occurs, if there is no current limiting measure, the current of each transmission line will increase sharply, and the transmission line is heavily overloaded and maintained for a long time. Before the fault is cleared, the stability and safety of the entire dc transmission system as well as the ac grid system can be threatened by a severe overload of the transmission line. In this embodiment, with the dc power flow controller of the present invention installed, as shown in fig. 9(b), after a fault is detected, the MMCL-DCPFC immediately enters a short-circuit fault current limiting state, and the bridge arm outputs a negative voltage to the power transmission line, so as to slow down the increase of the short-circuit fault current, and even realize the negative increase.
The fault is cleared after 8ms of fault occurrence, and when 9ms of fault occurrence, as shown in FIG. 9(c)And the direct current capacitor voltage of each submodule sequentially reaches the upper limit of the safe voltage, the overvoltage protection device of the submodule acts to bypass each submodule until the whole direct current power flow controller is switched into a direct current power transmission system by the bypass, and the direct current power transmission system is reduced to 0 after the power transmission line is temporarily increased. In the case of current-limiting measures, the main transmission line I1The fault peak current reaches 772A, and under the current limiting action of the direct current power flow controller, the main power transmission line I1The peak current of the fault is 723A, and the peak current is reduced by 6.35%. Therefore, the direct current power flow controller has certain fault current limiting capacity, and in addition, the current limiting of the direct current power flow controller is realized by absorbing fault current energy through the direct current capacitor of the submodule, so the current limiting effect can be adjusted by designing the direct current capacitor of the submodule, for example, the capacitance value of the capacitor is reduced, the upper limit of the safe voltage of the capacitor is increased, and the current limiting effect can be enhanced.
The above disclosure is only for the specific embodiments of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art can easily conceive of changes or modifications within the technical scope of the present invention, and shall be covered by the scope of the present invention.

Claims (8)

1. A dc power flow controller, comprising:
three bridge arms in triangular connection;
each bridge arm is formed by connecting a plurality of sub-modules in series, and the number of the sub-modules of each bridge arm is the same;
each submodule comprises an H-bridge circuit, a direct current capacitor and an overvoltage protection device, wherein the direct current capacitor is connected in parallel with the input end of the H-bridge circuit, and the overvoltage protection device is connected in parallel with the output end of the H-bridge circuit.
2. A dc power flow controller according to claim 1, characterized in that said overvoltage protection means are anti-parallel thyristors.
3. A control method of a direct current power flow controller is characterized by comprising the following steps:
step 1: connecting the dc power flow controller of claim 1 or 2 to a multi-terminal dc power transmission line, wherein the multi-terminal dc power transmission line at least comprises a master converter station 1 and two slave converter stations 2/3, and one leg a of the dc power flow controller1The other bridge arm A is connected in series in the power transmission line between the main converter station 1 and the slave converter station 22The power transmission line is connected between the main converter station 1 and the slave converter station 3 in series;
step 2: when the power flow is regulated and controlled, the overvoltage protection device of each submodule is in a blocking state, the output direct-current voltage of each bridge arm is controlled, the current flowing through the power transmission line is regulated, the power balance of the direct-current power flow controller is maintained, and the power flow state regulation and control are realized;
and step 3: when short circuit occurs, fault current is absorbed through the direct current capacitor of each submodule, and then bypass of the corresponding submodule is achieved through the overvoltage protection device of each submodule, so that fault current limiting and self-protection functions are achieved.
4. A control method of a dc power flow controller according to claim 3, wherein said step 2 comprises the following sub-steps:
step 2.1, controlling the output direct-current voltage of each bridge arm, and adjusting the output direct-current voltage of the direct-current power flow controller, so that the current of the power transmission line is adjusted, and power flow distribution control is realized;
step 2.2 control bridge arm A3Generating AC circulating current, bridge arm A1And A2Generating additional alternating voltage to realize power balance control among bridge arms;
and 2.3, collecting the direct current capacitor voltages of the n sub-modules in each bridge arm, summing the direct current capacitor voltages of the n sub-modules and averaging the direct current capacitor voltages of the n sub-modules, enabling the direct current capacitor voltage of each sub-module to follow the average value, and enabling the sum of the direct current capacitor voltages of the n sub-modules to follow n times of the rated voltage value of the direct current capacitor, so that voltage-sharing and voltage-stabilizing control of the sub-modules is realized.
5. The method of claim 4A control method of the dc power flow controller is characterized in that in the step 2.1, the bridge arm a1Output direct current voltage UA1,DCComprises the following steps:
UA1,DC＝I1R12(1-a)[b(1-a)-a-ac]+I2R12(1-a)c
bridge arm A2Output direct current voltage UA2,DCComprises the following steps:
UA2,DC＝-I1R12a[b(1-a)-a-ac]+I2R12ac
third bridge arm A3Output direct current voltage UA3,DCComprises the following steps:
UA3,DC＝-I1·R12[b(1-a)-a-ac]
wherein, I1Is the port current, I, of the main converter station 12For port current from the converter station 2, R12Is the equivalent resistance of the transmission line between the main converter station 1 and the slave converter station 2, a is the branch current proportionality coefficient,
Figure FDA0002319432020000021
b. c is the proportional coefficient of the branch resistance,
Figure FDA0002319432020000022
I12for the current flowing through the transmission line between the master converter station 1 and the slave converter station 2, R13Is the equivalent resistance R of the transmission line between the main converter station 1 and the slave converter station 323Is the equivalent resistance of the transmission line between the converter station 2 and the converter station 3.
6. A control method for a DC power flow controller according to claim 4, characterized in that in step 2.2, bridge arm A3The generated AC circulating current and the bridge arm A1Or A2The generated additional alternating voltage satisfies the following relation:
Figure FDA0002319432020000023
wherein, IACIs a bridge arm A3Generated AC circulating current, UAi,ACIs a bridge arm A1Or A2The generated additional ac voltage, i ═ 1,2 denotes bridge arm a1Or A2，
Figure FDA0002319432020000024
Is an alternating current circulating current IACWith additional AC voltage UAi,ACPhase difference between them, I1Is the port current, I, of the main converter station 12For port current from the converter station 2, R12Is the equivalent resistance of the transmission line between the main converter station 1 and the slave converter station 2, a is the branch current proportionality coefficient,
Figure FDA0002319432020000031
b. c is the proportional coefficient of the branch resistance,
Figure FDA0002319432020000032
I12for the current flowing through the transmission line between the master converter station 1 and the slave converter station 2, R13Is the equivalent resistance R of the transmission line between the main converter station 1 and the slave converter station 323Is the equivalent resistance of the transmission line between the converter station 2 and the converter station 3.
7. The control method of the dc power flow controller according to claim 4, wherein the specific operation of step 3 is:
3.1, enabling the power device of the H-bridge circuit of each submodule to be in a blocking state, and enabling the direct current capacitors of all the submodules to be connected into the power transmission line;
3.2, the direct current capacitor absorbs fault current, and when the voltage of the direct current capacitor is greater than the upper limit of the safety voltage of the direct current capacitor, the corresponding overvoltage protection device acts to bypass the submodule, so that self protection of the submodule is realized;
and 3.3, when the voltage of the direct current capacitors of all the submodules is greater than the upper limit of the safety voltage of the corresponding direct current capacitor, bypassing the direct current power flow controller to realize self protection of the direct current power flow controller.
8. A direct current transmission system, characterized by: the direct current power flow controller comprises the direct current power flow controller according to claim 1 or 2 and a multi-terminal direct current transmission line, wherein the multi-terminal direct current transmission line at least comprises a main converter station and two slave converter stations, one bridge arm of the direct current power flow controller is connected in series in a transmission line between the main converter station and one of the slave converter stations, and the other bridge arm of the direct current power flow controller is connected in series in a transmission line between the main converter station and the other slave converter station.
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